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Curved concrete elements
The use of SCC and textile reinforcement
In an earlier article (CPI issue of August 2011 [1]) a method was presented for producing precast curved panels with a thickness of a few centimeters using the 'flexible mould method'. This method is the central theme in the PhD study of the first author on the realization of free-form architecture in concrete (see fig. 1). The concrete panels manufactured with the flexible mould method can be applied for many archi-tectural purposes in which curvature is present, such as fagade cladding, precast plank floors or roof elements. The method essentially com-prises a reusable and bendable smooth surface that can be deformed into a wide range of geometries, including variable curvatures in one or two directions and freely shaped edge contours. In the earlier article in CPI it was concluded that some aspects needed further research, among which were the choice of suitable concrete mixtures and the investigation of fibre reinforcement. This article discusses the results of experiments investigating these aspects: the mixture choice was guided by measuring the workability in time and the ability to undergo deformations after casting without cracking. Furthermore, AR-glass textile was used to strengthen the panels with a flexible reinforcement that allowed deformation in the non-hardened state. Experimental work has been done to check the position of the reinforcement after defor-mation and the contribution of the textiles to the strength of thé panels.
•
Roel Schipper, Steffen Grünewald,
Marijn Kok, Rob Nijsse, Prashanth Raghunath,
Delft University of Technology, Nethedands •
Double curved panels, e.g. like the ones used in the buildings shown in fig. 1, ore more difficult to produce for a c c e p t a b l e prices than o r t h o g o n a l a n d flat concrete panels. Free-form architecture w o u l d benefit from e c o n o m i c a l l y feasible manufacturing methods of such elements [ 2 ] . Moss pro-duction of d o u b l e curved concrete elements has often been r e g a r d e d only possible after the realization of a flexible mould: on adjustable f o r m w o r k consisting of on elastic material that con be formed into the desired curved surface by the use of pistons, actua-tors, gravity, pin beds or other means. Several concepts for a flexible mould have been designed over the years, starting with the sketch of Renzo Piano (fig. 2 a ) , a l r e a d y m o d e in the 1960's. Others also hove w o r k e d on the concept, a m o n g which Lars Spuybroek (fig. 2 b , [ 3 ] ) a n d Florian-Peter Kosche (fig. 2 c ) .A t Delft University o f T e c h n o l o g y , a number of M S c thesis students a n d researchers have been w o r k i n g in the past years on conceptual or real models of the flexible mould, resulting in a g r a d u a l improvement a n d practical realization of the idea in w o r k i n g prototypes. A number of results of this w o r k are shown in f i g . 3.
All of these flexible moulds w o r k a c c o r d i n g to the some principle, outlined schematical-ly in f i g . 4 . The flexible materials of the mould ore supported by a subsystem con-trolling the desired final shape (step 1). The mould is filled with self-compacting con-crete (SCC) (step 2 ) ; fibres or textile can be used as reinforcement. During a short peri-o d peri-of structural build-up, the yield strength of concrete increases (step 3 ) . Then the mould is carefully d e f o r m e d into its final shape (step 4 ) . During this deformation, the fresh concrete has to follow the strain a n d stay stable under a certain slope. Concrete hardens in the d e f o r m e d mould (step 5) a n d finally the element is d e m o u l d e d (step
6). The flexible mould can be reused to pro-duce more elements, with identical or with altered curvature a n d geometry.
Experiments on deformation, rheology and reinforcement
Test setup
For the experiments (see fig. 5) moulds were prepared to produce small concrete panels (length x width x thickness: 8 0 0 x 4 0 0 x 2 5 mm or 8 0 0 x 4 0 0 x 5 0 mm). W i t h this test setup different curvatures a n d slopes could be realized. By using a setup with four iden-tical moulds, i t w a s possible to independent-ly vary parameters, such as time of defor-mation or curvature for each separate mould, which allows comparing the effect of mould setup.
In the l a b o r a t o r y equipment w a s available for dosing, mixing, casting a n d curing the desired concrete mixture, as well as for measuring the rheological behaviour a n d strength of the concrete.
