Influence of low vacuum and high temperature condition on moisture transport and dry shrinkage of mature concrete

Abstract

Low vacuum and high temperature typically promote the evaporation of moisture and exacerbate concrete shrinkage cracking, which highlights the issue of volume stability for concrete materials in some engineering environments (e.g. the operational environment of ultra-high-speed train in the low-vacuum tube). To evaluate its impact on moisture transport and drying shrinkage of mature concrete, different air pressure and temperature conditions were selected and the correspondent experiments of investigating moisture loss, internal relative humidity, and drying shrinkage after exposure were conducted. And the evolution of different types of moisture, hydration degree, pore structure, and morphology of matrix was analyzed through microscopic testing methods. Experimental results present that compared to the standard drying condition, the moisture loss and drying shrinkage of concrete under low vacuum and high temperature condition increased by more than 48.5 % and 77.4 % respectively. There exists a strong correlation between moisture loss and drying shrinkage of matrix after low vacuum and high temperature exposure. Besides, compared to the sole effect of low vacuum, the coupling effect of low vacuum and high temperature has a more significant impact on the moisture transport and drying shrinkage of concrete. Among them, after prolonged exposure to low vacuum and high temperature, the internal relative humidity of matrix drops to 50 %∼60 %, and the corresponding critical aperture reduces to 1.0–1.5 nm, thereby significantly increasing the shrinkage stress of concrete. Furthermore, low vacuum and high temperature condition reduces the content of capillary water, gel water, and chemically combined water, and leads to the loss of moisture in smaller pores. In addition, for mature concrete, low vacuum and high temperature condition not only impedes cement hydration but also induces the coarsening of pore structure, resulting in an increase in the number of macropores and pores around 50 nm, which correlates highly with the evolution of drying shrinkage.