Abstract An experimental study on load-induced temperature fields generated in cyclic compression tests was carried out to close an existing knowledge gap on the material behaviour of ultra-high-performance concrete (UHPC). The main result of the experimental part is an analytical model with which the misleading influence of specimen heating on the results of such fatigue tests can be eliminated. The theoretical work was based on the microplane model M4 for concrete. The model was calibrated on laboratory tests on the investigated UHPC. The results of the numerical simulations confirmed the influence of temperature on the fatigue strength.

Die Nachfrage nach der Rückgewinnung von Carbonfasern aus Altbauteilen oder nach der Verwertung von Carbonfaserabfällen steigt mit der zunehmenden Verwendung dieser Fasern in vielen Sektoren (z. B. Automobilindustrie, Luft‐ und Raumfahrt). Obwohl mit Carbonfaserhalbzeugen bewehrte Bauteile bereits seit einigen Jahren Anwendung finden, eröffnet die Nutzung von recycelten oder wiederverwendeten Carbonfasern (rCF) im Bausektor, insbesondere als Betonbewehrung, eine vielversprechende Möglichkeit, den Carbon‐Materialkreislauf zu schließen. In der vorliegenden Studie wurden Zugversuche an Garnen aus rCF, die mit zwei verschiedenen Imprägnierungen (thermoplastisch und duroplastisch) hergestellt wurden, durchgeführt und mit zwei ähnlich imprägnierten Garnen aus neuwertigen Carbonfasern verglichen. Die Ergebnisse zeigen signifikante Unterschiede in den mechanischen Eigenschaften, insbesondere hinsichtlich Zugfestigkeit und Steifigkeit. Dennoch stellen diese ersten Tests einen initialen Schritt in der Bewertung von rCF‐Garnen für strukturelle Anwendungen dar und weisen auf das Potenzial für eine weitere Optimierung der Prozesse für rCF hin, um ein Leistungsniveau zu erreichen, das mit dem von neuen Carbonfasern vergleichbar ist.

Im Anbetracht der gesellschaftlichen Herausforderungen des 21. Jahrhunderts, die an das Bauen der Zukunft gestellt werden, stellt sich die Frage, ob der Betonbau noch Teil dieser Zukunft ist. Das aber gerade mit dem Massenprodukt Beton ein ressourcenschonendes und klimaresilientes Bauen möglich ist, zeigen die in diesem Beitrag ausgewählten Beispiele, Ansätze und Möglichkeiten. Die Beleuchtung bereits vorhandener Erfahrungen aus der Tradition des Betonbaus und das Darstellen der aktuellen Entwicklungen skizzieren die zukünftigen Möglichkeiten. Es wird deutlich, dass die aufgezeigten Lösungen stets das Ziel verfolgen, in der Breite das Bauen mit Beton zukunftsfähig zu gestalten. So können auch einzelne Umsetzungsmöglichkeiten oder eine Kombination zielführend sein. Grundsätzlich gilt es, langlebige Bauwerke mit Lebensdauern über mehr als 100 Jahre sowie eine möglichst vollständige Umnutzung (Reuse) oder Wiederverwendung zu ermöglichen.

Der 11. September 2024 wird nicht nur der Brückenbaucommunity in Deutschland in Erinnerung bleiben. Der Teileinsturz einer Spannbetonbrücke ohne Vorankündigung schlug berechtigterweise gesamtgesellschaftlich hohe Wellen. Betroffen war die Dresdner Carolabrücke – ein ästhetisches und ausgesprochen schlankes Bauwerk, welches in der Fachwelt zurecht als eine Ikone der Ingenieurbaukunst ihrer Zeit angesehen wird. Die Konstruktion sowie der Bau wären auch unter den heutigen Möglichkeiten eine Herausforderung. Der vorliegende Teil 1 des Beitrags fasst die Entstehungsgeschichte, die Konstruktion selbst und den Bau der Brücke zusammen. Es werden die Maßnahmen zur Instandhaltung und Sanierung der Brücke vorgestellt. In einem Teil 2 wird der Versuch der Rekonstruktion des Einsturzvorgangs unternommen und die Suche nach der Einsturzursache beschrieben. Zudem wird auf das aktuelle Monitoring eingegangen und ein Ausblick auf das mögliche weitere Vorgehen gegeben.

On September 11, 2024, the partial collapse of the prestressed Carola Bridge in Dresden occurred without prior notice. The first part of the article presented the design and construction of the bridge, the measures for maintenance and renovation and the problem of stress corrosion cracking. In this second part, the comprehensive investigations into the cause of the collapse are described. An attempt is made to reconstruct the collapse process, and aspects of the further handling of bridges containing steel at risk of stress corrosion cracking are presented. In addition, the current monitoring is discussed, and an outlook on possible further action is given.