Figure 1: Two examples of free-form architecture (left: Verdana, NL Architects, right: Heydar Aliyev Cultural centre, Zaha Hadid Architects)
• Roel Schipper, MSc, received his civil engineering degree at TU Delft in 1993. He worked as a structural engi-neer. His main area of research is related to free-form architecture in precast concrete. At current time he is a lec-turer and researcher at TU Delft, Faculty of Civil Engineering, Dept. of Structural and Building Engineering.
h.r.schipper@tudelft.nl
• Steffen Grünewald, Dr.-lng., studied Civil Engineering at TU Darmstadt and received his PhD from TU Delft. He is an Assistant Professor at the Concrete Structures Group of TU Delft with research focus on the optimization,
pro-cessing and structural application of special concrete types. s.grunewald@tudelft.nl
• Marijn Kok, born in 1987, obtained his Master's degree in Civil Engineering at TU Delft in 2 0 1 3 . He is special-ized in the field of Building Engineering, Structural Design. For his Master's thesis he has researched textile rein-forced concrete elements produced with a reconfigurable flexible mould.
• Rob Nijsse graduated in Civil Engineering at the University of Delft and started to work in 1979 as a structur-al engineer at the consulting engineering company ABT. In 1 9 9 6 he became director at ABT and in 2 0 0 7 he start-ed to work as a port time professor at the University of Delft in Structural Engineering.
• Prashanth Raghunath, MSc, graduated from the Faculty of Architecture, TU Delft, with a specialization in Building Technology in 2 0 1 2 . He is currently working as a free lance consultam for Precast concrete, along with his father in India, with a focus on Precast/Prefab building solutions for moss housing in developing countries. prashanth.ppf@gmail.com
Figure 2: Three conceptual designs for a flexible mould
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Figure 3: Six prototypes of the flexible moulds designed and/or built at Delft University of Technology (clockwise, starting top left: Jansen, 2004; Quack, 2009; Vollers and Rietbergen, 2009; Janssen, 2011 (2x); Schoofs and Huyghe, 2010)
Deformation
M o r e tlion 5 0 elements were cost with sin-gle or d o u b l e curvature (curved in two directions, leading to e.g. a saddle shape or spherical shape). The radii of the curva-ture were in the range between 1.5 and 2.5 meter. M a n y elements were successfully d e f o r m e d after which they hardened a n d w e r e d e m o u l d e d . In some tests, the con-crete a p p e a r e d to be too fluid at the moment of deformation, leading to con-crete f l o w i n g out of the mould. In fig. 6 the placement of textiles in the mould, a num-ber of h a r d e n e d panels a n d a sawed cross-section with view on the textile ore visible. Generally, no cracks were detected due to
the d e f o r m a t i o n . In some coses small cracks, similar to the ones usually visible as a result of plastic shrinkage, were visible. These cracks a p p e a r e d not to be caused by the deformation itself, but most likely by the evaporation of w a t e r from the concrete sur-face in the p e r i o d between casting and deformation. After c h a n g i n g the procedure a n d covering all freshly cast elements with plastic to prevent evaporation from occur-ring, these cracks no longer a p p e a r e d . The aesthetic quality (e.g. surface smoothness and color, panel edges, air bubbles) was g o o d . The mould material however, a two-c o m p o n e n t silitwo-cone with Shore value 3 0 , chosen for its m a x i m u m flexibility, a p p e a r e d vulnerable for small d a m a g e s .
leading to quick w e a r after multiple costs. The amount of air bubbles was limited, and the edges of the panels were smooth and accurate in shape. To examine the influence of the mould surface quality, in some tests the some mixture as used for the panels was also cost in a smooth, hard-plastic (PE) con-tainer, leading to an extremely smooth and shiny concrete surface. Also a flexible polyurethane mould was used in a number of tests. It is likely that further improvement of the mould material will further improve the aesthetic quality. The d e f o r m a t i o n process itself a p p e a r s not to affect the qual-ity of the concrete.
Rheology of the concrete
The typical process of deformation after casting requires specific workability charac-teristics of the concrete during the different steps: preferably compacting a n d self-leveling during the casting stage (step 2 in fig. 4 ) , but viscous with a plastic behaviour during deformation (step 4 in fig. 4 ) . Due to the slope of the e d g e of the mould after deformation (up to on angle of 4 0 degrees with the horizontal) also a minimal yield strength is necessary, to prevent the con-crete from f l o w i n g out of the m o u l d (figure 7 ) .
The yield strength a n d plastic viscosity of the mixture are two rheological parameters that objectively characterize the workability [ 4 ] . In a number of tests using both tradi-tional slump (flow) tests a n d the BML
Figure 4: The flexible mould production process for double curved precast concrete panels explained in six steps
Viscometer [5] different mixtures w h e r e e x a m i n e d , with respect to the development of the rheological properties in time and to their behavior during deformation.
