Q: Is there a way to determine the compressive strength of concrete in-place, without removing samples for laboratory testing?

A: There are several nondestructive tests (NDT) that can be used to evaluate the strength of hardened concrete. Two of the most common NDT methods are the Schmidt rebound hammer and the Windsor probe, both of which measure the surface hardness of concrete as a quick indicator of relative compressive strength.

The rebound hammer test (ASTM C 805) measures the rebound of a spring-loaded plunger pressed against a concrete surface. The harder the surface, the greater the extent of the rebound, which correlates to relatively greater strength.

In the Windsor probe test (ASTM C 803), a powder-actuated gun drives hardened alloy probes into the concrete. The exposed length of the probe is measured, then related by a calibration table to the concrete's compressive strength.

The results of these tests can be influenced by a variety of factors, including surface smoothness, type and hardness of aggregates, age and moisture condition of the concrete, and the degree of carbonation at the surface. Therefore, it's important that results be compared with those from cored or cast specimens, and interpreted by NDT specialists who understand the limitations of the tests.