This paper describes the structural design, digital fabrication and construction of KnitCandela, a free-form, concrete waffle shell with KnitCrete, a falsework-less formwork approach using a custom prefabricated knitted textile as multi-functional, structural shuttering layer and a form-found cable net as the main load-bearing formwork. The digitally designed and fabricated textile provided integrated features for inserting and guiding elements such as cables and inflatables that helped shape the sophisticated mould. With a total weight of only 55kg, the 50m 2 formwork was easy and compact to transport. On site, the formwork was tensioned into a timber and steel rig, the pockets were inflated, and then coated with a thin layer of custom-developed, fast-setting cement paste. This paste served as a first stiffening layer for the textile, minimising the formwork's deformations during further concrete application. Fibre-reinforced concrete was manually applied onto the formwork to realise a 3cm-thick shell with 4cm-deep rib stiffeners. The novel approach, for the first time applied at architectural scale in this project, enables the building of bespoke, doubly-curved geometries in concrete, with a fast construction time and minimal waste, while also reducing the cost and labour of manufacturing complex parts.
Conventional construction of doubly-curved concrete structures is a time-, labour-and cost-intensive process. Flexible formworks have already been identified as a possible solution to produce such structures more efficiently. The KnitCrete technology developed at ETH Zurich uses 3D weft-knitted fabrics as stay-in-place formwork, which deliver multiple advantages over woven textiles due to their wider range of feasible geometries and possibility to include features and local material properties. The textile is initially coated with a fast-setting high-strength cement paste. The stiffened membrane is stable enough to serve as formwork for the final concrete layer. This paper discusses potential reinforcing strategies to guarantee structural safety and serviceability in KnitCrete structures. Possible approaches range from the use of the textile as a stay-in-place formwork as well as final reinforcement (by utilising high-strength fibrous materials such as aramid, glass or carbon fibre) to the implementation of geometric features, such as channels within the textile to guide conventional reinforcement or posttensioning tendons. The feasibility and efficiency of the proposed reinforcement strategies have to be experimentally verified, for which a systematic methodology is proposed. Preliminary analyses of the experimental campaign show the beneficial effect of the knitted reinforcement on the cracking behaviour of the textile-concrete composite material. Additional research is needed to exploit the potential of possible hybrid solutions using short steel fibres, post-tensioning or linear steel or glass fibre reinforcement.
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