土质与生物炭掺量对植被混凝土单向冻融特性的影响

[Objective] Vegetation concrete in alpine regions is prone to structural loosening and mechanical performance degradation after freeze-thaw cycles, which in turn limits the effectiveness of slope ecological restoration, while the coupled effects of soil type and biochar content on the freeze-thaw characteristics of vegetation concrete under unidirectional freeze-thaw conditions remain insufficiently understood. To address the above issues, this study investigates the effects of soil type and biochar content on the freeze-thaw characteristics of vegetation concrete, reveals the underlying mechanisms, and provides theoretical support for the optimization design of frost-resistant mix proportions in alpine regions. [Methods] Sandy soil and cohesive soil collected from Yichang were selected as planting substrates. Vegetation concrete specimens using sandy soil (VC-SS) and cohesive soil (VC-CS) were fabricated, respectively. Unidirectional freeze-thaw tests were conducted. The temperature field changes at different depths of the specimens were monitored in real time, and frost heave deformation data were collected using displacement sensors. The layered water content before and after freeze-thaw cycles was determined using the oven-drying method. The effects of soil type and biochar content on freezing temperature, frost heave amount, and water migration patterns of vegetation concrete were systematically analyzed, and the mechanisms were interpreted from the perspectives of thermal conduction, pore structure, and water transport. [Results] 1) Soil type had a significant effect on the freeze-thaw characteristics of vegetation concrete. Under the same biochar content, the freeze-thaw resistance of VC-SS was significantly better than that of VC-CS. The freezing center temperature of VC-SS was 0.2 ℃-1.8 ℃ lower than that of VC-CS, the maximum frost heave amount reduced by 5.6-7.0 mm, the water migration amount decreased by 0.2%-1.3%, and VC-SS reached the frost heave peak earlier. 2) During the freeze-thaw process, the water content of both types of specimens exhibited an “inverted C-shaped” distribution pattern. In freezing stage, water showed a unidirectional upward migration pattern from bottom to top, with the water content in the deep layer decreasing to 15.2%-19.83% and that in the shallow layer increasing to 20.2%-22.6%. In thawing stage, the water migration pattern shifted to bidirectional migration. The surface layer water content decreased by 0.05%-1.3%, the middle layer increased by 0.02%-1.4%, and the deep layer showed an overall decreasing trend of 0.4%-1%. 3) The effect of biochar content on the freeze-thaw characteristics of vegetation concrete exhibited a nonlinear pattern. With increasing biochar content, the freezing center point temperature, frost heave amount, and water migration amount of VC-SS and VC-CS all showed a trend of first decreasing and then increasing, with 0.5% being the optimal content. [Conclusion] Under unidirectional freeze-thaw conditions, sandy soil with low fine-particle content combined with 0.5% biochar content can significantly improve the freeze-thaw resistance of vegetation concrete and is an optimal scheme for mix proportion design in alpine regions. This mix proportion has relatively high thermal conductivity and low thermal insulation performance. Therefore, plant species with low-temperature germination characteristics should be selected in engineering applications to ensure the effectiveness of slope ecological restoration. The innovation of this study lies in the systematic clarification of the coupled regulatory mechanisms of soil type and biochar content on hydrothermal migration and frost heave deformation of vegetation concrete under unidirectional freeze-thaw action for the first time. This study clarifies the internal mechanisms of freeze-thaw deterioration under different mix proportions, and addresses the insufficient understanding of unidirectional freeze-thaw characteristics of vegetation concrete in alpine regions. The findings provide key theoretical support for frost-resistant design of vegetation concrete in slope ecological restoration of water conservancy and transportation engineering in alpine regions. Future studies can further investigate the evolution of geotechnical properties of vegetation concrete under freeze-thaw cycles to improve the engineering application system.