Effects of hybrid steel/bamboo fibers on the static performance and microstructure of UHPC

This study introduces a novel approach to ultra-high-performance concrete (UHPC) by hybridizing steel and bamboo fibers, providing a sustainable alternative to traditional fiber-reinforced UHPC without compromising mechanical integrity through the inclusion of biodegradable materials. UHPC was prepared and evaluated for macro- and micro-level properties using bamboo fibers of different lengths (6 mm, 12 mm, and 18 mm) combined with steel fibers, with a total fiber volume content of 2.0 %. Results show that combining 0.5 %-1.5 % of 12 mm bamboo fibers with 1.5 %-0.5 % steel fibers achieves an optimal balance, enhancing the flexural strength and toughness of UHPC while minimizing the trade-off in compressive strength caused by natural fibers. Due to its inherent hygroscopic properties, bamboo fiber reduces drying shrinkage in UHPC, forming a moisture-buffering system within the matrix that extends internal curing time and reduces volume changes. Microscopically, bamboo fibers increase UHPC's porosity, which promotes sustained hydration and the formation of calcium silicate hydrate (C-S-H) gel. Steel fibers provide the structural rigidity and crack resistance necessary to maintain UHPC's mechanical performance, creating a synergistic reinforcement mechanism. This study pioneers a green reinforcement strategy in ultra-high-performance concrete through the complementary use of natural and synthetic fibers, laying a foundation for further development of high-performance, eco-friendly concrete materials.