Downloaded From : www.EasyEngineering.net
t
ne
g.
in
er
ne gi
En
sy
a .E
w
w
w **Note: Other Websites/Blogs Owners Please do not Copy (or) Republish this Materials, Students & Graduates if You Find the Same Materials with EasyEngineering.net Watermarks or Logo, Kindly report us to
[email protected] Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
UNIT I PART A
1. What are the grades of concrete? (N/D 16) The mixes of grades M10, M15, M20 and M25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2) respectively.
w w
2. Mention any four destructive methods of testing concrete. (N/D 16), (M/J 12), (N/D 11) The methods of nondestructive testing of concretes are
w
Maturity test
b)
Ultrasonic pulse velocity method.
c)
Surface hardness method
d)
Ultra sonic tests for hardened concrete
e)
Concrete core cutting test
f)
Rebound hammer test
g)
Pull out test
h)
Vibration test
i)
Magnetic and electrical methods
in
3. What is the use of accelerators in concrete? (M/J 16)
er
e in
ng
yE as
.E
a)
g.
An admixture which, when added to concrete, mortar, or grout, increases the rate of hydration of strength development.
ne
hydraulic cement, shortens the time of set in concrete, or increases the rate of hardening or
t
Accelerating admixtures can be divided into groups based on their performance and application:
1. Set Accelerating Admixtures, Reduce the time for the mix to change from the plastic to the hardened state. Set accelerators have relatively limited use, mainly to produce an early set.
2. Hardening Accelerators, Which increase the strength at 24 hours by at least 120% at 20ºC and at 5ºC by at least 130% at 48 hours. Hardening accelerators find use where early stripping
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
of shuttering or very
early
access to pavements is required. They are often used in combination
with a high range water reducer, especially in cold conditions.
4. What is meant by hydration of cement? (N/D 15) The silicates and aluminates of cement react with water to form a binding medium, which solidifies into a hardened mass. This reaction is termed hydration which is exothermic with approximately 120 cal /g being librated.
w w
yE as
.E
w
5. List the various methods of concrete mix design. (M/J 12) The various methods of concrete mix design are, a) Bureau of Indian Standards (BIS) b) American concrete institute (ACI) c) British Development of Environment (DOE)
6. What are the various grades of cement? (N/D 11) Grade 33.Grade 43. Grade53
e in
ng
7. Define – Admixture (A/M 11)
8. What are the factors affecting the workability of concrete? (A/M 11)
t
ne
g.
The factors affecting the workability of a concrete are, Water content Mix proportions Size of aggregates Shape of aggregates Surface texture of aggregate Grading of aggregate
in
er
Admixture or concrete chemicals are the materials other than water, aggregates and cement, which are added to concrete immediately before or during its mixing. These are used to improve or give special properties to concrete.
9. What are the tests available for fresh concrete as per Indian Code of Practice? (N/D 10)
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
a) b) c) d) e) f)
The tests available for fresh concrete as per Indian Code of Practice are, The slump test The compacting factor test The Vee Bee consistency test The Kelly ball test The Flow test
10. Why curing is done for concrete? (A/M 10)
.E
w
d)
Following are the purposes of the curing of concrete: The curing protects the concrete surfaces from sun and wind The presence of water is essential to cause the chemical action which accompanies the setting of concrete. The strength of concrete gradually increases with age, if curing is efficient
w w
a) b) c)
11. What is the use of chemicals as retarders in concrete? (A/M 10)
yE as
In concrete, retarders are chemicals used in hot temperature and delays the setting time of concrete. 12. Define creep of concrete. (N/D 15)
ng
t
ne
g.
in
er
13. How is hardened concrete tested? (N/D 14) a) The hardened concrete are tested by b) Compressive strength test c) Flexural strength test d) Tensile strength test e) Concrete core test f) Pull out test g) Penetration test h) Ultrasonic pulse velocity test i) Rebound hammer test
e in
Inelastic deformation formed under a sustained load. It is partly due to the viscous flow of cement paste but largely due to the seepage of absorbed colloidal water from cement gel.
14. What are the factors affecting the workability of concrete? (N/D 14) a) The factors affecting the workability of a concrete are, b) Water content c) Mix proportions d) Size of aggregates
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
e) Shape of aggregates f) Surface texture of aggregate g) Grading of aggregate Use of admixtures
PART B
w w
1. Explain the various steps involved in the manufacture of concrete (N/D 16), (M/J 12) Manufacturing of concrete
.E
w
Introduction
yE as
Production of concrete requires meticulous care at every stage The ingredients of good and bad concrete are same but good rules are not Observed it may become bad Manufacturing of concrete includes the following stages
Transporting
Finishing Batching
t
The measurement of materials for making concrete is known as batching.
ne
g.
Curing
in
Compacting
er
Placing
e in
Mixing
ng
Batching
Methods of batching Volume batching Weigh batching
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Volume batching The required ingredients of conc. Are measured by volume basis Volume batching is done by various types of gauge boxes The gauge boxes are made with comparatively deeper with narrow surface Sometimes bottomless gauge boxes are used but it should be avoided Volume batching is not a good practice because of the difficulties it offers to granular material. Some of the sand in loose condition weighs much less than the same volume of dry compacted soil. For un important concrete or any small job concrete may be batched by volume.
w w
Weigh batching
w
.E
It is the correct method of measuring materials for concrete. Use of weight system in batching ,facilitates accuracy flexibility and simplicity.
yE as
The different types of weigh batching are there, they are used based on the different situation. In small works the weighing arrangement consist of two weighing buckets connected to the levers of spring
ng
loaded dials which indicates the load, The weighing buckets are mounted on a central spindle about
e in
which they rotate On large works the weigh bucket type of weighing equipment used ,the materials are fed from the over head storage hopper and it discharges by gravity.
in
er
Mixing
g.
Thorough mixing of materials is essential for the production of uniform concrete
Types of mixing
t
ne
The mixing should ensure that the mass becomes homogeneous uniform in color and consistency.
Hand mixing Machine mixing Hand mixing
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
It is practiced for small scale un important concrete works. Hand mixing should be done over a impervious concrete or brick floor sufficiently large size take one bag of cement. Spread out and measure d out fine aggregates and course aggregate in alternative layers. Pour the cement on the top of it and mix them dry by showel, turning the mixture over and over again until the uniformity of color is achieved. The uniform mixture is spread out in the thickness of about 20 cm. The water is taken and sprinkled over the mixture and simultaneously turned over. The operation is continued till such time a good uniform homogeneous concrete is obtained.
w w
Machine mixing
w
Mixing of concrete almost invariably carried out by machine ,for reinforced concrete work medium or
.E
large scale concrete works . Machine mixing is not only efficient it is also economical when quantity of concrete to be produced is large.
yE as
Type of mixer for mixing concrete Batch mixer
e in
ng
Continuous mixer
Concrete mixers are generally designed to run at a speed of 15 to 20 revolutions per minute For proper
er
mixing it is seen that about 25to 30 revolutions are required in a well designed mixer It is important that a mixer should not stop in between concreting operations for this requirement concrete mixer must be
t
ne
g.
Transporting of concrete
in
kept maintained
Concrete can be imported by variety of methods and equipments methods adopted for transportation of concrete Mortar pan Wheel barrow Crane, bucket and rope way
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Truck mixers and dumpers Belt conveyors Chute Skip and hoist Transit mixer Pump and pipe line Helicopter
w w
2. What are concrete chemicals? Explain in detail and discuss their uses. (N/D 16) (M/J 12)
w
Admixtures are the material, other than
Aggregates fiber reinforcement
ng
yE as
Water
.E
Cement
Which are used as an ingredient of concrete and is added to batch immediately before or during mixing.
in
er
• To reduce the cost of concrete construction.
e in
The major reasons for using admixtures are:
• To achieve certain properties in concrete more effectively than by other means.
g.
• To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in ad-
t
• To overcome certain emergencies during concreting operations.
ne
verse weather conditions.
Types of admixture for concrete • Chemical Admixtures • Mineral Additives • Polymers base
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Plasticizers (Water reducers) • In general, these chemicals act as dispersants for portland cement particles. By separating and spreading out the cement particles, internal friction is reduced, and slump and workability of the concrete is increased • Lowering w/cm is a key method for improving durability The organic substances or combinations of organic and inorganic substances, which allow a reduction in water content for the given workability, or give a higher workability at the same water content, are termed as plasticizing admixtures.
w w
The basic products constituting plasticizers are as follows: • Anionic surfactants such as lignosulphonates and their modifications and derivatives, salts of
w
sulphonates hydrocarbons.
.E
• Non ionic surfactants, such as polyglycol esters, acid of hydroxylated carboxylic acids and their modifications and derivatives.
yE as
• Other products, such as carbohydrates etc. Retarders
ng
A retarder is an admixture that slows down the chemical process of hydration so that concrete remains
e in
plastic and workable for a longer time than concrete without the retarder. 1. Retarders are used to overcome the accelerating effect of high temperature on setting properties of
er
concrete in hot weather concreting.
2. Very useful when concrete has to be place in very difficult conditions and delay may occur in
4. Other examples are: starches, cellulose products, sugars, acids or salts of acids
t
Accelerators
ne
g.
3. .Gypsum and Calcium Sulphate are well known retarders.
in
transporting and placing.
Accelerating admixtures are added to concrete to increase the rate of early strength development. 1. Permit earlier removal of formwork 2. Reduce the required period of curing
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
3. Advance the time that a structure can be placed in service 4. Partially compensate for the retarding effect of low temperature during cold weather concreting 5. In the emergency repair work. Commonly used materials as an accelerator:
Calcium chloride (Not used now)
Some of the soluble carbonates
Silicates fluosilicates (Expensive)
Some of the organic compounds such as triethenolamine (Expensive)
w w
Air-entraining Admixture One of the important advancements made in concrete technology was the discovery of air
w
.E
entrained concrete. (was made during the 1930s) In the United States and Canada, due to the recognition of the merits of air entrained concrete,
yE as
about 85% of concrete manufactured in America contains one or the other type of air entraining agent. By mixing a small quantity of air entraining agent or by using air entraining cement.
Minute spherical bubbles of size ranging from 5 microns to 80 microns distributed evenly in the
ng
entire mass of concrete. (10 and 1000 mm in ACI)
the spacing factor should not be greater than 0.2 mm
These incorporated millions of non-coalescing air bubbles, which will act as flexible ball
e in
bearings and will modify the properties of plastic concrete regarding workability, segregation, bleeding
in
er
and finishing quality of concrete.
1. Increased resistance to freezing and thawing.
t
2. Improvement in workability
ne
permeability
g.
It also modifies the properties of hardened concrete regarding its resistance to frost action and
3. Reduction in strength. 4. Reduces the tendencies of segregation. 5. Reduces the bleeding and laitance. 6. Decreases the permeability. 7. Increases the resistance to chemical attack.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
8. Permits reduction in sand content. 9. Improves place ability, and early finishing. 10.
Reduces the cement content, cost, and heat of hydration.
11.
Reduces the unit weight.
12.
Permits reduction in water content.
13.
Reduces the alkali-aggregate reaction.
14.
Reduces the modulus of elasticity.
w w
Damp-proofing & Waterproofing Admixture In practice one of the most important requirements of concrete is that it must be impervious to water
w
under two conditions;
.E
• Firstly, when subjected to pressure of water on one side. Permeability-reducing admixture for hydrostatic conditions (PRAH)
yE as
• Secondly, to the absorption of surface water by capillary action. Permeability reducing admixture for non-hydrostatic conditions (PRAN).
er
Factors Influencing Choice of Mix Design
e in
method. (M/J 16)
ng
3. What are the factors to be considered for mix design? Explain the step by step procedure for IS
According to IS 456:2000 and IS 1343:1980 the important influencing the design of concrete mix are
Maximum nominal Size of Aggregate
Grading of Combined aggregate
Maximum Water/ Cement Ratio
Workability
Durability
Quality Control.
t
Type of Cement
ne
g.
Grade of Concrete
in
IS Method of Mix Design The Bureau of Indian Standards, recommended a set of procedure for design of concrete mix. The
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
procedure is based on the research work carried out at national laboratories. Data for mix design The following basic data are required to be specified for design a concrete mix Characteristic Compressive strength only a few specified proportions of test results are
•
expected to fall of concrete at 28 days (fck) Degree of workability desired
•
Limitation on water/Cement Ratio with the minimum cement to ensure adequate durability
•
Type and maximum size of aggregate to be used.
•
Standard deviations of compressive strength of concrete.
•
Target Strength for Mix Design
w w
•
yE as
Where,
.E
Fck= f ck + t.s
w
The target average compressive strength (fck) of concrete at 28 days is given by
Fck= target average compressive strength at 28 days F ck= characteristics compressive strength at 28 days s= Standard deviation
ng
t= a stastical value, depending upon the accepted proportion of low results and the number of tests.
e in
According to Is 456: 2000 and IS 1343:1980 te characteristic strength is defined as the value below
The value of standard deviation is obtained from the table
t
ne
Selection of Water –Cement Ratio
g.
Fck= fck + 1.65 s
in
to
er
which not more than 5 percent of results are expected to fall. In such cases the above equation reduced
Since different cements and aggregates of different maximum sizes, grading, surface texture shape and other characteristics may produce concrete of different compressive strength for the same free water cement ratio, the relationship between strength and free water cement ratio should preferable be established for the material actually to be used. In the absence of such data, the preliminary free watercement ratio corresponding to the target strength at 28 days may be selected from the relationship shown below
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Alternatively, the preliminary free water cement ratio by mass corresponding to the average strength may be selected from the relationship shown below using the curve corresponding to the 28 days cement strength to be used for the purpose. However, this will need 28 days for testing of cement. The free water-cement ratio thus selected should be checked against limiting water-cement ratio for the requirements of durability as per table 5.4 and the lower of the two values should be adopted. Estimation of Air Content Approximate amount of entrapped air to be expected in normal concrete is given in table 9.6
w w
Selection of Water Content and fine to total aggregate ratio For the desired workability the quantity of mixing water per unit volume of concrete and the ratio of fine
w
aggregate (sand) to total aggregate by absolute volume are to be estimated from table below as applicable. Depending upon the nominal maximum size and type of aggregate.
.E
Selection of Water Content and fine to total aggregate ratio
yE as
For the desired workability the quantity of mixing water per unit volume of concrete and the ratio of fine aggregate (sand) to total aggregate by absolute volume are to be estimated from table below as applicable. Depending upon the nominal maximum size and type of aggregate.
ng
Calculation of Cement Content
e in
The cement content per unit volume of concrete may be calculated from the free water-cement ratio obtained in step- 2, and the quantity of water per unit volume of concrete obtained in step-4
er
The cement content so obtained should be checked against the minimum cement content for the
g.
Calculation of Aggregate Content
in
requirement of durability as per table 5 IS 456:2000 and the greater of the two value is adopted.
With the quantities of water and cement per unit volume of concrete and the ratio of fine to total
ne
aggregate already determined, the total aggregate content per unit volume of concrete may be
V= [ W + C + 1 x Sc p
Sfa
t
calculated from the following equations fa ] x 1 for fine aggregate …………………………1 1000
And V=[W+ C+ Sc (1-p)
1
x Ca ] x 1 for coarse aggregate …………..2
Sca 1000
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Where, V= Absolute volume of fresh concrete (m3) W= Mass of Water (kg) per m3 of concrete C= Mass of Cement (Kg) per m3 of concrete Sc= Specific gravity of cement say 3.15 P= ratio of fine aggregate to total aggregate by absolute volume Fa and Ca = Total masses of fine aggregate and coarse aggregate (kg) / m3 of concrete mass
w w
respectively
Sfa, Sca= Specific gravities of saturated surface dry fine aggregate and coarse aggregate respectively
.E
w
Normally Sfa= 2.6 and Sca= 2.7
Slump Test
ng
Definition
yE as
4. Explain any two tests for testing of fresh concrete. (M/J 16), (M/J 12)
e in
A slump test is a method used to determine the consistency of concrete. The consistency, or stiffness, indicates how much water has been used in the mix. The stiffness of the concrete mix should be matched to the requirements for the finished product quality
er
Slump is a measurement of concrete’s workability, or fluidity.
It’s an indirect measurement of concrete consistency or stiffness.
Principle
ne
g.
in
The slump test result is a measure of the behavior of a compacted inverted cone of concrete under the
t
action of gravity. It measures the consistency or the wetness of concrete. Apparatus
Slump cone : frustum of a cone, 300 mm (12 in) of height. The base is 200 mm (8in) in
diameter and it has a smaller opening at the top of 100 mm
Scale for measurement,
Temping rod(steel) 15mm diameter, 60cm length.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
w w
Procedure
The base is placed on a smooth surface and the container is filled with concrete in three
w
.E
layers, whose workability is to be tested . Each layer is temped 25 times with a standard 16 mm (5/8 in) diameter steel rod,
yE as
rounded at the end.
When the mold is completely filled with concrete, the top surface is struck off (leveled with
mold top opening) by means of screening and rolling motion of the temping rod.
ng
The mold must be firmly held against its base during the entire operation so that it could
brazed to the mold.
Collapse Slump
ne
g.
Types Of Slump
in
er
e in
not move due to the pouring of concrete and this can be done by means of handles or foot – rests
In a collapse slump the concrete collapses completely.
t
A collapse slump will generally mean that the mix is too wet or that it is a high workability mix, for which slump test is not appropriate.
Shear Slump
In a shear slump the top portion of the concrete shears off and slips sideways. OR If one-half of the cone slides down an inclined plane, the slump is said to be a shear slump. If a shear or collapse slump is achieved, a fresh sample should be taken and the test is repeated.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
If the shear slump persists, as may the case with harsh mixes, this is an indication of lack of cohesion of the mix. True Slump
In a true slump the concrete simply subsides, keeping more or less to shape This is the only slump which is used in various tests. Mixes of stiff consistence have a Zero slump, so that in the rather dry range no variation can be detected between mixes of different workability.
w w
However , in a lean mix with a tendency to harshness, a true slump can easily change to the shear slump type or even to collapse, and widely different values of slump can be obtained in different
.E
Uses
w
samples from the same mix; thus, the slump test is unreliable for lean mixes.
The slump test is used to ensure uniformity for different batches of similar concrete under
yE as
field conditions and to ascertain the effects of plasticizers on their introduction.
This test is very useful on site as a check on the day-to-day or hour- to-hour variation in
the materials being fed into the mixer. An increase in slump may mean, for instance, that the moisture
ng
content of aggregate has unexpectedly increases.
Other cause would be a change in the grading of the aggregate, such as a deficiency of
e in
sand.
Too high or too low a slump gives immediate warning and enables the mixer operator to
This application of slump test as well as its simplicity, is responsible for its widespread
use.
t
Compacting Factor Test
ne
g.
in
remedy the situation.
er
Introduction
These tests were developed in the UK by Glanville ( 1947 ) and it is measure the degree
of compaction For the standard amount of work and thus offer a direct and reasonably reliable assessment of the workability Of concrete . the test require measurement of the weight of the partially and fully compacted concrete and the ratio the partially compacted weight to the fully compacted
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
weight, which is always less than one, is known as compacted factor . For the normal range of concrete the compacting factor lies between 0.8 - 0.92
Apparatus
Trowels
Hand Scoop (15.2 cm long)
Rod of steel or other suitable material (1.6 cm diameter, 61 cm long rounded
w w
at one end ).
Balance.
w
Procedure
.E
Ensure the apparatus and associated equipment are clean before test and free from
Weigh the bottom cylinder to nearest 10gm, put it back on the stand and cover it up with
a pair of floats .
Gently fill the upper hopper with the sampled concrete to the level of the rim with use of a
scoop.
Immediately open the trap door of the upper hopper and allow the sampled concrete to
fall into the middle hopper.
er
e in
ng
yE as
hardened concrete and superfluous water.
Remove the floats on top of the bottom cylinder and open the trap door of the middle
Remove the surplus concrete above the top of the bottom cylinder by holding a float in
g.
in
hopper allowing the sampled concrete to fall into the bottom cylinder.
ne
each hand and move towards each other to cut off the concrete across the top of cylinder Wipe clean the outside of cylinder of concrete and weigh to nearest 10gm.
Subtract the weight of empty cylinder from the weight of cylinder plus concrete to obtain
t
the weight of partially compacted concrete.
Remove the concrete from the cylinder and refill with sampled concrete in layers.
Compact each layer thoroughly with the standard Compacting Bar to achieve full
compaction.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Float off the surplus concrete to top of cylinder and wipe it clean.
Weigh the cylinder to nearest 10gm and subtract the weight of empty cylinder from the
weight of cylinder plus concrete to obtain the weight of fully compacted concrete.
