Digital fabrication offers the opportunity to reintroduce force-flow-aligned reinforcement as a material-efficient solution in concrete construction. Nevertheless, it raises new questions regarding the structural design and performance of such elements. The current paper discusses design and structural performance of full-scale reinforced concrete beams of two types, with the same external dimensions but different reinforcement layout reflecting fabrication methods. One type was traditionally cast, and features modern orthogonal reinforcement composed of longitudinal bars and stirrups. The other type was digitally fabricated with Shotcrete 3D Printing (SC3DP) additive manufacturing method, and with force-flow aligned reinforcement in form of bent-up steel reinforcement bars. Both types were tested under three- and four-point bending, to investigate their response under shear force and bending moment respectively. The structural testing results prove that the two types of beams can be considered as equivalent under the bending moment at Ultimate and Serviceability Limit States, while the SC3DP beam contains only around half of steel reinforcement mass compared to the cast one. Furthermore, the challenges in calculation and verification of reinforced concrete beams with non-orthogonal reinforcement are discussed. Additionally, it is demonstrated that the same cementitious material exhibits higher mechanical performance when processed with SC3DP compared to casting in formwork, with similar or lower scatter. Finally, it is concluded that the reinforced SC3DP elements follow the same structural principles as the cast concrete ones, and as such can be designed using existing methods while respecting the reinforcement detailing as required per complex layouts.

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.

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.

The rapid development of climate change and the resulting EU climate targets require a significant reduction in CO2 emissions. The usage of fully recyclable, locally sourced low-carbon materials, such as earthen materials, can play a key role in reducing CO2 emissions. Currently, research in the field of additive manufacturing with earthenmaterials focuses on extrusion-based processes. However, with these new techniques, materials must be precisely tailored to the manufacturing process's requirements to achieve a functional material-process interaction. This results in the requirement for highly specialized materials, hindering the use of local earth resources with inherent varying compositions. As a traditional construction technique for earthen materials, rammed earth construction has been in use for thousands of years. Without adaptations to the ancient process to the current state of the art in industrialized countries, its use results in high construction costs. Therefore, research in advanced processing methods, including digital fabrication and automation approaches, could fully unlock the potential of local earthen construction and bridge the gap between traditional processes and the future demands of construction practices. The present research examines the challenges of sourcing and using local earth materials in a robust, automated, rammed earth manufacturing process. Therefore, investigations on the influence of process parameters such as layer height and impact energy on the material properties such as dry density and compressive strength using a range of raw materials are presented. The aim of this investigation is the identification of robustness criteria regarding the digitally controlled processing of local materials in the robotic rammed earth process on process and material level.

Bridges in Germany and across Europe face significant challenges due to increasing traffic loads and aging structures. The DFG Priority Programme 2388 "Hundred Plus - Extending the Lifetime of Complex Engineering Structures through Intelligent Digitalization" (SPP10 0+) aims to extend the lifespan of existing bridge structures through digital innovations and predictive maintenance strategies. This paper focuses on the SPP100+ affiliated cluster "Monitoring and Simulation," which encompasses seven sub-projects. These projects develop advanced methods for monitoring and assessing the condition of bridge structures using digital twins, high-resolution sensor technology, and numerical simulations. Innovative approaches such as nonlinear model adjustments, stochastic methods, and artificial intelligence facilitate precise and early identification of potential damages. The combination of continuous structural monitoring and efficient data evaluation is crucial for the long-term reliability and durability of existing bridges and contributes to resource conservation.

This study investigated the structural response of timber-plain concrete panels bonded with epoxy and PUR adhesives under a four-point bending load, both experimentally and numerically. Tests evaluating epoxy- and PUR-bonded glulam-plain concrete panels measured ultimate load (F-ult), effective bending stiffness (EIeff), and mid-span deflection (triangle(ult)). Shear stress within the adhesives was monitored using fiber optic sensors. Numerical simulations estimated F-ult and EIeff with errors of 5% and 14%, respectively. These discrepancies may stem from assuming defect-free wood (e.g., no knots), idealized boundary conditions, uniform adhesive thickness assumptions, and unmeasured mechanical and damage properties of wood and concrete (Detailed error source analysis is available in Section 4). Qualitative validation involved comparing sensor-recorded shear stress with simulation results. A parametric study evaluated the effects of adhesive thickness (0.5, 1, and 3 mm), concrete-wood depth ratio (50/85, 85/85, and 150/85 mm/mm), wood type (spruce (Picea), beech (Fagus), and Azobe (Lophira)), concrete strength class (C12/15 and C30/37), and span length (2, 4, and 8 m) on the bending behavior of the panel, and the shear and peel stress distribution within the adhesive bond line. Shear stress prevailed over peel stress in the adhesives, with peel stress approaching shear stress as span length increased (max. tau(a) / max. sigma(a) similar to 3.5-7). The ultimate load typically resulted in concrete compression damage and wood fiber damage, influenced by the concrete strength class and wood type. Higher depth ratios led to tension damage in concrete, while adhesive thickness had a minimal impact on stress distribution and failure modes.

