Concrete Strength Testing & Non-Destructive Methods

Assessing the structure of an existing building — before a retrofit, a risk assessment, a damage survey or adding a storey — requires knowing the concrete’s actual strength. The class written on the drawings and the strength achieved in the building are often not the same. This guide explains the non-destructive and semi-destructive methods used to measure it, with their strengths and limits.

Rebound (Schmidt) hammer: a fast screening tool

The rebound hammer measures how far a mass bounces back off the surface; that value relates to surface hardness and, indirectly, to strength. A reading takes seconds and causes no damage, which makes it ideal for scanning many points quickly. But it only reflects the near-surface zone: carbonated, damp, rough surfaces or points close to rebar skew the result. Hammer readings alone are therefore not used to declare strength — they must be calibrated against cores.

Ultrasonic pulse velocity (UPV): integrity inside the section

UPV measures the speed of a sound wave travelling through the concrete. High velocity indicates dense, void-free concrete; low velocity points to voids, cracks or deterioration. Because it reveals integrity through the section rather than surface hardness, UPV complements the rebound hammer. Using both together (the combined SonReb approach) reduces the error of either method alone.

Core sampling: the reference value

A core is a real cylindrical concrete sample taken from the structure and crushed in a laboratory. As a direct measurement, it provides the reference value against which non-destructive methods are calibrated. The cost is limited, repairable damage — core locations are chosen where they least affect structural behaviour and without cutting rebar, which is why rebar detection comes first.

Rebar detection and cover measurement

Before coring, drilling or cutting, you need to know where the reinforcement is. A cover meter locates rebar and measures cover magnetically; ground penetrating radar (GPR) reaches deeper and can also image complex layouts, post-tension cables and voids. These surveys prevent damage and feed the existing reinforcement layout into the retrofit design.

Carbonation and corrosion assessment

Concrete reacts with atmospheric carbon dioxide over time and carbonates inward from the surface, weakening the alkaline environment that protects the reinforcement and opening the door to corrosion. Spraying phenolphthalein on a freshly broken surface reveals the carbonation depth. Where corrosion is suspected, further measurements such as half-cell potential are used. Strength alone is not enough — the condition of the reinforcement governs the remaining service life.

Which method should you choose?

In practice the right approach is not choosing one method but sequencing them: rebar detection first, then wide-area non-destructive scanning (hammer plus UPV), then a limited number of cores from representative points with laboratory testing. This keeps damage minimal while keeping results calibrated. When hiring, ask how many points will be measured, how many cores will be taken and whether the laboratory is accredited — on Tenderely you can compare several providers’ scopes.

Frequently Asked Questions