Interlayer Strength Loss in 3D Printed Concrete Due to Time-Gap-Induced Macroporosity (2025-10)¶

Interlayer bond integrity governs the structural reliability of 3D printed concrete and is degraded by pauses between layers. This study used X-ray computed tomography to quantify interlayer pore morphology and conducted mechanical tests in splitting, shear, and flexure to measure interlayer strength. Pore statistics were used to parameterize a random pore reconstruction model that resolves stress fields and predicts strength, and its predictions agreed with measurements within 15%. The results showed that a 10 min gap transformed isolated pores into a 3 mm thick interlayer macro void band, which accelerated crack initiation and coalescence and caused pronounced strength loss. Longer gaps from 30 to 120 min promoted lateral proliferation and clustering of macro voids, after which shear and flexural strengths decreased approximately linearly with interlayer porosity, while splitting strength approached a plateau. For this concrete system, maintaining the time gap below 10 min or the interlayer macroporosity below 2% is advisable to preserve interlayer strength. Simulations also indicate a size effect driven by dense macro void clustering, with a fourfold increase in specimen size reducing predicted flexural strength by up to 14.2%.