Prestress Technology


Although concrete was used for a long time as a construction material, it was not comparable with steel, stone wood at all, because despite its high pressure resistance, it acts weakly when facing tensile stress. As a result, it was only used as a masonry material. Later, engineers applied a combination of reinforcing steel and concrete which resists tension and tensile pressure in order to get over this problem. They applied bar to reinforce concrete in early stages. However, they applied pre-stressed technology later in order to change concrete as one of the most efficient construction material. The idea of pre-stressing was presented in first decade of 20thcentury  and a variety of research was done during 1930 to 1940. This method has been applied since 1955 in various construction fields.

Theory of PreStressing
Pre-Stressing refers to imposing permanent compression force before imposing service loads in order to reduce and/ or remove tensile stresses.

Locating pre-stressing steel and anchoring it in both sides of the member creates constant (compression stress) in concrete section. Alongside the bending members, both the lower fiber (at the middle of the span) and upper fiber (on the support) have tensile stress. In pre-stressed sections, the distribution of tensile stress is controllable by replacing the cable position.

Prestreesing Methods

Pre-Stressing is done in 2 ways, pre tensioning and post tensioning. It means stressing the cables can be done before or after casting concrete. In pre tensioning method, at first cables stressed on prefabricated formworks between two ends. After casting concrete and obtaining the required compressive strength, they are cut and the existing force is transferred to concrete by pressure. This method is used in concrete pieces producing in factories and plants, and the products are usually prefabricated. In post tensioning method, cables are located in a structural element (inside duct). Then concreting and following its achieving to required compressive strength, it is stressed and anchored. This method can be applied in producing prefabricated and cast in situ pieces. Cables are placed inside post tensioning pieces as bonded or unbonded systems.

Bonded System

Here, several cables are located in one metal duct and all of them are ended to an anchor. The total series of these ducts are installed on the formwork and stressed after concrete casting and reaching required strength. To make a link between cable and concrete, grout is injected inside duct and consequently cable and duct are adequately stuck together. Thread like pattern on the duct is designed to create an appropriate bond with concrete. So, a reliable bond is established between cable and concrete, too. Bonded method is mostly used when high pre-stressing force is needed. Other items are among the usages of this system:
- pre-stressed bridges
- pre-stressed beams for large span
- Transfer Plates
- pre-stressed foundations

Unbonded System

In this method, stress force is transferred to concrete through ending anchors and cable has no link with concrete alongside it. Unlike bonded method, here, each cable has plastic coating and is held directly by an anchor. Since there is no need for bonding between cable and concrete, grout injection is omitted in this system. Unbonded system is mostly used when concrete members have small thicknesses and it isn’t possible to place bigger ending anchors used for anchoring several cable strands. The following items are among the usages of this system:
- Building Slab
- Slab on Ground