- Determine hardness of concrete, rock and masonry.
- Estimate compressive strength testing for concrete, rock and masonry.
- Locating defective or honeycombed concrete close to the surface.
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, 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. Schmidt hammer results reflect the condition in the top 30-50mm 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 Australia always recommends undertaking a number of correlation cores to apply correction factors to the hammer results. The Australian Standards AS1012.14 recommends at least three cores should be taken for each concrete structure.
Data analysis and presentation
The results are converted to strength using calibration curves and results are usually presented in tables.