Field-Calibrated Degradation Kinetics of Steel Fiber-Reinforced Shotcrete in Humid Underground Silver Mines

Abstract

Typical specifications for steel fiber-reinforced shotcrete primarily focus on early-age mechanical properties and do not provide much guidance on evaluating changes in capacity over time, particularly under humid underground service conditions. In addition, accelerated laboratory durability tests are rarely calibrated against measurements obtained from underground structures in service. This study addressed that gap with a 12-month dual-track approach: (i) maintaining controlled near-saturated conditions in the laboratory at 23 ± 2 °C and 95–100% relative humidity, with subsequent evaluation of the mechanical properties (compressive strength, splitting tensile strength, and single-fiber pull-out resistance) at 0, 3, 6, 9, and 12 months; and (ii) monitoring the compressive strength in three operating underground silver mines in Zacatecas, Mexico, using the mean 48 h production-control strength as the field reference and cores taken after approximately 12 months of service. The three laboratory trajectories were well described by first-order exponential models (R2 ≥ 0.99) throughout the 12-month monitoring period. The observed reductions were 22.0% for compressive strength, 21.4% for splitting tensile strength, and 26.3% for single-fiber pull-out resistance. The apparent pull-out rate constant was approximately 18% higher than the compressive-strength rate constant, though the mechanism for this difference was not identified independently. In the field, there was an apparent reduction in compressive strength of 8–10% after about 12 months. Comparison of the chamber and field compressive-strength rates produced an apparent acceleration factor, AF ≈ 2.7, with a per-mine range of 2.4–3.0. The high pairwise correlations among the three laboratory properties (r ≥ 0.996) and the first PCA component, which explained 99.8% of their standardized trajectory variance, reflected closely aligned temporal trends. However, since these were based on five exposure-age means, they should be considered only exploratory evidence of co-variation rather than causation. MANOVA demonstrated significant multivariate effects on the combined compressive and splitting tensile responses across exposure ages (p < 0.001). The proposed acceleration factor is preliminary and restricted to the materials, sites, exposure context, and observation period studied. Routine compressive-strength core testing may be useful as a practical screening indicator, but it cannot quantitatively replace direct bond or post-cracking evaluation.