Bending-active formwork systems for concrete shells - A classification and state-of-the-art review

Concrete shells with informed structural geometry allow for material-efficient and reduced embodied carbon structures. However, their construction is typically inefficient, uneconomic, and ecologically wasteful due to custom formworks and labour-intensive reinforcement. Active bending allows for the elastic deformation of initially straight elements into complex, curvilinear geometries without the need for formwork. Moreover, bending-active structures, such as strained gridshells, plate assemblies, and textile hybrids, are material-efficient, lightweight, compactly transportable, and rapidly erectable. Consequently, actively bent structures are highly compatible and found practical application as formworks for concrete shells. This paper presents a comprehensive overview spanning from historic to state-of-the-art examples of bending- active formwork systems for shaping concrete shells. Through a thorough comparison of these systems, the research identifies all aspects relevant to their complete characterisation and proposes a comprehensive classification introducing consistent nomenclature. The classification systematically describes all relevant aspects in four overarching categories - the general contextualisation of the approach, the bending-active system design, the construction design, and the resulting shell design. It critically reflects on the challenges and opportunities of bending-active formwork systems in shaping and potentially reinforcing concrete shells while mapping out potential strategies not yet explored for such systems. Based on all classification aspects, the paper summarises all presented projects in a catalogue and highlights their key innovations. It concludes by reflecting on the most critical common challenges such systems encounter, including shape control to precisely achieve the structural geometry, stiffening for both formwork and shell to enable relevant architectural scales, and pragmatic logistical construction efforts. The discussion situates these potentials into a broader context. Ultimately, this paper aims to serve as a guiding and inspirational foundation for future research and developments towards making bending-active formworks viable formwork solutions. Enabled by the structural geometry of both formwork and shell, these material-efficient systems could have the potential to circumvent the shortcomings of conventional custom formworks with exciting structural and architectural opportunities, thereby advancing the sustainability of the construction industry.