Rebound hammer calibration: why the factory curve is not enough
Updated: 2026-08-13 · ISKRIT
A rebound hammer does not measure strength. It measures how much energy the surface gives back, and everything between that number and a strength in megapascals is a correlation somebody fitted to somebody else's concrete.
What the instrument sees
The spring-loaded mass rebounds off the surface, and the rebound number R depends on the surface hardness of the top few millimetres. Carbonation, moisture, the direction of the blow, the coarse aggregate and the age of the concrete all move R without moving the strength of the member. That is why the scale printed on the hammer is a starting point, not an answer.
Why a factory curve drifts
The curve supplied with the instrument was fitted on a particular mix, a particular aggregate and a particular curing regime. Change the aggregate from limestone to granite, or test at 90 days instead of 28, and the same rebound number corresponds to a different strength. The published methods handle this in two ways: fit your own curve on companion specimens, or keep the factory curve and correct it by a coincidence coefficient obtained from a small set of paired readings.
Fitting your own curve
The procedure is the same whichever indirect method is used: rebound, impact impulse, ultrasonic pulse velocity, pull-off with spalling or rib shearing. Take at least a dozen specimens spanning the strength range of interest, record the indirect reading on each, then break it. Fit a straight line of strength against reading, screen the outliers, and check the fit before anything is read off it.
| Check | Threshold | If it fails |
|---|---|---|
| Correlation coefficient | 0.7 or better | the curve is not usable for strength assessment |
| Residual standard deviation | 15 % of mean strength or less | the scatter swamps the signal |
| Range of validity | the readings actually fitted | extrapolation beyond them is not evidence |
| Outliers | screened before fitting | one bad pair can rotate the whole line |
The mistake that survives audits
The common failure is not a bad hammer, it is a good curve applied outside its range. A calibration fitted between 20 and 35 MPa says nothing about a member reading 48 MPa, and a report that quotes such a figure is quoting extrapolation. The range belongs on the report next to the equation.
The concrete strength calculator fits the curve, screens outliers, reports the correlation and residual deviation against both thresholds, and refuses the curve when either fails.
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