Ultrasonic dispersion and attenuation in bubbly liquids

Ultrasonic monitoring of fresh cement-based materials is important as pulse speed and attenuation are indicative of the increasing stiffness of the medium, and enable characterization of the curing stage and projections to the mechanical strength from an early age. Despite its importance, practical application is not straightforward due to severe heterogeneity and inherent damping. One crucial parameter in the ultrasonic behavior of fresh cement is the air bubbles, which impose a frequency dependent phase velocity and attenuation, as also observed in all bubbly liquids. In this study, ultrasonic experiments take place in fresh mortar as well as in reference media like water and shampoo. Results show that both shampoo and mortar exhibit strong dispersion relatively to water, seen by the dependence of phase velocity on frequency. Gradually and as bubbles are released due to gravitational settlement (in shampoo) or constrained (hardening of cement) the dispersive trend weakens reaching towards a nearly flat dispersion curve like water. The results highlight the influence of cavities which are considered one of the strongest types of scatterers, while quantification of cement ultrasonic dispersion opens the way for more accurate characterization of the curing behavior.