Development of Dual-Layer Paving Blocks Using Waste Plastic as Aggregate Replacement with the Potential to Mitigate Microplastic Emissions

Abstract

Background and Objectives: The escalating global crisis of post-consumer plastic waste has driven the development of innovative solutions and strategic approaches across various sectors. In the field of construction materials, although the incorporation of plastic fragments as a partial aggregate replacement has gained a degree of acceptance, the environmental risks associated with the dispersal of secondary In the field of construction materials, although incorporating plastic fragments as a partial aggregate replacement has gained a degree of acceptance, the problem of the release of secondary microplastics induced by physical degradation and ultraviolet radiation has attracted increasing attention, posing long-term environmental risks during practical implementation. Consequently, this research studies the materials science and engineering feasibility and environmental safety of producing dual-layer paving blocks that utilize recycled plastic waste as a partial aggregate substitute. The primary objective is to develop a paving block with the potential to mitigate these environmental hazards through a specialized structural design: an inner core containing plastic waste is encapsulated within a high performance, plastic-free outer concrete shell. This design specifically aims to prevent the release of plastic fragments or microplastics into the ecosystem while ensuring that the physical and mechanical properties comply with the specified Thai industrial standards.

Methodology: The experimental program was divided into two sections. The first section consisted of three phases aimed at determining the optimal mix proportions to produce paving blocks, ensuring that the specimens achieved mechanical and physical properties in compliance with the Thai Industrial Standard for concrete paving blocks (TIS 827-2531). Phase 1 was a foundational study conducted to evaluate the influence of aggregate grading (fine sand and coarse sand) on the compressive strength of hydraulic cement mortar. In Phase 2, a polycarboxylate-type superplasticizer was introduced to refine the structure, optimizing the physical characteristics and workability required for the manual compaction molding process. In Phase 3, aggregates were partially replaced with three types of plastic waste, including low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polyethylene terephthalate (PET), at replacement ratios of 1.67% and 3.33% by weight of the total aggregate, investigating two particle size ranges: small (less than 5 mm) and large (5 – 15 mm). The second section of the research focused on the production process using the manual hand press technique, involving a comparative analysis of two distinct dual-layer molding approaches: (1) Sequential Casting, where the inner core was molded and cured to harden before being encased in the outer layer, and (2) Simultaneous Casting, a technique where both the inner and outer layers were consolidated and compressed concurrently. Test specimens were cured for 7 and 28 days and were analyzed using compressive strength testing according to ASTM C109/C109M, water absorption testing with TIS 109-2517, and microstructural analysis using Scanning Electron Microscopy (SEM) to investigate the interfacial bonding between the plastic fragments and the cement matrix.

Main Results: The results of the mix proportion evaluation phase indicated that the optimal weight ratio for the outer layer (plastic free) was hydraulic cement : water : coarse sand : superplasticizer at 1:0.35:3.00:0.007, which yielded the maximum compressive strength of 508.6 ksc at 28 days of curing. Regarding the influence of aggregate replacement with waste plastic, mechanical properties exhibited a continuous decreasing trend as the replacement content increased from 1.67% to 3.33% by weight of aggregate. The morphology and size of the plastic types played a crucial role in mechanical performance; specifically, specimens containing HDPE and PET shredded plastics with particle sizes smaller than 5 mm achieved higher compressive strength than those with larger sizes (5 – 15 mm) due to uniform dispersion within the matrix. Conversely, specimens incorporating LDPE plastic exhibited average compressive strengths below the minimum standard requirements across all mix proportions, with microstructural analysis revealing large voids and a high accumulation of needle-like shape of Ettringite. In the comparison of fabrication techniques for the dual-layer concrete paving blocks, where the inner layer utilized HDPE and PET plastics as aggregate replacement at 3.33% by weight of the outer layer mix, it was found that blocks manufactured via sequential casting encountered cold joint issues, leading to defects and interfacial delamination. Conversely, specimens fabricated through simultaneous casting effectively developed a monolithic bond. Consequently, the simultaneously cast dual-layer concrete paving blocks developed high compressive strengths of 481.6 ksc and 499.2 ksc, respectively, along with water absorption rates of 4.4% and 3.7%, respectively, both of which are below 5% and fully comply with the specified industrial standards.

Conclusions: This research demonstrates that the development of dual-layer paving blocks is an effective and viable approach for incorporating plastic waste into construction materials. Both the quantity and size of plastic fragments significantly influence the mechanical properties of the specimens, with HDPE and PET identified as the most suitable materials for aggregate replacement. The simultaneous casting technique is the optimal method for fabricating dual-layer paving blocks, as it eliminates the weak interfacial planes in the structure. Furthermore, the outer layer of the dual-layer structure serves as a protective barrier, shielding the plastic fragments from main degradation factors, such as abrasion and ultraviolet radiation, during practical applications. Adhering to Circular Economy principles, this research transforms plastic waste into high value, standardized construction products, fostering sustainable development that not only reduces plastic waste volume but also effectively possesses the potential to prevent microplastic pollution within the ecosystem