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A flexible mould for double curved

pre-cast concrete elements

Roel Schipper, MSc, lecturer/researcher #1 andprof. dipl.-ing. J.N.J.A. Vambersky, chair of Building Engineering #2

#Dept. of Structural and Building Engineering, Faculty of Civil Engineering and Geosciences, Delft University ofTechnology, P.O. Box 5048, 2600 GA, Delft, The Netherlands

1h.r.schipper@tudelft.nl

2j.n.j.a.vambersky@tudelft.nl

Abstract—The manufacturing of double curvedprecast concrete elements is still expensive, due tothe high costs and limited possibilities for repetitiveuse of the moulds or formwork. The goal of a PhDproject recently initiated at TU Delft is to develop aproduction method that overcomes these difficultiesby enabling the mould to be used many times andby making the shape of the mould adjustable in aflexible way. This paper describes the research goaland method of the PhD project.

I. Introduction

Freeform architecture is gradually gaining marketshare in everyday building practice. Until recentlyfreeform shapes and, more in general, complex geome-tries were merely designed either in an academic contextor in top budget buildings designed by famous archi-tects. In Germany freeform architecture examples arefor instance: Der Neue Zollhof in Düsseldorf (Frank O’Gehry Architects, 1998, see Figure 1), which is madein pre-cast concrete, the Mercedes-Benz Museum inStuttgart (UNStudio, 2006) and BMW Welt in München(Coop Himmelblau, 2007) in cast in situ concrete. InSpain it is for instance the -in mean time famous-Guggenheim museum in Bilbao (Frank O’ Gehry Ar-chitects, 1997), with the façade made in structural steelwith metal sheet cladding.

Figure 1. Der Neue Zollhof, Düsseldorf: final result and theunderlying pre-cast elements

In the past decade, a large number of freeform ar-chitecture projects have won design competitions world-wide [4],[5] and some of them have already been built.The possibilities of freeform shapes have apparentlyopened the eyes of a new generation of young ar-chitects, and a steady movement is now visible from

using freeform only in high profile architecture, suchas museums and corporate headquarters, towards usingit also in more common buildings. Freeform is slowlybecoming adopted as one of the architectural streamsor shape languages, as a natural choice among otherexisting possibilities.

Figure 2. Heydar Aliyev Cultural Centre, Baku (Zaha HadidArchitects, under construction)

II. Pre-cast concrete

and freeform architecture

Prefabricated concrete elements stand for a growingmarket share in the total European building construc-tion volume, both in high and low rise buildings. Pre-cast concrete elements have the advantage of combininghigh quality with relatively high strength and stiffnessand excellent heat accumulation properties. Usually,these pre-cast concrete elements are flat or prismaticin shape, that is, without curves, and the repetitionof these elements (achieved mostly by the use of the“master mould” principle in the manufacturing) makesit possible to reuse a single mould many times duringthe same project. The pre-cast concrete technology isan almost perfect construction technology for freeformarchitecture. Pre-cast concrete offers all of the desiredqualities required in our (environment conscious) humansociety: production by skilled workers in labor-friendlyoff-site environment, freedom of shape, high aesthetic

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value, high load bearing strength, small size tolerancesand safe and fast assembly on-site.

Due to the complexity of the freeform shapes, how-ever, it is often not possible to distinguish any repetitiveelements in the building at all [3]. This is illustrated bythe analysis of elements in the roof of a freeform buildingin 6 on the following page. Furthermore, most freeformbuilding designs consist of many single or double curvedsurfaces which require complex shapes for the mouldand element edges [6]. Since the costs of the moulds(formwork) for precast concrete make up a significantpercentage of the final price per element or per squaremeter, the feasibility of freeform buildings in precastconcrete is still far from optimal. The PhD researchcarried out at Delft University of Technology, Facultyof Civil Engineering, aims to improve the applicabilityof precast concrete through the development of a flexiblemould.

Figure 3. Several flexible mould sketches by (clockwise startingtop left) Renzo Piano, Hans Jansen, Van Roosbroeck, Lars Spuy-broek

III. Earlier research

Some attempts have already been made to design aflexible mould, see a.o. [7] and [1]. Mostly, those designsof a mould system were still rather conceptual (see (3)),not yet addressing all practical difficulties that occurif one tries to build the concept in reality. Dr. Vollersand ir. Rietbergen from Delft University of Technologyhave been working on a material-independent prototypefor a flexible mould. This patented system is based onthe principle of deformation of an originally flat sheetof any material into a double curved surface [8]. Theapplication of this principle on the material concrete in2009 by two Master students in the Stevin Lab in Delft(see Figure 4) has led to interesting observations and abetter insight in the questions that have to be solved [2]. A number of experiments have been performed using aprototype for a flexible mould built earlier by Rietber-gen. In these experiments the complete process startingfrom CAD geometry and ending with the final concrete

Figure 4. Experiments in the Stevin Lab with double curvedprecast concrete based on the patented system [8]

element has been covered, revealing many difficultiesand open ended research questions. The experimentshelped to formulate and clarify the following researchgoal.

IV. Research goal

A production method is to be developed that over-comes the difficulties caused by the curved shapes andlack of repetition. This implies that the method shouldsolve two obvious issues:

1) If the repetition can’t be found in the buildinggeometry, than the mould itself needs to be suit-able for multiple shapes: a reusable, flexible andadjustable mould.

