What is a Schmidt Hammer test?

A Schmidt hammer test (also called a rebound hammer test) is a non-destructive way to estimate the compressive strength of concrete. The device fires a spring-loaded mass against the concrete surface and measures the rebound distance: harder, stronger concrete rebounds the plunger further, giving a higher rebound number. To use a Schmidt hammer correctly, the test surface is first ground smooth, then the hammer is pressed perpendicular to the surface and triggered at least 10-12 times over a grid roughly 25mm apart, discarding obvious outliers and averaging the rest. The average rebound number is converted to an estimated cube or cylinder strength using the manufacturer's correlation curve, corrected for the hammer's orientation (horizontal, vertically up or down). The test is governed by BS EN 12504-2, and it is important to understand its limits: results are affected by surface carbonation (which hardens the outer skin and inflates readings on older concrete), moisture, and aggregate type, so a rebound survey indicates uniformity and identifies weak zones rather than proving strength. Where results matter structurally — before repairs, strengthening or change of use — Schmidt hammer readings should be calibrated against a small number of drilled cores. We use rebound surveys as a rapid first pass across slabs, columns and beams to map suspect areas before targeted intrusive testing.