Towards greener construction: a comprehensive evaluation of eco-friendly UHPC reinforced with hybrid fibers

One of the primary challenges with ultra-high-performance concrete (UHPC) is its high cement content, typically around 1000 kg/m³, which raises significant environmental concerns. Therefore, reducing cement content while maximizing its efficiency is essential for improving both the sustainability and performance of UHPC. This study focuses on developing an environmentally friendly UHPC mix with a reduced cement content of 700 kg/m³, reinforced with hybrid fibers: steel fiber (S_tF) and polypropylene fiber (PPF). The main objective of this research is to evaluate the effects of these two fiber types on the mechanical properties and durability of UHPC. It also aims to achieve a balance between enhanced strength, crack resistance, and load-bearing capacity under various conditions. Fiber volume percentages ranging from 0%, 0.25%, 0.5%, 0.75%, and 3% were incorporated for both S_tF and PPF, used as single or hybrid fibers. The study assessed several key mechanical and durability properties, including compressive strength, tensile strength, flexural strength, modulus of elasticity, porosity, water absorption, sorptivity, fire resistance, impact resistance, and energy absorption. Additionally, the microstructural properties were analyzed using scanning electron microscopy (SEM). In addition, the life cycle assessment (LCA) of UHPC was evaluated in terms of cost-effectiveness, energy efficiency, and carbon efficiency. Among the tested UHPC blends, despite the relatively low cement content, the mix containing 0.75% S_tF and 0.25% PPF demonstrated superior performance, achieving a compressive strength of 155 MPa, tensile strength of 5 MPa, and flexural strength of 4 MPa, which outperformed the mix containing 3% mono-S_tF. Furthermore, this hybrid fiber combination exhibited up to a 47% increase in initial and final kinetic energy absorption compared to its mono-S_tF counterpart. The hybrid blend of 0.75% S_tF and 0.25% PPF also showed reduced porosity (1.73%), lower water absorption (0.602%), and decreased saturation absorption (16.6%) compared to the monofilament S_tF mix. SEM analysis further confirmed that the hybrid fiber composition improved the fiber-matrix interface and reduced porosity. Furthermore, among all the mixes, the control and 3P mixes showed the highest environmental efficiency and reduced carbon emissions, energy consumption and costs. These results indicate that hybrid fiber systems can significantly enhance the mechanical performance, impact resistance, and durability of UHPC while simultaneously promoting the use of environmentally friendly cementitious compositions. This highlights the potential of hybrid fiber-reinforced UHPC for advanced structural applications.