NON DESTRUCTIVE TESTING OF CONCRETE STRUCTURES RAVI RANADE Practicing NDT , Structural & Rehabilitation consultant “Kanchan Bhavan” , 5 , Shilavihar Colony, Karve Road, Erandawane, PUNE 411038 Tel / Fax :- +91 - 20 - 2543 2643 Mobile :- +91 94220 07479 E-mail : -
[email protected] OR
[email protected] Web sites – www.conretepune.com , www.ndtconcrete.com , www.ndtindia.com
INDIA
INTRODUCTION :The need of testing hardened concrete has been felt for last three to four decades. The present methods of testing fresh concrete such as cube test, slump test are now being supported by various Non Destructive Testing methods. Many of the tenders now specify Non destructive testing as an additional quality control requirement. To assess the integrity of old or new concrete and reinforcement, Non destructive testing is one of the most powerful and reliable tools. The need of conducting non destructive testing for condition assessment of the RCC structures has grown considerably in recent times, due to increase in number of structures, showing signs of distress. The standard life of RCC frame structure is considered to be 60 – 80 years. But it has been reported that, many of the buildings completing just 20 – 25 years of their life, in Mumbai and other coastal areas, have observed in distressed condition. If one visits the chemical plant buildings, the RCC structures are in distressed condition within a span of only 7 to 10 years. The severe exposure condition is the main reason for this deterioration. For better durability performance of the structure, use additional cover , blended cement, anticorrosive coating to reinforcement are being recommended on one side and on the other hand use of quality controlled RMC, intensive and strict quality control & testing by using modern techniques is being suggested. The Non Destructive Testing is being fast, easy to use at site and relatively less expensive can be used for
Test actual structure instead of representative cube sample. Test any number of points and any locations Quality control tool Assess the structure for various distressed conditions Damage assessment due to fire, chemical attack, impact, age etc. Detect cracks, voids, fractures, honeycombs and week locations Monitor progressive changes in the properties of concrete , reinforcement etc. Assess overall stability of the structure Monitoring repair and rehabilitation systems Scanning for reinforcement location, stress locations.
In the recent years significant advances have been made in Non Destructive Testing techniques, equipment and methods.
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The various Non destructive / partial destructive tests can be identified as follows
Group - I A - Surface Hardness Methods
Rebound Hammer Test Penetration Test – Windsor Probe
Group - I B - Dynamic or Vibration Methods
Resonant Frequency – Only Lab method Ultrasonic Pulse Velocity Test Acoustic Emission Impact Echo Transient Dynamic Sonic / Pulse Echo , Impulse Response Cross Hole sonic logging Vibration Measurement
Group - C - Magnetic Methods
Reinforcement Detectors & Cover meters
Group - D - Electrical Methods
Dielectric Measurement ( Moisture Content ) Electrical Resistivity Half – Cell Potential Polarisation
Group - E - Electro- Magnetic Methods
Radar
Group - F - Radioactive / Infrared Methods
X – Radiography Gamma Radiography Infrared Thermography
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Group - G - Penetrability Methods
Air / Gas / water permeability Test
Group - H - Partially Destructive Tests
Penetration Resistance Test ( Windsor Probe ) Pull - out test Pull - off test Break – off test Core cutting
Group - I - Other properties of fresh / hardened concrete
Chemical tests Cement Content & Agg / Cement ratio Sulphate determination test Chloride determination test Alkalinity test Carbonation test Moisture Measurement Crack measurement & monitoring Endoscope / Bore scope / Underwater Inspection Underwater Inspection Abrasion resistance test Fresh concrete tests for W/C ratio and compressive strength
Selection of Non Destructive Testing Method Parameter
Test / Method
CONCRETE
Compressive strength
Rebound Hammer , Windsor probe, Ultrasonic Pulse Velocity, Core , Capo, Pull out Combined methods
Flexural Strength
Break – off
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Direct tensile strength Parameter
Pull off Test / Method
Concrete Quality, Homogeneity Honeycombing, voids,
Ultrasonic Pulse Velocity, Pulse echo, Endoscopy, Gamma Ray Radiography
Damages – Fire / blast
Rebound Hammer Ultrasonic Pulse Velocity Core Test
Cracks , Delaminations - Water tanks / Pavements / Bridges
Ultrasonic Pulse Velocity Infrared Thermography Acoustic Crack Detector Dye Penetration Test X – ray Radiography Gamma Ray Radiography Crack scope Vibration Measurement
STEEL
Location, cover, size
Re bar locator , Bar sizer
Corrosion
Half – Cell Potential Resistivity Carbonation Chloride Content
Condition
Endoscope / Borescope Underwater Inspection
Integrity & Performance
Tapping Pulse – echo Acoustic emission Radar Load Test
Rebound Hammer Test :- ( IS – 13311 ( Part 2 ) : 1992 ) Principle This is the most common method adopted for checking strength of concrete since 1940. The test is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which mass strikes. The plunger of hammer is pressed strongly and steadily against the concrete surface at right angles to its surface, until the spring loaded mass is triggered from the locked position. The spring controlled mass rebounds and the extent of such rebound depends upon the surface hardness of concrete. The distance traveled by the mass as a percentage is defined as rebound number.
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Rebound Hammer
Different Rebound Hammers along with Calibration Anvil
Procedure The test is sensitive to local variations in the concrete, for e.g. If plunger strikes the aggregate underneath, then it would indicate high rebound number, visa versa if it strikes a void / cement paste , then it will indicate low rebound number. For this reason it is desirable to take at least 12 readings per 300 Sqcm. ( as per BS 1881 : Part 202 (21)) and discard abnormally high / low readings from the average one. The impact / test points shall not be closer than 20 mm to each other.
