Interfacial debonding between fibre-reinforced polymer (FRP) and concrete is one of the most common failure modes in externally bonded FRP (EB-FRP) strengthened concrete structures, typically occurring within a thin layer of concrete near the interface. This study uses the scaled boundary finite element method (SBFEM), a semi-analytical numerical approach, to model the interfacial debonding process between FRP and concrete. The quadtree meshing scheme is used to smooth the mesh transition near the interface, and high computational efficiency is achieved by exploiting the advantages of SBFEM. The Mazars damage model, which considers the tensile and compressive damage separately, is integrated with a nonlocal model to eliminate mesh sensitivity, thereby enabling the accurate prediction of damage evolution in the concrete substrate. Several benchmarks, including three-point bending notched beams (TPBNB), a double-notched tension beam (DNTB) and single shear FRP-concrete specimens, are simulated to confirm the effectiveness and reliability of the proposed method. The numerical results align closely with both the experimental data and finite element modelling. Furthermore, the effects of internal length, bond length, FRP stiffness, and concrete strength on the interfacial bonding performance are investigated. The existence of the effective bond length and its relation to the bond length are confirmed. The results also reveal that the failure mode of the interface is sensitive to the internal length and that the ultimate debonding load depends critically on both FRP stiffness and concrete strength.

This paper presents a survey of engineering practice and culture in structural design relating to material efficiency. The survey included 44 questions and received 106 responses, predominantly from practising structural engineers in the UK. It builds upon a previous survey from 2017 with the same set of questions, allowing a longitudinal study of changes overtime. The results were analysed using statistical methods, including chi-square tests, Mann-Whitney U tests, two-sample t-tests of independence, regression analysis, and analysis of covariance. The findings showed that material efficiency ranks at the top of design priorities, alongside cost, a notable shift since the 2017 survey. There is also growing, though not yet universal, client demand for low-carbon designs. Despite these advances, overdesign remains prevalent, primarily due to the need to accommodate potential changes in span, loading, or layout prior to construction. The survey also revealed a disconnect between engineers' understanding of vertical floor loading and serviceability limit states and current code requirements. The results are complemented by research questions that emerged during the study, which require further input from the research community.

Imposed floor loads directly influence the embodied carbon of new building structures, and often determine the reuse potential of existing ones, with important sustainability impacts. Despite this, engineering design codes and practices predominantly adopt loads based on historical precedents, rather than empirical evidence or statistical analysis. Existing physical survey data for office loadings is scarce, outdated, and fails to capture extreme scenarios. This paper introduces a novel stochastic methodology based on realistic modern office layouts (from 29 industry fit-out drawings), probabilistic models of furniture and partition weights (derived from approximately 500 individual product specifications) and occupancy scenarios (informed by surveys and interviews). The resulting dataset contains over 55 million load samples, capturing spatial, temporal and specification variations across variety of office uses. A mean loading of 0.35-0.38 kN/m2 was determined, with increasing variance over smaller areas. Annual maximum loads were analysed, revealing that extreme loads are influenced by tall, heavily loaded items of storage furniture, particularly at smaller sampled areas, with high occupancy events increasingly important over larger areas. A standard minimum UK office imposed load of 2.5 kN/m2 was found to have a return period of 11 years for serviceability, or 397 years if factored for ultimate limit state design; however these return periods increase significantly if the maximum storage furniture pressure is limited. Partitions added a mean load of 0.25-0.26 kN/m2, increasing to approximately 0.5 kN/m2 for extreme loads with a 50-year return period, with little influence from area size. There is potential for this new approach to be applied to new building uses and across multiple storeys, with the study also revealing the need for additional extreme loading data regarding storage furniture and high occupancy events. The novel stochastic framework, and the resulting outputs, enable informed serviceability loading decision-making based on return-periods and show how offices can be designed and managed to minimise extreme loads. Moreover, they allow realistic load specification in buildings, guidance documents and design codes, avoiding over-design and unnecessary demolition and consumption of emissions-intensive structural materials.

