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-pl ace early-age compressive strength of concrete in unreinforced plate-l ike elements such as slabs and walls. In the study reported in this pa per, 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 reinforceme nt bar diameter and placement depth. For the bar diameters treated in this study, the presence of a single steel reinforcement bar may contr ibute to increased scatter in the measured value of the P-wave velocit y, but it does not cause a consistent alteration (i.e. consistent incr ease or decrease) in the value of the P-wave velocity determined by th e impact-echo method. The error that results in the predicted compress ive strength of the concrete due to any distortion in the value of the P-wave velocity depends upon the velocity at which the strength estim ation is being made. The general shape of the strength-velocity curve exhibits a significant increase in velocity accompanied by a relativel y small increase in strength at early ages, whereas at later ages, a s ignificant increase in strength is accompanied by a relatively small i ncrease in velocity. Thus, at early ages, a large distortion in veloci ty results in a relatively small error in the predicted strength. At l ater ages, even a small distortion in velocity leads to a relatively l arge error in the predicted strength. Thus the method is more appropri ately applied at earlier ages.