Prestressed concrete

Prestressed concrete is a form of reinforced concrete with an additional outer longitudinal force. This is applied by tensioned steel reinforcement in high strength prestressing steel, which " squeeze " the concrete. The design is primarily used for beams and bridge supports and enables the same construction here at heights compared to reinforced concrete larger spans.

• 2.2.1 In the concrete cross section (internal)
• 2.2.2 Outside of the concrete section (external)

Operation

The prestressed concrete differs from other reinforced concrete by a systematic bias ( = pre-extension ) of the steel reinforcement, the tendons. Here, the stretched tendons are supported by their anchors or directly via the bond with the concrete on the concrete, making this a compressive load and receive an instant exposure to a possible eccentricity of the anchor relative to the cross-section centroid line. In addition, deflection forces are generated at curved or kinked tendon guides. The component is loaded by the preload so that no or only small concrete tensile stresses in the concrete cross-section with overlay available with external influences such as dead weight. As concrete can absorb only a small tensile stress (about 10% in comparison to the compressive stress ) before breaking, but high compressive stresses, the biased ( pushed ) Concrete is more usable. The component is in the range of working loads stiffer due to a missing or greatly reduced cracking and therefore has smaller deformations ( deflections ) for large spans and high loads. An Increase of the load can be achieved by the use of prestressed steel, as this compared to normal reinforcing steel has a higher strength. Particularly in bridge construction, but also in container or in buildings with trusses, hollow core slabs or concrete slab of prestressed concrete finds its application today.

Clamping method

The clamping methods differ by the type of composite action, which gives them their names, and by the entry type of the clamping forces in a component.

PT with bonding

Immediate composite ( pre-tensioning )

The tensioning wires or wire strands are non-positively connected to the concrete, so that a relative displacement between the two materials, practically does not take place. When the bias voltage with immediate composite a direct bond between the prestressing steel and concrete is present. This method is used primarily in the prestressing bed of precast plants, in which concreted against external abutment tensioned prestressing wires or strands in the finished part. After casting and curing of the concrete the prestress is released. Through the bond between concrete and prestressing steel and wedging of the relaxed wire ( or braid) ( Hoyer effect), the clamping force is applied in the finished part. This type of bias is only possible with a straight prestressing steel guide. It is used for example for the manufacture of railway concrete sleepers and prestressed hollow core slabs.

Subsequent bond

In the post-tensioning the prestressing wires or strands are summarized in a profiled sheet metal or Kunststoffhüllrohr, the tendon or cable, and set in concrete with anchor heads at both ends without bias in a building. After the concrete has set the prestressing steel at the moving anchors are tensioned. Finally, the cladding tubes with a special cement grout, the grout, pressed, whereby a frictional connection between prestressing steel and concrete is produced. The tension cable guide can be in this process also curved, so this method has great potential applications.

Unbonded prestressing

The prestressing wires or strands can relatively move between the anchor points for the concrete. This will be the external bias, the tension cables except in anchoring or deflection region not in the concrete cross section, but are free exciting. In the internal bias voltage, the clamping members, as in the subsequent composite encased, but not coated with the cement, but with a fat, such as at the monostrand. Thus, the compression of the ducts, which enables a far smaller diameter of the wire ropes and thus an application of prestressed concrete for thin components such as ceiling construction is not required. A curved tensioning cable routing is possible. Because no network is present, occur with this Vorspannungsart by external loads hardly voltage changes in the prestressing steel at, there is also the possibility retighten.

In the concrete cross section (internal)

" An internal unbonded tendon consists of one or more cast- and post-tensioned wires, strands or rods in a corrosion protective envelope in which the prestressing steel is free to move in the longitudinal direction and is only connected to the anchor points fixed to the supporting structure. These tendons shall be interchangeable. "

Outside of the concrete section (external)

" An external clamping member is a post- biased clamping member is located outside of the concrete section, but within the envelope of the concrete support structure. The tendon is connected by anchor elements and deflection to the concrete superstructure " In Germany, for road bridges with prestressed concrete box-section since the late 1990s, mainly because of bad experiences with insufficient compressed sheaths only two variants allowed: . Bias exclusively with external tendons and the composite construction ( external and internal longitudinal prestressing ).

