According to the construction method, there are two kinds of prestressed concrete:
- Pre-tensioned concrete
- Bonded Post-tensioned concrete.
- Unbonded Post-tensioned concrete
In this method, wires or tendons are tensioned at first and concrete is poured later. It creates a good bondage between the tendon and concrete. As a result, the tendons are protected from corrosion and tensions are transferred directly. Tendons are anchored and stretched and the stress is transferred to the concrete when it is hard. Then the tendon tries to get back to the original length, but resisted by the bond between the concrete hence it induces compressive force in it.
Bonded Post-tensioned Concrete
Similar to pre-stressing but here concrete is poured first then tendons are tensioned. Tendons are placed at suitable places in the member and then casting is done. After the concrete becomes hard, the tendons are tensioned by hydraulic jacks against the concrete. When the tendons have tensioned sufficiently, according to design, they are fixed in position. After the jacks are removed, tension remains and it transfers pressure to the concrete. This method is widely used in building monolithic slabs for mega house construction projects where expansive soil sometimes creates problems. Moreover, post-tensioned concrete is also used in bridges.
Unbonded Post-tensioned Concrete
It is little different from bonded post-tensioned concrete. It allows freedom to move the cables. For this, each tendon is coated with grease and covered by plastic. Stress transfer to the concrete is achieved by the cables through anchors. There are some advantages to this like-
- The ability to distress tendons before trying to repair work.
- The ability to individually adjust cables
Also, there are some disadvantages like if damaged, one or more than cable can be distress itself and burst out of the slab.