Structural design and testing of material optimized ribbed RC slabs with 3D printed formwork

Abstract

[EN] Most of the concrete volume in multistorey buildings is cast in solid slabs, which are frequently flat slabs supported on columns. By using two-way spanning ribbed slabs, concrete consumption could be significantly reduced. However, due to the high costs associated with formwork, such a complex rib configuration is rarely used nowadays. With the advent of technologies for automated formwork fabrication, the material-saving potential inherent in this structural system could again be exploited. This paper investigates the feasibility of material-efficient ribbed concrete slabs on a building scale using conventional concrete and steel reinforcing bars cast inside a three-dimensionalprinted plastic-based formwork. To that end, the code-compliant design of ribbed slabs is first discussed, followed by the introduction of a concept for an automated design-to-production workflow. The sustainability of this slab system is compared to a solution using conventional formwork in a case study consisting of a multibay office building with slabs spanning 8 m in both directions, revealing that ribbed slabs use 40% less concrete than solid slabs. Several representative structural elements of the case study (ribs, slab-column transition) were produced at full-scale and tested until failure to investigate the feasibility of production and structural performance. Three T-beams with various rib shapes (straight, kinked with diaphragms, curved) were tested in a threepoint bending configuration, showing a ductile behavior with longitudinal reinforcement yielding and indicating the relevance of torsional effects in curved ribs. Punching tests on two slab-column connections (ribbed, solid) revealed that the optimized ribbed slab could prevent brittle punching failures and achieve an ultimate load 105% higher than the solid reference slab. All specimens' load-bearing behavior could be predicted using established design formulas, showing the feasibility of producing code-compliant ribbed slabs with the applied technology.