Optimising combined waste polymer in warm asphalt mixture using response surface methodology

This study optimises the incorporation of Combined Waste Polymer (CWP) into asphaltic concrete using response surface methodology to enhance the mechanical and durability performance of Warm Mix Asphalt. Waste plastic bottles and water sachets were processed, blended in equal proportions, and used as a composite modifier with Sasobit wax. Centred Central Composite Design with four independent variables: bitumen content (4.5–8.0%), CWP content (0–18% by bitumen weight), Sasobit dosage (0–4%), and mixing temperature (110–160 °C) was employed to evaluate eleven performance responses: Marshall stability, flow, marshall quotient, air voids, bulk density, indirect tensile strength, tensile strength ratio, resilient modulus, fatigue life, compressive strength, and skid resistance. Quadratic polynomial models were developed and validated using ANOVA at 95% confidence level. All models were statistically significant ( p < 0.0001) with high coefficients of determination (R 2 = 0.9239–0.9677), Adequate Precision values exceeding 27, and low coefficients of variation (< 6%), confirming excellent predictive capability. The CWP emerged as the most influential factor across nearly all responses, significantly enhancing stiffness, tensile properties, moisture resistance, fatigue life, and rutting resistance. Positive interaction effects between CWP and Sasobit further improved mixture cohesion and elastic recovery. Quadratic terms confirmed the presence of optimum modifier dosages, beyond which excessive stiffness and reduced workability marginally diminished performance. Optimised mixtures demonstrated superior structural performance at reduced production temperatures, confirming the technical viability of integrating recycled polymer composites into energy-efficient and waste-diverting pavement mixtures.