Elastic wave dispersion for heterogeneity characterization in cement-based media

Ultrasound has been extensively used to monitor the structural condition of cementitious materials and structures. Wave velocity among other parameters carries valuable information, as it depends on the material stiffness and heterogeneity. Although it is used for a first estimate of the performance, still, the full inspection capacity, especially in-situ has not been reached. Specifically, even when lower-than-expected wave velocities are measured, they cannot be attributed directly to a specific deterioration mechanism or heterogeneity size. Reasons include the limited sensitivity of pulse velocity to early signs of damage, limited capacity to interpret the waveform in terms of the structural condition, as well as technical issues including acoustic coupling. The present paper demonstrates that proper ultrasonic dispersion interpretation can pave the way to reliable assessment of the typical void size, allowing maintenance decisions based on engineering criteria in addition to established but rough empirical correlations of pulse velocity to concrete quality. The contribution of theoretical investigations in the explanation of the experimentally obtained behaviour is highlighted. The importance of the applied frequency is paramount because of the interaction between the wavelength and the heterogeneity or typical void size.