Adaptive-binder-aggregate mixing (ABAM): Concept for extrusion-based multi-material 3D concrete printing

3D concrete printing (3DCP) enables layerwise fabrication with digital control, offering geometric freedom and material efficiency. However, conventional pump-based 3DCP is constrained by conflicting material requirements, namely sufficient workability for pumping and extrusion versus sufficient resistance to flow and early-age structural build-up for buildability after deposition. This paper introduces Adaptive-Binder-Aggregate Mixing (ABAM), a process concept that avoids long-distance pumping of an aggregate-rich printable cementitious composite (PCC), which can be critical for porous lightweight aggregates and can limit feasible aggregate size and volume fraction. Instead, a pumpable cementitious compound (CC) without aggregates is prepared in the stationary environment and conveyed to the end-effector, where aggregates are stored and incorporated near the nozzle to form the PCC shortly before deposition. The process enables functional material gradation by switching aggregate type during printing, allowing spatial property tailoring within a monolithic element. A prototype implementation is presented together with an initial feasibility demonstration.