<p>In this study, High-Density Polyethylene (HDPE) plastic aggregates were used to partially replace natural coarse aggregates (NCA) in concrete at replacement levels of 10%, 30%, and 50%. Previous studies have shown that incorporating HDPE aggregates reduces concrete strength. To overcome this limitation, various strength enhancement techniques were applied, including sand coating of HDPE particles, addition of Styrene Butadiene Rubber (SBR) latex (1–3%), and partial replacement of cement with silica fume (10–20%). Additionally, steel fibers were introduced in four proportions (0.5–2% by weight of cement) for each case. The results revealed that mixes with uncoated HDPE and SBR latex exhibited higher workability, whereas sand coating, silica fume, and steel fibers reduced it. Hardened properties—compressive, splitting tensile, and flexural strength—improved significantly with the inclusion of SBR latex, silica fume, and steel fibers. The maximum increases were observed as follows: compressive strength rose by 74% for concrete with 50% HDPE along with 20% silica fume, splitting tensile strength by 54% for concrete with 30% HDPE along with 15% silica fume, and flexural strength by 89% for concrete with 50% HDPE along with 3% SBR and 10% silica fume. The addition of steel fibers further enhanced compressive and tensile strength up to 1% and flexural strength up to 0.5%, though at the cost of reduced workability. Unlike previous studies that investigated single modification methods, this research demonstrates the synergistic effect of combining multiple strength enhancement techniques to enhance the mechanical performance of HDPE concrete. Future research study is recommended to investigate durability properties of HDPE concrete at same or higher replacement levels along with synergistic influence of same strength enhancement techniques.</p>