5. Discuss in detail about Ultrasonic Pulse velocity test. (N/D 11) (M/J 15) Ultrasonic test on concrete is a recognized non-destructive test to asses the homogeneity and
w w
integrity of concrete. With this ultrasonic test on concrete, following can be assessed: 1. Qualitative assessment of strength of concrete, its gradation in different locations of structural members and plotting the same. 2. Any discontinuity in cross section like cracks, cover concrete delamination etc.
w
3. Depth of surface cracks.
.E
This test essentially consists of measuring travel time, T of ultrasonic pulse of 50 to 54 kHz, produced
yE as
by an electro-acoustical transducer, held in contact with one surface of the concrete member under test and receiving the same by a similar transducer in contact with the surface at the other end. With the path length L, (i.e. the distance between the two probes) and time of travel T, the pulse velocity (V=L/T) is calculated (fig.2). Higher the elastic modulus, density and integrity of the concrete, higher is the pulse
ng
velocity. The ultrasonic pulse velocity depends on the density and elastic properties of the material
e in
being tested.
Though pulse velocity is related with crushing strength of concrete, yet no statistical correlation can be
t
ne
b) Lateral dimension of the specimen tested
g.
a) Path length
in
The pulse velocity in concrete may be influenced by:
er
applied.
c) Presence of reinforcement steel d) Moisture content of the concrete The influence of path length will be negligible provided it is not less than 100mm when 20mm size aggregate is used or less than 150mm for 40mm size aggregate. Pulse velocity will not be influenced by
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
the shape of the specimen, provided its least lateral dimension (i.e. its dimension measured at right angles to the pulse path) is not less than the wavelength of the pulse vibrations. For pulse of 50Hz frequency, this corresponds to a least lateral dimension of about 80mm. the velocity of pulses in steel bar is generally higher than they are in concrete. For this reason pulse velocity measurements made in the vicinity of reinforcing steel may be high and not representative of the concrete. The influence of the reinforcement is generally small if the bars runs in a direction at right angles to the pulse path and the quantity of steel is small in relation to the path length. The moisture content of the concrete can have a small but significant influence on the pulse velocity. In general, the velocity is increased with increased
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w
moisture content, the influence being more marked for lower quality concrete.
Fig.1: Method of propagating and receiving pulses Measurement of pulse velocities at points on a regular grid on the surface of a concrete structure provides a reliable method of assessing the homogeneity of the concrete. The size of the grid chosen will depend on the size of the structure and the amount of variability encountered.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Table: 1 – General Guidelines for Concrete Quality based on UPV CONCRETE QUALITY
PULSE VELOCITY
Very good to excellent
3.5 – 4.0 km/s
Good to very good, slight porosity may exist
3.0 – 3.5 km/s
Satisfactory but loss of integrity is suspected
.E
w
w w
>4.0 km/s
Poor and los of integrity exist.
ng
yE as
<3.0 km/s
Table 1 shows the guidelines for qualitative assessment of concrete based on UPV test results. To
e in
make a more realistic assessment of the condition of surface of a structural member, the pulse velocity can be combined with rebound number. Table 2 shows the guidelines for identification of corrosion
er
prone locations by combining the results of pulse velocity and rebound number.
in
Table:2 – Identification of Corrosion Prone Location based on Pulse Velocity and Hammer Readings
1
Test Results
Interpretations
High UPV values, high rebound number
Not corrosion prone
t
No.
ne
g.
Sl.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
2
3
Surface delamination, low quality of surface concrete,
numbers
corrosion prone
Not corrosion prone, however to be confirmed by chemical
Low UPV, high rebound numbers
tests, carbonation, pH
Corrosion prone, requires chemical and electrochemical
w w
4
Medium range UPV values, low rebound
Low UPV, low rebound numbers
tests.
yE as
.E
w Detection of Defects
When ultrasonic pulse travelling through concrete meets a concrete-air interface, there is a negligible transmission of energy across this interface so that any air filled crack or void lying directly between the
ng
transducers will obstruct the direct beam of ultrasonic when the void has a projected area larger than the area of transducer faces. The first pulse to arrive at the receiving transducer will have been directed
er
Estimating the depth of cracks
e in
around the periphery of the defect and the time will be longer than in similar concrete with no defect.
An estimate of the depth of a crack visible at the surface can be obtained by the transit times across the
in
crack for two different arrangements of the transducers placed on the surface. One suitable
g.
arrangement is one in which the transmitting and receiving transducers are placed on opposite sides of
ne
the crack and distant from it. Two values of X are chosen, one being twice that of the other, and the transmit times corresponding to these are measured. An equation may be derived by assuming that the
t
plane of the crack is perpendicular to the concrete surface and that the concrete in the vicinity of the crack is of reasonably uniform quality. It is important that the distance X be measured accurately and that very good coupling is developed between the transducers and the concrete surface. The method is valid provided the crack is not filled with water. This test is done as per IS: 13311 (Part 1) – 1992.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Procedure for Ultrasonic Pulse Velocity i) Preparing for use: Before switching on the ‘V’ meter, the transducers should be connected to the sockets marked “TRAN” and ” REC”. The ‘V’ meter may be operated with either: a) the internal battery, b) an external battery or c) the A.C line. ii) Set reference: A reference bar is provided to check the instrument zero. The pulse time for the bar is
w w
engraved on it. Apply a smear of grease to the transducer faces before placing it on the opposite ends of the bar. Adjust the ‘SET REF’ control until the reference bar transit time is obtained on the instrument
w
read-out.
iii) Range selection: For maximum accuracy, it is recommended that the 0.1 microsecond range be
.E
selected for path length upto 400mm.
yE as
iv) Pulse velocity: Having determined the most suitable test points on the material to be tested, make careful measurement of the path length ‘L’. Apply couplant to the surfaces of the transducers and press it hard onto the surface of the material. Do not move the transducers while a reading is being taken, as
ng
this can generate noise signals and errors in measurements. Continue holding the transducers onto the surface of the material until a consistent reading appears on the display, which is the time in
e in
microsecond for the ultrasonic pulse to travel the distance ‘L’. The mean value of the display readings should be taken when the units digit hunts between two values.
er
Pulse velocity=(Path length/Travel time)
v) Separation of transducer leads: It is advisable to prevent the two transducer leads from coming into
in
close contact with each other when the transit time measurements are being taken. If this is not done,
t
6. Explain the concept of mix design (A/M 11) (N/D 15)
ne
incorrect display of the transit time.
g.
the receiver lead might pick-up unwanted signals from the transmitter lead and this would result in an
Concrete Mix Design As Per Indian Standard Code Concrete Mix Design Introduction The process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability, and workability as economically as possible, is termed the concrete mix design. The proportioning of ingredient of
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
concrete is governed by the required performance of concrete in 2 states, namely the plastic and the hardened states. If the plastic concrete is not workable, it cannot be properly placed and compacted. The property of workability, therefore, becomes of vital importance. The compressive strength of hardened concrete which is generally considered to be an index of its other properties, depends upon many factors, e.g. quality and quantity of cement, water and aggregates; batching and mixing; placing, compaction and curing. The cost of concrete is made up of the cost of materials, plant and labour. The variations in the cost of materials arise from the fact that the cement is several times costly than the aggregate, thus the aim is to produce as lean a mix as
w w
possible. From technical point of view the rich mixes may lead to high shrinkage and cracking in the structural concrete, and to evolution of high heat of hydration in mass concrete which may cause cracking.
yE as
.E
w
The actual cost of concrete is related to the cost of materials required for producing a minimum mean strength called characteristic strength that is specified by the designer of the structure. This depends on the quality control measures, but there is no doubt that the quality control adds to the cost of concrete. The extent of quality control is often an economic compromise, and depends on the size and type of job. The cost of labour depends on the workability of mix, e.g., a concrete mix of inadequate
for the particular site conditions
in
er
e in
ng
workability may result in a high cost of labour to obtain a degree of compaction with available equipment. Requirements of concrete mix design The requirements which form the basis of selection and proportioning of mix ingredients are : a ) The minimum compressive strength required from structural consideration b) The adequate workability necessary for full compaction with the compacting equipment available. c) Maximum water-cement ratio and/or maximum cement content to give adequate durability
t
ne
g.
d) Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass concrete. Types of Mixes 1. Nominal Mixes In the past the specifications for concrete prescribed the proportions of cement, fine and coarse aggregates. These mixes of fixed cement-aggregate ratio which ensures adequate strength are termed nominal mixes. These offer simplicity and under normal circumstances, have a margin of strength above that specified. However, due to the variability of mix ingredients the nominal concrete for a given workability varies widely in strength. 2. Standard mixes The nominal mixes of fixed cement-aggregate ratio (by volume) vary widely in strength and may result
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
in under- or over-rich mixes. For this reason, the minimum compressive strength has been included in many specifications. These mixes are termed standard mixes. IS 456-2000 has designated the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40. In this designation the letter M refers to the mix and the number to the specified 28 day cube strength of mix in N/mm2. The mixes of grades M10, M15, M20 and M25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2) respectively. 3. Designed Mixes In these mixes the performance of the concrete is specified by the designer but the mix proportions
w w
are determined by the producer of concrete, except that the minimum cement content can be laid down. This is most rational approach to the selection of mix proportions with specific materials in mind possessing more or less unique characteristics. The approach results in the production of concrete
yE as
.E
w
with the appropriate properties most economically. However, the designed mix does not serve as a guide since this does not guarantee the correct mix proportions for the prescribed performance. For the concrete with undemanding performance nominal or standard mixes (prescribed in the codes by quantities of dry ingredients per cubic meter and by slump) may be used only for very small jobs, when the 28-day strength of concrete does not exceed 30 N/mm2. No control testing is necessary
in
er
e in
ng
reliance being placed on the masses of the ingredients. Factors affecting the choice of mix proportions The various factors affecting the mix design are: 1. Compressive strength It is one of the most important properties of concrete and influences many other describable properties of the hardened concrete. The mean compressive strength required at a specific age, usually 28 days, determines the nominal water-cement ratio of the mix. The other factor affecting the strength of concrete at a given age and cured at a prescribed temperature is the degree of compaction. According to Abraham’s law the strength of fully compacted concrete is inversely proportional to the water-
ne
g.
cement ratio.
t
2. Workability The degree of workability required depends on three factors. These are the size of the section to be concreted, the amount of reinforcement, and the method of compaction to be used. For the narrow and complicated section with numerous corners or inaccessible parts, the concrete must have a high workability so that full compaction can be achieved with a reasonable amount of effort. This also applies to the embedded steel sections. The desired workability depends on the compacting equipment available at the site. 3. Durability The durability of concrete is its resistance to the aggressive environmental conditions. High strength
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
concrete is generally more durable than low strength concrete. In the situations when the high strength is not necessary but the conditions of exposure are such that high durability is vital, the durability requirement will determine the water-cement ratio to be used. 4. Maximum nominal size of aggregate In general, larger the maximum size of aggregate, smaller is the cement requirement for a particular water-cement ratio, because the workability of concrete increases with increase in maximum size of the aggregate. However, the compressive strength tends to increase with the decrease in size of aggregate.
w w
IS 456:2000 and IS 1343:1980 recommend that the nominal size of the aggregate should be as large as possible. 5. Grading and type of aggregate
yE as
.E
w
The grading of aggregate influences the mix proportions for a specified workability and water-cement ratio. Coarser the grading leaner will be mix which can be used. Very lean mix is not desirable since it does not contain enough finer material to make the concrete cohesive. The type of aggregate influences strongly the aggregate-cement ratio for the desired workability and stipulated water cement ratio. An important feature of a satisfactory aggregate is the uniformity of the
in
er
e in
ng
grading which can be achieved by mixing different size fractions. 6. Quality Control The degree of control can be estimated statistically by the variations in test results. The variation in strength results from the variations in the properties of the mix ingredients and lack of control of accuracy in batching, mixing, placing, curing and testing. The lower the difference between the mean and minimum strengths of the mix lower will be the cement-content required. The factor controlling this difference is termed as quality control. Mix Proportion designations The common method of expressing the proportions of ingredients of a concrete mix is in the terms of
t
ne
g.
parts or ratios of cement, fine and coarse aggregates. For e.g., a concrete mix of proportions 1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or the mix contains one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The proportions are either by volume or by mass. The water-cement ratio is usually expressed in mass 7
Factors to be considered for mix design. (A/M 11) (N/D 12)
ð The grade designation giving the characteristic strength requirement of concrete. ð The type of cement influences the rate of development of compressive strength of concrete. ð Maximum nominal size of aggregates to be used in concrete may be as large as possible within the limits prescribed by IS 456:2000.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
ð The cement content is to be limited from shrinkage, cracking and creep. ð The workability of concrete for satisfactory placing and compaction is related to the size and shape of section, quantity and spacing of reinforcement and technique used for transportation, placing and compaction. Procedure 1. Determine the mean target strength ft from the specified characteristic compressive strength at 28-day fck and the level of quality control. ft = fck + 1.65 S
w w
where S is the standard deviation obtained from the Table of approximate contents given after the design mix. 2. Obtain the water cement ratio for the desired mean target using the emperical relationship between
yE as
.E
w
compressive strength and water cement ratio so chosen is checked against the limiting water cement ratio. The water cement ratio so chosen is checked against the limiting water cement ratio for the requirements of durability given in table and adopts the lower of the two values. 3. Estimate the amount of entrapped air for maximum nominal size of the aggregate from the table. 4. Select the water content, for the required workability and maximum size of aggregates (for aggregates
durability, and greater of the two values is adopted.
in
er
e in
ng
in saturated surface dry condition) from table. 5. Determine the percentage of fine aggregate in total aggregate by absolute volume from table for the concrete using crushed coarse aggregate. 6. Adjust the values of water content and percentage of sand as provided in the table for any difference in workability, water cement ratio, grading of fine aggregate and for rounded aggregate the values are given in table. 7. Calculate the cement content form the water-cement ratio and the final water content as arrived after adjustment. Check the cement against the minimum cement content from the requirements of the
t
ne
g.
8. From the quantities of water and cement per unit volume of concrete and the percentage of sand already determined in steps 6 and 7 above, calculate the content of coarse and fine aggregates per unit volume of concrete from the following relations:
where V = absolute volume of concrete = gross volume (1m3) minus the volume of entrapped air Sc = specific gravity of cement
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
W = Mass of water per cubic metre of concrete, kg C = mass of cement per cubic metre of concrete, kg p = ratio of fine aggregate to total aggregate by absolute volume fa, Ca = total masses of fine and coarse aggregates, per cubic metre of concrete, respectively, kg, and Sfa, Sca = specific gravities of saturated surface dry fine and coarse aggregates, respectively 9. Determine the concrete mix proportions for the first trial mix. 10. Prepare the concrete using the calculated proportions and cast three cubes of 150 mm size and test them wet after 28-days moist curing and check for the strength. 11. Prepare trial mixes with suitable adjustments till the final mix proportions are arrived at.
w w
8. Why is compaction necessary in concrete? Explain in detail the various methods of compaction of
w
concrete. [N/D 14]
yE as
.E
Compaction is a process of expelling the entrapped air. If we don’t expel this air, it will result into honeycombing and reduced strength. It has been found from the experimental studies that 1% air in the concrete approximately reduces the strength by 6%.
DIFFERENT METHODS OF CONCRETE COMPACTION There are two methods of compaction adopted in the field as given below.
Mechanical Compaction
er
(A) HAND COMPACTION
e in
Hand compaction
ng
1. RODDING
ne
g.
in
Hand compaction is used for ordinary and unimportant structures. Workability should be decided in such a way that the chances of honeycombing should be minimum. The various methods of hand compaction are as given below.
t
It is a method of poking with 2m long, 16 mm dia rod at sharp corners and edges. The thickness of layers for rodding should be 15 to 20 cm. 2. RAMMING It is generally used for compaction on ground in plain concrete. It is not used either in RCC or on upper floors. 3. TAMPING
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
It is a method in which the top surface is beaten by wooden cross beam of cross section 10 cm x 10 cm. both compaction and levelling are achieved simultaneously. It is mainly used for roof slabs and road pavements. (B) MECHANICAL COMPACTION Vibration is imparted to the concrete by mechanical means. It causes temporary liquefaction so that air bubbles come on to the top and expelled ultimately. Mechanical vibration can be of various types as given under. 1. INTERNAL VIBRATION
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w
It is most commonly used technique of concrete vibration. Vibration is achieved due to eccentric weights attached to the shaft. The needle diameter varies from 20 mm to 75 mm and its length varies from 25 cm to 90 cm. the frequency range adopted is normally 3500 to 5000 rpm. The correct and incorrect methods of vibration using internal vibration needles are shown below.
2. EXTERNAL VIBRATION This is adopted where internal vibration can’t be used due to either thin sections or heavy reinforcement. External vibration is less effective and it consumes more power as compared to the internal vibration. The form work also has to be made extra strong when external vibration is used.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
3. TABLE VIBRATION It is mainly used for laboratories where concrete is put on the table. 4. PLATFORM VIBRATION It is similar to table vibrators but these are generally used on a very large scale. 5. SURFACE VIBRATION These are also called screed board vibrators. The action is similar to that of tamping. The vibrator is placed on screed board and vibration is given on the surface. It is mainly used for roof slabs, road pavements etc., but it is not effective beyond 15 cm depth.
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
UNIT II PART A 1. Define damp proof course. What are its causes of dampness? (N/D 16) Definition :-The courses which are laid to check the entry of water or moisture into the building are called damp proof courses. Causes:- faulty design of structure faulty construction or poor workmanship
w w
use of poor quality materials in construction. 2. What is the purpose of providing construction joints? (N/D 16)
w
Joints in concrete building construction are construction joints, expansion joints, contraction joints and
.E
isolation joints. They prevent cracking of concrete.
yE as
3. Define Slip form. (M/J 16), (N/D 10).
This is a formwork system which can be used to form any regular shape or core. The formwork rises
ng
continuously, at a rate of about 300mm per hour, supporting itself on the core and not relying on support or access from other parts of the building or permanent works
er
e in
4. Define scaffolding. (M/J 13), (M/J 12)
5. Mention the significance of bond in masonry structures. (M/J 12)
g.
in
Scaffolding:-It is defined as the temporary structure employed in the building construction for supporting workers, materials and tools etc., during its construction alteration, demolition, painting and repair etc.,
ne
Masonry is defined as the construction of building units bonded together with mortar. The selection of material for the masonry will depend on strength, water proofing, thermal
t
insulation, fire resistance, durability and economy. 6. Define – Stretcher bond. (N/D 11)
In this bond, all the bricks are laid with their length parallel to the longitudinal direction of the wall. Since stretchers are visible in duration it is known as stretcher bond. 7. What are the causes of dampness? (N/D 11)
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Moisture up from the ground to the walls. Rain travel from wall tops. Rain beating against external walls.
8. Define plinth. (N/D 10) Plinth:- It is the horizontal course of stone or brick provided at the base of the wall above ground level. It indicates the height of the ground floor level above the natural ground level. It protects the building from dampness.
w w
9. Define acoustics. (N/D 14) The term acoustics is defined as the science of sound, and it describes the origin, propagation and
.E
w
sensation of sound.
yE as
10. What are the conditions for good acoustics of an auditorium? (N/D 12) Conditions for good acoustics of an auditorium: - The initial sound should be of adequate intensity or loudness. It is important for a speaker to be heard over a long distance. The sound produced
ng
should be evenly spread over the whole area covered by audience. If the sound is not evenly distributed echoes will be established. The initial sound should be clear and distinct. For music hall, should be reduced.
in
er
11 Write the types of scaffolding. (N/D 11)
e in
the initial sound should reach the audience with same frequency and intensity. All undesired sound
g.
Types of scaffolding: - Single scaffolding or Brick layer scaffolding, Double scaffolding or Masons scaffolding, Ladder scaffolding,Cantilever or Needle scaffolding.
ne
12 Write any some materials used for joints. (N/D 10)
t
Bituminous felt Metal strips Fiber board these are some of the materials used for joints. 13 Define flooring:- (N/D 12)
The properly supported horizontal surfaces which divide the building into different levels for providing accommodation one above the other within space are called floors. 14 What are monolithic wall? Write its classifications? (M/J 13) Monolithic walls:-Walls built of a material requiring some kind of shuttering in the initial stages.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Masonry can be classified into the following types stone masonry. Brick masonry. Hollow block concrete masonry. Reinforced masonry, composite masonry.
PART B 1.
Explain in detail, the different types of bonds in brick masonry with sketches. (N/D 16) (M/J 15)
w w
Bond is the arrangement of bricks or stones in each course, so as to ensure the greatest possible interlocking and to avoid the continuity of vertical joints in two successive courses, both on the face and
w
in the body of a wall.
OBJECTIVES OF BONDS
t
ne
g.
in
er
e in
ng
yE as
.E Types of Bonds used in Masonry
A bond is provided to achieve the following objectives: (a) The primary objective of providing a bond is to break the continuity of the vertical joints in the successive courses both in the length and thickness of masonry structure. As a result, the structure will
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
act as a bounded mass and its load will be transmitted uniformly to the foundations. (b) To ensure longitudinal and lateral strength of the structure. (c) To provide pleasing appearance by laying bricks symmetrically. (d) To do masonry work quickly by engaging more masons on a job at a time.
