Hong Life CalculatorsUnderstanding the rebound hammer strength estimator — concept and principle
A rebound hammer (often called a Schmidt hammer) is a quick, non-destructive way to check the surface hardness of hardened concrete. A spring-loaded mass strikes a plunger pressed against the concrete, and the distance it bounces back is read as a rebound number R. Because harder, denser concrete tends to be stronger, R can be converted into a rough compressive strength estimate.
How it is calculated
The calculator applies the basic curve for the rebound hammer from EN 13791:2007, where R is the average rebound number (dimensionless) and f is the estimated in-situ compressive strength in MPa (N/mm²):
- f = 1.25 × R − 23 for 20 ≤ R ≤ 24
- f = 1.73 × R − 34.5 for 24 ≤ R ≤ 50
Outside 20–50 the curve is not defined, and the calculator says so instead of extrapolating. The basic curve is a deliberately conservative lower bound, meant to be shifted upward once it has been calibrated against cores from the same concrete. EN 13791:2019 later replaced this curve with an approach based on reference tables and screening tests calibrated with cores.
The principle
When the hammer mass hits the plunger, part of its energy is absorbed by local crushing and plastic deformation of the concrete surface, and the rest is returned as rebound. A harder surface absorbs less energy, so the mass rebounds further. Surface hardness and compressive strength are both governed by the cement paste quality, porosity and aggregate, so they are correlated, but only statistically. That is why EN 12504-2 and ASTM C805 treat R mainly as an indicator of uniformity.
Worked example
- Default input: rebound number R = 35, which lies in the 24–50 range.
- Multiply by the slope: 1.73 × 35 = 60.55.
- Subtract the offset: 60.55 − 34.5 = 26.05 MPa.
How to read the result
Treat 26.05 MPa as a conservative lower-bound estimate, not a measured strength. The most reliable use is comparative: on the same element, measured the same way, a zone that reads several units lower than its neighbours deserves closer investigation.
Common mistakes
- Testing on rough, wet, plastered or heavily carbonated surfaces; carbonation hardens the skin and raises R.
- Ignoring hammer orientation; testing upward or downward changes the rebound compared with horizontal impacts.
- Using the result instead of core tests for acceptance or structural assessment.
Frequently asked questions
How many readings should I take at one location?
Take the full set your standard requires, never strike the same spot twice, and combine the readings as the standard specifies.
Why is my rebound number higher on old concrete?
Carbonation and drying harden the surface layer, so R can overstate the internal strength. Cores give the real answer.
Sources: EN 12504-2, Testing concrete in structures — Part 2: Non-destructive testing — Determination of rebound number · EN 13791:2007, Assessment of in-situ compressive strength in structures and precast concrete components · ASTM C805/C805M, Standard Test Method for Rebound Number of Hardened Concrete