Compressive strength and cost-effectiveness of confined waste plastic bottle brick masonry walls

Abstract

Uganda faces persistent challenges of housing shortages, high construction costs, and environmental degradation resulting from unsustainable resource extraction and poor waste management. Large quantities of plastic bottles and saw dust waste remain underutilized, yet their potential as construction materials has not been sufficiently explored. This study investigated the compressive strength and cost-effectiveness of Plastic Bottle Brick (PBB) masonry walls as a sustainable alternative to conventional concrete block walls in Mbale City, Uganda. The research addressed the gap in empirical data on the structural performance of confined PBB walls with varying filler materials (uncompressed air, saw dust and pit sand). The pit sand PBB walls attained a compressive strength of 0.6 ± 0.02 MPa, which was comparable to that of the hollow concrete block walls (0.6 ± 0.06 MPa). The sawdust and air PBB walls, however, recorded lower mean strengths of 0.3 ± 0.05 MPa and 0.3 ± 0.03 MPa, respectively, which were well below the solid concrete block walls (0.8 ± 0.03 MPa). All PBB variants exhibited greater ductility, with failure strains of 1.8–2.0%, compared to 1.0–1.2% in concrete block walls. Although the PBB blocks did not meet the minimum compressive strength requirements of DS/EN 1996-1-1 DK NA:2019 for load-bearing masonry units, pit sand PBB blocks demonstrated adequate performance for small-scale or low-rise applications, while the air and sawdust filled variants were suitable for non-load-bearing infill walls. A cost-benefit analysis, based on materials, labour, time utilisation, and carbon emission costs, revealed that air PBB blocks were the most economical at UGX 11,694, followed by pit sand filled units, while solid concrete block walls were the least economical at UGX 28,759. The sawdust filled units were relatively more expensive (UGX 27,634) due to higher carbon emission costs. The study also developed a practical production method that enables PBB unit casting at a rate only 17% slower than that of conventional blocks, demonstrating commercial viability and compatibility with existing formwork systems.