Overcoming Environmental Stress Cracking of FDM 3D Printed Formwork for Counter-Pressure Casting of Concrete.

The rapid growth of interest toward concrete digital fabrication reflects the current aspiration for better, smarter, faster, and greener construction means. Among a broad variety of techniques developed by our community, digital casting presents clear advantages regarding dimensional precision, geometrical freedom, and surface finish of the produced elements. In contrast to robotic slip forming, the usage of digitally fabricated formworks requires simpler equipment. It, however, calls for easily shaped formworks, typically best three-dimensional (3D) printed, for example, by fused deposition modeling. While such molds can be easily fabricated with a wide range of commercially off-the-shelf available 3D printers, a shortcoming is the susceptibility of many polymers to environmental stress cracking, particularly when in contact with high pH solutions typical for cementitious materials. This article confirms the problem posed by this type of environmental stress cracking and presents two very effective means of circumventing it: A silicone coating and cyclic olefin copolymer. Apart from this, in the specific case of counterpressure casting (CPC), hydrostatic pressure must be resisted by a powder bed surrounding the formwork. The efficiency of such beds is examined and a particular mixture of sand and lead is shown to be particularly effective, provided its density is regulated to balance stress principles derived from soil mechanics. Presented applications include the successful CPC of thin prismatic formworks with a concrete height up to 3 m as representative of typical interfloor load-bearing elements. The combination of counterpressure and stress control is shown to be essential for such achievement, highlighting the potential of this approach as a viable member of the concrete digital casting family.