Active and passive monitoring of the early hydration process in concrete using linear and nonlinear acoustics

Microstructural changes occurring in freshly poured concrete during curing have been monitored on a laboratory scale using a combination of the Acoustic Emission (AE) Technique with linear and nonlinear ultrasonic/elastic wave spectroscopy. The AE technique is a passive ultrasonic signal recording technique capable of online monitoring the internal microstructural activity of young concrete during the hydration process. Ultrasonic wave spectroscopy is traditionally used to evaluate the material's longitudinal and shear wavespeed and attenuation properties (providing properties such as Young's Modulus of Elasticity, Poisson's Ratio and Quality factor) by means of an active excitation of a medium with pulsed sound waves. In addition to these traditional techniques, we have implemented a nonlinear version of ultrasonic wave spectroscopy which probes the nonlinear elastic properties of the microstructure (offering information about the micromechanical behaviour) through the analyses of the harmonic generation from a continuous wave transmission through the concrete sample. The evolution in the AE events, and in the linear and nonlinear ultrasonic behaviour of young concrete is analyzed as a function of the degree of hydration for various initial compositions during the first three days of the curing process. The results show a good correlation between the linear and nonlinear acoustic properties and the phase changes in the concrete due to chemical reactions and mechanical setting seen in the temperature profile.