For the experiments, two types of mixture were used: a medium-grained and a fine-g r a i n e d concrete with a maximum afine-gfine-gre- aggre-gate diameter of 8 mm a n d 1 mm, respec-tively. Concrete and cement paste in partic-ular are thixotropic materials, which means that a time-dependent structural build-up of fine particles takes place w h e n the concrete
is at rest. To enhance this thixotropic behav-iour, fly ash a n d O m y o Betoflow (ultra-fine calcium c a r b o n a t e ) were used as fine parti-cles in a d d i t i o n to cement C E M I 5 2 , 5 R. Utilizing this characteristic allows casting concrete at a much lower yield value (a higher fluidity) than the critical yield stress. After that, the thixotropic structural build-up is utilized to a l l o w deformation o f t h e mould in the p e r i o d between 3 0 to 6 0 minutes after casting. The use of SCC is on obvious choice for relatively slender precast
archi-tectural concrete elements. SCC initially has a very low yield strength, but due to the high p o w d e r a n d fine particle content t h i x o t r o p y increases the y i e l d strength quickly after casting. This results in the abil-ity of the concrete to keep its position after the deformation of a mould. The workabili-ty w a s first tested by determining the slump flow with A b r a m ' s cone. Both mixtures were self-compacting with slump flows of 7 0 0 a n d 6 8 5 mm respectively, measured direct-ly after mixing, for the medium-grained and
Figure 6: left: laminating of AR-glass textile as reinforcement prior to deformation ofthe panels; middle: curved, demoulded and aligned ele-ments; right: 4 layers of textile visible in cut section
fine-grained mixtures, hlowever, a l r e a d y in tfie first fiour, the quick structural build-up increases the yield strength. Repeated slump tests were carried out during the first hour, still using A b r a m ' s cone, filled directly after mixing, but kept at rest in the cone during a certain time span until the cone w a s lifted. The slump values of both mix-tures clearly decreased from a slump of 2 0 to 21 cm at t = 3 0 min-utes after mixing to a slump of only 5 to 8 cm at t = 6 0 minmin-utes after mixing. W i t h i n 3 0 minutes both mixtures c h a n g e d from fluid to half-plastic (from class S5 to class S2 a c c o r d i n g to [ 7 ] ) . The contribution of hydration progress to the increase in the yield strength in the dor-mant p e r i o d is small. The concrete w o u l d become very f l o w o b l e a g a i n after slightly t a p p i n g the slump cones (see fig. 8 ) .
To relate the slump values to Sl-values of yield strength (Pa) a n d plastic viscosity (Pa-sec) BML viscometer tests w e r e carried out. The BML is a c o a x i a l cylinder viscometer with which the rheological val-ues can be determined by creating a controlled a n d measured shear f l o w pattern in the concrete mixture. This is d o n e by rotating a cylindrical container with concrete a r o u n d a stationary torque sensor a n d plotting rotation speed against torque in a g r a p h . From this g r a p h the rheological parameters can be calculated through regression. Directly after mixing, the yield strength w a s in the range of 0 to 10 Pa a n d the plastic viscosity w a s about 4 0 Po-s. During a period of rest of a b o u t 3 0 minutes, the yield strength increased to about 6 0 0 Pa as a result of thixotropic behaviour. This yield strength is sufficient for the start of the deformation process, d e p e n d i n g on the exact curvature a n d panel size.
