Understanding Concrete Cube Testing - A scientific way By - Er. Ravi Ranade
Structural Auditor, Concrete & NDT Consultant (
[email protected] ) Abstract -
As per IS – 456 – 2000, Concrete cube testing is the most important acceptance criteria for concrete. Apparently cube casting and testing procedure seems to be very simple, but there are several factors that affect the cube strength. It has been observed that, in majority projects the cube casting, testing procedures and acceptance criteria are not followed as per IS code and hence in reality, cube strength do not truly represent in-situ strength. The variation in true intrinsic in-situ strength and cube strength is noticed to be very high. This paper highlights the correct scientific method of cube casting and valid testing, and explains in detail loopholes and factors affecting these procedures. Prelude – Durability of concrete is being talked for last few decades. Concrete is supposed to be a durable material, which should last for at least 80 to 100 years. But at the same time number of concrete structures are showing signs of distress at much early stage. We have been noticing, number of building collapses in last few years across the country. A new clause for Durability was added in IS – 456 – 2000, which specifies a minimum grade of concrete as M-20 and above based on the various exposure conditions. Before year 2000, most of the buildings in India are constructed with M-15 grade of concrete. Based on the statistical data of thousands of structures, on which various Non Destructive tests were conducted, it has been observed that, deterioration of concrete is more in the structures constructed after 1980 and even in newly constructed buildings. This is very surprising. In spite of increasing the grade of concrete to M-20 from M-15, still the reduction observed in concrete strength is more in building with 0 to 20 years of age. Where are we going wrong?. Is the material, to be blamed or the construction techniques or the workmanship?. In most of the cases a total casual approach towards quality is found right from worker to the chief engineer level. The cube results of all these structures indicate a sufficient strength, but the ground reality is different. In most of the cases the cube test procedure and results are managed. In many case it has been observed that, a separate batch is casted for filling concrete cubes with low w/c ratio and higher cement content than mentioned in mix design. Once the cube are filled with the GOOD concrete, the real production of “Sambar Concrete” is continued for entire rest of the day. Thus on paper the structures are Durable, but in ND test, the structure are noticed to have sub-standard quality concrete. The author has noticed that, in the majority of the testing labs at construction sites, institutes and other laboratories, the cube testing is done in a very crude manner. Most of the IS code requirements are not followed. Infact most of the testing persons are totally unaware about codal provisions. In many cases the testing is done by an unskilled labour without any supervision by a trained engineer. This paper elaborates Do’s and Don’ts in Cube casting and Testing procedure Durability Requirements Cl. No.- 8, of IS – 456- 2000 talks of Durability aspects of the concrete and specifies various requirements. The exposure conditions classification , Minimum Cement content, Max w/c ratio & Minimum Grade of concrete for various exposure conditions as per IS – 456-2000 are as below –
Sampling of Concrete – Sample of concrete for test specimen shall be taken at the end discharge point. The samples thus obtained shall be mixed on a non-absorbent base with shovel until it is uniform in appearance. In case of RMC supply, most of the time the concrete samples are collected from the chute of transit mixer, which is wrong, the samples shall be collected at the end discharge point only. If pumping / placing is not in the scope of the RMC supplier, then RMC supplier may collect additional sample for his contractual requirements. But for final acceptance of concrete as per IS – 456, the samples should only be taken at end discharge point. Sampling should be spread over the entire period of concreting and the frequency of sampling of concrete of each grade shall be as following:
