Recycled PET Fibers with Dopamine Surface Modification for Enhanced Interlayer Adhesion in 3D Printed Concrete

Three-dimensional printed concrete (3DPC) is increasingly recognized in the construction industry for its high design flexibility and the elimination of conventional formwork. However, weak interlayer adhesion remains a significant challenge. The potential of recycled polyethylene terephthalate (PET) fibers for reinforcing 3DPC is being explored, driven by their environmental sustainability and economic advantages. However, there is an inadequate interfacial adhesion between these recycled fibers and the 3DPC matrix. This study investigated the use of dopamine modification to address this issue and enhance the interlayer adhesion of fiber-reinforced 3DPC. Recycled PET fibers were surface-modified using dopamine treatment, forming a polydopamine (PDA) film that improved surface roughness and hydrophilicity. Both unmodified and modified fibers were incorporated into 3DPC at various volume fractions (0.1%, 0.3%, 0.5%). The effects on interlayer adhesion strength, compressive strength, and flexural strength were systematically evaluated and compared. The results showed that the inclusion of 0.3 vol% dopamine-modified fibers resulted in a 22.5% increase in interlayer adhesion strength compared to the control group, and a 14.8% improvement over unmodified fibers at the same content. Additionally, the compressive strength and flexural strength of 3DPC with 0.3 vol% MPET fibers increased by 22.5% and 27.6%, respectively, compared to the control group. Microstructural analysis using SEM and XRD revealed that the dopamine modification significantly improved the interfacial adhesion between fibers and the concrete matrix, explaining the superior performance of modified fibers. This study demonstrates that recycled PET fibers modified with dopamine can effectively enhance the interlayer adhesion of 3DPC. The findings affirm that surface modification techniques can significantly elevate the utility of recycled PET fibers in 3DPC, contributing to the sustainable advancement of construction materials.