Water vapour sorption experiments on hardened cementitious materials Part I: Essential tool for analysis of hygral behaviour and its relation to pore structure

摘要

This paper forms the first part of a series. In this first part, a broad range of normal and high-performance (HP) hardened cement pastes and concretes is studied under both laboratory and in-situ conditions. Water vapour desorption-adsorption experiments are carried out by means of the saturated salt solution method on very thin specimens. The effect of various parameters and in particular of the mix-composition is studied on the so-called water vapour sorption isotherms (WVSIs). It is found that the presence of aggregates does not influence the curves. Likewise, the peculiarities of HP materials are highlighted: within the high relative humidity (RH) range, significant lower water contents are measured than for normal materials, and large RH changes induce only slight variations of water content. More generally, desorption isotherms can be partitioned in several ranges, which are influenced or not by W/C: a unique partition is pointed out, valid for every material tested. Moreover, the hysteretic behaviour of the materials is investigated by plotting scanning isotherms within various RH ranges and by performing first and second desorption-adsorption cycles. Furthermore, a pore structure analysis is carried out from WVSIs. The bulk porosity accessible to water, the C-S-H "gel" porosity, the BET specific surface area, and the BJH pore size distribution are thus assessed, along with the C-S-H "gel" amount. In addition, a master curve is exhibited as regards the average adsorbed water layer thickness vs. RH, for RH< =63.2 percent. Moisture profiles in structural elements exposed to various drying conditions in laboratory and in natural environments (RC test specimens and bridge deck) are assessed by means of gamma-ray attenuation measurements. A perfect consistence is pointed out between the water contents measured by this technique on 70- to 150-mm thick cylinders and those provided by the desorption experiments previously mentioned (on 1 - to 3 -mm thick specimens). The very similar moisture profiles and the high degrees of liquid water saturation recorded in HP materials, whatever the mixture, the age and the environmental conditions, confirm that these materials are weakly sensitive to environmental hygral changes within a broad RH range.