Prestressed concrete | EPFL Graph Search

Prestressed concrete is a form of concrete used in construction. It is substantially "prestressed" (compressed) during production, in a manner that strengthens it against tensile forces which will exist when in service. This compression is produced by the tensioning of high-strength "tendons" located within or adjacent to the concrete and is done to improve the performance of the concrete in service. Tendons may consist of single wires, multi-wire strands or threaded bars that are most commonly made from high-tensile steels, carbon fiber or aramid fiber. The essence of prestressed concrete is that once the initial compression has been applied, the resulting material has the characteristics of high-strength concrete when subject to any subsequent compression forces and of ductile high-strength steel when subject to tension forces. This can result in improved structural capacity and/or serviceability compared with conventionally reinforced concrete in many situations

Design for SLS according to fib Model Code 2010

Olivier Burdet, Ekkehard Fehling

This paper provides an overview of serviceability specifications given by the fib Model Code for Concrete Structures 2010 (fib MC2010 [1]). First, the reasons behind crack control and deflection control are discussed, then specific design rules are provided. Simple rules as well as detailed models are also presented. Numerical examples are provided in order to assist in the application of the design recommendations for crack control and deflection control (reinforced and prestressed concrete elements). Simple rules mean indirect control of cracking or deflections without calculations. Indirect crack control may include limitation of stresses and selection of maximum bar diameter or maximum bar spacing. Indirect deflection control normally means limiting the span-to-depth ratio. Detailed models are based on physical and mathematical approaches to cracking and deflections. The design crack width is expressed as the maximum bond transfer length multiplied by the mean strain between cracks. Deflection analysis can be provided by integrating curvatures or by using a simplified or refined method. Vibrations and numerical modelling of cracking are also briefly discussed.

Ernst & Sohn2013

Zum Querkraftwiderstand von Stahl- und Spannbetonträgern mit dünnen Stegen

Eckart Hars

The design of reinforced and prestressed concrete girders is usually performed so as to avoid any non ductile failure mode. Amongst these failure modes, one of the most undesirable occurs by crushing of concrete in the web. To avoid this brittle type of failure, it is of paramount importance to know precisely the effective compressive strength of concrete in the web. It is reduced by cracking (transverse strains), which is due to the shear deformations. In the presence of post-tensioning cables in the web, cracks may form along the cables inside the web, which also weakens the concrete of the web. Numerous bridges have been built in Switzerland by using profiled girders with thin webs containing only a minimal amount of stirrups but large post-tensioning ducts, which occupy a considerable part of the web width. Both phenomena mentioned above play a role for these bridges, especially for more brittle high performance concrete. The two phenomena are only partly understood. A sound physical model for the shear behaviour as well as a failure criterion are necessary to achieve a uniform safety level for new structures and to evaluate the strength of existing structures. Large-scale laboratory tests on prestressed concrete girders have been conducted in order to improve understanding and to investigate both phenomena in detail. Web crushing along the post-tensioning cables was the mode of failure of all girders. In addition, laboratory tests have been conducted on prismatic specimens to investigate the effect of the presence of a post-tensioning cable in an isolated manner. A physical model has been developed for the shear behaviour of reinforced and prestressed concrete girders. The variation of loading, stresses and strains along the girder axis is taken into account. The shear capacity of the compression flange is considered on the basis of equilibrium and compatibility. The load increase in the post-tensioning cables is calculated with a bond condition. A physical failure criterion has been developed for the effect of the presence of a posttensioning cable. The failure load of the conducted prism tests and a large number available in the literature has been estimated with a good precision for various post-tensioning duct types and concrete cylinder strengths. Based on physical considerations a failure criterion has been developped that takes into account the effect of transverse strains. Both effects are more pronounced for a higher cylinder strength. With the developed physical model and the failure criteria, the failure load of a large number of reinforced and prestressed concrete girders (laboratory and from the literature) has been estimated with good precision. The interaction of the effects of the presence of a post-tensioning cable and of transverse strains has been taken into account for the development of a combined failure criterion. Its format is compatible with the current design codes.

EPFL2006

Revisiting Detailing Rules for Bending Bars, Anchorage and Minimum Shear Reinforcement in Concrete Structures

Frédéric Monney

Reinforcement detailing rules describe the shape, geometrical dimensions, and amount of steel to be placed in reinforced concrete structures. They allow for simple and fast designs, account for several effects neglected in the design, ensure a satisfactory behaviour under serviceability conditions, a sufficient robustness, and an adequate behaviour in case of unexpected actions. Most detailing rules provided in codes have not been updated to correspond to current manufacturing processes, material performances and scientific knowledge. They are often based on "rules of good practice" which, while satisfactory, lack a sound scientific basis and may not be needed. This is one reason why detailing rules may differ amongst countries and design codes. Some of these rules are overly conservative, in particular when evaluating existing structures, while others may neglect significant effects. Even though these rules play a major role in the economy and safety of concrete structures, little research has been performed in this domain. Several detailing rules have been identified as needing additional investigations to verify their adequacy to current practical needs and recent technological evolutions. This thesis presents a research programme on three main detailing rules: bend detailing and required mandrel diameter, anchorage of shear reinforcement with bends and hooks and minimum amount of shear reinforcement. This research aims at developing mechanical models, simplified formulas and detailing provisions, on the basis of experimental results. These were obtained using state-of-the-art measurement techniques such as Digital Image Correlation or Fibre-Optic Measurements.Bends and hooks of reinforcing bars are usually obtained by plastic bending of the bars against mandrels. Codes specify minimum mandrel diameters to avoid splitting or spalling failures that may limit the resistance of the detail. The thesis includes a research programme on the detailing of bends and the required mandrel diameter to avoid concrete failures leading to spalling of the concrete cover. A mechanical model for the design of bent reinforcement was developed. A simplifying standard bending procedure is proposed, in which details formerly requiring various bend diameters can be obtained by using a single mandrel, allowing for faster automated manufacturing of bent reinforcement.Bends and hooks at the end of the bars are efficient solutions for the anchorage of reinforcement. However, these details are relatively sensitive to the cracking state of the surrounding concrete. For shear reinforcement, spalling failure of the concrete cover for bars close to the concrete surface can occur. The thesis includes an investigation on the mechanical response and performance of bend and hook anchorages. A mechanical model and practical considerations on the activation of shear reinforcement in beams are presented.The required minimum amount of shear reinforcement in beams and slabs has been discussed for decades. It is crucial to ensure economic and safe new structures and to accurately assess existing ones. The investigation shows that the shear behaviour is strongly dependent on the amount and the post-yield response of the shear reinforcement (ductility class). For all investigated detailing rules, the implementation of these findings into codes is discussed, highlighting the consistency of the recent changes, particularly with the new generation of Eurocode 2.

EPFL2022