Postec Academy
A little theory on the design and execution of post-tensioning and its uses in Chile.
What is post-tensioning?
Post-tensioned concrete is a specialty of structural calculation that considers in the design of the structural element the incorporation of steel cables that are tensioned after the concrete has been poured, with approximately 21 tons of force each (For the case of 0.6" cables). These steel cables are also known as strands.
By tensioning the cables, the aim is to reduce tensile stresses in the concrete, stiffen the element to decrease deformations, increase the element's breaking strength, and reduce the effects of cracking.
This allows:
- Generate savings in kg of reinforcement and m3 of concrete.
- Greater architectural freedom. Allows for large spans and less invasive structural elements. Ideal for height restrictions in residential and commercial buildings.
- Concrete with fewer cracks and deformations. A great advantage for the new considerations of reducing inertia due to cracking in code ACI 318-19.
- Faster construction. Formwork can be recovered early.
Where is post-tensioning used?
Post-tensioning can be used in slabs, beams, columns, foundations, and footings, for both static and seismic design. Although the design of post-tensioned elements is more common in static elements, it can also be used in elements with seismic requirements, since the dynamic stresses can be absorbed by the contribution of the passive reinforcement.
In Chile it has been used in residential buildings, offices, commercial buildings, educational buildings, hospitals, industrial foundations, bridges, among others.
Among the more extravagant uses, one can highlight the use to achieve 10x10 m open floor plans, 16 m spans between columns, suspended systems (post-tensioned column), and 5 m cantilevered terraces.
A typical post-tensioned slab (for parking, room or office use) can be designed for spans of 8x8 m in thicknesses of 16 cm and 32 cm (cap), or spans of 9x9 m in thicknesses of 18 cm and 36 cm (cap).
Advantages of post-tensioning
Free plants
It allows for the design of open-plan, beam-free floors. Greater freedom in the design of mixed-use buildings.
Overhangs
Achieve designs that are impossible to replicate in traditional concrete. You can take advantage of the square meters of cantilevers that don't add to the project's area.
Terms of service
Concrete with a lower level of cracking. Ideal for avoiding after-sales problems.
Speed of construction
Once it has tightened (approx. 3 days or less) the molding system can be recovered.
Savings in concrete
Reduced thickness of post-tensioned elements compared to traditional solution.
Optimize the structural design
It reduces concrete stress, improves design at break and decreases deformations.
Types of post-tensioning
Post-tensioning can be adherent or non-adherent , with the adherent system being the most typical in Chile.
In the bonded post-tensioning system, once the cables are tensioned and their elongation is verified, the sheaths containing the cables are injected. This system is also known as injected post-tensioning. This bonds the cable to the slab, providing advantages such as corrosion protection, uniform long-term stress distribution, accident prevention, and greater flexibility for future installations. When the cable is cut, it only loses tension locally, as the strength of the injection prevents further strain through adhesion.
The non-bonded post-tensioning system, also known as greased post-tensioning, is characterized by maintaining the cable's sliding motion within the sheath. This system consists of a greased cable protected by a plastic sheath. This system is more common in the USA.
The cables are typically 0.5" or 0.6" in cross-section, depending on the system the company uses. At Postec, we design post-tensioned slabs with 0.6" cables and use patents from BBV Systems (a German-made post-tensioning system).
