Gradientenbetontechnologie

The technology of graded concrete – From the development of concrete mixtures and the conceptual design to the automatized manufacturing In accordance with the principle form follows force, the DFG Priority Programme (SPP) 1542 – Leicht Bauen mit Beton – focuses on energy efficient building elements which are designed specifically for desired operational demands. This concept is a specific adaption of the component's inner structure and works in parallel to the optimization of its outer shape which is often limited due to the outer geometry of slabs, walls and supporting pillars. The technology of graded concrete provides an innovative possibility to adapt the inner structure of a concrete component in order to meet defined static and structural‐physical requirements. The challenging demands imposed by the technology of graded concrete require a holistic approach including the development of the concrete mixtures, the design of the building components and finally the automatized manufacturing. In collaboration between the departments ILEK, IWB and ISYS of the University of Stuttgart a complete process chain for the manufacturing of functionally graded concrete components was realized. This approach for concrete results in fully recyclable building elements with significant savings in mass and energy.     

Verstärkung mit vorgespannten eingeschlitzten Lamellen

Oberflächig geklebte Kohlefaserlamellen (CFK) werden seit vielen Jahren zur Bauteilverstärkung eingesetzt. Die hohe Festigkeit dieser Lamellen kann aufgrund der begrenzten Zugkapazität des Betons nicht zur Gänze ausgenutzt werden. Eingeschlitzte CFK Lamellen hingegen bieten wesentliche Vorteile, die zusammen mit Vorspannung noch weiter optimiert werden können. Für den Vorspannvorgang wurde eine spezielle Verankerung, auf Basis der Composite Wedge Technik, entwickelt, die einfaches und rasches Vorspannen erlaubt. Die Verankerung ist zudem nur 8 kg schwer, was besonders vorteilhaft beim Hantieren an der Tragwerksunterseite ist. Nach dem Vorspannen wird die Verankerung entfernt, die permanente Verankerung der Lamellenenden wird durch nachträgliches Verkleben gewährleistet. Es verbleiben keine Stahlteile im oder am Bauwerk. So zeichnet sich die Konstruktion durch geringen Wartungsaufwand und hohe Dauerhaftigkeit aus. Die Effizienz der entwickelten Systeme wurde an vorgespannten Plattenstreifen experimentell untersucht. In den Untersuchungen konnte die Funktionsfähigkeit nachgewiesen werden. Strengthening using Prestressed Near Surface Mounted Strips Exernally bonded carbon fiber reinforced plastic (CFRP) strips are used since several years for strengthening of structures. The high strength of the strips cannot be exploited in most applications. Near surface mounted strips offer several advantages, which allow for further optimization together with prestressing. For the prestressing action a special anchoring device, based on the Composite Wedge Principle, was developed, which allows a simple and quick prestressing procedure. Additionally the anchorage weighs only 8 kg, which is especially beneficial when manipulations on the lower surface of the structure are necessary. The anchorage will be removed after prestressing and the permanent anchorage of the ends of the strips will be established by bond. No steel parts remain in the structure, after completing the strengthening. This results in low maintenance and a high durability. The efficiency of the developed systems and the prestressed plate strips was examined experimentally. In these investigations a perfect functionality could be demonstrated.     

Korrosionsbeständigkeit eines nichtrostenden Chromstahls in karbonatisiertem Normal‐, Leicht‐ und Recyclingbeton

Um der Forderung nach Nachhaltigkeit nachzukommen, wird in den nächsten Jahren der Einsatz von Betonen mit rezyklierten Gesteinskörnungen (Beton‐ und Mischabbruch) zunehmen. Wegen den wärmedämmenden Eigenschaften wird auch der Bedarf an Betonen mit Leichtzuschlägen (z. B. Blähglas) steigen. Als wesentliches Element dieser Entwicklung werden für die Zementund Betonproduktion zunehmend Zemente mit reduziertem Portlandzementklinkergehalt sowie Zusatzstoffe wie Flugasche und Hüttensand verwendet. Damit nimmt der Karbonatisierungswiderstand der Betone tendenziell ab und das Risiko für Korrosionsschäden zu. Die nachfolgend vorgestellten Untersuchungen hatten zum Ziel, die Korrosionsbeständigkeit eines nichtrostenden Chromstahls (Top12, Zusammensetzung entspricht ungefähr dem Stahl mit der Werkstoffnummer 1.4003) in karbonatisiertem Beton mit unterschiedlicher Zusammensetzung zu evaluieren, mit normalem Betonstahl zu vergleichen und zu beurteilen. Die Ergebnisse lassen den Schluss zu, dass der Top12, im Gegensatz zum normalen Betonstahl, in allen untersuchten Betonen und damit auch in stark karbonatisierten Recyclingbetonen beständig ist. Corrosion Resistance of a Stainless Chromium‐Steel in Carbonated Ordinary, Light‐Weight and Recycling Concrete In order to achieve the goals for a sustainable development, concrete with recycled aggregates (concrete and mixtures from concrete and masonry, e.g. clay bricks and calcium silicate blocks etc.) is going to be used more and more in the future. Due to its thermal insulating properties, the demand for concrete with light‐weight aggregates (e.g. foam glass) will also increase. As an essential element of this development, an increasing amount of cements with a reduced clinker factor as well as of mineral additions such as fly ash and ground granulated blast furnace slag is used for concrete production. Therefore, as a general tendency, the carbonation resistance of concrete mixes decreases while the risk of corrosion damages increases. The goal of the investigations described in this paper was to evaluate and to assess the corrosion resistance of a stainless rebar (Top12, composition corresponds approx. to steel grade 1.4003) in various carbonated concrete mixes and to compare the results with common rebars. The results lead to the conclusion that, in contrast to common rebars, Top12 is durable in all investigated concrete mixes, including strongly carbonated recycling concretes.     

Experimental‐numerical research on the seismic performance of URM buildings made of lightweight AAC blocks / Experimentell‐numerische Untersuchung zum seismischen Verhalten von unbewehrten Mauerwerksgebäuden aus Porenbetonblöcken

Masonry walls constructed with lightweight AAC blocks and thin‐layer mortar meet the increasingly strict requirements of energy efficiency and sustainability. In this sense, they represent an excellent solution for modern buildings, not only for external cladding but also as loadbearing elements. Despite the possible advantages of using lightweight AAC masonry, a specific assessment of its seismic performance is mandatory in order to set design recommendations allowing to reach safety levels consistent with those required for other masonry types complying with EN 1998 standard requirements. A comprehensive study on the seismic performance of unreinforced masonry buildings made of lightweight AAC was carried out in an integrated experimental‐numerical approach. The experimental campaign provided the necessary information to setup a reliable numerical model to be extensively used to assess the seismic performance of a number of prototype AAC masonry buildings with different characteristics, by means of both linear and nonlinear static (pushover) analysis. The results of this systematic numerical assessment were eventually used to draft design recommendations, to set parameters (behaviour factors) to be used in linear analysis and to calibrate rules for simple buildings.