Development and Performance Characterisation of a Recycled Iron Plastic Aggregate (RIPA) for Modified Asphalt Mixtures

Abstract

Growing concerns over plastic waste accumulation and the disposal of iron-oxide industrial residues have intensified efforts to develop alternative pavement materials that support circular resource use and reduce reliance on natural aggregates. This thesis investigates the development of a dual-waste composite aggregate, termed Recycled Iron Plastic Aggregate (RIPA), produced by melt-mixing waste High-Density Polyethylene (HDPE) onto Magnetite (Iron Oxide) (Fe3O4) grinding by-product, and evaluates its suitability as a partial replacement for natural aggregate in asphalt mixtures. The research follows a structured three-stage framework covering (i) material production, (ii) material-level physical and chemical characterisation, and (iii) the incorporation of RIPA as a 3%, 5% and 10% replacement for granite in 14 mm Stone Mastic Asphalt (SMA-14), a widely used road surfacing material in the United Kingdom (UK). A comprehensive programme was undertaken to assess the mechanical and functional performance of RIPA-modified mixtures, as well as its long-term behaviour with respect to fatigue, rutting, ageing and moisture resistance. At the mixture level, the research also examined surface-erosion resistance under water flow and assessed the subsequent impacts on water quality, ensuring that the use of RIPA would not raise concerns regarding microplastic contamination in discharged water.

RIPA was produced through controlled melt extrusion, generating composite particles with angularity comparable to granite, water absorption below 0.5% and densities between 1.50 and 1.80 g/cm remained within the SMA-14 grading limits. Material characterisation confirmed a dual-phase composite in which Fe3O4 contributed rigidity and surface texture, while the HDPE fraction contributed to lower moisture uptake and improved bitumen interaction. Pull-off tests recorded bonding strengths of 0.98-1.09 MPa, values close to those of granite (1.20-1.25 MPa).

Mixture-level testing showed that RIPA can be incorporated into SMA-14 using the standard dry process. At 3-5% replacement, the mixtures displayed consistent gains, with tensile strength increasing by 5-12%, stiffness by 4-10%, and fatigue life improving by 15-35% compared with the control. Rutting resistance also remained stable, staying within ±5% of the control mixture. At 10% replacement, the mixtures became more open and the aggregate structure weakened, leading to 15-30% lower strength and stiffness. These results indicate that the mechanical benefits of RIPA are maintained only when used at moderate levels.

Surface and environmental assessments indicated that 3-5% RIPA maintained friction levels close to the control, with British Pendulum Number (BPN) values staying within ±3 units, and limited hydraulic-erosion losses to below 1.5%. At 10%, erosion losses exceeded 4%, and the surface texture became noticeably smoother. Small polymer fragments were observed only when the mixture experienced severe erosion, suggesting that any release is linked to structural deterioration rather than the presence of RIPA itself.

Durability testing further indicated the use of moderate RIPA contents. After ageing, stiffness retention improved by 10-18%, and moisture conditioned tensile strength remained between 85-92% of original strength, compared with 78% for the control mixture. Under freeze–thaw cycling, the 3-5% RIPA mixtures showed only 10-18% loss in strength, while the 10% mixtures experienced 30-45% loss and visible cracking.

Overall, the findings indicate that RIPA may offer as a viable and environmentally favourable partial aggregate replacement for SMA-14 when used within a controlled 3-5% range. Its performance arises from the combined contribution of the Fe3O4 and HDPE phases, which together support interlock, moisture stability and improved stress distribution. These outcomes suggest a possible pathway for scaling the material, support wider circular-economy objectives and provide a basis for further field trials and industrial evaluation.

Date of Award	Dec 2025
Original language	English
Awarding Institution	Aston University
Supervisor	Mujib Rahman (Supervisor) & Ahmed Abed (Supervisor)