About drying effects and poro-mechanical behaviour of mortars

The drying of mortar and concrete generally affects their elastic properties and strength. Such property variations are of interest, since they affect the durability of structures designed with these materials. Previous studies, conducted on two mortars with different W/C ratio, showed that drying such material led to decreases in their elastic modulus and their Poisson ratio. Although generally attributed to micro-cracking, these decreases are nevertheless consistent with simple poro-mechanical effects i.e. transition from undrained behaviour toward drained behaviour. The main objective of this experimental study was therefore to state whether or not pore fluid pressure played a direct role in the variation of elastic properties measured on saturated or dried material. Poro-mechanical measurements of drained and undrained bulk moduli, Biot and Skempton coefficients were carried out. They actually revealed that poro-mechanical coupling effects could explain a part of the differences observed between elastic properties. Ethanol was chosen as the pore fluid to avoid possible unexpected chemical effects on the materials tested (often observed with water). To conclude about the saturation effects, a complementary experimental procedure was conducted in two steps: compression tests were performed on previously dried samples, which were then re-saturated with ethanol before being mechanically tested again. The results obtained are unambiguous and showed that poro-mechanical effects are not involved in the change in elastic properties. On an other hand, the major roles played by microcracks induced by drying and by capillary suction are highlighted: (a) The decrease in elastic properties, observed as drying occurs, is shown to be due to induced micro-cracking; (b) the increase in capillary pressure, brought about by drying, leads to an increase in the mortar failure strength.