Crack initiators in reinforced concrete structures can facilitate fulfilling the serviceability requirements. They can be used as a design parameter to diminish the minimum reinforcement for members subject to imposed deformation and exposed to the environment 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 digital fabrication technologies: the tensile strength is typically reduced locally in interfaces between layers. Rather than trying to avoid these weak interfaces, this paper discusses the potential of taking advantage of them to act as crack initiators reducing the minimum reinforcement content. A tension chord-based model is 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 finding, the model predicts a reduction of the required minimum reinforcement ratio proportional to the locally decreased concrete tensile strength for a specified maximum crack width requirement under imposed deformations. An experimental campaign on five layered and three reference tension ties confirmed the clearly positive impact of weak interfaces on crack spacings and widths.

The construction industry needs to reduce its large environmental footprint drastically. Building with less material is one of the main levers for reducing this negative impact. This material reduction can be achieved with structurally efficient geometries requiring a higher degree of complexity than typically applied in conventional construction practices. Digital fabrication with concrete has been proposed as one of the solutions to facilitate the fabrication of efficient structures. Over the last few years, extensive research has been conducted within the National Centre of Competence in Research (NCCR) Digital Fabrication at ETH Zurich to investigate digital fabrication with concrete for structural applications. Various digital technologies were investigated, including 3D concrete printing, digital casting, Mesh Mould, printed polymer formworks and knitted formworks. This contribution highlights the main findings of these investigations with a particular focus on the development of reinforcement strategies, as these strategies are an essential step to ensure compliance with existing design guidelines and ease of mass-market adaptation. Promising future research areas are identified based on the assessment of the technology readiness and sustainability potential of the investigated approaches.

EN 1992-1-2 generally limits the redistribution of bending moments from the intermediate supports to the span for continuous reinforced concrete slabs and beams in fire conditions to 15%. While higher redistributions are allowed if sufficient rotation capacity is provided, EN 1992-1-2 does not indicate how to assess the rotation capacity. However, plastic hinges caused by the rotation demand under fire conditions are highly relevant when predicting the global response and structural safety of a structure (partially) exposed to fire. Rotation capacity is specifically necessary at support regions subjected to negative bending and fire, where concrete in compression undergoes thermal degradation while the tension chord remains close to ambient temperature. This article presents a comprehensive model for the behavior of statically indeterminate members in fire conditions, enabling to estimate whether sufficient rotation capacity is provided. Material properties specified by EN 1992-1-2 are applied combined with complementary considerations concerning (i) the biaxial compressive strength of concrete, (ii) strain hardening and limitations of the ultimate strain of reinforcement, as well as (iii) tension stiffening. Tension stiffening detrimentally influences the ductility of the tension chord, limiting the rotation capacity. When comparing predictions obtained by the model to experimental results given in the literature, the correlation is good for the investigated one-way continuous slabs and beams. However, considerable uncertainty exists regarding the type of concrete aggregate used. Moreover, uncertainties concerning the behavior of concrete under compression and fire conditions are highly relevant for modeling the region of supports with rotational restraint.

This paper investigates the effect of material properties, boundary conditions, and related modeling and design uncertainties on the fire behavior of statically indeterminate reinforced concrete beams and slabs by means of a parametric study based on a comprehensive model. This model uses material properties specified by EN 1992‐1‐2, complemented by considerations concerning the biaxial compressive strength of concrete, strain hardening and limitations of the ultimate strain of reinforcement, as well as tension stiffening. The parametric study identifies and explains the most influential parameters governing the fire behavior of statically indeterminate reinforced concrete beams and slabs. The implementation and generalization of these parameters are evaluated and compared to current design rules in EN 1992‐1‐2 derived from the evaluation of experimental testing and real fire cases. Overall, the detailing rules given in EN 1992‐1‐2 are found to be reasonably safe, and they can be easily used for practical applications. Furthermore, the results of the study indicate that model predictions for the studied statically indeterminate systems are subject to considerable uncertainty because (i) information on the used material is possibly incomplete and (ii) the models given in design codes do not (or only insufficiently) cover all relevant aspects of the thermo‐mechanical behavior. Specifically, the concrete aggregate type with its corresponding thermal expansion, the strain hardening properties of the reinforcement and tension stiffening with its detrimental effect on the ductility of the tension chord may affect the rating across several standard fire resistance times of statically indeterminate reinforced concrete members subjected to bending.

