Application of elastic waves for the analysis of setting and hardening in cement composites

Concrete setting and hardening processes are the most critical phases during construction works, influencing properties of a concrete structure. The point at which final set occurs is important since it provides an estimate when the development of concrete strength and stiffness starts. Monitoring strength development enables us to determine time when the structure is ready to carry loads so that removal of formwork or prestressing can be done without the risk of collapse, cracking or large deformations. In this research ultrasonic measurements are used to characterize a hydration process of cement based materials. Measurements are performed on concrete with different cement types and varying water-cement ratios. Ultrasonic pulse velocity method and acousto-ultrasonic method are used in the analysis. Complementary techniques used to characterize setting and hardening are Proctor penetration resistance method and compressive strength tests. A new approach in determining initial setting time of concrete is presented. It is shown that stress wave factor amplitude can be used in monitoring hardening of concrete.