Study on the pore structure and salt freeze-thaw resistance of the surface layer and interior of concrete for bridge expansion joints in cold climatic areas

In cold climatic areas, bridge expansion joint concrete is susceptible to damage from freeze-thaw (F-T) cycles and chloride-salt erosion. This paper investigates the differences between the surface layer of concrete (C-S) and the interior of concrete (C-I) under various curing conditions, focusing on pore structure characteristics. The salt freeze-thaw (sF-T) resistance of the C-S and C-I was analyzed using the surface scaling, mass growth rate, and apparent chloride diffusion coefficient after one-sided sF-T cycles (Dapp′). The results indicate that the pore structure of the C-S was more intricate than that of the C-I. However, the paste structure of the C-I was denser. The fractal dimension (DF) of the C-I was lower than that of the C-S under different curing conditions, with a reduction of approximately 10 %. Additionally, it was observed that the C-I exhibited significantly higher sF-T resistance than the C-S. To obtain a more precise evaluation of the concrete's sF-T resistance, it is recommended to remove 2 mm from the edges of the specimen. Furthermore, the Dapp′ is a more reliable indicator of concrete's sF-T resistance when the F-T medium is a chloride-salt solution. The Dapp′ of the C-I decreased by 26.8 %, 33.2 %, and 49.9 % under the curing conditions of 20℃-95 %RH, 5℃-70 %RH, and 0℃-50 %RH, respectively, when using concrete specimens with 2 mm of edges removed, compared to the C-S. The study results can serve as a fundamental basis for designing and constructing of concrete for bridge expansion joints in cold climatic areas.