Prestressing
Pre-tensioned concrete is cast around already tensioned tendons. This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension.
The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction. However, it requires stout anchoring points between which the tendon is to be stretched and the tendons are usually in a straight line. Thus, most pretensioned concrete elements are prefabricated in a factory and must be transported to the construction site, which limits their size.
Pre-tensioned elements may be balcony elements, lintels, floor slabs, beams or foundation piles. An innovative bridge construction method using pre-stressing is the stressed ribbon bridge design.
The advantages of bonded system over unbonded post-tensioning are:
The unbonded post-tensioned systems can be used in typical levels, while the bonded post-tensioning systems can be specified for the transfer girders on different levels to provide optimum crack and deflection control features essential for transfer girders required to carry the loads from the multistory structure.
In corrosive environments, bonded systems offer significant design advantages that lead to life-cycle savings then unbonded post tensioning.
Bonded post-tensioning provide resistance to progressive collapse then unbonded post tensioning
Bonded post-tensioning tendon has the capability of developing its force in a relatively short distance along its length compared to unbonded post tensioning.
Bonded post-tensioning systems offer flexibility when future design modifications to the building are needed more than unbonded post tensioning.
There is a reduction in deflection of an unbonded beam in comparison with a bonded beam.
There is a reduction in Shear of the unbonded beam in comparison with the bonded beam.
There is a reduction in top stress of an unbonded beam in comparison with a bonded beam.
There is a reduction in bottom stress of an unbonded beam in comparison with a bonded beam.
There is a reduction in bending moment of an unbonded beam in comparison with a bonded beam.
