The production of double-curved precast concrete elements for cladding or shell structures requires expensive CNC (computer numerical control)-milled formwork. As an alternative method, the innovative flexible mould for economically efficient and sustainable production of such elements is discussed in this paper. This method comprises the use of a flexible, CNC-controlled formwork, which is filled with self-compacting concrete. After a short period of thixotropic stabilization in the fresh state, the flexible mould is then deformed into its desired geometry, typically having a strong curvature radius of only a few metres in one or two direction(s). After hardening and de-moulding, the flexible mould can be reused for elements with the same or different curved geometry. The present paper describes the outcomes of a study focussing on two aspects relevant for the abovementioned production method: effect of change of rheological properties in the first 90 min after casting and assessment of the risk of cracking and development of cracks during the deformation process. In an experimental study the following parameters were modified: radius of deformation, moment of deformation in time, panel thickness and water-cement ratio. The presence of cracks after deformation was investigated quantitatively, using a petrographic technology. The results show that for the application of the flexible mould method the plastic stage of concrete is important to be considered.@en

Expensive CNC (computer numerical controlled)-milled formwork is required for the production of double-curved precast concrete elements for cladding or shell structures. The innovative flexible mould method for economically efficient and sustainable production of such elements, developed at Delft University of Technology, comprises the use of a flexible, CNC-controlled formwork, which is filled with self-compacting concrete (SCC). This paper describes how curved precast concrete elements can be manufactured in this open and reusable flexible mould. The proposed method reduces formwork costs of architectural freeform elements made with concrete. First, the method is described briefly, then tests are discussed, demonstrating that by measuring the rheological parameters of the concrete during the process, the right moment of deformation can be determined. The measurements show that thixotropic behaviour of concrete for this manufacturing method is very helpful, since it leads to a quick increase of the yield strength of the fresh concrete, but still leaves concrete deformable in order to prevent cracking caused by the deformation of the mould. The change of the rheological behaviour of concrete in the period between mixing and deformation of the mould was assessed; an additional study was executed in order to assess the integrity of the concrete after the deformation of the mould.@en