Enhancing impact resistance of self-compacting concrete using recycled PET fibers and metallized plastic fibers: an integrated experimental and analytical investigation

The intrinsic brittleness of cementitious composites under localized dynamic loading, combined with the accumulating environmental burden of post-consumer polyethylene terephthalate (PET) and metallized plastic (MP) packaging waste, motivates the search for concrete mixtures that simultaneously provide energy-absorption capacity and material circularity. This study presents an integrated experimental and analytical assessment of self-compacting concrete (SCC) modified with recycled PET particles as a partial replacement for fine aggregate (0–15% by mass) and two families of discrete polymeric reinforcement, recycled PET fibers and metallized plastic fibers, incorporated at volume fractions of 0–2.0%. Twenty mixtures were proportioned at a constant water-to-cementitious-material ratio of 0.36, with silica fume contents of 5–20% and scoria lightweight coarse aggregate occupying approximately 40% of the total aggregate volume. A total of 240 specimens were characterized through fresh-state assessments (slump flow, V-funnel, L-box, J-ring, and U-tunnel), hardened mechanical testing (compressive, splitting tensile, and flexural strength, UPV, density, and water absorption), and repeated drop-weight impact loading adapted from ASTM D2444 using a 4.54 kg hammer released from 457 mm onto 150 × 65 mm discs. Filling and passing ability deteriorated monotonically with plastic content, with slump flow declining from 679 to 586 mm and V-funnel time increasing from 5.6 s to 10.8 s; linear regression demonstrated that the fiber-bearing series lost flowability more than twice as rapidly per unit PET addition (− 5.24 and − 5.60 mm/%) as the fiber-free series (− 2.49 and − 2.47 mm/%). Compressive strength peaked at 43.4 MPa in the unmodified reference, whereas the peak splitting tensile strength (9.2 MPa), flexural strength (5.28 MPa), and maximum impact resistance (486 blows; 10.8 kJ) occurred in PET-fiber mixtures with substantially lower compressive capacity. This mechanistic decoupling establishes that compressive strength is an unreliable proxy for impact toughness and identifies 10% PET replacement with 1.0–1.5% fiber as the governing design envelope.