Connections are a persistent challenge for traditional reinforced concrete construction and even more for digital fabrication with concrete (DFC). The latter, on the other hand, opens up new possibilities for producing con-nections with tailored geometries and surface roughness. Based on the findings of an exploratory, experimental campaign, this paper discusses the design, fabrication and structural testing of digitally fabricated joints using the DFC technology Eggshell to 3D print the formwork for joint surfaces in an additive manufacturing process. The programme includes (i) unreinforced and reinforced construction joints whose joint surface texture was varied to control the roughness and (ii) digitally fabricated dry keyed joints with varying geometry. Both, the construction joints as well as the dry keyed joints, were produced using set-on-demand concrete with a relatively small maximum aggregate size of 0 to 4 mm - i.e., strictly speaking, mortar rather than concrete - as typically applied in DFC. The experimental campaign included thorough instrumentation: a 3D scan of the joint surface before testing and high-resolution digital image correlation (DIC) combined with distributed fibre optical sensing (DFOS) during testing. The deviations from the targeted surface geometry, determined with 3D scans, were below 2 mm, with a standard deviation between 0.4 mm and 0.7 mm for keyed joints. The high-resolution DIC allowed measuring the compressive strains locally at (i) the corrugation of unreinforced or reinforced con-struction joints and (ii) the key chamfer of dry joints and, hence, studying strain variations over the height of the joints and the efficiency of different keys with varying dimensions. Finally, the shear transfer capacity, evaluated in a push-off test setup, showed that all digitally fabricated joints met or even outperformed the requirements defined in pertinent design codes despite the smaller aggregate size.

Structural ConcreteVolume 24, Issue 3 p. 3119-3123 ISSUE INFORMATIONFree Access Issue Information: Structural Concrete 3/2023 First published: 25 June 2023 https://doi.org/10.1002/suco.202370311AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Volume24, Issue3June 2023Pages 3119-3123 RelatedInformation

Most current technologies in digital fabrication with concrete (DFC) rely on controlled environmental conditions and, thus, have been used in prefabricated construction. Prefabricated reinforced concrete elements produced in factories need assembly and connection on-site. Using DFC for producing tailor-made geometries and applying surface roughness generates new possibilities for the design of connections. DFC enables (i) fabricating dry connections, for example, by using exact formworking or milling processes, and (ii) the relatively straightforward preparation of rough construction joints, for example, by using extrusion processes. In a recent study, a series of different specimens incorporating connections were tested using deformation-controlled push-off tests. This contribution presents the experimental campaign including design, preparation and test results employing keyed connections produced with the Eggshell technology, a fabrication process using 3D printed thin plastic formwork.

Providing reinforcement is essential for the structural integrity of concrete elements and for safely handling, transporting, and assembling prefabricated concrete parts. However, the integration of reinforcement is a persisting challenge in 3D concrete printing with extruded concrete. This paper presents a production process consisting of 3D printing around pre-installed reinforcement. The reinforcement is composed of conventional reinforcing steel bars, which can be pre-assembled in cages independently of casting, boosting the specialisation and efficiency in production. This approach was used to produce a 3.4 m span T-beam with optimised topology, consisting of three segments connected with matching surfaces. The beam segments were printed upside down, with an open web on top of the flange. Each segment featured reinforcing steel installed in the flange and web. After printing and assembling the segments, a conventional reinforcing bar was inserted in the web as bending reinforcement and grouted subsequently. The structural performance was assessed in a six-point bending test. The fabrication and structural testing of this case study beam showed that pre-installed reinforcement imposes several challenges to the extruder precision, the precision of the bent reinforcement, and – if applied – the casting after printing.

The construction industry produces buildings and infrastructure. These construction works are typically 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 efficiency. Accordingly, traditional construction methods, along with the resulting construction works, 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 its 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 is a young yet already broad discipline that brings about the potential for the necessary reduction of the environmental impact and further industrialisation of the construction industry while being compatible with the multifaceted requirements for construction works. Still, it has not penetrated the construction mass market, which is paramount for making a significant difference towards improving the environmental impact of the construction industry. The present study 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 a value-driven ideation process to identify relevant mass-market levers of digital fabrication in the construction industry. The presented methodology indicates two exemplary applications of how traditional construction and digital fabrication processes can be combined to tackle the persistent environmental sustainability challenges.

