Rebound hammer or ultrasonic: which test, and when
Two non-destructive methods, routinely treated as interchangeable. They measure different properties, and the difference decides which one answers your question.
Non-destructive testing gets requested as a single item — "can you test the concrete" — as though the methods were graded by accuracy and you simply pick the best one you can afford. They are not graded. They measure different physical properties, and each is close to useless at the other's job.
What each one is actually measuring
A rebound hammer measures surface hardness. A calibrated mass strikes the concrete and the instrument records how far it bounces back. Harder surface, higher rebound. It reads the outer few centimetres and nothing beyond them.
Ultrasonic pulse velocity measures how fast sound crosses the element. A pulse is sent in one face and timed at the other. Sound travels quickly through sound, dense concrete and slowly through concrete containing voids, cracking or poor compaction — so UPV reads the whole thickness, but reads it as a single averaged path.
Choosing between them
- Uniformity across a large pour, or a quick comparative survey of many elements: rebound hammer. It is fast, needs one face, and is well suited to finding the outlier among fifty columns.
- Suspected voids, honeycombing, internal cracking or depth of damage: UPV. Surface hardness will tell you nothing about a void behind it.
- An actual strength figure you intend to rely on: neither, alone. Both are correlations. They need calibrating against cores taken from the same concrete before a number from either means anything defensible.
The pairing that works
In practice the two are complementary and are usually run together. UPV locates where the concrete is not behaving as it should; the rebound hammer covers ground quickly across everything else; and a small number of cores, taken where the other two disagree, anchor the whole survey to a real measured strength. That combination is what lets a survey cover an entire phase of completed units while opening only one of them.
Both methods are correlations. A number from either, uncalibrated, is an opinion with a decimal point.
