ABSTRACT
Cement hydration and particles flocculation are responsible for concretes solidification. Several physiochemical mechanisms are activated from the moment that cement comes in contact with water until the material becomes solid. Autogenous shrinkage consists of chemical shrinkage and self-desiccation. Chemical shrinkage occurs because the hydrated materials occupy less space than the unhydrated constituent materials. Shortage in water due to low w/b ratio increases self-desiccation. Self-desiccation is governed by two physical mechanisms. The reduction of disjoining pressure will cause the material to contract, whereas, the meniscus formation within the pore will cause tension in the pores fluid (Laplace’s Law). The disjoining pressure will be greater when the Relative Humidity (RH) is high and therefore a drop in RH will reduce the disjoining pressure causing shrinkage. The availability of water within the pores will prevent RH drop and disjoining pressure reduction. Pores partially filled with water within the materials structure cause the creation of water-air menisci and RH drop (Kelvin’s Law). The water-air menisci will result in tension stress development within the pore fluid and consequently the surrounding material will develop compressive forces to restore equilibrium. The surrounding compressive stress will lead to further contraction.
