Influences of high temperature action on the bonding properties between cement-based repair materials and substrates

To reveal the influence mechanism of elevated temperatures on the bonding performance between different types of cement-based repair materials and substrates, this study prepared two types of fiber reinforced cementitious composites (FRCC), namely OPC-FRCC and SAC-FRCC using Ordinary Portland cement (OPC) and Sulphate aluminium cement (SAC), respectively. Additionally, SAC mortar (SAC-M) was prepared. The research focuses on investigating the effects of repair material type, the degree of high temperature damage to the substrate, and the water content of the substrate on the interfacial shear strength. The water exchange process between fresh cement-based repair materials and heat-damaged concrete substrates was investigated using neutron radiography. Finally, the deterioration law and mechanism of the interfacial shear strength between cement-based repair materials and concrete substrates after high-temperature exposure were investigated. The results indicate that the interfacial shear strength between OPC-FRCC and water-cooled substrates is consistently lower than that with air-cooled substrates treated by the same temperature, and the former decreases with increasing substrate water saturation. When the treatment temperature of substrates rises from 105℃ to 200℃, the interfacial shear strength between substrates and SAC-based materials decreases significantly. However, once the temperature exceeds 200℃, it shows no significant further change. The interfacial shear strength between OPC-FRCC, SAC-FRCC, and SAC-M and the same types of substrates decreases in that order. During water exchange, the water absorption coefficient of substrates is enhanced with increasing treatment temperatures. After the substrate is exposed to 105℃~400℃, the water loss rates of the different repair materials are similar. Exposure of the substrate to 600℃ significantly promotes water loss from the repair materials. High-temperature exposure between 105℃ and 600℃ causes a continuous decrease in the interfacial shear strength between OPC-FRCC and the substrate. Exposure to temperatures between 60℃ and 200℃ leads to a rapid decrease in interfacial shear strength between SAC-based materials and the substrate, which is attributed to the thermal decomposition of ettringite in the SAC-based materials. To ensure effective interfacial bonding, the thermally damaged substrate should be kept dry before repair with OPC-FRCC, and concrete structures repaired with SAC-based materials should not be exposed to high-temperature environments.