Simulation of moisture transport in hydrating cement-based overlay systems

Drying of cement-based overlay systems is a critical issue, because it causes differential shrinkage between the overlay material and the concrete substrate and may induce cracking or debonding of the overlay material. In this paper the mechanisms of moisture transport in hydrating cement-based overlay systems are studied. A model is proposed for simulating the moisture transport. A parameter study has been conducted to quantitatively investigate the influence of the thickness of the overlay material and the curing conditions on hydration of the overlay materials. The evolution of the moisture profile in the overlay system and the development of the degree of hydration (DOH) of the overlay material have been calculated. The change of water content in the overlay material is investigated, in terms of the water absorbed by the substrate, the water consumed by hydration of the overlay material and the water evaporated to the environment. The simulation results show that the water evaporation is a dominant factor that causes water loss of the overlay material, while the water absorption by the substrate plays only a minor role. Moist curing is much more effective than sealed curing (e.g. by using sealing agent) for hydration of the overlay material. The DOH of the overlay material is significantly increased with longer moist curing. Under the same curing condition (e.g. moist curing + drying), thinner overlay materials are more vulnerable to water loss and exhibit a lower DOH. It suggests that for proper hydration of cement-based overlay materials, moist curing is recommended rather than applying a sealing agent.