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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.
Chindasiriphan et al. (Sun,) studied this question.