Stray current leakage poses a significant threat to the service life of underground electrified engineering structures. Although ultra-high-performance concrete (UHPC) exhibits superior durability, compared to ordinary concrete, the corrosion mechanism of internal steel fibers induced by stray current remains unclear, thus restricting its broader application in underground environments. Under the influence of stray current, chloride ion transport in UHPC is governed by the combined effects of free diffusion, electromigration, and changes in pore structure caused by steel fiber corrosion. The mechanisms of free diffusion and electromigration are extensively studied, but the influence of steel fiber corrosion on chloride ion transport is not thoroughly investigated. In particular, the impact of corrosion-induced microstructural changes on chloride migration is not quantitatively assessed, and the micro-morphological characteristics of steel fiber corrosion under stray current are not fully revealed.

The materials used included Portland cement (P·II 52.5), silica fume (particle size: 0.1-0.2 μm), fly ash (Type F), quartz powder (particle size: 20-100 μm), quartz sand (particle size: 450-900 μm), a superplasticizer, and steel fibers. The steel fibers were added at a volume fraction of 2%, with a length of 8 mm and a diameter of 0.12 mm. The microstructural characteristics of steel fiber corrosion and the chloride ion transport mechanism in UHPC under the influence of stray current were determined by rapid chloride migration test (RCM), water absorption test, atomic force microscopy (AFM), and X-ray computed tomography (X-CT), respectively. The electrochemical corrosion of UHPC was simulated via the RCM test, and the changes in effective porosity after corrosion were determined via the water absorption test. The micro-morphological features of corroded steel fibers were characterized by AFM, and the corrosion-induced microcrack propagation and changes in porosity within the UHPC matrix were analyzed by X-CT.

The corrosion pits on the surface of the steel fibers are primarily distributed in the range of -250 to 200 nm at an external voltage of 30 V. As the voltage increases to 50 V, the pit depth expands to -430 to 700 nm, and it further extends to-580 to 770 nm at 70 V. Correspondingly, the effective porosity of UHPC increases from 0.50% to 0.58% at 30 V, from 0.60% to 1.31% at 50 V, and from 0.58% to 1.88% at 70 V.

The expansion of corroded steel fibers induces the formation and propagation of microcracks within the UHPC matrix, leading to an increase in apparent porosity. At different applied voltages of 30 V, 50 V, and 70 V, the apparent porosity increases to 1.49-1.95%, 1.50-2.43%, and 1.47-2.96%, respectively. The effective chloride diffusion coefficient measured is 3.68×10^-12 m²/s at 30 V, increases to 9.11×10^-12 m²/s at 50 V, and further increases to 28.3×10^-12 m²/s at 70 V. These results demonstrate that stray current significantly accelerates the chloride ion transport in UHPC. Neglecting the effect of porosity variation can result in a significant overestimation of the service performance of UHPC structures within their intended design life.

Stray current induced changes in the micro-morphology of steel fibers, with maximum pit depths concentrated in the range of -580 nm to 770 nm. The corrosion became more severe closer to the exposed surface, potentially causing a fracture of the steel fibers and a complete loss of their load-bearing capacity. The results of water absorption tests indicated that stray current increased the effective porosity of UHPC. This increase could be primarily attributed to the expansion of corroded steel fibers, leading to microcrack propagation within the UHPC matrix. The apparent porosity caused by steel fiber corrosion was quantified. In specimens unaffected by stray current, the apparent porosity ranged from 1.47% to 1.56%. A higher applied voltage led to more significant changes in apparent porosity.