Rebound Hammer Test on Pre-cast elements
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Rebound Hammer Test on Bridge Girder
Influence of Test Conditions The test is significantly influenced by following factors : Type of Cement :Concrete made with high alumina cement can give strength 100 % higher or that with supersulphated cement can give 50 % lower strength than that with ordinary Portland cement. Type of Aggregate :The influence of aggregate type and proportion can be considerable. Surface condition & Moisture content :Trowelled & floated surfaces are harder than moulded surfaces. Exposed aggregate surfaces are unsuitable for this methods. A wet surface will give rise to underestimation of strength of concrete by about 20 % lower than calibrated under dry condition. Curing & Age of concrete :The relation between hardness and strength varies as a function of time, subsequent curing and exposure condition will further influence this relationship. But this effect can be ignored for concrete up to 3 months old. Carbonation of Concrete surface :Concrete exposed to atmosphere will normally form a hard carbonated skin. The strength predicted for carbonated concrete may overestimate up to 60 %. Application :The rebound hammer method may be used for Assessing the likely compressive strength of concrete with the help of suitable calibration charts. Assessing uniformity of concrete Assessing the quality of concrete in relation to specified standard requirements. Assessing the quality of one element of concrete to another.
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Reliability & Limitations :The test determines the hardness of the surface only. The impact of hammer is sensitive to the surface layer of about 25 to 50 mm only. The reliability of this test as alone, for compression strength is less, but it can generally indicate uniformity of concrete. The rebound hammer results are very sensitive to the spring constant, thus after a use of hammer for about 15000 impacts, it is recommended to calibrate the hammer on Anvil. The influence of aggregate type and proportion can be considerable. Thus it is strongly recommended that every lab, NDT consultant using rebound hammer shall develop their own graphs for compressive strength verses Rebound number, and shall not use blindly, the graphs given by manufacturers. The probable accuracy of prediction of concrete strength in laboratory level is 15 % and that of structure is 25 %. It is recommended to use of Rebound hammer along with ultrasonic concrete testing or core test.
Penetration Resistance Test (Windsor probe ) :Principle A test commonly known as Windsor probe test, estimates the strength of concrete from the depth of penetration by a metal rod driven into the concrete by a specific amount of energy generated by standard charge of powder. The penetration is the function of surface hardness, punching shear resistance and hardness of aggregate expressed as Mohr No. The first two variables are closely related to compressive strength. The penetration is inversely proportional to the compressive strength of concrete, but the relation depends on the hardness of the aggregate. Procedure The probe diameter is 6.35 to 7.94 mm and length is about 79.5 mm. Probes are driven with a velocity of 183 m/s. The strong aggregate near surface may obstruct the probe, thus at least three tests are recommended at a location. The probes shall be driven at least 100 to 175 mm apart to avoid overlapping of zones of influences. This test compared to rebound hammer test is better in the sense that, it does not test only the surface, but it in depth the probe actually fractures the aggregate and the compression of material is taken into account.
Windsor probe Reliability & Limitations :-
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This test is also not widely accepted ,as it tests only thin layer to the tune of three times the impregnated depth. It is claimed that concrete between 25 to 75 mm below surface can be assessed by this method. The surface damage caused by this test is about 50 – 100 mm. It is reported that, this test is not reliable for concrete with strength less than M – 10. The probable accuracy of prediction of concrete strength of structure is 25 %.
Ultrasonic Pulse Velocity Test :- ( IS – 13311 ( Part 1 ) : 1992 ) Principle This is one of the most commonly used methods, in which the ultrasonic pulses are transmitted through the concrete. In solids, the particles can oscillate along the direction of sound propagation as longitudinal waves, or the oscillations can be perpendicular to the direction of sound waves as transverse waves. At surfaces and interfaces, various types of elliptical or complex vibrations of the particles occur. A complex system of stress waves is developed which includes longitudinal (Compression) , shear ( transverse ) and surface ( releigh ) waves. Piezoelectric transducers are designed to generate longitudinal and transverse (shear) waves. The active element of most acoustic transducers is piezoelectric ceramic These transducers converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode). The travel time is measured with an accuracy of +/- 0.1 microseconds. Transducers with natural frequencies between 20 kHz and 200 kHz are available, but 50 kHz to 150 kHz transducers are common. This instrument basically is dependent on the Dynamic Young’s Modulus , density , Poisson’s ratio of the material. Under certain specified conditions the velocity and strength of concrete are directly related. V2 =
Ed (1-v) p (1+v) (1-2v)
Where Ed = Dynamic Young’s Modulus v = Dynamic Poisson’s ratio As V Ed , Ed Es Thus V fck
p = Density
& V fck
Thus it is clear from the above equations that pulse velocity is directly proportional to the compressive strength of concrete. Procedure There are three possible ways of measuring pulse velocity :
Direct Transmission ( cross probing )
Semi-direct Transmission
Indirect Transmission ( Surface probing ) UPV - Different Test Methods
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The transducers are placed on the smooth concrete surface to measure the time required for travel. A coupling media such as petroleum jelly, grease are applied to the surface to have good acoustical coupling. The velocity is calculated as V = L / T Where,
L is the distance between two probes and T is the time required to travel the distance between two transducers.
If the quality of concrete in terms of density, homogeneity, strength and uniformity is good then the velocity obtained is higher In practice it is convenient to establish the relation between strength of cube and pulse velocity. Calibration charts are prepared based on the above relationship by testing sufficient numbers of cubes under various conditions and for various mix designs. This calibration charts also has to applied proper modifications / correction factor for various influencing conditions such as Grade of cement , Moisture content, Type of aggregate, water cement ratio, % reinforcement etc. At Construction Diagnostic Centre (CDC), we have carried out an extensive research on quality assessment and it has been observed that, the quality gradation as per IS – 13311 (part-1)- 1992 is valid ONLY for M – 15 grade concrete. For concrete with more than M – 20, we recommend to grade the quality of concrete as per below given table – Gradation of Quality of concrete ( as per CDC ) - Direct & Semi Direct velocity Km/Sec. Quality of Concrete M - 20 to M - 25 M - 30 to M - 35 > M - 40 Excellent More than 4.400 More than 4.600 More than 4.900 Good 3.750 to 4.400 3.900 to 4.600 4.150 to 4.900 Medium 3.400 to 3.750 3.600 to 3.900 3.800 to 4.150 Doubtful Less than 3.400 Less than 3.600 Less than 3.800
Gradation of Quality of concrete ( as per CDC ) - Indirect velocity Km/Sec. Quality of Concrete M - 20 to M - 25 M - 30 to M - 35 > M - 40 Excellent More than 3.900 More than 4.100 More than 4.400 Good 3.250 to 3.900 3.400 to 4.100 3.650 to 4.400 Medium 2.900 to 3.250 3.100 to 3.400 3.300 to 3.650 Doubtful Less than 2.900 Less than 3.100 Less than 3.300
Influence of Test Conditions Moisture content : The pulse velocity through saturated concrete is higher by about 2 to 10 % than dry concrete. For high strength and well-compacted concrete the influence is less as against low strength and less compacted concrete. Type of Aggregate : For aggregate having higher specific gravity pulse transmission is faster. Type of Cement : It has observed by the author that, the pulse velocity does not have any significant effect with the different grades of cement.
