Influence of steel reinforcing bars on the evaluation of early-age concrete strength using the impact-echo method

The impact-echo method has been used in the past to estimate the in-place early-age compressive strength of concrete in unreinforced plate-like elements such as slabs and walls. In the study reported in this paper, tests were performed to evaluate the influence of the presence of steel reinforcing bars on impact-echo measurements of P-wave velocity in early-age concrete slabs. Variables considered include reinforcement bar diameter and placement depth. For the bar diameters treated in this study, the presence of a single steel reinforcement bar may contribute to increase scatter in the measured value of the P-wave velocity, but it does not cause a consistent alteration (i.e. consistent increase or decrease) in the value of the P-wave velocity determined by the impact-echo method. The error that results in the predicted compressive strength of the concrete due to any distortion in the value of the P-wave velocity depends upon the velocity at which the strength estimation is being made. The general shape of the strength-velocity curve exhibits a significant increase in velocity accompanied by a relatively small increase in strength at early ages, whereas at later ages, a significant increase in strength is accompanied by a relatively small increase in velocity. Thus, at early ages, a large distortion in velocity results in a relatively small error in the predicted strength. At later ages, even a small distortion velocity leads to a relatively large error in the predicted strength. Thus the method is more appropriately applied at earlier ages.