Experimental and theoretical studies on postfire behavior of functionally graded ultra-high performance concrete

In order to improve structural fire-resistant behaviors, this paper designed a two-layer functionally graded ultra-high performance concrete (FGUHPC) structure composed of a UHPC layer and a lightweight aggregate concrete (LWAC) layer. UHPC layers are adopted to provide structural bearing capacity and protected by LWAC layers from elevated temperature. Splitting tensile tests and three-point flexural tests were conducted under ambient and elevated temperatures to evaluate interfacial bond performance and flexural bearing capacity, where two interfacial treatments were adopted and compared. The experimental results revealed that FGUHPC members exhibited good integrity during heating, no explosive spalling occurred and the maximal temperature at interfacial regions was 266°C. The interfaces showed desirable bond performance under ambient temperature while the splitting tensile strength was decreased by around 85% in the case of high temperature. Flexural test results indicated that the structural stiffness would be reduced by around 42% under elevated temperature, as a result, the maximal deflection was increased from 2.5 mm to 3.7 mm. SWM could significantly improve interfacial bond performance and prevent debonding failure of specimens at the postfire state, leading to higher structural bearing capacities. The bearing capacities of specimens with and without interfacial treatments were 42.7 kN and 38.4 kN respectively under ambient temperature, which remained about 88% after elevated temperature.