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Type of Mix & water / cement ratio : With the change in mix proportion the % change in velocity is less as compared with the % change in compressive strength. For change in w/c ratio, % change in velocity is comparable with % change in the strength. Reinforcement : The pulse velocity through steel is about 5.9 Km/sec. as against 3.0 to 4.5 Km/sec. of concrete. Thus it can be seen that, for weaker concrete the effect of reinforcement is more than for higher strength concrete. As far possible reinforcement shall be avoided while selecting the test point. If the axis bar is parallel to the pulse path ( which is a rare case ), the increase in velocity may be to the extent of 5 to 20 % depending on the bar size and the location of bar from the path of the pulse. If the axis of bars is perpendicular to the pulse path, the increase in velocity may be to the extent of 1 to 5 % depending on the bar size. Stress :At higher stress level significant reduction in pulse velocity is observed. Path length :The minimum path length is governed by the frequency of the transducers. Natural frequency transducers kHz
Minimum path length ( mm ) For concrete with V = 3.5 Km/sec. V = 4.5 Km/sec. 23 30 35 45 70 90 175 225
150 100 50 20
There is no significant drop in velocity up to 3.0 m path length. For 3m to 6 m path length, reduction of 5 % in pulse velocity has been reported. Application The ultrasonic pulse velocity method is used to assess The homogeneity of the concrete The presence of cracks, voids and other imperfections, depth of crack Changes in the structure of the concrete which may occur with time The quality / compressive strength of concrete of one element in relation to another element / standard requirement. The values of dynamic elastic modulus of concrete
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Ultrasonic Pulse Velocity testing of Bridge Girder – Direct Method
Ultrasonic Pulse Velocity testing of Residential Building– Direct Method
Ultrasonic Pulse Velocity testing of Overhead Water Tank wall– Indirect Method
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Ultrasonic Pulse Velocity testing of Slab – Direct Method – Probe from Top & Bottom
Ultrasonic Pulse Velocity testing of Slab – Indirect Method
Ultrasonic Pulse Velocity testing of Concrete Road – Indirect Method for crack depth detection
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Ultrasonic Pulse Velocity testing of Silo
Reliability & Limitations Ultrasonic pulse velocity has been found to be a valuable and reliable method of detecting interior of concrete member in truly a non destructive way. If the data about factors influencing pulse velocity as stated above is not available , then, the assessment of compressive strength will have limitations. The accuracy of the strength prediction by this method at about 95 % confidence level is about 15 - 20 % depending upon the concrete mix design data available and the correction factors applied. It is recommended to use this test in combination with rebound hammer & core test for new concrete and with core test for old concrete.
Pull out test :Principle & Procedure The force required to pull out a previously embedded rod ( LOK Test )or cut & expanded in situ ring (Capo test) is measured and the same is related to the compressive strength of the concrete. After the test the cone hole is patched with non- - shrink grout/ mortar. The concrete is actually subjected to tension & shear, but the calculated pull out strength is approximately near to the shearing strength of the concrete. The pull out test need not be carried out up to removal of concrete core, but if the rod / ring do not come out for the required force, then the required strength is assumed to exist. This test is assumed to be better over Rebound hammer test or Penetration resistance test.
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Pull out Test Similar to the above test, special cylindrical moulds are inserted in the slab shuttering. Thus the actual concrete mix with same curing can be tested. These moulds are removed from the slab and tested for compression strength. Reliability & Limitations But these tests has got limitations that all bolts / moulds needs to embedded before hand ( except for Capo Test ). The moulds can be inserted only in slab. All these tests require pre planning.
Pull – off Test : Principle & Procedure This method measures the in situ tensile strength of concrete by applying a direct tensile force. The calibration charts of pull – off tensile strength to cube compressive strength are available. A metal disk is fixed to the surface by epoxy resin adhesive to the concrete surface and is then removed by jack to measure the pull – off force. To avoid surface carbonation effect a partial core of suitable depth can be drilled. It is reported that, the effect of age, type of aggregate, curing etc. is very marginal . This method can be used for testing bond strength of repair work.
Pull – Off test Break – off Test :Principle & Procedure This test determines the flexural strength in a plane parallel at a certain depth from surface. A partial core hole is cut into the concrete or a removable cylinder is introduced in the 14
concrete. Using a hydraulic unit a transverse force is applied at the top of surface , which breaks up the core left in the concrete. The break – off force is related to the bending tensile strength. The core obtained from test may also be tested for compression in laboratory.
Break – Off Test
Core Test :- ( IS – 516 – 1959 , IS – 1199 – 1959 & IS - 456 - 2000 ) Principle & Procedure This is one of the very reliable tests adopted for checking the compressive strength of the ‘In situ concrete”. Other physical properties such as density, water absorption can also be measured from the core concrete. In addition chemical properties of concrete specimen for its cement content, carbonation depth, chloride and sulphate content may be measured. Though this test may become partially destructive for beams / columns , but it can be used for slabs, walls, where partial destruction of concrete due to core cutting do not disturb the stability of the member. In this method concrete cores of sizes raging from 20 mm to 150 mm in diameter and 50 mm to 500 mm long are drilled out by a diamond cutters. The recommended diameters are 100 to 150 mm, but if the drill depth is insufficient as in of case slabs , then smaller diameters may be used but not less than three times nominal aggregate size. The core diameter to length ratio shall be normally between 1.0 to 2.0 ( preferably 2.0) The core diameter shall be at least three times the nominal maximum size of aggregate. Reinforcement shall be avoided in the core. At least three cores shall be tested for acceptable accuracy. These cylindrical concrete cores are then made smooth at both ends ( if required ) and then tested for compressive strength. If required capping of the faces shall be done. The strength of capping material shall be higher than that of concrete in the core. Cap shall be as thin as practicable. The specimen shall be cured in water for 48 hours before testing. The cylindrical strength is then co-related to cube strength. IS – 516 suggest a multiplying factor of 1.25 for converting cylindrical strength to equivalent cube strength. In addition a correction factor for height to diameter ratio shall be applied as given in IS – 516. IS – 456 states that the concrete in the member represented by a core test shall be considered acceptable, if the average equivalent cube strength of core is equal to at least 85 % of the
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cube strength of the grade of concrete specified, but no individual core has a strength less than 75 %.
