The western region of China is characterized by high altitude, significant diurnal temperature variations, and extensive saline soil distribution. Under the coupled effect of freeze-thaw cycles and salt attack, the service life of local concrete structures decreases significantly, resulting in substantial economic losses. Although steel fiber-reinforced ultra-high performance concrete (UHPC) exhibits an enhanced freeze-thaw resistance, it faces a challenge of underutilization of fiber strength due to the limitations imposed by fiber-matrix interface degradation and inherent matrix risks. In this study, ethylenediaminetetraacetic acid (EDTA) treated steel fibers replaced conventional steel fibers to enhance the mechanical properties and freeze-thaw resistance of UHPC via improving the fiber-concrete matrix interface performance.

In this study, steel fibers were modified using the EDTA method. First, 0.05 mol/L CaCl₂ and 0.05 mol/L Na₂CO₃ solutions were prepared separately, and 1% EDTA was added to Na₂CO₃ solution. Subsequently, the steel fibers were immersed in a mixed solution of CaCl₂, Na₂CO₃, and EDTA for 6 h. Finally, the fibers were washed with distilled water. Based on the measured concentrations of SO₄^2- and Cl- in saline soil from Golmud, this study employed a 133.6 g/L NaCl and 19.5 g/L MgSO₄ solutions to simulate the corrosive environment for freeze-thaw tests. This study investigated the effect of combined salt attack-freeze-thaw cycles on the fiber-matrix interfacial bond strength, pull-out energy, relative dynamic elastic modulus, mass loss ratio, compressive strength, and flexural strength of UHPC. The phase composition and microstructure were characterized by X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM), revealing the microscopic mechanisms of performance degradation. In addition, a freeze-thaw damage model for UHPC was also established based on the two-parameter Weibull distribution function.

The EDTA modification induces a longitudinal exfoliation on steel fiber surfaces, resulting in a significantly increased roughness. The interfacial bond strength and pull-out energy are enhanced by 22.2% and 66.7%, respectively. Correspondingly, the compressive strength and flexural strength of UHPC are improved by 2.5% and 12%. The enhanced fiber-matrix interfacial bond strength and pull-out energy increase the ultimate tensile stress required for unstable propagation of microcracks in UHPC, effectively inhibiting the initiation and development of microcracks. This mechanism thereby enhances its freeze-thaw resistance. After 500 cycles of water-freezing and salt solution freezing tests, the EDTA-treated steel fiber-reinforced UHPC demonstrates significantly superior performance, compared to conventional steel fiber UHPC in all critical metrics i.e., interfacial bond strength, pull-out energy, compressive strength, flexural strength, relative dynamic elastic modulus, mass loss ratio, and porosity. After 500 freeze-thaw cycles in water, the bond strength of plain steel fiber UHPC and modified steel fiber UHPC are decreased by 40.4% and 27.8%, respectively. Their pullout energy are declined by 29.7% and 50.6%, respectively. The relative dynamic elastic modulus are decreased by 5.26% and 4.57%, respectively. The masses are reduced by 0.46% and 0.45%, respectively. The compressive strengths are decreased by 16.6% and 16%, respectively, and the flexural strengths are decreased by 26.1% and 20.6%, respectively. After 500 salt solution freeze-thaw cycles, the bond strength of plain steel fiber UHPC and modified steel fiber UHPC are decreased by 41.3% and 39.3%, respectively. The pullout energies are declined by 40.5% and 54.2%, respectively. The relative dynamic elastic modulus are dropped by 5.44% and 5.25%. The masses are reduced by 0.5% and 0.48%. The compressive strengths are decreased by 21.4% and 20.3%, respectively, and the flexural strengths are decreased by 29.3% and 25.2%, respectively. The XRD patterns reveal that after 500 salt solution freeze-thaw cycles, some expansive products such as ettringite and gypsum are formed within the UHPC. The expansive stresses generated by these products induce crack initiation and propagation, thereby degrading the performance of the UHPC. MIP indicates that freeze-thaw cycles result in an increased porosity in UHPC. In both water-freezing and salt solution freeze-thaw environments, the porosity of modified steel fiber UHPC is lower than that of plain steel fiber UHPC. SEM images reveal that during debonding of modified steel fibers from the matrix, a greater amount of weak matrix material adhering to the fiber surfaces is carried away. This phenomenon demonstrates that modified steel fibers establish a stronger interfacial bonding with the cementitious matrix. Based on the predictions by the Weibull model, the critical freeze-thaw cycles for EDTA-treated and conventional steel fiber-reinforced UHPC are predicted to be 1610 and 1270 cycles under water-freezing exposure, and 1220 and 1150 cycles under salt solution-freezing exposure, respectively. The results indicate that in the both environments, fiber modification treatment can effectively enhance the freeze-thaw resistance of UHPC and significantly improve its durability.

The results demonstrated that EDTA modification treatment enhanced the strength and fiber-matrix interfacial bonding performance of UHPC, inhibited micro-crack propagation, thereby slowing the deterioration rate of properties such as relative dynamic elastic modulus and strength during freeze-thaw cycles, reduced mass loss, and improved the freeze-thaw resistance of UHPC. The freeze-thaw damage model predicted that EDTA-modified steel fiber UHPC could have a 26.8% increase in freeze-thaw cycle resistance under water-freezing conditions and a 6.1% improvement under salt solution-freezing environments, compared to conventional steel fiber UHPC. This study could offer a reference for the durability de-sign of UHPC structures in cold saline soil regions of western China.