This paper proposes a method for determining the characteristics of microcracks in wet mortars caused by frost damage, from ultrasonic velocities measured at multiple frequencies. In partially water-saturated mortar, P-wave propagation induces pressure gradients between the saturated and dried regions. This is followed by pore water flow, called mesoscopic-scale wave-induced fluid flow (WIFF). It results in the viscoelastic behavior of the dynamic moduli, and their frequency dependency reflects the mobility of pore water related to the crack properties. The integration of the cracked effective medium (CEM) theory and mesoscopic-scale WIFF model enables the description of the frequency-dependent moduli of cracked mortars as functions of crack porosity, width, and water saturation. Numerical simulations were conducted initially to evaluate the effects of crack properties and saturation on the moduli. The simulations demonstrated that the crack aperture significantly influenced the reduction in mortar frame moduli and that the frequency dependency of the P-wave velocity was sensitive to the water saturation, as well as the crack properties. Subsequently, P- and S-wave velocity measurements were performed on mortar specimens subjected to freeze–thaw cycles (FTCs). The crack porosity, width, and water saturation were evaluated simultaneously using the proposed method. The obtained crack porosity was consistent with the porosity variation before and after FTCs. The obtained crack width agreed with the variation in pore size distribution. The CEM-WIFF method can be used to evaluate the generation and progress of microcracks in mortar constructions by continuously or periodically measuring the ultrasonic velocities at multiple frequencies.
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