Experimental and Analytical Investigation of the Shear Behavior of the Developed BFRP Reinforced 3D Printed Composite Beams with U-Shaped Seawater Sea-Sand ECC Formwork (2025-12)¶

Construction in coastal and reef-island regions is constrained by limited material availability, the demand for rapid construction, and stringent requirements on long-term durability. To address these challenges, this study develops a 3D printed seawater and sea-sand engineered cementitious composite (3DP-SSECC) compatible with existing concrete 3D-printing systems. BFRP-reinforced composite beams were fabricated comprising permanent U-shaped 3DP-SSECC formwork reinforced with basalt fiber-reinforced polymer (BFRP) bars and filled with seawater and sea-sand concrete (SSC) as the core material. Four-point shear tests were conducted to examine the effects of shear-span ratio, stirrup ratio, and concrete type on shear behavior, including failure modes, crack propagation, load–displacement response, and shear capacity. Results show that 3DP-SSECC enhances shear resistance through three primary mechanisms: (i) the printed interlayer texture provides strong mechanical interlock with the SSC core, effectively suppressing interfacial debonding; (ii) extrusion-induced fiber alignment enables efficient crack control, maintaining service crack widths ≤ 0.25 mm; and (iii) at larger shear-span ratios—where shear resistance is governed primarily by longitudinal reinforcement, the aligned fibers markedly improve longitudinal crack control, increasing shear capacity by up to 83.2 % relative to cast, non-printed seawater-and-sea-sand ECC (SSECC) beams. Furthermore, an analytical model is proposed for predicting the shear capacity of the composite beams, which incorporates fiber-bridging effects and printing-induced anisotropy and shows good agreement with the experimental results over the investigated ranges of shear-span and stirrup ratios.