Direct Tensile Behavior of High Recycled-Glass Ultra-High-Performance Concrete Reinforced with Recycled Polyethylene and Commercial Fibers

Ultra-high-performance concrete (UHPC) provides exceptional strength and durability; however, its high cement and silica fume contents raise cost and environmental concerns. This study investigates the direct tensile behavior of a sustainability-driven UHPC in which 52% of the solid constituents are replaced with recycled glass and tensile performance is tailored using recycled and commercial fiber systems. A previously optimized recycled-glass UHPC matrix complying with ASTM C1856 was reinforced with recycled polyethylene fibers of varying lengths, commercial polypropylene and polypropylene–polyethylene fibers, brass-coated high-strength steel microfibers, and hooked-end steel macrofibers at fiber volume fractions of 1%, 2%, and 3%. Direct tensile tests were performed under displacement control in accordance with JSCE-08, and first-crack stress, peak tensile stress, tensile strain capacity, and energy absorption were derived from the stress–strain response. The most significant finding is that stable strain-hardening and multiple cracking can still be achieved in a UHPC matrix incorporating very high recycled-glass contents when appropriate steel fiber systems are used. Hooked-end steel fibers at 3% volume fraction reached peak tensile strengths of approximately 12 MPa and toughness values close to 40 kJ/m3, demonstrating that post-cracking performance comparable to conventional UHPC can be preserved despite aggressive matrix modification. In contrast, polymeric and recycled polyethylene fibers primarily enhanced first-crack stress but did not generate sustained post-cracking hardening, indicating their suitability for crack control and serviceability rather than structural tensile strengthening.