35th: Pre-stress Concrete

This is a method is replacing traditional reinforced concrete of steel reinforcement bars to overcome the concrete’s natural weakness in tension. Prestressing tendons are used to provide a clamping load which produces a compressive force that offsets tensile force due to loading. It could be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete.

Bonded post-tensioned concrete is a method of applying compression after concreting. Basically, the cables [p1] are isolated by tubes [p2] before they are tensioned by hydraulic jacks [p3-4] after concrete was hardened. When the tendons have stretched sufficiently according to design, they are to be maintained tension after the jacks are removed, thus pressuring the concrete. The tubes will be cured by cement.

Unbonded post-tensioned concrete is providing each cable freedom of movement relative to concrete by greased and covered by plastic. The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab. The main disadvantage over bonded post-tensioning is the fact that a cable can distress itself and burst out of the slab if damaged. It is usually used to temporarily repair a damaged building by holding up a damaged wall or floor until permanent repairs can be made.

Prestressed concrete lowered the cost in a project, especially high-rise building. It provides thinner slabs meanwhile diminishing the number of joints which eventually leads to lower maintenance costs since joints are the major locus of weakness in concrete buildings.


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