Analysis of Ultrasonic Pulse Velocity Correlations for Estimating Residual Compressive Strength of Concrete Subjected to High-Temperature Hysteresis: Influence of Various Aggregates

In this study, the compressive strength and ultrasonic pulse velocity (UPV) behaviors of concrete at high temperatures by aggregate type were comparatively analyzed in accordance with the CEN and CEB Codes. The aggregates adopted were divided into limestone, granite, river gravel, and crushed stone, and the target temperatures were set to 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, and 600 °C. The analysis items were specified as compressive strength (Fc), UPV, ratio of compressive strength (Fcratio), and UPV ratio (UPVratio); in addition, the correlations between Fcratio and UPV were analyzed and compared to the previously proposed estimation equations for concrete at room temperature. Accordingly, the Fcratio of limestone and river gravel aggregates exceeded the UPVratio at temperatures lower than 300-400 °C; however, UPVratio tended to be higher at subsequent temperatures. For granite aggregates, UPVratio was higher than Fcratio over the entire temperature range, while crushed stone aggregates exhibited higher Fcratio than UPVratio. Both Fcratio and UPVratio of granite aggregates exceeded those of limestone aggregates, while the UPVratio and Fcratio of river gravel aggregates were higher and lower than those of crushed stone aggregates, respectively. Consequently, a strength estimation equation was proposed by analyzing the correlations between Fcratio and UPV, and was inferred to be better than the existing strength estimation equations.