Types of Bonds
w w
(1) ENGLISH BOND
w
The bond, in which headers and stretchers are laid in alternate courses, is called "English bond".
.E
The following are the salient features of English bond:
yE as
(i) Headers and stretchers are laid in alternate courses. (ii) In each heading course, a queen closer is placed next to quoin header and the remaining bricks are laid as headers.
(iii) Every alternate header in a course comes centrally over the joint between two stretchers in the
ng
course below, giving an approximate lap of 2 ¼ in.
even multiple of half bricks (e.g. 9 in, 18 in, 27 in, etc.)
e in
(iv) The same course will show headers or stretchers on face and back, if the thickness of the wall is an
er
(v) The same course will show headers on the face and stretchers on the back and vice versa, if the thickness of the wall is an odd multiple of half brick. (13 1/2 in , 22 1/2 in , etc )
t
ne
g.
(vii) Every transverse joint is continuous from face to face.
in
(vi) The middle portion of the thicker walls consists entirely of headers.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
(2) FLEMISH BOND The bond, in which headers and stretchers are laid alternately in the same course, is called "Flemish bond". The following are the salient features of Flemish bond: (i) Headers and stretchers are laid alternately in the same course. (ii) Every header in each course lies centrally over every stretcher of the underlying course. (iii) In every alternate course a queen closer is placed next to quoin header, so as to provide a lap of
w w
approximately 2 1/4 in.
(iv) Brick bats are to be used in walls having thickness equal to an odd multiple of half brick.
English Bonds
ng
yE as
.E
w Sr
This bond consists of headers and stretchers
stretchers laid in alternative courses.
laid alternatively in each course.
It is strongest of all the bonds.
It is less strong for walls having thickness more
g.
in
2
This bond consists of headers and
er
1
e in
No.
Flemish bond
than 13 ½ inches. It provides rough appearanceespecially
It provides good appearance for all thickness of
for one brick thick walls.
walls.
There are no noticeable continuous
There are partly continuous vertical joints in the
vertical joints in the structure built in this
structure built in this bond.
t
4
ne
3
bond. 5
Much attention is not required in
Special attention is required in providing this bond.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
providing this bond. 6
Progress of work is more.
Progress of work is less.
7
It is costly because the use of brick bats
It is economical because brick bats are allowed for
is not allowed.
forming this bind.
g.
in
(a) DOUBLE FLEMISH BOND
er
e in
ng
yE as
.E
w
w w Types of Flemish Bond
of the wall, is called Double Flemish Bond.
t
(b) SINGLE FLEMISH BOND
ne
The bond in which headers and stretchers are laid alternately in each course, both in the face and back
The bond provided in a wall with Flemish bond in facing and English bond in backing is called "Single Flemish bond" or "Cross bond".
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
This bond combines the advantages of both English and Flemish bonds and simultaneously eliminates their disadvantages. This bond is recommended where costly bricks are specified for facing in order to provide good appearance to the wall. Also, it can be made more economical by using cheap quality of bricks on the back of wall. On the other hand, it weakens the overall strength of the wall because of maximum use of brick and
w w
existence of continuous vertical joints. Also, it can not be provided in walls having thickness less than 13 ½ in.
yE as
.E
w (3) HEADING BOND
The bond in which all the bricks are laid as headers in every course of a wall is called "Heading bond".
ng
3/4 bats are laid as quoin bricks in the alternate courses to break the continuity of vertical joints, which
e in
increases the transverse strength but weakens the longitudinal strength of the wall. This bond is commonly used for constructing steining of wells, footings of walls and columns, corbels,
ne
g.
in
er
cornices, etc.
t
(4) STRETCHING BOND
The bond in which all the bricks are laid as stretchers in every course is called "Stretching bond”. This bond is provided for constructing 4 ½ in thick partition walls.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
(5) GARDEN WALL BOND This bond is used for constructing one brick thick garden walls, boundary walls, and other walls such as
w w
outer leaves of cavity walls to provide good appearance. Types of Garden Wall Bond
w
.E
(a) ENGLISH GARDEN WALL BOND
"English Garden Wall Bond"
t
ne
g.
in
er
e in
ng
yE as
The garden wall bond in which a heading course is provided after 3 or 5 stretching courses is called
(b) FLEMISH GARDEN WALL BOND In this bond a header is provided after 3 or 5 stretches in each course. This bond is also known as “Sussex or Scotch Bond".
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
yE as
.E
w
w w (6) RAKING BOND
The bond in which all the bricks are laid at an angle other than 900 to the facing and backing of the wall is known as "Raking bond".
ng
strength.
e in
This bond is used for doing inner filling of walls at suitable intervals to improve their longitudinal
transverse strength of the wall.
in
er
The angle of rake between any two adjacent courses should be 90 degree to attain maximum This bond can also be used as paving in case of brick floors, 4 ½ in thick.
ne
(a) HERRING BONE BOND
g.
Types of Raking Bond
t
The raking bond in which bricks are laid at an angle of 45 degree , strating at the central line and proceeding towards the facing and backing of the wall, is called "Herring Bone Bond"
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
(b) DIAGONAL BOND
w w
The raking bond in which bricks are laid starting from the corner in parallel rows inclined to the facing
(7) HOOP IRON BOND
e in
ng
yE as
.E
w
and backing of the wall is known as "Diagonal bond".
er
The bond in which, after every fourth or fifth course of masonry, reinforcement in the form of longitudinal ties is provided for additional strength of the wall, is called " Hoop Iron Bond".
t
ne
g.
in
This bond is provided for constructing 4 1/2 in thick partition walls
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
(8) MONK BOND This bond in which two stretchers and one header are laid alternately in each course is called "Monk bond". This bond is used in the construction of boundary walls.
yE as
.E
w
w w (9) RHOM BOND
This bond in which brick/ tiles are laid in order to have straight horizontal and vertical joints in the facing
ng
is called "Rhom bond".
t
ne
g.
in
er
e in
This bond is used only in facing work to provide architectural beauty.
2. Explain in detail any two types of joints in buildings with sketches.
(N/D 16) (M/J 15)
Types of joints in concrete constructions are:
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
1.
Construction Joints
2.
Expansion Joints
Construction Joints in Concrete: Construction joints are placed in a concrete slab to define the extent of the individual placements, generally in conformity with a predetermined joint layout. Construction joints must be designed in order to allow displacements between both sides of the slab
w w
but, at the same time, they have to transfer flexural stresses produced in the slab by external loads. Construction joints must allow horizontal displacement right-angled to the joint surface that is normally
w
caused by thermal and shrinkage movement. At the same time they must not allow vertical or rotational
t
ne
g.
in
er
e in
ng
yE as
joint.
.E
displacements. Fig.1 summarizes which displacement must be allowed or not allowed by a construction
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Expansion Joints These are structural separation between building elements that allow independent movement without damage to the assembly. Expansion joints are designed to safely absorb the thermal expansion and contraction of various construction materials, to absorb vibration. They are commonly provided in bridges, railway tracks, piping systems, and other structures.
ng
yE as
.E
w
w w 3. What is scaffolding? Mention its various components and types. (M/J 16)
e in
Scaffolding
Temporary support required to provide a safe and convenient working surface is known as scaffold or
er
scaffolding. There is a limit to the safe working height at which a worker can access the
g.
in
building work from ground level.
Scaffolding is used on new-build projects and for work to existing structures, including maintenance
t
rapid erection, disassembly and reuse.
ne
and repair work. The temporary structure needs to be structurally safe yet also capable of
Functional requirements
Provide a safe working horizontal platform
Provide safe horizontal and vertical access to buildings
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Scaffold may be owned and maintained by a contractor, although it is more common for the scaffolding to be hired from a scaffolding subcontractor as and when required. Temporary structures must be designed to suit their purpose by a competent person (e.g. certified structural engineer) Scaffold components The scaffold is usually constructed from aluminium or steel tubes and clips, with timber or metal scaffold planks used to form a secure and level working platform. Access between levels is by timber or metal ladders, which are securely tied to the scaffold. Other common components are scaffold boards
w w
and edge protection. Scaffold boards
w
A standard scaffold board is 225mm wide by 38mm thick with a maximum span of 1.5m. The board is
.E
made from sawn softwood. Lightweight metal scaffold boards are used in some systems. Greater spans
yE as
can be achieved by using thicker boards; the distances between transoms on which the scaffolding boards span must not exceed the maximum span allowed for each board. Each board must be closely butted together so that there is no chance of the board slipping off the
ng
supporting tubes. Each board must overhang the ledger by 50mm, but the overhang must not exceed
e in
four times the thickness of each scaffolding board.
Scaffolding boards are butted together to make a working platform; the minimum working
er
platform depth is three boards. When materials are loaded onto the platform, the clear passage
in
for workers should be at least 430mm. If the materials are to be manoeuvred on the scaffold, a distance of 600mm clear pedestrian passage must be maintained at all times.
g.
When laying bricks, the scaffold platform should be at least five boards wide (1150mm). Hop-up
ne
brackets may be used to increase the working height of the lift and to increase the working width of the
t
scaffolding platform. When using hop-up brackets, care must be taken not to overload the scaffold. The cantilevered bracket induces bending moments in the standards.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
er
Toe boards
e in
ng
yE as
.E
w
w w Hop-up brackets
in
Toe boards must be used at the end of the scaffolding to ensure that materials and tools do not fall off
g.
the scaffold. The toe boards must be a minimum height of 150mm. The boards also prevent the for materials and workers, but must be replaced immediately afterwards.
t
Safety for pedestrians
ne
possibility of people slipping off the edge of the platform. Toe boards may be removed to allow access
When scaffolding is positioned in areas accessed by pedestrians it is necessary to use high visibility warning tape and lighting. Lagging and padding can be placed around the standards to reduce the risk of injury from accidental impact with the scaffold. Close boarding, netting and sheeting must be used to prevent objects from falling onto anyone below.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
While the scaffolding is being erected it will be necessary to use a physical barrier to protect the area. Warning signs should be used to prevent access by unauthorised persons and signage should be positioned at access points.
with
g.
in
Scaffold types
scaffolding
er
safety signs, lights and hazard warning
e in
ng
yE as
.E
w
w w Independent
Putlog scaffolds
ne
Putlog scaffolds are erected as the external wall is constructed. The scaffolding uses the external wall
t
as part of the support system . Standards and ledgers are tied to the putlogs. Each putlog has one flat end that rests on the bed or perpendicular joints in the brick or blockwork.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Putlog scaffold The blade end of the putlog is usually placed horizontally and inserted fully into the brickwork joint, ensuring a full bearing is achieved. Where putlog scaffolds are used on refurbishment work, joints may be raked out to insert the blade end. In such works the blade may also be placed vertically. Where the
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
putlog scaffold is used in new works, the putlog is placed on the wall at the required lift height and the wall is constructed around the blade end of the putlog. While the system uses less scaffolding and is less expensive than independent scaffolding, it is essential that the erection of the scaffold is coordinated with the sequencing of brickwork. The scaffold lifts must progress at the same speed as the masonry work. Health and safety requirements call for competent and certified scaffolding erectors to construct and alter scaffolding; thus this system is not used as much as it used to be.
w w
At one time, bricklaying gangs would have a labourer who could also erect the scaffold as the brickwork progressed. With good scheduling and coordination of brickwork and scaffold lifts, the system can still
w
prove economical. Zigzag (sway) bracing is applied diagonally to the face of the scaffold, tying the
t
ne
g.
in
er
e in
ng
yE as
.E
ledgers and standards together. Plan bracing and ledger bracing should be used where specified.
Putlog scaffolding
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
yE as
Independent scaffolding
.E
w
w w Zigzag (sway bracing)
These scaffolds are erected ‘independently’ of the building structure, unlike putlog scaffolds, and are
ng
tied to the structure through window openings. Ties are required to ensure horizontal stability is
t
ne
g.
in
er
e in
maintained.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
e in
ng
yE as
.E
w
w w Independent tied scaffold
Independent scaffolds are constructed from two parallel rows of standards tied by transoms, which bridge the width of the scaffold, and ledgers, which run along the length of the scaffold. A space is
er
usually maintained between the scaffold and the building to allow the masonry to progress unhindered
g.
in
by the scaffold.
The gap allows the brickwork to be checked for plumb and also helps to reduce damage to
ne
the brickwork caused by mortar snots splashing off the scaffold and onto the wall. On long stretches of scaffold, continuous diagonal tubes can be used to run from the top to the bottom of the scaffold
t
structure. These act as façade bracing. Bracing is used to resist horizontal loads and to stiffen the structure. The bracing prevents distortion to the rectangular grids. Sway (zigzag) bracing may be applied diagonally to the face of the scaffold, tying the ledgers and standards together. Lateral bracing is also applied across the ledgers.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Independent scaffold
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
g.
in
er
e in
ng
yE as
.E
w
w w ne
Scaffolding components
t
Proprietary scaffold systems
Proprietary systems are another type of independent scaffold. It rely on the same principles as independent scaffold but use standard lengths for ledgers, transoms and standards, all of which are capable of being clipped together and dismantled easily and quickly.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
e in
ng
yE as
.E
w
w w Proprietary system with hop-up brackets
The standards often come with spigot ends, which allow the next standard to be located over the
er
locating spigot very quickly. The jointing systems vary depending on the manufacturer. Proprietary
in
systems rely on ledgers and transoms having a locating lug or bracket fixed to each end; these ends can be quickly dropped into the clips, sockets or cups, which are fixed at regular intervals on the
ne
g.
standards.
Components, such as ledgers and transoms, are designed so that they can be interchangeable.
t Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Proprietary scaffolding system – based on the SGB cuplok system
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
w w method
of
connecting
the SGB cuplok system
standards,
ledgers
and
transoms
–
based
on
.E
w
System
yE as
For small scaffolds, bracing may not be required across the width of the scaffold since the frame is very rigid. Bracing must, however, run across the bays in accordance with the manufacturer’s instructions. Where loads are increased or hop-ups are used, additional bracing is required.
ng
Each proprietary system varies with manufacturer and system, and the manufacturer’s instructions must
t
ne
g.
in
er
e in
be carefully followed to ensure that the scaffold is erected safely.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Proprietary
yE as
.E
w
w w (a)
connection
for
ledgers
and
transoms.
(b)
ng
Standards with adjusting jack and base plate
standards,
To aid the flow of work, ‘hop-up’ scaffolding units can be used to increase the height that the workforce
e in
can access at each lift. These can be used between the standards or can be used between the internal standard and wall, providing a platform that is closer to the area of work and not reducing the width of
t
ne
g.
in
er
the standard platform.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w System scaffolding
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Proprietary scaffolds have the benefit of rapid erection and disassembly; however, their use is limited to relatively standard operations due to the size of the components. Where loads are known to be considerable and the scaffolding arrangement is complicated due to specific project layout/geometry, then traditional scaffolding designed by a structural engineer may provide a more flexible and appropriate solution. 4. What are the methods of providing damp proof course? (A/M 10) (N/D 14)
w w
Damp proof course (DPC) is generally applied at basement levels which restricts the movement of moisture through walls and floors. Selection of materials for damp proof course and its various methods of applications in buildings is discussed.
w
.E
Materials for Damp Proof Course (DPC): Properties of Materials for DPC
yE as
An effective damp proofing material should have the following properties; 1. It should be impervious. 2. It should be strong and durable, and should be capable of withstanding both dead as well as live loads without damage. It should be dimensionally stable.
4.
It should be free from deliquescent salts like sulphates, chlorides and nitrates.
e in
Types of Materials for Damp Proof Course
ng
3.
The materials commonly used to check dampness can be divided into the following three
er
in
categories: 1. Flexible Materials: Materials like bitumen felts (which may be hessian based or fiber/glass fiber based), plastic sheeting (polythene sheets) etc. Semi-rigid Materials: Materials like mastic, asphalt, or combination of materials or layers.
3.
Rigid Materials: Materials like first class bricks, stones, slate, cement concrete etc.
t
Selection of Materials for Damp Proof Course in Buildings
ne
g.
2.
The choice of material to function as an effective damp proof course requires a judicious selection. It depends upon the climate and atmospheric conditions, nature of structure and the situation where DPC is to be provided.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
e in
ng
yE as
.E
w
w w The points to be kept in view while making selection of DPC materials are briefly discussed below:
in
er
1. DPC above ground level
For DPC above ground level with wall thickness generally not exceeding 40 cm, any one of the type of
g.
materials mentioned above may be used. Cement concrete is however commonly adopted material for under normal conditions.
t
ne
DPC at plinth level, 38 to 50mm thick layer of cement concrete M15 (1:2:4 mix) serves the purpose
In case of damp and humid atmosphere, richer mix of concrete should be used. The concrete is further made dense by adding water proofing materials like Pudlo, Impermo, Waterlock etc. in its ingredients during the process of mixing. It is used to apply two coats of hot bitumen over the third surface of the concrete DPC. 2. DPC Material for floors, roofs etc.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
For greater wall thickness or where DPC is to be laid over large areas such as floors, roofs, etc., the choice is limited to flexible materials which provide lesser number of joints like mastic, asphalt, bitumen felts, plastic sheets etc. The felts when used should be properly bonded to the surface with bitumen and laid with joints properly lapped and sealed. 3. DPC Material for situations where differential thermal movements occur
w w
In parapet walls and other such situations, materials like mastic, asphalt, bitumen felts and metal (copper or lead) are recommended.
w
It is important to ensure that the DPC material is flexible so as to avoid any damage or puncture of the
.E
material due to differential thermal movement between the material of the roof and the parapet.
yE as
4. DPC material for Cavity Walls
In cavity wall construction, like cavity over the door or window should be bridged by flexible material like bitumen felt, strips or lead etc.
ng
e in
Methods of Damp Proof Course Installation in Construction General principles to be observed while laying damp proof course are: 1. The DPC should cover full thickness of walls excluding rendering.
er
2. The mortar bed upon which the DPC is to be laid should be made level, even and free from projections. Uneven base is likely to cause damage to DPC.
in
3. When a horizontal DPC is to be continued up a vertical face a cement concrete fillet 75mm in radius should be provided at the junction prior to the treatment.
ne
g.
4. Each DPC should be placed in correct relation to other DPC so as to ensure complete and continuous barrier to the passage of water from floors, walls or roof. Figures 1 to 7 explain provision of damp proof course under different conditions
t Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Figure 1: Air Drain
Figure 2: Damp Proof Course Treatment for basement on undrained soils
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
.E
w
w w t
Figure 4: Damp Proof Course Treatment for Basement in Damp Soil
ne
g.
in
er
e in
ng
yE as
Figure 3: Plan of building showing DPC
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
.E
w
w w t
ne
Figure 6: Damp Proof Course for Flooring
g.
in
er
e in
ng
yE as
Figure 5: Asphalt tanking for basement
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
w w
.E
w
Figure 7: Damp Proof Course for Internal Walls
yE as
5. What are the types of Shuttering? (N/D 10) (M/J 15)
Formwork in concrete construction is used as a mould for a structure in which fresh concrete is poured only to harden subsequently. Types of formwork for concrete construction depends on formwork material and
ng
type of structural element.
e in
Formworks can also be named based on the type of structural member constructionsuch as slab formwork for use in slab, beam formwork, column formwork for use in beams and columns respectively etc.
er
The construction of formwork takes time and involves expenditure upto 20 to 25% of the cost of the
in
structure or even more. Design of these temporary structures are made to economic expenditure. The operation of removing the formwork is known as stripping. Stripped formwork can be reused. Reusable
ne
g.
forms are known as panel forms and non-usable are called stationary forms.
Timber is the most common material used for formwork. The disadvantage with timber formwork is that it
t
will warp, swell and shrink. Application of water impermeable cost to the surface of wood mitigates these defects. A good formwork should satisfy the following requirements: 1. It should be strong enough to withstand all types of dead and live loads. 2. It should be rigidly constructed and efficiently propped and braced both horizontally and vertically, so as to retain its shape.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
3. The joints in the formwork should be tight against leakage of cement grout. 4. Construction of formwork should permit removal of various parts in desired sequences without damage to the concrete. 5. The material of the formwork should be cheap, easily available and should be suitable for reuse. 6. The formwork should be set accurately to the desired line and levels should have plane surface. 7. It should be as light as possible. 8. The material of the formwork should not warp or get distorted when exposed to the elements. 9. It should rest on firm base.
w w
Economy in Formwork
The following points are to be kept in view to effect economy in the cost of formwork:
w
.E
1. The plan of the building should imply minimum number of variations in the size of rooms, floor area etc. so as to permit reuse of the formwork repeatedly.
yE as
2. Design should be perfect to use slender sections only in a most economical way. 3. Minimum sawing and cutting of wooden pieces should be made to enable reuse of the material a number of times. The quantity of surface finish depends on the quality of the formwork. Formwork can be made out of timber, plywood, steel, precast concrete or fiberglass used separately or in
ng
combination. Steel forms are used in situation where large numbers of re-use of the same forms are
e in
necessary. For small works, timber formwork proves useful. Fibre glass made of precast concrete and aluminium are used in cast-in-situ construction such as slabs or members involving curved surfaces.
t
1. well seasoned
ne
It should be
g.