Corrosion protection

Since creep and shrinkage of concrete reduce the biasing forces of the tendons, particularly high pre-strains of the prestressing steel are required. That is, for a given clamping force, the cross-sectional area of the tendon to be as small as possible. This is achievable only through using high-strength steels. The high tensile steels under the tendons of prestressed concrete components are especially sensitive to corrosion. The corrosion protection by grout, concrete is therefore to be designed very carefully. In unbonded prestressing corrosion protection is usually achieved by a Factory- Fettverpressung the plastic tube in which the prestressing steel is.

Problems

Due to lack of experience with the new technology and underestimation of the environmental impacts have occurred in the post-war period Collapsing, necessary interruptions or costly repairs of various prestressed concrete structures. Here, for example, played also problems with stress corrosion cracking in prestressing steels (eg Neptune steel), lack of knowledge of material properties (different moduli of elasticity of concrete, depending on the used rock aggregates) and imperfections of the method of calculation ( neglecting temperature gradients in the cross-section ) play an important role. Today, these problems are considered to be largely solved. By the use of the replaceable external bias voltage to further improve the robustness and thus extend the life is to be achieved in the bridge construction. Moreover, it is possible to detect by the method of clamping a wire break location in the existing, potentially critical, the clamping structures cracks steels.

History

The first proposal to harness concrete was made ​​in 1886 by the Americans Jackson. 1888 W. Döhring reported from Berlin for a patent, which provided tensioned wire inserts into boards, laths and trabeculae to crack minimization in the prestressing bed. As of 1907, first experiments were performed with a cast- in reinforcement tensioned state on a proposal by Mathias Koenen at Stuttgart. However, the biasing force applied was almost completely ineffective due to low steel stress of 60 N / mm ² due to shrinkage and creep. 1919 used a Bohemian engineer Karl Wettstein for thin concrete planks piano strings made ​​of high strength steel with high voltage and succeeded, for the reasons he did not saw. Only the Americans Dill recognized in 1923 that high-strength wires with high voltage are required. As a father of today's French engineer Eugène Freyssinet of prestressed concrete must be described. He quickly realized the importance of harnessing and devoted himself intensively this technique. Freyssinet created by his investigations into the creep and shrinkage of concrete, and the use of high-strength strongly biased wires, the necessary conditions for successful prestressing of concrete. From 1928 to 1936 he settled various inventions relating to hydraulic jacks and anchorages for tendons with high-strength steel and steel stress of 400 N / mm ² patented. Freyssinet built the first prestressed concrete structures, which consisted of both prestressed in tension bed and cable prestressed elements.

The first prestressed concrete bridge in Germany was the railway bridge floodplain. They converted a road with a maximum span of 69 meters above the station premises and was designed by Franz Dischinger with an external bias voltage and built in 1937. The first German prestressed concrete bridge with bias in the composite, according to the method of Freyssinet was that Built 1938 transfer path Hesseler between Beckum and Oelde, the Federal Highway 2 spanned with 33 meters span and since 2012 as a technical monument at the service area Vellern (south side, driving direction Hannover) is. 1965, a pre-stressed concrete girder bridge was with the span of 208 m, then the largest in the world, built with the Bendorfer bridge.

Europe's first street with prestressed bridge decks were released between Herbrechtingen and Mergelstetten in November 1953. In this section of the B19 also the first roundabout was constructed in 2013 with a concrete road in Germany.

When nuclear power plant THTR -300 by Rudolf Schulten, which went into operation in 1985, a reactor pressure vessel made of prestressed concrete was used with steel liner ( liner), which, however, not proven in operation.