Prestressed concrete

Prestressed concrete is a variant of reinforced concrete in which the reinforcing steel is put under tension or pre-stretched in the unloaded state in various processes. The prestressing of the reinforcement results in a longitudinal force acting in the component, which "compresses" the concrete. If there is no load, this can lead to an overelevation (deformation) of the component. If the component is loaded, the compressive load in the component caused by the prestressing counteracts the tensile forces that usually occur in reinforced concrete, which are caused by dead load and live load. The steel used in prestressed concrete has a higher strength compared to conventional reinforcing steel.

The use of prestressed reinforcement makes additional use of the good compressive strength of the concrete. The occurrence of cracking is minimized in prestressed concrete components, and deflections are significantly lower in large spans. Prestressed concrete is used in particular in bridge and building construction. It is often used for precast concrete elements such as prefabricated ceilings or trusses made of reinforced concrete.

The prestressing of the reinforcement can be achieved in different post-tensioning methods:

• Post-tensioning bed method (with immediate bonding): This method is usually used in precast plants. After the reinforcing steel has been laid, it is clamped using external abutments. This is followed by the concreting process. Once the component has hardened, the steel is relaxed. The bond takes place directly between prestressing steel and concrete.
• Post-tensioning method with subsequent bonding: tension cables are laid in the formwork. These are ducts made of sheet metal or plastic, which are limply concreted into the component with anchor bodies at both ends. This contains the tension wires or tension strands, which are only tensioned at the anchors after the concrete has hardened. Subsequently, a bond between the prestressing steel and the concrete is achieved in the cladding pipe by grouting with mortar.
• Post-tensioning without bonding: Here, too, tension cables with tension wires or strands are laid in the formwork, but there is no need for grouting after the concrete has hardened. There is no bond between concrete and prestressing steel, the prestressing steels can be retensioned at any time.

Source: bauwion