Improving the Mechanical Performance of 3D-Printed UHPFRC with Thermally Modified Carbon Fibers (2025-11)¶

Additive manufacturing enables automated, formwork-free, and geometrically complex construction. Integrating ultra-high-performance fiber-reinforced concrete (UHPFRC) with 3D concrete printing allows slender, high-strength elements without conventional steel reinforcement. Carbon fibers offer high specific strength and corrosion resistance, but weak fiber-matrix bonding limits efficiency. This study examines thermal surface modification of chopped and milled carbon fibers (600 °C, 3h and 5h) in cast and 3D-printed UHPFRC (3DP-UHPFRC). Fourier-transform infrared spectroscopy, scanning electron microscopy, and water-contact-angle measurements show that 3h treatment increases oxygen-containing surface groups and hydrophilicity (30% improvement), with rougher topography, while 5h causes surface degradation. Correspondingly, 3h treatment improves compressive, flexural, and direct tensile strengths in both cast and printed specimens, whereas 5h treatment reduces performance. These results underscore the need for precise carbon fiber thermal treatment to enhance fiber-matrix bonding and ensure 3DP-UHPFRC compliance with the ACI 239R-18 standard, which requires 150 MPa compressive strength for 28 days and 6.2 MPa direct tensile strength.