Diamond Core Cutting Machine
Diamond Core Bits
Application The core cutting is mainly conducted for :
Determining “In situ” compressive strength of structure. Small cores for chemical tests Inserting water supply, plumbing pipes inserting conducts for electrical cables Making pockets for machine foundation for inserting bolts Making weep holes in walls
Core Sample Extraction from a Dam
Core Sample Extraction from a Beam
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Core Extraction from a RCC Chimney
Core Extraction from Box Girder Bridge
Reliability & Limitations As this test gives actual in situ strength of concrete is more acceptable, but due to partial destructiveness needs to be used very carefully. The reliability of small cores i.e. 40 – 50 mm is less as compared to normal cores. The detection of reinforcement shall be perfect. If the quality of concrete is not good, one may not even get a complete core for testing. The cost of core cutting is more compared with other ND tests, as it consumes diamond bits, which are costly.
Other properties of fresh / hardened concrete In addition to the compressive strength , many other properties need to be tested. To assess the durability of the structure tests such as chemical tests, crack monitoring, moistures content are conducted. Many of the chemicals require fully equipped laboratories, but now days “kits” available for site testing may be used. Chemical tests :Cement content and aggregate / cement ratio :It is well known that, lime compounds and silicates in Portland cement are generally decomposed by and soluble in hydrochloric acid. The quantity of soluble silica or calcium
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oxide is determined by simple analytical procedures, and if the composition of the cement is known, the cement content can be calculated. It is virtually impossible to find out the relative ingredients of blended cement. Even the accuracy of cement content is reported to be about 30 to 50 kg/ m3
Chloride determination :Principle & Procedure The excessive amount of chloride in concrete is a great potential corrosion of embedded reinforcement. The small quantity may be present in concrete , but chlorides may increase due to use of creek sand, saline air, seawater, and severe exposure conditions as in case of chemical industries. Various methods for measuring chloride content are available. Laboratory analysis is a simple and reliable method, but it requires a specialised laboratory set up. ‘Hach’ simplified method and ‘Quantab; simplified methods uses the commercially available test kits / strips for site determination of chloride content. These methods are quick are have good accuracy Reliability & Limitations An adequate number of samples representing members, needs to be collected. The chloride distribution may vary point to point depending upon the vicinity of the exposure condition. It has been reported that ‘Quantab’ method can predict chloride content with an accuracy of 0.03 to 1.2 %.
Carbonation test :Principle CO2 present in the atmosphere reacts with the hydrated cement minerals to reduce the alkalinity of the concrete and increase the risk of reinforcement corrosion. Carbonation of concrete results in increase in strength and reduction in permeability. Reinforcement in concrete will not corrode, if the pH of concrete is maintained around 13. Carbonation reduces the pH level of concrete. Reinforcement corrosion may possibly commence at a pore solution pH of about 11. Carbonation rate is normally high in dry weather than in moist weather. The depth of carbonation increases with an increase in water / cement ratio. It is to be noted that corrosion of reinforcement will start if entire cover to the steel is carbonated , but presence of moisture and oxygen is essential. Procedure The extent / depth of carbonation is determined by treating freshly broken / core cut concrete surface or a drilled powder obtained from various depths, with a phenolphthalein indicator. A purple red colouration will be obtained where the highly alkaline concrete has been unaffected, but carbonated portion will remain uncoloured. The change of colour corresponds to a pH of about 8.3.
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Purple colour indicates Un-Carbonated concrete and Colourless Concrete is a Carbonated Concrete Crack monitor & Moisture content meter:To measure the crack width and movement , Crack scope and Crack movement devices are available. The crack scope magnifies the crack about 25 times the crack and measures up to a level of 0.0025 mm. The moisture content in the hardened concrete can be measured by meter within its magnetic field of about 25 mm. Fresh concrete tests for W/C ratio and compressive strength :Now a days fresh concrete testers are available . This instrument consists of probe with two small bowls at bottom attached to a rotary drill. The probe is inserted into fresh concrete and the probe is rotated for about ten times. While rotating the resistance of concrete and other properties of concrete such as temperature , fluidity are sensed by a software and are analysed to give ,finally the slump / W/C ratio / Approximate 28 days strength and the temperature of the concrete. Group - III - Reinforcement location , size and corrosion Steel shares about 40 to 70 % of the load in RCC, that’s why it is equally important that condition assessment of steel is also carried out along with concrete. During last few decades it has been observed that, corrosion of reinforcement in severe in structures near seashore and in the vicinity of chemical industries. A lot of attention is needed for detecting this detoriation and protecting it with proper treatment. Thus due importance shall be given for measuring the size of bar and the amount of corrosion. Rebar Locator & bar sizer :Principle & Procedure The reinforcement bar is detected by magnetising it and inducing a circulating “ eddy current” in it. After the end of the pulse, the eddy current dies away, creating a weaker magnetic field as an echo of the initial pulse. The strength of the induced field is measured by a search head as it dies away and this signal is processed to give the depth measurement. The eddy current echo is determined by the depth of the bar , the size of bar and the orientation of the bar. This detection of location of reinforcement is required as a pre process for core cutting.
