Performance and mechanisms of concrete incorporating polymer-based waste aggregates under mechanical, environmental, and thermal exposure

Alhamdan, Ahmad 2026. Performance and mechanisms of concrete incorporating polymer-based waste aggregates under mechanical, environmental, and thermal exposure. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Polymer-based waste materials, including recycled plastics, polyurethane foam, and end-of-life tyre rubber, are accumulating globally due to their persistence and limited recycling pathways. Incorporating these materials into concrete provides a route to reduce landfill disposal and dependence on natural aggregates while improving concrete performance. However, polymer aggregates differ fundamentally from conventional aggregates in chemical and physical composition, stiffness, density, surface characteristics, and interaction with the cementitious matrix, resulting in complex and sometimes conflicting effects on mechanical performance and durability. A clear understanding of these mechanisms under realistic exposure conditions remains limited. This thesis examines the performance and governing mechanisms of concrete incorporating polymer-based waste aggregates across four material systems. Experimental evaluation under laboratory and field conditions is combined with a mechanism-led interpretation of behaviour, focusing on mechanical performance, physical structure, transport properties, durability indicators, and thermal response. The first part of the study examines low-carbon concrete formulations incorporating recycled plastic as a partial replacement of coarse aggregate, exposed to both laboratory curing and marine field conditions. Plastic aggregate inclusion reduced compressive, tensile, and flexural strength due to low stiffness and weak interfacial bonding. Improvements in durability-related properties were consistently observed. Despite increased total porosity, plastic-modified concrete exhibited reduced chloride ingress and higher bulk electrical resistivity, indicating that pore connectivity, rather than total porosity, governs transport behaviour. Marine exposure further highlighted the influence of environmental conditions, where salt crystallisation and pore infilling contributed to changes in density and surface characteristics over time. These findings confirm that plastic aggregates can enhance durability performance through physical disruption of capillary pathways, even when mechanical strength is reduced. The second part of the thesis focuses on concrete incorporating waste rigid PU foam as a fine aggregate replacement, evaluated under laboratory curing, freeze thaw cycling, and elevated temperature exposure at 300 °C. Under laboratory conditions, increasing PU content resulted in systematic reductions in strength and stiffness, accompanied by increases in porosity and reductions in density. However, the material exhibited enhanced ductility and resistance to brittle failure. Under freeze-thaw conditions, the presence of PU foam improved resistance to surface scaling and mass loss due to its ability to accommodate internal stresses through its deformable structure. In contrast, exposure to elevated temperature revealed a different degradation mechanism, where thermal instability of the polymer phase led to increased porosity and strength loss. PU-modified concrete exhibited exposure-dependent performance, with improved resistance to cyclic environmental loading but increased vulnerability to thermal degradation. The third part of the thesis investigates concrete incorporating coarse tyre rubber aggregates under laboratory curing and moderate elevated temperature exposure. Similar to plastic and PU systems, the inclusion of rubber resulted in reduced mechanical strength and stiffness due to its low modulus and weak interfacial transition zone (ITZ). However, improvements in ductility, energy absorption, and crack resistance were observed. A key contribution of this work is the evaluation of a dry cement-based surface coating technique for rubber aggregates. The coating significantly improved interfacial bonding, leading to measurable gains in strength, stiffness, and durability-related properties, including reduced porosity and increased electrical resistivity. Under elevated temperature exposure, rubberised concrete exhibited complex behaviour, with strength reductions linked to thermal degradation of the polymer phase, while coated systems demonstrated improved stability compared to uncoated counterparts. The final part of the thesis explores a hybrid concrete system combining PU foam as fine aggregate replacement and tyre rubber as coarse aggregate replacement. This approach balances the beneficial and detrimental effects of each material. The results indicate that the hybrid system exhibits reduced strength and stiffness relative to conventional concrete, but maintains structural continuity without significant internal defects, as confirmed by ultrasonic pulse velocity measurements. The combined presence of two polymeric phases leads to a more compliant material with enhanced deformability, while transport-related properties remain governed by pore connectivity and microstructural continuity. This demonstrates the feasibility of hybrid polymer aggregate systems for applications where functional performance, rather than high structural strength, is prioritised. Across all investigated systems, polymer-based aggregates modified concrete behaviour primarily through their low stiffness, low density, hydrophobic nature, and altered interfacial characteristics. These factors reduce strength and stiffness but can enhance durability by disrupting fluid transport pathways and increasing electrical resistivity. The results highlight the importance of distinguishing between total porosity and pore connectivity, as well as the critical role of the ITZ in governing both mechanical and durability performance. Overall, this thesis provides a comprehensive experimental and mechanistic evaluation of concrete incorporating polymer-based waste aggregates under multiple exposure conditions. The findings contribute to a clearer understanding of how such materials influence performance across different property domains and demonstrate their potential for use in sustainable construction applications where durability, resilience, and functional performance are prioritised over maximum strength.

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Schools > Engineering
Uncontrolled Keywords:	1. Recycled polymer aggregates 2. Sustainable concrete 3. Recycled plastic aggregate 4. Polyurethane foam waste 5. Tyre rubber aggregate 6. Concrete durability
Date of First Compliant Deposit:	28 August 2026
Last Modified:	01 Sep 2026 09:52
URI:	https://orca.cardiff.ac.uk/id/eprint/189177

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