Durability of micro-cracked UHPC under the effects of loads and environments: A state of the art review

Due to the high cement content, uneven fiber distribution and the action of external loads, Ultra-High Performance Concrete (UHPC) is prone to cracking, i.e. working with many microcracks in real engineering. This poses a great challenge to the durability of UHPC structures in harsh environments. This review summarizes the latest progress in the durability degradation of micro-cracked UHPC under the effects of service loads and corrosive environments. The results show: (1) Moderate corrosion of steel fibers in micro-cacked UHPC can temporarily enhance the tensile properties (corrosion threshold: 4–6 % for straight fibers and up to 12 % for deformed fibers). In contrast, excessive corrosion results in performance degradation. (2) The coupling effects of freeze-thaw cycles, dry-wet cycles, high temperatures, and corrosive ions substantially expedite the degradation of the matrix and fibers, especially when the crack width exceeds 0.1 mm. High temperatures further increase crack permeability and the depth of ion penetration. (3) The coupling effects of environmental and mechanical loads lead to nonlinear degradation behavior, where the combined effect surpasses the individual effects, posing a serious risk to structural integrity. Finally, considering that the durability of micro-cracked UHPC is affected by multiple factors, the direction and approach to further improve the durability of micro-cracked UHPC are given, including reducing the water-cement ratio to enhance the pore structure, selecting aggregates with compatible thermal expansion coefficients, and adding nanomaterials or supplementary cementitious materials (SCMs) such as sulfoaluminate cement (CSA) to improve matrix density and mitigate shrinkage and corrosion channels.