September 11, 2024 will not only be remembered by the bridge construction community in Germany. The partial collapse of a prestressed concrete bridge without prior notice rightly made waves throughout society. The bridge in question is the Dresden Carola Bridge - an aesthetic and extremely slender structure that is justifiably regarded by experts as an engineering icon of its time, whose design and construction would still be a challenge under today's conditions. This first part of the article summarizes the history of the bridge's genesis, its design and construction, and presents the measures taken to maintain and renovate the structure. The second part, an attempt will be made to reconstruct the collapse process, and the search for the collapse causes will be described. In addition, the current monitoring will be discussed and an outlook on possible further action will be given.

Assessment of structural responses to dynamic loads, such as impact, is essential because these loads can cause severe damage to infrastructure and pose risks to human lives. Important elements of structures, like bridge piers and building columns, are particularly vulnerable to impact loads from vehicle collisions or rockfalls. To address such critical loading, we conducted impact tests to analyze the responses of post-tensioned steel-reinforced concrete (RC) column sections under controlled impact loads. A large drop tower was used to accelerate a rigid cylindrical projectile with a flat nose, having a diameter of 100 mm, a length of 380 mm and a weight of 21.6 kg. Reaction forces were measured using load cells, while accelerometers captured high dynamic accelerations during impact. Both the reinforced concrete columns and the impactor were equipped with a speckle pattern, facilitating Digital Image Correlation (DIC) analysis. The DIC system was used to track the impactor velocity, to measure deflections, and to observe of the cracking patterns on the column surfaces. In total, six 200 mm × 300 mm × 1500 mm different column specimens were tested under two distinct impact velocities: 25 m/s and 33 m/s. The clear span was 1000 mm and the longitudinal and transverse reinforcement ratios were approximately 2 % and 0.7 %, respectively. Four columns were post-tensioned to two levels of 34 % and 67 % of their axial capacity and compared to two reference specimens with no axial force. This range of axial force was chosen to have a detailed evaluation of how different levels of post-tensioning influenced structural performance, specifically in terms of reaction force, lateral deflection and cracking patterns under impact loading. We observed that the mass of debris generated by the impact increased with impact velocity. In most cases, the debris mass also increased with a higher axial force ratio. This trend is likely due to the release of elastic energy stored within the post-tensioned specimen during the impact event, which intensified the dynamic response. Specifically, we noted a pronounced spalling of the concrete cover, primarily on the rear side of the impact, which led to the exposure of the reinforcement. The results of this study can serve as basis for analytical and numerical models and as guideline for testing additional parameters in similar specimens.

This study compares the blast performance of reinforced concrete (RC) slabs with and without strengthening on the impact-facing side. The strengthening strategy employed the application of two thin layers of materials with a high mutual stiffness offset, i.e., high-contrast layers. The first is a low-strength, low-modulus damping layer made of infra-lightweight concrete, followed by a second layer of high-ductility fiber-reinforced concrete. The plain RC slabs under investigation vary in thickness of either 40 mm or 100 mm. The layered specimens consist of a 40 mm thick RC slab strengthened with a 40 mm damping layer and a 20 mm cover SHLC3 layer. This configuration enables a comparison of its behavior with the unstrengthened specimen (a plain 40 mm thick RC slab) and a specimen with a similar eigenfrequency (the plain 100 mm thick RC slab). The employed shock tube subjects the specimens to two rapidly rising areal pressures: a low-pressure wave reaching approximately 0.4 MPa and a high-pressure wave peaking at around 1.2 MPa. The study assesses the specimens' response in terms of accelerations, velocities, and deformations. Additionally, it evaluates damage by analyzing crack patterns, Ultrasonic Pulse Velocity (UPV) measurements, and damping analysis. Overall, the layered specimens exhibited performance nearly equivalent to the 100 mm thick specimens, displaying similar deformations and velocities despite having lower mass and bending stiffness. The high-pressure shock wave hardly damaged layered specimens, unlike the 40 mm thick slabs.

In civil engineering, carbon is typically regarded as a modern material to serve as reinforcement in concrete structures. Compared to steel reinforcement, it features two substantial benefits: It is not sensitive to corrosion, and has an enormously increased tensile strength. In contrast, carbon reinforcement is sensitive to lateral pressure and lacks the property of strain hardening. As a first step of establishing carbon reinforced concrete as a new building composite material, carbon reinforcement has basically served to replace the state‐of‐the‐art steel reinforcement. This target led to pioneering findings with respect to determining the material properties of the composite and developing advanced individual components. However, barely substituting steel by carbon does not allow to fully utilize the carbon's benefits while its disadvantageous properties reveal the limits of this approach. Instead, novel design principles are required to meet the material's nature aiming at appropriately using its beneficial properties. Currently, new construction principles are being researched for high‐performance building material combinations such as textile and carbon reinforced concrete. This paper provides an overview of baselines in the preliminary stages of this research. The overview includes history, inspiration, concrete matrices, non‐metallic reinforcement, structural elements, modeling, production, tomography, and sustainability. The objective of the study is to provide a baseline for the envisaged development of principles for future construction: radically new concepts for the design, modeling, construction, manufacturing, and use of sustainable, resource‐efficient building elements made of mineral building materials with the aim of entirely benefiting from the materials' potential.

