Mechanical performance of M20 and M30 grade concrete reinforced with recycled polyethylene terephthalate (PET) fibers at extended curing ages

This study evaluates the mechanical performance of M20 and M30 concrete reinforced with recycled polyethylene terephthalate (PET) fibers at volume fractions of 0, 0.5, 1.0, 1.5, and 2.0%. The fibers, cut from post-consumer bottles, measured 45 × 1 × 0.37 mm. A total of 360 specimens were tested for compressive, split tensile, and flexural strength at 7, 28, 56, and 90 days, while an additional 18 cubes were tested for water absorption and permeable voids. Each reported value represents the mean of three specimens, accompanied by the standard deviation (SD), coefficient of variation (CV), and 95% confidence interval (CI). A three-way analysis of variance (ANOVA) was performed to evaluate the effects of concrete grade, PET dosage, and curing age. Among the tested dosages, 1.0% PET produced the highest strengths in both grades at all curing ages. At 28 days, compressive strength increased by 13.13% for M20, from 26.66 to 30.16 N/mm², and by 13.05% for M30, from 38.40 to 43.41 N/mm². At the same age, split tensile strength increased by 23.41% and 19.08%, while flexural strength increased by 19.21% and 20.25% for M20 and M30, respectively. Strength declined at dosages above 1.0%. At 2.0% PET, the 28-day compressive strength decreased by approximately 9.4% in both grades, possibly because of fiber agglomeration; however, this mechanism was not directly measured. Quadratic regression estimated an optimum dosage of approximately 1.0% for both grades. An indicative carbon assessment suggested approximately 10% lower carbon emissions per unit of strength at 1.0% PET and the diversion of 13.8 kg of plastic waste per cubic meter of concrete. These findings characterize performance up to 90 days for the two concrete grades and the specific fiber geometry investigated.