Pretension concrete speaks to what is pretension concrete, why pretension concrete and explain pretension concrete.
This may be accomplished in two distinct ways and is used with precast concrete members. High strength steel strands are stretched tightly between abutments in a precast plant and the member or series of concrete members which are arranged end to end is cast around the stretched steel.
The curing concrete sticks to the strands along their full length. After the concrete has cured to a specified minimum compressive strength the strands are cut off at the end of each member. This releases the external tension on the steel allowing it to recoil slightly, which squeezes all of the concrete of the member into compression. If as is usually the case, the steel is placed as close as possible to the tension side of the member, the member takes on a decided camber which is the lengthwise arching, at the time the steel strands are cut.
Much or all of this camber disappears later when the member is subjected to loads in a building.
The strong abutments needed to hold the tensioned strands prior to the pouring of the concrete are expensive to build except in a single fixed location where many concrete members can be created within the same set of abutments. For this reason, pretensioning is used only for concrete members cast in precasting plants.
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 river bridge design.
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