English

Abstract

The reliability of the non-destructive Schmidt Hammer Test as a means of estimating the compressive strength of concrete is investigated by testing three groups of concrete cube specimens. The first group was exposed to cycles of alternate drying and wetting in brackish water; the second group, to continuous immersion in brackish water; and the third (control) group, to normal room condition. Results show that the average Schmidt Hammer Rebound Number (RN) for samples in the first and second group is significantly less than that of the third group. These indicate the reliability of the Schmidt Hammer Test in predicting the reducing effect of exposure to brackish water on the compressive strength of concrete. Moreover, results show that in each of the three groups, the average estimated compressive strength of concrete based on Schmidt Hammer Test (using the rebound curve provided by the manufacturer of the device) underestimates the average actual compressive strengths based on direct compression test. Thus, in a quick strength and safety assessment of existing concrete structures, the Schmidt Hammer Test is fairly reliable in determining whether the further use of the structure would still be safe, but may not be so in concluding whether the use is no longer safe. In the latter case, the Schmidt Hammer Test should be supplemented by core sampling and testing. O determine the strength of concrete in existing structures, cylinder specimens are usually taken from the structures and brought to laboratories where they are loaded to failure to obtain actual compressive strength. This procedure is the most accurate way but it requires considerable time and expenses. In order to assess in-situ concrete strength in a faster manner, non-destructive testing (NDT) techniques have been developed and adopted. These techniques estimate the strength of existing structures by measuring some concrete properties other than its strength, and then relate these properties to strength or other mechanical properties of concrete (1). Among the many available NDT techniques, the most widely employed is the one using a device called the rebound hammer, also known as the Schmidt Hammer, due to the