Three test specimens shall be made for each sample for testing at 28 days. Additional samples may be required for various purposes such as to determine the strength of concrete at 7 days or at the time of striking the formwork, or to determine the duration of curing, or to check the testing error. Cube Casting & Testing Procedure –
Size of Test Specimens— Test specimens shall be cubical in shape of size 150 × 150 × 150 mm. Cube Moulds — The mould shall conform to IS: 10086-1982. The Mould shall be of Cast Iron or Mild steel with a thickness wall plate as 8 mm. The tolerance for Distance between opposite faces is +- 0.2 mm Tamping Bar — The tamping bar shall conform to 6.1(a) of IS: 10086-1982. The tamping rod shall be of 16 mm diameter & 600 mm long with bullet head at the tamping head Compaction - Fill the cube in three equal layers of 50 mm thick. Compact it, either manually or by Vibrating table ( IS – 2514 - 1963 - Specifications for Vibrating Table). For cubical specimens, in no case shall the concrete be subjected to less than 35 strokes per layer for 15 cm cubes or 25 strokes per layer for 10 cm cubes. For compacting on Vibrating Table , each layer is filled and vibrated till no more bubbles are on the surface of the layer, this is repeated for the 3 layers. It is very important, not to over vibrate the layers as it may lead to segregation / disruption of the concrete mix Curing— The test specimens shall be stored in a place, free from vibration, in moist air of at least 90 percent relative humidity and at a temperature of 27°± 2°C for 24 hours ± ½ hour from the time of addition of water to the dry ingredients. After this period, the specimens shall be marked and removed from the moulds and, unless required for test within 24 hours, immediately submerged in clean, fresh water or saturated lime solution and kept there until taken out just prior to test. The water or solution in which the specimens are submerged shall be renewed every seven days and
shall be maintained at a temperature of 27°± 2°C. The specimens shall not be allowed to become dry at any time until they have been tested. Test Procedure— Specimens stored in water shall be tested immediately on removal from the water and while they are still in the wet condition. Surface water and grit shall be wiped off the specimens and any projecting fins removed. Specimens when received dry shall be kept in water for 24 hours before they are taken for testing. The dimensions of the specimens to the nearest 0.2 mm and their weight shall be noted before testing No packing shall be used between the faces of the test specimen and the steel platen of the testing machine. One of the plates, preferably top plate, of CTM shall be a rocker & roller plate.
The load shall be applied without shock and increased continuously at a rate of approximately 140 kg/sq cm/min until the resistance of the specimen to the increasing load breaks down and no greater load can be sustained. The maximum load applied to the specimen shall then be recorded and the appearance of the concrete and any unusual features in the type of failure shall be noted. Visual Observation of Failure pattern - The failure shape can indicate whether it’s a satisfactory / unsatisfactory failure. The image below shows the various failures of a cube as show in BS EN 12390-3:2009
Fig. 1 - Satisfactory failures of Cube Specimens
Fig. 2 - Some Unsatisfactory failures of Cube Specimens
Factors Affecting Concrete Cube Test –
Numbers of factors affect the concrete test, such as influence of size, geometry, and the moisture state of concrete; loading parameters include stress level and duration, the rate at which stress is applied etc. A compression test assumes a pure state of uniaxial loading. However, this is not the case, because of frictional forces between the load plates and the specimen surface. The affect is to restrain the specimen from expanding One of the surveys to identify the effects of deviating from the standard procedures for determining cube strength and indicates the importance of the method of test
Fig. 3a - Effects of deviation from standard procedures for determining Cube strength
Fig. b - Statistical data on likely reasons of cube failure in one of Power plant project
Some of the factors, which influence the test results are listed below –
Age – Chemical composition, addition of chemical / mineral admixtures and fineness of cement influences the age – strength relationship. Below table give an indicative strength gain with the time. These values are indicative & they can vary from brand to brand of cement and on number of parameters stated above.
