What Special Designs Are Required to Achieve UHPC's Fire Resistance?

The fire resistance of UHPC is a key consideration when it is applied to building structures. Its challenges mainly stem from the fact that its high density is prone to causing "explosive spalling" at high temperatures, as well as high temperatures degrading the bond between steel fibers and the matrix and deteriorating its mechanical properties. Through special design strategies such as "anti-spalling design, high-temperature fiber selection, optimized protective layers, and microstructure design", Qinglong Group has significantly improved the fire resistance of UHPC, broadening its application in scenarios with fire protection requirements.

Preventing high-temperature spalling is the primary task of fire-resistant design. The extremely low permeability of UHPC means that under high fire temperatures, the steam pressure generated by vaporization of internal moisture cannot escape quickly, which can easily lead to sudden collapse of the surface layer or even deeper layers. Qinglong's solution is to add a small amount of low-melting-point organic fibers, such as polypropylene (PP) fibers. When the temperature rises to their melting point (approximately 160-170°C), the fibers melt and form a fine interconnected network in the matrix, providing relief channels for the steam pressure and thereby effectively preventing spalling. This is a key technical measure for improving the fire safety of UHPC.

Addressing the degradation of mechanical properties at high temperatures requires a multi-pronged approach. On one hand, when steel fibers must be used, their anchoring performance at high temperatures can be improved through surface treatment (such as copper plating or shaped end hooks). On the other hand, in areas with extremely high fire resistance requirements, high-temperature-resistant non-metallic fibers can be explored, such as basalt fibers or heat-resistant synthetic fibers, which can still maintain a certain strength at high temperatures and provide residual toughness for the matrix. Meanwhile, designing adequate fire protection layers for UHPC components (such as fire-resistant boards or thick-film fireproof coatings) is an effective engineering measure to delay the temperature rise of the core and protect both the fibers and the matrix.

In addition, microstructure optimization is also beneficial. By optimizing the mix ratio, components that are prone to decomposition or instability at high temperatures are reduced, while high-temperature-stable phase compositions are increased. Qinglong's fire-resistant UHPC research focuses not only on laboratory testing but also on integration with overall fire protection design, ensuring that in real fire scenarios, UHPC components can maintain sufficient integrity and load-bearing capacity, buying valuable time for personnel evacuation and firefighting rescue.