Timber for formwork should satisfy the following requirement:
in
Timber Formwork:
er
Types of Formwork (Shuttering) for Concrete Construction:
2. light in weight 3. easily workable with nails without splitting 4. free from loose knots Timber used for shuttering for exposed concrete work should have smooth and even surface on all faces which come in contact with concrete.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Normal sizes of members for timber formwork: Sheeting for slabs, beam, column side and beam bottom
25 mm to 40mm thick
Joints, ledges
50 x 70 mm to 50 x 150 mm
Posts
75 x 100mm to 100 x 100 mm
Plywood Formwork Resin bonded plywood sheets are attached to timber frames to make up panels of required sizes. The cost
w w
of plywood formwork compares favourably with that of timber shuttering and it may even prove cheaper in certain cases in view of the following considerations:
w
1. It is possible to have smooth finish in which case on cost in surface finishing is there.
.E
2. By use of large size panels it is possible to effect saving in the labour cost of fixing and dismantling.
Steel Formwork
yE as
3. Number of reuses are more as compared with timber shuttering. For estimation purpose, number of reuses can be taken as 20 to 25.
ng
This consist of panels fabricated out of thin steel plates stiffened along the edges by small steel angles. The panel units can be held together through the use of suitable clamps or bolts and nuts. The panels can
e in
be fabricated in large number in any desired modular shape or size. Steel forms are largely used in large projects or in situation where large number reuses of the shuttering is possible. This type of shuttering is
er
considered most suitable for circular or curved structures.
ne
2. Steel forms can be installed and dismantled with greater ease and speed.
g.
in
Steel forms compared with timber formwork: 1. Steel forms are stronger, durable and have longer life than timber formwork and their reuses are more in number.
t
3. The quality of exposed concrete surface by using steel forms is good and such surfaces need no further treatment. 4. Steel formwork does not absorb moisture from concrete. 5. Steel formwork does not shrink or warp. Construction of Concrete formwork: This normally involves the following operations: 1. Propping and centring
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
2. Shuttering 3. Provision of camber 4. Cleaning and surface treatment Order and method of removing formwork: The sequence of orders and method of removal of formwork are as follows: 1. Shuttering forming the vertical faces of walls, beams and column sides should be removed first as they bear no load but only retain the concrete.
w w
2. Shuttering forming soffit of slabs should be removed next. 3. Shuttering forming soffit of beams, girders or other heavily loaded shuttering should be removed in the end.
w
Rapid hardening cement, warm weather and light loading conditions allow early removal of formwork. The
.E
formwork should under no circumstances be allowed to be removed until all the concrete reaches strength of atleast twice the stresses to which the concrete may be subjected at the time of removal of formwork. All to concrete.
yE as
formworks should be eased gradually and carefully in order to prevent the load being suddenly transferred
Figure 1 to 6 shows formwork for different types of members in civil engineering construction.
t
ne
g.
in
er
e in
ng Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Figure 1(a): Details of timber formwork for RCC beam and slab floor
yE as
.E
w
w w t
ne
g.
in
er
e in
ng
Figure 1(b): Details at section (A) shown in above figure
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Figure 2(a): Elevation
Figure 2(b): Details of timber formwork for circular RCC column
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Figure 3(b): Details of timber formwork for square or rectangular RCC column
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Figure 3(a): 150 3D View
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
w
w w Figure 4: Sectional plan showing details of timber formwork for an octagonal column
t
ne
g.
in
er
e in
ng
yE as
.E Figure 5: Details of formwork for stair
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Walls, columns and vertical sides of beams
2
Slabs (props left under)
3
Beam soffits (props left under)
4
Removal of props to slabs
5
1 to 2 days 3 days 7 days
t
1
Period of time
ne
Description of structural member
g.
S. No.
in
Table: Period of removal of formwork
er
e in
ng
yE as
.E
w
w w Figure 6: Timber formwork for RCC wall
(a) For slabs spanning upto 4.5 m
7 days
(b) For slabs spanning over 4.5 m
14 days
Removal of props to beams and arches (a) Spanning upto 6 m
14 days
(b) spanning over 6 m
21 days
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
6. Compare English and Flemish bond. (A/M 10) (N/D 14) (1) ENGLISH BOND
The bond, in which headers and stretchers are laid in alternate courses, is called "English bond".
w w
The following are the salient features of English bond: (i) Headers and stretchers are laid in alternate courses. (ii) In each heading course, a queen closer is placed next to quoin header and the remaining bricks are
w
laid as headers.
.E
(iii) Every alternate header in a course comes centrally over the joint between two stretchers in the
yE as
course below, giving an approximate lap of 2 ¼ in.
(iv) The same course will show headers or stretchers on face and back, if the thickness of the wall is an even multiple of half bricks (e.g. 9 in, 18 in, 27 in, etc.) (v) The same course will show headers on the face and stretchers on the back and vice versa, if the
ng
thickness of the wall is an odd multiple of half brick. (13 1/2 in , 22 1/2 in , etc )
e in
(vi) The middle portion of the thicker walls consists entirely of headers. (vii) Every transverse joint is continuous from face to face.
t
ne
g.
in
er
(2) FLEMISH BOND The bond, in which headers and stretchers are laid alternately in the same course, is called "Flemish bond".
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
The following are the salient features of Flemish bond: (i) Headers and stretchers are laid alternately in the same course. (ii) Every header in each course lies centrally over every stretcher of the underlying course. (iii) In every alternate course a queen closer is placed next to quoin header, so as to provide a lap of approximately 2 1/4 in. (iv) Brick bats are to be used in walls having thickness equal to an odd multiple of half brick.
English Bonds
No.
This bond consists of headers and
stretchers laid in alternative courses.
This bond consists of headers and stretchers
It is strongest of all the bonds.
laid alternatively in each course.
e in
2
Flemish bond
ng
1
yE as
.E
w
w w Sr
It is less strong for walls having thickness more
It provides good appearance for all thickness of
for one brick thick walls.
walls.
There are no noticeable continuous
There are partly continuous vertical joints in the
vertical joints in the structure built in this
structure built in this bond.
t
ne
g.
4
It provides rough appearanceespecially
in
3
er
than 13 ½ inches.
bond. 5
Much attention is not required in
Special attention is required in providing this bond.
providing this bond. 6
Progress of work is more.
Progress of work is less.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
7
It is costly because the use of brick bats
It is economical because brick bats are allowed for
is not allowed.
forming this bind.
er
e in
(a) DOUBLE FLEMISH BOND
ng
yE as
.E
w
w w Types of Flemish Bond
The bond in which headers and stretchers are laid alternately in each course, both in the face and back
t
ne
g.
(b) SINGLE FLEMISH BOND
in
of the wall, is called Double Flemish Bond.
The bond provided in a wall with Flemish bond in facing and English bond in backing is called "Single Flemish bond" or "Cross bond". This bond combines the advantages of both English and Flemish bonds and simultaneously eliminates their disadvantages.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
This bond is recommended where costly bricks are specified for facing in order to provide good appearance to the wall. Also, it can be made more economical by using cheap quality of bricks on the back of wall. On the other hand, it weakens the overall strength of the wall because of maximum use of brick and existence of continuous vertical joints. Also, it can not be provided in walls having thickness less than 13 ½ in.
w w
7 Explain contraction and isolation joint. (N/D 16) (M/J 14)
in
er
e in
ng
yE as
.E
w Contraction Joints or Control Joints
g.
Control Joints (often confused with expansion joints) are cuts or grooves made in concrete or asphalt at regular intervals. These joints are made at locations where there are chances of cracks or where the
ne
concentration of stresses are expected, so that when a concrete does crack, the location will be known
t
to you. In such a way a concrete will not crack randomly but in a straight line (i.e. control joint). In other words Contraction or Control Joints are Pre-Planned Cracks. The cracks may be due to temperature variations or drying shrinkage or other reasons. Joints depth should be 25% of the depth of the slab. For instance a 4" thick slab should have 1" deep cut. Joints Interval (taken in feet) should not be more than 2 - 3 times the slab thickness (in inches). Let
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
say a 6" slab should have joints 2 x 6=12 to 3 x 6 = 18 feet apart. For fresh concrete grooving tools are used while saw is used for hardened concrete. Isolation joints Isolation joints have one very simple purpose—they completely isolate the slab from something else. That something else can be a wall or a column or a drain pipe. Here are a few things to consider with isolation joints:
ne
g.
in
er
e in
ng
yE as
.E
w
w w Even wooden columns should be isolated from the slab.
t
Walls and columns, which are on their own footings that are deeper than the slab subgrade,
are not going to move the same way a slab does as it shrinks or expands from drying or temperature changes or as the subgrade compresses a little.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
yE as
.E
w
w w If slabs are connected to walls or columns or pipes, as they contract or settle there will be
e in
ng
Very long unjointed sections can expand enough from the hot sun to cause blowups, but this is rare.
restraint, which usually cracks the slab—although it could also damage pipes (standpipes or floor drains).
er
Expansion joints are virtually never needed with interior slabs, because the concrete doesn't
Expansion joints in concrete pavement are also seldom needed, since the contraction joints
g.
in
expand that much—it never gets that hot.
open enough (from drying shrinkage) to account for temperature expansion. The exception might
ne
be where a pavement or parking lot are next to a bridge or building—then we simply use a slightly wider isolation joint (maybe ¾ inch instead of ½ inch).
t
Blowups, from expansion of concrete due to hot weather and sun, are more commonly
caused by contraction joints that are not sealed and that then fill up with noncompressible materials (rocks, dirt). They can also be due to very long unjointed sections.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
yE as
.E
w
w w Isolation joints are formed by placing preformed joint material next to the column or wall or
e in
ng
Polyethylene foam isolation joint material comes in various colors. C 2 Products
standpipe prior to pouring the slab. Isolation joint material is typically asphalt-impregnated fiberboard, although plastic, cork, rubber, and neoprene are also available.
er
Isolation joint material should go all the way through the slab, starting at the subbase, but
For a cleaner looking isolation joint, the top part of the preformed filler can be cut off and the
g.
in
should not extend above the top.
space filled with elastomeric sealant. Some proprietary joints come with removable caps to form
ne
this sealant reservoir.
At columns, contraction joints should approach from all four directions ending at the isolation
t
joint, which should have a circular or a diamond shaped configuration around the column. For an I-beam type steel column, a pinwheel configuration can work. Always place the slab concrete first and do not install the isolation joint material and fill around the column until the column is carrying its full dead load.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
8. What are the qualities of good formwork? (N/D 10) A good formwork should satisfy the following requirements: 10. It should be strong enough to withstand all types of dead and live loads. 11. It should be rigidly constructed and efficiently propped and braced both horizontally and vertically, so as to retain its shape. 12. The joints in the formwork should be tight against leakage of cement grout. 13. Construction of formwork should permit removal of various parts in desired sequences without damage to the concrete.
w w
14. The material of the formwork should be cheap, easily available and should be suitable for reuse. 15. The formwork should be set accurately to the desired line and levels should have plane surface.
w
16. It should be as light as possible. 18. It should rest on firm base. Economy in Formwork
yE as
.E
17. The material of the formwork should not warp or get distorted when exposed to the elements.
The following points are to be kept in view to effect economy in the cost of formwork:
e in
ng
4. The plan of the building should imply minimum number of variations in the size of rooms, floor area etc. so as to permit reuse of the formwork repeatedly. 5. Design should be perfect to use slender sections only in a most economical way.
er
6. Minimum sawing and cutting of wooden pieces should be made to enable reuse of the material a number of times. The quantity of surface finish depends on the quality of the formwork.
in
Formwork can be made out of timber, plywood, steel, precast concrete or fiberglass used separately or
g.
in combination. Steel forms are used in situation where large numbers of re-use of the same forms are necessary. For small works, timber formwork proves useful. Fibre glass made of precast concrete and
ne
aluminium are used in cast-in-situ construction such as slabs or members involving curved surfaces.
t Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
UNIT III PART A
1. What is an under-reamed pile? (N/D 16) It is a pile with one or more bulbs in its vertical shaft .These bulbs are known as under-reams and it increases the bearing capacity of the soil.
w w
2. What is a shoring? (N/D 16) Shoring is a temporary structure used to support tilted or endangered walls .The walls might have been
w
endangered due to unequal settlement of foundation, removal of adjoining structures or making large
yE as
.E
opening in the walls
3. What is meant by coffer dam? (M/J 16)
ng
Cofferdam is a temporary structure constructed to exclude water from the site to construct a permanent sub-structure, without the interface of water. It is used when the well foundation is to be carried in
er
4. What is meant by pipe jacking? (M/J 16)
e in
running water.
in
Pipe jacking is a method of installing a pipe under roadway , railway or highways without using an open cut trench .The pipe jacking procedure uses a casing pipe of sheet or reinforced concrete ie, jacked
t
ne
5. State the uses of Box jacking. (M/J 12)
g.
through the soil.
Box jacking is a method of constructing a Box like concrete structure for transportation purposes – namely below railway, roadway. Box is jacked following the procedure of Pipe jacking. 6. What are sheet piles? (M/J 12) Sheet piles are installed in sequence to design depth along the planned excavation perimeter or seawall alignment. The interlocked sheet piles form a wall for permanent or temporary lateral earth support with
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
reduced groundwater inflow. Anchors can be included to provide additional lateral support, if required. 7. What is called Caissons? (N/D 11) It is a special type of foundation used for the construction of bridge piers in v prevent ery deep water, where it is either difficult to construct a cofferdam or to prevent its leakage. Types: a) Box caisson b) Open caisson c) Pneumatic caisson. 8. How will you increase the frictional resistance of piles? (N/D 11).
w w
A box pile displaced a large volume of soil. In such a pile, the frictional resistance is rapidly developed. In clayey strata, both H pile and box piles from a plug of clay between the flanges of H pile and between
.E
friction.
w
the walls of the box pile. The plug of clay is as hard mass acts along with the pile and cause additional 9. What are the methods used for tunnel driving? (N/D 12).
yE as
Following are the methods generally used for driving a tunnel, a) Full face heading b) Heading and bench method c) Drifts method d) Pilot tunneling
ng
10. What is mucking? (N/D 14).
The operation of removal of excavated material in tunneling operation is called mucking.
in
er
e in 11 What are the advantages of drift method? (N/D 15).
Drift method of tunnel excavation has the following advantages: a) It helps to determine the
g.
region of bad rock or excessive ground water before actually taking up the full excavation, so as to
ne
enable to take up the corrective measures. b) The drift provides ventilation while driving the main
t
tunnel. c) It reduces the consumption of explosives. 12 Explain about cement grouting .Uses. (N/D 16).
In this method, cement grout which is a mixture of cement, sand and water is used. The process consists of making a number of holes in ground and then filling these holes by cement grout under pressure. This process is continued till no grout is coming up through the hole. Uses: a) The grouting procedure can be used in stopping leakages from rock. b) It can also be used to fill the
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
voids in soil so as to strengthen the soil and to make the rock or soil water tight. 13 Write the situations under which pile foundation is recommended. (N/D 12). The pile foundation is recommended for the following situations: a) When spread footing, raft and grillage foundations are uneconomical. b) When heavy concentrated loads are to be transmitted by the foundations. c) Where there is scouring in the soil near the foundations. c) Where the soil is made up and of a compressible nature. 14 Write the essential features of a pump to be used for dewatering. (N/D 15).
w w
The pump to be used for dewatering process should have the following features: a)The pump should be portable so that it can be easily moved as and when required. b) The pump should be
w
capable of handling water mixed with impurities such as sand, earth, etc., c) The pump should be
.E
of strong make. 20) What is the equipment used for driving a pre-cast pile in a sandy soil? The equipment used for driving a pre-cast pile in a sandy soil is a hammer. Hence maximum stresses
yE as
are developed at the top due to direct strokes and at the point in overcoming the resistance to penetration. Therefore additional reinforcement is provided.
t
ne
g.
PART B
in
er
e in
ng
15 What are the various methods to dewater deep excavations? The various methods of dewater deep excavations are, a) Sumps and ditches b) Deep well sumps c) WellPoint systems d) Deep well drainage e) Horizontal drainage f) Vacuum – dewatering system g) Drainage by electro-osmosis
1. Explain the various types of sheet piles (N/D 16) (M/J 15)
Sheet piles may he made up of wood, concrete or steel. Steel piles are driven side by side into the ground to form a continuous vertical wall for retaining soil. The alignment and resistance or thrusts are normally provided by horizontal wallers, braces or tiebacks. Factors affecting the choice of a particular
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
type of pile include nature of ground, cost, ease of installation, availability of material, ability to withstand driving, lateral strength and ease of making connections. Depending upon the material used in their manufacture, some of the types of sheet piles are, 1.
Wooden sheet piles
2.
Precast concrete Sheet piles
3.
Prestressed concrete sheet piles
4.
Steel sheet piles
1. Wooden sheet piles:
w w
Wooden sheet piles are made in various sizes and forms. The nature of site conditions determine, the choice of a particular type, In places where excavation is small and the ground water problem is not
w
serious, 5 cm x 30 cm to 10 cm x 30 cm wooden planks arranged in a simple row will serve the purpose.
.E
If the water-tightness is required to a great extent, lapped sheet piling is used. In this case, each pile is made up of two planks, either spiked or bolted to one another. Thus if only earthen banks of small
yE as
height are to be supported, a single or double row of planks properly erected will perform the function of sheet piling. If complete water tightness is desired or pressure of the retained material wakefield or tongue and grooved sheeting is generally used. To facilitate the driving of the piles, they are usually
Wakefield piles:
e in
stratum.
ng
bevelled at foot. This not only assists in driving but also prevents bruising, if the piles encounter hard
This type of pile is made with three planks, 5 cm, 8 cm or 10 cm in thickness. The planks are nailed
er
together with the middle plank offset forming a tongue on one edge and a groove on the other. The
in
planks are connected by using a pair of staggered bolts at 80 cm centre to centre at intermediate points.
g.
The triple lap piles prove stronger in driving. There is no wastage in forming the tongue and groove
ne
joints and the piles have less tendency to warp. Timber sheet piles have light weight and as such the equipment required for pile driving is also light. This is considered to be an important advantage timber
t
piles have over piles of other materials.
Sectional Plan of a Wakefield Pile 2. Precast concrete sheet piles:
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Precast concrete piles are made in square or rectangular cross-section and are driven similar to wooden piles to form a continuous wall. The interlock between two piles is normally provided with the help of tongue and groove joint. The tongue and groove extend to the full length of the piles in most of the cases. An alternative method of providing joint between two piles is shown below. In this method, after the piles are driven to the required. depth, the joint is grouted with cement mortar 1: 2 (1 cement : 2 sand).
yE as
.E
w
w w Sectional Plan of Different Types of Precast Concrete Piles The piles are reinforced to avoid formation of cracks due to rough handling or shrinkage stresses. In
ng
order to reduce the possibility of damage due to driving impact, the stirrups should be spaced closely near the top and bottom of the piles. The piles are normally bevelled at their feet to facilitate tightly close
e in
driving of a pile against the already driven one. Reinforced concrete sheet piles are bulky and heavy and as such they are gradually being superseded by prestressed concrete piles.
g.
in
er
3. Prestressed concrete sheet piles:
ne
On account of the numerous advantages the prestressed concrete members have over the conventional type of reinforced concrete members, prestressed concrete sheet piles are commonly used for sheet
t
piling jobs. Similar to concrete sheet piles, they are reinforced on both the faces so that they could be handled from either side. They are comparatively lighter in weight, more durable and economical in the long run. They are advantageously used in sea water, since the danger of cracking of concrete is negligible and also the corresponding danger of corrosion of pile reinforcement is reduced. 4. Steel sheet piles:
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Steel sheet pile is a rolled steel section consisting of a plate called the web with integral interlocks on each edge. The interlocks consist of a groove, one of whose legs has been suitably flattened. This flattening forms the tongue which fits into the groove of the second sheet. Commonly used sheet piles can be broadly divided into the following three categories,
Straight-web type
Shallow or deep arched-web type
Z web type
Special shapes and sizes of steel sheet piles are manufactured for meeting the requirement of
w w
junctions and other similar situations. Each of the above mentioned type of piles is manufactured in varying widths and lengths. The selection of the type of pile and the section to be adopted depend upon
w
the depths up to which the pile is to be driven, the nature of soil to be penetrated the elevation of the
.E
earthen embankment, ground water level etc. In general, Straight web type of piles are used where the piles are liable to he subjected to tensile forces
yE as
and interlocking strength is of prime importance (Cellular cofferdam etc); Arched-web type are used where the piles are required to resist bending stresses (in cantilever retaining walls etc,) and Z-web type of piles arc used where the piles are required to resist bending stresses of very large magnitude.
t
ne
g.
in
er
e in
ng
Steel Sheet Piles Steel sheet piles are driven with the help of pile drivers which may be of drop hammer type or single or double acting hammer driven by steam or compressed air. The outstanding feature of steel sheet piles is that they can be used for greater depths. The continuous interlocking arrangement of the piles gives
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
strength and rigidity to the supported structure. A wall made from properly driven sheet piles leaks very little, hence steel sheet piling is used with advantage in the construction of deep cofferdams. They are commonly used in coastal defence works which are likely to be subjected to tidal action
2. Explain in detail about tunneling techniques (N/D 16) (M/J 12) Methods of Tunnel Construction
w w
The
method
of
Tunnel
construction
adopted
for
a
project
depends
on
various
factors. Tunnel construction and Tunnel Engineering is considered to be one of the most sophisticated
w
and specialized art in the field of Civil Engineering. Unpredictable ground conditions, environmental
t
ne
g.
in
er
e in
ng
yE as
.E
requirements and geological factors makes Tunneling a challenging job.
