Uniaxial and triaxial performance of concrete incorporating oxygen plasma-modified recycled plastic particles

The increasing accumulation of plastic waste and the depletion of natural mineral aggregates have driven interest in the use of recycled plastic particles as alternative aggregates in concrete. However, weak interfacial bonding between hydrophobic plastic particles and the cement matrix remains a critical limitation. This study investigates the effect of oxygen plasma surface modification on the mechanical performance of concrete incorporating recycled polypropylene flakes (PPF) and polyethylene agglomerates (PEA). Compressive and flexural strength tests, together with triaxial tests under confined conditions, were performed. Surface wettability was evaluated using contact angle measurements, and interfacial morphology was analysed by scanning electron microscopy (SEM). Although plastic particles reduced compressive strength compared to reference concrete, plasma treatment improved mechanical performance, particularly for PEA mixtures, with compressive strength increasing by up to 33% relative to untreated counterparts. Under triaxial loading, concrete specimens containing plasma-treated PEA particles exhibited reduced deformations and improved elastic recovery, indicating enhanced pressure transfer under confinement. SEM analysis confirmed improved interfacial bonding in treated specimens. The results demonstrate that oxygen plasma treatment effectively enhances interfacial properties and multiaxial mechanical performance of concrete incorporating plastic particles, providing novel insights into its behaviour under triaxial loading.