What Special Designs Are Required to Achieve the Fire Resistance of UHPC? Fire Resistance Optimization Technologies and Practical Solutions

UHPC has inherently weak fire resistance. In high-temperature environments, its strength can drop sharply due to the decomposition of hydration products and fiber softening, so special design is required to improve fire performance and meet building fire code requirements. Targeted fire-resistant design is key to UHPC applications in public and high-rise buildings. Drawing on 28 years of industry experience, Qinglong has summarized its core optimization techniques and practical solutions.

1. High-Temperature-Resistant Aggregate and Admixture Optimization

The selection of aggregates and admixtures directly affects UHPC's high-temperature resistance, so components with excellent fire resistance should be prioritized. For aggregates, high-temperature-resistant minerals such as quartz sand and basalt should be used; their melting points above 1,500°C mean they are unlikely to soften or deform under fire conditions. Qinglong uses high-purity quartz sand in all projects with fire-resistance requirements, replacing ordinary aggregates that easily decompose at high temperatures. For active admixtures, fly ash or slag powder is added to replace part of the silica fume. The glassy phase in fly ash forms a dense molten layer at high temperatures that blocks heat transfer, while slag powder improves the material's high-temperature stability. Qinglong's tests show that UHPC with 20% fly ash added has a 30% lower strength loss rate at 600°C. In addition, adding an appropriate amount of refractory clay can further improve the material's fire resistance limit and extend its high-temperature endurance.

2. Fiber Selection and Composite Reinforcement Design

Fibers are a core factor affecting UHPC's fire performance; fiber types that soften easily at high temperatures should be avoided. Steel fibers soften significantly above 500°C and lose their reinforcing effect, so they should be replaced with high-temperature-resistant fibers such as basalt fiber and carbon fiber. Basalt fiber can withstand temperatures above 600°C, and carbon fiber above 1,000°C. In a high-end landmark project in Shenzhen, Qinglong replaced steel fibers with basalt fiber, raising the UHPC fire resistance limit to 2.5 hours. For scenarios where steel fibers must be used, a combined 'steel fiber + basalt fiber' solution is adopted, with steel fibers providing normal-temperature strength and basalt fibers providing high-temperature reinforcement, while a high-temperature-resistant coating is applied to the fiber surface to slow the softening of the steel fibers. Qinglong adopted this solution in the UHPC sculpture project for the Shanghai Astronomy Museum, balancing normal-temperature performance with fire-resistance requirements.

3. Cover Thickness and Cross-Section Design Optimization

Increasing cover thickness is a direct way to improve fire performance and must be designed reasonably according to fire-resistance rating requirements. For load-bearing UHPC members, the cover thickness should be ≥30mm, and for non-load-bearing members ≥20mm, an increase of 5-10mm over conventional designs. Thickening the dense UHPC matrix delays heat transfer to the interior, protecting the fibers and hydration products in the core area. The cross-section design adopts a 'solid + ribbed plate' structure, avoiding thin and slender shapes: solid cross-sections increase resistance to heat conduction, while ribbed structures enhance the overall stability of members and reduce deformation and cracking at high temperatures. In the UHPC member design for the Yangshengtang Pharmaceutical Hangzhou Industrial Park, Qinglong optimized the cross-section and cover thickness so that the fire resistance limit meets Class I fire-resistance requirements.

4. Surface Fire-Retardant Coatings and Protective Treatment

Surface fire-retardant coatings are a supplementary means of improving fire performance, and the appropriate type can be selected according to the scenario. For interior members, ultra-thin steel-structure fire-retardant coatings are preferred, with a thickness of 3-5mm; they do not affect appearance and offer excellent fire resistance. For exterior members, intumescent fire-retardant coatings are used, which form a dense carbonized insulation layer at high temperatures to block heat transfer. In the UHPC exterior wall project for the Ouargla hotel in Algeria, Qinglong used weather-resistant intumescent fire-retardant coatings, which both improve fire performance and enhance resistance to salt-spray corrosion. In addition, applying a high-temperature-resistant sealing primer to the surface reduces moisture evaporation and gas escape at high temperatures, preventing blistering and cracking on the member surface and further improving fire-resistance stability. Through multi-dimensional special design, Qinglong's UHPC products can meet fire protection needs in different scenarios and expand their range of applications.