Portal Structure for Cumberland Gap Tunnel Like immersed tunnels are used for crossing water bodies, it is important to plan and do a complete feasibility study on which tunnel is appropriate for the project. The type of tunnel and method used depends on various factors, some of them are listed below.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
1.
Geometrical configuration;
2.
Geology
3.
Ground condition;
4.
Type of crossing;
5.
Project Requirement;
6.
Environmental requirements.
er
e in
ng
yE as
.E
w
w w Complete List of Different Methods of Tunnel Construction
g.
in
Cut-and-cover tunnels
ne
In this type of tunnels, the tunnel structure is cast-in-situ or precast in an excavation. After construction, the structure is back-filled with new or excavated soil. Cut and cover construction is adopted when the
t
depth of tunnel is shallow and the safe excavation is possible from the surface with out collapsing the walls of excavation and when it is economical and acceptable. This methodology is usually used for the construction of underpasses, approach sections of other tunnels & tunnels in flat terrain or shallow depth. The tunnels may be constructed in place or by using precast sections. Two types of cut and cover construction are; bottom-up and top-down.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
yE as
.E
w
w w e in
Bored or Mined Tunnels
ng
Cut and Cover Construction using Side Slopes Excavation- Ft McHenry Tunnel
These tunnels are built without excavating the ground surface. These tunnels are named according to
er
the type of material through which the tunnel is being excavated. When a tunnel passes through
in
different types of material it is known as mixed face construction. In bored tunneling, the excavation takes place at the portal or at a shaft, thus the is a minimum impact on usual traffic, air & noise quality,
g.
and utilities. Linings are the most important component of these kind of tunnels. For depths 10 m to 12
t
ne
m, cut-and-cover is usually more economical and practical than mined tunneling.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
ng
yE as
.E
w
w w Rock Tunnels
er
e in
A Typical Horseshoe Section for a Two-lane Tunnel
Rock tunnels are excavated through the rocks either by drilling or by blasting. The tunneling method
in
utilizes mechanized excavators in case of soft rocks or rock tunnel boring machines (TBM). Sequential
g.
Excavation Method (SEM) is also used in some cases. The behavior of rocks can change place to place
ne
and type to type so depending on this stabilization measures ranging from no support at all to anchor bolts to steel sets to even heavily reinforced concrete lining and combination of all these are used. It is
t
one of the most challenging tunneling geology.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
w
w w yE as
Soft Ground Tunnels
.E
Unlined Rock Tunnel in Zion National Park, Utah
When tunnels are excavated in soil using a shield or pressurized face TBM, or by mining methods commonly known as sequential excavation method (SEM) are used they are called soft ground tunnels.
ng
Soft ground includes cohesive soils as well as cohesionless soils and silty sands. Very soft ground tunnels when excavated sequentially by small drifts and openings, it is known as New Austrian
er
Immersed Tunnels
e in
Tunneling Method (NATM).
in
Immersed tunnels usually consist of very large pre-cast concrete or concrete filled steel tunnel elements which are fabricated in the land and later installed under water. After installation, these tunnels are
g.
backfilled. There are lot of immersed tunnels around the world used for road or rail connections.
ne
Immersed tunnels are fabricated in required lengths in dry docks or in improvised floodable basins or on
t
shipways. The ends of the elements are sealed with bulkheads at each end, and then floated out and towed to the installation location.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
w w
Immersed Tunnel
Jacked Box Tunnels
w
Jacked Box Tunnel Structure Construction Operation
t
ne
g.
in
er
e in
ng
yE as
.E
In these types of tunnels, prefabricated box structures are jacked horizontally through the soil against a thrust wall using methods to reduce surface friction, like bentonite slurry. These are often used for construction beneath runways or railroads embankments where surfaces are shallow but the must not be disturbed since it can disrupt their the normal services. The method was developed from pipe jacking technology. The Jacked box tunneling is used in soft ground and for short lengths of tunnels.
3. Describe the procedure involved in underwater construction of diaphragm walls and basement. (M/J 16) (N/D 14)
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
In structural engineering, a diaphragm is a structural system used to transfer lateral loads to shear walls or frames primarily through in-plane shear stress . These lateral loads are usually wind and earthquake loads, but other lateral loads such as lateral earth pressure or hydrostatic pressure can also be resisted by diaphragm action. The diaphragm of a structure often does double duty as the floor system or roof system in a building or the deck of abridge, which simultaneously supports gravity loads. Diaphragms are usually constructed of plywood or oriented stand board in timber construction; Metal deck or composite metal deck in steel construction; or concrete slab in concrete construction. The two primary types of diaphragm are flexible and rigid. Flexible diaphragms resist lateral forces depending on
w w
the tributary area, irrespective of the flexibility of the members that they are transferring force to . On the other hand, rigid diaphragms transfer load to frames or shear walls depending on their flexibility and
w
their location in the structure.
.E
Parts of a diaphragm include:
yE as
the membrane, used as a shear panel to carry in-plane shear the drag strut member, used to transfer the load to the shear walls or frames The chord, used to resist the tension and compression forces that develop in the diaphragm, since
er
Diaphragm wall construction methods
e in
ng
the membrane is usually incapable of handling these loads alone.
Diaphragm wall construction requires that a proper sequence of works is followed. Specialized
in
excavating equipment has to be used. This equipment requires more available space when compared to
t
ne
1. Guide wall installation for diaphragm walls
g.
other construction methods.
Guide walls are constructed in-situ typically as lighly reinforced concrete elements. Guide walls maintain the horizontal allignment and wall continuity of a diaphragm wall while the provide support for the upper soils depth during panel excavation. This temporary support is important as the slurry levels vary during construction and the upper few feet or one meter of the wall tends to be unstable. Equally important,
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
guide walls help guide the diaphragm wall grabs vertically and aid in the positioning of the final structure.
2. Pre-excavation for diaphragm wall installation Prior to the diaphragm wall grabs starting excavation, the slurry pump must be fully submerged in
w w
bentonite slurry. To achieve this, a small initial excavation by the grab is carried out that is filled with slurry. Occassionally, some preexcavation might be required before guide walls are installed to remove
.E
w
certain obstructions.
3. Primary panel excavation for diaphragm wall construction
yE as
The primary panels are excavated first. The minimum length of a panel depends on the grab equipment size and is generally in the order of 3.0m (15ft). If soils are stable, the primary panels can be
ng
constructed in multiple bites. In such a case, a panel can be subdivided into three bites with the left and right panels excavated first while the middle bite is excavated last. With this approach, diaphragm wall panels are constructed.
er
e in
panels in the order of 6.5m to 8.0m are achieved. Multiple bites are also required when corner or T
4. Slurry cleaning and desanding for diaphragm wall construction
in
g.
Prior to tremieying the concrete, and while the panel is excavated, the supporting slurry fluid must be
ne
cleaned so that it's properties are within acceptable levels (density, sand content, viscocity, PH). Slurry is circulated at regular intervals throughout the construction period through the regeneration plant.
t
Otherwise, fresh slurry fluid can also be used although this approach is not the most economical. 5. Joint constuction methods for diaphragm wall construction
Diaphragm wall joints need to receive special attention do detail. Various joint types are available for diaphragm walls. Joint selection depends on the excavating equipment as much as contractor
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
preference Joints can be flat, circular, with steel beams, or special grooved type with water stops. Grooved type joints with water stops are typically preferred while in the US it is also very common to use steel I beams for water stops. Flat panels and circular joints are generally avoided. 6. Reinforcement cage lowering and concrete tremieing Once the bottom of the panel is reached (and cleaned), the reinforcement cage can be lowered into position. The reinforcement cage is typically suspended from the guide wall panels, and must have
w w
enough transverse and diagonal reinforcement to permit it to be properly lifted and lowered into place. Sufficient space must be left for at least two or three tremie pipes so that tremieing can take place.
w
Concrete tremieing refers to the process of replacing the supporting slurry with the permanent concrete
.E
with the use of vertical pipes called tremies. With the tremies, concreting of a diaphragm wall starts from
yE as
the bottom and the tremies are lifted progressively as the concrete level rises. During this process the tremies are maintained within the freshly poured concrete for a minimum depth of 2ft or (0.6m). Overpouring might be required to make sure that all slurry is displaced from the panel by concrete. Poor
ng
tremieing can result in slurry pockets getting entraped within the diaphragm wall concrete. These pockets can then lead to excessive and costly groundwater leaks or even blowouts. This has been the repairs and delays.
7. Secondary panel excavation for diaphragm wall construction
g.
in
er
e in
case in certain portions of the Central Artery Project in Boston, MA (Big dig) and has led to costly
Secondary panels are constructed between primary diaphragm wall panels. When trench cutters are
ne
used, the primary panel is formed with a single bite excavation. With trench cutters a flat panel joint is
t
typically used, but the trench cutter eats into unreinforced concrete of the adjacent primary panels. After the specified depth is reached, the reinforcement cage is lowered into position and concrete is tremied with tremie pipes from the bottom up. 4. What is dewatering? And briefly explain the various dewatering techniques. (M/J 16) (N/D 14)
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
DEFINITION When water table exists at a shallow depth below ground surface, it is essential to lower the water so as to carry out construction of foundation, basement, and metro tunnels etc. This is achieved by pumping out water from multiple wells installed at the site. The process is called as dewatering. Types of dewatering method Dewatering can be done by adopting one of the following four strategies Dewatering of soil by temporary lowering of water table using wells and pumps prior excavation as depleted .Allowing water
w w
to reap into excavation area, collecting it in sumps and pumping it out. Before that adequate steps have to be taken to support the soil on sides of the excavated area, to prevent washing away of fines and have sufficient space for the work area. Making the soil around excavated zone impermeable by
w
technique such as grouting are freezing so that inflow of water is stop are minimized.
.E
INSTALATION TECHNIQUE
yE as
Sufficient size and capacity of dewatering system is necessary to lower and maintain ground water table and to allow material to be excavated in a reasonable dry condition. Excavation slopes to be stabilized where sheeting is not required Dewatering system is to be operated continuously until backfill
ng
work has been completed. Then, the structure to be constructed at the excavated area has to be finished The complete stand by have to be available for immediate operation as may be required, to
e in
adequately maintain dewatering on continuous basis and in the event that all or any other part of the system may become inadequate or fail The water removed from the excavation to be disposed in such
er
a manner as will not endanger portions of work under construction or completed. For dewatering
g.
WELL POINTS DEWATERING
in
purpose, well points deep well, caissons and tunnels are used.
ne
When construction operation have to be excited below the ground water table level. Dewatering of soil can be done by the following methods Collecting water in sumps and pumping it out. Installing well
t
points small or deep wells and pumping out ground water Using special technique in fine grained soils such as vaccum dewatering and electro osmosis WELL POINTS To pump out the ground water small sized wells called well points are used for a more dry working area the two methods used most often for lowering water table below the excavation level are the well point method and the deep well method.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
WELL POINT METHOD : This is economical and useful for lowering the water table by 15m or less. Incase of well point method or deep well method it is based on the fact that removal of water by continuous pumping from a well causes the water table level to become depressed and result in the formation of draw down. When a series of wells are placed close to each other, the overall effect is lowering of the water table level. Well points, being smaller, are easy to install. Well points, can lower the water table by only 6.7m because the pump, is located at the ground surface and connected to group of well points through a pipe, cannot lift water from greater depth. Beyond 7m, multistage well points are used. DEEP WELL METHOD This
w w
method is useful for lowering the water table by more than 15m. Deep wells have larger diameter more depth and greater spacing. The pump is located at the bottom of well and hence can pump out water
.E
w
from greater depth. Deep wells become more economical if more points are required.
yE as
5. What are Caissons and cofferdam. (M/J 12) (N/D 15) A caisson foundation also called as pier foundation is a watertight retaining structure used as a bridge pier, in the construction of a concrete dam, or for the repair of ships. It is a prefabricated hollow box or foundation.
e in
ng
cylinder sunk into the ground to some desired depth and then filled with concrete thus forming a Caisson foundation is Most often used in the construction of bridge piers & other structures that require foundation beneath rivers & other bodies of water. This is because caissons can be floated to the job
er
site and sunk into place.
in
Caisson foundations are similar in form to pile foundations, but are installed using a different method. It
g.
is used when soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat. It is a form of deep foundation which are constructed above ground level, then sunk to the
t
ne
required level by excavating or dredging material from within the caisson.
Caissons (also sometimes called “piers”) are created by auguring a deep hole into the ground, and then filling it with concrete. Steel reinforcement is sometimes utilized for a portion of the length of the caisson. Caissons are drilled either to bedrock (called “rock caissons”) or deep into the underlying soil strata if a geotechnical engineer finds the soil suitable to carry the building load. When caissons rest on soil, they are generally “belled” at the bottom to spread the load over a wider area. Special drilling bits are used to remove the soil for these “belled caissons”.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
The caisson foundations carry the building loads at their lower ends, which are often bell-shaped.
Functions of Caisson Foundation The foundation system of and the soils beneath the building prevent the complex from moving vertically. When a load is placed on soil, most soils settle. This creates a problem when the building settles but the utilities do not. Even more critical than settlement is differential settlement. This occurs when parts of your building settle at different rates, resulting in cracks, some of which may affect the structural integrity of the building. Conversely, in some rare instances soils may swell, pushing your building upwards and
w w
resulting in similar problems. Therefore, the foundation system must work in tandem with the soils to support the building.
w
Types of Caissons:
Floating Caissons
Open Caissons
Pneumatic Caissons
Sheeted Caissons
e in
Excavated Caissons
ng
yE as
Box Caissons
.E
er
Box caissons are watertight boxes that are constructed of heavy timbers and open at the top. They are
in
generally floated to the appropriate location and then sunk into place with a masonry pier within it. Excavated caissons are just as the name suggests, caissons that are placed within an excavated site.
g.
These are usually cylindrical in shape and then back filled with concrete. cavities.
t
ne
Floating caissons are also known as floating docks and are prefabricated boxes that have cylindrical Open caissons are small cofferdams that are placed and then pumped dry and filled with concrete. These are generally used in the formation of a pier. Pneumatic caissons are large watertight boxes or cylinders that are mainly used for under water construction.
Advantages and Disadvantages of Caissons:
Advantages of Caissons:
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Economics
Minimizes pile cap needs
Slightly less noise and reduced vibrations
Easily adaptable to varying site conditions
High axial and lateral loading capacity
Disadvantages of Caissons: Extremely sensitive to construction procedures
Not good for contaminated sites
Lack of construction expertise
Lack of Qualified Inspectors
Drilled Pier Foundations
yE as
.E
w
w w
A drilled pier is a deep foundation system that is constructed by placing fresh concrete and reinforcing steel into a drilled shaft. The shaft is constructed by rotary methods using either a self-contained drill
ng
unit or a crane mounted drill unit. The hole is advanced through soil or rock to the desired bearing stratum. Temporary or permanent steel casings may be used to maintain the sides of the drilled
e in
excavation if caving soils or water infiltration becomes a problem.
er
Drilled shafts can be used to sustain high axial and lateral loads. Typical shaft diameters range from 18
in
to 144 inches. Drilled shafts (also called caissons, drilled piers or bored piles) have proven to be a cost effective, excellent performing, deep foundation system, that is utilized world-wide. Typically they are
g.
used for bridges and large structures, where large loads and lateral resistance are major factors.
ne
Concrete Caissons:
t
A 10″ or 12″ diameter holes are drilled into the earth and embedded into bedrock 3 to 4 feet. Usually used for the structural support for a type of foundation wall, porch, patio, monopost, or other structure. Two or more “sticks” of reinforcing bars (rebar) are inserted into and run the full length of the hole and then concrete is poured into the caisson hole. A caisson is designed to rest on an underlying stratum of rock or satisfactory soil and is used when unsatisfactory soil exists
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Caisson Construction Process:
After some initial form work and concrete pours, the cutting edge is floated to the breakwater by towboat and fastened to the caisson guide. Concrete is placed (poured) into steel forms built up along the perimeter of the box. With every concrete placement, the box becomes heavier and sinks into the water along the caisson guide.
Forms are also built inside the box around the air domes and concrete is placed in between. The resulting open tubes above the air domes are called dredge wells.
When the caisson finally touches the river bottom, the air domes are removed and earth is excavated through the long dredge well tubes, as shown in the animation below. The caisson sinks into the river bottom. Excavation
w w
continues until the caisson sinks to its predetermined depth. As a final step, concrete is placed (poured) into the bottom 30 feet of the hollow dredge wells and the tops are
t
ne
g.
in
er
e in
ng
yE as
.E
w
sealed.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
ng
yE as
.E
w
w w Coffer Dam
There are various types of cofferdams used for construction of structures in water. Construction details
e in
of these cofferdams are provided in this article.
er
The basic needs of human being are food, air, water, shelter and transport. To fulfill the basic needs of
in
shelter and transport every inch of the earth land is being used for the construction of roads, building or
g.
other structures.
ne
Nowadays even structure on water are being constructed. But the construction in water is a very tedious used to overcome this problem. One the methods used for this purpose are Cofferdams.
t
job. As the structure is hard to build in water as concrete doesn’t set in water. Many methods are being
Cofferdam can be defined as the temporary structure that is built to keep the water away from the execution site, so that the structure can be built on the dry surface.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
The cofferdams should have walls that exclude water from building site. For this the walls must be water proof and the height of the wall must be more than the maximum water level. These types of cofferdams are preferred where the area of building site is large and the dry soil bed is at reasonable depth
e in
ng
yE as
.E
w
w w Types of Cofferdams and Their Construction Details
er
Coffer dams can be classified into many types depending upon the depth, soil conditions, and
ne
g.
Types of Cofferdams
in
fluctuations in the water level and type of material used.
Considering the material used in their construction, cofferdams may be divided into the following
Earthen cofferdam
Rockfill cofferdam
Single-walled cofferdam
Double-walled cofferdam
Braced cofferdam
t
categories.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Cellular cofferdam (Circular or diaphragm type)
Earthen Cofferdam Earthen cofferdams are constructed at the place where the height of the water is less say 3m and the current velocity is low. These dams are built using the local available material such as clay, fine sand or even soil. The height of the dam is kept 1m more than that of max water level. Freeboard of the dam or the top of the dam is kept 1m so that the water doesn’t enter the other side even when waves arise.
w w
The slope is usually given but 1:1 or 1:2. The slope of the water side is pitched with rubble stones so the
w
water action doesn’t score the embankment. Even sheet piles are driven in the center of the dam to
.E
resist water seepage. After the construction of earthen cofferdam, the water from the other site is pumped out and construction is executed.
g.
in
er
e in
ng
yE as ne
Fig: Cross-Section of an Earthen Cofferdam
t
Rockfill Cofferdam
Rock-fill cofferdams are better than that of earthen dams. These dams are preferred when the rock is available easily at the construction site. These dams are very pervious, to prevent water from seeping an impervious membrane of soil is provided in the dam. The height of the dam is can be up to 3m. The slope can be maintained at 1:1.5 to 1:125. The slope on the water side is pitched so as to protect dam from wave action.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
w
w w Fig: Cross-Section of Rockfill Cofferdam
.E
Single-Walled Cofferdam
yE as
This type of cofferdam is preferred when the depth of the water is more than 6m and area of construction is less. Usually this is used in construction of bridges. Wooden or timber sheets are driven into the river bed on the perimeter of the area of construction. On
ng
the inside steel or iron sheets are driven into the river bed. This inside sheets are placed at equal
e in
distance with the help of wales which are bolted to both sheets for either sides. To improve the stability of this types of dam, half-filled bags of sand are placed on the both side of the
er
walls. The water from the inside is pumped out and the construction process is undertaken.
t
ne
g.
in Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Fig: Construction Details of Single Walled Cofferdam
Double-Walled Cofferdam
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Double-walled types of cofferdams are used when the area of construction site is large and depth of water is high. In this place use of single walled cofferdam becomes uneconomical as the supports are to be increased. So double walled cofferdam is used. The difference in one wall and double wall dam is that her it has two walls instead of walls for extra stability. This type of dams can hold water up to 12m high. Two piles are driven inside the water bed with a space in between and attached each other with wales
w w
with bolted connection. As the water depth increases the space between the walls increases. The space between the walls are filled with soil. To prevent the leakage from the ground below, the
t
ne
g.
in
er
e in
ng
yE as
.E
w
sheet piles are driven to a good depth in the bed.
