Abstract High-performance concrete (HPC) is increasingly adopted in the construction of concrete structures for bridges, towers, offshore rigs, high-rise buildings and wind turbines. HPC enables slender cross-sections, lighter structural components and longer spans for structures. However, due to their inherently reduced self-weight, these structures are often subjected to cyclic loads throughout their lifetime. Despite being an advanced material, the potential of using HPC under fatigue loading cannot be fully exploited due to conservative design standards. The fracture mechanical behaviour of HPC, containing basalt aggregate (AG) and coarse mortar (CM), was investigated under static and cyclic tensile loading. The matrix material CM was developed as a concrete equivalent mortar using the excess paste theory. The test results show clear differences between the HPC and its components, basalt AG and CM, in terms of mechanical properties and fracture mechanical behaviour under static and cyclic loading. A novel and efficient hybrid formulation for phase-field modelling in brittle fracture is introduced. Both helping to understand the influence of the internal mesostructure on the fracture mechanical properties of the HPC on the macroscale.

In accordance with the basic principles of applicable regulations, a reinforced concrete component can be considered as good as new if it has been repaired in a professional manner. To ensure this, it is important to select suitable repair materials in addition to defining an effective repair procedure and ensuring flawless execution. Planners and contractors are supported in this by the specifications of the regulations, which specify the required strength, stiffness or penetration resistance, for example, depending on the material, environmental influences and the properties of the old concrete. However, these specifications do not consider the respective usage and load conditions during and after repair in combination with the specified material characteristics. Current regulations lack information on the influence of prevailing loads and the associated stress distribution in repaired component cross-sections on the resulting degradation effects. The planning aid presented in this article makes it possible to estimate the stress distribution between old concrete and repair material and, based on this, to select a material that considers the component load in the interests of permanent repair. Realistic case studies illustrate the application of the planning aid.

As part of the global effort to tackle climate change, Low-Carbon Concretes (LCC) are becoming increasingly important in the construction industry. They differ from standard concretes in terms of their significantly reduced carbon footprint. This can be achieved directly by (I) using CO2-negative SCM and/or (II) using blended binders with reduced cement clinker content (e.g. substitution by SCM). Disadvantages associated with these strategies, such as reduced workability or durability, can be compensated through optimizing the particle size distribution or reducing the water-to-clinker ratio while increasing the superplasticizer dosage. Regarding limitations of the practical feasibility of these approaches and/or the availability of SCM, the substitution of cement clinker by high amounts of ground limestone in LCC seems to be particularly suitable for reducing CO2-emissions. However, high amounts of ground limestone in LCC result in a significant reduction in mechanical properties and durability due to the lower C-S-H formation, resulting in a less dense hardened structure as well as less alkaline environment in the pore solution. A novel approach for producing LCC involves the use of Hydration Control Additives (HCA). These additives modify the hydration kinetics of OPC and enhance its strength-forming potential by converting the aluminate phases into additional solids, i.e. ettringite (‘aluminate-boost’). Therefore, the use of HCA is a promising approach to realize LCC with low-clinker binders containing high amounts of limestone powder. This study investigates the application of HCA for LCC with up to 50 vol.

This paper discusses the flexural and tensile strength properties of 3D printed concrete, based on the results of a RILEM TC 304-ADC interlaboratory study on mechanical properties. These properties are determined using different testing techniques, including 3- and 4-point flexural tests, splitting tests, and uniaxial tension tests, on specimens extracted from large 3D printed elements in accordance with a prescribed study plan. The relationship between compressive and flexural or tensile strengths, cast or printed samples, different types of tests, and different loading orientations, are analysed to understand the influence of 3D printing. As expected, the strength can reduce significantly when the main tensile stress is acting perpendicular to the interface between layers. The role of deviations from the standard study procedure, in terms of the time interval between the placing of subsequent layers, or the adoption of a different curing strategy, are also assessed. While the increased time interval significantly impacts the strength in the critical direction, the use of variable curing conditions does not seem to have a clear-cut effect on the strength ratios of the printed to cast specimens. Additionally, the paper looks at the variability in the results for the printed specimens, in order to emphasize the need for multiple replicates for obtaining a proper result. An extensive insight into the aspects affecting the variability is presented in the paper. Finally, with the limited dataset available for specimens tested at a larger scale, it is difficult to arrive at a clear understanding of the role of specimen size (i.e., greater number of layers).