Lightweight ceiling elements made of carbon-reinforced concrete In concrete components under bending, a large proportion of the material used is underutilized. More material efficiency can be achieved, e. g., by resolving the solid interior of such components. In the presented project, this is realized by load-bearing filigree carbon-reinforced concrete membranes. At first, in the article, the method of producing such filigree structures by casting under negative air pressure is briefly explained and the material properties achieved in standard tests are discussed. Then, the design of two 1.8 m × 1.8 m large slabs and their load-bearing behavior in 9-point bending tests are described and compared with numerical simulations. Finally, an outlook on future research is given.

The paper presents a series of impact experiments performed on reinforced concrete (RC) beams with and without stirrups and additionally without and with lateral strengthening layers made of two types of strain-hardening cement-based composites (SHCC). The impact tests were performed in an advanced drop tower facility with accelerated steel projectiles. Four impact velocities were applied, ranging from 17 m/s to 30 m/s, corresponding to kinetic energies from 2.1 kJ to 6.4 kJ. The difference among the 20 mm-thick, normal-strength SHCC layers with 2 % fiber volume content consisted in the type of reinforcing fiber: polyvinyl alcohol (PVA) or ultra-high molecular weight polyethylene (UHMWPE). The RC beams with no shear reinforcement yielded shear failure with displaced punching cones under all impact velocities. The stirrups notably increased their impact resistance and damage extent, allowing multiple impact loads without structural collapse. The lateral strengthening layers of SHCC substantially increased the load-bearing capacity, diminished the damage extent, and increased the damage tolerance under repeated impact events of the RC beams both with and without stirrups, proving to be a promising strengthening solution for existing structures prone to severe mechanical loading.

Lightweight ceiling elements made of carbon-reinforced concrete In concrete components under bending, a large proportion of the material used is underutilized. More material efficiency can be achieved, e. g., by resolving the solid interior of such components. In the presented project, this is realized by load-bearing filigree carbon-reinforced concrete membranes. At first, in the article, the method of producing such filigree structures by casting under negative air pressure is briefly explained and the material properties achieved in standard tests are discussed. Then, the design of two 1.8 m x 1.8 m large slabs and their load-bearing behavior in 9-point bending tests are described and compared with numerical simulations. Finally, an outlook on future research is given.

In the case of solid slabs made from reinforced concrete that are usually subjected to bending, large areas of the structure are stressed well below their load-bearing capacity or remain stress-free. Contrary to this are shell structures, which can bridge large spans with little material if designed well. To improve the efficiency of ceiling slabs, we want to utilize the shell load-bearing behaviour on a smaller scale by dissolving the solid interior accordingly. In order to be able to study a wide range of such constructions virtually, a parametric multi-objective simulation environment is to be developed in an ongoing research project, the basic analysis approaches of which are presented in this paper. In addition to the basic workflow and the programs used, the material models for TRC material compared and their calibration are described on the basis of tests on textile reinforced concrete (TRC) samples. Various material models were implemented within the commercially available software RFEM (Version 5.19). Laboratory tests on two different geometry solutions of TRC structures served to verify the models. The structures were selected in a way that differentiates between the bending and membrane actions to indicate the application fields for various approaches in the numerical modelling of TRC structures.

Concrete as one of the most widely used materials has a significant impact on the emission of CO2 and resource consumption. To reduce these negative aspects, concrete structures need to become more efficient and lightweight. Textile-reinforced concrete (TRC) as a novel material promotes new possibilities in the design of filigree load-bearing structures. In turn, such a shift demands new methods in the manufacturing of concrete elements to ensure an appropriate level of quality compared to common methods. As one possibility it was proposed to rethink a vacuum-assisted die-casting method, which is widely used in other industries such as automotive, for the production of thin-walled TRC components. In this regard, the question arose, if the specific manufacturing conditions result in different material properties of the high-performance concrete matrix. Thus, current research focused on detecting differences and similarities in the material properties of concrete and TRC samples cast under standard conditions and under reduced air pressure conditions or how it is referenced further in the text negative air pressure condition (APC). On themeso scale, no noteworthy differences were determined. To dive deeper into understanding how the altered environmental conditions impact the high-performance concrete matrix, a series of experiments were conducted to scrutinize the microstructure of samples that were extracted from large-scale shell elements cast under different grades of air pressure using a computed tomography (CT) and X-ray diffraction (XRD) method. The results are presented in the paper.

