Production of Curved Precast Concrete Elements for Shell Structures and Free-form Architecture using the Flexible Mould Method

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Free-form buildings tend to be expensive. By optimizing the production process, economical and well-performing precast concrete structures can be manufactured. In this paper, a method is presented that allows producing highly accurate double curved-elements without the need for milling two expensive mould surfaces per single element. The flexible mould is fully reusable and the benefits of applying self-compacting concrete are utilised.The flexible mould process work as follows: Thin concrete panels are cast in a horizontally positioned flexible mould, using a self-levelling concrete. After a certain initial hardening, the mould is deformed and the concrete is allowed to harden further. The knowledge about rheological characteristics is essential during casting and to find the suitable moment for the mould to be deformed. The behaviour of the concrete in the plastic stage is important: A) to allow the concrete to follow the deformation of the flexible mould, B) to counteract its movement under a slope and C) to prevent cracking in an early phase. After the flexible mould has reached its final position, the concrete develops its strength and can be demoulded in a short production-cycle; aesthetically attractive elements of different and complex geometries can be produced with the same reusable mould.

Introduction

This paper discusses a very recent innovative technology that is in the focus of architects, inspiring and enabling them to realize new and complex shapes: the flexible mould system. Although architecture with curved geometry, found for example in domes, vaults and shell structures, has been appreciated throughout the centuries because of their inspiring and appealing shapes and structural benefits, in the last decades of the previous century they have become more and more rare. It seems that double-curved structures in concrete, as for example seen in the famous shells built by Torroja, Isler and Nervi, slowly became economically unfeasible (ref. 1), partly as a result of the increased labour and formwork costs, related to the complex shape, and partly because of the upcoming trend to precast concrete structures. Interestingly enough, three parallel developments have recently refreshed the interest for complex and double-curved geometry again: 1) recent CAD paradigms offer powerful modelling tools for parametric and complex-shaped 3D-modelling, 2) rapidly improving computational power of engineering tools enable the structural analysis of such structures, 3) these technological boosts enable and inspire architects and structural designers to apply these shapes in real buildings and structures, to realize shapes that are beautiful and functional at the same time (ref. 2). One problem, however, is not solved for buildings and structures in concrete: how to reduce the formwork costs, that have remained extremely high as a result of the complex shapes with limited repetition?


Concrete has always been a material that was very suitable for this type of architecture, but in the last decade it has even become 'cool' again. Two recent examples of (formwork for) buildings and bridges in concrete can be found in Figure 1. The shown structures use the state of the art regarding present formwork technology: Timber, steel, or plastics to construct the formwork are applied, in many cases CNC-milled. For many free-form structures, the available budget is above average: it is accepted that for the more complex and appealing shape, a certain price needs to be paid. Free-form design simply results in complex shapes with very limited repetitive elements, as can be easily observed analysing Figure 1. Although modern technology such as CNC-milling and perhaps in the near future also 3D-printing may offer accurate solutions with limited labour costs, these technologies are relatively slow for large projects, material- and time-consuming and thus expensive as well. The potential market is growing: free-form architecture is without doubt upcoming. Present formwork technology however, unfortunately, is not yet equipped for this large variation in forms. In this paper, an innovative new formwork method will be presented: the flexible mould. It was developed from idea to feasible and operating method during the PhD research of the first author at Delft University of Technology, supported by both practical and theoretical work of the other authors.