Critical threshold fiber content for freeze-thaw resistance in 3D-printed concrete

The durability performance of fiber-reinforced 3D-printed concrete (3DPC) under freeze-thaw cycling remains poorly understood, limiting its application in cold climates. This study systematically investigates the effects of steel micro-fiber content (0.3–1.3 % by volume) on freeze-thaw resistance through mass loss, gas permeability, and microstructural analysis. Two critical fiber content thresholds govern performance transitions: 0.6 % for mechanical optimization achieving 25 % permeability reduction at 12 MPa versus 44 % for control specimens, and 1.0 % minimum for freeze-thaw protection, above which permeability degradation remains manageable (K₁₅₀/K₀ ∼ 15–31) compared to catastrophic failure (>550-fold increase) below this threshold. These thresholds represent transition zones where dominant mechanisms shift rather than absolute boundaries. Printing-induced fiber alignment transforms 3DPC from durability liability to advantage only above the 1.0 % threshold, with controlled fiber orientation achieving up to 25 times better freeze-thaw resistance than cast specimens by converting interlayer weaknesses into reinforced zones. X-ray computed tomography revealed preferential damage in the 89–356 μm pore range, with pore evolution patterns varying with fiber content and printing orientation, establishing that 0.6 % optimizes general 3DPC applications while 1.0–1.3 % is required for extreme freeze-thaw environments. This threshold-based framework provides evidence-based design guidance for developing durable 3DPC systems across varying climatic conditions.