The influence of steel reinforcement on ultrasonic pulse velocity measurements in concrete of different strength ranges

Abstract

The present paper reports on the assessment of concrete strength, using non-destructive testing devices. The study focuses on Ultrasonic Pulse Velocity (UPV) as a non-destructive tool for estimating the compressive strength of the concrete in real structures, using a UPV-strength correlation model. The objective is to highlight the steel reinforcement influence on the ultrasound pulse velocity (UPV) measurement as a mean of a non-destructive strength assessment of the concrete. UPV-strength correlation models are built on the basis of laboratory plain concrete specimens tested to destruction. However, the real life structures contain steel reinforcement which interferes with UPV measurements, resulting in the distortion of these measurements, particularly when the reinforcement bars lie in the wave propagation path. Thus, when assessing the quality of the concrete in real structures, it is recommended to avoid reinforcement rebars, which is often difficult, if not impossible to ensure, especially for highly reinforced structural elements. This work proposes an alternative which consists of assessing the influence of steel reinforcement on UPV measurements through a correction factor for UPV readings that will allow the user to take account of the presence of steel reinforcement when assessing the concrete strength of real structures through an ultra-sound pulse velocity testing. In the study, concretes in three different strength ranges were targeted; the aim being to assess the role of the concrete density and the manner with which it interacts with reinforcement rebars when measuring UPV in reinforced concrete structures. The strength ranges considered vary from ordinary, to moderate, to the high strength concrete. Cylindrical moulds made of concrete from three different strength ranges were cast; some of these moulds were reinforced with different longitudinal steel ratios (low, moderate, high) to simulate real practical cases. These concrete moulds were then subjected to UPV tests in the parallel and perpendicular directions to the rebars disposition in order to study the reinforcement influence on UPV from the two directions. A correction factor was then developed to work out of the real ultrasonic pulse velocity of concrete that can be used in the UPV-strength correlation model to determine the compressive strength. For the three concrete strength ranges considered in this study, the presence of steel reinforcement was found to affect the UPV measurements. The results show also that the effect of steel rebar on UPV measurements is more accentuated with a low concrete density, particularly in the presence of a high steel ratio and rebar lying in the parallel direction to the wave propagation path. Indeed, for high performances concrete, which is a higher density concrete, a high steel ratio was found to slightly modify the pulse velocity; moderate and low steel ratios did not affect the pulse velocity. For moderate and normal strength concretes, however, high and moderate steel ratios had noticeable effects on the UPV measurements, especially in the parallel direction; showing that UPV testing is not only affected by the presence of steel reinforcement in concrete, but it is also related to the steel ratio, the rebar disposition, and the concrete density. A correction factor is then necessary for UPV measurements to correlate the strengths of reinforced concrete structures.