The interfacial bonding performance of engineered cementitious composites (ECC) and existing concrete exposed to high temperature: Effects of exposure temperature, substrate moisture, and curing conditions

Engineered cementitious composites (ECCs), renowned for their crack resistance and tensile properties, are extensively employed in restoration work. However, research on the interfacial bonding properties between ECC and existing concrete exposed to high temperatures (ECC-HTC) is limited. Factors such as exposure temperature, substrate moisture, and curing conditions significantly influence the properties of ECC-HTC. This paper investigates the effects of these factors on the macro-mechanical properties, microstructure, and physical phase changes of ECC-HTC. This paper reveals that elevated exposure temperatures from 20 degrees C to 600 degrees C increase substrate damage, reducing compressive strength from 50.9 MPa to 29.9 MPa, increasing interfacial porosity from 2.7 % to 8 %, and increasing pore structure complexity, with the fractal dimension rising from 1.11 to 1.3. This results in a decrease in interfacial bonding strength from 3.48 MPa to 2.54 MPa. High substrate humidity adversely affects bonding strength at 20 degrees C due to increased interfacial porosity from a higher water-cement ratio. Conversely, at 600 degrees C, it favors the formation of C-S-H, enhancing bonding strength. Water-cured (20 degrees C water soaking) substrates exposed to 600 degrees C improve interfacial bonding strength by 0.3 MPa compared to those subjected to standard condition curing (20 degrees C, RH > 95 %). This is attributed to the moisture availability, which favors the formation of C-S-H with a lower Ca/Si ratio in the interfacial transition zone (ITZ). Better pre- curing conditions help recover strength of substrate exposed to high temperatures but do not improve interfacial bond strength because healing of substrate pores and cracks hinders ECC hydration product penetration. These findings offer valuable theoretical insights and practical guidance for repairing and reinforcing fire-damaged concrete structures.