Rebound Hammer Results Disagree with Cores
A technician uses a rebound hammer on a 90-day-old slab to estimate in-place strength after a cylinder set came in low. Rebound numbers average 34, which the technician's correlation table converts to roughly 4200 psi. Cores taken from the same area and tested per ASTM C42 give 3300 psi. The slab surface is carbonated and slightly rough from a broom finish. The technician wants to report the rebound result as the controlling value because it is higher and avoids a repair.
1. Which result controls, and why?
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Cores control. Rebound hammer is a surface hardness index, not a strength measurement. The surface here is carbonated and broom-finished, both of which bias rebound readings - carbonation hardens the surface layer and inflates the rebound number, and a rough finish scatters impact energy. The correlation table used was not established for this mix, this age, or this surface condition. Cores test the actual concrete and are the accepted basis for in-place strength evaluation under ACI 318. Judgment: the core result of 3300 psi governs. The rebound number is not a substitute and should not be reported as controlling.
2. What is the proper role of the rebound hammer in this investigation?
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Rebound hammer is useful as a screening tool to map variability across a large area - it can identify zones that warrant coring. It is not a substitute for cores and must be correlated to cores from the same concrete before any strength claim is made. In this case, the hammer could be used to locate the lowest-rebound zones so coring is targeted efficiently. Judgment: use rebound to guide where to core, then let cores answer the strength question. Reporting the higher number because it avoids repair inverts the purpose of testing.