Correlation between Destructive Compressive Testing (DT) and Non Destructive Testing (NDT) for Concrete Strength
Abstract
Concrete is the most widely used construction material worldwide. Strength of a concrete structure may have to be assessed without causing physical damage to it, due to various reasons like its monumental importance or the legal dispute on whether the strength of the concrete in the structure is satisfactory enough or not. In an attempt to meet the above demand, correlation and comparison between Destructive Test (DT) and Non Destructive Test (NDT) were carried out.
A total of 24 concrete cubes (150 mm x 150 mm x 150 mm) were cast with concrete mix ratio of 1:2:4. 12 cubes were tested destructively for compressive strength with compression machine and 12 cubes were also tested non-destructively with Schmidt Rebound Hammer. Compressive strength test results at curing ages (7, 14, 21 and 28days.) were collated and analysed.
The results obtained from the non-destructive testing method were correlated with the results obtained from destructive testing method. The coefficient of correlation between the two set of compressive strength was 0.988 which indicates a perfect relationship between compressive strength results from both methods.
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Introduction
The standard method of determining strength of hardened concrete consists of testing concrete cubes in compression. It is very important to ascertain the compressive strength of concrete before subjecting to its anticipated loads. Compressive strength of hardened the concrete can be determined using the destructive (DT) and non-destructive testing (NDT) method. The destructive testing (DT) method is carried out by crushing the cast specimen to failure while non-destructive testing (NDT) is carried out without destroying the concrete specimen. The rebound (Schmitz) hammer is one of the most popular non-destructive testing (NDT) method used to test the strength of the concrete.
This is due to it relatively low and simplicity in use. Although the non-destructive testing (NDT) result are much quicker compare to the destructive methods. They are more of an approximation than exact compressive value. In as much as the rebound hammer result are quicker and do not destroy the surface of concrete tested, there is no established relationship between compressive strength obtained using NDT and DT.
Various non-destructive methods of testing concrete have been developed, which include, Firing method, Skramtayev’s method, Polakov’s method, Magnitostroy method, Fizdel ball hammer, Einbeck pendulum hammer, Ball indentation hammer, Rebound hammer, Pull out techniques, Windsor probe, Ultrasonic pulse velocity methods, Radioactive and nuclear methods, Magnetic and electrical methods. In all these methods of tests, due to simplicity, rebound hammer test based on surface hardness becomes most popular in the world for non-destructive testing of in-situ concrete.( Kaushal Kishore 2000).
The rebound (Schmitz) hammer is one of the most popular non-destructive testing (NDT) method used to test the strength of the concrete. This is due to it relatively low and simplicity in use. Although the non-destructive testing (NDT) result are much quicker compare to the destructive methods. They are more of an approximation than exact compressive value. In as much as the rebound hammer result are quicker and do not destroy the surface of concrete tested, there is no established relationship between compressive strength obtained using NDT and DT.
Conclusion
The correlation among the strength values obtained by DT and NDT test methods on concrete cubes has been established. Schmidt Hammer test method has been used as a non-destructive test. The following conclusions have been drawn:
There exist a strong relationship between the compressive strength results from both methods with Correlation coefficient of R2 = 0.988.
The predictive equation y = 0.6951x + 3.7852
The following recommendations were made based on result and observation recorded in this study.
Increased number of test sample cubes is suggested to for better correlation of both rebound hammer test and compressive strength test values.
The Schmidt rebound hammer is recommended for the assessment of compressive strength of hardened concrete to rule out difficulties (delayed feedbacks on laboratory analysis, epileptic power supply for running tests on concrete cubes.) in testing cubes for their strength using compression testing machine.