Correlation between Non-destructive Test Results and Mechanical Properties of Concrete

Abstract

The NDT of concrete in today's scenario has received a great importance in terms of practical and engineering value. The subject has received a growing attention dur- ing recent years, especially the quality characterization of damaged structure made of concrete using NDT testing. Massive structure like dams, bridges requires their continous quality checking. For this purpose many techniques are involved which causes damage to the structure such as extraction of cores for checking the strength. Extraction of core is not advisable as it would results in damaging the structure and also very expensive procedure. In non-destructive testing, number of advantages re- duction in nos. of labours, lesser structural damage, prediction of concrete strength without extraction of cores,lesser cost of testing etc. These techniques are used to predict the strength of the structure with acceptable approximation. Therefore, in order to find the strength of the concrete structure, investigations were carried out for generating the correlation between the NDT results and the mechan- ical properties of concrete. The aim of present investigation is to develop correla- tion curves between compressive strength and NDT testing for concrete of different grades(M15, M20, M25, M30, M40, M50). It is proposed to develop multiple re- gression curves from the results of UPV and Rebound Hammer in determining the compressive strength of concrete at different ages 7, 28, 90 days for better assessment of structure. Total 252 specimens are casted for the present investigation. M15, M20, M25, M30, M40, M50 grades of concrete using OPC and PPC are used for the present investiga- tion. A standard size of cubes, beams and cylinders as per IS provisions are used for evaluating the compressive strength, modulus of elasticity, exural strength and split tensile strength. Cube specimens have been tested at the age of 7, 28 and 90 days respectively. Beam and cylinder specimens have been tested at the age of 28 days for both non-destructive and destructive testing. For cube specimens, both direct and semi-direct methods are adopted for finding out UPV results. In surface hardness test, cube specimens are tested by keeping position of rebound hammer both vertical and horizontal. Same methodology has been adopted for beam and cylinder speci- mens for rebound hammer test. UPV results are taken by direct method for beam & cylinder specimens. Correlations are carried out between compressive strength and NDT results i.e. rebound no. & UPV at different ages 7, 28, 90 days. For other mechanical properties correlation with NDT results are also carried out. Compilation of results of RCC members are carried out from different sites. Also NDT results are taken on RCC laboratory specimens of different grades. Correlation of NDT results are carried out with compressive strength of RCC members both in-situ and labora- tory specimens. Combination of results of RCC and PCC specimens are carried out for finding the correlation of compressive strength with NDT results. From the Correlation curves indicated that, there is good linear correlation between mechanical properties and NDT results. Combination of both NDT methods gives reliable results than one method. NDT method is most suitable for predicting the strength results without disturbing the structure.