Noncontact inspection of the curing process of cementitious materials by laser doppler vibrometry

An essential issue in materials research, quality control, and in practical planning and implementation of construction projects, is the understanding of the curing process of fresh cement-based materials. Immediately after mixing, cementitious materials exhibit a significant damping effect on ultrasonic wave propagation together with low pulse velocity. During the curing process, ultrasonic waves, especially the nonlinear acoustic behavior of the material, are sensitive to the point at which the solid phase appears. After this point, the ultrasonic pulse velocities and signal amplitudes increase continuously. The point of solidification is of practical significance since the connectivity of the solid phase is responsible for the load-bearing capacity of the cement composite and its long-term behavior. The aim of this study is to monitor the early stages of fresh cement-paste composites during the hydration process using nonlinear elastic waves. The measurements in this work were performed using a combination of contact ultrasonic transducer and noncontact optical detection measurement device. The principle of operation of the detection device is based on the doppler effect. Using this technique, the amplitudes of harmonic vibrations of an elastic wave with a fundamental frequency propagating through the material can be assessed. This leads to the evaluation of important materials characteristics, such as the changes in internal microstructure of fresh concrete during curing, the evolution of higher order elastic contants of the material expressed in the form of nonlinear parameters, as well as the longitudinal wave velocity.