Non-equilibrium dynamics in concrete: Effects of pump-induced conditioning on nonlinear coda wave interferometry parameters

Tight cracks and microcracks often go undetected by linear ultrasonic testing, motivating nonlinear ultrasonic techniques, such as nonlinear coda wave interferometry (NCWI). NCWI combines high-frequency coda wave interferometry with high-amplitude low-frequency pumping, gradually increasing the pump amplitude, to probe changes in wave velocity due to crack shearing/opening. The question is whether dynamic in-service loading could be used as pump to increase NCWI's utility for in situ monitoring. Since high-amplitude pumping induces transient time-dependent changes in the material, the pumping duration and history influence the material's response. This study presents a systematic investigation of the pumping protocol's influence on NCWI results and discussions of the possible underlying mechanisms. We conduct NCWI with varying duration and pump amplitude orders on two concrete blocks—one intact and one containing a closed crack—under controlled temperature and humidity. Our findings show that longer pump durations cause larger relative velocity changes, likely due to progressive crack sliding/opening, while descending amplitudes yield smaller changes as the material may not fully recover from prior higher-amplitude pumps. Nevertheless, relative velocity changes measure consistently higher for the damaged block compared to the intact one. This work highlights the time-dependent material response and the importance of consistent pumping protocols when deploying NCWI.