Waste Plastics as Alternative Paving Material

Abstract

The significant accumulation of plastic waste underscores the urgent need for its innovative repurposing to preserve natural resources and alleviate landfill pressures. This dissertation examines the incorporation of waste plastics, specifically low-density polyethylene (LDPE) and polypropylene (PP) , along with devulcanized rubber (DVR) sourced from end-of-life tires, as upcycled composite modifiers in asphalt binders. The aim was to enhance pavement sustainability without compromising service life performance. The UAE has historically relied on unmodified bitumen (NB) in their Hot mixed asphalt (HMA) compositions. However, the increasing traffic levels paired with the hyper-arid climate is driving the local agency to transition to virgin SBS polymer-modified bitumen (PMB). This transition presented an ideal opportunity to explore alternative sustainable binders incorporating waste materials.

The thesis hypothesizes that combining waste DVR with plastics (DVR-LDPE and DVR-PP) can create a synergistic balance between elasticity and stiffness. This combination is anticipated to yield a high-performance binder that maintains flexibility while resisting deformation at high-temperature. This composite approach is envisioned to overcome the limitations of each individual waste component—such as poor compatibility with bitumen and potential performance trade-offs—by harnessing their complementary properties to deliver superior performance comparable to the locally used alternatives (NB and PMB).

The experimental phase involved modifying NB with the upcycled waste composites at varying concentrations (8%, 16%, and 24% by weight of NB). Results showed a progressive improvement in the non-recoverable creep compliance (Jnr), signifying the development of a more effective polymeric network with the inclusion of DVR-LDPE and DVR-PP. According to the Multiple Stress Creep Recovery (MSCR) criteria, both composites, at 16% and 24% modification levels, achieved “E” grade classification at 64°C, on par with the performance of the PMB used locally. Furthermore, DVR-LDPE demonstrated exceptional fatigue cracking resistance, surpassing both NB and PMB across multiple strain levels. This was confirmed through fatigue characterization using the Linear Amplitude Sweep (LAS) test, highlighting its remarkable durability under cyclic loading conditions. The research also addressed phase separation issues often associated with modified binders produced with these wastes. DVRLDPE consistently emerged as the most effective modified binder, achieving stable blends suitable for storage and transportation, followed by the DVR-PP modified bitumen. In contrast, binders modified exclusively with LDPE or PP demonstrated pronounced phase separation. These findings were validated across a range of modifier concentrations, effectively addressing the phase separation challenges inherent in waste-modified bitumen. Interestingly, the simultaneous addition of plastics and DVR (in a non-composite form) to bitumen also resulted in phase separation issues. Optimal compatibility was ultimately only realized when DVRLDPE and DVR-PP were utilized as upcycled pellets, with the former being a preferable solution.

Based on the comprehensive investigation, the optimal modifier concentration was selected to be 20% DVR-LDPE and 20% DVR-PP, which lies at the midpoint of the highest tested range studied previously. These formulations were rigorously validated using two distinct base binders sourced from different suppliers to ensure the observed enhancements are not binder-sensitive. The composites showed similar levels of adaptability to both the base binders. Yet again, the 20% DVR-LDPE combination emerged as the best solution, evidenced by the linear viscoelastic (LVE), rutting, and fatigue cracking response. In terms of LVE characterization, modifications incorporating 20% DVR-LDPE consistently surpassed the mechanical performance of conventional PMB across the entire frequency spectrum. Moreover, the 20% DVR-LDPE modification achieved rutting performance comparable to that of the PMB, with both being classified as “E” according to the MSCR criteria. In addition, DVR-LDPE modified binders displayed nearly 2.5 times the number of cycles to failure at a lower strain level of 2.5% compared to PMB, indicating significantly enhanced durability. This performance superiority extended to higher strain levels, compellingly advocating for DVRLDPE as a viable alternative in the UAE. Additionally, the Glover-Rowe analysis, projected onto the black space (G* vs δ), revealed a lower propensity for non-load induced cracking in DVR-LDPE binders compared to both DVR-PP and PMB after long-term aging, further substantiating the performance of bitumen modified with DVR-LDPE. Here, G*, the complex modulus, quantifies the overall stiffness of the bitumen, while the phase angle (δ) represents the lag between applied stress and strain, indicating the material's viscoelastic balance between elastic and viscous responses. Overall, DVR-LDPE was identified as the optimal composite solution and is recommended over DVR-PP for future applications.

Despite the promising results achieved by DVR-LDPE, industries across all sectors need an accurate estimation of the economic and environmental burdens prior to its widespread adoption, and the pavement sector is no exception. Life Cycle Assessment (LCA) and Life Cycle Cost Assessment (LCCA) are systematic methodologies that have emerged as indispensable tools in quantifying and comparing the environmental burden and economic viability of asphalt pavements throughout their service life. Consequently, this study assessed and contrasted the environmental emissions and economic profiles of conventional binders (NB and PMB) against the proposed DVR-LDPE in potential local HMA setting. The LCA indicated a substantial reduction in potential environmental impacts when using HMADVR+LDPE compared to HMA-NB and HMA-PMB, with potential greenhouse gas (GHG) emission reductions of 14.4% and 51.6%, respectively. Moreover, the DVR-LDPE modified HMA also proved to be the most cost-effective among the formulations studied. Additionally, GHG emissions obtained from the LCA were monetized to calculate the total environmental savings (TES) in accordance with the social cost of carbon. Specifically, the HMADVR+LDPE configuration yields a TES of $36,955 and $5,177 per km-lane of the HMA layer when substituted for HMA-NB and HMA-PMB, respectively. Collectively, these findings support the hypothesis that upcycled waste, particularly DVR-LDPE, not only improves the durability of bitumen but also enhances its environmental and economic viability.

The LCA in this dissertation was designed not only to advance theoretical sustainability assessments but also to come up with a strategy to enforce their real-world implementation. The study thus introduces a novel approach by using environmental savings from the impact monetization to incentivize suppliers who leverage or bid for green technologies, demonstrating significant reductions in their carbon footprint. Hence, an incentive design structure is proposed that integrates these environmental savings into the contracting mechanism. This approach will align buying strategies with environmental goals, promoting the shift towards greener infrastructure projects. Such integration ensures that environmental considerations are given equal weight in financial evaluations, supporting a more comprehensive approach to infrastructure development.

Date of Award	12 Dec 2024
Original language	American English
Supervisor	Ahmed Mahmoud (Supervisor)