Application of ultrasonic P-wave reflection to measure development of early-age cement-paste properties

Ultrasonic wave reflection (UWR), well established for the study of stiffening and strength development of cement-based materials, normally uses shear waves (S-waves) because they are sensitive to microstructural development. This study demonstrates possible application of UWR with longitudinal waves (P-waves) using a low impedance buffer for investigating stiffening in hydrating cement paste. The P-wave reflection coefficient was seen to increase modestly as hydration progressed. Also, the P-wave reflection coefficient showed higher values for pastes with lower water to cement ratios, which are primarily attributed to the higher density of low w/c pastes, and similar effects with addition of fly ash and entrained air. Partial debonding between paste and buffer was observed in most pastes at a time that coincided with final set as measured using S-waves; and the debonding appears to be associated with the development of pore water under-pressure that occurs after solidification (due to chemical shrinkage).