Post Tensioning Systems
Post tensioning is a technique used in reinforced concrete structures for reinforcing concrete.
Concrete has high compressive strength but relatively low tensile strength. Post tensioning method can increase bearing capacity of concrete structure by applying lateral loads to concrete using high-strength steel cables.
This is called pretensioned concrete as intentional tension is applied prior to service loads.
Post-tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms in accordance with the static design made before the concrete is placed. Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete. This increases bearing capacity of concrete. This is the most common post-tensioning method. There are also special post-tensioning methods such as external post-tensioning for special structures and renovations.
Advantages and Application Areas
As post-tensioning increases bearing capacity of the structure, it has several advantages over standard reinforcing steel (rebars) in terms of design and specific requirements. Some of these advantages are listed below:
- It allows slabs and other structural members to be thinner
- It allows to design longer spans in elevated members, like floors or beams
- It allows to design larger cantilevers
- It allows to design smooth slabs
- It reduces loads on bearing elements allowing to design thinner structures
- It reduces rebars and concrete used
- It reduces or eliminates shrinkage cracking and therefore no joints or fewer joints are needed
- It allows us to design larger slabs without expansion joint
These advantages vary based on requirements of the structure and project and demands of designer and contractor.
Post-tensioning has become increasingly popular over the past 30 years or so as the technology has been perfected. Some of the more common applications are:
- Bridges and viaducts
- Slabs
- Foundation
- Strengthening of bridges and buildings to resist seismic forces
- Renovation of old structures
- Silos
- Car Parks
In addition to the above, post-tensioning is used in many projects to decrease rebar and concrete costs in various structures, to design longer spans and use thinner structural elements.
Types of Post-Tensioning
There are two types of post-tensioning; unbonded post-tensioning and bonded post-tensioning.
Unbonded post-tensioning systems use mono-strand systems which are coated with specially formulated grease with an outer layer of seamless plastic extruded in one continuous operation to provide protection against corrosion. Depending on the application and the level of protection needed, anchorages of unbonded mono-strand systems may also be encapsulated. Light and flexible, unbonded mono-strand can easily and rapidly be installed – providing an economical solution.
Bonded post-tensioning systems are comprised of tendons from one to multiple strands (multi strand) or bars (PT bars). For bonded systems the pre-stressing steel is encased in a corrugated metal or plastic duct. After the tendon is stressed, cementitious grout is injected into the duct to bond it to the surrounding concrete.
Together with its business partners, Tekbem has been doing design, materials supply, supervisorship and application of post-tensioning systems.
For more information on Post-Tensioning Systems and types, please follow the related link on the left, download our Post-Tensioning Systems Brochure or contact us.