Glass-fibre textile reinforcement
Since the use of concrete without reinforcement is only a c c e p t a b l e in situations w h e r e brittle behaviour w o u l d not lead to safety prob-lems, it is generally necessary to o d d reinforcement to improve the ductility a n d the strength of the concrete. For example, in f a c a d e c l a d d i n g , stresses due to self-weight, w i n d a n d temperature differ-ences could lead to sudden failure of elements. For this reason, a study w a s carried out on the usability of textile reinforced concrete (TRC) in combination with the typical production method of the flex-ible mould [ 7 ] . Since little concrete covering of the reinforcement is necessary, TRC has the a d v a n t a g e of being suitable for thin ele-ments, such O S the ones manufactured in this research. Research at RWTF^ A a c h e n ( [ 8 ] , [ 9 ] ) indicated the possibilities a n d mechanical properties of TRC. It w a s d e c i d e d to perform a number of deforma-tion experiments, using a very flexible alkali resistant (AR) glass fibre textile o b t a i n e d from 3T TextilTechnologieTransfer G m b H in A a c h e n , a spinoff c o m p a n y of the RWTFH A a c h e n research g r o u p . The textile (cem-FIL 5 3 2 5 ) consists of 2 , 4 0 0 tex AR-glass fibre yarns structured in a bi-axiol mesh. The distance between the yarns is 8.3 mm in both directions. Each yarn consists of 1,600 fibres with a cross-sectional area of 0 . 0 2 7 mm^. The tensile strength of the textile is 1,700 M P a . The textiles w e r e p l a c e d in the (earlier used) fine grained concrete using a lamination process, alternating thin layers of concrete (3-6 mm) with a layer of textile, until the desired element thickness a n d number of textile layers w a s reached (see fig. 6 ) . After this, the standard deformation procedure, as e x p l a i n e d earli-er in fig. 4 , w a s f o l l o w e d . Both single-curved a n d double-curved ele-ments were cost. It w a s observed that the textiles follow the defor-mation into the desired geometry without problems. After harden-ing, the elements were s o w e d in smaller strips to investigate whether the textiles h o d remained in the expected positions, which i n d e e d was the cose. The l o a d bearing behavior of TRC depends on a spe-cific combination of textile a n d fine grained concrete. To investigate the mechanical properties of the TRC used in the deformation tests, bending a n d tensile tests w e r e conducted. As expected the
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Figure 7: Minimal yield strength necessary to prevent concrete from flowing out of mould
forced concrete showed strain hardening behavior, increasing both strength a n d duc-tility.
Conclusion
The p r o d u c t i o n m e t h o d of the flexible mould promises to be a feasible addition to the a l r e a d y a v a i l a b l e manufacturing tech-niques for free-form concrete elements. By using a concrete mixture with self-compact-ing a n d thixotropic behaviour, the deforma-tion of elements after casting into accurate single- or double-curved shapes is possible. By measuring the workability using slump tests or measuring rheological parameters
using 0 viscometer, the right moment of deformation can be determined. Elements with a radius between 1.5 a n d 2.5 meter w e r e manufactured without problems. The deformation process does not cause crack-ing or other d a m a g e of the concrete. By using gloss fibre textiles, the elements can be reinforced to fulfill the design require-ments. The gloss fibre textile does follow the deformation accurately. The number of tex-tile layers can be chosen based on the stresses expected from structural calcula-tions. The mechanical properties of the TRC were checked a n d were a c c o r d i n g to the expectation.
Acknowledgements
The project w a s kindly supported by Co-novation BV, 0 Dutch c o m p a n y specialized in concrete innovation.
• Literature
[1 ] H.R. Schipper and B. Janssen. Manufacturing double
curved precast concrete panels. Concrete Plant
International, 4/2011 p:32-38
[2] H.R. Schipper and J.N.J.A. Vambersky. A Flexible
mould for double curved pre-cast concrete elements.
In Precast2010 Het nieuwe bouwen in prefab beton
-Assembling freeform buildings in precast concrete,
Delft University of Technology, 2011
[3] L. Spuybroek. NOX: machining architecture.
Thames and Hudson, London, 2004
[4] F. de Larrard. Why rheology matters. Concrete
International, 8:79-81,1999
[5] 0. Wallevik. Rheology of fresh concrete such as SCC
-Dr. Wallevik Rheology course. The Island Building
Research Institute, 2005
[6] NEN-EN 206-1 (en) - Concrete - Part 1: Specification,
performance, production and conformity
[7] M.A.D. Kok. Textile reinforced double curved concrete
elements - manufacturing free-form architecture wilh a
flexible mould. Master thesis, Technische Universiteit
Delft, Faculty of Civil Engineering and Geosciences,
Department of Structural and Building Engineering,
2013
[8] J . Hegger and S. Voss. Investigations on the bearing
behaviour and application potential of textile
rein-forced concrete. Engineering Structures, 30:2050¬
2 0 5 6 , 2 0 0 8
[9] J . Hegger, M. Horstmann, S. Voss, and N. Will.
Textilbewehrter Beton - Tragverhalten, Bemessung und
Anwendung. Beton- und Stahlbelonbau, 102:362-370,
2007
Figure 8: Slump tests on a thixotropic mixture: slight vibration by tapping the cone increases the fluidity of the fresh mixture
im
FURTHER INFORMATION
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