The two major themes in the current construction industry are digital construction and low environmental impact. As a prominent digital construction technology, concrete 3D printing has attracted increasing attention. However, the current understanding of the durability of 3D printed cement-based materials (3DPCM) remains limited, which hinders its wider application, especially as load-bearing, reinforced concrete structures. This work shares the knowledge acquired during a broad interlaboratory study regarding the durability of 3DPCM with 15 laboratories from 13 countries participating, under the framework of TC 304-ADC ‘Assessment of Additively Manufactured Concrete Materials and Structures’. Anisotropy in water absorption capacity, carbonation and chloride ingress resistance of 3DPCM were evaluated by 15 institutes with their own printable materials and printing equipment. Additionally, the impacts of cold joints on these properties were investigated and a comparison between printed and cast samples was carried out. The outcome of this study indicates that the water absorption test provides information on the bulk porosity of the samples, while the carbonation and chloride ingress tests are more effective and visually reflect the local defects, especially the layer interfaces and cold joints. The water ingress depth of cast samples prepared with printable mixtures is an order of magnitude higher compared to conventional concrete, while their carbonation and chloride ingress resistance are comparable. The sorptivity and estimated water ingress height of printed samples measured in the direction parallel to the filaments is generally higher than that measured in the perpendicular direction and in cast samples. Similarly, the carbonation and chloride ingress depth and rate of printed samples measured in the direction parallel to the filaments is generally higher than that measured in the perpendicular direction or in cast samples. The overall durability of 3DPCM is weakened by anisotropy, these effects can be addressed with targeted mixture design and processing strategies. Due to the variations in printers, printing parameters and materials, three types of cross-section geometries were observed in printed samples with cold joints. The carbonation depth that measured from the maximum carbonation ingress point near the cold joint to the sample edge effectively captures the effect of cold joints in all these three types of cross-section geometries of printed samples. Finally, the participants identified areas of improvement in the methodology and suggestions were made to refine the procedure for adoption in future research.

Accurate and efficient structural health monitoring of infrastructure objects such as bridges is a vital task, as many existing constructions have already reached or are approaching their planned service life. In this contribution, we address the question of the suitability of UAV-based monitoring for SHM, in particular focussing on the geometric deformation under load. Such an advanced technology is becoming increasingly popular due to its ability to decrease the cost and risk of tedious traditional inspection methods. To this end, we performed extensive tests employing an 18.5 m long research reinforced concrete bridge that can be exposed to a predefined load via ground anchors. Very high resolution image blocks have been captured before, during and after the application of controlled loads. From those images, the motion of distinct points on the bridge has been monitored, and in addition, dense image point clouds were computed to evaluate the performance of surface-based data acquisition. Moreover, a geodetic control network in stable regions is used as control information for bundle adjustment. We applied different sensing technologies in order to be able to judge the image-based deformation results: displacement transducers, tachometry and laser profiling. As a platform for the photogrammetric measurements, a multi-rotor UAV DJI Matrice 600 Pro was employed, equipped with two RTK-GNSS receivers. The mounted camera was a PhaseOne iXM-100 (100 MP) with an 80 mm lens. With a flying height of 30 m above the terrain, this resulted in a GSD of 1.3 mm, while a forward and sideward overlap of 80% was maintained. The comparison with reference data (displacement transducers) reveals a difference of less than 1 mm. We show that employing the introduced UAV-based monitoring approach, a full area-wide quantification of deformation is possible in contrast to classical point or profile measurements.

This study investigates the influence of particle surface modifications on particle bed concrete 3D printing by Selective Cement Activation (SCA). Prior research suggests that enhancing particle wettability and reducing bulk porosity of the particle bed increases the performance of SCA components. In this work, the effects of dry coatings (nanoscale SiO2 and TiO2) and a liquid grinding aid (diethylene glycol-DEG) on the wettability and bulk density of two sand fractions and a sand/cement mixture were examined. Nanoscale SiO2 was found to improve the wettability of the sand/cement mixture, while DEG and nanoscale TiO2 increased the bulk density and, consequently, reduced the bulk porosity. Based on the results, specimens were 3D printed using the modified materials. Surface modification by DEG increased the compressive strength of printed specimens by 30 % compared to the unmodified, reference material. This increase is attributed to a higher bulk density and, thus, particle bed density compared to the reference material. We conclude that surface modifications enhancing the bulk density can significantly increase the compressive strength of SCA components, thereby expanding SCA's potential and facilitating its use in construction. (c) 2025 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/).

