The effect of external load on internal relative humidity in concrete

In poromechanical modeling of unsaturated concrete, the effect of external load on internal relative humidity (RH) is usually neglected. In this study we present experimental measurements of RH changes in a low w/c mortar during uniaxial compressive loading. We observed an instantaneous RH increase of almost 2% RH inside sealed mortar samples at loading with a stress/strength ratio of 30%. The initial RH increase is further recovered over time while the sample is under constant load. Upon unloading, an instantaneous RH decrease of roughly the same magnitude takes place. These results confirm that the hygro-mechanical coupling is relevant for understanding concrete deformations, both quasi-immediate and possibly also volumetric creep and drying creep. The observed coupling supports the theory of local water redistribution from load-bearing water layers in hindered adsorption areas toward larger pores as a result of applied compressive stress.