A sequential computational framework for recycled-material 3D printing concrete: From pumping flow to buildability limit

Abstract

Recycled sand is increasingly used as a sustainable substitute for natural fine aggregate in 3D printing concrete
(3DPC), yet the ultra-fine fraction generated during recycling, recycled concrete crushed fine (RCCF, < 63 µm),
remains underexploited as a partial cement substitute. Existing studies on this material are limited to experimental characterisation, without extending to computational prediction of behaviour throughout the printing process. This study addresses that gap by developing a sequential Computational Fluid Dynamics (CFD) and Finite Element Method (FEM) framework to evaluate an optimised mortar mix incorporating recycled sand
and 10% RCCF (mix FIN 10). The fresh-state flow within the pumping pipe was simulated in OpenFOAM using a Bingham model and validated against nine physical printing trials, with an average deviation of 22% in volumetric flow rate. The post-extrusion structural response was then simulated in CalculiX using a layer-wise activation scheme and a Mohr–Coulomb plasticity model calibrated from fresh-state compression and direct shear tests. The simulation showed that the printed wall could be successfully built up to a height of 540 mm (36 layers) without collapse, reaching a numerically predicted buildability limit of 555 mm at layer 37. The results demonstrate the feasibility of RCCF as a cement substitute and establish an experimentally calibrated, process-scale computational methodology linking pumping flow to post-deposition structural stability for this recycled-material 3DPC mix.