2011 Volume 65 Issue 1 Pages 392-399
Concrete is inherently a durable materials. If concrete properly is designed for environment where it will be exposed, and if carefully produced with good quality control, concrete possesses maintenance-free performance for several decades, except in highly corrosive environment. This implies that the majority of concrete durability issues are focused on the later ages, when the concrete has gained adequate strength. However, the early age shrinkage of concrete induces cracking, which could severely decrease concrete life expectancy；the shrinkage microcracks not only affect the mechanical properties but also increase the permeability and facilitate ingress of aggressive substances such as chlorides and carbon dioxide gas. Hence, the investigation on shrinkage cracking is necessary to control early deterioration for ensuring safety of construction. Fundamentally drying shrinkage is caused by the migration of water in hardened cement paste. This leads to a conclusion that the change in water migration with various external environments, namely water adsorption-desorption isotherm, is of large importance to clarify the mechanism on drying shrinkage. This study focuses on the water adsorption-desorption isotherm of hardened cement paste. The isotherm usually shows hysteresis loop. This hysteretic behavior is often explained by the existence of constrictions or bottlenecks between pores that cause a delay in the desorption process compared to the adsorption. However, it has been reported that the hysteresis loop of isotherm is observed for regular unconected pores, which does not have constrictions and bottleneck pore. A simple phenomenological model, which is proposed by Pellenq et al. and based on a generalized Gibbs surface dividing theory for adsorption and capillary condensation/evaporation in cylindrical meso-pores, can be interpreted the hysteresis loop of isotherm for regular unconnected pore. In this study we extend the Pellenq’s model form unconnected pore to the connected complicated pore system such as hardened cement paste. As a result, it is evident that modified Pellenq’s model can assess the essential migration of water?in hardened cement paste under any ambient?environment. This will be a stepping stone to make clear the mechanism of drying shrinkage.
