PERMEABILITY OF PORTLAND CEMENT CONCRETE WITHOUT ADDITIONS
Fluid transfers across concrete are fundamentally the same as for all porous materials. Powers [4.8] has shown a relationship between permeability and capillary porosity (Fig. 4.4). Gel pores also contribute to the possibilities of transfer across cement paste, but in a very limited way. According to Powers [4.8] the water permeability of this phase is only 7×10−16 m/s. Mehta [4.
1] has likewise shown the influence of cement paste hydration and of the water/cement ratio on capillary porosity. Permeability of hardened cement paste (hcp) is practically nil for capillary porosities of less than 10%, which corresponds to overall porosity in the region of 35% [4.9]. This threshold is generally interpreted as a transitional stage between a system of
interconnected pores and a system of isolated pores. With equal overall porosity, permeability depends on pore size distribution, pore continuity and isotropy [4.10]. According to Massazza [4.11], concrete permeability depends largely on pores whose diameter exceeds 0.1 μm; an essential role is also played by their shape and connectivity. Mention is also made of the influence of the specific area of hydration products [4.12]; cement pastes with a large specific surface area can trap a large amount of water, thus slowing down the migration of humidity in pastes with a high capillary porosity [4.13].