METHODOLOGY FOR NON DESTRUCTIVE TESTING OF RCC STRUCTURES

Non Destructive Testing (NDT) involves such testing procedures which do not damage the structure in any way. These tests can be carried out to evaluate condition of concrete and steel, or to determine the size of members or spacing, location and cover of steel bars and other utility lines etc.

NDT on RCC structures can be carried out using different equipment. Accuracy of the equipment used and skill of the operator are key to accurate testing, however, it should be kept in mind that NDT will only yield as good a result as the robustness of methodology adopted for carrying out these tests.

Following discussion pertains to use of some of the equipment and establishing a testing methodology to carry out NDT on RCC structures.

Description of Equipment used for NDT of RCC Structures:

The most commonly used NDT instruments for RCC testing are Schmidt Hammer (also known as Concrete Rebound Hammer), Ultrasonic Pulse Velocity Tester (PUNDIT), Ground Penetration Radar (GPR) and Eddy Current Rebar Tester.

1.    Schmidt Hammer

Schmidt Hammer works on the principle of measuring rebound of hammer when it is struck against the concrete surface by release mechanism of spring within the instrument. It basically measures surface hardness of concrete and gives a Rebound Number (R) which can then be correlated using graphs to strength of concrete. This test has been standardized by ASTM C-805 standard. Another type of Hammer has also been developed which optically measures the Rebound speed and gives Q value.

Schmidt Hammer Mechanism

Use and Limitations:

The instrument is easy to use and gives quick and consistent results. However, it has certain limitations as follows:

It measures surface hardness of concrete which does not necessarily translate into strength. In old structures, concrete at the surface can undergo carbonation which will result in higher rebound number but this carbonation of concrete does not equate to increase in concrete strength.

The instrument only gives rebound number based on the surface properties of concrete and no information is obtained regarding condition of concrete within the structure. Instrument can give erratic readings if test is conducted directly over hard aggregate or soft cement paste in concrete.

Without project specific correlation curves, the strength determined through rebound number will have a wide margin of error.

2.    PUNDIT

PUNDIT works on the principle of measuring the time taken by ultrasonic pulse to travel within the RCC structure. This is accomplished by placing two transducers in contact with the member. One of the transducer emits the pulse and the other transducer receives the pulse and the time taken to travel within the concrete member is determined by the equipment. Pulse speed is directly proportional to the Elastic Modulus and hence strength of concrete. The pulse travel time can be used to determine strength of concrete by using correlation curves. This test has been standardized by ASTM C-597 standard.

PUNDIT Test on Concrete

Use and Limitations:

The instrument requires skill and experience for proper use. It can give accurate information for the whole depth of concrete member like quality and strength of concrete and any honeycombing/cracks in concrete. However, its strength determination is only accurate if project specific correlation curves are developed otherwise strength calculated using PUNDIT can vary in a range of ±20%. With project specific correlation curves this error can be reduced to less than ±5%. The results of PUNDIT are affected by presence of moisture and steel reinforcement in concrete among other things. This aspect should be considered when developing project specific correlations

3.    Ground Penetration Radar

GPR is based on the principle of measuring reflection of radio waves from within the concrete structure based on dielectric properties of concrete and embedded objects. The polarity of reflected waves depends on the type of material encountered by the wave, which is then used to ascertain position of reinforcement, cavities, utility lines etc. within the structure. This instrument is useful in making a 3D map of RCC structure thereby determining orientation, position and depth of reinforcement, anomalies (honey combing, voids etc.) within concrete, and orientation and position of utility lines.

GPR test on RCC block with rebars and void

GPR test on Post Tensioned slab with Rebars

Use and Limitations:

The instrument requires skill and experience for proper use. It can give detailed information regarding position and orientation of embedded reinforcement and utility line etc. and any cracks/voids in concrete and depth of concrete member. However, it cannot be used to determine strength of concrete, whereas advanced techniques such as dual polarization or digital imaging are required to assess diameter of rebars, however the accuracy decreases as the embedment depth increases.

4.    Eddy Current Rebar Tester

Eddy Current Rebar Tester works on the principle of creation of eddy current when steel is subjected to a magnetic field. The instrument generates a magnetic field and as the instrument is moved along the surface of structure, eddy currents are generated when it passes over embedded steel reinforcement. These eddy currents are detected by the instrument and location, depth and size of steel bar is determined. It does not require any prior calibration.

Eddy Current Rebar Test

Use and Limitations:

The instrument requires skill and experience for proper use. It can give diameter of rebars within a range of ±1 no. bar dia., however, data regarding existing bar size has to be entered into the instrument for it to determine bar dia. accurately. It can accurately determine the position, cover and orientation of rebars. However, its range is limited to the upper reinforcement layer and it cannot detect any reinforcement underneath the upper layer. It gives no information regarding quality of concrete.

Need for Project Specific Correlations for NDT of RCC structures:

Developing project specific correlations is corner stone of carrying out reliable non-destructive testing. This is due to the fact that concrete is made of various components and is not a homogenous material. Any change in ratio or properties of each constituent material will impact the test results obtained by using NDT equipment. Correlations are necessary to equate strength of concrete to Pulse Velocity obtained by PUNDIT or Rebound Number obtained by Schmidt Hammer.