2) Since curvature is one of the main characteristicsof freeform architecture, the mould needs to enablethe manufacturing of (double) curved elements.

Based on the earlier researches and experiments theobjective of the PhD project is to push the developmentof the flexible mould concept for the material concretefrom theoretical concept towards a working industrialprototype: the necessary developments and open endedresearch questions have been identified. At the end ofthe research the design specifications of vital parts ofthe flexible mould should be described, and supportedby theoretical and -if possible also- experimental studyas a proof of concept. A number of practical and morefundamental questions, however, have to be solved first,as described in the following paragraph.

V. Practical and fundamental questions

By examining the results of the earlier experiments inthe Stevin Lab, a number of important questions havebeen identified, some more practical, others of a morefundamental character. These questions will have to beanswered first in order to determine what developmentsare necessary and which boundary conditions should bechosen to design a useful and working manufacturingprocess:

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Figure 5. Simplified research map

• Which manufacturing options for making DoubleCurved Concrete (DCC) elements are availableother than using a flexible mould? What other typesof concrete panel techniques can be distinguished incurrent practice?

• Which obstacles are standing between the theoret-ical concept of a flexible mould for concrete andthe practical implementation of the concept in aworking prototype?

• What specific properties would make a flexiblemould suitable for the industrial manufacturing ofprefabricated DCC elements?

• What properties do the desired prefab DCC ele-ments have or need in terms of size, weight, looks,application field, texture, color, curvature, struc-tural behaviour?

• What type of concrete mixture is suitable for ap-plication in the flexible mould?

• In which applications is it necessary to reinforceelements? Which materials can be used for rein-forcement of DCC elements?

• How to deal with the link between complex geom-etry in CAD models and an automated productionprocess?

• How to deal with mechanical aspects and practicaldifficulties with the machinery that were exposedby the earlier experiments?

The research has been divided in five main aspects(see Figure 5): As can be seen, the development ofa functional machine is depending on a number ofaspects: in the first place a number of recent examplesof freeform architecture will be analyzed in order tomake sure that the developed solution will be practicallyapplicable. Secondly, the more mathematical side of theprocess will be addressed by looking at data exchangebetween CAD and machine, and also at the way complexgeometry can be broken down into elements that can bemanufactured and assembled. Furthermore of course theaspect of concrete mixtures and reinforcement should betaken into consideration. And at last, also the functionalaspects of the manufactured elements form a boundarycondition in the development of any machine. Althougha number of aspects are of a more theoretical characterand need serious desk study, also the more practicalapproach (“just start and do it”) has proved very usefuland inspiring in the earlier experiments. A number ofnew lab experiments is foreseen, as described in thefollowing paragraph.

VI. Planned experiments

The experiments will be centered around the fivethemes shown in (5). Since the Stevin Lab offers ex-cellent facilities for composing and casting differentconcrete mixtures, and the production process in mindrequires a very specific constitution, this part of theresearch will start in the coming months. Some existingarchitectural CAD models have already been examinedin order to, for example, determine the desired elementsize and curvature (See Figure 6). For now, the research

Figure 6. Analysis of a CAD model of a residential building inHet Funen in Amsterdam (NLArchitects) - the project has beenrealized last year using cast in site concrete

will focus on structural elements with a load bearingfunction, such as structural façade elements or roofelements. During the initial lab experiments the rightflexibility and material for the mould will be examinedusing smaller test setups. For the development of a fullscale prototype industrial partners will be invited toparticipate as soon as some tangible results follow fromthe theoretical study and first lab experiments. Theresearch has to come to a final result within the comingthree years.

VII. Conclusion

The PhD research described in this paper is aiming forthe development of a working industrial prototype for aflexible concrete mould. This flexible mould will be ableto produce double curved concrete elements that can beapplied for realizing freeform architecture projects. Theresearch is now in the stage that practical experimentswill be carried out in the Stevin Lab in Delft in orderto determine the right concrete mixtures and mould

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material. Parallel a number of more theoretical aspectswill be taken into consideration. In the Precast2012symposium we hope to report on the results.

References

[1] E. Den Hartog. Prefabrication of concrete shells.Master’s thesis, Delft University of Technology, 2008.

[2] K. Huyge and A. Schoofs. Precast double curvedconcrete panels. Master’s thesis, Delft University ofTechnology, 2009.

[3] L. Iwamoto. Digital Fabrications, Architectural and

Material Techniques. Princeton Architectural Press,2009.

[4] Y-T. Liu. Distinguishing Digital Architecture: 6th

Far Eastern International Digital Architecture De-

sign Award. Birkhäuser, 2007.[5] Y-T. Liu and C-K. Lim. New Tectonics, Towards

a New Theory of Digital Architecture: 7th Feidad

Award. Birkhäuser, 2009.[6] H. Pottmann, A. Asperl, M. Hofer, and A. Kilian.

Architectural Geometry. Bentley Institue Press–Exton–Pennsylvania–USA, 2007.

[7] M.K.H.M. Van Roosbroeck. Construction of prefabconcrete shells. Master’s thesis, Delft University ofTechnology, 2006.

[8] K.J. Vollers and D. Rietbergen. Patent number

2000699, Werkwijze en inrichting voor het vormen

van een dubbelgekromd paneel uit een vlak paneel. NLOctrooicentrum, 2008.

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