How does the rebound hammer test work and what are its limitations?

The rebound hammer (Schmidt hammer) measures surface hardness by impacting the concrete with a spring-loaded plunger and measuring rebound distance. Rebound number is correlated to compressive strength using charts. Limitations include: only measures surface (30-50mm depth), affected by surface conditions, aggregate type, moisture, carbonation, and age. Results have high variability (15-25%) and should be calibrated with core tests. It's a non-destructive screening tool, not a substitute for core testing.