Macro steel fiber effect on water permeability of concrete under coupled freeze-thaw cycles and compressive loading

Abstract

The durability and damage mechanisms of concrete in cold regions are influenced by multiple coupled factors, including freeze-thaw (FT) cycles, mechanical loading, and moisture penetration. Water permeability directly plays a decisive role in durability, as water acts as the primary medium for aggressive agents to penetrate the matrix. This study investigates the water permeability of plain concrete (PC) and steel fiber reinforced concrete (SFRC) under compressive stress, both with and without exposure to FT cycles. The results show that the evolution of permeability with stress can be divided into three stages: a slight decrease, a slight increase, and a sharp increase. FT cycles substantially elevate permeability; for PC, the initial permeability rises from 2.99 × 10⁻¹¹ to 1.88 × 10⁻⁶ m/s, while the ultimate permeability increases 7.15 × 10⁻⁹ to 3.18 × 10⁻⁵ m/s after 100 FT cycles. Although steel fibers increase the initial permeability at 0 FT cycles (e.g., SFRC60 is 137% higher than PC), their beneficial effect becomes increasingly evident as the number of FT cycles increases. After 100 FT cycles, the initial and ultimate permeability of SFRC60 decrease by 71.6% and 97%, respectively, compared with those of PC. Moreover, under high stress levels (stress level > 0.65, where stress level is defined as the ratio of applied compressive stress to the peak compressive stress of each specimen), SFRC consistently exhibits lower permeability than PC, with the difference becoming more evident as the number of FT cycles increases. These findings demonstrate that steel fibers significantly enhance the frost resistance and overall durability of concrete under the combined actions of compressive stress and FT cycling.