A post-tension slab consists of laying a grid of high-strength steel strands in bundles to form tendons. After casting, these strands are tensioned at the appropriate pressure to provide compression to the slab and increase its capacity.
In unbonded constructions, 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.
Post tension unbonded systems are used in buildings, they allow for flat slabs eliminating then need of drop beams and increasing the span length while staying more economical then conventional systems such as hourdi slabs or RC solid slabs. In slab on grade, they allow the removal of crack control joints, providing a seamless flat surface over large areas.
Mega Prefab post tension 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. In some situations, the height of the building is limited, in which the reduced story height has allowed additional stories to be constructed within the building envelope.
A Mega Prefab designed PT slab contain 20 to 30% less concrete and a minimal quantity of steel reinforcement, leading to significant savings in the overall structure costs.
The main advantages are summarized as follows:
- Increased clear spans.
- Thinner slabs.
- Lighter structures.
- Reduced cracking and deflections.
- Reduced story height.
- Rapid construction.
- Better water tightness
To obtain maximum benefits from post-tension slabs, it is recommended that the system is incorporated into the building structure at early design stages.
- Preliminary design and cost estimation.
- Execution design and shop drawings.
- Supply and supervision / installation of post tensioning systems.
Bonded or Unbonded PT system
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:
- Develops higher ultimate flexural strength
- Does not depend upon the anchorage after grouting
- Localized the effects of damage in case of a severed tendon
- Simple technique for demolishing or providing future opening in slab
- Provides greater available lever arm
- Reduces friction losses
- Can be constructed faster