KNIT CONCRETE FORMWORK

ABHIPSA PAL1, WI LEEN CHAN2, YING YI TAN3,PEI ZHI CHIA4 and KENNETH JOSEPH TRACY51,2,3,4,5Singapore University of Technology and Design1,2{abhipsapal.96|chanwileen}@gmail.com3yingyi_tan@mymail.sutd.edu.sg 4,5{peizhi_chia|kenneth_tracy}@sutd.edu.sg

Abstract. The manufacture of concrete funicular shells often relieson traditional formwork construction techniques to provide a sculpturedcavity for the fluid material to occupy (Bechthold, 2004). While thisenables a predictable geometric outcome, the extensive use of timberand/or steel to construct these formworks account for up to 60% ofthe total production cost of concrete and are discarded after the castingis complete (Lloret et al. 2014). Thus, we propose an alternativemethod to create prefabricated modular systems out of concrete castedin customised tubular knitted membranes. These perform as a networkof struts that can be affixed onto 3D printed nodes of a singulardesign. Altogether, these components serve as a kit-of-parts that canbe transported to site and assembled together to create shell geometries.

Keywords. Knitted Textile; Fabric Formwork; Concrete Casting.

1. INTRODUCTIONThis paper demonstrates the design and fabrication of a modular system composedof prefabricated concrete struts that are cast within knitted tubular formwork andassembled together with 3D printed joints. In this scenario, we seek to capitaliseupon the customisability of additive manufacturing methods (i.e. machine knittingand 3D printing) to create a kit-of-parts for the formation of shell-like framegeometries.

RE: Anthropocene, Proceedings of the 25th International Conference of the Association for Computer-AidedArchitectural Design Research in Asia (CAADRIA) 2020, Volume 1, 213-222. © 2020 and published by theAssociation for Computer-Aided Architectural Design Research in Asia (CAADRIA), Hong Kong.

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Figure 1. Lattice shell.

The fluid nature and structural properties of concrete have led to its prevalenceas a construction material in the building industry. As such, concrete is used inthe manufacture of planar and freeform geometries alike. However, the making ofconcrete forms usually entails the use of rigid formwork materials (e.g. timber andsteel) which account for up to 60% of the total production cost (Lloret et al. 2014). These are discarded after the casting is complete, further resulting in increasedmaterial waste.

In response, we propose the use of a strut-to-strut configuration of cylindricalconcrete forms. These can be assembled into a network of beam-like componentsthat can be interconnected to form shell-like geometries, inspired by the FunicularFunnel Shell typology (Rippman & Block, 2013). Each individual strut unitinvolves a bespoke textile machine-knitted out of Ultra-High-Molecular-WeightPolyethylene (UHMWPE) to serve as a flexible tubular formwork. We sew 3Dprinted interface nodes onto the edge of the fabric, enabling it to be linked toneighbouring strut nodes via a snap-fit joint. Lastly, we pretension the fabric tubewith an adjustable jig, before casting it with concrete.

Thus, by integrating bespoke flexible membranes with an adjustable system,this potentially presents a sustainable alternative in the manufacture of complexforms in concrete. Such enables the formation of volumetric cylindricalgeometries with varying diameters or lengths within a single jig. Thus, there is noreliance on traditional formworkmakingmethods and this subsequently minimisesthe generation of waste.

For this paper, we focus primarily on the design of the knitted formworkwith the aim to attain a physical model that has a good fidelity with its digitalcounterpart. Using CNC knitting technology, we investigated this objectiveby changing the input yarn materials and stitch patterns and recording theresultant geometry after concrete was cast into it. This was followed by afurther refinement of the textile formwork to match our target geometry whileincorporating 3D-printed snap fit joints. This cumulated in a 1:3 prototype with 3

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interconnected struts. Furthermore, as part of a master’s thesis, a building designwas proposed for a full scale implementation of this fabrication method. This willalso briefly demonstrate the computational tools employed in the design of thissystem.

2. LITERATURE REVIEWFabric formwork for concrete casting has been an ongoing research domain in botharchitecture practice and academia. In addition to minimising the amount of rigidformwork material, the use of fabric membranes for casting concrete brings aboutother advantages, such as facilitating the formation of curved forms.

Fabric concrete casting can be categorised into two groups: (i) open-facedmoulds and (ii) closed moulds. The first type (i) employs the fabric membraneas a base that can be stiffened in place for more concrete to be applied uponit. Recent works include van Hennik & Houtmann’s thin-walled concrete shellsusing pneumatic formwork (van Hennik, P. C., Houtman, R., 2008), Veenendaal’scable-net system (Veenendaal, D., Block, P., 2014) and Popescu’s KnitCandela(Popescu, 2018). The second group (ii) involves concrete being poured intothe cavity of a bounded fabric to solidify. Works that adopt this strategy areWest’s variable concrete truss (West, 2007), Yo_cy’s ‘Cast Thicket’ (Tracy, K.,Yogiaman, C., 2014) and Culver& Sarafian’s ‘Fabric Forms’ (Culver, R., Sarafian,J., 2016). We review the latter two built works which we find relevant to ourproposal.

