Effects of vacuum exposure on the mechanical properties and microstructures of concrete with and without surface coating

Cement concrete has been used for the construction of vacuum tunnel in the hyperloop transportation system. However, concrete behavior in vacuum environment is largely unexplored. This paper carried out systematic research on the mechanical properties and microstructures of concrete with and without surface coating in vacuum environment. Compressive and flexural tensile tests were conducted on concrete cylinders exposed to vacuum for different periods. Scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and thermogravimetric analysis (TG) were employed to evaluate the effects of vacuum exposure on cement hydration and microstructural characteristics. The test results indicate that the vacuum exposure at early ages of concrete inhibits cement hydration reactions and deteriorates the microstructure and mechanical properties of concrete. However, vacuum exposure after 28 d’s curing in standard conditions had beneficial effects on concrete due to the densification of the matrix. It was also found that the surface coating significantly enhanced the flexural tensile strength of concrete in both atmospheric and vacuum environment. In vacuum environment, the surface coating effectively inhibits moisture evaporation and the formation of porous structure of hydration products, but weakens the positive effects of later age-vacuum exposure on concrete. It was concluded that both vacuum exposure and surface coatings had dual effects on the mechanical properties and microstructures of concrete.