Residual Interface Adhesion Properties between Lightweight Engineered Cementitious Composites and Concrete after Elevated Temperatures

Lightweight engineered cementitious composites (LECCs) can provide many advantages compared with engineered cementitious composites (ECCs), for instance, high strength-to-weight ratio, low resilience, good thermal insulation, and energy saving for buildings. Consequently, LECC is gaining increasing popularity in reinforcement engineering. As the temperature rises, the mechanical properties of LECC deteriorate, leading to a decrease in its bond strength with concrete. A LECC with good thermal insulation properties was developed in this study, and its residual bonding properties with concrete after elevated temperatures exposure were investigated. The LECC shows strain-hardening characteristics and high thermal insulation properties, with an apparent density of 1,556 kg/m3, compressive strength of 36.2 MPa, a tensile strain capacity of 7.3% under standard curing conditions, and thermal conductivity of 0.69 W/(m·K), much lower than the traditional ECC of 1.3–1.5 W/(m·K). LECC–concrete composite specimens were subjected to split tension as well as direct shear tests at the LECC–concrete interface after specific heating-cooling procedures (The gradient increases from 20°C to 600°C and then cools naturally). Different damage modes between specimens after split tension and direct shear tests were compared and analyzed. The effect of temperatures and LECC matrix strength on the residual interfacial bonding performance was investigated. The experimental results show that low-density LECC has better bonding strength at room temperature, and has good integrity and bonding with concrete after elevated temperatures. The relationship between the residual compressive strength of LECC matrix after elevated temperatures and the residual direct shear strength and split tensile strength of LECC–concrete interface is established, which provides a test basis for the performance loss evaluation of LECC–concrete interface after fire.