Fig: Construction Details of Single Walled Cofferdam
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Braced Cofferdam When it’s difficult to drive piles inside the bed in the water, then this type of cofferdam is used. In braced cofferdam two piles are driven into the bed and they are laterally supported with the help of wooden cribs installed in alternate courses to form pockets. The empty pockets here are filled with stone and earth. The framework of the cofferdam (made from, logs of wood) is prepared on ground and then floated to the site where the cofferdam is to be constructed.
w w
The layers of sand and the other loose material overlying the impervious hard bed is dredged out. Crib
w
is then sunk to the position, the bottom of each crib is given a shape to fit in the variation in the surface of bedrock. After the pit is dewatered, the structure is concreted. When concreting has been completed
.E
above the water level, the cofferdam is removed.
t
ne
g.
in
er
e in
ng
yE as Fig: Braced Cofferdam Construction Details
Cellular Cofferdam
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
When the water layer is more than 20m, common types of cofferdams are uneconomical to use. In this situations cellular cofferdams are used. This type of dam is used in construction of dams, locks, weirs etc. Cellular cofferdam is made by driving straight web steel sheet piles, arranged to form a series of interconnected cells. The cells are constructed in various shapes and styles to suit the requirements of site.
w w
Finally, the cells are filled with clay, sand or gravel to make them stable against the various forces to which they are likely to be subjected to.
w
The two common shapes of the cellular cofferdam are,
.E
(i) Circular type cellular cofferdam.
yE as
(ii) Diaphragm type cellular cofferdam. (i) Circular Type Cellular Cofferdam
This type of cellular cofferdam consists of circular arcs on the inner and outer sides which are connected by straight diaphragm walls. The connection between the curved parts and the diaphragms
ng
are made by means of a specially fabricated Y-element.
e in
The cofferdam is thus made from interconnected steel sheet piles. The empty spaces are filled with non pervious materials like clay or sand. Due to the filling material the self weight of the membrane
in
er
increases and leakage is reduced.
g.
One advantage of the diaphragm type is that the effective length of the cofferdam may be increased easily by lengthening the diaphragm. Hence in case, from design consideration it is necessary to have
ne
effective width of the cofferdam more than 21 meter, diaphragm type of cofferdam must be used.
t Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
ne
(ii) Diaphragm Type Cellular Cofferdam
g.
in
er
e in
ng
yE as
.E
w
w w Fig: Plan and Section Details of Circular Type Cellular Cofferdam
t
It consists of a set of large diameter main circular cells interconnected by arcs of smaller cells. The walls of the connecting cells are perpendicular to the walls of the main circular cells of large diameter.
The segmental arcs are joined by special T-piles to the main cells. The circular type cellular cofferdams are self-sustaining, and therefore independent of the adjacent circular cells. Each cell can be filled independently.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
The stability of such cells is much greater as compared with that of the diaphragm type. However, the circular cells are more expensive than the diaphragm type, as these require more sheet piles and greater skill in setting and driving the piles. Because the diameter of circular cells is limited by interlock tension, their ability to resist lateral pressure due to high heads is limited.
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Fig: Plan and Section Details of Diaphragm type Cellular Cofferdam
Straight Shaft Drilled Piers (Caissons)
yE as
.E
w
w w Used in moderate to high swelling soils. (This is one of the most effective foundation designs for use in sites that contain expansive soils.)
Purpose is to attain required penetration into zone where there is little or no seasonal moisture variation. Current
e in
ng
standard of care in the area is a minimum penetration of 6 feet into bedrock and minimum length of 16 feet. Dead reinforcing in grade beam.
in
er
loads should be as high as practical. This design requires relatively long spans between piers and more
Caissons into bedrock
Friction Piers into stiff clays
End Bearing Belled Piers
Appropriate Voiding – Should be constructed with void material of appropriate strength and thickness
t
ne
g.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
w w
Fig: A series of 1.2-metre thick diaphragm wall panels were joined to form a 24-metre diameter caisson
w
shaft. Four of these caissons were built to provide a sound base for the foundation of the main structure one of the caisson shafts.
t
ne
g.
in
er
e in
ng
yE as
.E
of the building tower. The photo shows the excavation work using typical excavating machines inside
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
.E
w
w w 6. Describe the procedure involved in underwater construction of diaphragm walls. (M/J 12) (N/D 15)
yE as
Diaphragm wall Construction
Diaphragm wall is a continuous wall constructed in ground in to facilitate certain construction activities,
a) As a retaining wall
b) As a cut-off provision to support deep excavation
er
e in
ng
such as:
c) As the final wall for basement or other underground structure (e.g. tunnel and shaft) c) As a
t
ne
Diaphragm wall
g.
d) As a form of foundation (barrette pile – rectangular pile)
in
separating structure between major underground facilities
Diaphragm wall is a reinforced concrete structure constructed in-situ panel by panel. The wall is usually designed to reach very great depth, sometimes up to 50m, mechanical excavating method is thus employed. Typical sequence of work includes: a) Construct the guide wall
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
b) Excavation to form the diaphragm wall trench c) Support the trench cutting using bentonite slurry d) Inert reinforcement and placing of concrete to form the wall panel Guide wall – guide wall is two parallel concrete beams constructed along the side of the wall as a guide to the clamshell which is used for the excavation of the diaphragm wall trenches. Trench excavation – In normal soil condition excavation is done using a clamshell or grab suspended by
w w
cables to a crane. The grab can easily cut through soft ground. In case of encountering boulders, a gravity hammer (chisel) will be used to break the rock and then take the spoil out using the grab.
w
Excavation support – the sides inside the trench cut can collapse easily. Bentonite slurry is used to
.E
protect the sides of soil. Bentonite is a specially selected fine clay, when added to water, forms an impervious cakelike slurry with very large viscosity. The slurry will produce a great lateral pressure
yE as
sufficient enough to retain the vertical soil.
Reinforcement – reinforcement is inserted in the form of a steel cage, but may be required to lap a few
ng
sections in order to reach the required length.
Concreting – placing of oncrete is done using tremie pipes to avoid the segregation of concrete. As
e in
Concrete being poured down, bontonite will be displaced due to its lower density than concrete. Bontonite is then collected and reused.
er
in
Joining for the diaphragm wall panel – Diaphragm wall cannot be constructed continually for a very long section due to limitation and size of the mechanical plant. The wall is usually constructed in alternative
g.
section. Two stop end tubes will be placed at the ends of the excavated trench before concreting. The
ne
tubes are withdrawn at the same time of concreting so that a semi-circular end section is formed. Wall
t
sections are formed alternatively leaving an intermediate section in between. The in-between sections are built similarly afterward but without the end tube. At the end a continual diaphragm wall is constructed with the panel sections tightly joined by the semi-circular groove. Using hydrofraise (reverse circulation trench cutter) to form diaphragm wall panel. Bored piles of square section can be installed using the Hydrofraise or similar drilling techniques. The bore hole is stabilised by drilling mud. The "Hydrofraise" is a drilling machine powered by three down-
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
the-hole motors, operating with reverse circulation. A heavy metal frame, serving as a guide, is fitted at its base with two cutter drums carrying tungsten carbide tipped cutters. These rotate in opposite directions and break up the soil. A pump is placed just above the drums and evacuates the loosened soil, which is carried up to the surface by the drilling mud. The mud with cuttings is continuously filtered (desander unit) and then poured back into the trench. A heavy crawler crane supports and manipulates the machine. It carries the power pack supplying the hydraulic power, which is conveyed through hoses to the three down-the-hole motors, two of them driving the cutter drums and the third driving the pump. The hydraulic cutting device is designed to give the cutter drums a high torque at low speed of rotation.
w w
The guide frame is suspended from the cable-operated crane. A hydraulic feed cylinder is used to achieve a constant rate of advance or to maintain a constant weight on the cutter drums. Another
w
important advantage is that the drilling mud is constantly screened and desanded during excavation.
.E
Thus the reinforcement can be placed and concreting carried out as soon as the required depth has been reached. This excavation system makes it possible to drill piles panels or diaphragm wall elements
yE as
in a very wide range of soils, from cohesion less soils to hard rock. 7. What are the advantages of belt conveyors? (M/J 12) (N/D 11)
Some of the advantages are:
e in
ng
Belt conveyors have many advantages over other types of bulk material handling equipment. Belt conveyors are capable of handling a wide range of bulk materials from very fine to large lump sizes. Very fine materials such as portland cement are loaded at terminals using belt conveyors. Large lump
er
size materials such as coal are transported from mines using belt conveyors.
Belt conveyors can be designed to handle capacities for any operation. It is common for belt conveyors
in
batching operations or to convey a small amount of material between processes.
ne
g.
to unload ships at capacities up to 10,000 tons per hour. Belt conveyors can also be designed for Belt conveyors can be configured to fit almost any application. A belt conveyor can convey material
t
horizontally, on an incline or a combination of both. It is common to use a single belt conveyor to
transport material horizontally a certain distance, then elevate the material on an inclined section of belt conveyor and then horizontally again.
Belt conveyors can be used to stock-pile or reclaim bulk materials. Radial stackers are used for creating large piles of materials such as wood chips, coal or ore. Reclaim belt conveyors are located under the piles to carry the materials into the plant for processing.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Belt conveyors require less horsepower to operate than other types of conveyors. Bulk materials are carried on top of the belt and remain static, therefore requiring much less energy to move.
Belt conveyors have proven to be a reliable method of conveying bulk materials. Industry standards for the design of belt conveyors have been developed by the Conveyor Equipment Manufacturer’s Association (CEMA).
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
UNIT IV PART A 1. What are shells? (N/D 16), (M/J 12) Shells are three dimensional structures constructed as storage tanks or roof for large column free areas, such as exhibition halls, sports complex or theatres.
w w
2. What are launching girders? (N/D 16) For erection of large beams in buildings or bridges, temporary girders are used. Such girders are called launching girders. Launching girders are usually of steel as it would be light compared to concrete
.E
w
girders
yE as
3. Write a note on offshore platform (M/J 16)
Offshore platforms are structures constructed in the ocean to explore or to produce oil and gas from the sources found below the sea. Offshore platforms are in steel or in concrete
ng
4. Define articulated structures (M/J 16) sliding joint or joints.
er
e in
A structure in which relative motion is allowed to occur between parts, usually by means of a hinged or
in
5. What are the precautions to be taken while erecting light weight components on tall structures? (M/J
g.
12)
ne
The precautions to be taken while erecting light weight components on tall structures are, a) Excellent coordination and site organization have to be maintained b) All heavy equipments like generators,
t
lightning system, twists, etc., are to be in working condition c) Adequate communication facility should be coordinated between ground level, crane drivers, ship format and twist operators. 6. What are the three common tower crane configurations? (N/D 11) The three common tower crane configurations are, a) Static tower cranes b) Travelling tower cranes c) Climbing tower cranes
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
7. What are cooling towers? (N/D 10) Cooling Towers are used to cool the water that is used to recon dense the steam that is used to generate electricity. 8. Define – Braced Domes. (N/D 10)
w w
Braced domes are composed either of members lying of a surface of revolution of straight members with their connecting points lying on such a surface, an arrangement which avoids any obstruction of the inner space. This arrangement generally results in a dome of circular tone or in one truncated into a polygonal base, domes with elliptical or oval plan have been used in rare cases. 9. What are the systems of pre stressing? (N/D 11)
w
.E
1. Freyssinet System 2. Magnel-blaton System 3. Lee-Mc. Call or stress steel system 1. What are the advantages of pre stressed cement concrete? (N/D 15)
yE as
1. It is possible to take the full advantage of compressive strength of concrete and high tensile strength of the steel used. 2. 15 to 30% of the concrete is saved. 3. 60 to 80% of the steel is saved. 4. Prestressed concrete members are thinner in section and hence there is greater reduction of the How are domes erected? (N/D 12)
e in
2.
ng
self-weight of the member.
Domes are usually erected with a central temporary support on which the supporting ring rests. If
er
the span is greater than 40 – 50m, the tower of an erecting frame serves the support. What are shells? (N/D 16)
g.
in
3.
Shells are three dimensional structures constructed as storage tanks or roof for large column free 4.
How are shells classified? (N/D 11)
t
ne
areas, such as exhibition halls, sports complex or theatres.
1. Singly curved shells like cylindrical shells 2. Doubly curved or spherical shells 5.
What is a sky scraper? (N/D 15) A skyscraper is a tall, continuously habitable building of many storeys, usually designed for office and commercial use. There is no official definition or height above which a building may be classified as a skyscraper. One common feature of skyscrapers is having a steel framework that supports curtain walls. These curtain walls either bear on the framework below or are possibly suspended from the framework above, rather than load-bearing walls of conventional construction.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
6.
What are tall structures? (N/D 13) Transmission towers are tall structures with relatively small cross section and with a large ration between the height and the maximum. Tall buildings are generally multi storeyed structure where greater part of the construction is composed of beams and stancheons.
7.
Define – Braced Domes(A/M 11) Braced domes are composed either of members lying of a surface of revolution of straight members with their connecting points lying on such a surface, an arrangement which avoids any obstruction of the inner space. This arrangement generally results in a dome of circular tone or in one truncated into a polygonal base, domes with elliptical or oval plan have been used in rare cases
w w
8.
yE as
.E
w
Distinguish between silos and bunkers. (N/D 11) Silos A silo is a structure for storing bulk materials. Silos are used in agriculture to store grain or fermented feed known as silage. Silos are more commonly used for bulk storage of grain, coal, cement, carbon black, woodchips, food products and sawdust. Silos are mostly above the ground
Bunkers A bunker is a defensive military fortification designed to protect the inhabitants from falling bombs or other attacks. They were used extensively in World War I, World War II, and the Cold War for weapons facilities, command and control centers, and storage facilities. Bunkers are mostly below the ground
1. Describe in detail about shell roof structures (N/D 16) (M/J
in
15)
er
e in
ng PART B
Classification
Singly curved
Double curved
Cylindrical shells
t
ne
stadiums, exhibition halls, theatres, complex churches etc
g.
Shells are 3d structures constructed on storage tanks or roof for large column area such as indoor
Singly curved It can be used for rectangular shape buildings, shells represents the roof of the building Dome storage
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
tank for water and petroleum is example for single curved Doubly curved For doubly curved structures the super structure should be in hexagonal or circular shape Cylindrical shape These are just modification of pitched roof and frequently employed in modern age construction It has two types North light shell roof Barrel vault shell roof Both are different to provide lighting effect in factories In barell vault ventilation s provided in middle Special Forms for Concrete Shells
w w
A thin shell concrete structure, is a structure composed of a relatively thin shell of concrete, usually with no interior columns or exterior buttresses. The shells are most commonly flat plates and domes, but may also take the form of ellipsoids or cylindrical sections, or some combination thereof.
w
Types and Forms of Shell Structure Folded Plates
Barrel Vaults
Short Shells
Domes of Revolution
Folded Plate Domes
Intersection Shells
Warped Surfaces
Combinations
Shell Arches
t
ne
g.
in
er
e in
ng
yE as
Folded Plates
.E
The elements of a folded plate structure are similar to those of a barrel shell except that all elements are planar, and the moments in the slab elements are affected by the differential movement of the joints. For the structure shown, the end supports and the side supports are both complete walls
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w Barrel Shells
The elements of a barrel shell are: (1) The cylinder, (2) The frame or ties at the ends, including the columns, and
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
(3) The side elements, which may be a cylindrical element, a folded plate element, columns, or all combined. For the shell shown in the sketch, the end frame is solid and the side element is a vertical beam. A barrel shell carries load longitudinally as a beam and transversally as an arch. The arch, however, is supported by internal shears, and so may be calculated. The elements of a folded plate structure are similar to those of a barrel shell except that all elements are planar, and the moments in the slab elements are affected by the differential movement of the joints. For the structure shown, the end supports and the side supports are both complete walls
yE as
.E
w
w w ng
The elements of a short shell are the barrel, which is relatively short compared to radius, the element at the base of the cylinder to pick up the arch loads, and the arches or rigid frame to pick up the entire
e in
ensemble. In this case it is a rigid frame arch. The size of the arch could have been reduced by horizontal ties at the springings. There may be multiple spans. (1) As an arch carrying load to the lower elements. and (2) As as a curved beam to the arches.
(N/D 11) [N/D-
t
1. Explain Domes
ne
The thickness of the shell can be quite thin due to these properties.
g.
in
er
The short shell carries loads in two ways:
14] Domes
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
w w
Domes are membrane structures, the internal stresses are tension and compression and are statically determinate if the proper edge conditions are fulfilled. In a dome of uniform thickness, under its own
w
weight, the ring stresses are compression until the angle to the vertical is about 57 degrees. If the dome Translation Shells
e in
ng
yE as
.E
is less than a full hemisphere, a ring is required at the base of the dome to contain the forces.
er
A translation shell is a dome set on four arches. The shape is different from a spherical dome and is easier to form than a spherical dome.
g.
in
generated by a vertical circle moving on another circle. All vertical slices have the same radius. It is The stresses in a translation shell are much like a dome at the top, but at the level of the arches, tension
t
Advantages of Concrete Shells
ne
forces are offset by compression in the arch. However there are high tension forces in the corner. Like the arch, the curved shapes often used for concrete shells are naturally strong structures, allowing wide areas to be spanned without the use of internal supports, giving an open, unobstructed interior. The use of concrete as a building material reduces both materials cost and a construction cost, as concrete is relatively inexpensive and easily cast into compound curves. The resulting structure may be immensely strong and safe; modern monolithic dome houses, for example, have resisted hurricanes and fires, and are widely considered to be strong enough to withstand even F5 tornadoes.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Disadvantages of Concrete Shells Since concrete is porous material, concrete domes often have issues with sealing. If not treated, rainwater can seep through the roof and leak into the interior of the building. On the other hand, the seamless construction of concrete domes prevents air from escaping, and can lead to buildup of condensation on the inside of the shell. Shingling or sealants are common solutions to the problem of exterior moisture, and dehumidifiers or ventilation can address condensation
w w
3.. Explain the general requirements in launching girders. (M/J 16)[N/D-13] Launching girders are most commonly used for placing pre-cast post-tensioned concrete box segments to form viaducts and bridges and are especially useful for lofty structures in marine or congested urban
w
conditions due to their ability to move themselves forward to the next span - hence they are particularly
.E
economic for multi-span structures. Curvature can be accommodated by moving laterally on cross-
yE as
beams and modest gradients can also be accommodated. For most situations the balanced cantilever method is the favoured sequence of construction. Description and Sequence:
ng
LGs are relatively large pieces of equipment, their size being based on the maximum spans and segment weights to be erected. A large LG might typically weigh in excess of 800 tonnes and be in the
e in
order of 150 to 180 metres in length (as a rule of thumb just over twice the length of the main spans unless intermediate temporary support systems are to be used). Regular inspection maintenance of
er
this equipment to an approved schedule is fundamental to ensure trouble-free and safe operation.
• Pick-up and winching of segment into its approximate position
t
ne
• Delivery of a segment to the LG (at deck level or from ground level)
g.
