Towards a model for structural performance in concrete printing based on buildability and toolpath design

Characterizing the early age viscoelastic behavior of extruded cementitious materials remains a critical issue for 3D concrete printing (3DCP) technologies because the fabrication of large-scale structures requires modeling the behavior beyond the fresh state. This paper provides a model for structural performance that highlights the connection between rheological and mechanical testing results in the fresh state, the relevance of the post-fresh state in buildability for 3DCP, and the impact of toolpath design on hardened state behavior. The overall results show that buildability for 3DCP is largely dependent on post-fresh state behavior and is characterized by thixotropy, early structuration, setting, and hardening properties. The rheological results indicate that oscillatory strain rheology combined with a pre-shear protocol that replicates the shear history of the printing components is a valid method to evaluate fresh state stiffness. In addition, post-fresh state buildability was shown to depend on a mixture of both flexural and plastic collapse in the design of overhang elements. Finally, the toolpath design settings that resulted in a filament overlap ranging from 9% to 25% improved the mechanical performance by promoting the bonding of filaments and minimized anisotropic behavior. The model resulting from this study will be used to create a computational tool to support designers in the design of structures for 3DCP, which will be the subject of a future publication.