Effects of water-to-binder ratio and polyethylene terephthalate (PET) microfibers on the performance of in-situ carbonated concrete

Abstract The increasing demand for sustainable construction materials has encouraged the reuse of waste plastics in concrete to reduce environmental impacts while enhancing performance. Among these, recycled polyethylene terephthalate (PET) microfibers show potential to improve mechanical properties and durability, especially in conjunction with mix design parameters such as the water-to-binder (w/b) ratio. This study investigates the combined effects of water-to-binder (w/b) ratio and recycled polyethylene terephthalate (PET) microfiber inclusion on the fresh properties, mechanical performance, durability and microstructure of in situ carbonated concrete. Lower w/b ratios (0.35–0.40) produced denser mixes with reduced workability, further decreased by PET fiber addition (0.5–1.0% by binder weight), necessitating increased superplasticizer dosages. PET fibers increased air content from 1.8% in plain mixes to 2.6% at 1.0% fiber dosage, while concrete density slightly decreased from approximately 2405–2410 kg/m 3 to 2395 kg/m 3 . Mechanical tests showed up to 20% improvement in compressive strength with lower w/b ratios and up to 15% enhancement in splitting tensile and flexural strengths with PET fibers. Accelerated carbonation testing revealed that at 28 days, carbonation depths reduced from 8.0 mm to 6.6 mm at w/b 0.35 and from 17.5 mm to 14.9 mm at w/b 0.50 when 1.0% PET fibers were incorporated. SEM analysis confirmed well-dispersed fibers with strong bonding and reduced microcracking in low w/b mixes, while XRD results indicated pronounced carbonation through increased calcite (CaCO 3 ) peak intensities and decreased portlandite (Ca(OH) 2 ) peaks, especially in fiber-reinforced, low w/b samples. These findings demonstrate that combining low w/b ratios with recycled PET microfibers significantly enhances concrete’s mechanical properties and durability against carbonation, offering a sustainable approach to producing high-performance concrete.