Hammer rebound is affected by hardness of aggregates and amount of coarse aggregate in concrete. Similarly, pulse velocity is affected by ratio of coarse aggregates and moisture content in concrete. It is a fact that each and every project has different type of concrete and coarse and fine aggregates are also different, hence no general correlation can be developed which will give accurate results for all types of concretes.

As an example following chart can be reviewed which shows wide variation in strength – rebound number curves developed by different researchers.

Therefore project specific correlations should be developed for each and every type of concrete used in the project. This is done by testing project concrete cylinders or cubes or cores using PUNDIT and Schmidt Hammer and then crushing them to determine compressive strength. Hence a correlation is developed between strength of concrete and instrument reading. Details of methodology to establish project specific correlations can be found in ACI 228.1-03. It states that “in planning the correlation testing program, six to nine strength levels should be considered” and “the range of strengths used to establish the correlation should cover the range of strengths that are to be estimated in the structure”. Further, it mentions that at least two replicate tests should be carried out at each strength level to determine average strength of concrete at that strength level.

If project specific correlations are not developed, then these NDT tests will have limited utility, however, they will still give us information regarding uniformity and homogeneity of concrete in the structure.

Examples of project specific correlations for Schmidt Hammer and PUNDIT are presented below:

Methodology for NDT of RCC structures:

As can be seen in the preceding discussion regarding various instruments used for NDT, it is evident that no one instrument can be regarded as giving complete range of tests for RCC structure. Further, all the instruments have their limitations which need to be taken into account. Hence, it is necessary to formulate a proper methodology regarding NDT of RCC structures.

Let us consider a slab which is to be investigated using NDT for which we do not have any prior data. We can proceed through the following steps to conduct NDT for the slab.

1.    The first step would be to scan the slab using GPR. This will give us a 3D image of depth of concrete, position and orientation of reinforcement and any utility lines, voids or honeycombing in the slab.

2.    The next step will be mark the areas which are clear of rebars and utility lines and which show honey combing and voids or other defects which we want to investigate.

3.    These areas can be further precisely marked for location of rebars using Eddy Current Rebar Tester. During this test, the diameter of bars and cover can also be verified.

4.    Once location of rebars and defective concrete is established, Schmidt Hammer can be used to take rebound readings at the marked locations of interest.

5.    Next coring should be done to take representative cores from the slab at some of the marked locations. These cores should be tested using PUNDIT. If separately cast cylinders or cubes are available which represent the concrete of slab, these should also be used to establish correlations for PUNDIT and Schmidt Hammer tests. The higher the number of samples having a wide range of compressive strength, the more accurate will be the correlations.

6.    Next the cores/cylinders/cubes should be crushed to determine the compressive strength of concrete.

7.    Project specific correlations should be developed both for Schmidt Hammer and PUNDIT using the compressive strength of cores/cylinders/cubes and Rebound Number of Schmidt Hammer, and Pulse Velocity of PUNDIT. These project specific correlations should not be used for other projects.

8.    Further tests should be carried out on the slab using Schmidt Hammer and/or PUNDIT to determine strength of concrete at these locations. Strength should be determined using project specific correlations developed in step 7.

9.    If required, further coring should be done to verify the results of non-destructive testing.

Horizontal Coring

The above methodology has been devised based on the assumption that all four equipment are available for NDT. However, if only two of these equipment are available e.g. PUNDIT and Eddy Current Rebar Tester so what can be the methodology of carrying out NDT? This can be outlined as follows:

1.    Establish a project specific correlation for concrete strength and PUNDIT readings

2.    Conduct PUNDIT survey of whole slab at suitable intervals (preferably a uniform grid) and get pulse velocity readings.

3.    Identify weak areas in concrete based on PUNDIT readings. Due to the fact that we are no using GPR, we will not be able to identify whether these weak areas represent honeycombing or if there is utility line running underneath. Hence, it will be important to refer to construction as-built drawings to identify these.

4.    Conduct Eddy current test to locate rebars in these areas of interest. In the process rebar cover and diameter is also determined.

5.    After areas clear of rebars has been established, conduct coring at specified weak locations to verify results of PUNDIT.

It can be seen that test methodology can be altered based on the equipment available for the same and based on the type of investigation being carried out.

For detailed methodology regarding NDT and destructive testing using various types of in-situ tests refer to ACI 228.1-03.

NDT should never be used as a replacement or alternative to destructive testing, rather it complements destructive testing. NDT is a very good tool to evaluate structures; however, its results should always be confirmed with destructive testing. NDT reduces the need for extensive destructive testing by highlighting areas which require further testing, hence, frequency of destructive testing is greatly reduced by use of NDT. However, if correct methodology is not adopted, NDT will give erroneous results which will be of limited use to ascertain the actual condition of a structure.

References:

1. American Society for Testing and Materials Standards (ASTM)
2. American Concrete Institute (ACI) Manual of Concrete Practice
3. Handbook on Non Destructive Testing of Concrete (CRC)
4. PROCEQ (https://www.proceq.com/)