This study explores the use of recycled high-density polyethylene (HDPE) plastic and robo sand to fabricate lightweight plastic coarse aggregates (PCA) for concrete. These PCAs were produced by melting HDPE plastic and mixing it with robo sand in a 1:3 ratio, forming aggregates sized 2.36 mm to 20 mm. The main aim was to replace conventional coarse aggregates entirely and assess structural and sustainability benefits. Three concrete cubes were cast using 100% HDPE-based aggregates, and tests were conducted to evaluate workability, compressive strength, and physical properties like specific gravity and impact value. The results showed a notable reduction in concrete density and a 28-day compressive strength averaging around 29 MPa, indicating promising potential for lightweight structural applications. The project highlights the environmental benefits of plastic reuse in concrete and supports eco-friendly alternatives in construction.