Interface bonding enhancement for FRP grid reinforced 3D-printed concrete structures

The layer-by-layer deposition characteristic of 3D-printed concrete (3DPC) provides a promising route for fabricating functionally graded concrete (FGC); however, the difficulty in placing conventional reinforcement results in poor flexural performance. In-process embedded flexible fiber-reinforced polymer (FRP) grids have emerged as a potential solution to enhance flexural capacity, yet their effectiveness is often constrained by weak bonding at the FRP grid–concrete interface and insufficient interlace bonding between warp and weft yarns. To address these limitations, this study proposes applying functional surface coatings (i.e., epoxy resin and expansive cement) on FRP grids to simultaneously improve the grid–3DPC interface bond and strengthen inter-yarn bonding, thereby enhancing the flexural performance of flexible FRP grid-reinforced 3D-printed FGC structures. Three-point bending tests evaluated flexural response, while pull-out and interlayer bonding tests quantified interface performance. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were used to reveal the underlying microstructural mechanisms. The results show that the functional coatings significantly improve both the FRP grid–3DPC interface bonding strength and inter-yarn bonding, leading to a maximum increase of 188.3% in flexural strength. Microstructural analyses indicate that the coatings refine the pore structure in the interlayer region, reducing porosity. Moreover, a theoretical model was developed to predict flexural strength, achieving prediction accuracies ranging from 91.40% to 98.12%. This study demonstrates the effectiveness of functional coatings in enhancing flexible FRP grid-reinforced 3DPC structures and provides guidance for developing advanced reinforcement strategies. • Functional coatings markedly enhance FRP grid–3DPC interface bonding. • Flexural strength increases up to 188% via optimized epoxy coating. • Coatings suppress yarn slippage and promote ductile multiple cracking. • Microstructural refinement explains improved bond and flexural response.