Monitoring the setting and hardening of early-age mortar using a pulse-echo ultrasonic guided wave approach

A pulse-echo ultrasonic guided wave approach that monitors the setting and hardening of early-age mortar during the first twenty-four hours of hydration is presented. The method transmits a torsional wave mode on one end of a cylindrical steel rod embedded in mortar and then receives the reflected signals. Both the reflection from the end of the rod and the reflection from the point where the waveguide enters the mortar are monitored. The development of the mortar's mechanical properties is related to both the energy leaked into the surrounding mortar and the energy reflected at the entry point. Experiments were performed on mixtures with varying water-cement ratios (w/c = 0.40, 0.50, and 0.60), chemical admixtures (accelerant and retardant), and mineral admixtures (silica fume and fly ash). The time of setting and compressive strength of the different mortar mixtures was determined in accordance with ASTM standards. The change in signal strength of the end- and entry-reflection of the guided wave appears to be correlated to the setting times and compressive strength of the mortar. The ability of this method to only require access to one side of the specimen makes it attractive for the development of a portable system that monitors early-age cementitious materials in the field.