An Experimental Study on the Influence of Waste Tyre Metal Fibre on Asphalt Mixture's Performance Properties

The disposal of waste tyres presents a significant environmental challenge, necessitating sustainable, high-value recycling solutions. This study explores the incorporation of waste tyre metal fibre (WTMF) into hot mix asphalt (HMA) to enhance mechanical performance while reducing landfill burden. WTMF-modified mixes containing 0%, 0.375%, 0.75%, 1.125%, and 1.50% fibre were evaluated through Marshall and volumetric testing, indirect tensile strength (ITS) and tensile strength ratio (TSR) for moisture damage, stiffness modulus at varying temperatures, and fatigue life under cyclic loading. Microscopic analysis revealed WTMF's irregular, rough surface with microcracks and pits, aiding crack-bridging and stress transfer. Marshall testing showed that the optimum binder content of WTMF-modified mixtures was approximately 5% higher than that of the control (conventional HMA without WTMF); however, stability decreased while flow increased, resulting in a reduced Marshall quotient due to fibre conglomeration affecting porosity and bulk specific gravity. ITS results indicated that the control mixture exhibited the highest cracking resistance, whereas WTMF-modified mixtures demonstrated improved moisture resistance (TSR > 80%). The maximum improvement was observed at 0.75% WTMF-induced HMA, with an 11% increase in TSR, while a slight reduction of 2.4% occurred at 1.50% WTMF-induced HMA. Stiffness testing showed that the mixture containing 0.375% WTMF achieved the highest modulus, exhibiting up to a 70% increase at 5 °C and more than a twofold increase at elevated temperatures compared to the control mixture. With increasing temperature, stiffness decreased by approximately 84% for the control mixture and 80% for the 0.375% WTMF-modified mixture. Fatigue analysis showed that the control mixture achieved a fatigue life of 115,529 loading cycles at low stress, followed by substantial reductions in fatigue life with increasing stress levels, whereas moderate WTMF contents improved strain performance; however, excessive fibre content increased permanent deformation under high stress. Stress- and strain-based empirical power-law relationships were established for predicting the fatigue life of each investigated mixture. Results demonstrate that WTMF's controlled dosage within the optimum range of 0.375 to 0.75% has the potential to improve HMA's performance indicators, offering a sustainable recycling pathway for waste tyres.