Rethinking Structural Concrete for Digital Fabrication

Abstract

Construction works are typically large, immobile and customised, and must meet many criteria to provide value to modern society: structural safety, durability, serviceability, aesthetics and integration, environmental sustainability and construction effciency. Traditional construction methods have been adapted to comply with these multifaceted requirements for more than a century. However, the construction industry has a considerable environmental impact, with reinforced concrete as the primary driver due to its extensive use, and will be facing an ever-increasing responsibility to tackle climate neutrality in the upcoming years. Digital fabrication with concrete (DFC) is a young yet already broad discipline that brings about the potential for reducing the environmental impact and further industrialising of the construction industry. Still, it has not penetrated the construction mass market, which is paramount for having a signifcant impact. The frst part of the present thesis tackles this issue by (i) assessing traditional construction and digital fabrication on a value-driven basis, identifying and summarising their inherent strengths and challenges, and (ii) proposing an ideation process to identify relevant mass-market levers of digital fabrication in the construction industry. The second part of the thesis investigates two exemplary applications conceived within this ideation process: digitally fabricated structural connections and weak interfaces for reducing minimum reinforcement. Connections of structural members are a persistent challenge for traditional reinforced concrete construction and even more so for DFC to meet the promise of delivering geometrically optimised structures with minimised environmental impact. At the same time, DFC opens up new approaches for producing connections, such as for transferring shear forces across interfaces with tailored geometries. An experimental exploration of such shear joints was performed using a push-off test setup to showcase the feasibility of DFC for connections. The programme included the design, fabrication and structural testing of joints using the technology Eggshell to 3D print the formwork, providing (i) unreinforced and reinforced construction joints with varying joint surface texture and (ii) digitally fabricated dry keyed joints with varying key geometry. The joints were produced using set-on-demand concrete with a relatively small maximum aggregate size of 4 mm, as typically applied in DFC. The experimental campaign included thorough instrumentation (i) proving the high geometric accuracy of the produced joint interfaces and (ii) allowing to study the effciency of the varying surface textures. Despite the small aggregate size used, the shear transfer capacity of all tested joints met or even outperformed the requirements defned in pertinent design codes. Crack initiators can be used to diminish the minimum reinforcement for members subjected to imposed deformations and environmental exposure as they reduce the crack spacing and width when arranged close enough. While crack initiators in conventional concrete construction are cumbersome to provide (e.g. by construction joints or taperings), they are inherent to layered extrusion processes with many DFC technologies: the tensile strength is typically reduced locally in the interfaces between layers. An analytical method based on the Tension Chord Model was developed to (i) account for the local strength reduction and (ii) predict the effect of weak interfaces on the expected crack spacing and width. As a key fnding, the model predicts a reduction of the required minimum reinforcement ratio proportional to the locally decreased concrete tensile strength for a specifed maximum crack width requirement under imposed deformations. An experimental campaign on fve layered and three reference tension ties confrmed the clearly positive impact of weak interfaces on crack spacings and widths. These two applications studied in detail served as prototypes to indicate how traditional construction and digital fabrication processes can be combined to tackle the persistent environmental sustainability challenges.