Relative Ultrasonic Pulse Velocity-Based Prediction of Residual Compressive Strength in Thermally Damaged Loess-Substituted Concrete with Different Target Strengths

This study investigated the elevated-temperature deterioration of loess-containing concrete with different target strengths and evaluated ultrasonic-pulse-velocity (UPV)-based models for predicting residual compressive strength. Six mixtures combining target strengths of 30 and 45 MPa with loess replacement levels of 0, 15, and 30% were exposed to 23, 100, 200, 300, 500, and 700 °C. The dataset comprised 108 individual measurements representing 36 mixture-temperature conditions. Bulk density, UPV, and compressive strength were measured after natural cooling. Two normalization schemes were distinguished: a normal-concrete-based relative performance index, which retains both the initial penalty caused by loess replacement and subsequent thermal deterioration, and a mixture-specific residual ratio referenced to the initial value of each mixture. Experimental variability was quantified using standard deviations, coefficients of variation, and 95% confidence intervals. The effects of target strength, loess replacement, and temperature were examined using three-way ANOVA and Kruskal–Wallis tests. In addition, leakage-free condition-wise group cross-validation was performed so that the three replicates from each mixture-temperature condition were never divided between training and validation sets. UPV and compressive strength decreased markedly between 300 and 500 °C. Absolute compressive strength was significantly affected by all three factors, whereas exposure temperature was the dominant main effect for the mixture-specific residual strength ratio. Under condition-wise cross-validation, the normal-concrete-based relative model retained R2 = 0.906, MAPE = 10.81%, and MPE = 0.60%, while the mixture-specific residual-ratio model achieved R2 = 0.957 and MAPE = 7.94%. The proposed models are therefore suitable as preliminary screening-level tools within the investigated material and temperature ranges, but not as stand-alone bases for final structural safety decisions.