Experimental investigation of the first desorption isotherm of a high performance concrete with thin sliced samples

The aim of present study is to determine experimentally the first desorption isotherm of a high performance concrete based on CEM I cement for long-term underground nuclear waste storage structures. Thin sliced samples are used in order to reduce the time needed to obtain the complete isotherm. To ensure the experimental representativeness, a great amount of sliced samples with thickness h = 1, 2, and 3 mm have been desaturated from two different initial saturated states till reference dry state. The analysis of size effect in kinetics mass variation for fully water-saturated samples shows that thinner samples lose mass more quickly in the first days, but the size has no clear influence on the total relative mass loss once mass stabilization is reached. An important influence of initial curing condition is observed due to the creation of new surfaces and an increase in porous connectivity. The results obtained with the present experimental technique compare well with previous works on different sample geometries. A pore network model on the basis of Kelvin-Laplace's capillary theory is implemented and shows the existence of two main desaturation modes corresponding to capillary pores and C-S-H porosity.