Rebound hammer (Schmidt hammer)
A handheld device that estimates concrete strength from the rebound of a spring-driven plunger against the surface. It is a screening tool, not an acceptance test.
What does rebound hammer (schmidt hammer) mean?
The rebound number reflects only the surface layer, so carbonation, moisture, finish quality, and nearby aggregate all skew results. It is genuinely useful for comparing relative uniformity across a large area or finding a suspect region to core. Correlating it to actual strength requires calibration against cores from the same concrete.
In practice
An inspector will sweep a wall with one to find soft spots, then core the worst ones rather than reporting the hammer numbers as strengths.