A jackhammer is mainly used to assess the uniformity of the strength of concrete within a structure and localize areas of poor quality.
When using a rebound hammer to estimate the compressive strength in place, the reference curves provided by the manufacturer should be used with some caution. The correlation between rebound number and compressive strength is very dependent on the concrete mix under test. Calibration is required to obtain an on-site strength rating using a jackhammer according to basic standards. The recommended method is to correlate measurements of a jackhammer with destructive tests on core samples or cubes / cylinders from the same concrete mixture as in the design.
For details on the requirements for creating such a correlation curve, see the following standards and recommendations: EN 13791 (Europe), ASTM C805, ACI 228.1R-03 (North America), JGJ T23-2001 (China).
The resulting data is used either to offset the reference conversion curve, or to define a custom curve for this particular mix. Typically, a curve is determined to provide a margin of safety in order to take into account various factors that may affect in-situ tests. In EN 13791, the lower 10th percentile curve is recommended. This means that 90% of the data pairs are above the curve and below 10%.
Carbonation forms a hardened layer on the surface and as this layer increases, it can lead to a significant overestimate (possibly up to 50%) of the compressive strength of reinforcing concrete when measured with a jackhammer. Either the carbonated layer should be removed before the rebound test, or the rebound test should be carried out before and after removing the carbonated layer using a grinder on a surface with a diameter of about 120 mm. This allows for a correction factor (sometimes called a “time factor”).
