Enhancing the Durability of Bituminous Concrete Using Plastic Waste on Soft Rock Aggregates

The use of marginal sedimentary aggregates in pavement construction remains a major challenge in mountainous regions due to their high porosity, weak lamination planes, and susceptibility to moisture-induced deterioration. This study investigates the potential of low-density polyethylene (LDPE) plastic waste to enhance the engineering performance of laminated Miocene soft rock aggregates used in bituminous concrete. Aggregates sourced from the Surma Group (Bhuban Formation) in Mizoram, India, were characterized through physico-mechanical, geochemical, and mineralogical analyses to evaluate their durability and moisture sensitivity. X-ray fluorescence (XRF) analysis revealed elevated feldspar and total alkali contents (≈5.15%), indicating a mineralogical composition prone to hydrophilic behavior and stripping within bituminous mixtures. To mitigate these limitations, aggregates were coated with varying proportions of LDPE plastic using the dry process. An optimum LDPE content of 9% by weight of aggregate produced significant improvements in aggregate performance, resulting in a 70.03% reduction in Aggregate Impact Value (from 17.72% to 5.31%), a decrease in Los Angeles Abrasion Value from 42.93% to 31.45%, and an 89.82% reduction in water absorption (from 4.52% to 0.46%). The polymer coating effectively sealed lamination planes and reduced moisture ingress within the sedimentary structure. Bituminous concrete mixtures incorporating LDPE were further evaluated using Marshall stability and indirect tensile strength tests. The addition of 1.1% LDPE by weight of mix significantly enhanced moisture resistance. For mixtures with nominal maximum aggregate sizes (NMASs) of 13 mm and 19 mm, the Tensile Strength Ratio (TSR) increased from 52.59% and 58.58% in the control mixtures to 82.81% and 87.10%, respectively, thereby satisfying the minimum requirement of 80% specified by MoRTH. The results indicate that LDPE functions as a hydrophobic barrier and structural sealant that improves binder-aggregate adhesion and prevents stripping along weak lamination planes. The findings demonstrate that LDPE-modified bituminous concrete provides a sustainable and technically viable strategy for upgrading marginal sedimentary aggregates into durable pavement materials while simultaneously promoting the beneficial reuse of plastic waste.