Improved freeze-thaw modification of recycled concrete aggregate originally from frost resistive concrete

Air-entrained recycled concrete aggregate (AE-RCA), with strong frost resistance, cannot be effectively modified using the freeze-thaw modification that combines thermo-drying and freeze-thaw cycles. To address this issue, the modification technique was improved by introducing the pre-treatment of vacuum saturation. Subsequently, experimental investigations were conducted to validate the modified effect of the improved freeze-thaw modification. The results demonstrated that the improved method shows higher efficiency in treating AE-RCA, particularly for the aggregates with a low water-to-cement ratio. A theoretical analysis was conducted on various influential factors within the new modification method, utilizing the thick-walled spherical model and the theory of pore water freezing. The primary factors under consideration included the water-to-cement ratio, air content, natural aggregate radius, thickness of attached mortar, thermo-drying temperature, minimum temperature, and saturation method. The distribution of tangential strain in the attached mortar and the displacement at the Interfacial Transition Zone (ITZ) were chosen as comparative indices. Finally, the integration of theoretical analysis and experimental results led to the development of a predictive model capable of estimating the required number of freeze-thaw modifications under various conditions quickly.