Double-curved precast concrete elements: Research into technical viability of the flexible mould method

The production of precast, concrete elements with complex, double-curved geometry is expensive due to the high costcosts of the necessary moulds and the limited possibilities for mould reuse. Currently, CNC-milled foam moulds are the solution applied mostly in projects, offering good aesthetic performance, but also resulting in waste of material, relatively low production speed and fairly high costs per element. The flexible mould method aims to offer an economic alternative for this state of art technology by allowing repeated reuse of the same mould, and if necessary, reuse in adapted shape. A patent and literature review and comparison of state-of-art formwork methods reveals that, although the idea of a flexible formwork already dates from the mid-20th century, in building industry it has not yet found widespread application, and is still experimental to a large extent. In other industries, such as aerospace and automotive, flexible moulds are occasionally used for rapid prototyping purposes, mostly for the forming of thin metal sheets. The understanding of the flexible mould principle in terms of mechanics is still in development. In combination with concrete, the flexible mould has been industrially applied only on occasion. Deliberately imposed deformation of concrete after casting allows the use of only one single-sided flexible mould, but - being a method quite alien to normal precast concrete production - has hardly been investigated. Therefore, models are needed both for the flexible layer as well as it's use in combination with concrete. By analysing a number of architectural cases in terms of geometrical aspects, more information is gathered about building size, element thickness, curvature radius and number and type of elements. This information is used to define the type of shapes for which the flexible mould method would be suitable. Through the last 80 years, the shape of curved architecture has changed; whereas the early famous shell designers such as Isler and Torroja aimed for structurally optimized and material-efficient shapes, nowadays these shapes have mostly made place for free-form curves, in which parametric design or sculptural influences are leading. For larger projects, several hundreds to even thousands of uniquely curved elements are manufactured, varying in curvature radius in a range between 0.75 m and 45 m. Furthermore the contours and edge position can vary from element to element. Prediction of each element's edge position is non-trivial for the flexible mould method, especially not for elements with strong curvature. The deformation process can be described mathematically by analysing thecurvature parameters. An important and meaningful parameter is the Gaussian curvature. Depending on the change in Gaussian curvature, the imposed deformation of the mould surface and the concrete results in certain amounts of bending action (B) and in-plane surface stretching (S). Bending tensile strains in the still plastic concrete can be in the range of 25 to 50‰ for an element with 50 mm thickness, which is far more than the values normally encountered in concrete after casting. The application of in-plane shear deformation appears to be helpful to deform the mould from flat to double-curved. The exact positioning of the element edges can be determined from this in-plane shear deformation. The shape of the mould, in the present research, is controlled by a grid of actuators - extendible support points that follow the intended architectural shape. As mould surface, a thin rubber layer can be used, that, however, has to be supported by a material that is capable of carrying the weight of the concrete without visible deflection between the actuators. Various solutions are investigated for this support material, of which the strip mould offers the most accurate results and predictability. As said, the concrete in this method is deliberately deformed after casting in an open, single-sided mould. This requires control over both the fluidity and strain capacity of the fresh concrete: if the concrete is too fluid, it will flow out of the mould after deformation due to the slope of the mould, if it is already too stiff, cracks may occur. Various experiments are conducted to investigate the viability of the principle as well as the parameters that influence the risk of either flow or cracking. It appears that the use of a self-compacting concrete with thixotropic properties reduces both the risks: as a result of quick stabilisation after casting, the yield strength build-up will prevent flow once the mould is deformed and put at a certain slope. Thanks to it's plastic strain capacity, this type of concrete will be able to undergo the imposed deformation without cracking. An important measure to prevent this cracking is the curing of the concrete directly after casting and a deformation that takes place before initial setting time. Thin steel rebar, glass-fibre textiles or mixed fibres are all applicable as reinforcement, the latter two giving the best results. For the measurement of yield strength development of the concrete mixture before and after casting, various methods are investigated. Literature research and experiments demonstrate that, once the rheological behaviour of a mixture has been determined with a viscometer accompanied with slump (flow) tests, the correct moment of deformation of the flexible mould can later be determined from repeated slump (flow) tests with sufficient reliability. However, as soon as the mixture constituents will be adapted, new viscometer measurements have to be carried out again. The flexible mould method has been successfully tested on single- and double-curved precast concrete elements with a radius down to 1.50 m and an element thickness up to 50 mm. Until this moment, the maximum element size tested was approximately 2 x 1 m2, but larger elements are expected to be feasible. An integrated design-to-production process is required: due to the complex geometry and the impact of this geometry on all aspects of the manufacturing, all parties involved should cooperate to make the use of this method possible. Computational skills are needed to determine design parameters and control the manufacturing process. Several new questions were identified during the research, but at this moment, implementation of the flexible mould method in an industrial environment in cooperation with a concrete product manufacturer is the best way to determine the priorities for further research. From the full research it is concluded that the flexible mould method is viable for the production of double-curved concrete elements