Temporal and spatial variations of mechanical performance and microstructure of mature concrete under long-term low vacuum condition

Abstract

Rapid moisture loss under low vacuum condition (LVC) would alter the microstructure of concrete materials, presenting substantial challenge to their long-term mechanical performance and receiving increasing attention. In this study, a two-year exposure experiment was conducted to investigate the spatiotemporal effects of long-term LVC on the static and dynamic mechanical properties, phase composition, multi-scale pore structure, and interfacial transition zone (ITZ) of mature concrete cured for 28 days. Experimental results show that compared to normal air condition (NAC), short-term (1 month) LVC exposure initially enhances the compressive strength of mature concrete, whereas prolonged LVC exposure leads to a decline of 2.4∼10.4 % in compressive strength and 5.0∼22.1 % in flexural strength. Correspondingly, compared to NAC, the ultimate strain and impact toughness of concrete after long-term LVC exposure are reduced by as much as 35.2 % and 36.8 %, respectively. Moreover, LVC does not alter the type of hydration products, but it inhibits further hydration. This inhibition not only leads to an increase in mesoscopic pores of 750–1100 μm but also results in an increase in the total porosity and the most probable aperture at the micro-nano scale. Furthermore, prolonged LVC exposure produces a looser morphology of the ITZ between the coarse aggregate and paste, along with a reduction in indentation modulus and an increase in thickness by 10μm. Additionally, in space terms, the adverse effect of prolonged LVC on strength and microstructure of mature concrete significantly decreases with increasing exposure depth. Lastly, a physical model based on dynamic moisture loss is proposed to explain the microstructure changes in the matrix after LVC exposure, offering deeper insights into the spatiotemporal evolution mechanism of mature concrete’s mechanical performance under LVC.