An improved ultrasonic coda wave method for concrete behavior monitoring under various loading conditions

Monitoring of concrete behavior is an important task to evaluate the safety of concrete structures. This paper proposes a new stretching factor accumulation method based on the stepwise coda wave interference (CWI) to accurately calculate the relative velocity change Δv/v. The Δv/v and residual decorrelation coefficient K_d are used to study the concrete behavior in step loading experiment and short-time fixed loading experiment. The results show that: (1) In the loading stage of step loading experiments, both Δv/v and K_d can be used to monitor the load and the microcrack development; (2) In the relaxation stage of step loading experiments, the slow dynamics effect of concrete specimens can be characterized by Δv/v and K_d; (3) In the short-time fixed loading experiments, both Δv/v and K_d increase with the time of fixed loading, and the increasing rate of Δv/v decreases with the increase of concrete strength. This work can be used to promote the development of ultrasonic testing techniques that aid in concrete behavior monitoring.