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Reinforcement Detection Reliability & Limitations With this instruments a cover to reinforcement can be measured up to 100 - 150 mm with an accuracy of 15 % and Manufacturers claim accuracy of a bar diameter measurement to be less than 2 to 4 mm. But it has observed that, most of the available detectors do not accurately measure the bar diameter. Proper calibration of these instruments is very essential. The factors which affect the accuracy are – Very closely spaced bars or bundled bars or bars in layers, Binding wire, aggregate containing iron or magnetic properties
Corrosion mapping Reinforcement in concrete will not corrode if the protective iron oxide film formed by the high alkaline condition of the concrete pore fluid with a pH around 13 is maintained. This film gets destroyed by chlorides or by carbonation, if moisture and oxygen are present, resulting in corrosion. In the corrosion process anodic and cathodic areas are formed on the reinforcement, causing dissolution of the steel and the formation of expansive corrosion products at the anode. Half – Cell Potentiometer :- – ASTM – C-876 Principle & Procedure The instrument measures the potential and the electrical resistance between the reinforcement and the surface to evaluate the corrosion activity as well as the actual condition of the cover layer during testing. The electrical activity of the steel reinforcement and the concrete leads them to be considered as one half of weak battery cell with the steel acting as one electrode and the concrete as the electrolyte. The name half-cell surveying derives from the fact that the one half of the battery cell is considered to be the steel reinforcing bar and the surrounding concrete. The electrical potential of a point on the surface of steel reinforcing bar can be measured comparing its potential with that of copper – copper sulphate reference electrode on the surface. Practically this achieved by connecting a wire from one terminal of a voltmeter to the reinforcement and another wire to
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the copper sulphate reference electrode. Then generally readings taken are at grid of 1 x 1 m for slabs, walls and at 0.5 m c/c for Column, beams
Working Diagram of Half-Cell Potential Test
Half-Cell Potential Test
Copper – Copper Sulphate Half-Cell
One end of Half-Cell connected to the reinforcement
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Half-Cell Potential Test of Column
Marking of Slab and Pre-wetting for Half-Cell Potential Test
Half-Cell Potential Test of Overhead Water Tank Wall An Equi-potential contour map can be plotted to get an overall picture of the member
Half – Cell Equi-potential Contour Map
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The risk of corrosion is evaluated by means of the potential gradient obtained, the higher the gradient , the higher risk of corrosion. The test results can be interpreted based on the following table Half – cell potential ( mv) relative to Cu-Cu sulphate Ref. electrode
% chance of corrosion activity
Less than - 200 Between – 200 to – 350 Above – 350
10 % 50 % ( uncertain ) 90 %
Significance and use This method may by used to indicate the corrosion activity associated with steel embeded in concrete. This method can be applied to members regardless of their size or the depth of concrete cover. This method can be used at the any time during the life of concrete member. Reliability & Limitations The test does not actual corrosion rate or whether corrosion activity has already started, but it indicates the probability of the corrosion activity depending upon the actual surrounding conditions. If this method used in combination with resistivity measurement , the accuracy is higher. If the concrete surface has dried to the extent that it is dielectric , then pre wetting of concrete is essential.
Resistivity :- – Principle & Procedure Corrosion is an electrochemical process. For corrosion of steel reinforcement to occur in concrete, an ionic current (a flow of ions) must pass between anodic and cathodic regions of the concrete. The electrical resistivity of the concrete affects the ionic flow and the rate at which corrosion can occur; a higher concrete resistivity decreases the current flow. An empirical relationship between corrosion rate and concrete resistivity has been derived from measurements on actual structures Concrete electrical resistivity can be obtained by applying a current into the concrete and measuring the response voltage. Four probes On-site electrical resistivity of concrete is commonly measured using four probes in a Wenner array. The reason for using four probes is the same as in the laboratory method - to overcome contact errors. In this method four equally spaced probes are applied to the specimen in a line. The two outer probes induce the current to the specimen and the two inner electrodes measure the resulting potential drop. The probes are all applied to the same surface of the specimen and the method is consequently suitable for measuring the resistivity of bulk concrete in situ. The resistivity is given by:
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V is the voltage measured between the inner two probes (measured in volts, V) I is the current injected in the two outer probes (measured in amps, A) a is the equal distance of the probes (measured in metres, m).
Wenner 4 probe - Concrete Resistivity Meter
Interpretation of Results Resistivity (kΩ – cm ) More than 200 20 to 200 10 to 20 5 to 10 Less than 5
Indication of Corrosion Rate Very Low Corrosion Rate Low Corrosion Rate Low to Moderate Corrosion Rate High Corrosion Rate Very High Corrosion Rate
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Resistivity Contour Map
Significance and use
Indication of corrosion rate Correlation to chloride permeability On site assessment of curing efficiency Determination of zonal requirements for cathodic protection systems Identification of wet and dry areas in a concrete structure Indication of variations in the water/cement ratios within a concrete structure Identification of areas within a structure most susceptible to chloride penetration Correlation to water permeability of rock
Reliability & Limitations The resistivity is affected by various factors such as rebars on electrical resistivity measurements, aggregate size, moisture content, temperature, carbonation
Group - IV - Miscellaneous Test Impact Echo Principle & Procedure Receiver adjacent to impact point monitors arrival of stress as they undergo multiple reflections between surface and opposite side of plate like member or from internal defects. Frequency analysis permits determination of distance to reflector if wave speed is known. Significance and use Locate a variety of defects within concrete elements such as delaminating, voids, honeycombing or measure element thickness
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Impact Echo Pile Integrity Test (Pulse Echo Method ) Principle & Procedure
Hammer impact on the surface and a receiver monitors reflected stress wave. Time domain analysis used to determine travel time. Based on One Dimensional Stress Wave approach, a low strain integrity testing The pile head motion is measured as a function of time. The time domain record is then evaluated for pile integrity.
Significance and use • • • • • • • • • • •
Suitable for - Slender structural elements like structural columns, driven concrete piles, cast in place concrete piles, concrete filled steel pipe piles, timber piles. Evaluation of Pile integrity and pile physical dimensions i.e. cross-sectional area, length, continuity, and consistency of the pile material Piles shall be trimmed to cut off level or sound concrete level before the test with all laitance removed. For Pile Dia. > 500 mm additional readings ( @ 3 to 5 Nos ) are required. Application of Impact shall be within a distance of 300 mm The software filters the signal to eliminate High & Low frequencies In a few cases where piles are too long or skin fiction is high, low strain method does not provide sufficient information particularly the toe reflection. In such cases high strain method can be used by giving higher impact energies The cast-in-situ piles should not be tested normally before 14 days of casting as per IS code & not before 7 days as per ASTM. The reasonably correct assessment of Stress Wave Velocity of pile concrete is essential as an input. A Complementary Ultrasonic Pulse velocity test may be carried out at the head of the pile in order to arrive at the speed of sound propagation. The test being a low cost and speedy, it is recommended to carry out testing of 100 % piles. The further tests such as Dynamic or Static Load tests may be decided upon the results of Pile Integrity Tests.