In this paper, we report on the effect of adhesive type on the bending behavior of adhesively bonded glulam-concrete composite panels. Steel-reinforced and unreinforced concrete were bonded to glued-laminated timber (GL-24 h) panels with polyurethane (PUR) and epoxy adhesives using the wet bonding technique. The PUR and epoxy adhesives (both were two-component adhesives) were selected to represent brittle and ductile adhesive behaviors. Four-point bending load was applied to glulam-concrete composite panels to investigate their bending behavior. The bending behavior measured includes bending load vs. mid-span deflection, strain distribution at the mid-span of the panel, and strain distribution between the loading point and its adjacent support. These were determined by linear variable differential transformers (LVDTs), strain gauges, and digital image correlation (DIC) technique. The experimental results showed that polyurethane-bonded glulam-unreinforced concrete panels showed similar load vs. mid-span deflection curves to epoxy-bonded glulam-unreinforced concrete panels and epoxy-bonded glulam-reinforced concrete panels. The shear stress, shear deformation, and peel strain analyses at the adhesive bond line showed that debonding at the glulam-concrete interface was more likely to occur for panels bonded with the ductile polyurethane.

Currently, the effect of the concrete substrate on the load-bearing capacity of externally bonded carbon fiber-reinforced polymer (CFRP) strips is determined in the guidelines solely on the basis of the concrete strength. In addition to strength, surface preparations can improve the bond resistance by exposing the aggregates of the substrate. However, the interlocking also depends on the parameters that derive from the inner concrete structure such as the aggregate grain dimension and shape, as well as the air void distribution. Therefore, the influence of the concrete inner structure on the bond is investigated with double shear tests by comparing specimens with equivalent concrete strength and varying grain shape and dimension. Bond parameters are calculated using a novel procedure based on optical measurements. Furthermore, crack paths are analyzed by means of computed tomography (CT) scans, which prove to be an essential inspecting technique for characterizing the inner concrete structure. The fracture energy is approximately 30% higher for concrete with a maximum grain size of 8 mm compared to an equivalent mortar with 4 mm maximum grain size. (C) 2022 American Society of Civil Engineers.

High-performance concrete (HPC) enables slender cross sections, lighter structures and wider spans for buildings characterised due to its advanced mechanical properties. These structures are often subjected to high cyclic loads. Despite being an advanced material, the opportunities that arise from the use of HPC under fatigue loading cannot be fully exploited because of conservative design standards. This could be mitigated by implementing fracture mechanical properties in structural analysis. In the following paper, the fracture behaviour of HPC is investigated using a modified compact tension test inside a computed tomography system. The tensile strength and fracture energy are measured. The fracture energy is of particular interest in this context as it is a key parameter in determining the damage laws of brittle materials. The morphological characteristics of aggregates and their mechanical interaction with the cement-based matrix strongly influence the crack formation and fracture behaviour of composite materials such as HPC. In order to identify the initial mesostructure comprising aggregate particles, cementitious matrix and air voids, a 3D image analysis technique based on computed tomography (CT) has been integrated. CT scans are performed under loading, and the 3D damage and crack propagation phenomena are quantitatively observed during the test. The static tests are carried out in displacement control and CT images are generated in predetermined displacement steps.

Obtaining the mesostructure of concrete from X-ray computed tomography (CT) requires segmentation of the data into distinct phases, a process complicated by the limited contrast between aggregates and mortar matrix. This paper explores the possibility to add baryte or hematite into the concrete mixture to enhance the contrast between cement paste and aggregates in CT, thus allowing for a semi-automatic segmentation. Raw and segmented CT images of plain and modified concrete mixtures are obtained and compared to assess the validity of the proposed approach. Characterization tests are also performed in order to ensure that the concrete characteristics are not appreciably affected by the presence of the enhancers.

