Mechanical Performance and Compressive Behaviour of Plastic Waste–Based Brick under Horizontal and Vertical Loading Orientations

Abstract

Plastic waste–based bricks (PW-Bricks) offer a promising pathway for plastic waste valorisation in the construction sector; however, their mechanical performance is commonly evaluated using a single compressive strength metric that does not reflect actual masonry loading conditions. This study investigates the compressive behaviour of PW-Bricks under horizontal and vertical loading and proposes an orientation-sensitive evaluation framework for mix optimization. PW-Bricks incorporating 20–50% plastic waste were fabricated alongside conventional cement–sand bricks as control specimens and tested at curing ages of 3, 7, and 10 days following ASTM C109/C109M-21. The results demonstrate a pronounced orientation-dependent response. Under horizontal loading, compressive strength increased with plastic waste content and curing age, reaching 30.15 MPa for PW-Bricks containing 50% plastic waste at 10 days, compared with 22.74 MPa for conventional bricks. In contrast, under vertical loading, PW-Bricks exhibited relatively low and nearly constant compressive strengths ranging from 1.08 to 1.22 MPa, whereas conventional bricks achieved 2.48 MPa at 10 days. To evaluate this trade-off, normalized horizontal and vertical strength indices were integrated into a composite performance index balancing enhanced horizontal resistance with minimum vertical stability requirements. Based on this evaluation, PW-Bricks containing 40% plastic waste were identified as the optimal mix, providing the most favourable balance between mechanical performance, material homogeneity, and sustainability. The proposed framework offers a transferable methodology for evaluating heterogeneous waste-based masonry materials and supports application-oriented mix design for non-load-bearing and semi-structural masonry systems.