Tailoring Ultra-High-Performance Concrete Toughness with Carbon Fibers: Linking Microstructure and Fracture Energy

This paper presents a multiscale experimental study on the effect of carbon microfiber reinforcement on the fracture behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC).Various fiber lengths (0.1–6 mm) and contents (1.2–20 kg/m³) were tested, linking microstructural features obtained via X-ray computed tomography (CT), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) to mechanical performance. Results show that a dosage of 6 kg/m³ of 3–6 mm fibers offers a balance between workability and fracture resistance, with fracture energy increasing up to 195% compared to plain concrete. This research provides insights for optimizing fiber type and content in UHPFRC for structural applications.