Measurement and prediction of shotcrete degradation in thermal environments: A mechanistically supported RSM approach

ABSTRACT The performance degradation of shotcrete in harsh thermal environments poses a significant challenge to structural integrity. To quantify the coupled effects of environmental temperature and relative humidity on this degradation, this study employs Response Surface Methodology (RSM) based on a Central Composite Design (CCD). Compressive strength and Ultrasonic Pulse Velocity (UPV) were selected as response indicators to monitor mechanical deterioration. Numerical regression models were constructed and validated via Analysis of Variance (ANOVA) and residual analysis, revealing significant interactive effects between temperature and humidity. Optimization analysis using the Desirability function pinpointed the critical environmental conditions causing maximum degradation: 64% relative humidity and 80°C. Subsequent confirmatory experiments yielded relative errors of only 8.02% for compressive strength and 3.88% for UPV, demonstrating the model’s reliable predictive capability. Furthermore, integrated multi-scale characterizations—including SEM, MIP, quantitative XRD/TGA, internal moisture tracking, and long-term permeability tests—elucidated the physical and chemical mechanisms of this degradation. The results revealed that a steep internal moisture gradient at 64% RH triggers a detrimental “tug-of-war” mechanism: moisture-driven AFt recrystallization generates outward expansion, while simultaneous C-S-H depletion and drying shrinkage induce inward tension. This superimposition causes severe micro-cracking and pore coarsening, creating irreversible transport pathways that compromise long-term performance. These findings establish a robust framework integrating macroscopic prediction with microstructural observation, providing a validated methodology for performance assessment in geothermal engineering applications.