Injection 3D Concrete Printing (I3DCP) is an emerging fabrication technique that enables spatial concrete extrusion within a carrier liquid, reducing gravitational effects and allowing the creation of complex space trusses. However, I3DCP introduces new challenges in toolpath planning due to material rheology and mechanical constraints. This paper introduces an automated planning method tailored for I3DCP, integrating a constraint satisfaction problem (CSP)-based sequence planner with a Cartesian motion planner. The sequence planner uses heuristic local search with forward checking and backtracking, while the motion planner addresses end-effector redundancy with kinematic and velocity constraints. The method is validated by fabricating a 3-meter-span pedestrian bridge using a stationary 6-axis robotic arm and tested on multiple prototypes of increasing geometric complexity through simulation, demonstrating its effectiveness and scalability for intricate structural designs.

Traditional construction techniques, such as in-situ casting and pre-cast concrete methods, have well-established testing protocols for assessing compressive strength and modulus of elasticity, including specific procedures for sample preparation and curing. In contrast, 3D concrete printing currently lacks standardized testing protocols, potentially contributing to the inconsistent results reported in previous studies. To address this issue, RILEM TC 304-ADC initiated a comprehensive interlaboratory study on the mechanical properties of 3D printed concrete. This study involves 30 laboratories worldwide, contributing 34 sets of data, with some laboratories testing more than one mix design. The compressive strength and modulus of elasticity were determined under three distinct conditions: Default, where each laboratory printed according to their standard procedure followed by water bath curing; Deviation 1, which involved creating a cold joint by increasing the time interval between printing layers; and Deviation 2, where the standard printing process was used, but the specimens were cured under conditions different from water bath. Some tests were conducted at two different scales based on specimen size—“mortar-scale” and “concrete-scale”—to investigate the size effect on compressive strength. Since the mix design remained identical for both scales, the only variable was the specimen size. This paper reports on the findings from the interlaboratory study, followed by a detailed investigation into the influencing parameters such as extraction location, cold joints, number of interlayers, and curing conditions on the mechanical properties of the printed concrete. As this study includes results from laboratories worldwide, its contribution to the development of relevant standardized testing protocols is critical.

This paper presents a form-finding approach for Injection 3D Concrete Printing (I3DCP) using Vector-based Graphic Statics (VGS). This approach adopts a top-down strategy, initiating a preliminary global design in the form of a space truss and integrating structural and fabrication constraints specific to I3DCP. A form-dependent self-weight load is applied throughout the form-finding process until the structure achieves static equilibrium. As the current I3DCP setup is mounted on a robotic arm with a stationary base, the feasibility of the designed structure for I3DCP is assessed, ensuring compatibility with the robotic arm's workspace. Structures exceeding the workspace boundaries are segmented and individually optimised, subject to topological and geometrical constraints. The optimised segments are then merged into a single assembly to complete the process. This approach is demonstrated through the design and construction of a 3-metre-span pedestrian bridge. This prototype is 3D scanned and then analysed via the finite element method to evaluate its mechanical performance.

Distributed Fiber-Optic Sensing (DFOS) is emerging as a key technology for Structural Health Monitorings (SHMs). DFOS offers strain and temperature measurement over long range with high spatial resolution. This technology is most suitable for large civil infrastructure such as bridges or tunnels Structural Health Monitoring. But early applications were limited to one-dimensional measurements along principal load paths, but recent work has enhanced DFOS with full-field monitoring through fiber-optic grids or with multi-sensor fusion technics. This paper reviews the state-of-the-art in DFOS-based damage detection for concrete, steel, and steel-composite bridges and highlights new opportunities for enhanced DFOS applications. First, the most common damage characteristics on different bridge types are summarized. A concise collection of published DFOS applications on bridges follows, illustrating integration either surface-bonded or embedded within structural components for short- and long-term monitoring. It is relived that DFOS is routinely used to validate design assumptions, locate crack initiation in concrete members, and monitor crack propagation in steel bridges. Despite these successes, full-field monitoring with fiber-optic grids remains limited to laboratory scale. Consequently, the paper presents enhanced methodologies for monitoring common damage types: bonding failure, surface cracks,tendon breaks, shear failure, debonding of Carbon Fiber Reinforced Polymer (CFRP) strips, based on embedded DFOS, enhanced 2D fiber-optic grids and multi-sensor fusion. These developments are placed in the context of the Cluster ”Damage detection” of the SPP 100+ project “Extending the Lifetime of Complex Engineering Structures through Intelligent Digitalization”, which aims to transfer laboratory advances into robust, field-deployable SHM solutions on the Rhine River bridge ”Nibelungenbr¨ucke Worms”.

