How does prestressed concrete work?

Written by collin fitzsimmons
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How does prestressed concrete work?
(Photo by Shakespeare. http://en.wikipedia.org/wiki/File:Post-Tensioning-Cables-4.jpg)

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Concrete, Reinforced Concrete and Prestressed Concrete

Concrete is an essential building material, and it is with little surprise that advances in technology allow us to use this material even more fully. "Prestressing" refers to a procedure to overcome the natural tension weakness of concrete, enabling longer beams, larger floors and wider and longer bridges than ordinary concrete or reinforced concrete, concrete with steel reinforcement bars inside. This prestressing procedure can be done in several ways, but ultimately all techniques seek to add compression using tensile rods, thereby increasing tensile strength.

Three Ways to Stress Concrete

There are roughly three ways that concrete can be stressed into having more tensile strength. First, pre-tensioned concrete refers to a procedure whereby tensile rods are put in place first and tightened, followed by concrete pouring. When the tensile rods are loosened, the static friction of the rods against the load ensures compression, increasing tensile strength. However, this method can be problematic because it requires sturdy anchoring before pouring, which sometimes is impossible. This method is common with prefabricated building elements. Secondly, compression can be applied after pouring concrete using bonds. Tendons are threaded through the material, which are tightened using hydraulic jacks once the concrete is dry. These tendons are then wedged into position. This is a common method. Finally, compression can be applied after pouring concrete, without bonds -- in other words, tensile cords can be individually placed to allow freedom of movement and are only connected to the concrete with steel anchors along the perimeter. This is another common method, although the main disadvantage is that a cable can burst out of the slab, such as during repairs, if the anchoring system fails.

Advantages

Prestressed concrete shows several advantages that make it a useful building material. Prestressed concrete cracks less frequently and can be made into thinner, stronger slabs, highly important in high-rise building floors. Thinner slabs in high-rise buildings mean additional floors for the same or lower cost. By increasing tensile strength and therefore maximum span length, prestressed concrete increases the amount of usable floor space in buildings. Furthermore, joints between slabs require maintenance and are costly; by enabling longer slabs and thinner floors, joints can be used less frequently, saving on construction and maintenance costs.

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