A percolation-threshold model for evaluating permeability in Portland pastes with high water-cement ratio

In a perspective of improved sustainability, the definition of policies for the management of the heritage of concrete buildings and infrastructures, aiming at the suitable mitigation of the harm to environment and at the reduction of societal costs, requires reliable methodologies to estimate the service-life and maintenance periodicity time for a given class of concrete structures. Concrete watertightness is a basic property for assessing durability of the built heritage to weathering exposure and is mainly dependent on permeability of the cement paste embedding the aggregates which typically have much lower permeability in an ordinary concrete. Fundamental research of the past century has shown that a reliable experimental determination of permeability in Portland concretes and pastes can be achieved in terms of intrinsic permeability by uniaxial water-permeation tests under open-flow conditions. Powers and Brownyard have early assessed in 1947 the ability of the Kozeny-Carman relation in providing predictions of intrinsic permeability of Portland cement pastes, although achieving predictivity only below a water-cement ratio (w/c) of about 0.5. Above such threshold, as they observe, their model is no longer predictive since permeation through the capillary space formed due to excess water becomes predominant over permeation through the gel-space. In the present study an enhancement of Powers and Brownyard's canonical model is proposed by addressing capillary percolation and the possibility of simultaneous flow at the gel and capillary scales. The onset of capillary percolation for w/c slightly above 0.5 is naturally predicted. Agreement of the proposed model is assessed against experimental results and discussed in relation to optimal management of existing RC constructions with a view towards sustainability.