A rebound hammer is primarily used to assess the uniformity of concrete strength within a structure and to localize areas of inferior quality.
When using a rebound hammer to estimate in-situ compressive strength, the reference curves provided by the manufacturer must be used with some caution. The correlation between rebound number and compressive strength is very much dependent on the concrete mixture under test. In order to obtain an estimate of in-situ compressive strength using a rebound hammer in accordance with major standards, a calibration is necessary. The recommended method is to correlate the rebound hammer measurements with destructive tests made on core samples, or cubes/cylinders made from the same concrete mixture as that used in the structure.
Please refer to the following standards and guidelines for details of the requirements for creating such a correlation curve: EN 13791 (Europe), ASTM C805, ACI 228.1R-03 (North America), JGJ T23-2001 (China).
The resulting data is used either to shift a reference conversion curve or to define a custom curve for that particular mix. Typically the curve is defined to provide a safety margin to take into account the various factors that may affect the in-situ tests. EN 13791 recommends the use of a lower 10th percentile curve. This means that 90% of the data pairs lie above the curve and only 10% lie below.
Carbonation forms a hardened layer on the surface and as this layer increases it can lead to a significant over-estimation (possibly as high as 50 %) of the compressive strength of the underlying concrete when measuring with a rebound hammer. Either the carbonated layer must be removed before rebound testing, or the rebound test should be carried out before and after removal of the carbonated layer using a grinding machine over a surface area of about 120 mm diameter. This allows a correction factor to be considered (sometimes referred to as a “time coefficient”).