Monitoring of Stresses in Concrete Using Ultrasonic Coda Wave Comparison Technique

Condition or health monitoring of concrete structures has experienced increasing interest over the last decade. While conventional sensors such as strain gauges are accurate and reliable, they only allow for surface observations. In contrast, ultrasonic waves propagate through the thickness of a member and can thus detect internal changes. In this paper we present an ultrasonic monitoring approach that uses a coda wave comparison (CWC) technique, which makes use of the highly sensitive diffuse (or coda) portion of a recorded ultrasonic waveform. In this study, the changes in the applied stress were correlated to the changes observed in the ultrasonic waveforms, which were estimated using magnitude-squared coherence (MSC). The CWC technique was evaluated by investigating key influence parameters that affect the relationship between MSC and the applied stress. First, two concrete cylinders were cast and tested to study the effect of maximum aggregate size. Second, two concrete prisms were used to study the effect of the frequency of the transmitted pulse. Finally, we discuss a field test involving a prestressed concrete bridge girder and a column. The results show that MSC is capable of discriminating minute stress changes in a laboratory as well as a field setting.