Characterizing the Effect of Applied Stress in Concrete by Magnitude-Squared Coherence of Ultrasonic Full-Waveforms

Abstract

Ultrasonic testing is widely used to evaluate concrete structures. Namely, the ultrasonic pulse velocity method, which is based on the time-of-flight between transducers, is used in practice to determine the uniformity of concrete in structural members. While this approach is capable of detecting relatively significant deterioration, it is insensitive to small internal changes, such as a geometric change due to an applied stress. The objective of this paper was to study the effect of applied stress in concrete on ultrasound using digital signal processing on the recorded fullwaveforms. For this purpose, the magnitude-squared coherence (MSC) was used to determine the similarity between two recorded waveforms: a reference recorded waveform under no stress and one taken at a certain level of applied stress. The experimental work for this study consisted in applying a monotonically increasing compression load on four Ø 152 x 305 mm concrete cylinder specimens up to failure. A pitch-catch setup was used during the loading process consisting of two identical ultrasonic normal-wave transducers to continuously transmit an ultrasonic pulse and receive its response. The results show that looking at the recorded data in a traditional fashion does not show any differences before an ultimate load of approximately 80% is reached. By using MSC to compare entire recorded waveforms, however, a linear relationship to approximately 70% of ultimate stress can be observed immediately, starting at very low levels of applied stress.