Determination of initial degree of hydration for improvement of early-age properties of concrete using ultrasonic wave propagation

During the first few hours after mixing, the properties of concrete change between different types of material behaviour. Fresh concrete is during mixing a Bingham material, gradually attaining solid body properties with considerable compressive strength and stiffness. The development of mechanical properties can be described by the degree of hydration. For the prediction of mechanical properties of early-age concrete as well as for the prediction of stresses caused by differences of temperature and autogenous shrinkage, it is essential to know the initial degree of hydration, from which on the development of strength and stiffness can be assumes to begin. This paper deals with the determination of the end of the dormant phase by using ultrasonic pulse velocity techniques. Using compression wave and shear wave transducers the hardening of concrete is observed under adiabatic curing conditions. From the development of dynamic Young’s modulus and Poisson’s ratio a model of the initial degree of hydration is derived to improve existing models of the development of tensile strength and modulus of elasticity for very early-age concrete. A procedure to determinate an upper and lower bound for the end of setting time is presented. Typical results are presented for different concrete compositions, especially for high strength concrete.