When cube results come back low, or nobody took cubes, the question becomes: how strong is the concrete actually in the building? Non-destructive tests give a first answer without cutting anything.
How the Test Works
- Prepare a smooth, clean test area on the member.
- Hold the hammer perpendicular to the surface.
- Take a set of readings spread over the area — typically a dozen or so.
- Discard outliers and average the rest.
- Convert the rebound number to an estimated strength using the instrument's or calibrated correlation.
The test is covered in Indian Standards on non-destructive testing of concrete (IS 516 Part 5, which superseded the earlier IS 13311 series).
What Affects the Result
| Factor | Effect |
|---|---|
| Surface finish | Rough or plastered surfaces give misleading readings |
| Moisture | Wet surfaces read low |
| Carbonation | Old concrete's carbonated surface reads high |
| Aggregate type | Changes the correlation |
| Hammer angle | Must be corrected for |
Rebound, UPV and Cores
| Test | Measures | Use |
|---|---|---|
| Rebound hammer | Surface hardness | Fast screening and comparison |
| Ultrasonic pulse velocity (UPV) | Uniformity, voids, cracks | Finding poor compaction or honeycombing inside |
| Core test | Actual in-place strength | Confirmation where strength is in doubt |
Combined rebound and UPV readings give a better estimate than either alone; cores remain the reference.
The Short Version
Rebound hammer readings are a quick, non-destructive indicator of surface quality — good for screening and comparing members, not for proving strength. Use UPV for uniformity and cores for the final answer.