Concrete is a composite material whose structure at the mesoscale, i.e. at a scale where the cementitious matrix, aggregates and pores are separately identified, plays a significant role on the observed macroscopic behavior. In particular, it is nowadays widely accepted that a deeper understanding of the aging processes in concrete (e.g., cracking or chemical degradation) can be gained by modeling explicitly its mesostructure [1]. To date, the mesostructure of concrete is mainly obtained through artificial generation [2] or 3D imaging techniques such as the X-ray computed tomography (CT) [3]. While the former is usually computationally expensive and/or oversimplified, the latter has become an attractive option in the past few years. However, the identification of the various phases, i.e. the segmentation process, is not trivial [3], especially because of the limited contrast between cementitious matrix and aggregates due to their similar composition. Also, since the dimensions of the inclusions in concrete span a few orders of magnitude (from 0.1 mm up to 35-40 mm), a threshold length above which the heterogeneities are explicitly modeled is needed. Another challenge to be faced is the generation of meshes as input for the numerical codes. In particular, the large amount of degrees of freedom generated by directly converting the voxelized CT images into voxel-based hexahedral meshes often limits the possibility of analyzing a representative volume of the material [3]. Hence, a technique to describe the voxelized images through surfaces and volumes should be adopted to allow for a standard mesh generation. In the present work, we explore the possibility to add highly absorptive baryte powder into the concrete mix to enhance the contrast between the cementitious matrix and the aggregates, allowing for an easier segmentation and mesh generation. A threshold length for the explicit description of the heterogeneities is chosen accounting for the CT image resolution and the maximum aggregate size. Calorimetry and mechanical tests are performed in order to ensure that the concrete mix is not appreciably affected by the presence of baryte. The results of both normal (i.e., without baryte) and modified concrete are compared to demonstrate the validity of the method. Acknowledgments. The authors gratefully acknowledge Dale P. Bentz of the U.S. NIST for the useful suggestions and the discussion provided.

This chapter gives an overview on the state-of-the-art about verifications of reinforced concrete structures using Externally Bonded (EB) Fibre Reinforced Polymers (FRP) under particular loading condition. Focus is mainly put on flexural strengthening, nowadays the most common application field for composite materials in structural engineering. The items discussed in this chapter are:Serviceability limit states;Fatigue behaviour;Effects of fire and high temperature;Long term behaviour;Anchoring systems;Mechanically Fastened Systems

Eine effektive Methode um Alterungserscheinungen und Überbelastungen von Stahlbetonbauteilen zu begegnen ist das Verstärken mit aufgeklebten Kohlefaserkunststoffen (CFK). Voraussetzung für die Wirksamkeit einer solchen Verstärkung ist ein intakter Verbund zwischen aufgeklebter Bewehrung und Beton. Insbesondere bei schwingenden Beanspruchungen ist dieser von Ermüdungserscheinungen bedroht. In Dauerschwingversuchen zeigt sich, dass schon bei circa 40 Prozent der statischen Verbundbruchkraft eine Rissbildung im Betonsubstrat entlang des aufgeklebten CFK eintreten kann, die zu einer Entkopplung der aufgeklebten Bewehrung führt. Die für die Kraftübertragung noch verbleibende Verbundlänge hängt dabei von der Ober-, der Unterlast und der Lastspielzahl ab. Dieser Zusammenhang wird anhand von Versuchsergebnissen und daraus abgeleiteten Modellvorstellungen erläutert. Aus den Versuchsergebnissen und den Modellvorstellungen geht das Bemessungskonzept für Ermüdung aus der DAfStb-Richtlinie für das Verstärken von Betonbauteilen mit geklebter Bewehrung [1] hervor. Das Konzept und der Ablauf der Nachweisführung wird in einem zweiten Abschnitt dargestellt. Mit dem beschriebenen Bemessungskonzept ist es seit Einführung der neuen Generation allgemeiner bauaufsichtlicher Zulassungen [2], [3], [4] im Januar 2015 für Verstärkungssysteme mit schubfest aufgeklebten CFK-Lamellen wieder möglich Stahlbetonbauteile auch unter nicht ruhender Belastung zu verstärken.

