The introduction of strut-and-tie models in fib Model Code 1990 as a design basis for discontinuity regions constituted a significant step toward promoting consistent design methods for reinforced concrete structures. In fib Model Code 2010, the scope was broadened through the stress field method that was introduced as a complementary tool. The present article summarizes subsequent evolutions in both methods, which will be incorporated in the upcoming fib Model Code 2020. Besides emphasizing their suitability for the structural design and assessment, their adaptability to the “Levels-of-approximation” approach is also depicted. This article presents the theoretical ground of both methods and looks on their potential for computer-modeling implementation. With this respect, several strategies are presented by discussing their advantages and optimum field of application.

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Thin-walled textile-reinforced concrete beams have recently emerged as a promising approach for material-efficient design. However, the increased complexity of the formwork is a major challenge in implementing such elements for broader use in the construction industry. This study presents a novel type of stay-in-place flexible formworks with integrated textile reinforcement. The use of weft-knitted textiles allows the integration of continuous high-strength rovings as shear reinforcement and the introduction of spatial features within the fabric to guide bending-active rods to shape the complex cross-section geometry. The manufacturing procedure and the structural performance were investigated in an experimental campaign consisting of four concrete beams with I-profile cross sections tested in three-point bending, where aramid rovings were used for the shear and conventional deformed steel bars for the flexural reinforcement. The transverse reinforcement ratio proved to be essential in increasing the shear strength. Thereby, the use of digital image correlation measurements of the surface deformations allowed the direct assessment of the strains and, thus, the mechanical activation of the textile reinforcement. The full tensile capacity of all the aramid rovings crossing the governing crack could not be exploited due to the brittle material behavior, resulting in a progressive failure once the first roving reached its tensile strength. Compatibility-based stress fields were used to predict the load-deformation and failure behavior of the tested beams, which resulted in an excellent agreement of the ultimate loads and failure modes obtained from the model with the observations and results from the experiments.

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The form freedom enabled by digital fabrication with concrete technologies provides advantages for a wide range of concrete based objects, from architectural to structural elements. The current chapter focuses on the specifics of structural design and engineering of DFC with emphasis on those technologies based on Additive Manufacturing with extrusion. Since it is a new and innovative way to build, a clear common approach to structural engineering has not yet been developed. As a result, this chapter aims to introduce the specific challenges of structural design and engineering with the additive manufacturing technology, providing an overview of structural typologies that have been developed (especially concerning the reinforcement strategies, including fibre reinforcement). Furthermore, the structural principles adopted in DFC and the codified approaches used in conventional reinforced concrete is compared, and putative structural testing procedures and validation methods for DFC are reported.@en

Conventional construction of doubly-curved concrete structures is a time-, labour- and cost-intensive process. Flexible formworks have already been identified as a possible solution to produce such structures more efficiently. The KnitCrete technology developed at ETH Zurich uses 3D weft-knitted fabrics as stay-in-place formwork, which deliver multiple advantages over woven textiles due to their wider range of feasible geometries and possibility to include features and local material properties. The textile is initially coated with a fast-setting high-strength cement paste. The stiffened membrane is stable enough to serve as formwork for the final concrete layer. This paper discusses potential reinforcing strategies to guarantee structural safety and serviceability in KnitCrete structures. Possible approaches range from the use of the textile as a stay-in-place formwork as well as final reinforcement (by utilising high-strength fibrous materials such as aramid, glass or carbon fibre) to the implementation of geometric features, such as channels within the textile to guide conventional reinforcement or post-tensioning tendons. The feasibility and efficiency of the proposed reinforcement strategies have to be experimentally verified, for which a systematic methodology is proposed. Preliminary analyses of the experimental campaign show the beneficial effect of the knitted reinforcement on the cracking behaviour of the textile-concrete composite material. Additional research is needed to exploit the potential of possible hybrid solutions using short steel fibres, post-tensioning or linear steel or glass fibre reinforcement.@en