Effect of heat-treatment and hydrostatic loading upon the poro-elastic properties of a mortar

This contribution aims at identifying experimentally the poro-elastic properties of a cement-based material under different levels of confining pressure, and after a heat-treatment up to 400 °C. The model material used is a normalized mortar, with a (W/C) ratio of 0.5. After a given heating/cooling cycle, drained bulk modulus K_b, solid matrix bulk modulus K_s and Biot's coefficient b are measured at different confining pressure levels (with a maximum of 25 MPa).Results show that under drained conditions, mortar stress-strain relationship evolves with increasing heat-treatment temperature from linear elastic with brittle failure (up to 105 °C heat treatment) to plastic and ductile (from 200 °C and above). Plastification testifies of material degradation under gradual confining pressure. At the microstructure scale, this is attributed to thermal damage after heat treatment above 105 °C, which consists mainly in various micro-cracking. This leads to easier failure of solid skeleton bridges (or trabecules), and to pore network collapse. Concomitantly to this, at given confining pressure P_c, secant drained bulk modulus K_b decreases monotonously, for heat-treatment temperatures above 105 °C. On the opposite, at given heat-treatment temperature above 105 °C, secant drained bulk modulus K_b increases when confining pressure is increased. This testifies of a solid matrix rigidification in the elastic domain, and it is attributed to increased skeleton compactness linked with pore network collapse. This is directly attributable to heat treatment followed by confinement.At given confining pressure P_c, matrix bulk modulus K_s and Biot's coefficient b increase with heat-treatment temperature above 105 °C. The increase in b means that mortar becomes less and less cohesive and more and more of a granular nature. Moreover, Biot's coefficient b and solid matrix bulk modulus K_s are independent of confining pressure P_c for intact mortar, whereas they decrease for heat-treated mortars when P_c increases. From literature analysis alone, it was quite unexpected that after heat treatment, K_s should vary under confinement. This is interpreted as the closure, under confining pressure, of micro-void connections and of micro-cracks created by heat-treatment. Therefore, increasing confinement induces more and more occluded pores in the solid matrix, whereby K_s diminishes.