Extrudability window and offline test methods to predict buildability of 3D printing concrete

This study investigates the effect of thixotropy and stiffness evolution on the extrudability and buildability of 3D printing concrete. Different types of supplementary cementitious materials and limestone filler were used to prepare mortars with varying levels of thixotropy and early-age stiffness. Mixtures with yield stress and flocculation (τ_floc) of 180–400 and 420–950 Pa, respectively, exhibited adequate extrudability without plastic collapse. The printing was completed 8–15 min after mixing at a vertical build-up rate of 138 mm/min. Several process parameters, including rest time to secure a penetration resistance of 150 kPa (T₁₅₀) and penetration resistance at various rest times were developed to assess early-age stiffness. Results indicate that the penetration resistance at rest time of 10–30 min and T₁₅₀ can be correlated with the buildable height at elastic buckling. A prediction model of buildable height based on penetration resistance was proposed. The early-age hydration was analyzed to evaluate the early-age stiffness.