Age in Days 1 3 7 14 21 28
% Gain of Strength Fly Ash OPC Blended Cement 41.6 25.5 61.5 49.4 76.8 67.9 89.3 83.0 96.7 91.9 100 100
Fig. 4 - % Gain of concrete strength for different types of Cement – Two research studies
Now a days variety of cements are available in the market with various grades and with various blended mineral admixtures like, fly ash, GGBS etc. There is a lot of variation in the gain in strength at early ages especially upto first 15 days. Thus in IS – 456-200, the clause for testing and acceptance criteria for 7 days has been removed. It is now optional to test the cubes at 7 days and the acceptance criterion for this age has to be established at site for that particular grade of cement and mix design. Size of Specimen – Specimen size is important for the simple fact, that as the specimens become larger it is more likely to contain an element that will fail at a low load
Fig. 5 - Compressive strength of Cubes & Cylinders of Different Sizes If the maximum size of aggregate is large in relation to the size of the mould, the compaction of concrete & uniformity of the distribution of the large particles of aggregates are affected. IS – 516 allows to use 100 mm cube for aggregate size of max 20 mm and 150 mm for max size of aggregate as 40 mm Shape – Geometric factors such as volume of concrete, shape of concrete and h/d ratio (height to lateral dimension) of specimen affects the concrete cube and cylinder strength ratio. The following figure shows the effect of height/diameter ratio to concrete strength ratio.
The restraining effect of the platens of the testing machine extend over the entire height of the cube, but leaves unaffected a part of test cylinder, thus the strength of cylinder is lower than the cube shape specimen As per IS – 516 the conversion factor is - Cube Strength = 1.25 x Cylinder Strength
Fig – 6 Frictional restraint at the ends of cylinders result in state of triaxial compression shown in the shaded region
Mould material & Texture of moulds
Not much of research data is available on effect of mould material and texture of moulds. But it has been reported that the concrete casted in paper and plastic moulds give lower compressive strength than the steel moulds. The texture of the mould can affect friction between steel plates and the concrete & in turn can affect the restraining effect of the platens of the testing machine
The moulds for casting concrete specimens must have the following properties: 1) Made of a non-absorbent material, 2) Nonreactive with the concrete (aluminium and magnesium are examples of reactive materials), 3) Hold their dimensions and shape, and 4) Be watertight. The mould material used is a cast iron, which is totally different than the site shuttering material. At site, there is a practice to use variety of shuttering materials, such as wooden planks especially for beams, plywood for columns and slabs, steel plates for slabs and now days even plastic & aluminium formwork material is available.
Compaction – Layers, no. of blows, dia. of compaction rod
The compressive strength of concrete cast in structures is generally determined by standard cube or cylinder tests. However, these standard tests on specimens represent the potential quality of concrete and they should include full compaction. On the other hand, compaction received by concrete structures and standard specimens may be different. The strength of moist-cured concrete mainly depends on the water-to-cement ratio and degree of compaction. The presence of voids in concrete due to improper compaction significantly reduces the strength. As per standard test procedure, each layer of concrete in the cube mould should be vibrated till no more bubbles are on the surface of the layer and for this minimum 35 blows are required. If the number of blows are reduced than the specified, then there will be a possibility of some entrapped air left inside, which can reduce the strength. The presence of even 5 % voids due to incomplete compaction may reduce the strength by 35 %. Similarly over compaction is also not recommended, as it will segregate the concrete.
Curing temperature – 27
0
C ( +- 2 C) 0
Curing temperature plays an important role on the compressive strength development of concrete. Generally, higher curing temperatures promote an early strength gain in concrete but may decrease its 28-day strength. In fact, not only curing temperature but also casting temperature (i.e., the temperature during the first 2 h after making concrete) influence on strength development of concrete and presented in the following three scenarios. Cast and cured at the same temperature: It is generally observed that up to 28 days, the higher the temperature the more rapid the cement hydration and the strength gain. Cast at different temperatures but cured at a normal temperature: The concrete castes at lower temperature can attain ultimate compressive strengths at 180-day compared with concrete casted at high temperature. It is because, at low temperature casting, the microstructure of the hydrated cement paste is relatively more uniform. Cast at normal temperature but cured at different temperatures: The lower the curing temperature, the lower would be the strength up to 28 days. At a curing temperature near freezing, the 28-day strength is about one-half of the strength of the concrete cured at normal temperature. At curing temperature below freezing the strength development is hardly any.
Curing period
It is recommended to cure the concrete cubes for 28 days. But for site concrete, as per IS – 4562000, the curing period shall be minimum 10 days for OPC & 15 days for Blended cements. It has been found that for 7 to 14 days moist curing concrete can get a strength upto 85 - 90 % that of 100 % strength for 28 days full moist curing.
