Unbonded post-tension

What is a Post-Tension Slab?

Before a post-tension slab is poured, high-strength steel strands or cables, called tendons, are laid in a tight grid.These help support and give strength to the slab once the concrete has been cured and they have been tensioned at the proper pressure.

Advantages of Post-tensioned slab systems

Mega Prefab slabs bring unique architectural freedom when compared to conventional building methods. They allow increased spans to be achieved with thinner concrete sections. by reducing the need for columns the structure becomes more efficient and functional.

A Mega Prefab designed slab can contain less concrete (20% to 30%), a minimal quantity of reinforcement and less complicated rebar shaping, leading to a cost-effective sustainable structure.

To obtain maximum benefits from Mega Prefab slabs, it is recommended that the system is incorporated into the building structure at early design stages.

The main advantages of post tensioned slab over conventional reinforced concrete slab, may be summarized as follows:

- Increased clear spans.
- Thinner slabs.
- Lighter structures.
- Reduced cracking and deflections.
- Reduced story height.
- Rapid construction.
- Better water tightness

These advantages can result in significant savings in overall costs. There are also some situations where the height of the building is limited, in which the reduced story height has allowed additional stories to be constructed within the building envelope.

Mega Prefab services

- Project feasibility study for post tension application.
- Preliminary design and cost estimation.
- Final design and shop/working drawings.
- Complete supply and installation of post tensioning systems.

Bonded or Unbonded

Post tensioned slab systems can be constructed using either bonded or unbonded tendons. The relative merits of the two techniques are subject to debate. The following points may be made in favor of each:

Bonded Advantages:

- Develops higher ultimate flexural strength
- Does not depend upon the anchorage after grouting
- Localized the effects of damage
- Simple technique for demolishing or providing future opening in slab

Unbonded Advantages:

- Provides greater available lever arm
- Reduces friction losses
- Simplifies prefabrications of tendons
- Grouting not required
- Can be constructed faster

Technical details

In unbonded construction, the plastic sheathing acts as a bond breaker between the concrete and the prestressing strands. It also provides protection against damage by mechanical handling and serves as a barrier that prevents moisture and chemicals from reaching the strand. The strand coating material reduces friction between the strand and the sheathing and provides additional corrosion protection. Anchorages are another critical element, particularly in unbonded systems.
After the concrete has cured and obtained the necessary strength, the wedges are inserted inside the anchor casting and the strand is stressed. When the jack releases the strand, the strand retracts slightly and pulls the wedges into the anchor. This creates a tight lock on the strand. The wedges thus maintain the applied force in the tendon and transfer it to the surrounding concrete. In corrosive environments, the anchorages and exposed strand tails are usually covered with a housing and cap for added protection.

Download: • Post-tension catalog

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