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Pile Integrity Test (Pulse Echo Method )
Typical Pile Integrity Testing Instrument in use
Defective Piles
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0m
1
2
3
4
5
6
7
Good Pile – PIT Signal
0m
1
2
3
4
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Defective Pile – PIT Signal Limitations • Evaluation is approximate and not exact. • No information about load carrying capacity of pile. • It does not provide information regarding verticality or displacement in position of piles. • Minor deficiencies like local loss of cover, small intrusions or type of conditions of materials at the base of piles are undetectable. • Integrity testing may not identify all imperfections, but it can be useful tool in identifying major defects within the effective lengths. • The test may identify minor impedance variations that may not affect the bearing capacity of piles. In such cases, the engineer should use judgment as to the acceptability of these piles considering other factors such as load redistribution to adjacent pile, load transfer to the soil above the defect, applied safety factors and structural load requirements. • Soil stiffness or founding on rock of similar density as the pile, will attenuate the signals such that there will be little or no toe reflection. • The low strain integrity method is applicable to cast-in-situ concrete bored and driven piles. Conclusive results are rarely obtained in case of segmented precast reinforced concrete driven piles or precast piles in prebored holes. It may not detect • The toe reflection when the L/D ratio roughly exceeds 20 (In hard soils) to 60 (In very soft soils) • Progressive changes in cross-section • Minor inclusions and changes in cross-section smaller than 25% . • Variations in length smaller than 10%. • Features located below a crack or a major (1:2) change in impedance • Debris at the toe • Deviations from the straight line and from the vertical Impulse Response Principle & ProcedureTest similar to sonic echo method except the signal processing involves frequency domain analysis of the received signal and the impact force history. Significance and use Determine the length of deep foundations ( pile & piers ) , determine the location of cracks or constriction ( Neck-in). Provides information on the low-strain dynamic stiffness of the shaft / soil system.
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Cross Hole Sonic Logging ( Pile Integrity Testing ) Principle & ProcedureAnalogous to the ultrasonic pulse velocity test, but transducers are positioned within tube cast into the deep foundation or holes drilled after construction. Crosshole Sonic Logging (CSL) uses high frequency compressional sonic waves as the energy source. The sonic source produces an impulse whose frequency content is usually 30 to 40 kHz. Sonic waves passing through concrete are influenced by the density and elastic modulus of the concrete. Fractured or "weak" concrete zones lower the velocity of the sonic waves and, therefore, can be detected. In addition, the amplitude of a seismic pulse is affected by these defects although this is not extensively used at the present time. The frequency content of the seismic energy pulse determines the resolution and penetration of the signal. High frequencies have high amplitude attenuation but can image small targets. Conversely, lower frequencies have less attenuation but image larger targets. Significance and use Determine the location of low-quality concrete along the length of shaft and between transducers. With drilled holes permits direct determination of shaft length.
Limitations Access tubes must be installed prior to concrete placement and special care must be taken to avoid tube debonding between concrete and the tubes. Tube debonding condition can occur with PVC access tubes above the water table. No signal is obtained in the tube debonding zone. Only defects along the path of the sonic wave will be detected; it cannot detect anomalies outside the rebar cage. It cannot be used to detect shaft bulbing (increase in diameter
Radar Principle & ProcedureAnalogous to the ultrasonic echo method except that electromagnetic waves are used instead of stress waves. Interface between material with different dielectric properties results in reflection of a portion of incident electromagnetic pulse.
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Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. This nondestructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum, and detects the reflected signals from subsurface structures. GPR can be used in a variety of media, including rock, soil, ice, fresh water, pavements and structures. It can detect objects, changes in material, and voids and cracks. GPR uses high-frequency (usually polarized) radio waves and transmits into the ground. When the wave hits a buried object or a boundary with different dielectric constants, the receiving antenna records variations in the reflected return signal. The principles involved are similar to reflection seismology, except that electromagnetic energy is used instead of acoustic energy, and reflections appear at boundaries with different dielectric constants instead of acoustic impedances.
Radar The depth range of GPR is limited by the electrical conductivity of the ground, the transmitted center frequency and the radiated power. As conductivity increases, the penetration depth decreases. This is because the electromagnetic energy is more quickly dissipated into heat, causing a loss in signal strength at depth. Higher frequencies do not penetrate as far as lower frequencies, but give better resolution. Optimal depth penetration is achieved in ice where the depth of penetration can achieve several hundred metres. Good penetration is also achieved in dry sandy soils or massive dry materials such as granite, limestone, and concrete where the depth of penetration could be up to 15-metre (49 ft). In moist and/or clay-laden soils and soils with high electrical conductivity, penetration is sometimes only a few centimetres. Ground-penetrating radar antennas are generally in contact with the ground for the strongest signal strength; however, GPR air-launched antennas can be used above the ground. Significance and use The depth range of GPR is limited by the electrical conductivity of the ground, the transmitted frequency and the radiated power. As conductivity increases, the penetration depth decreases. This is because the electromagnetic energy is more quickly dissipated into heat, causing a loss in signal strength at depth.
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Higher frequencies do not penetrate as far as lower frequencies, but give better resolution. Dry sandy soils or massive dry materials such as granite, limestone, and concrete where the depth of penetration could be up to 15 m. In moist and/or clay-laden soils and soils with high electrical conductivity, penetration is sometimes only a few centimetres Good Radar media ( Low Conductive Low Attenuation ) Dry salt Snow Ice and fresh water wet or dry sand Dry rocks
Poor Radar media ( High Conductive High Attenuation ) Salt water Metals clay Clay-rich soils Conductive Mineral
Applications – • • • • • • • •
Locate pipes, cables below ground , buried foundations Locate steel in concrete, Find internal defects, voids, delaminations and cracks Locates zones of water penetration and corrosion within defective concrete, Determine thickness of members. Find integrity of concrete / masonry walls Mapping of subsurface soil and rock layering Locating buried archaeological structures
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Radiographic Tests (
X - Ray / Cobalt Gamma ray ) :-
Principle & ProcedureThese are the recent development for scanning the concrete. Radioactivity methods are being increasingly used for the identification of defects in concrete hidden from the eye, which develop during construction. The techniques of using X- ray linear acceleration and cobalt 60 gamma radiography can be used to identify various defects, such as cracks, voids in concrete or grout, position of reinforcements, corrosion of steel , bond stresses in pre-stressed beams and degree of compaction etc. A radiographic image is essentially a two-dimensional shadow picture or display of the intensity distribution of the X-rays or Gamma rays that have passed through a object. The Object attenuates radiation according to mass and the type and size of the defects present. When a beam of X-ray or gamma-radiation passes through concrete, more radiation is absorbed by the denser parts of concrete. The radiation pattern can be made visible with photographic film or fluorescent screens. Gamma-rays differ from X-rays only in their origin. The two main techniques used are radiography and radiometry. In both cases, gamma-radiation is preferred because its sources are more portable than Xray sources, easier to use on sites, thicker section of concrete can be penetrated, gives clear details at shorter exposure time and cheaper than X-ray sources.
