The estimation of mechanical properties and strength of concrete can be carried out by several methods; destructive and non destructive. In this study crushing strength of concrete cubes is the usual destructive test to determine the compressive strength of concrete. The rebound hammer test, ultrasonic pulse velocity and core test are the non destructive tests to determine compressive strength of concrete and to obtain correlation between them. In this work using M-20 grade of concrete twelve

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... mbers of concrete cubes of size (150mmx150mmx150mm) were cast using the water cement ratio 0.55. The five numbers of slabs of size 500mmx500mm with 120mm thickness were made by same water cement ratio. An experimental study was conducted to determine the correlation between actual compressive strength of concrete cubes and strength estimated from cores having different H/D ratio (1.5, 1.75 and 2.0) and diameter of 50mm. totally thirty numbers of cores were extracted from the concrete slabs, fifteen numbers of cores were extracted from concrete portion (without reinforcement) and other fifteen numbers of cores were extracted with reinforcement. The results indicate that the cores with reinforcement of H/D ratio 1.5, 1.75 and 2.0 indicate greater compressive strength as compared to cores free from reinforcement for the same H/D ratio and core compressive strength increases with increase in the H/D ratio. The very good correlation coefficient (0.979) is obtained. www.iosrjournals.org 32 | Page Core drilling: The cores were cut by means of a rotary cutting tool with diamond bits size 50mm. The core cutting equipment is heavy and it is firmly supported and braced against the concrete surface to prevent relative movement which will result in a distorted or broken core, a continuous water supply is provided to lubricate the core cutter. By applying the uniform pressure in a vertical direction, totally thirty numbers of cores were extracted from the marked concrete slabs. In that fifteen numbers of cores are extracted from only in concrete portion (without reinforcement) and other fifteen numbers of cores are extracted with reinforcement. Visual inspection: The drilled cores from the slab elements were observed, the aggregates are equally distributed and there is no cracks and damage caused to the drilled cores. End preparation: The drilled cores are marked at suitable heights and diameter. The marked cores are trimmed with three different height/diameter ratios such as 1.50, 1.75 and 2.0. The trimmings of cores were done by water lubricated diamond saw. The end surfaces of the trimmed cores were smoothened for compression test.