LG is in place the basic steps for a typical span construction are:-
in
Once the
• Application of epoxy resin to segment faces to be joined
• Final positioning and temporary stressing for self-support (allowing the segment to be released from LG) • Internal permanent post-tensioning sufficient to allow placing of the next segment • Repetition for further segments until completion of the cantilevers
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
• Form and stress a concrete stitch at mid-span to complete the span • Launch the LG to next span • Final post-tensioning possibly continuous through more than one span Launching the girder to the next span is usually a multi-stage process involving tiedowns, counterbalancing with pre-cast segments and winches and the use of temporary support legs but the precise procedure to be followed will vary from one piece of equipment to another and must be clearly set out in method statements, and preferably certified by an independent checking engineer.
w w
Launching girder in balanced cantilever mode Insurance Aspects: For insurance purposes launching girders may be considered either as contractor’s plant or temporary works and this can be an important factor when preparing the policy documents. However, whether treated as plant or temporary works, a
w
failure can have very serious insurance implications including:-
.E
• Injury or loss of life by operatives and members of the public
yE as
• Third party property damage
• Damage and delay to the contract works
• Clearance of debris Claims can arise, and have arisen, either as a result of procedures not being
ng
strictly followed or due to failure of the equipment itself and hence the development of detailed reduce the risks to their lowest achievable level.
er
Training and Access:
e in
procedural steps and their very strict implementation using experienced operatives is essential to
in
Operating and moving LGs is a specialised process requiring staff with extensive training and experience. Whilst main contractors might wish to allocate some staff to the erection process they
g.
should be under the direct command of a specialist from the manufacturer or a company specialising in
ne
this type of work. In addition to the task of lifting and placing the segments these workers need to receive training in several related operations including gluing and post-stressing of the segments
t
together with the numerous safety requirements for standard construction such as ventilation requirements, working at height, PPE and communications. All trained staff (including resident site staff) who are permitted to access the LG working areas, should be clearly identifiable (usually by means of a “truss permit label” on their helmets) without which access to the fenced-off working areas above and below should be denied. In the case of shift-working a period of supervision hand-over is important to ensure on-going operations follow the correct sequence and the agreed procedures.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Detailing the Erection Procedures: Method statements, including risk assessments, should set out the procedural steps to be followed in detail and it is considered important for the manufacturer or specialist company to be directly involved in this process. Setting out the multi-stage operations is best undertaken by means of a general method statement which can then be developed into a more detailed and specific MS. These statements will invariably require diagrammatic as well as descriptive elements covering the erection sequence for each span and highlighting the particular stressing required at different times, as certified by the independent checking engineer.
w w
4. Write a note on bridge decks (M/J 12) (N/D 10)
.E
w
BRIDGE DECKS
The principal function of a bridge deck is to provide support to local vertical loads (from highway traffic,
yE as
railway or pedestrians) and transmit these loads to the primary superstructure of the bridge, (1). As a result of its function, the deck will be continuous along the bridge span and (apart from some railway bridges) continuous across the span. As a result of this continuity, it will act as a plate (isotropic or
ng
orthotropic depending on construction) to support cal patch Continuity ensures that whether or not it has been designed to do so, it will participate in the overall structural action of the superstructure. The
e in
overall structural actions may include: Contributing to the top flange of the longitudinal girders, Contributing to the top flange of cross girders at supports and, where present in twin girder and cross
er
girder structures, throughout the span, Figure 1(3). Stabilising longitudinal and cross girders Acting
in
as a diaphragm to transmit horizontal loads to supports. Providing a means of distribution of vertical
g.
load between longitudinal girders, It may be necessary to take account of these combined actions
ne
when verifying the design of the deck. This is most likely to be the case when there are significant stresses from the overall structural actions in the same direction as the maximum bending moments
t
from local deck actions, e.g. in structures with cross girders where the direction of maximum moment is along the bridge. The passage of each wheel load causes a complete cycle of local bending stresses. The number of significant stress cycles is, therefore, very much higher for the deck than for the remainder of the superstructure. In addition, some of the actions of the deck arising from its participation in the overall behavior are subject to full reversal; an example is the transverse distribution of vertical load between girders. For both these reasons, fatigue is more likely to govern the design of
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
the bridge deck than the remainder of the superstructure.
5. What are the different Types of Material Handling Equipment? (N/D 10) (M/J 14)
Categories of Material Handling Equipment
yE as
.E
w
w w
Material handling equipment encompasses a diverse range of tools, vehicles, storage units, appliances and accessories involved in transporting, storing, controlling, enumerating and protecting products at any stage of manufacturing, distribution consumption or disposal.
The four main categories of material handling equipment include: storage, engineered systems, industrial trucks and bulk material handling.
ng
Storage and Handling Equipment
Examples of storage and handling equipment include:
ne
g.
in
er
e in
Storage equipment is usually limited to non-automated examples, which are grouped in with engineered systems. Storage equipment is used to hold or buffer materials during “downtimes,” or times when they are not being transported. These periods could refer to temporary pauses during long-term transportation or long-term storage designed to allow the buildup of stock. The majority of storage equipment refers to pallets, shelves or racks onto which materials may be stacked in an orderly manner to await transportation or consumption. Many companies have investigated increased efficiency possibilities in storage equipment by designing proprietary packaging that allows materials or products of a certain type to conserve space while in inventory.
t
Racks, such as pallet racks, drive-through or drive-in racks, push-back racks, and sliding racks Stacking frames Shelves, bins and drawers Mezzanines
Engineered Systems Engineered systems cover a variety of units that work cohesively to enable storage and transportation. They are often automated. A good example of an engineered system is an Automated Storage and Retrieval System, often abbreviated AS/RS, which is a large automated organizational structure
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
involving racks, aisles and shelves accessible by a “shuttle” system of retrieval. The shuttle system is a mechanized cherry picker that can be used by a worker or can perform fully automated functions to quickly locate a storage item’s location and quickly retrieve it for other uses. Other types of engineered systems include: Conveyor systems Robotic delivery systems Automatic guided vehicles (AGV)
Industrial Trucks
w w
yE as
.E
w
Industrial trucks refer to the different kinds of transportation items and vehicles used to move materials and products in materials handling. These transportation devices can include small hand-operated trucks, pallet-jacks, and various kinds of forklifts. These trucks have a variety of characteristics to make them suitable for different operations. Some trucks have forks, as in a forklift, or a flat surface with which to lift items, while some trucks require a separate piece of equipment for loading. Trucks can also be manual or powered lift and operation can be walk or ride, requiring a user to manually push them or to ride along on the truck. A stack truck can be used to stack items, while a non-stack truck is typically used for transportation and not for loading. There are many types of industrial trucks:
g.
in
er
e in
Bulk Material Handling Equipment
ng
Hand trucks Pallet jacks Pallet trucks Walkie stackers Platform trucks Order picker Sideloader Many types of AGV
t
ne
Bulk material handling refers to the storing, transportation and control of materials in loose bulk form. These materials can include food, liquid, or minerals, among others. Generally, these pieces of equipment deal with the items in loose form, such as conveyor belts or elevators designed to move large quantities of material, or in packaged form, through the use of drums and hoppers. Conveyor belts Stackers Reclaimers Bucket elevators Grain elevators Hoppers Silos
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
6
Explain in detail, the process of in-situ pre-stressing of high rise structures? (A/M 10) (N/D 12)
w
w w
PRESTRESSING METHOD IN MULTI-STORIED BUILDING FRAME
History of Pre-stressing
.E
yE as
The art of pre-stressing concrete evolved over many decades and from many sources, but we can point to a few select instances in history that brought about this technology.
ng
In the United States, engineer John Roebling established a factory in 1841 for making rope out of iron wire, which he initially sold to replace the hempen rope used for hoisting cars over the portage railway in central Pennsylvania. Later, Roebling used wire ropes as suspension cables for bridges, and he developed the technique for spinning the cables in place.
er
e in
During the 19th century, low-cost production of iron and steel, when added to the invention of portland cement in 1824, led to the development of reinforced concrete. In 1867, Joseph Monier, a French gardener, patented a method of strengthening thin concrete flowerpots by embedding iron wire mesh into the concrete. Monier later applied his ideas to patents for buildings and bridges.
ne
g.
in
Swiss engineer Robert Maillart’s use of reinforced concrete, beginning in 1901, effected a revolution in structural art. Maillart, all of whose main bridges are located in Switzerland , was the first designer to break completely with the masonry tradition by putting concrete into forms technically appropriate to its properties – yet visually surprising. His radical use of reinforced concrete revolutionized masonry arch bridge design.
t
The idea of pre-stressing concrete was first applied by Eugene Freyssinet, a French structural and civil engineer, in 1928 as a method for overcoming concrete’s natural weakness in tension. Pre-stressed concrete can now be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. PRE-STRESSED CONCRETE Pre stressed concrete, like reinforced concrete, is a composite material which uses to advantage the compressive strength of concrete, whilst circumventing its weakness in tension. Pre
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
stressed concrete is made from structural concrete, usually of high strength, and high strength steel tendons which may or may not be grouped together. Prior to external loading the tendons are tensioned in one of two ways. With pretensioning the tendon are tensioned prior to the casting of the concrete and using post tensioning techniques the tendons are tensioned after the concrete has hardened. Some ordinary reinforcing steel is also often included both as subsidiary longitudinal reinforcement and as transverse stirrups to resist shear.
t
ne
g.
in
er
e in
ng
yE as
.E
w
w w
Pre-stressed concrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Pre-stressing tendons (generally of high tensilesteel cable or rods) are used to provide a clamping load which produces a compressive stress that offsets the tensile stress that the concrete compression member would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steelreinforcement bars, inside poured concrete. The basic purpose of pre-stressing is to improve the performance of concrete members and this is achieved by inducing in the beam initial deformation and stresses which tend to counteract those produced by the service loads.
Since concrete is weak in tension in normal reinforced concrete construction cracks develop in the tension zone at working loads and therefore all concrete in tension is ignored in design.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Pre-stressing involves inducing compressive stresses in the zone, which will tend to become tensile under external loads. This compressive stress neutralizes the tensile stress so that no resultant tension exists, (or only very small values, within the tensile strength of the concrete). Cracking is therefore eliminated under working load and all of the concrete may be assumed effective in carrying load. Therefore lighter sections may be used to carry a given bending moment, and pre-stressed concrete may be used for longer span than reinforced concrete. The pre-stressing force also reduces the magnitude of the principal tensile stress in the web so that thin-webbed I - sections may be used without the risk of diagonal tension failures and with further savings in self-weight.
w
w w
The pre-stressing force has to be produced by a high tensile steel, and it is necessary to use high quality concrete to resist the higher compressive stresses that are developed. As the name itself suggests pre-stressing is the technique of stressing a structural member prior to loading to resist excessive tensile stresses.
g.
PRINCIPLE OF PRESTRESSING
in
·
er
·
e in
·
Maximum utilization of provided section of the member. Provision of slender member for long span beams as compared to RCC. Use of high strength materials contribute to the durability of the structure. Pre-stresses concrete has considerable resilience and impact resistance. Proves to be economical only in long span beam-column frames compared to other materials. The intermediate distance between the columns can be in increased by using prestressed concrete as compared to reinforced cement concrete. Architectural design provisions and specifications can be achieved using pre-stressed concrete. Dead weight of concrete is reduced to a higher rate using pre-stressed concrete.
ng
· · · · ·
yE as
.E
The advantages of pre-stressed concrete as a construction material in multi storied frame can be listed as follows:
t
ne
The function of pre-stressing is to place the concrete structure under compression in those regions where load causes tensile stress. Tension caused by the load will first have to cancel the compression induced by the pre-stressing before it can crack the concrete. Figure (a) shows a plainly reinforced concrete simple-span beam and fixed cantilever beam cracked under applied load. Figure (b) shows the same unloaded beams with pre-stressing forces applied by stressing high strength tendons. By placing the pre-stressing low in the simple-span beam and high in the cantilever beam, compression is induced in the tension zones; creating upward camber. Figure (c) shows the two pre-stressed beams after loads have been applied. The loads cause both the simple-span beam and cantilever beam to deflect down, creating tensile stresses in the bottom of the simple-span beam and top of the cantilever beam. The structural Designer
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
balances the effects of load and pre-stressing in such a way that tension from the loading is compensated by compression induced by the pre-stressing. Tension is eliminated under the combination of the two and tension cracks are prevented. Also, construction materials (concrete and steel) are used more efficiently; optimizing materials, construction effort and cost.
yE as
.E
w
w w Fig 1. - Comparison of Reinforced and Prestressed Concrete Beams
ng
t
There are two methods of pre-stressing concrete: -
ne
METHODS AND SYSTEM OF PRE-STRESSING
g.
in
er
e in
Pre-stressing can be applied to concrete members in two ways, by pre-tensioning or posttensioning. In pre-tensioned members the pre-stressing strands are tensioned against restraining bulkheads before the concrete is cast. After the concrete has been placed, allowed to harden and attain sufficient strength, the strands are released and their force is transferred to the concrete member. Pre-stressing by post-tensioning involves installing and stressing pre-stressing strand or bar tendons only after the concrete has been placed, hardened and attained a minimum compressive strength for that transfer.
1) Pre-cast Pre-tensioned 2) Pre-cast Post-tensioned Both methods involve tensioning cables inside a concrete beam and then anchoring the stressed cables to the concrete.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Pre-cast Pre-tensioned: Pre-tensioning is a method of pre-stressing in which the steel tendons are tensioned before the casting of the member. In this method the tendons are tensioned using hydraulic jacks, which bear on strong abutments between which the moulds are placed. After the concrete attains full strength the tendons are released and the stress is transferred to the concrete by bond action. Procedure of precast pre-tensioned concreting Stage 1
yE as
.E
w
w w
Tendons and reinforcement are positioned in the beam mould.
Stage 2 Tendons are stressed to about 70% of their ultimate strength.
er
e in
ng t
ne
g.
in
Stage 3 Concrete is cast into the beam mould and allowed to cure to the required initial strength.
Stage 4 When the concrete has cured the stressing force is released and the tendons anchor themselves in the concrete.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
1. Explain in detail, the merits and demerits of various types of shells. (A/M 11) (N/D 12)
w w
Advantages of Concrete Shells
w
Like the arch, the curved shapes often used for concrete shells are naturally strong structures, allowing
.E
wide areas to be spanned without the use of internal supports, giving an open, unobstructed interior. The use of concrete as a building material reduces both materials cost and a construction cost, as
yE as
concrete is relatively inexpensive and easily cast into compound curves. The resulting structure may be immensely strong and safe; modern monolithic dome houses, for example, have resisted hurricanes and fires, and are widely considered to be strong enough to withstand even F5 tornadoes.
ng
Disadvantages of Concrete Shells
Since concrete is porous material, concrete domes often have issues with sealing. If not treated,
e in
rainwater can seep through the roof and leak into the interior of the building. On the other hand, the seamless construction of concrete domes prevents air from escaping, and can lead to buildup of
er
condensation on the inside of the shell. Shingling or sealants are common solutions to the problem of
t
ne
g.
in
exterior moisture, and dehumidifiers or ventilation can address condensation
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
UNIT V PART A 1. What are the factors influencing compaction? (N/D 16) The factors which influence compaction are: static weight ,number of vibratory drums, roller speed, drum diameter, frequency and amplitude relationship between frame and drum weight driven or non driven drum centrifugal force and total applied force
w
2. What are the types of dredgers? (N/D 16)
w w
Dipper dredger, Ladder dredger and Suction dredger 3. Name the equipment’s used for earth moving operations. (M/J 16)
Bulldozers
Skid steer loaders
Motor graders
Trenchers (M/J 16)
e in
4. Define compaction
ng yE
Excavators
as .E
er
Compaction is defined as the process of densifying or increasing the unit weight of a soil mass through the application of static or dynamic force, with the resulting expulsion of air.
g.
in
5. Give a list of construction equipments needed in the construction of tall structures. (M/J 12) Mobile crane, Tower crane, Chain Hoist, Winch Hoist, Lift crane, Hydraulic crane.
t
ne
6. What are the types of equipments used for trenching and tunneling? (M/J 12) Trenching: Wheel trenches, Ladder type trenches Tunneling Tunnel Boring machine 7. Name the equipments used for volume batching (N/D 11)
The equipments used for volume batching are aggregate feeders, cement silo, water measuring device
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
and mixing unit
8. What are the various types of conveyors? (A/M 10) The various types of conveyors are: belt conveyor, roller conveyor, chain or cable conveyor,pipe line conveyor, screw conveyor and elevating conveyor. 9. What are the advantages of using belt conveyors? (N/D 10)
w w
w
The advantages of using belt conveyors are: It is capable of handling light or heavy, fine or coarse, wet or dry material. It can handle, thousands of tons of material per hour for several kilometers. It can handle not materials up to 1600c. It operates without noise. (N/D 16)
as .E
9. What are the factors influencing compaction?
ng yE
The factors which influence compaction are: static weight, number of vibratory drums, roller speed, drum diameter, frequency and amplitude relationship between frame and drum weight driven or non-driven drum centrifugal force and total applied force 10. Name the equipments used for volume batching (N/D 15)
e in
The equipments used for volume batching are aggregate feeders, cement silo, water measuring device and mixing unit
er
11. What are the various types of conveyors? (N/D 13)
ne
12. What are the advantages of using belt conveyors? (N/D 12)
g.
in
The various types of conveyors are: belt conveyor, roller conveyor, chain or cable conveyor, pipe line conveyor, screw conveyor and elevating conveyor.
t
The advantages of using belt conveyors are: It is capable of handling light or heavy, fine or coarse, wet or dry material. It can handle thousands of tons of material per hour for several kilometers. It can handle not materials up to 1600c. It operates without noise. 13. What is a tractor? Mention its types? Tractor is earthmoving equipment which converts engine energy into tractive energy. The two types of tractors are crawler or tract type and wheel or pneumatic type.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
PART B
1. Explain the various equipment’s for pile driving. (N/D 16), (M/J 12) (N/D 15)
Pile Driving Equipment: Piles are installed by a special pile driving device known as a pile hammer. The hammer may be suspended from the boom of a crawler crane, supported on a large frame called a pile driver or carried on a barge for construction in water.
w w
w
In all cases, the hammer is guided between two parallel steel members called leads. The leads may be adjusted at various angles for driving vertical and batter piles.
Types of Hammer: 1. Drop hammer:
as .E
Several types of hammers are in use and each of which are different sizes. The hammer types are:
t
ne
g.
The double-acting hammer employs steam or air for lifting the ram and for accelerating the downward stroke. The energy of a double-acting hammer is equal to the (weight of the ram I mean effective pressure I the effective area of ram) 1 times the height of fall.
in
3. Double-acting hammer:
er
In a single acting hammer a heavy ram is lifted up by steam or compressed air but dropped by its own weight. The energy of a single acting hammer is equal to the weight of the ram times the height of fall.
e in
2. Single acting hammer:
ng yE
The drop hammer in the pile driving equipment consists of a heavy ram in between the leads. The ram is lifted up to a certain height and released to drop on the pile. This type is slow and therefore not in common use. It is used in the cases where only a small number of piles are driven.
4. Diesel hammer: The diesel hammer is a small, light weight and highly mobile. They use gasoline for fuel. To start the operation, the ram is raised, and the fuel is injected. As the ram is released, the ram falls and compresses air and fuel. The air and fuel becomes hot because of the compression and the air-fuel mixture is ignited. The resulting explosion 1. Advances the pile and 2. Lifts the ram. If the pile advance is very great as in soft soils, the ram is not lifted by the explosion sufficiently to ignite the air-fuel mixture on the next cycic, requiring that the ram be again manually lifted.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
5. Vibratory hammer: The principle of the vibratory driver is two counter-rotating eccentric weights. The driving unit vibrates at high frequency and provides two vertical impulses, one up and one down. The downward pulse acts with the pile weight to increase the apparent gravity force. These hummers have reduced driving vibrations, reduced noise, and great speed of penetration.
Hammer Selection: Generally the size of hammer is more important factor than type of hammer. A heavy pile should be driven by a heavy hammer delivering large energy. Preferably the weight of HKjmcr should beat ^H HB1 the total weight of the pile and the deriving energy should be at Hpiie foot-pound for each pound of pile weight. Each type of hammer has its use under suitable conditions, The advantages and disadvantages of cach type are summarized below:
Single-acting hammer :
w
w w
They arc advantageous when driving heavy piles in compact or hard soils; the heavy ram striking at - tow velocity produces least damage due to impact. The disadvantages arc low driving speed and large headroom requirement.
Double-acting hammer:
as .E
They are generally used to drive piles of light or moderate weight in soils of average resistance against driving. This type of hammer can drive piles at fast speed, requires less headroom and can be used to extract piles by turning them [i.e. the double-acting hammer] upside down.
ng yE
Diesel hammer:
They are similar in application as double-acting hammers, but driving may become difficult in extremely soft ground.
Vibratory hammer:
e in
They have fairly good results in silty and clayey deposits. They are used in heavy clays or soils with appreciable numbers of boulders. See above for other advantages.
Efficiency (ɳh) 0.7 - 0.85
Diesel hammers
0.8 - 0.9
Drop hammers
0.7 - 0.9
t
ne
g.
in
Single and double acting hammer
er
Hammer Type
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
2. Mention the various types of compaction equipment. Mention their uses (N/D 16) (M/J 15) (N/D 12) There are different types of rollers and other soil compaction equipments available. Use of these compacting machines depends on soil types and moisture conditions. Different Types of Soil Compaction Equipments: The soil compaction equipments can be divided into two groups: 1.
Light soil compacting equipments
2.
Heavy soil compacting equipments
w
1. Light Soil Compacting Equipments:
w w
These equipments are used for soil compacting of small areas only and where the compacting effort needed is less. Below are light equipments for soil compaction:
as .E
(i) Rammers:
Rammers are used for compacting small areas by providing impact load to the soil. This equipment is light more.
t
ne
g.
in
er
e in
ng yE
and can be hand or machine operated. The base size of rammers can be 15cm x 15cm or 20cm x 20cm or
For machine operated rammers, the usual weight varies from 30kg to 10 tonnes (6 lbs to 22000 lbs). These hammers with 2- 3 tonnes (4400 to 6600 lbs)weights are allowed to free fall from a height of 1m to 2m (3ft to 7ft) on the soil for the compaction of rock fragments.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Rammers are suitable for compacting cohesive soils as well as other soils. This machine in areas with difficulty in access. (ii) Vibrating Plate Compactors:
ng yE
as .E
w w
w Vibrating plate compactors are used for compaction of coarse soils with 4 to 8% fines. These equipments are used for small areas. The usual weights of these machines vary from 100 kg to 2 tonne with plate areas between 0.16 m2 and 1.6 m2. (iii) Vibro Tampers:
e in
Vibro tampers is used for compaction of small areas in confined space. This machine is suitable for compaction of all types of soil by vibrations set up in a base plate through a spring activated by an engine
er
driven reciprocating mechanism. They are usually manually guided and weigh between 50 and 100 kg (100 to
g.
