Assessing the degree of accuracy of the SCHMIDT’S rebound hammer in estimating the compressive strength of concretes in Uganda
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A rebound hammer is a non-destructive testing apparatus whereby the rebound of the spring-driven mass is measured after its impact with a concrete surface. The output of the rebound hammer is referred to as the rebound number and is correlated with the surface hardness of concrete. It is one of the most widely used techniques to estimate the compressive strength of concrete in Uganda. The rebound hammer is affected by parameters like concrete carbonation, surface moisture content, surface condition, age of concrete, specimen size, elevated temperature, exposed fire, and others. These parameters affect surface hardness but don’t necessarily affect the actual compressive strength. This research was aimed at determining the degree of accuracy of a rebound hammer in estimating the compressive strength of concrete using the crashing test as the control. The original rebound hammer N/L series was used in this study. The rebound hammer test was carried out on cube samples of 150 mm x 150 mm that were brought by clients to the Makerere structure laboratory and Teclab limited. The results obtained from the crashing test were compared with the estimated strength from the calibration curve using the average rebound number. The factors considered in this research were those that have a big effect on the rebound number, for example, surface moisture content, surface texture, and angle of orientation of the hammer. The rebound numbers for moisture content were obtained when the cubes were removed from the curing tank, and then more readings were taken after 2 hours and after 24 hours. A sample of 50 samples were tested the rebound number was reduced by 51.37% of the actual rebound number for the concrete is removed from where it is submerged in water in the curing tank and immediately tested. Rebound numbers were also obtained for both rough-textured surfaces and smooth-textured surfaces, and angle of orientation and different graphs were obtained. The results showed both a higher average error of 58.87% compared to the smooth surface that was 55.85%. The angle orientation of 900 and 850 had an error difference close to 7% which can cause a big effect to the results. The other rebound numbers obtained were those where all parameters were constant, and these were smooth surfaces at ninety degrees and dry surfaces. The results that were obtained showed a variation of the estimated compressive strength from the actual compressive strength. These findings show that the rebound hammer is inaccurate in measuring the compressive strength of concrete because it only considers surface hardness, which is not the actual compressive strength. It is recommended that a Rebound Hammer should not be used alone to come up with conclusive judgement on the compressive strength of concrete but should be used alongside cores to obtain best results.