Full-Scale Spun-Cast Cementless Concrete Pipes: Effects of Manufacturing Parameters and Hybrid Fibers Reinforcement on Structural Performance

Spun-cast production is widely used for precast concrete pipes, but the structural performance of cementless geopolymer pipes remains highly sensitive to manufacturing control. The main objective of this study was to identify how mixture design, interruption during casting, rotation regime, casting duration, and hybrid reinforcement influence the structural performance of full-scale fly ash-based cementless concrete pipes manufactured in an industrial plant. A three-phase experimental program was carried out. First, a 3 3 factorial matrix was used to optimize the geopolymer mixture by varying NaOH molarity, Na₂SiO₃/NaOH ratio, and alkaline activator-to-fly ash ratio. Second, full-scale pipes were manufactured under different spun-cast conditions, including continuous and interrupted casting, different rotation speeds, and casting durations. Third, the effect of reducing the conventional steel cage and adding steel and polypropylene (PP) fibers was evaluated. In total, 29 pipes of 450 mm internal diameter were tested under three-edge bearing and patch loading after 56 days. The optimum mixture was obtained at 14 M NaOH, Na₂SiO₃/NaOH =1.5, and AA/FA = 0.55, giving compressive strengths up to 28 MPa. Interrupting the spun-cast process at 50% filling significantly reduced the capacity; whereas, interruption at 75% had a little effect, indicating a practical consolidation threshold. The best mechanical response was obtained with an initial speed of 100-125 rpm, a final speed of 300-350 rpm, and a total casting time of 18 min. Reducing the steel cage decreased the cracking and ultimate loads, but fibers partially recovered the lost capacity. Among the tested configurations, the pipe with full steel cage plus 10 kg/m³ PP fibers achieved the highest three-edge bearing performance, reaching 141 kN at 0.30 mm crack width and 206 kN at ultimate, satisfying ASTM C76 Class IV requirements. The study establishes practical production thresholds and reinforcement-selection guidance for manufacturing sustainable cementless concrete pipes at full scale. • Full-scale mechanistic control of spun-cast geopolymer pipe production • Identifies critical 75% consolidation threshold in centrifugal casting • Optimized rpm regime maximizes structural load capacity • Hybrid fibers offset reduced steel cage reinforcement • Achieves ASTM Class IV with low-carbon cementless concrete