Evolution of acoustic emission characteristics in concrete with damage progress and loading type for geological disposal of high-level nuclear wastes

Abstract

>90% of yield stress). In addition, the moving average PF of PTST in the early damage stage appears to be particularly lower than that of UCT and BTT. The loading in BTT renders distinctive responses in the slope of the maximum RA–cumulative AE count (or tan Θ). The slope value shows a sudden increase to ~0.48 when the stress is close to 90% of yield stress in BTT

for rest the slope mostly remains less than 0.25 in all tests. This dissertation advances our understanding on AE responses of concrete materials with well-controlled laboratory-scale experimental AE data, and provides insights into further development of AE-base real-time diagnostic monitoring of structures made of rocks and concretes.

Deep geological disposal of high-level radioactive wastes (HLW) critically requires prediction and monitoring of long-term changes in the mechanical properties and behaviors of surrounding rocks and engineered barrier systems to ensure the structural integrity of the disposal system. The acoustic emission (AE) method is considered as one of the ways to diagnose and monitor the mechanical damage progress in rocks and concrete. The objectives of this dissertation are (a) to identify the AE characteristics emitted from concretes as concrete materials under different types of loading, (b) to suggest AE parametric criteria to determine loading types and estimate the failure stage, and finally (c) to examine the feasibility of using AE method for real-time monitoring of geologic disposal system of HLW. This study performs a series of the mechanical experiments on concrete samples simultaneously with AE monitoring, including the uniaxial compression test (UCT), Brazilian tensile test (BTT) and punch through shear test (PTST). These mechanical tests are chosen to explore the effect of loading types on the resulting AE characteristics. This study selects important AE parameters which includes the AE count, average frequency (AF) and RA value in the time domain, and the peak frequency (PF) and centroid frequency in the frequency domain. The result reveals that the cumulative AE counts, the maximum RA value and the moving average PF show their potentials as indicators to damage progress for a certain loading type. The observed trends in the cumulative AE counts and the maximum RA value show three unique stages with an increase in applied stress: the steady state stage (or crack initiation stage

< 70% of yield stress), the transition stage (or damage progression stage

70–90% of yield stress) and the rising stage (or failure stage