Evaluating the fire resistance potential of functionally graded ultra-high performance concrete

Relevant standards stipulating the concrete surface temperature less than 380 degrees C and the defects of UHPC prone to fire spalling both limit the direct application of UHPC in specific tunnel fire prevention occasions. In view of this, a two-layered functionally graded ultra-high performance concrete (FGUHPC) was proposed in this paper, where lightweight aggregate concrete (LWAC) serves as the fire-resistant layer and UHPC functions as the structural load-bearing layer. The results showed that LWAC can control its backfire surface temperature to meet the standard requirements under a thickness of 40 mm, and the residual mechanical properties of UHPC before 400 degrees C were comparable to those at ambient temperature. The straight shear test was adopted to evaluate the FGUHPC interfacial bond strength. The results showed that the interfacial strength of FGUHPC reaches 3.05 MPa at ambient temperature, which is close to or even slightly higher than that of homogenous LWAC. When FGUHPC was treated at 1200 degrees C for 2h, FGUHPC can still maintains its integrity with its interfacial strength 120.0 % higher than that of LWAC. Compared to the homogenous UHPC member, FGUHPC composed of 40 mm LWAC and 120 mm UHPC effectively avoided explosive spalling, and maintained the interface temperature at around 200 degrees C. After static loading failure of the post-fire FGUHPC member, the two layers of concrete still remained bonded without delamination, and the loss in load-bearing capacity was less than 20 %. Finally, the design method of FGUHPC members under elevated temperature was discussed.