Properties of the Transition Region in 3D Printed Functionally Graded Concrete Through Near Nozzle Binder-Aggregate Mixing (2026-03)¶

3D Concrete Printing (3DCP) with Near Nozzle Binder-Aggregate Mixing (NNBAM) enables continuous functional grading of mortars by controlled aggregate substitution and adjustment of the paste-to-aggregate ratio. This study investigates the transition region between a lightweight mortar (LWM) and a natural sand mortar (NSM), where the material gradation occurs gradually. Cast specimens of known LWM to NSM proportions were tested and complemented with a printing trial that included the intended material transition. Material tests included fresh state yield stress and penetration resistance, fresh and hardened state density, water absorption and the mechanical properties flexural strength, compressive strength and E-modulus. The printing trial object was submitted to photogrammetry 3D-scanning to determine several geometrical parameters, and samples were taken at equidistant spacing to determine density and water absorption. The results show that the transition region consists of a rapid transition zone followed by a more gradual one. The fresh state strength properties (yield strength, penetration resistance) develop significantly during gradation, but the filament geometry was only moderately affected as the density also increases. At increasing substitution of lightweight aggregate with normal sand, the dry density ranged from 930 to 2134 kg/m3 and water absorption decreased from 19.2% to 7.3%. Mechanical properties increased with the normal sand substitution ratio, with compressive strength ranging from 15.3 to 72.6 MPa, flexural strength from 1.9 to 10.5 MPa, and elastic modulus from 6.9 to 31.1 GPa. Importantly, the results demonstrate that mechanical properties follow distinct non-linear trends rather than proportional scaling with aggregate substitution.