Radiograph of a Pre-stressed Beam Reliability & Limitations Delaminations are almost always undetectable. Radiation health hazard and Expensive. Cracks should be oriented parallel to the X-ray beam.
Infrared Thermography Principle & ProcedureInfrared Infrared energy is part of the electromagnetic spectrum and behaves similarly to visible light. It travels through space at the speed of light and can be reflected, refracted, absorbed, and emitted. The wavelength of IR energy is about an order of magnitude longer than visible
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light, between 0.7 and 1000 µm (millionths of a meter). Other common forms of electromagnetic radiation include radio, ultraviolet, and x-ray. Infrared radiation Everything on this planet contains thermal energy and therefore has a specific temperature. This thermal energy is emitted from the surface of the material. This energy is called is infrared (IR) radiation. The amount of IR radiation emitted at a certain wavelength, from the surface of an object, is a function of the object's temperature. Thus we can calculate the temperature of an object by measuring the infrared radiation emitted from it. How is infrared energy related to problem detection? Detectors in the infrared camera convert this incoming infrared energy from the infrared spectrum to the visual spectrum so we can see the infrared energy. These visual maps that co-relate image intensity or colour to the amount of infrared radiation received from that object. The amount of radiation received, along with other parameters, is used to calculate the actual surface temperature of the target object. The detectors are extremely sensitive to small temperature differences and, with a trained and experienced thermographer, inspections are accurate and very valuable. Infrared Thermography Infrared Thermography is the technique for producing a visible image of invisible (to our eyes) infrared energy emitted by objects. The higher the temperature, the more energy emitted. The camera provides a basic grey scale image which is converted to false color images to make interpretation of thermal patterns easier. More cameras will also measure the temperatures of the target object. The thermal image produced by an infrared camera is called a thermogram. Infrared Thermography – Use in Construction Thermal heat loss inspections for buildings, plants, facilities, refineries. Moisture contamination evaluations in buildings, condo's, plants facilities Concrete integrity inspections Locate missing or damaged insulation Identify air leakage energy losses Evaluate the thermal performance of retrofits Locate radiant heating wires or pipes Detect delaminations in concrete bridge deck Locate and identify mold growth areas in building structures
Electromagnetic Spectrum
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Normally Thermal imaging cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 9000–14,000 nanometers or 9–14 µm)
Sample Thermographical Image of Buildings showing Hot & Cool locations
Thermographical Image of O. H. Tank indicating water level
Reliability & Limitations Small Delaminations / cracks are almost undetectable. Most of the time only surface defects can be identified. Sometimes external heating may be required. Accurate temperature measurements are hindered by differing emissivities and reflections from other surfaces
Vibration Measurement – (BS 7385- Part 1 & 2 :1990, ISO 4866:1990(E), BS 6472:1992 ) Structural vibration in buildings can be detected by the occupants and can affect them in many ways: their quality of life can be reduced as also can their working efficiency. Ground borne vibration from sources such as blasting, piling, machinery or road/rail traffic can be a source of concern for occupants of buildings in the vicinity. This concern can lead to a need to assess the effect of the imposed vibration on the building structure to ascertain whether damage could occur.
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Sources of Vibrations
It is increasingly recognized that buildings must sustain vibrations, and recognition of this is needed both in design for structural integrity, serviceability and environmental acceptability, and in the preservation of historic buildings. Measurement of vibration in a building is carried out for a variety of purposes: a) Problem recognition Where it is reported that a building is vibrating at such a level as to cause concern to occupants, it may be necessary to establish whether or not the levels warrant concern for structural integrity. b) Control monitoring Where maximum permitted vibration levels have been established by some agency and those vibrations have to be measured and reported. c) Documentation Where dynamic loading has been recognized in design and measurements are made to verify the predictions of response and provide new design parameters. These may use ambient or imposed loading. Strong motion seismographs, for example, may be installed so as to indicate whether or not the responses to earthquake warrant changes on operating procedure in a structure. d) Diagnosis Where it has been established that vibration levels require further investigation, measurements are made in order to provide information for mitigation procedures. Another diagnostic procedure is to use structural response to ambient or imposed loading to establish structural condition, for example, after a severe loading, such as an earthquake.
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In general, building vibration are measured in acceleration terms. In some cases, such as impulsive events, it may be found convenient to measure in terms of particle velocity so that peak values may be identified.
Vibration Meter Vibration Measurement is carried out for various structures such as Buildings / Structures, Bridges, Chimney / Stacks, Water Tanks, Heritage Structures Endoscopy – Endoscopy surveys provide a fast, cost effective means of identifying problematic / unaccessible areas where bear minimum destruction is allowed for inspection such as – Concrete pipes, Water Pipes Storm pipes Sewer Lines, Culvert Cavity walls, Air ducts, Vents, Chimneys, Ceiling, roof and floor voids, Water tanks, Heritage structures, Timber structures, Concrete or Masonry structures with large voids, honeycombing etc.
Bore scopes / Endoscopes 36
Endoscopy of Masonry
Endoscopy of False Ceiling
Bore scope survey - rusting inside pipes
Bore scope survey – Boiler Tube
Endoscopes allows to conduct surveys with the very minimum of disruption to the structures, whilst providing with the detailed information. This technique has been found to be very useful for identifying corrosion inside pipes, hollow areas in the timber structures caused by decay of timber due to wet rotting, termites These endoscopes / Bore Scopes can take a sharp snap shots along with continuous video recording. Underwater Inspection – Many a times inspection and testing of underwater structures becomes very difficult. Most of the ND methods can not be conducted underwater and we are required to depend only on the visual inspection of the part of structure above water level. One way of inspecting underwater structures is to send the non technical divers and capture images or to capture video / images by underwater camera operated by a technical person sitting outside the water. The cost of second option is less and is recommended as the on site inspection is carried out by a technical person. Such underwater inspection is required for many structures like – Dams, Bridges, Water Tanks, Effluent & Water Treatment Plants, Jetty etc.