The integration of reinforcement in digital fabrication with concrete has led to the development of various approaches, many of which are constrained by the requirements of the concrete printing process. In contrast, the aim here was to investigate the robotic production of complex reinforcement structures as a primary process which supports the application of concrete and therefore acts as stay-in-place formwork that creates shaping potential beyond conventional printing processes. Based on the concept of combining the robotic processes of Fibre Winding and Shotcrete 3D Printing (SC3DP), the presented methodology comprised design explorations, fabrication variations, the realisation of a real-scale demonstrator and the assessment of the structural performance. Accordingly, an automated process for the fabrication of thin-shell double-curved reinforced concrete elements with controlled thickness and homogeneous concrete distribution was developed and characterised. Following this approach in the future will not only contribute to fabrication-informed design but also minimise concrete use and formwork waste.

The construction industry is facing a dual challenge: an increasing demand for new buildings on the one hand and the urgent need to drastically reduce emissions and waste on the other. One promising field of research to face these challenges comprises additive manufacturing (AM) technologies. Through these advanced methods, digital workflows between design and fabrication can be implemented to optimise the form and structure, unlocking new architectural freedom while ensuring sustainability and efficiency. However, to drive this transformation in construction, the new technologies must be investigated in large-scale applications. One of these fast-emerging AM techniques is Shotcrete 3D Printing (SC3DP). The present research documents the 1:1 scale manufacturing process, from digital to real, of a building section utilising SC3DP. A workflow and production steps, spanning from design over manufacturing to assembly, are introduced. The architectural design, reinforced by computational methods, was iteratively refined to adapt to manufacturing constraints. The paper also emphasises the importance of a digital twin in ensuring seamless data integration and real-time adjustments during construction. By incorporating reinforcement techniques such as short rebar insertion and robotic fibre winding, this study demonstrates the structural capabilities achievable with SC3DP. In summary, the implementation of comprehensive digital workflows utilising computational design, automated data acquisition and data flow, as well as robotic fabrication is presented to demonstrate the potential of AM methods in construction. Furthermore, this paper provides a perspective on potential future research paths and opportunities inherent in leveraging the innovative SC3DP technique.

Architects and engineers have historically developed and reinvented concrete's technologies, formwork and aesthetics to suit the pragmatic and philosophical aims of their times. Architect and computational design researcher Norman Hack and his co-authors introduce a contemporary method for the fabrication of concrete structural elements using the Injection 3D Concrete Printing (I3DCP) process formulated at TU Braunschweig and discuss its benefits over other types of printed concrete.

Die Brückeninfrastruktur in Deutschland und Europa steht aufgrund steigender Verkehrslasten und alternder Bauwerke vor erheblichen Herausforderungen. Das DFG-Schwerpunktprogramm 2388 „Hundert plus – Verlängerung der Lebensdauer komplexer Baustrukturen durch intelligente Digitalisierung“ (SPP100+) strebt an, durch digitale Innovationen und prädiktive Instandhaltungsstrategien die Nutzungsdauer bestehender Brückenbauwerke zu verlängern. Der vorliegende Beitrag fokussiert sich auf das SPP100+ zugehörige Cluster „Monitoring und Simulation“, das sieben Teilprojekte umfasst. Die Projekte entwickeln fortschrittliche Methoden zur Überwachung und Zustandsbewertung von Brücken mittels Digitaler Zwillinge, hochauflösender Sensortechnik und numerischer Simulationen. Innovative Ansätze wie nichtlineare Modellanpassungen, stochastische Methoden und künstliche Intelligenz ermöglichen eine präzise und frühzeitige Identifizierung potenzieller Schäden. Die Kombination aus kontinuierlichem Bauwerksmonitoring und effizienter Datenauswertung ist entscheidend für die langfristige Sicherheit und Langlebigkeit bestehender Brücken und trägt darüber hinaus zur Ressourcenschonung bei.

