The STEM course focuses on Scientific and Technical Writing. It is a project-based program centered on an independent research project. In Dr. Crowthers’s STEM I class, students complete one major research project that begins over the summer with topic selection and continues throughout the academic year.

During A Term, students analyze and present a peer-reviewed research paper to develop scientific reading and presentation skills. In B and C Terms, students present original data from their own projects during regular update meetings. Peer discussions and feedback are built into the course, helping students refine their research methods, analysis, and communication. Overall, the course emphasizes the full research process and prepares students for advanced scientific research and technical writing.

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Simulating Recycled Polyethylene Terephthalate-Modified Warm Mix Asphalt

This project investigates how recycled polyethylene terephthalate (PET) can be used to improve the performance and sustainability of warm mix asphalt (WMA). The study develops a multi-scale computational framework that combines particle-level discrete element modeling (DEM) with pavement-scale finite element modeling (FEM). At the microscale, DEM simulations quantify how adding 0–10% shredded PET (as a partial replacement for fine aggregate) affects stiffness and Poisson’s ratio. These homogenized properties are then implemented in FEM simulations (via Abaqus from Dassault Systèmes) to evaluate rutting resistance, flexural response, and thermal conductivity. Results show that moderate PET contents (around 3–5%) increase stiffness, reduce permanent deformation, and lower thermal conductivity while maintaining flexural strength. Overall, the project demonstrates that recycled PET can enhance WMA performance and provides a transferable simulation workflow for optimizing sustainable asphalt mixtures without extensive laboratory testing.

Abstract

The increasing accumulation of polyethylene terephthalate (PET) plastic waste presents a critical environmental challenge, necessitating sustainable reuse strategies. Warm mix asphalt (WMA), produced at lower temperatures than conventional hot mix asphalt, reduces energy consumption and emissions but can suffer from performance limitations such as rutting susceptibility. This study investigates the feasibility of incorporating recycled PET into WMA using a multi-scale simulation approach. Discrete element modeling (DEM) simulations were conducted using YADE to model particle-scale behavior of WMA mixtures containing 0–10% shredded PET as a partial replacement for fine aggregate. Virtual uniaxial compression tests were applied to determine effective modulus and Poisson’s ratio, enabling homogenization of the mixture into representative material properties. These DEM-derived properties were then implemented in finite element modeling (FEM) simulations using Abaqus to evaluate pavement-level mechanical and thermal behavior, including rutting under repeated loading, flexural response, and heat transfer through the asphalt layer. Preliminary DEM results for the control WMA mixture were consistent with published values, validating the modeling framework. FEM simulations indicate that PET-modified WMA is expected to exhibit reduced deformation while having consistent flexural stress relative to the control mixture. In addition, reduced heat flux was observed, suggesting lower thermal conductivity. Overall, the results suggest that recycled PET can enhance both the mechanical and thermal performance of WMA while supporting sustainability goals through plastic waste recycling. This work establishes a foundation for optimizing PET dosage and demonstrates a transferable simulation framework for evaluating modified asphalt mixtures without extensive physical testing.

Graphical Abstract

Research Question

How does the incorporation of recycled polyethylene terephthalate (PET) affect the stiffness, rutting resistance, flexural strength, and thermal conductivity of warm mix asphalt, as evaluated through simulation?

Background Infographic

Methodology

Hypothesis

Recycled polyethylene terephthalate (PET) will improve stiffness, rutting resistance, and thermal conductivity of warm mix asphalt while preserving deformation behavior, with optimal performance at moderate PET contents.

Background

Plastic pollution is accelerating globally, with plastic production doubling since 2000 and projected to triple by 2060 (United Nations Environment Programme, 2025). Polyethylene terephthalate (PET) dominates many waste streams, accounting for approximately 60% of plastic waste and creating significant environmental challenges (Usman & Kunlin, 2024). One promising mitigation strategy is incorporating recycled plastics into pavement materials. Warm Mix Asphalt (WMA) reduces production temperatures by approximately 20–40 °C compared to Hot Mix Asphalt (HMA), leading to decreased fuel consumption and emissions (Rubio et al., 2012). Previous studies indicate that recycled PET can reinforce binder–aggregate structure while diverting plastic from landfills, and polymer additives such as high-density polyethylene have been shown to improve asphalt durability (Usman & Kunlin, 2024; Chegenizadeh et al., 2021). To investigate PET-modified WMA across multiple length scales, this project integrates Discrete Element Modeling (DEM) using YADE to capture particle-scale interactions and Finite Element Modeling (FEM) using Abaqus to evaluate macro-scale mechanical and thermal behavior.

