Monitoring Early Age Properties of Cementitious Material Using Ultrasonic Guided Waves in Embedded Rebar

The evaluation of early age properties of concrete is critical for ensuring construction quality. This paper presents a sensing method to use ultrasonic guided waves in a rebar for monitoring the early age properties of cementitious material. An EMAT sensor was used to excite the longitudinal mode L(0,1) wave in a rebar embedded in cement/mortar, and an ultrasonic transducer was used for receiving the echo signals. Guided wave dispersion curves were developed to select appropriate frequency range. The leakage attenuations of the L(0,1) mode wave from the rebar to the surrounding cement materials were continuously monitored for the first 10 h. The evolution of the shear wave velocity was also monitored simultaneously. The leakage attenuation from experimental measurements was compared with the theory-predicted attenuation in both time and frequency domains, and showed good agreement. Experiments were performed on three cement paste samples and three mortar samples. The results indicated that attenuation is nearly linearly related to the shear wave velocity, and shear wave velocity is linearly related to the penetration resistance (ASTM C403) in logarithmic scale. These results suggest that mechanical properties and hardening process of cement materials can be monitored by using the ultrasonic guided waves in a rebar.