The concrete used in floor slabs accounts for large greenhouse gas emissions in building construction. Solid slabs, often used today, consume much more concrete than ribbed slabs built by pioneer structural engineers like Hennebique, Arcangeli and Nervi. The first part of this paper analyses the evolution of slab systems over the last century and their carbon footprint, highlighting that ribbed slabs have been abandoned mainly for the sake of construction time and cost efficiency. However, highly material-efficient two-way ribbed slabs are essential to reduce the environmental impact of construction. Hence, the second part of this paper discusses how digital fabrication can help to tackle this challenge and presents four concrete floor systems built with digitally fabricated formwork. The digital fabrication technologies employed to produce these slab systems are digital cutting, binder-jetting, polymer extrusion and 3D concrete printing. The presented applications showcase a reduction in concrete use of approximately 50% compared to solid slabs. However, the digitally fabricated complex formworks produced were wasteful and/or labour-intensive. Further developments are required to make the digital processes sustainable and competitive by streamlining the production, using low carbon concrete mixes as well as reusing and recycling the formwork or structurally activating stay-in-place formwork.

The construction industry produces buildings and infrastructure and, hence, satisfies basic needs of modern society. It provides a considerable portion of total value added and worldwide jobs. At the same time, it is responsible for more than 11% of CO2 energy-related carbon dioxide (CO2) emissions worldwide. Digital concrete fabrication is a young, yet already broad research discipline which brings about potential for the necessary reduction of the ecological impact and for further industrialisation of the construction industry, while being compatible with the specific requirements of flexibility and individuality. Still, it has not succeeded to penetrate the mass-market, largely due to lacking competiveness and compliance with structural integrity requirements. The present contribution comprehensively considers features of conventional construction processes to identify benefits when using digital concrete fabrication. Thereby, it addresses not only a complete substitution of conventional construction methods, but also solutions taking advantages of the synergic combination of traditional and novel technologies. It assesses traditional construction methods (in-situ construction and prefabricated construction) and clusters their features in order to elaborate their strengths and persistent challenges. Following a customised review on the digital fabrication methods, it identifies some new levers and opportunities for mass-market penetration of digital concrete fabrication technologies in structural and civil engineering construction works.

Digital technologies overcome typical constraints of traditional concrete construction processes caused by the high impact of labour costs and bring about many new possibilities to the conceptual design, dimensioning, detailing, and production of concrete structures. While the potential of geometric flexibility is being extensively explored, most digital technologies encounter difficulties in penetrating the market due to lacking compliance with structural integrity requirements. To maximise their impact, it is essential that digital concrete processes (i) integrate reinforcement resisting tensile forces and (ii) address conventional structures with geometric simplicity. This paper discusses the potential of digital concrete fabrication processes to reduce the quantity of reinforcement required in concrete structures. For example, "minimum reinforcement" can be tremendously reduced by (i) tailoring the concrete grade locally to the actual needs and (ii) ensuring small crack spacings and correspondingly reduced crack widths by means of crack initiators. An experimental study shows that the strength reduction in the interfaces between layers from extrusion processes can be quantified with reasonable accuracy, which allows using these weak interfaces as crack initiators. A mechanical model to quantify the corresponding potential for saving "minimum reinforcement" when using 3D printing is presented. It is found that weak interfaces in layer joints with 33% of the concrete tensile strength inside the layer allow reducing up to 80% the minimum reinforcement for a given maximum crack width requirement under imposed deformations.

Conference paper written by Patrick Bischof, Walter Kaufmann and Primo De Zanet presented at IABSE Conference: Creativity and Collaboration – Instilling Imagination and Innovation in Structural Design, Bath, United Kingdom, 19-20 April 2017.

This paper contains a seismic assessment at urban scale of the cities of Sion and Martigny in Switzerland. These two cities have been identified for the present research based on their importance regarding size and the characteristics of the building stock for which information was available. Moreover, microzonation investigations are available for both cities. This results in a more accurate characterization of local expected ground shaking, which is expressed through specific response spectra. Sion and Martigny represent, respectively, the capital and second largest city of the canton of Valais. This region is characterized by the highest seismicity within Switzerland. The paper focuses on the assessment using Risk-UE methodology, namely the empirical method LM1 and the mechanical method LM2. The obtained results are compared in order to assess the related accuracy. Firstly, buildings of the two cities were surveyed in order to collect main structural characteristics in a database. Building stock is typical of that region and can be found similar to many other medium-sized Swiss cities. Around half of the buildings are unreinforced masonry buildings, while several others are reinforced concrete buildings with shear walls. Results show the most vulnerable part of the cities regarding earthquake. There are significant differences in global results between LM1 and LM2 methods. The mechanical LM2 method is more pessimistic since it predicts damage grades of about one degree higher than LM1 method. However, the main drawback of the empirical LM1 method is that an a priori determination of an adequate value of the macroseismic intensity is required. Nevertheless, LM2 method may lead to a global overestimation of damage prediction.