Elastic wave characterization of controlled low-strength material using embedded piezoelectric transducers

Ultrasonic wave methods have been widely used to monitor the hydration process of cementitious materials. However, the conventional ultrasonic wave methods require a coupling agent and pressure or a buffer material to maximize the coupling of the transducer. The objective of this study is to apply two types of embedded piezoelectric transducers to controlled low-strength material (CLSM) and to monitor the elastic wave characteristics during the hydration process. The CLSM mixture is composed of silica sand, calcium sulfoaluminate cement, fly ash, and water, and then, the CLSM mixture is prepared with three different fine contents. Using piezoelectric disk and bender elements installed in cuboid containers, the compressional and shear waves are measured for 72 h at various intervals. The monitoring results show that during the hydration process, the evolution of shear wave velocities obtained using the bender element is less variable than that in compressional wave velocities obtained using the piezoelectric disk element. As a power function, the shear wave velocities increase with an increase in the elapsed time. As the fine content of the CLSM mixture increases, the water content required for the flowability of the CLSM mixture increases and the shear wave velocities then decrease. The results demonstrate that the CLSM mixture with high water content induces a delay of the interconnection of the cementitious particles at the very early stage of the hydration. The geometric boundary condition of the container is considered as an aspect of the estimation of the shear wave velocity obtained by the bender element. This study demonstrates that the embedded elastic wave transducers can be effectively used for monitoring the hydration process of CLSMs.