When using a rebound hammer to estimate the strength of in-place concrete, does it make a difference where you take the reading? For instance, will you get a lower reading near the bottom of a wall because of the dead weight of concrete above?
Readings may vary depending on position in the structure, but the variations aren't caused by differences in dead weight. Concrete near the bottom of an 8-foot-high wall feels a stress of only 8 psi caused by the dead weight of the concrete above (150 pounds per cubic foot 2 8 feet = 1,200 pounds per square foot. 1,200 pounds per square foot/144 square inches per foot = 8 psi). It's more likely that concrete near the top of the wall will have a lower rebound reading. That's because some of the bleedwater may not get to the surface before the concrete sets. This trapped water slightly lowers the strength. Remember that the rebound hammer gives only a crude estimate of concrete strength, with an accuracy of perhaps 15% to 25%.