Procedure

This study employs a fully computational framework to simulate warm mix asphalt (WMA) incorporating recycled polyethylene terephthalate (PET). The asphalt mixture is modeled as a composite of mineral aggregates, bitumen binder, and PET particles. Aggregates are represented in the Discrete Element Method (DEM) as spherical particles following a dense-graded distribution, with 55% coarse aggregate (>2.36 mm) and the remainder fine aggregate (<2.36 mm), which is held constant across all simulations. PET replaces the fine aggregate fraction by volume at five dosages (0%, 1%, 2%, 5%, and 10%) and is assigned lower stiffness, reduced density, higher ductility, and lower friction to reflect its polymeric behavior. Asphalt binder effects are incorporated through interparticle cohesion, viscous damping, and rolling resistance. Particle-scale simulations are conducted using YADE, where 50 mm cubic specimens are generated via gravity deposition and isotropic compaction, with five independent realizations per PET level to account for packing variability. Mechanical testing consists of uniaxial compression to determine effective Young’s modulus and lateral deformation tracking to calculate Poisson’s ratio. These homogenized properties are then passed to Abaqus for pavement-scale Finite Element Modeling (FEM), where PET-modified WMA is treated as a homogeneous elastic material with viscoelastic behavior introduced through a Prony series. Rutting performance is evaluated under 800 moving load cycles applied to a 0.1 m cubic asphalt block, while flexural strength is assessed through three-point bending of a beam specimen. Thermal behavior is modeled using a two-layer system subjected to a temperature gradient from 40 °C at the WMA surface to 20 °C at the base, enabling calculation of heat flux and temperature fields. Outputs including stiffness, rutting depth, flexural stress, midspan deflection, tensile strain, and surface heat flux are extracted and compared against the 0% PET control to evaluate changes in mechanical and thermal performance. Trends across PET content are analyzed to identify an optimal dosage, and model validation is performed by comparing DEM-derived elastic properties and FEM stress–strain responses with reported values for conventional WMA.

Figures

Figure 1: Effective modulus versus PET content — Figure 1. Effective modulus versus PET content for warm mix asphalt obtained from DEM uniaxial compression simulations. Bars show mean values from five randomized packings; error bars represent ± SEM.

Figure 2: Poisson's ratio versus PET content — Figure 2. Poisson’s ratio versus PET content for warm mix asphalt obtained from DEM uniaxial compression simulations. Bars show mean values from five randomized packings; error bars represent ± SEM.

Figure 3: Rutting depth as a function of loading cycles for warm mix asphalt containing 0–10% recycled PET. Curves represent FEM-predicted accumulated surface displacement under repeated vertical loading up to 800 cycles for each PET content.

Figure 4: Flexural stress versus PET content — Figure 4. Flexural stress versus PET content for warm mix asphalt, obtained from FEM beam simulations under identical geometry and loading conditions.

Figure 5: Midspan deflection versus PET content — Figure 5. Midspan deflection versus PET content for warm mix asphalt, obtained from FEM beam simulations under identical geometry and loading conditions. Bars show maximum midspan displacement.

Figure 6: Effective thermal conductivity versus PET content — Figure 6. Effective thermal conductivity of warm mix asphalt as a function of recycled PET content (0–10%), obtained from FEM heat transfer simulations. Points represent FEM-derived values; the solid line indicates linear regression.

Analysis

Simulation results indicate that incorporating recycled PET into warm mix asphalt alters stiffness and deformation behavior while preserving overall material integrity. DEM uniaxial compression showed that effective modulus increased from approximately 842 MPa in the control mixture to about 930 MPa at 5% PET, with statistically significant gains observed at 3% and 5% PET, while Poisson’s ratio remained relatively constant across all dosages. This suggests that PET primarily influences stiffness rather than volumetric deformation. FEM rutting simulations further demonstrated reduced permanent deformation at moderate PET contents, with rut depth decreasing from 2.57 mm in the control to 2.47 mm at 2% PET after 800 loading cycles. Flexural modeling showed that tensile stress remained approximately constant, while midspan deflection decreased up to 5% PET, indicating improved bending stiffness without compromising flexural strength. Thermal analysis revealed a monotonic decrease in effective thermal conductivity with increasing PET content, from 0.998 W/m·K (0%) to 0.856 W/m·K (10%), consistent with PET’s lower intrinsic conductivity. Collectively, these results identify an optimal PET range of approximately 3–5%, which improves stiffness and rutting resistance while reducing heat transfer, with minimal impact on flexural performance.

