Environmental Benefits of Concrete Floor Slabs Produced with Digitally Fabricated Formworks (2024-09)¶

A key lever to reduce the negative environmental impact of the construction industry is increasing the structural efficiency of load-bearing structures. Floor slabs are particularly impactful as they (i) entail around half of the material of a typical building and (ii) offer considerable potential for savings through the choice of structurally efficient floor systems [1], such as ribbed slabs. Ribbed concrete slabs are currently predominantly built using standardised, prefabricated elements, which limits the floor plan layout, restricting their use to regular and repetitive floor plans and constraining the load-transfer direction. Digital fabrication (dfab) promises to overcome these limitations and improve the applicability of ribbed slabs by enabling the fabrication of customised geometries. However, several factors restrain the implementation of digitally fabricated concrete, including the lack of code-compliant, generally applicable reinforcement strategies and the high per-volume emissions and cost of digitally fabricated concrete. Using digitally fabricated formworks promises to tackle these challenges by combining the geometric freedom enabled by digital fabrication with the advantages of conventional construction methods. This potential was recently showcased by (i) the use of 3D printed polymeric formwork, enabling concrete savings of around 40 %, while increasing the structural performance [2, 3] and (ii) the use of 3D concrete printed void elements as lost formwork, enabling savings in embodied carbon of around 30 % [4]. These findings are based on single prototypes and idealised floor layouts. However, potential savings through the change in structural system strongly depend on geometry, particularly the span [1]. The two dfab approaches mentioned above are thus applied to a case study of a typical Swiss building with realistic boundary conditions to assess their potential for practical application. For a direct comparison, designs using conventional solid slabs and ribbed slabs are considered. Also, straight and curved rib layouts are included to determine the potential savings through a more direct force flow enabled by optimised rib layouts and the geometrical freedom made possible by dfab. For the given floor layout of the case study, for each fabrication method described above, suitable materials, dimensions, and – in the case of ribbed slabs – rib layouts are selected in pre-design. As the possible combinations are numerous, a semi-automated data generation workflow as illustrated in Fig. 1 was developed: A linear elastic FE-Analysis is conducted for each entity, followed by reinforcement design, including verifications for both the ultimate (ULS) and the serviceability limit state (SLS) in compliance with Swiss Building Codes. Subsequently, comprehensive checks are carried out to ensure constructability. For entities fulfilling all checks and verifications, a bill of quantity (BOQ) is compiled, based on which the environmental impact, including the impact of the formwork, is calculated. The comparison of the environmental impact associated with different solutions allows assessing (i) the potential of digitally fabricated formworks and (ii) the impact of different rib layouts to improve the environmental sustainability of floor slabs. These findings help to promote the implementation of digitally fabricated formworks and highlight areas requiring further research.