Performance evaluation and optimum asphalt content of asphalt mixtures with polypropylene-based multilayer packaging waste

Multilayer plastic packaging is difficult to recycle because of its complex composition. Asphalt mixtures tolerate heterogeneous inputs, offering a practical valorization route. This study evaluates polypropylene-based multilayer plastic waste as a bitumen modifier for polymer modified bitumen (PMB). Mixtures containing 0, 3, 4, and 5 wt% plastic were assessed by the Marshall method. Plastic content significantly affected all seven Marshall parameters (two-way ANOVA, p < 0.001), most strongly Marshall Quotient ( η 2 = 0.76) and flow ( η 2 = 0.54). Tukey HSD contrasts showed the 4 and 5 wt% mixtures differed from the unmodified control while 3 wt% did not, placing the effective threshold between 3 and 4 wt%. Flow and volumetric properties (VMA, VIM, VFB) remained within Bina Marga 2018 limits throughout. Optimum asphalt content decreased stepwise from 5.93 wt% (unmodified) to 5.90, 5.83, and 5.70 wt% at 3, 4, and 5 wt% PMB – a 0.23 wt% reduction at equivalent specification compliance. The 5 wt% mixture combined full compliance with the highest stability (1,619 ± 67 kg) and lowest binder demand. Polypropylene-based multilayer waste is, therefore, viable as an asphalt modifier at mix-design level; durability and binder-level characterization remain necessary before field application.