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