Microstructures and properties of concrete surfaces under different exposure conditions in complex natural environments of high-altitude regions

The quality of concrete surfaces is negatively affected by the plateau climate. The purpose of present study was to investigate the mechanism of the climate effect on the microstructures and properties of concrete surfaces under different exposure conditions. Scanning electron micro-scopy, X-ray diffraction, thermogravimetry, nitrogen adsorption porosimetry, mercury intrusion porosimetry, and microhardness tests were used for microstructure analysis. Rebound strength and abrasion resistance tests were conducted to assess the mechanical properties. Tests of sorp-tivity under various absorption conditions, chloride ion impenetrability after vacuum saturation and immersion, and deicing salt scaling with natural and standard freeze-thaw cycling, were conducted to assess the durability. The results showed that both cement hydration and micro-structure densification of the exposed concrete surfaces were lower than those of the concrete interior, leading to worse mechanical properties and durability. The rapid moisture loss, rather than the low temperature, was the dominant cause of the weak surface layer, with a depth be-tween 10 mm and 15 mm. The performance of a south-oriented surface was slightly better than that of a north-oriented surface, which was due to the stronger solar radiation for the former surface. However, the difference was so small that it could be ignored. Furthermore, the concrete sorptivity under the conditions of semi-immersion and vacuum saturation was lower in the high-altitude regions, which was due to the higher evaporation and smaller pressure difference after vacuum absorption. Thus, significant errors were expected in all tests of chloride ion impene-trability assessment conducted in high-altitude regions.