Repurposing plastic bottle cap waste in controlled low-strength material for pavement base applications

Plastic waste, particularly polypropylene bottle caps, presents a growing disposal challenge due to limited recycling potential. This study offers a sustainable solution by repurposing these caps as plastic aggregate (PA) in controlled low-strength material (CLSM) for pavement applications. Both cement-based and alkali-activated material (AAM)-based binders were evaluated. The AAM binder was produced from high-calcium fly ash activated with sodium hydroxide, without requiring heat curing. Natural river sand was partially replaced with PA at 10–30 % by volume. Experimental evaluations covered flowability, bleeding, unit weight, unconfined compressive strength (UCS), resilient modulus (M R ), and free-free resonance tests. Results showed that increasing PA content reduced slump flow, density, and mechanical performance, while bleeding behavior varied with binder type. At 10 % PA, cement-based CLSM exhibited a 31.6 % reduction in 28-day UCS and a 20 % drop in M R . AAM-based CLSM showed a reduction of 1 MPa in UCS and 4.7 % in M R for every 10 % PA added. Free-free resonance tests confirmed reduced stiffness in both systems. Despite the strength reduction, cement-based CLSM with 10 % PA met the strength requirements for both base and subbase layers, while AAM-based mixtures with up to 10 % PA met subbase criteria. Cost analysis indicated that PA reduced material costs, and its use in AAM-based CLSM reduced CO 2 emissions by up to 65 % compared to conventional mixes. Overall, this study demonstrates that incorporating PA provides dual environmental and economic benefits while achieving acceptable engineering performance, supporting its use in sustainable pavement construction.