Fire resistance of 3D printed ultra-high performance concrete panels

Ultra-high performance fiber-reinforced concrete (UHPFRC) is highly suitable for 3D concrete printing (3DCP) due to its high flexural strength, thereby reducing the need for reinforcements. However, UHPFRC is susceptible to spalling under exposure to fire, limiting its application as structural members. In this paper, the effect of 3D printing process on the fire behavior of UHPFRC is studied, benchmarking against mold-cast panels. The insulation, integrity, and structural adequacy of the panels are investigated using the heat transfer mechanisms, failure modes, and the post-fire compressive strength of the panels, respectively. The presence of interlayers reduced the spalling of UHPFRC under fire and improved the structural integrity of 3D printed specimens compared to mold-cast specimens. Further, the addition of 0.5 % polypropylene (PP) fibers eliminated the interlayer delamination and spalling in 3D printed UHPFRC panels. Similarly, 3D printed panels showed improved structural adequacy than mold-cast specimens. The residual compressive strength of 3D printed UHPFRC panels after being exposed to fire was observed to be above 50 % of the initial mean compressive strength. However, the insulation property of 3D printed panels was reduced compared to that of the mold-cast counterparts due to the high rate of heat transfer via the porous interlayers. The addition of PP fibers improved the insulation resistance of the interlayer region and surface of the 3D printed panels. The strength anisotropy of the 3D printed UHPFRC reduced significantly following the fire exposure. Further, the thermal bowing of 3D printed panels was higher with an increased dosage of PP fibers due to the increase in the thermal strain of UHPFRC. Therefore, adequate care must be given in the design of 3D printed structures for potential fire resistant wall, slab, and façade elements.