Moisture transport in concrete

After presenting the fundamentals in the previous chapters constituting Part I, we begin here Part II dealing with advanced topics. In this chapter, we study concrete as porous material, the mechanical behavior of which is strongly affected by the presence and migration of moisture (i.e., various phases of water) through the pore space. We present the basic concepts and equations characterizing the moisture transport under isothermal conditions. We discuss the thermodynamic aspects and briefly describe various transport mechanisms. Then, we focus attention on relatively simple models with a limited number of parameters, particularly on the classical Bažant–Najjar model, which can be effectively used in practical applications and is recommended in design codes. After deriving a nonlinear moisture diffusion equation, we study various problems of practical interest by combining analytical and numerical techniques. The cases we cover include drying of a slab or half-space (under constant or variable ambient humidity), steady flux of moisture through a wall, and spreading of a hydraulic pressure front into unsaturated or self-desiccated concrete. The link between moisture transport and shrinkage is also discussed. Finally, we briefly comment on the changes required to take into account the effects of self-desiccation and autogenous shrinkage and outline the diffusion processes affecting the alkali–silica reactions (ASR).