Environmental-Mechanical Trade-offs in Fibre-Reinforced Fly Ash Concrete: A Comparative Life Cycle Assessment

While fibre reinforcement and fly ash individually improve concrete sustainability, their comparative environmental-mechanical trade-offs, particularly distinguishing virgin from recovered fibres under a unified mixture framework, remain inadequately quantified. To bridge this gap, this study provides a comprehensive cradle-to-gate life cycle assessment (LCA) integrating 28-day compressive strength, strength-normalised eco-efficiency indices, and robust Monte Carlo uncertainty propagation across 38 distinct concrete formulations. Results demonstrate that fly ash utilisation strategies are environmentally divergent: cement replacement reduced global warming potential (GWP) by up to 49.7% at the expense of early strength (up to 22.4% decrease), whereas mass addition improved compressive strength by up to 20.7% while maintaining a GWP within 0.6% of the control. Most significantly, a profound environmental disparity emerged among fibre origins. While manufactured and recovered steel fibres yielded statistically comparable peak compressive strengths (65.3 MPa and 64.4 MPa, respectively), manufactured steel imposed an endpoint environmental burden increase of approximately 839% relative to the control at the maximum 1.25% dosage. In stark contrast, recovered steel and polypropylene caused marginal endpoint increases of only 5.1–5.2% and 20.7%, respectively. Recovered steel fibres within a 0.50–1.00% volume dosage range offered the optimal strength-to-impact balance, achieving a minimum deterministic strength-normalised endpoint intensity of 0.129 Pt/MPa for the 60 mm fibre. By capturing these stark disparities and propagating inventory uncertainty, this study contributes a reliable, multi-criteria decision-making framework for optimising sustainable, high-performance concrete composites.