The reduction of 15 % in strength, for lesser period of curing at site, is being accounted in partial safety factor for concrete in structural design. Hence curing of site concrete though recommended more than 10 days, but is not mandatory. Sometimes the cubes are placed at the site so as to receive the same curing condition, that of site. Such cubes may indicate an in-situ strength of concrete, but these samples shall be casted in addition to the standard cubes. The final acceptance criteria shall be only on the standard cube specimens fully cured.
Moisture Condition –
The standard procedure requires that the specimens continue to be in a moist condition at the time of testing. In compression tests it has been observed that air-dried specimens show 20 to 25 percent higher strength than corresponding specimens tested in a saturated condition. The lower strength of the saturated concrete is attributed to the disjoining pressure within the cement paste & lubricating effect, moisture has on the concrete particles
Test procedure & Compression Testing Machine Issues -
One-sided (bending) modes of failure are associated with reduced strengths and can be caused by one or more of the following faults Placing cubes off-centre and faults in cube moulds (Figure – 7 ) Faults in testing machine (loose or misaligned parts) Insufficient lateral stiffness of the testing machine Premature locking of the ball seating before mating with cube Rotation of the ball seating under load Excessive lateral movement of the hydraulic ram Excessive lack of planeness of loaded surfaces of platens or specimen with the influence on fracture and strength depending on the quality (stiffness) of the concrete (Figure 8) Fins on cube (particularly at later ages)
Fig. 7 - Strength & Failure type for 100 mm Cubes placed eccentrically in testing machine
Fig. 8 - Fracture modes for Cubes of different qualities of concrete tested using non-plane platens The effect on measured cube strength of the behaviour illustrated in Figure 8 is given in Figure 9.
Fig – 9 - Effect of platen planeness defects on measured cube strength of concrete of differing quality A hard or stiff plate will concentrate stress at the outer edges whereas a softer plate will have higher stress at the center. This same concepts of hard and soft at applicable to the testing machines themselves. A soft machine will release the stored energy of its deformation to the specimen as it fails whereas a hard machine will not. Packing is not recommended, because it results in an appreciable lowering of the apparent mean strength of the concrete. Similarly lubrication of the test surfaces is also not recommended as it reduces the strength of the specimen. The use of rubber of lubricant between the specimen and the loading plate can induce lateral tensile load at the end of specimen. This will cause vertical splitting and reduce apparent strength It is recommended to calibrate the CTM every year from an ISO – 17025 accredited calibrating Lab. The specimens shall be tested in the range of 10 to 90 % of the capacity of CTM. For testing small core samples, it is recommended to use a smaller capacity CTM, say less than 35 to 50 Tons
Direction of casting & Direction of Testing
The cylinders are cast and tested in the same direction, whereas in a cube the testing direction is perpendicular to the direction of casting. In the structure, for compression members, the situation is similar to the cylinder specimen, and that’s why cylinders are more realistic
Fig – 10 - Casting and Testing direction for Cylinder & Cube
Fig – 11 - Relation between mean strength of concrete cubes loaded in the direction of casting & in the standard direction Rate of loading – Rate of loading is quite important in the test for compressive strength. In general, the higher the loading rate the higher the measured strength. The reasons for this are not completely clear, however, it is thought that under slow loading rates more subcritical cracking may occur or that slow loading allows more creep to occur which increase the amount of strain at a given load. A project was conducted under the guidance of the author, to assess the effect of rate of loading on the compressive strength of concrete. The data was collected from the various sites to know the rate of loading with which the cubes are tested in site labs. Using this data concrete cubes of various grades from M-20 to M_40 were tested with different rate loading. As per IS – 516 the standard rate of loading should be 140 Kg/Sqcm/ Min i.e. @ 315 KN/Min for 150x150x150 cube. It can be seen from the Fig 12, that, with increase in rate of loading a compressive strength can get increased upto 50 %. It has been further observed that, in India presently there are hardly any labs, which have the Compression Testing Machines, which can control the rate of loading.
