Probing materials damage at various depths by use of Time Reversal Elastic Nonlinearity Diagnostic: Application to concrete.

In the context of license renewal in the field of nuclear energy, maintaining in service and re-qualifying existing concrete structures for the period of long term operations is challenging. This paper aims at providing some quantitative information related to the degree of micro-cracking of concrete and cement based materials in the presence of thermal damage. Time Reversal Elastic Nonlinearity Diagnostic (TREND) is based on the use of time reversal to focus energy at a prescribed location. This focused elastic wave energy is then analyzed for nonlinear frequency content. By varying the frequency content of the focused waveforms, the technique can be used to probe different depths relative to the surface, i.e., the TREND will probe the surface and penetrate to a depth defined by the wavelength of the focused waves. To validate this approach, we need reference measurements. We develop a methodology based on Resonant Ultrasound Spectroscopy, numerical simulations and Nonlinear Resonant Ultrasound Spectroscopy to get quantitative values of nonlinearity. These results are quantitatively correlated to the evolution of concrete microstructure in the presence of thermal damage. We then show the validity of using TREND at various depths by the good correlation between the nonlinearity measured using NRUS and TREND.