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Prestressed concrete is a strong material made of high strength concrete and steel strands, invented in 1886. It can be made by pre-tension or post-tension methods and is used in various structures to withstand heavy loads and prevent cracking. Post-tensioned concrete is used for houses in areas with expansive soils and for more elaborate designs.
Prestressed concrete is a combination of high strength concrete and steel strands. This combination makes for a very strong structural material that is used in the construction of roof slabs, bridge girders and railway sleepers. Prestressed concrete was invented and patented in 1886 by Henry Jackson, an engineer from San Francisco. This type of concrete became the material of choice in Europe after World War II, due to the shortage of steel. The Walnut Lane Memorial Bridge, built in 1951 in Philadelphia, Pennsylvania, became the first structure in America made of this type of material.
An ordinary concrete beam, even without a load, struggles to support its weight. When service loads are added to the weight of the beam, hairline cracks develop. Over time, these cracks will get bigger and eventually the concrete will crumble. The thin concrete pads tend to sag as extra weight is added, resulting in the house moving. These changes are why prestressed concrete was invented.
Prestressed concrete can be made by two different methods; pre-tension and post-tension. The pre-stressing method involves stretching high-tensile steel strands between abutments located at both ends of the concrete pour bed. After the wires have been taught, concrete is poured into the beds, where it surrounds and adheres to the wires. Once the concrete is dry it will have bonded to the steel. After the concrete has reached the desired strength, the strands are released, causing the concrete to develop a slight arc which makes it more resistant to heavy loads.
There are several reasons for using this type of concrete in various structures. The beams are made of pre-stressed concrete so they can withstand the stresses without cracking. A pile is pre-compressed so it won’t fall apart with the forces of heavy hauling and driving. The columns are made of prestressed concrete so they will not deform under the weight of heavy loads. Thin pads of concrete are prestressed to keep them from bending under normal weight.
Creating prestressed concrete using the post-tensioning method involves applying compression after the concrete has been poured and hardened. The concrete is poured around a curved duct with steel strands running through it. After curing, tension is applied to the strands using hydraulic jacks. The strands are then butted into place so that tension remains after the hydraulic jacks are removed.
Concrete created by the post-tensioning method is used as monolithic slabs for houses located in areas with expansive soils such as adobe clay. This type of concrete is also used for creating structures where the seasonal expansion and contraction of the soil is a problem. Post-stressed concrete is highly efficient for building construction with more elaborate design.
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