Relationship between ultrasonic Rayleigh wave propagation and capillary porosity in cement paste with variable water content

The relationship between Rayleigh wave velocity at ultrasonic frequencies and capillary porosity is investigated in dry and water saturated cement paste. Capillary porosity is varied by changing the water/cement (w/c) ratio. Water saturation is set at four levels - 100, 70, 30 and 0% - by drying the fully saturated specimen. Rayleigh wave (RW) wavefront and phase velocities are measured using air-coupled ultrasonic transducers as transmitter and receiver. The measurement accuracy is evaluated using a reference specimen made up of a non-dispersive material. As expected, a linear decrease of RW velocity vs. porosity is observed for the intermediate saturation states, but not for the fully and dry states, possibly due to micro-cracks induced during the drying phase and to the presence of bubbles in the specimens with low w/c ratio. The comparison of phase velocity in specimens with low and high w/c ratio shows an inversion of the porosity gradient perpendicular to the surface of measurements, which is due to segregation of cement at high w/c ratio. Finally, the correlation between RW velocity and capillary porosity is analyzed and compared with results obtained in mortar. (C) 2012 Elsevier Ltd. All rights reserved.