Rate of Loading Kg / Sqcm. / Min
Fig – 12 - Relation between Rate of Loading & Compressive strength of concrete for different Grades of Concrete
Acceptance criteria –
As per IS – 456 – 2000 , the concrete shall be deemed to comply with the strength requirements when both the following condition are met:
Conclusions – The entire process of Concrete cube Test right from sampling, casting of cubes and testing of cubes is very sensitive to number of factors as stated above. In many of the cases, the scientific procedure is not followed and thus there is a doubt about the correctness of the cubes strength. It has been observed that, most of the time the cube strength is on higher side, than the true cube strength. Some of the standard practices followed at sites are – A truly representative concrete is not used for filling the cubes. A special batch with lower w/c ratio and higher cement content is made to cast the cubes. The samples are very rarely collected at the end discharge point. The sample size is always less than the IS – 456 requirements. The cube samples do not have the correct size and shape. It is very important to measure the dimensions of the specimens to the nearest 0.2 mm as per IS – 516. In most of cases dimensions not are not measured, but are simpy assumed be perfect 150 x 150 x 150 mm, which is NEVER the case. It is very important to know the type of fracture, which can indicate that, whether testing is properly carried out or not or whether the size and shape of the cube is responsible for erroneous results. At allmost ALL sites, temparature control facility for curing tanks is not available.
Most of the time, cubes are taken out from the tank much before the testing , and they do not have the saturated surface condition (SSD). The resulted strength is on higher side. Hardly any labs in India have Rate of loading controled testing machines. As reported above, rate of loading with which cubes are etsted at site or in labs is much higher than the requried. Thus the resulted strength is on very much higher side.
These all observation give rise to a doubt about the reliablity of the Cube test. Further the site conditions are totally different. It’s a big question mark, that whether Cube strength represents the Insitu strength? There is a in general very casual attitude towrds entire process of concreteing activity right from selection of raw material, testing of raw materials, mix design, control on w/c ratio, correction in mix design for moisture / absorption, concreteing activity, curing and testing of concrete. The most important - curing is slowly getting vanished from the sites. “Bhisti” is vanished. The concrete is hardly cured. And if you ask the site engineers about this, we get the answer as ‘we do not have time to do all such bookish things at site, we have to achive the target completion of project’. We are running madly after time, at the cost of Quality What we can do – Train all concern the civil engineers from site, owner’s engineers and engineers in the structural designers office Increase awareness to accredit the Labs by NABL ( ISO – 17025) Reduce the speed of construction – especially concreting activity, which is never on critical path. Conduct Non Destructive tests in addition to get a fair picture about realistic In-situ strength of concrete The making and casting of concrete seems to be much easy, but proper understanding of a concrete and making a “Durable Concrete” requires sinsere efforts. Otherwise structures will remain “Durable”, only on paper !!!! --------------------------------------------References – Properties of Concrete – By Adam Neville Concrete – Microstructure, Properties & Materials – By P. Kumar Mehta & Paulo J. M. Monteiro Advanced Concrete Technology - Testing and Quality – Edited by John Newman & Ban Seng Choo Significance of Tests & properties of concrete & concrete making materials - Editors - Joseph F. Lamond and James H. Pielert, Concrete Technology – M L Gambhir Casting Mould influence on Concrete Strength - luke M.Snell and H.AldridgeGillespie Concrete Test Cubes – Ambuja Technical Literature Series – 18 http://theconstructor.org/concrete www.engineeringcivil.com Project on “Effect of Rate of Loading on compressive strength” under guidance of Er. Ravi Ranade IS – 516- 1959 - Method for Test of Concrete IS – 456 – 2000 – Plain & Reinforced concrete – Code of Practise - Section 2 - Materials, Workmanship, Inspection and Testing IS – 10086 – 1982 - Specifications for Moulds for use in Test of Cement & Concrete IS – 2514 - 1963 - Specifications for Vibrating Table IS:14858-2000 – Compression Testing Machine used for testing concrete & Mortar
