EXPERIMENTAL STUDY ON CORE STRENGTH OF CONCRETE

Abstract

Core test is commonly required in the area of concrete industry to evaluate the concrete strength and sometimes it becomes the unique tool for safety assessment of structural adequacy of existing concrete structures. Core test is therefore introduced in most codes. All studied codes provisions seem to be unreliable for predicting the in-situ concrete standard cylinder strength from the results of core tests. Therefore an experimental study has been performed to see the influence of different factors that affect the interpretation of core test results. For this purpose a number of concrete cores have been drilled from concrete blocks and RC beams. Six beams were tested under two point loading to produce the first crack and hence the microcrack at the interfaces of aggregate and mortar matrix in tension zone. The blocks were stepped in order to maintain the similar condition for all cores. The beams and blocks were cast for different mix ratios with different coarse aggregates (stone chips and brick chips- ¾ inch downgraded). The minimum reinforcement was provided in beams just to resist the temperature, shrinkage and stresses due to handling. For each mix ratio, the cores with three different diameters (2 -inch, 3-inch and 4-inch) were drilled along parallel and perpendicular to the direction of concrete casting from blocks compression, neutral zone and tension zone of beams. From beam only 4-inch diameter core were drilled from different locations which experience different stress condition under bending test. Height to diameter 2.0 was maintained. The w/c ratio was 0.42 in order to maintain slump was 3 to 4 inch. For each ratio, three standard cylinders were cast. The concrete cores and cylinder were tested in the laboratory following standard method specified in ASTM C39 and comparison has been made between the cores strength and standard cylinder strength to study the drilling effect on concrete core strength. The concrete core tests indicate that the average strength of 3-inch diameter core is almost similar to the average strength of 4-inch diameter core. Where, 2-inch diameter cores experience relatively low strength and the results are more scattered than the corresponding 3-inch and 4-inch diameter cores. Analysis of results show that with the increase of the ratio of drilling surface and volume of the core, the core strength becomes lower and provide more scattered result in comparison with the corresponding larger diameter cores. The strengths of cores drilled from RC beams are not appeared to follow normal trends. Generally, the cores in compression zone experienced higher strength than the core in tension zone. The cores drilled from tension zone experience relatively lower strength with scattered results.