This research investigates the fatigue behaviour and fracture mechanics of high-performance concrete (HPC), including various compositions such as HPC with basalt aggregates (HPC-B), HPC with gravel (HPC-G), and high-strength coarse mortar (CM) under static and cyclic tensile loading within the special priority program SPP 2020. The study aims to integrate fracture mechanics into structural analysis to enhance design guidelines for slender cross-sections and safety-related high-performance structural components. The experimental investigations reveal HPC-B’s remarkable superiority, displaying its higher compressive strength, modulus of elasticity, and tensile strength compared to HPC-G and CM. A modified disk-shaped compact tension (MDCT) based on ASTM standards, aided by digital image correlation (DIC) unveils fracture behaviour, emphasizing fracture energy as a crucial parameter. HPC-B exhibits improved crack resistance and notch sensitivity reduction attributed to crushed basalt aggregates and an enhanced interfacial transition zone (ITZ). The research scrutinizes factors like material characterization, aggregate morphology, stress levels, and the displacement rate on crack formation. High-cycle fatigue tests show HPC-B’s superior performance, and the post-fatigue analysis reveals enhanced residual fracture toughness attributed to nano-level structural changes, stress redistribution and aggregate-matrix interaction. A 3D image analysis via Computed Tomography (CT) scans captures mesostructural crack propagation and provide quantitative insights. This research marks a significant shift from conventional aggregate-focused approaches and introduces a novel approach by integrating excess paste theory and mesoscale analysis, highlighting the critical role of aggregate choice in material characterization and mesoscale design in enhancing the structural efficiency of HPC. Furthermore, the study advances the understanding of HPC fatigue behaviour, emphasizing the interplay of aggregate types and morphologies and their dynamic response to cyclic loading, offering valuable insights for optimizing design guidelines and fostering innovation in structural engineering.

This contribution deals with particle-bed 3D printing and presents a numerical approach to predict and optimize the printing process. The process studied here, Selective Paste Intrusion (SPI), has been successfully used to print small and medium-sized objects, but it has not been widely implemented. Before widespread adoption in the construction industry, fundamental questions regarding process optimization and paste/aggregate properties must be addressed. To optimize the process, the SPI process has been studied numerically and a computational model was developed to predict the propagation of the fluid through the particle bed. The model describes the cement paste as a Bingham fluid and the particle bed as a porous medium. In parallel, the rheology of the paste and the properties of the porous medium were investigated experimentally. The developed numerical tool was validated through printing experiments. It was shown that the tool was able to predict the final penetration depth, which is crucial for the quality of the printed component.

Digital fabrication technologies, such as 3D concrete printing, are currently making their way into the construction industry. The primary focus in this field is often on the depositing processes, such as extrusion 3D concrete printing, where material is typically applied in horizontal planar layers. This area has seen substantial progress in recent years. However, numerous research and development projects are specifically targeting the additive manufacturing of unreinforced raw concrete components. When implementing these technologies in practice, it has become clear that additional processes, such as fully automated process-parallel reinforcement integration, application of cover layers and formative and subtractive post-processing of the components, are essential for successful application. In addition, by varying the orientation, characteristics and arrangement of the layers, new shapes and functions can be realised. Examples include angled layer orientation for producing vaulted geometries without support structures, as well as non-planar layer formation for complex component geometries or assembly joints. Moreover, alternative innovative manufacturing processes, such as KnitCrete, Smart Dynamic Casting or Injection 3D Printing, reveal new potential for the application of digital manufacturing technologies in the construction industry. This article aims to demonstrate the possibilities offered by digital fabrication with concrete beyond the stacking of horizontal planar layers, and how these technologies can complement and expand a future digital fabrication strategy in the construction industry.

Selective Cement Activation (SCA) is a powder bed 3D printing technique that offers the possibility to precisely manufacture free-form building elements with a high degree of complexity and surface quality. Expanding upon initial findings, we aim to further improve the dimensional accuracy. This fundamental research will explore the effect of vertically adjusting the water content over height to account for the often observed uneven dimensional deviations across the height of a component printed with SCA. Therefore, we investigated three types of w/c-ratio gradation: (a) core-shell gradation, (b) core-shell gradation with shell thickness adaption and (c) core-shell gradation with core water content adaption. The samples produced in this way are compared with samples printed with a uniform w/c-ratio in terms of dimensional accuracy, compressive strength and homogeneity. The results show that grading the w/c-ratio, in general, can increase the dimensional accuracy compared to specimens printed with a high uniform w/c-ratio while simultaneously not significantly decreasing the compressive strength. The adapted gradation concept with core water content adaptation further increased the dimensional accuracy, resulting in just slightly less accuracy than the reference specimens printed with a low uniform w/c-ratio of 0.3. However, these specimens achieved 51