In the case of solid slabs made of reinforced concrete that are usually subjected to bending, large areas of the structure are stressed well below their load-bearing capacity. Contrary to this are shell structures, which can bridge large spans with little material if designed according to the force flow. To improve the efficiency of ceiling slabs, we want to utilize the shell load-bearing behavior on a smaller scale by resolving the solid interior accordingly. In order to study a wide range of such constructions virtually, a parametric multi-objective simulation environment is being developed in an ongoing research project. The basic analysis approaches that were implemented are presented in this paper. The basic workflow, the used programs and material models, and their calibration on the tests on textile-reinforced concrete (TRC) samples are described.

Im Beitrag werden die Baumaterialien, die für die beiden Gebäudeteile BOX und TWIST des Carbonbetongebäudes CUBE verwendet wurden, grundlegend beschrieben. Der Fokus liegt dabei auf der Vorstellung und Darlegung der maßgebenden Materialeigenschaften von denjenigen Baustoffen, die noch keiner Produktnorm unterliegen und deren Anwendung damit einer Zustimmung im Einzelfall (ZiE) unterlag. Darunter fielen sieben verschiedene Betone, drei Carbongelege, zwei unterschiedliche Carbonstäbe, Glasfaserstäbe und drei Hochleistungsdämmstoffe.

Ende September 2022 wurde an der TU Dresden das weltweit erste Gebäude aus Beton eröffnet, bei dem ausschließlich endlosfaserbasierte Bewehrung, hauptsächlich aus Carbon, zum Einsatz kam. Für die Herstellung des Gebäudes wurden eine Zustimmung im Einzelfall (ZiE) und eine vorhabenbezogene Bauartgenehmigung gemäß Sächsischer Bauordnung bei der Landesstelle für Bautechnik (Leipzig) erwirkt. Im Teil I der ZiE wurde der Gebäudeteil BOX des CUBE geregelt. Es wurden die mechanischen Kennwerte der einzelnen Carbonbetonkomponenten und das rechnerische Vorgehen zur Bestimmung der Gebrauchs‐ und Tragfähigkeit der Carbonbetonbauteile in Fertigteil‐ bzw. Halbfertigteilbauweise festgelegt. Zudem erlaubte die ZiE den Einsatz nicht geregelter, neuartiger Wärmedämmungen und legte Parameter für die Berechnung des Wärmeschutzes und Energiebedarfs für das Gebäude fest. Im Beitrag werden ausgewählte Untersuchungen zu den Carbonbetonbauteilen der BOX vorgestellt, die Basis für die ZiE waren.

The article provides a general description of the building mate-rials used for the two building parts BOX and TWIST of the car -bon reinforced concrete building CUBE. The focus of the article is on the presentation and explanation of the decisive material properties of those building materials that are not yet subject to a product standard and whose use was therefore subject to approval in individual cases (in German: Zustimmung im Einzel-fall, ZiE). These included seven different concretes, three car -bon fabrics, two different carbon rods, glass fibre rods and three high-performance insulating materials.

Multiscale techniques allow for the efficient numerical investigation of the structural behavior considering a complex reinforcement distribution. The present contribution compares two multiscale methods in terms of their applicability for thin-walled, carbon-reinforced concrete structures. The first is a coupled multiscale method that simultaneously solves multiple finite element problems (FE2) and provides a smeared material model. The second is the multiscale projection method (MPM) which is capable of reproducing localization effects within a certain domain and their effect on the overall failure of the structure. For both methods, the problem is divided into a macroscopic and a mesoscopic scale. The former describes the statical system of the shell. The latter considers the distribution and geometry of the reinforcement. Micro-CT data acquired and processed in the scope of CRC/TRR 280 give detailed insight into the mesoscopic scale. The coupled multiscale model aims to define a representative volume element (RVE) that captures the mesoscopic behavior at each macroscopic point. Shell elements cover the macroscopic behavior of the statical system, while scaled boundary elements represent the mesoscopic model. The MPM magnifies certain, spatially limited areas where localization phenomena might occur. The overall mesoscopic effects are incorporated by the projection of the mesoscopic stresses onto the macroscale. Here, both scales are modelled using three-dimensional finite elements. On the mesoscale, the extended finite element method (XFEM) is used to reproduce the reinforcement heterogeneities. In future work, both methods will be used for the analysis of shell-like dissolved concrete structures.