Non Destructive and Partially Destructive Testing Of Concrete Structures

The major non destructive and partially destructive methods of concrete testing have been discussed here. I hope it will be beneficial to everyone interested in this broad topic.

1. NON- DESTRUCTIVE AND PARTIALLY DESTRUCTIVE TESTING METHODS PRESENTED BY GAUTAM CHAURASIA

2. Non Destructive Testing (NDT ) AND PARTIALLY DESTRUCTIVE TESTING Non destructive test is a method of testing existing concrete structures to assess the strength and durability of concrete structure In the non destructive method of testing, without loading the specimen to failure (i.e. without destructing the concrete) we can measure strength of concrete Nowadays this method has become a part of quality control process This method of testing also helps us to investigate crack depth, micro cracks and deterioration of concrete Non destructive testing of concrete is a very simple method of testing but it requires skilled and experienced persons having some special knowledge to interpret and analyze test results Partially destructive test require material samples to be taken or any other minor breach of structural integrity In partially destructive testing, concrete surface is deteriorated a bit without structural failure The capacity of the existing building can be evaluated using these methods and hence whether retrofitting is requied in the existing structure or not can be assessed

3. DIFFERENT METHODS OF NON-DESTRUCTIVE TESTING OF CONCRETE REBOUND HAMMER TEST ULTRASONIC PULSE VELOCITY METHOD PROFOMETER TEST/ REBAR LOCATOR TEST DIFFERENT METHODS OF PARTIALLY DESTRUCTIVE TESTING OF CONCRETE PENETRATION RESISTANCE TEST PULL OUT TEST MAGNERIC AND ELECTRICAL METHOD

4. REBOUND HAMMER TEST Definition: Rebound hammer test is a non destructive testing method of concrete which provides a convenient and rapid indication of the compressive strength of concrete. The rebound hammer is also called as Schmidt hammer that consists of a spring controlled mass that slides on a plunger within a tubular housing.

5. PRINCIPLE The rebound of an elastic mass depends on the hardness of the surface against which its mass strikes When the plunger of the rebound hammer is pressed against the surface of the concrete, the spring- controlled mass rebounds and the extent of such a rebound depends upon the surface hardness of the concrete. The surface hardness and therefore the rebound is taken to be related to the compressive strength of the concrete. The rebound value is read from a graduated scale and is designed as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer. Surface hardness measured during the test gives an idea about the soundness and quality of cover concrete Locations having very low rebound numbers indicate weak surface concrete and vice versa

6. PROCEDURE Before commencement of a test, the rebound hammer should be tested against the test anvil, to get reliable results, for which the manufacturer of the rebound hammer indicates the range of readings on the anvil suitable for different types of rebound hammer Apply light pressure on the plunger- it will release it from the locked position and allow it to extend to the ready position for the test Press the plunger against the surface of the concrete, keeping the instrument perpendicular to the test surface. Apply a gradual increase in pressure until the hammer impacts Do not touch the button while depressing the plunger. Press the button after impact, in case it is not convenient to note the rebound testing in that position Take the average of about 5 readings

7. INTERPRETATION OF TEST RESULTS After obtaining the correlation between compressive strength and rebound number, the strength of the concrete can be assessed Generally, rebound number increases as the strength increases and it is also affected by several other parameters such as type of cement, aggregate, surface condition, moisture content, curing, etc. The rebound index is indicative of compressive strength of concrete up to a limited depth only The internal cracks, flaws or heterogeneity across the cross-section will not be indicated by rebound number Also the estimation of strength of concrete by rebound hammer method cannot be held to be very accurate There may be a variation of about 25% in the test results

8. 2 . ULTRASONIC PULSE VELOCITY METHOD Definition It is a non destructive test to assess the homogeneity and integrity of concrete It is used for qualitative assessment of strength of concrete, its gradation in different locations of structural members and plotting the same Any discontinuity in the cross section like cracks, voids, delamination can Known Depth of surface cracks can also be assessed

9. PRINCIPLE Ultrasonic pulse velocity test consists of measuring travel time, T of a ultrasonic pulse of 50 to 54 KHz, produced 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 Higher the elastic modulus, density and integrity of the concrete, higher is the pulse velocity. The ultrasonic pulse velocity depends on the density and elastic properties of the material being tested. Though pulse velocity is related with the crushing strength of concrete, yet no statistical correlation can be applied The pulse velocity may be influenced by: Path length Lateral dimension of the specimen tested Presence of reinforcement steel Moisture content of the concrete

10. PROCEDURE Preparing for use : before switching on the “V” meter, the transducers should be connected to the sockets marked” TRAN” and “REC” Set reference : a reference bar is provide to check the instrument zero. The pulse time for the bar is engraved on it. Adjust the “SET REF” control until the reference bar transmit time is obtained on the instrument read- out Range selection : for maximum accuracy, it is recommended that the 0.1 microsecond range be selected for the path length up to 400 mm 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 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 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. Pulse velocity= ( path length/ travel time)

11. Separation of transducer leads: it is advisable to prevent the two transducer leads from coming into closer contact with each other when the transit time measurements are being taken. If this is not done, he receiver lead might pick-up unwanted signals from the transmitter lead and this would result in an incorrect display of transit time

12. INTERPETATION OF RESULT PULSE VELOCITY CONCRETE QUALITY >4.0 KM/S Very good to excellent 3.5 -4 KM/S Good to very good, slight porosity may exist 3.0 -3.5 KM/S Satisfactory but loss of integrity is suspected <3.0 KM/S Poor and loss of integrity exists S. N. Test Results Interpretations 1 High UPV values, high rebound number No corrosion prone 2 Medium UPV values, low rebound number Surface delamination, low quality of surface concrete, corrosion prone 3 Low UPV, high rebound number Not corrosion prone, however to be confirmed by chemical tests, carbonation, PH 4 Low UPV, low rebound number Corrosion prone, requires chemical and electrochemical tests

