Beton- und Stahlbetonbau最新文献

Forced carbonation effects on recycled concrete aggregates and resulting concrete

In this paper, the effects of forced carbonation after the processing of recycled concrete aggregates were investigated on three levels. In the first step, the effects at the grain level were analyzed. In the second step, concretes with different substitution ratios (replacement of natural aggregates with recycled aggregates) were produced from recycled carbonated and non-carbonated aggregates. The fresh concrete properties and workability characteristics were determined for these concretes with replacement ratios of 50 %, 75 %, 100 % without fine particles <1 mm, and 100 % for the entire grading curve. In the third step, the decisive hardened concrete properties were determined. The concrete was produced using a low-emission cement (CEM II/C–M). In addition to measurable effects at the grain level, a partial reduction in the need for concrete admixtures to achieve comparable fresh concrete properties was also observed. No positive or negative effects resulting from carbonation were observed in the investigation of the hardened concrete properties. With the integration of forced carbonation into processing, three aspects were considered in order to achieve concrete mixtures with the lowest possible environmental impact: 1) use of recycled aggregates, 2) CO₂ storage in the recycled material and 3) use of low-emission cements.

Experimental investigations on the creep behavior of old concrete

Current normative creep prediction models are based on test data from comparatively young, previously unloaded concrete. Therefore, the applicability of these models to existing concrete structures that are loaded again is not assured. However, reliably predicting creep deformations is crucial for preserving existing structures, for example when planning subsequent strengthening measures using external prestressing. This article presents the initial results of creep tests conducted on drill core samples taken from three dismantled prestressed concrete bridges. Despite having a load age of over 40 years, the samples exhibited pronounced creep behavior with creep coefficient values of approximately 0.3 to 0.6 after 90 days of loading. The measured creep deformations were up to 145% greater than the creep prediction model's expected values acc. to Model Code 2020.

Jointless concrete pavements with overlays made of carbon-reinforced concrete (CRC)

Concrete surfaces in industrial areas, parking lots, roads, and motorways are typically provided with joints to prevent uncontrolled cracking due to stress gradients and restrained deformations. Thermal or hygric deformations are concentrated at these joints. To prevent the ingress of harmful substances into the concrete and its substrate, the joints are usually sealed with flexible polymer sealants. However, the durability of these sealing systems is limited despite regular maintenance. Moreover, the concrete in joint areas often exhibits premature damage caused by the combined effects of mechanical loading and environmental exposure. This article presents an alternative concept in which existing joints are covered with thin overlays made of carbon-reinforced concrete (CRC). These CRC layers enable the transformation of local joint openings into quasi-ductile deformations by multiple fine cracks within a predefined area, thereby extending the service life of the concrete pavement. Firstly, the study investigates the load-bearing and deformation behavior of the CRC overlays under biaxial tensile loading at various load ratios. Secondly, the bond properties between the CRC layer and the underlying concrete, as well as the ductility and cracking behavior under quasi-static uniaxial tensile loading are examined using large-scale specimens.

Modular circularity: Build, reuse, repair, convert

Building is characterized by its individuality and manual processes. This is driven by economically viable and demand-oriented structural designs. It leads to monolithic, unique structures that can no longer be used if their purpose changes or if they suffer local damage. While multiple uses of resources were common in construction methods such as masonry (reuse of individual bricks), today's concrete structures are demolished at the end of their service life and largely deposited or recycled as filling material. The paper presents a modular approach to transform concrete construction into a circular process. The key is modular construction, which turns common concrete blocks into repairable and convertible structures that can be adapted to changing utilization requirements. The only way to do this sustainably is through serial production and the use of digitalization and automation. To ensure that this step is not only implemented in the future, but that the existing building stock—over 16 billion tons of concrete structures in Germany alone—is also transferred into the cycle, the building stock is included in the concept and integrated into modular construction through the principle of reuse as separated and reconditioned concrete elements (modules). The paper introduces the concept of modular circularity and presents the associated elements of serial construction, automated production, reuse as well as the convertibility and reparability of load-bearing structures.

Enhancing the sustainability of non-metallic reinforced concrete through the 3R framework

The construction industry is transitioning from a linear to a circular economy, focusing on waste reduction, material reuse, and recycling. Fiber-reinforced composites (FRCs) support this shift by extending reinforcement durability through corrosion-resistant fibers and reducing concrete demand. However, recycling FRCs remains challenging due to high energy consumption, harmful by-products, and degradation of mechanical properties—often leading to downcycling. Furthermore, recycling does not reduce the embodied carbon and grey energy from production, transport, and installation. For sustainable construction, non-metallic reinforcement should be preserved as long as its structural integrity remains intact—e.g., through the reuse of entire concrete slabs. This contribution highlights technologies that aim to implement the 3R principles (Reduce, Reuse, Recycle) in construction, including robotic manufacturing, recycled carbon fibers (rCF), reinforcement repair, automated deconstruction, and reuse-oriented design. Integrating these technologies into non-metallic reinforcement enhances the circularity of concrete structures and promotes material sustainability in the building sector.

Hybrid M - Special considerations when planning a multifunctional infrastructure project in an inner-city location

The new bus depot in Munich's Moosach district replaces the old bus depot in Laim. Covering an area of around 27,000 square metres, the new building complex is divided into bus operations facilities, an office block for external letting (2nd to 5th floors of the western and northern shell construction) and a multi-storey car park. The construction project posed a particular challenge for the structural design, both due to the integration of a wide range of technical requirements on the part of the users and the difficult boundary conditions on the site. The solutions developed for this project are discussed in the present article.

Investigation of the load-bearing behaviour of different notch connections in a new, demountable TCC ceiling system

The construction industry contributes significantly to global energy and resource consumption. In view of climate targets, sustainable materials and the concept of Design for Disassembly (DfD) – developing components that can be dismantled and reused with minimal effort – are gaining increasing relevance. This is particularly true for hybrid elements that combine the advantages of different materials within a single cross-section. In this study, prefabricated timber-concrete composite (TCC) floor elements were developed with a focus on DfD. The key objective was to optimize the shear connection between timber and concrete, as it largely governs the structural behaviour. Load-bearing capacity, ductility and disassembly of different notch connections were investigated as an alternative to bonded or mechanical connectors. The results show that notched connections provide a structurally efficient and reversible solution. The birdsmouth-shaped notch proved especially effective, offering high load-bearing capacity, good ductility, and ease of disassembly. The newly developed TCC elements demonstrated strong composite action and simple separability – clear advantages in terms of reusability and environmental performance.

Collapse of the Carola Bridge in Dresden Part 2: Investigations into the causes of the collapse and the consequences

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.

Tensile tests on yarns made from recycled carbon fibers: Semi-finished products for construction

The demand for the recovery of carbon fibers from old components or the recycling of carbon fiber waste is increasing with the growing use of carbon fibers in many sectors (e.g., automotive, aerospace). Although components reinforced with semi-finished carbon fiber products have already been in operation for several years, the use of recycled or reused carbon fibers (rCF) in the construction sector, especially as concrete reinforcement, opens up a promising opportunity to close the carbon material cycle. In the present study, tensile tests were carried out on yarns made of rCF produced with two different impregnations (thermoplastic and thermoset) and compared with two similarly impregnated yarns made of virgin carbon fibers. The findings indicate substantial disparities in the mechanical properties, particularly in tensile strength and stiffness. These findings signify an inaugural phase in the assessment of rCF yarns for structural applications and underscore the feasibility of subsequent enhancement of rCF processes to attain performance benchmarks commensurate with those of virgin carbon fibers.