A variety of Non Destructive Testing (NDT) methods have been developed or are under development for investigating and evaluating concrete structures. These methods are aimed at estimation of strength and other properties; monitoring and assessing corrosion; measuring crack size and cover; assessing grout quality; detecting defects and identifying relatively more vulnerable areas in concrete structures.
Many of NDT methods used for concrete testing have their origin to the testing of more homogeneous, metallic system. These methods have a sound scientific basis, but heterogeneity of concrete makes interpretation of results somewhat difficult. There could be many parameters such as materials, mix, workmanship and environment, which influence the results of measurements. Moreover, these tests measure some other property of concrete (e.g. hardness) and the results are interpreted to assess a different property of concrete e.g. strength, which is of primary interest.
Thus, interpretation of results is very important and difficult job where generalization is not possible.
As such, operators can carry out tests but interpretation of results must be left to experts having experience and knowledge of application of such non-destructive tests.
Non-Destructive Testing of Concrete (NDT on Concrete)
What is Non Destructive Test?
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. Now days 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.
Purpose of Non-destructive Tests:
The non-destructive evaluation techniques are being increasingly adopted in concrete structures for the following purposes:
(i) Estimating the in-situ compressive strength
(ii) Estimating the uniformity and homogeneity
(iii) Estimating the quality in relation to standard requirement
(iv) Identifying areas of lower integrity in comparison to other parts
(v) Detection of presence of cracks, voids and other imperfections
(vi) Monitoring changes in the structure of the concrete which may occur with time
(vii) Identification of reinforcement profile and measurement of cover, bar diameter, etc.
(viii) Condition of prestressing/reinforcement steel with respect to corrosion
(ix) Chloride, sulphate, alkali contents or degree of carbonation
(x) Measurement of Elastic Modulus
(xi) Condition of grouting in prestressing cable
Different Methods of Non-Destructive Testing of Concrete
Various non-destructive methods of testing concrete have been developed to analyze properties of hardened concrete, which are given below.
1. Surface Hardness Test
These are of indentation type, include the Williams testing pistol and impact hammers, and are used only for estimation of concrete strength.
2. Rebound Hammer Test
The rebound hammer test measures the elastic rebound of concrete and is primarily used for estimation of concrete strength hand for comparative investigation.
3. Penetration and Pullout Techniques
These include the use of the simbi hammer, spit pins, the Windsor probe, and the pullout test. These measure the penetration and pullout resistance of concrete and are used for strength estimation, but they can also be used for comparative studies.
4. Dynamic or Vibration Tests
These include resonant frequency and mechanical sonic and ultrasonic pulse velocity methods. These are used to evaluate durability and uniformity of concrete and to estimate its strength and elastic properties.
5. Combined Methods
The combined methods involving ultrasonic pulse velocity and rebound hammer have been used to estimate strength of concrete.
6. Radioactive and Nuclear Methods
These include the X-ray and Gamma ray penetration tests for measurement of density and thickness of concrete. Also, the neutron scattering and neutron activation methods are used for moisture and cement content determination.
7. Magnetic and Electrical Methods
The magnetic methods are primarily concerned with determining cover of reinforcement in concrete, whereas the electrical methods, including microwave absorption techniques, have been used to measure moisture content and thickness of concrete.
8. Acoustic Emission Techniques
These have been used to study the initiation and growth of cracks in concrete.
Types of Non Destructive Tests:
According to their use, non-destructive equipment can be grouped as under:
i) Strength estimation of concrete
ii) Corrosion assessment and monitoring
iii) Detecting defects in concrete structure
iv) Laboratory tests
It is of outmost importance to ensure the structural integrity of the concrete structures of buildings are assessed for safety and structural competence.
Handbook on Non Destructive testing of concrete
Handbook of Concrete Engineering
Guide on NDT on Concrete structures
Evaluation of Compressive strength using Non Destructive test methods