Prestressed concrete

Prestressed concrete is concrete that has is internally stressed to enable service loads to be offset to a degree. It is ideal for things subject to bending or tension. It can be used to produce: beams, slabs, floors, bridges, and tanks. Two different types of methods to produce prestressed concrete are pretensioning and posttensioning. The most used type of prestressing is linear prestressing.

History
In the late 19th century, while reinforced concrete was still in its beginning stages, people began to realize how beneficial prestressing reinforcement could be. In 1886 P.H Jackson applied and received a patent for tightening steel tie rods in stones or arches for floor slab use. These attempts and several for years to come failed because the prestress level was not high enough and was difficult to maintain due to concrete shrinkage. It was not until 1923 that the first experiment with pretensioned concrete was successful. R. E. Dill was capable of accounting for shrinkage in the concrete with coated steel as reinforcement. While Dill patented this process, the father of modern prestressed concrete was the French engineer Eugene Freyssinet, who used high-strength steel to reinforce the concrete. This technique was the beginning of the posttensioning methods; pretensioning methods followed closely, being discovered by a German (Hoyer) in the 1930s. The first true application of prestressed concrete was in large tanks and pipes that used circular prestressing methods in the late 1930s. The Preload Company began creating these pipes with specially designed equipment that could force tension into the wires that wrapped round the tanks. Proper stressing of the steel and anchoring the locking wire ends is the most important aspects of properly posttensioning concrete. The development of cone-shaped wedges for anchoring the ends and other alternative anchoring systems were developed in 1939, then 1940. Gustave Magnel, Belgian Engineer that found a substitute anchoring system, visited the United States to give lectures on reinforced concrete, causing a plan for Walnut Lane Bridge to develop. This bridge is the first linearly prestressed structure. After the 1950s, prestressed concrete rose in popularity. After World War II, the building industry boomed and the steel shortage during the Korean War all helped the industry grow significantly. High strength, low slumping concrete was a necessity to the success of prestressed concrete; once this problem was fixed, prestressed concrete use grew exponentially.

Manufacturing Process
When pretensioning concrete, steel is positioned in the forms and is stretched to the necessary stress and held in place while concrete is applied like regular reinforced concrete. The concrete is then allowed to cure and the tension in the steel is released, causing the load to transfer to the concrete. Posttensioned concrete is usually created differently. The steel is placed in the concrete encased in conduits or sleeves so that the steel and concrete are not touching, and is meant to be unstressed. Once the concrete reaches its compressive strength, a jack helps to develop necessary tension. A free end is locked in place and the force of the tension in transferred to the hard concrete. The transfer is possible by concentrated loads are placed at each end of the member so the end blocks and anchoring devices are required to keep the compression present. Grout is usually put in the conduits to keep water out, and prevent corrosion. Posttensioning can be applied either internally or externally by opposite means. It can be useful to compress varied sections or provide support over larger projects. It is used to join large precast segments, such as bridges.

Uses and Installation
Prestressed concrete had initially been used for industrials tanks and pipes but when the Walnut Lane Bridge proved how dependable prestressed concrete could be it grew into a viable construction material. Prestressed concrete is a typical medium used for creating thousands of bridges. The Tampa Bay Bridge was also useful in highlighting the finer qualities of prestressed concrete. It is 17,500 feet long with 46 foot long spans. This helped to develop a standard of prestressed sections that are still used today. Florida prestressed concrete companies helped to develop a popular “double-tee” section. In 1950, the concrete Products Company of America created the first pretensioned bridge beam in the United States. It was a hollow box beam with circular voids. With heavy experimentation, pretensioning techniques grew. A seven-wire unit was soon developed with the assistance of the U.S Steel Corporation and soon became the industry standard. Piano wire was initially used but seemed to fail, as did larger wires that did not have enough surface area to create a strong bond. The use of twisted strand that was roughly .25 inches was created using seven smaller wires. The strands were soon made large, allowing hollow box beams as long as 100 feet to be produced. Shorter spans and smaller wires are used in school buildings, parking garages, and warehouses during the 1950s. Prefabricated and pretensioned wires that were capable of being brought to the construction site were in high demand. The maximum length of the members was only limited by what mode of transportation would be moving them. The Concrete Technology Corporation in Washington pioneered prestressed concrete for buildings. Arthur Anderson, the founder, thought that prestressed concrete production required plant manufacture and control. The Boeing Company Development Center was the biggest industrial prestressed concrete during the 1950s. The creation of the monorails at Seattle World’s Fair and Disney World Orlando were also notable for their curved prestressed girders, that required special manufacturing techniques.