Experimental and numerical simulations of ultrasonic wave- multi-phase matter interactions in cement-based composites

In the case of concrete, the predominant damages develop at the microscale level, mostly at a length lesser than the size of inclusions. Again, if the vibrational energy induced by the dynamic loading does not get effectively dissipated by cement-based materials, it may prolong the original micro-defects that strike down the structural durability and safety. Such nontrivial and sensitive investigations require detailed inspection. The incoherent diffuse ultrasonic wave energy carries essential information about the material microstructure. Analysis based on statistical diffusion theory derives two parameters- diffusivity (D) and dissipation (σ), where, D reflects the material microstructure and σ indicates the material's viscoelastic property and damping effect. Initially, a numerical study is performed for understanding on wave-matter interaction in two scales of heterogeneity (concrete, cement paste) using diffusion-based analysis, so that role of inclusion (in cementitious matrix) on diffusion can be brought out in an uncoupled manner. The diffusivity parameters depend on tortuosity of wave path, transmission modes and the corresponding excitation frequencies. For experimental purpose, prismatic specimens are cast in two scales; ultrasonic measurements are taken with two excitation frequencies (200 and 300 kHz) under three transmission modes (direct, semi-direct, indirect). Signal processing in the Time-Frequency domain is performed to find the characteristic differences in the material microstructures at the same age of hydration (28 days) for both samples. The Spectral Energy Density of the signals provides information on spectral energy over the time window for a particular wave field. In this study, the influence of microstructure in wave propagation is explicitly demonstrated, and theoretical explanations are quantitatively validated with the actual experimental measurements.