A spring-loaded hammer hits the concrete surface with a known repeatable force and the rebound of the hammer from the surface is recorded. The rebound is inversely proportional to the energy absorbed by the concrete which is related to its hardness and therefore its compressive strength. The method is reasonably quick to collect, with the hammer rebound results being digitally recorded. The method is particularly suited as a comparative test.
The accuracy of the method depends upon the surface condition, moisture content, the extent of carbonation and the concrete mix. For good quality concrete, the estimated values can vary by as much as 20 to 30% of actual strength determined from core tests. For low strength concrete, lower accuracies are expected as the strength is more influenced by the aggregate. Rebound hammer results reflect the condition in the top 30-50 mm of the concrete surface. Ultrasonic Pulse Velocity (UPV) or Impact Echo measurements can be used in conjunction with a rebound hammer to improve estimates of concrete compressive strength.
GBG Group also always recommends undertaking several correlation cores to apply correction factors to the hammer results.
The results are converted to strength using calibration curves and results are usually presented in tables.
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