Yo_cy’s ‘Cast Thicket’ explores the principle of a fully cast-in-placeinstallation consisting of multiple slender members. Yogiaman and Tracyassembled strips of polypropylene formwork with tabbed seams around internalsteel bars, enabling the entire structure to be cast as a single piece with embeddedreinforcement.

Culver & Sarafian’s ‘Fabric Forms’ project takes a different approach throughthe use of robotic arms to form individual Y-shaped struts using stretched Lycrafabric. With one ‘leg’ secured onto a stationary jig and the other two affixedto 6-axis robotic arms, they poured concrete onto the internal cavity of thepretensioned formwork. The hydrostatic pressure from the concrete deformed thestretched elastic fabric in accordance with their simulated model. Upon curing, thephysical ‘Y’ components were affixed together into a lattice space frame using 3Dprinted connectors that could be bolted together with their adjacent neighbours.

While both projects demonstrate successful formation of lightweight concreteelements using tensile membrane formworks, each reveals how the materialinteraction between the formwork and hydrostatic pressure from concreteinfluences the final outcome. ‘Cast Thicket’, due to the use of largely inelasticpolypropylene sheets, experiences buckling of the surface when concrete is pouredinto the formwork. This causes geometric inaccuracies in the installation. On thecontrary, the resultant geometry of ‘Fabric Forms’ is dependent on the interactionbetween the tensioned elastic Lycra fabric and the weight and hydrostatic pressureof the concrete. The interplay of material and forces introduces interestinggeometric outcomes, but it is challenging to attain a target geometry without the

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use of physics-based simulations.Thus, we can see that it is vital to take into account the materiality of the

fabric formwork in order to enhance the fidelity of the physical cast prototype.The capacity for the fabric to stretch needs to be stiff enough to resist the pressureexerted by the concrete, while also being adequately elastic to be pretensioned tominimise the effects of buckling. As such, we turn to a machine-knitted formworkof high performance yarns where we can customise its elasticity in differentdirections and also its initial shape. This tunability of both the fabric’s structureand material will be explored more in the next section.

3. EXPERIMENTATIONIn this research, we explored using CNC-knitting technology to alter the elasticityof our fabric formwork in the vertical (walewise) and horizontal (coursewise). Thisfabrication technology offers a stitch-by-stitch level of control to modify the yarnmaterial, stitch pattern and loop lengths. We input these designs into a proprietarysoftware which converts the code into machine instructions. The knitting machinereads the machine instructions and then knits a bespoke textile seamlessly in asingle ‘print’. For our experiments, we produced our samples on a 15-gauge ShimaSeiki MACH2XS weft knitting machine.

Our goal was to create a relatively inelastic tube that could resist the hydrostaticpressure exerted by concrete yet could still be tensioned to prevent surfacewrinkling. Thus, we decided upon a tubular jersey stitch pattern composed fromUHWMPE yarns twisted with spandex and nylon to form a cylindrical cavity. Thischosen stitch pattern coupled with the low extension of the yarn material resultedin a fabric that is stiff in the course-wise direction and could be stretched in thewalewise axis.

Figure 2. (Left) Outer face of knit mould, (Right) Inner face of knit mould.

3.1. INITIAL TESTS AND PARAMETER VARIATIONS

In our first test, we knitted a tube of 65mm in diameter with the above-mentionedknitting parameters. We tensioned this upon a custom-made jig made from

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threaded rods and MDF bases for the top and bottom plates. We affixed the fabricwith 3D-printed nodes, with the top being open and the bottom being closed, andattached each end to the plates. This was done in such a way that the orientation ofthe fabric with its walewise axis in the vertical direction (i.e. parallel to the rods)and the coursewise axis parallel to the plates. Such a configuration resulted in thestiffer coursewise axis resisting bulging from the poured concrete.

Figure 3. Knit tube being stretched.

In addition, we varied the knitting parameters and walewise stretch ratios.Specifically, these included inlays in the coursewise direction where a continuousUHWMPE yarn is woven in front of every alternate loop to restrict the extension inthe coursewise axis. We hypothesised that this will be more effective in counteringthe effects of hydrostatic pressure. The walewise stretch factors were variedbetween zero stretch and the maximum of 1.5 times stretch. We believed thatthe stretching of the knit works together with the inlays to maintain the cylindricalform of the cast element, while preventing wrinkling of the surface.

In these casting attempts, we poured our concrete mix incrementally in smallbatches to prevent the tube from deforming excessively under the full weight ofthe concrete. This also meant that the bottom layers had time to harden and couldtherefore resist the load from the subsequent pours.

During the cast, we observed that water and small amounts of cement wouldseep out from the tube. This could be attributed to the porous nature of theknitted membrane structure despite a relatively tight knit. Our experiments alsorevealed that inlays are effective in restricting the expansion at the bottom halfof the cylinders. Without the addition of inlays, the cylinder expands at thebase and tapers three-quarters towards the top, which could be the result of theaforementioned hydrostatic pressure (see Fig. 4a). The walewise stretching alsofacilitates the control of the geometry where the bulging is negated while providinga consistent symmetrical geometry (see Fig. 4c,d). Thus, our subsequent castsutilised the parameters reflected in Fig 4d. The main knit structure is UHMWPEtwisted with spandex and nylon.

