Effect of Polyethylene Terephthalate (PET) Fibers on Concrete Properties

This study aims to evaluate the effect of PET fibers, incorporated as a volumetric replacement of sand (0–3%), on the behavior of concrete, in order to identify a dosage range that provides optimal performance. The reference concrete mix was designed using the Dreux–Gorisse method, without the addition of admixtures or reinforcement. This mix served as the basis for the fiber-reinforced concretes. The PET fiber contents investigated were 0, 0.5, 1, 1.5, 2, 2.5, and 3% (0% corresponding to the plain control concrete). Fibers were introduced by volumetrically replacing sand with PET fibers. The water-to-cement ratio was kept constant at 0.55, with a cement dosage of 350 kg/m3. The fibers were obtained from post-consumer plastic bottles and cut into strips 1–2 mm in width and 30–50 mm in length. The experimental program included the slump test (Abram’s cone), fresh concrete density, water absorption (0–192 h), water loss (24–192 h), and mechanical tests in compression and splitting tensile strength at 7 and 28 days. Slump decreased from 6 cm (0 to 0.5%) to 3–3.5 cm (≥ 2%). Density declined almost linearly between 0 and 3% (−6% in the fresh state and −9% in the hardened state). Water absorption reached a minimum at 1.5% (~0.25%) before increasing sharply beyond 2%, with saturation occurring between 144 and 192 h. Water loss reached zero around 144 h, with the longest terminal phase observed at 2.5%. In terms of mechanical performance, splitting tensile strength peaked at 1% (6.94 MPa at 7 days and 8.70 MPa at 28 days), while compressive strength peaked at 2% (18.26 MPa at 7 days and 27.19 MPa at 28 days). Beyond 2–2.5%, the combined effects of low workability and reduced compactness (due to ITZ weakening and entrapped air) negatively affected density, water absorption, and mechanical strengths. Keywords: fiber-reinforced concrete, recycled PET fibers, sand replacement, water absorption, density, mechanical properties