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Underwater camera
Underwater Structures
Corrosion of Underwater Structures captured by Camera
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Inspection of Jetty & Bridge Foundations by Underwater Camera
Ultrasonic Thickness Gauge – (IS : 15435: 2003 & IS : 2417 ) A very useful tool in the structural audit / survey. In most of the Industries, especially in Chemical factories corrosion is a very serious issue for the structural stability of the structural steel structures, M S Chimney , Stacks, Gantry girders, Pipe racks, Pipelines etc. Using the Ultrasonic thickness Gauge one can find out the precise thickness of all such steel / metal structures, components.
Ultrasonic Thickness Gauge
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Moisture Measurement Moisture is know to be one of the main source causing deterioration of masonry and timber structures. To identify the source of leakages and dampness moisture meters are very helpful. If the concrete, masonry , timber structures are found to be wet, then appropriate repairs can be carried out to avoid further decay / deterioration of the material and structure as a whole.
Moisture Measurement of Timber
Moisture Measurement of Concrete
Infrared Thermometers – Used in combination with Moisture meters to identify the temperature gradient and spot temperatures from a long distance by just sighting the object, a very useful tool in structural audits of the buildings, water structures, chimney etc.
Infrared Thermometer
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Load Test – (IS – 456 – 2000 ) Though not a Non Destructive Test, IS – 456 – 2000 recommends Load testing of flexural members to be carried out In case the core test results do not satisfy the requirements. Many a times Load test is carried out on building & Bridge components when – • • • • • • •
The strength of concrete is below the acceptable norms Structural Design data is not available. Load carrying capacity of the flexural member needs to be assessed. The members is to be subjected to a higher loads The members are noticed to have cracks, deflections The structure is damaged due to fire, earthquake, blast, corrosion etc. Change in use of structure.
The Load Test for Building is carried out as per IS – 456. For Bridges Load test is carried out in accordance with IRC – SP – 51 – 1999, the load test can be performed by either loading with simulation of specific IRC vehicle ( IRC – 37 ) or by other type of static load which produces the design forces.
Load Test of RCC Beam
Load Test of Bridge
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Recommended Numbers of Readings for various test methods Test method
Nos. of Individual readings recommended at a location
Standard cores Schmidt hammer Ultrasonic Pulse Velocity Windsor Probe Pull – out Pull – off Break – off
3 12 2 3 1 3 1
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47.
BS1881:Part 201:1986 - Guide to the use of nondestructive methods of test for hardened concrete BS1881:Part 203:1986 - Recommendations for measurement of velocity of ultrasonic pulses in concrete BS1881:Part 207:1992 - Recommendations for the assessment of concrete strength by near-to-surface tests IS 13311 : Part 1 :1992 – NDT of Concrete – Method of test - UPV IS 13311 : Part 2 :1992 – NDT of Concrete – Method of test – Rebound Hammer IS – 456 – 2000 – Plain & RCC – Code of Practice IS – 1199 – 1959 - Method of sampling & Analysis of Concrete IS – 516 – 1959 – Methods of tests for Strength of Concrete ACI – SP 82 – In Situ / Non Destructive Testing of Concrete ACI 311.4 4R-00 – Guide for Concrete Inspection ACI 365.1 R-00 – Service life Prediction – State of Art Report ACI 214.4 R-03 – Guide for obtaining Cores & Interpreting Compressive strength results. ACI 228.2 R-98 – NDT Methods for Evaluation of Concrete in Structures ACI 228.1 R-03 – In-Place Methods to Estimate Concrete Strength ACI 437 R-03 – Strength Evaluation of Existing concrete buildings ACI – SP - 168 – Innovations in NDT of concrete ACI – SP – 112 - NDT ACI – Monogram No – 9 – Testing of Hardened concrete – NDT methods ACI – Monogram Series – Evaluation of Concrete Properties from Sonic Tests Malhotra V. M. (Ed.) - Testing Hardened Concrete: Non-destructive Methods, ACI, monograph No. 9, Detroit, US, 1976. Malhotra V. M. – Handbook of NDT of concrete – Second Edition J. H. Bungey - The Testing of Concrete in Structures N. J. Carino – NDE to Investigate Corrosion status in Concrete Structures P.A.M. Basheer – Near Surface Testing for strength & Durability of Concrete R. Halmshaw – NDT – Second Edition H. W. Reinhardt – Testing during Concrete Construction ACI – India Chapter – National Seminar on “Quality Assurance in Civil Engineering through Material testing” - 2003 ACI – India Chapter – National Symposium on “NDT of Structures” - 2000 ACI – India Chapter – National Seminar on “Potential of NDT for Quality Assurance & Diagnosis” - 2004 ACI – India Chapter – International Seminar on “NDT- 2006” ICI – Proceedings of Indo-US Workshop of NDT –1996 IRC – Highway Research Board – Special Report on “ State of Art : NDT techniques of concrete bridges” – 1996 The British Institute of NDT – Proceedings of International Conference on “NDT in Civil Engineering” Vol 1 & 2 - 1997 BAM – Proceedings Vol 1 & 2 - International Symposium ( NDT – CE – 1995 ) BAM - Proceedings International Symposium ( NDT – CE – 2003 ) ISNT – Proceedings of “Workshop on NDE of Concrete Structures” - 2002 SPIE – Vol 2457 - Proceedings “NDE of Aging structures & Dams” COEP – National Symposium on “Diagnosis & Evaluation of Concrete Structures using NDT “ - 2004 Grasim Cement Publication No – 8 on NDT CECR – Compilation of paers on “Instrumentation & NDT” 1988-98 Controls – Product Catalogue NDT James – Product Catalogue FLIR – Product Catalogue Central Federal Lands Highway (Website) www.piletest.com Wikipedia Sensors and Softewares (Website)
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