In recent years, FRP (Fibre-Reinforced Polymer) bars have been considered to be a promising alternative to conventional steel bars for reducing the CO2 emissions of concrete structural elements, whilst their environmental performance compared to traditional steel bars requires quantitative assessment. This paper introduces a multi-level comparative framework to evaluate the carbon efficiency of FRP bars compared to steel bars, from the material level to the section level and structural element levels, leading to quantitative comparative outcomes useful for decision-making when selecting the most carbon-efficient rebar type for achieving the lowest embodied carbon (cradle-to-gate) of structural elements. Each comparison level has its applicability depending on the specific context of construction projects. Because relative environmental performance can vary significantly in function of material and environmental data, the developed comparative framework aims to provide the rigorous methodology to follow, which can be adapted to any construction project by updating material and environmental datasets, rather than declaring a universally superior rebar type. Steel and BFRP (basalt FRP) bars are compared, based on the assumed material and environmental data, as an example to demonstrate how to apply the proposed comparative formwork. Results indicate that the optimal solution depends mainly on the structural parameters (such as span and slab thickness) as well as the assumed carbon data. For prismatic elements, the optimal choice between steel and BFRP bar is strongly influenced by the selection of slab thickness. For non-prismatic elements, it is found that non-prismatic BFRP-reinforced slabs have 4.8%–14.4% lower CO2 than non-prismatic steel-reinforced slabs and can save up to 63% of CO2 compared to the traditional solution (prismatic steel-reinforced slabs) in the presented examples.

In this study, fabric formwork is used to cast I-shaped and non-prismatic tapered reinforced concrete (RC) beams that have up to a 40% reduction in concrete volume, resulting in lower embodied CO2 than a rectangular prismatic beam. The primary aim of this research is to use distributed sensing to characterize the behavior of these shape-modified beams to an extent that was not previously possible and compare their behavior to that of a conventional rectilinear beam. FourRC beams (a rectangular control and three fabric-formed sections) were tested in three-point bending. Distributed fiber-optic strain sensors were used to measure strains along the full length of the longitudinal reinforcement, and digital image correlation (DIC) was used to acquire crack patterns and widths. The results indicate that fabric-formed RC beams can achieve the same load-carrying capacity as conventional rectilinear prismatic beams and meet serviceability requirements in terms of crack widths and deflections. The longitudinal reinforcement strains along the full length of the specimens were captured by Canadian concrete design equations, which account for the effects of both flexure and shear on reinforcement demand.

Concrete shells can be efficiently designed through form-finding techniques to create shapes where the structure functions predominantly in compression under the applied load. While such concrete shell floor systems can reduce material consumption by replacing traditional flexural load transfer with efficient membrane action, their construction is challenging with conventional formwork methods. This study conceptualises, develops, and experiments the use of flat auxetic grids as formwork for casting compressive-dominant concrete shells. Due to their negative Poisson's ratio, auxetic meshes exhibit dome-like synclastic behaviour, curving towards the same side in all directions when subjected to out-of-plane deformations. Therefore, the feasibility of transforming a flat auxetic grid into the most efficient shapes for concrete shells solely under the self-weight of concrete or with additional constraints is explored in this study. A parametric nonlinear finite element model linked to an evolutionary optimisation algorithm is developed to design a flat auxetic grid that deforms to approximate a form-found compression-dominant shell geometry. The construction of concrete shells using the designed auxetic geometry is experimentally demonstrated. A form-finding algorithm coupled with an evolutionary algorithm is also used to verify that the experimental geometries and the deformed finite element models of different auxetic formwork designs approach compression-dominant shell geometries. Results show that semi-flexible flat auxetic grids can be feasible, versatile, reusable, and less bulky as a formwork system for casting concrete into cast compression-dominant shell geometries.

Reinforced concrete is a major contributor to the environmental impact of the construction industry, due not only to its cement content, but also its steel tensile reinforcement, estimated to represent around 40% of the material embodied carbon. Reinforcement has a significant contribution because of construction rationalisation, resulting in regular cages of steel bars, despite the availability of structural-optimisation algorithms and additive-manufacturing technologies. This paper fuses computational design and digital fabrication, to optimise the reinforcement layout of concrete structures, by designing with constrained layout optimisation of strut- and-tie models where the ties are produced with robotic filament winding. The methodology is presented, implemented in open-source code, and illustrated on beam and plate reinforcement applications. The numerical studies yield a discussion about parameter selection and constraint influence on material and construction efficiency trade-offs. Small-scale physical prototypes up to 50 cm x 50 cm provide a proof-of-concept.

Reducing carbon emissions from the construction sector is vital amid the climate emergency. Shape-optimisation can reduce concrete usage compared to prismatic designs, but formwork and reinforcement need unconventional solutions when curved and complex geometries are required. This study proposes dual-purpose formwork with CFRP textile, serving as both the reinforcement and a part of the formwork, for shape-optimised concrete beams. A prototype shape-optimised concrete beam with a stay-in-place CFRP textile formwork is constructed and tested, and an analysis method is developed to estimate the Ultimate Limit State capacity. Pouring concrete onto dry CFRP textile and resin-curing post-hardening concrete created a promising bond between the concrete surface and formwork. The CFRP textile showed no observable debonding from the concrete surface under load testing to failure, and overall strain behaviour aligned well with the analysis method presented. The prototype beam experienced a premature brittle flexural failure initiated by rupture in CFRP textile due to stress concentrations, but still reaching 75 % of the predicted ultimate capacity. Stay-in-place participating formwork with CFRP textile is a promising construction method for shape-optimised beams without internal reinforcement, however, further research is needed to provide further insights on deformability, stress concentrations, and bond behaviour.

