MONITORING DAMAGE IN CONCRETE USING DIFFUSE ULTRASONIC CODA WAVE INTERFEROMETRY

Abstract

Coda Wave Interferometry (CWI) is a nondestructive evaluation technique for monitoring small velocity changes in multiple scattering media, as demonstrated in previous seismic and acoustic experiments. Velocity changes of ultrasonic waves in the medium, obtained through an acoustoelastic measurement, are seen more markedly in the arrival time of the late coda waves. Velocity changes can be extracted accurately from two diffuse fields obtained at different stress levels using CWI. The stress‐velocity relationship is well known and given by Hughes’ and Kelly’s equations. In this work, the authors apply CWI based on the stretching technique to a heterogeneous, nonlinear, highly‐scattering and multiphase medium, concrete, to detect damage in an early stage of degradation. Ultrasonic signals are measured in different concrete samples using two transducers: one transducer acting as a source and the other transducer as a receiver. The relative velocity change obtained can be used to calculate the nonlinearity parameter β and to characterize the damage level. This research investigates the relative velocity changes in concrete samples due to the following material degradation: applied dynamical load and chemical reaction.