2. Heavy Soil Compaction Equipments:
in
220 lbs).
ne
These compacting machines are used for large areas for use on different types of soils. The heavy different types of these equipments:
t
compaction equipments are selected based on moisture content of soil and types of soil. Following are
I) Smooth Wheeled Rollers: Smooth wheeled rollers are of two types:
Static smooth wheeled rollers
Vibrating smooth wheeled rollers
The most suitable soils for these roller type are well graded sand, gravel, crushed rock, asphalt etc. where crushing is required. These are used on soils which does not require great pressure for compaction. These
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
rollers are generally used for finishing the upper surface of the soil. These roller are not used for compaction of uniform sands.
w w
w The performance of smooth wheeled rollers depend on load per cm width it transfers to the soil and diameter
as .E
of the drum. The load per cm width is derived from the gross weight of the drum. The smooth wheeled rollers consists of one large steel drum in front and two steel drums on the rear. The
ng yE
gross weight of these rollers is in the range of 8-10 tonnes (18000 to 22000 lbs). The other type of smooth wheel roller is called Tandem Roller, which weighs between 6-8 tonne (13000 to 18000 lbs). The performance of these rollers can be increased by increasing the increasing the weight of the drum by
e in
ballasting the inside of drums with wet sand or water. Steel sections can also be used to increase the load of the drum by mounting on the steel frame attached with axle.
er
The desirable speed and number of passes for appropriate compaction of soil depends on the type of soil and
in
varies from location to location. About 8 passes are adequate for compacting 20 cm layer. A speed of 3-6
ne
g.
kmph is considered appropriate for smooth wheel rollers. Vibrating smooth wheeled rollers
t
In case of vibrating smooth wheeled rollers, the drums are made to vibrate by employing rotating or reciprocating mass. These rollers are helpful from several considerations like:(i) Higher compaction level can be achieved with maximum work (ii) Compaction can be done up to greater depths (iii) Output is many times more than conventional rollers
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
e in
ng yE
as .E
w w
w Although these rollers are expensive but in the long term the cost becomes economical due to their higher outputs and improved performance. The latest work specifications for excavation recommends the use of
(ii) Sheepsfoot roller Roller:
in
er
vibratory rollers due to their advantage over static smooth wheeled rollers.
g.
Sheepsfoot rollers are used for compacting fine grained soils such as heavy clays and silty clays. Sheepsfoot construction projects.
t
ne
rollers are used for compaction of soils in dams, embankments, subgrade layers in pavements and rail road
Sheepsfoot rollers are of static and vibratory types. Vibratory types rollers are used for compaction of all fine grained soils and also soil with sand-gravel mixes. Generally this roller is used for compaction of subgrade layers in road and rail projects.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
as .E
w w
w As seen in picture above, sheepsfoot rollers consist of steel drums on which projecting lugs are fixed and can base, prismatic and clubfoot type.
ng yE
apply a pressure upto 14kg/sq cm or more. Different types of lugs are namely spindle shaped with widened
The weight of drums can be increased as in the case of smooth wheeled rollers by ballasting with water, wet
e in
sand or by mounting steel sections.
The efficiency of sheepsfoot rollers compaction can be achieved when lugs are gradual walkout of the roller
er
lugs with successive coverage. The efficiency is affected by the pressure on the foot and coverage of ground
in
obtained per pass. For required pressure and coverage of ground, the parameters such as gross weight of feet per drum are considered.
ne
g.
the roller, the area of each foot, the number of lugs in contact with the ground at any time and total number of
maximum when a foot is vertical.
t
The compaction of soil is mainly due to foots penetrating and exerting pressure on the soil. The pressure is
(iii) Pneumatic Tyred Rollers: Pneumatic tyred rollers are also called as rubber tyred rollers. These rollers are used for compaction of coarse grained soils with some fines. These rollers are least suitable for uniform coarse soils and rocks. Generally pneumatic tyred rollers are used in pavement subgrade works both earthwork and bituminous works.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
as .E
w w
w Pneumatic rollers have wheels on both axles. These wheels are staggered for compaction of soil layers with
ng yE
uniform pressure throughout the width of the roller.
The factors which affects the degree of compaction are tyre inflation pressure and the area of the contact. The latest rollers have an arrangement to inflate the tyre to the desired pressure automatically. The total
e in
weight of the roller can be increased from 11.0 tonne to 25.0 tonne or more by ballasting with steel sections or other means.
in
er
(iv) Grid Rollers:
Grid rollers are used for compaction of weathered rocks, well graded coarse soils. These rollers are not
g.
suitable for clayey soils, silty clays and uniform soils. The main use of these rollers are in subgrade and sub-
t
ne
base in road constructions.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
As the name suggests, these rollers have a cylindrical heavy steel surface consisting of a network of steel bars forming a grid with squire holes. The weight of this roller can be increased by ballasting with concrete blocks. Typical weights vary between 5.5 tonnes net and 15 tonnes ballasted. Grid rollers provide high contact pressure but little kneading action and are suitable for compacting most coarse grained soils. (v) Pad Foot / Tamping Rollers: These rollers are similar to sheepsfoot rollers with lugs of larger area than sheepsfoot rollers.
e in
ng yE
as .E
w w
w er
The static pad foot rollers also called tamping rollers have static weights in the range of 15 to 40 tonnes and
in
their static linear drum loads are between 30 and 80 kg/cm. These rollers are more preferable than sheepsfoot roller due to their high production capacity, and they are replacing sheepsfoot rollers.
g.
ne
The degree of compaction achieved is more than sheepsfoot rollers. The density of soil achieved after compaction with this roller is more uniform.
t These rollers operate at high speeds, and are capable to breaking large lumps. These rollers also consists of leveling blades to spread the material. Pad foot or tamping rollers are best suitable for compacting cohesive soils
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
3. Mention the various types of earthwork equipment. Mention their uses. (M/J 16), (M/J 12) Excavator An excavator is a construction vehicle used to excavate or move large objects. An Excavator is basically made up of 2 parts: a driving base associated a powerful boom arm with an attachment designed for excavating. The operator sits within a small cab connected to the base and controls the arm. The excavator uses a Hydraulic system to generate a Hydraulic force to control the mechanical arm of the machine. It also uses a chain wheel system for it’s movement.
w w
w
Excavators are been used in large and small scale constructions. They are used for small housing projects to do a cut and fill, used in road construction, used in marine structures to place armors and large rocks, and also been used in larger sites to excavate, move construction material, remove construction waste etc….. There are several types of excavators which are classified on use, brand and purpose. Usage of Excavator in Construction.
as .E
The excavator probably is the most commonly used machine in the Construction Industry. There are many uses of an Excavator. Digging of trenches, holes, foundations
Demolition
Material handling
General grading/landscaping
Brush cutting with hydraulic attachments
Forestry work
Lifting and placing of pipes
Mining, especially, but not only open-pit mining
River dredging
ne
g.
in
er
e in
GRADER
ng yE
t
A grader is a construction machine with a long blade used to create a flat surface. It is commonly called in names such as road grader, a blade, a maintainer and motor grader,
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
w w
w
Graders Used in construction Graders are mostly been used in road construction for construction and maintenance of dirt roads and gravel roads. The grader typically consist of three axles, with the engine and cab situated top, the rear axles at one end of the vehicle and a third axle at the front end of the vehicle, with the blade in between.
LOADER
ng yE
as .E
In the construction of paved roads they are used to prepare the base course to create a wide flat surface for the asphalt to be placed on. They are also used to set native soil foundation pads to finish grade prior to the construction of large buildings. Many countries use grader for the flatting process that is done before the placing of Asphalt. Graders too have various types; some consist a large fork front, and some consist of a flat blade front, which vary from different sizes.
t
ne
g.
in
er
e in
A loader is a heavy equipment machine frequently used in construction industry, mainly used to Load material (such as demolition waste, feed, gravel raw minerals, used material, rock, and plywood) into or onto another type of machinery (such as a dump truckload).Loaders have a very higher productivity and a lower maintenance cost unlike most of the other large scale construction machines.The Loader has a large bucket with a shorter moving arm. Large quantities of material such as soil, construction material, etc can be moved from this.The loader consist of four large wheels for its movement.Loaders are best suited for earth moving, road construction, agricultural purposes, and also in large and small scale constructions which include marine structures. Loaders too are available in various types based on it’s use and brand. Similar names : Front end loader, bucket loader, scoop loader, or shovel.
Loader in construction Loading materials into trucks. Usage of Loaders in construction Laying pipe Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
Clearing rubble Moving of Construction waste and other materials Small excavations Bulldozer A bulldozer is a crawler (continuous Tracked tractor) with a substantial metal plate used (known as blade) fitted to push large amounts of soil, sand, dirt or other materials when construction or remodeling and usually push on the back with a claw mechanism (Known as ripper) to loosen densely-compacted materials.
er
e in
ng yE
as .E
w w
w TRENCHERS
t
ne
g.
in
It is sometimes called as Ditchers or Trenches, are similar to excavators in the sense that penetrate the soil, break soil and rock, and from the earth. They differ from excavators in that the soil is removed in one continuous movement. Digging trenches for pipes used specifically for, but other machines have been improvised in the past in order to serve this purpose.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
w w
w as .E
Trenches in construction Trenchers can come in two types Ladder trenchers
Wheel trenchers can dig trenches at speeds that other machines cannot compare to.
e in
ng yE
SCRAPERS
t
ne
g.
in
er
The scraper is a large piece of equipment used in mining, construction, agriculture and other earth moving applications.
Scraper used in construction
When the hopper is full it is raised, and closed with a vertical blade (other name : apron). The scraper can carriage its load to the fill area where the blade is raised, the back panel of the hopper, or the ejector, is hydraulically pushed forward and the load tumbles out. Then the empty scraper returns to the cut site and repeats the cycle.
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
4. Describe in detail the various equipments used for compaction, batching.
[M/J – 10] [N/D – 12]
Concrete production equipment According to specific conditions on the construction work, the needs of the user, the environmental and security conditions or economic factors, the concrete can be produced in the following categories related to location of production: • Concrete plant in relative distance of the construction work; • Concrete plant in the construction work;
w
• Concrete mixers on-site with small and medium dimensions.
w w
ng yE
as .E
There are no significant technical differences in terms of production of concrete, in relative distance of the concrete plant and construction work and the concrete plant at construction work. The relevant selection criteria for concrete production equipment, which can be divided in two distinct but interrelated levels are: − Selection criteria regarding the location of concrete production; − Selection criteria regarding technical aspects of each equipment comparable between each other. In both cases, the decision is based in the required concrete quantity, concrete quality, concrete type, costs and deadlines. The aspects related to equipment maintenance and safety of are very important because the concrete production equipment are also working machines with associated costs and dangers, which if not treated could cause injuries. Description and characterization of concrete production equipment
e in
Concrete production equipment on concrete plant.
t
ne
g.
in
er
The concrete mixers are made of high resistance steel drums or tanks, especially on the inside where is coated with special steel plates. Drums or tanks are available in several dimensions. The dimension of the drum is the factor that determines the capacity of the mixer in quantitative terms. Inside are equipped with mixing paddles or blades on the shaft power transmission placed in arms that rotate around that axis. The power transmission axes can be in vertical or horizontal axis and there may be more than one mixer shaft. The introduction of relative motion with the right intensity and the proper proportion of constituents are used to reach to adequate concrete mixture. Regarding the concrete production in concrete plant, there are two major categories of concrete mixing, these groups can be used for the classification of concrete mixers: • Simple production mixers; • Continuous production mixers. The simple production mixers have the characteristic of producing one mixture at the time, therefore have to be completely emptied after each mixing cycle (and cleaned if possible) and recharged with materials for the next mixing. In terms of production capacity per cycle can reach from 1 m3 to 6 m3 . The concrete mixer can be tailor-made for special cases. Within the category of simple production mixers still exists the reversible mixer that can achieve capacities of 9 m3 per cycle. The simple production mixers has always been the most used, they exists in several types depending of the direction and number of rotation axes. The continuous production mixers as the name implies, are continuously fed with the constituents of concrete at the same
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
rate at which concrete is discharged. They are generally composed by not tilting drums and have blades or paddles rotating axis or axes in case of the mixer be a twin shaft mixer. The continuous production mixers are used only when large amounts of concrete are required and when the required concrete quality is weaker, is the case of paving or tops of dams. The main selection criteria to consider in selection of the concrete production equipment in plant are: − Required concrete volume; − Required concrete quality; − Time and costs; − Mixing speed; − Level of automation; − Number and type of mixing paddles (mixing system). Mixing systems are suitable for certain types of concrete, for example, concrete mixers with satellites mixing system are suitable for dry high-strength concrete usually suitable for precast concrete elements. A good selection of mixing equipment is crucial to the success of the construction work, because it determines the compliance or non-compliance with deadlines, costs and quality of concrete. Concrete production equipment on-site
w
as .E
w w
The portable concrete mixers can be electricity, petrol or diesel powered, with capacities of mixing drum ranging from 100 liters to 750 liters. The concrete mixers used in the current construction works are, based on the needs of each project, the mixers of 180 liters, 260 liters and 400 liters. There are many different types of portable concrete mixers ranging in characteristics such as capacity, number of cycles/hour, power supply and mobility. This equipment is characterized by its " portability " - i.e., ease of movement and deploymentthis is due to their small size and weight. It can be easily transported from place to place in the same construction work. [M/J – 10] [N/D14]
ng yE
1. Various Equipment’s for mixing of concrete.
The mixers can be classified according to the technical aspects of overall operation, namely: • Mobility; • Discharge method; • Mixing system; • Power supply.
e in
t
ne
g.
in
er
ployed and easily moved and handled. In terms of location in the jobsite it is, whenever possible, close to raw material constituents of concrete, and near the discharge locations, since the drums must be loaded before each discharged immediately after mixing. A concrete mixer with great advantages in terms of work mobility is the mini-truck mixer. The discharge method is also important because it determines the placement of concrete in its final position, and a defective discharge method can lead to loss of properties required for concrete, like the occurrence of concrete segregation The mixing system is important because there are concrete with characteristics that are only possible to reach by a specific blending method. The type of power supply is an aspect that can be important depending on the availability of energy resources on site. The main technical aspects (selection criteria) for portable concrete mixers are: − Mobility; − Mixing capacity; − Mixing speed; − Mixing Method; − Discharge method; − Concrete Properties Maintenance and safety of concrete production equipment Regular maintenance is essential for the reliability and service life of concrete production equipment. The proper maintenance also helps to eliminate the hazards associated to workplace due to the typical supervision activities of the maintenance processes. Lack of maintenance or improper maintenance can cause dangerous situations, accidents and health problems. So there is a direct link between maintenance and safety. This type of equipment has a high mechanical rotation operation. This rotational movement and easy access to input and output local of concrete feeding devices makes it dangerous process. The active
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
maintenance of this equipment is considered of high risk and must be performed safely. Before any maintenance the equipment must be disconnected from any power source. Concrete transportation equipment
as .E
w w
w
The process of transporting concrete is an extremely important and determining step for the success of a construction work. It is necessary to run an effective and efficient transport to ensure that the concrete reaches its final destination in the prescribed conditions, i.e. without losing properties inherent to its fresh state and later its hardened state The transportation of concrete shall be done with equipment suitable for the type of concrete, the distance between the place of production and the jobsite, the type of route, the weather and other conditions that could be predicted, such as traffic or times hold due to other deliveries of concrete. The perishable material such as concrete requires careful planning with respect to transportation and placement so the discharging process should not coincide with the arrival of other concrete fillers. The time factor is crucial, it is necessary to be a preparation even in the level of correction of unexpected equipment failures, for example, a flat tire can be sufficient to delay the delivery. Currently there are several fleet management software that aids the fleet management and even the preventive and corrective maintenance. Given the importance of the transportation process it is also important to define selection criteria, with the objective of adapting the equipment to the construction work and to the type of concrete. The aspects of selection are related to transportation capacities, required concrete quality, time and costs. Description and characterization of concrete transportation equipment
ng yE
t
ne
g.
in
er
e in
Within the concept of transportation the equipment used for this purpose is the concrete mixer truck. The concrete mixer truck is characterized by being transport equipment and simultaneously a mixing equipment, the mixing process is usually useful to maintain the consistency of concrete. The concrete mixer truck is a vehicle with a drum/mixing tank mounted on its chassis or on semitrailer, are generally mounted with hydraulic pump piston with variable displacement and fixed hydraulic motor. Drums are available with capacities varying from 1 m 3 to 12 m3 , and its interior is controlled in terms of temperature, humidity, pressure and consistency. The rotation speed of the drum is controlled and is also a very important factor for the mixture control. The mixing drums are fitted with helical blades that are an integral part of the drums and are responsible for mixing the concrete when revolve around one direction and are responsible for unloading and cleaning when turning in the reverse direction. These blades are made of highly resistant steel because they are in permanent contact with the concrete. The selection criteria for concrete transportation equipment are very important to ensure the success of the construction work, being necessary to consider the transport equipment taking into account the location of the construction work, its size required properties for concrete and deadlines to meet. Thus the selection criteria should include: − Required quantity and quality of concrete - This criteria is important to determine the capacity of the concrete mixer truck and/or the type and the number of trips required for completion of the concrete placement; − Use method - this relates hypothesis of hire only concrete mixer trucks or concrete mixers truck for transportation and production or concrete truck mixers with pump system. Concrete placement equipment The process of concrete placement in the jobsite - concreting - is one of the most important in the concrete cycle because if the requirements and rules are not followed all the work so far can be putted in question even if the concrete has left the production in excellent condition and transportation has been successful. This stage is therefore one of the most demanding in terms of supervision. It is understood by concrete placement on-site, the unloading of concrete to its final position, compacting and finishing. The finishing of the concrete is done through leveling, grinding and possible application of products in its surface layer. The main ways of
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
placing concrete in its final position are: − Discharged by the concrete mixer truck in to its final position; − Through concrete gutters discharge; − Discharged by buckets, after reception of concrete mixer truck and crane transportation; − Pumped using concrete pumps and discharged in its final position by means of piping. 6. Explain in detail, the factors governing the selection of equipment for earthwork.
(N/D 15) (M/J 14)
One of the problems encountered by a contractor as he plans to construct a project is the selection of the most suitable equipment to be used. For a beginner contractor, he cannot spend all the money for equipment in a particular investment. A contractor can never afford to own all types and sizes of equipment. A contractor does not have to pay for the construction equipment but it must pay for itself by earning for the contractor more money than it costs.
ng yE
as .E
w w
w Factors Affecting the Selection of Construction Equipment
er
e in
1. Standard type of Equipment. There is no such definition of standard type of equipment. An equipment maybe standard to one contractor but may not be to another. It depends on the operations of a contractor/company.
in
2. Special Equipment. One definition of special equipment is one that is manufactured for use on a single project or a special type of operation
t
ne
g.
3. Replacement of Parts. One factor in the Selection of branded equipment is the availability of replacement parts. When buying equipment, spare parts must be readily available or could be easily purchased. A truck with a broken axle is useless until the axle is replaced. A contractor should know where to obtain spare parts for his equipment, if not, then it may be wise to purchase or rent another equipment. 4. Cost of Owning and operating construction equipment. There is no exact method of determining the probable cost of owning equipment. Carefully kept records should give information as a guide as to the type of equipment you need to purchase. The number and the kind of projects a contractor is involved in should indicate what kind of equipment he has to purchase. 5. Economic life of construction equipment. The owner of the construction equipment should be interested in obtaining the lowest possible cost per unit of production. In order to accomplish this objective, he must follow an informed program of equipment replacement. How many years will he use his equipment?? Do his maintenance costs warrant the operation of the equipment or is there a need to dispose the equipment and buy another on?? The owner must consider all the costs related to the ownership and operation of the equipment. The costs to be considered are: depreciation and replacement, investment, maintenance and repairs, downtime, obsolescence. An analysis of the effect which hours of usage will have on each of these
Downloaded From : www.EasyEngineering.net
Downloaded From : www.EasyEngineering.net
costs will establish the time at which a machine should be replaced. 6. Sources of Construction Equipment. Contractors are frequently concerned about whether to purchase or lease construction equipment. Under certain conditions, it is financially advantageous to purchase whereas under the conditions it is more economical and satisfactory to rent it.
t
ne
g.
in
er
e in
ng yE
as .E
w w
w Downloaded From : www.EasyEngineering.net