Das Verbundverhalten aufgeklebter Bewehrung unter zyklischer Beanspruchung hat einen wesentlichen Einfluss auf die Verformungen und die Tragfahigkeit verstarkter Betonbauteile. Der Schadigungsprozess des CFK-Lamellenverbundes auf Beton wird unter nicht ruhender Beanspruchung bei mehr als zwei Millionen Lastwechselzahlen bei variierenden Ober- und Unterlasten in Dauerschwingversuchen untersucht. Anhand von Dehnungs- und Verschiebungsmessungen werden vorhandene Modellvorstellungen zur Rissbildung an der Betonoberflache teilweise bestatigt und in Bezug auf den Einfluss der Unterlast erweitert. Die Vorgehensweise der Versuchsdurchfuhrung und -auswertung zeigt eine Moglichkeit auf mit wenigen Versuchen die notwendigen Parameter fur das Bemessungskonzept zu identifizieren. Daruber hinaus wird ein Ansatz zur Berucksichtigung des unterschiedlichen Verbundverhaltens von einbetonierter und aufgeklebter Bewehrung bei schwingender Beanspruchung formuliert. Es wurden Dauerschwingversuche an Zug-Druckkorpern mit einer Verbundlange von 1100 mm unter mehr als zwei Millionen Lastwechseln durchgefuhrt. Dabei wurden drei unterschiedliche Betonfestigkeitsklassen und zwei unterschiedliche Typen geklebter Bewehrung aus Kohlefaserkunststoffen berucksichtigt. Anhand der Versuchsergebnisse konnte ein Modell entwickelt werden, welches es ermoglicht die Schadigung des Lamellenverbundes in Abhangigkeit der Unterlast und der Lastspielzahl zu beschreiben. Mit dem Modell lasst sich aus der zu erreichenden Lastspielzahl und dem Unterlastniveau die Schwingbreite der Lamellenkrafte bestimmen, die zu einer bestimmten entkoppelten Lange der Lamelle fuhrt. Die entwickelten Formeln und Modellvorstellungen beinhalten ebenfalls die Moglichkeit, die Rissfortschrittrate oder die erreichbare Lastspielzahl in Abhangigkeit der entkoppelten Lange und der Ober- und Unterlast zu bestimmen. Die vorliegende Arbeit beinhaltet die Entwicklung eines Modells fur den Klebeverbund von Faserkunststoffen auf Beton unter schwingender Beanspruchung, einen fur die Praxis anwendbaren Bemessungsansatz und die versuchstechnische Erkundung der fur das Modell und den Bemessungsansatz notwendigen Parameter.

Strengthening with externally bonded carbon fiber reinforced plastics (CFRP) is an effective technique to counteract aging and overloading of reinforced concrete structures. Prerequisite for the effectiveness of such reinforcement is an intact bond between reinforcement and concrete. In particular, under cyclic loads this is threatened by fatigue. Cyclic tests show, that already at about 40 % of the static bond strength cracking in the concrete substrate along the bonded CFRP occurs, which leads to decoupling of the bonded reinforcement. The remaining bond length depends on the upper and lower load level and the number of load cycles. In following experiments, results and model concepts are presented which illustrate this relationship. The design concept for fatigue from the DAfStb guideline for strengthening of concrete members with adhesively bonded reinforcement [1] was developed on the base of the experimental results and the model concepts. The concept and the process of the structural analysis is shown in a second section. Since the introduction of the new generation of general building approvals [2], [3], [4] for strengthening systems with externally bonded CFRP strips in January 2015 it is possible again to strengthen reinforced concrete structures also under non static loading with the described design concept.

Autogenous shrinkage of cement paste and concrete is defined as the macroscopic length change occurring with no moisture transferred to the exterior surrounding environment. It is a result of chemical shrinkage affiliated with the hydration of cement particles and the ongoing process of self‐desiccation. The process of self‐desiccation can be modeled starting from the formation of the capillary pore space during hydration in the cement paste. In this proposal a working model will be introduced explaining the difficulties to obtain the autogenous shrinkage strains directly from a simulated or measured microstructure of cement paste. In a second step the autogenous shrinkage of a hardening cement mortar was described on a mesoscopic level. It based on measurements on cement paste. The mortar simply consists of cement paste and a defined fraction of spherical aggregates with a known modulus of elasticity. Furthermore the influence of the interfacial transition zone (ITZ) is studied in numerical simulations. The results of these finite‐element‐calculations are introduced and compared with testing results of the autogenous shrinkage of hardening mortar samples. (© 2009 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)