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Figure 4. Experiments with inlays and stretch factors.

3.2. INTEGRATION WITH ADJACENT MEMBERS

Our next series of casts investigated connectivity with adjacent members. For this,we designed a strut configuration which consisted of 3 valences coming togetherat an intersection point. The struts were flared at their ends to interface withtheir neighbours and we performed this by modifying the textile’s initial shapeby narrowing and widening the perimeter edges. The concrete mix used was sand(64%), cement (23%), water (13%).

Our first design (see Fig. 5) presented a funnel shape on both the top andbottom of the knitted fabric, allowing the cast element to fit with the othermemberswith a 120 degree angle. However, the design proved to be problematic due to thelarge cross-sectional profile at the flared regions, resulting in chunky ends with anarrow and thin central ‘neck’ which was prone to cracking.

Figure 5. First flared funnel-like strut.

To fix the initial design, we edited the knit profile to reduce the large flaredends. The longer, sharp ends of the knit profiles were slightly shortened so that

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the problem areas would be stiffer after tensioning; hence they would bulge less.We also straightened the knit profile which made the fabric more cylindrical. Thiswill ensure that the smallest cross sections of each cast would not be too narrow.

Figure 6. Refined cylindrical strut.

4. Scale PrototypeOur experiments led to themaking of a 1:3 scaled prototypemodel with three strutsconnected at a single intersecting point. Wemade the three struts out of the profilesin Fig. 6 and designed 3D printed joints that functioned as the interface betweeneach strut member. The design of these 3D printed joints had curved indentationswith holes that could be locked together with several steel threaded rods (see Fig.7).

Figure 7. 3D printed joint and connector detail.

These joints also had additional functionalities where they acted as holders toposition the embedded steel rods in place which serves as reinforcement. Theedges of the joints consisted of tiny holes for the fabric to be sewn onto beforecasting was performed. Moreover, the joints also incorporated a central hole topour the concrete into the internal cavity.

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Figure 8. Scaled 1:3 prototype with 500mm length struts and having an overall size of750x750mm.

4.1. DESIGN PROPOSAL

The research gathered in the previous sections was implemented in a masters thesisproject where this system of concrete struts was used to compose a funicular roofstructure of a flower market based in India. Using planned circulation lines toderive the starting mesh, the roof was generated using a particle-springs simulationvia the Kangaroo plug-in.

Figure 9. Mesh refinement for particle-springs simulation.

The starting mesh was further refined with the strut density being highest at thecentral axis and decreases in density away from this axis. The pattern is formedby connecting the centre of each mesh face, so a custom mesh was made to varythe pattern density. Using Weaverbird, a surface frame was made, which was

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thickened and smoothened to create the final structure. We also chose a colourscheme that was relevant to the building context, given that it was a stay in placeformwork that could be any colour.

Figure 10. Proposed Lattice Roof Structure.

5. Discussion and AssessmentOur proposed fabrication system presents a method to create prefabricated strutelements with a single knitted fabric formwork on an adjustable tensioning jig.Thus, this has the advantage of making customisable strut members with a singlebespoke fabric formwork. This also essentially breaks down the overall latticestructure into modular components that can be easily transported and assembledon-site.

The creation of individual strut components also overcomes the limitations ofthe knitting machine in terms of the maximum needle bed width of 1.5m. Thispotentially enables the creation of each member without the need for sewing toother pieces of fabric, especially when doing a large-scale single cast.

Additionally, our current research examines assessing our concrete strutsquantitatively in terms of its structural performance. Using 4 point loading tests,we are examining the static and dynamic flexural stiffness of the tubular cast.These assessments look into the possibility of the outer textile contributing as atensile component which works with the bonded concrete to improve its flexuralproperties. We hypothesize that the textile layer could also help in resisting theeffects of creep since it is bonded with the concrete.

Another benefit is the haptic quality of the knitted fabric which alters theappearance of our concrete struts . The soft fabric contrasts with the hardappearance of bare-faced concrete and the fabric can be designed by altering theyarn colour andmaterial. This provides an added layer of design over the aestheticsof the struts, which further enhances the spatial quality of the structure. However,an extra step of scrubbing off the concrete bits that bled out and hardened onto thesurface was necessary to achieve a desirable finish.

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6. Future WorkIn summary, our proof-of-concept prototypes that present the possibility ofcreating new formal qualities and spatial configurations using fabric formwork.Our research proves, at a single strut scale that it is feasible to control thegeometry of each strut and this therefore offers the potential to create relativelyhigh fidelity physical models when compared to their digital counterparts. Ona larger scale, these network lattice configurations show potential for makinglightweight structural systems that could ease transportation and on-site assembly.

Future work would involve refining the connection details to accommodatenon-planar strut arrangement as well as a higher valence. We also plan to test on1:1 prototypes that assemble into overall funicular structural systems like archesor domes constructed out of multiple casted struts.

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