Shell structures are primarily subjected to compressive stresses if their geometry is adjusted to the applied loading. Still, the geometry in a real environment usually deviates from an optimal form for load-bearing due to tolerances and architectural demands, thus causing additional bending moments (m) and normal forces (n). The use of textile-reinforced concrete (TRC) makes it possible to reduce thickness and thus the dead weight of a shell, though it also increases the sensitivity for deviations from perfect conditions. Additionally, textile reinforcement typically consists of two orthogonally aligned reinforcement layers of yarns, while the principal stress directions relevant to the shell design typically diverge from the yarn orientation. Hence, the cross-section suffers a reduced resistance due to the anisotropic material properties of the textile reinforcement. In this paper, we investigate the load-bearing capacity of existing TRC-shell structures that were created with a free form-finding approach. Given the anisotropy of the textile reinforcement, the analysis of the shell must not only consider the n-m interaction but also the principal stress direction for each specific location and load situation within the structure. This is considered by the verification concept presented here, which follows the semi-probabilistic design format from the European Codes. It could be shown that this new methodology for assessing free form shells can be used identify the most critical parts of such shells.

Shape optimisation of concrete elements will reduce concrete consumption and hence embodied carbon. However, providing both formwork and reinforcement for shape optimised concrete elements is challenging due to their curved geometries. This paper explores a novel design and construction method for shape optimised concrete beams where flexible CFRP textile is used as a stay-in-place participating formwork, i.e. serving as both the formwork and reinforcement. The technical feasibility of the system was assessed by generating a series of beam designs to discuss the scale of the CFRP textiles required. Environmental feasibility was also assessed by estimating the embodied carbon of several designs against conventional beam designs. Due to the lack of readily available options in the present market, further studies are needed to assess the technical feasibility of using CFRP textiles up to 2 mm thick as participating formwork to reinforce beam designs required in practice. Shape optimisation can reduce concrete volume by up to 36% compared to equivalent prismatic beam designs with similar midspan depths. The midspan depth of the design with minimum embodied carbon for a given design criteria may not coincide with the depth of a conventional prismatic beam design, yet embodied carbon reductions up to 33% are possible.

The Automating Concrete Construction (ACORN) project explored digital workflows from the design to the construction of reinforced concrete building floor elements, reducing carbon emissions and increasing efficiency of building processes. The resulting digital tool, named SQUIRREL, enabled the design of shells, composed of prefabricated segments, through an interactive framework, composed of parametric design tools, and informed by architectural, structural, and construction requirements, including building integration, fabrication, transport, assembly, and resource reuse. This paper presents the design and implementation of the SQUIRREL tool, focusing on the main design tasks for a segmented reinforced concrete shell, including formfinding and segmentation layout definition. This paper also documents the development and application of a Design Space Visualisation module within SQUIRREL, which streamlined parametric studies used to inform the design decisions behind the modelling and implementation process. Finally, we discuss the right balance between design automation and user interaction, which should inform the development of future construction-aware computational design tools.

A significant portion of the environmental impact of a building’s superstructure lies in its structural flooring. By leveraging funicular forms such as thin concrete shells, a materially and carbon-efficient alternative to bending-active flooring systems can be attained. In addition, through segmentation and the use of dry jointed interfaces, a segmented concrete shell allows for ease of disassembly compatible with circular economy principles for the built environment. This paper presents a novel segmented concrete shell flooring system that leverages the symmetry of revolution of the classical fan vault form to facilitate future design flexibility through increased reconfigurability. The design and form-finding of the segmented fan concrete shell are detailed through the use of an evolutionary algorithm and finite element analysis. Quarter-scale prototypes were digitally fabricated using a robotic concrete spraying process which were then assembled and tested to assess its structural potential, evaluate the limitations, and identify areas of future work. An embodied carbon analysis demonstrates that the system provides a mass and embodied carbon saving compared to conventional flooring systems while adding approximately a 20

To reduce the significant environmental impact of the construction industry, building floors designed as concrete shells that work mainly in compression and that are segmented for prefabrication and disassembly offer a promising alternative to reinforced thick flat slabs. The OAK prototype, a 4.5 m × 4.5 m segmented concrete shell with reversible dry joints for reusable building floors, offers such potential. The non‐linear behavior of the concrete material and the segmented shell system make understanding the mechanics of such a structural system challenging for practical design. In particular, the compressive stresses and the slenderness of shells, along with their fabrication and assembly imperfections, make them prone to instability. This article reports the methodology and results from a set of physical structural assessments on the OAK prototype, including material, serviceability, robustness, and stability tests.