Discussion

This study demonstrates that incorporating recycled PET into warm mix asphalt can enhance mechanical performance while improving thermal insulation properties. DEM results showed that effective modulus increased at moderate PET dosages (3–5%) while Poisson’s ratio remained stable, indicating that PET primarily improves stiffness without significantly altering deformation behavior. FEM simulations supported these findings, showing reduced rutting depth and decreased midspan deflection under bending at moderate PET contents. Thermal analysis revealed a consistent decrease in effective thermal conductivity with increasing PET, reflecting PET’s lower intrinsic conductivity and suggesting improved insulation performance. An optimal PET range of approximately 3–5% appears to balance increased stiffness and rutting resistance with minimal impact on flexural strength. These results suggest potential service-life extension and improved sustainability through plastic waste diversion and reduced aggregate demand. However, several limitations remain. The binder was modeled implicitly rather than as a fully viscoelastic phase and potential microplastic release and long-term aging effects were not evaluated. Overall, the DEM–FEM framework provides a cost-effective method for screening sustainable pavement materials prior to laboratory validation. Future work includes experimental testing and lifecycle simulations to further validate PET-modified WMA performance.

References

American Association of State Highway and Transportation Officials. (2013). A policy on design standards — Interstate system. Access Management. https://accessmanagement.info/document/policy-design-standards%C2%97interstate-system/

Chegenizadeh, A., Peters, B., & Nikraz, H. (2021). Mechanical properties of stone mastic asphalt containing high-density polyethylene: An Australian case. Case Studies in Construction Materials, 15, e00631. https://doi.org/10.1016/j.cscm.2021.e00631

Enfrin, M., Myszka, R., & Giustozzi, F. (2022). Paving roads with recycled plastics: Microplastic pollution or eco-friendly solution? Journal of Hazardous Materials, 437, 129334. https://doi.org/10.1016/j.jhazmat.2022.129334

Moghaddam, T. B., Karim, M. R., & Syammaun, T. (2012). Dynamic properties of stone mastic asphalt mixtures containing waste plastic bottles. Construction and Building Materials, 34, 236–242. https://doi.org/10.1016/j.conbuildmat.2012.02.054

Phoenix Technologies. (2008). PET properties [Datasheet]. https://www.phoenixtechnologies.net/media/371/PET%20Properties%202008.pdf

Rubio, M. C., Martínez, G., Baena, L., & Moreno, F. (2012). Warm mix asphalt: An overview. Journal of Cleaner Production, 24, 76–84. https://doi.org/10.1016/j.jclepro.2011.11.053

Šmilauer, V., et al. (2021). Yade Documentation (3rd ed.). The Yade Project. https://doi.org/10.5281/zenodo.5705394

Some useful numbers on the engineering properties of materials (geologic and otherwise). (n.d.). GEOL 615 course handout, Pennsylvania State University. https://cpb-us-e1.wpmucdn.com/sites.psu.edu/dist/1/57960/files/2016/10/Some-Useful-Numbers-1g1rkuu.pdf

United Nations Environment Programme. (2025). Plastic pollution. https://www.unep.org/topics/chemicals-and-pollution-action/plastic-pollution

United States Department of Transportation, Federal Highway Administration. (2014). Standard specifications for construction of roads and bridges on Federal Highway projects (FP-14), Section 703—Aggregate. https://highways.dot.gov/federal-lands/specs/wfl-los/fp-14-library/703fp14.docx

Usman, I. U., & Kunlin, M. (2024). Influence of polyethylene terephthalate (PET) utilization on the engineering properties of asphalt mixtures: A review. Construction and Building Materials, 411, 134439. https://doi.org/10.1016/j.conbuildmat.2023.134439

Stem I