13. 3. PROFOMETER TEST Definition Profometer test is a non- destructive testing technique used to detect location and size of reinforcements and concrete cover quickly and accurately In this test a small, portable, and handy instrument which is known as profometer or rebar locator is used The equipment weight is less than 2 Kgs, and works on normal batteries and thus does not require any electrical connection. This instrument is available with sufficient memory to store measured data

14. PRINCIPLE The instrument is based upon measurement of change of an electromagnetic field which is caused by steel bars embedded in the concrete The basic principle in this method is that the presence of steel affects the electromagnetic field is directed by profometer device TEST PREPARATIONS It is essential to conduct proper assessment of the structure before the test For this purpose. Proper staging, ladder or a suspended platform may be provided. Before actual scanning, marking is done with chalk on the concrete surface by dividing it into panels of equal areas

15. PROCEDURE Determine Steel Bar Location Path measuring device and spot probes are used together for path measurements and scanning of rebars These are connected to profometer via cables and are moved on the concrete surface for scanning the rebars and measuring the spacing As soon as the bar is located, it is displayed on the screen Once the bar is located, it is marked on the concrete surface

16. B . Measure Bar Diameter Diameter probe is used for measuring the diameter of the bars. It is connected with profometer by a cable After finding out the location of rebar, the diameter probe is placed on the bar parallel to the bar axis Four readings are displayed and the mean value of these readings is taken as diameter of bars

17. C. Determine Concrete Cover Depth probe of the profometer is used to measure the cover It is also connected with profometer by cable and is placed exactly on the bar As soon as the depth probe is above rebar or nearest to it, it gives an audio signal through a short beep and visual display Simultaneously, the measured concrete cover is stored in memory

18. 4. PENETRATION RESISTANCE TEST Definition Penetration resistance test is conducted on concrete structures using Windsor probe test machine The penetration is inversely proportional to the strength of concrete The result of test is influenced by aggregate strength and nature of formed surfaces of concrete The purpose of the penetration resistance test is used to determine the uniformity of concrete, specify the poor quality or deteriorated concrete zones

19. PROCEDURE Place the positioning device on the surface of the concrete at this location to be tested Mount a probe in the driver unit Position the driver in the positioning device Fire the probe into the concrete Remove the positioning device and tap the probe on the exposed end with a small hammer to ensure that it has not rebounded and to confirm that it is firmly embedded Place the measuring base plate over the probe and position it so that it bears firmly on the surface of the concrete without rocking or other movement

20. INTERPRETATION OF RESULT The penetration resistance of concrete is computed by measuring exposed length of probes driven into concrete. In order to estimate concrete strength, it is necessary to establish a relationship between penetration resistance and concrete strength Such a relationship must be established for a given test apparatus, using similar concrete materials and mixture proportions as in the structure Procedures and statistical methods provided in ACI 228.1R can be used for developing and using the strength relationship

21. 5. CORE TEST Definition The core test provides the visual inspection of the interior of the concrete and direct measurement of the compressive strength Physical properties such as density, water absorption, indirect tensile strength and expansion due to alkali-aggregate reaction can also be measured. After the sample is broken, it can be used as samples for chemical analysis- PH value and chloride content In core test, the determination of core size and its location is a very crucial factor as core cutting is liable to cause some damage to the structure

22. Principle The test should be taken at points where minimum strength and maximum stress are likely to coincide The accuracy of test increases with the with the ratio of core diameter to the aggregate size Generally the recommended length to diameter ratio of the cores is between 1 to 2 A core having a maximum height of less than 95% of its diameter before capping or a height less than its diameter after capping must be rejected

23. PROCEDURE The number, size and location of core samples should be carefully selected to permit all necessary laboratory tests. If possible, use virgin samples for all tests so that there will be no influence from prior tests During drilling of cores, the electrical conduits or prestressing steel must not be disturbed Water soaking of 40 hrs. is recommended prior to testing It is better to drill cores through full depth of member to avoid need of its breaking for extraction An extra 50 mm is usually drilled to allow for possible damage at the base of the core At least three cores must be removed at each location in the structure for strength determination The hole drilled to take the core is filled by packaged repair material. In some cases, a precast cylinder of concrete may be fitted in the core hole by using cement grout or epoxy resin

24. INTERPRETATION OF RESULT For horizontally drilled cores: Corrected cylinder strength= core strength*[(2.5*0.8)/(1+(1/R))] For vertically drilled cores: Corrected cylinder strength= core strength*[(2.3*0.8)/(1+(1/R))] If the ratio of length to diameter of specimen is less than 1.94 , apply the correction according as below table L/D Strength Correction Factor 1.75 0.98 1.50 0.96 1.25 0.93 1.00 0.87

25. 6. PULL OUT TEST Definition The test measures the force required to pull out a previously cast in steel insert with an embedded enlarged end in the concrete. In this operation, a cone of concrete is pulled out and the force required is related to the compressive strength of concrete . PRINCIPLE This test is based on the principle that the force required to pull out a cone of steel embedded in concrete is proportional to the strength of concrete .

26. PROCEDURE Pull out insert is embedded in plain concrete during pouring. When the strength is required to be determined, the force is applied at the embedded end. The force required to pull out the assembly is measured.

27. INTERPRETATION OF RESULT The relationship between the pullout force in KN and compressive strength in MPa is given alongside The pullout test produces a well defined in the concrete and measure a static strength property of concrete. The compressive strength can be considered as proportional to the ultimate pullout test