The possibility of achieving auxetic behaviour in conventional materials is currently attracting significant research interests in the form of auxetically enhanced cementitious composites and, especially, in re-entrant honeycomb structures. Most studies on the behaviour of re-entrant steel honeycomb auxetic reinforcement for concrete focus on achieving uniplanar auxetic behaviour perpendicular to the direction of loading. This study focuses on investigating multi-planar auxetic behaviour throughout the composite. The auxetic behaviour was achieved by perforating the steel sheets before making the re-entrant honeycomb structures. The re-entrant honeycomb was fabricated by folding mild steel strips 70 mm wide into a re-entrant profile and welding the strips onto one another. Three types of perforations were investigated, representing three categories of samples: circular perforation, orthogonal elliptic perforations and orthogonal peanut perforations. The perforated samples were compared to the non-perforated re-entrant honeycomb (control) through compression strength tests of the composites. Direct tension tests were also carried out on each steel strip in order to understand their respective stress–strain behaviour. The study demonstrates that perforating the re-entrant honeycomb walls could be a simple method for achieving multi-planar auxetic behaviour in re-entrant steel honeycomb-reinforced cementitious composites.

Imposed floor loading directly influences structural material requirements for new buildings, and often determines the adaptation potential of existing ones, with important sustainability implications. However, the loads adopted in current design codes and practice are largely based on historical precedent, rather than evidence. This project aims to provide a robust statistical analysis of contemporary office loading, and introduces a novel methodology modelling both furniture and occupancy based on fit-out drawings of modern offices. Using a Monte Carlo approach, a large dataset capturing a variety of office layouts, furniture specification and occupancy scenarios is created. The results characterise typical and extreme loading, revealing the importance of storage furniture, high occupancy events and acceptable return periods in determining an appropriate design load, and the critical influence of the loaded area size.

This paper presents a parametric study on the pullout performance of twisted steel fibre in normal concrete. The twisted fibre is 40 mm long with a rectangular cross-section with a diagonal length of 1.12 mm. The experiment parameters include the number of twists (4 and 6) and the fibre inclination angle, θ, (30°, 45°, 60°, 75°, and 90°). The results indicate that the inclination angle has a great effect on the pullout response contrary to the number of twists. Such a database can then be employed to develop analytical models of the behaviour of Fibre Reinforced Concrete (FRC) elements (compressive, tensile, and flexural), facilitating the optimisation process of such elements and leading to optimum designs.

Will Arnold, Mike Cook, Duncan Cox, Orlando Gibbons and John Orr reflect on progress made around carbon targets over the past four years.

Auxetic materials expand in the lateral direction when stretched axially and contract laterally when compressed axially, thereby resulting in a negative Poisson's ratio. This counter-intuitive behaviour results in such materials having a very wide range of potential benefits such as lateral confinement and improved bonding with cementitious matrices. This phenomenon has resulted in a proliferation of research in the use of auxetic materials in cementitious construction. However, numerous studies have focused on laboratory-scale auxetic cementitious composite samples for non-structural applications, while only very few recent studies have attempted to achieve auxetic behaviour for full-scale structural elements. Studies on auxetic cementitious materials have continued to be exploratory, with a variety of reported findings together with differing recommendations. This paper, therefore, reviews the state of the art on the application of auxetics in cementitious construction, the challenges and opportunities associated with the development and use of these innovative materials, and recommendations for future research to encourage uptake of these materials by engineers. It examines more than 100 primary research articles on the mechanical properties, design, optimisation, and specific applications of auxetics in cementitious composites. An important finding from the review is that the benefits derived from auxetic reinforcements require deformations which far exceed serviceability limits specified for structural elements in static loading. Therefore, the application of a chosen auxetic geometry will require bespoke design procedures to satisfy both strength and stiffness requirements, especially for cementitious composites. Furthermore, the finite element modelling approach of concrete damage plasticity is also noted as an essential tool for analysing the significant deformation behaviour of the structures. The review concludes that there is great potential for auxetic materials and structures through the careful selection of application-specific materials, and the enhancement of bonding between the cement matrix and the auxetic phase. Moreover, hybridising the geometries of the auxetic reinforcement can maintain a balance between the stiffness essential for load-bearing members and the advantages derived from auxeticity.