Graphical Construction Glossary >> Concrete and Concreting. >> >> Prestressed Concrete
First patented in America in 1886 prestressed work did not come into it's own until the post WWII reconstruction effort in Europe.
The principal behind prestressing can be illustrated by lifting s section of books off a shelf by pressing inwards on the two end books. If one presses hard enough quite a few can be lifted and carried, without the inner books falling out.
The photos here are of a job that is a cast in-situ suspended slab that is post tensioned. That is the steel cables are tensioned after the concrete has gained some compressive strength to resist the tensioning of the cables.
Pre tensioned concrete is a different ball game and it usually applies to work done in specialised casting factories with the enormously strong jacking points and the safety measures in place in case of cable failure. The cables are tensioned in ducts and then the ducts are filled with pressure injected grout.
The process has been used for long enough that for many years it has been a major concern in demolition of building containing prestressed beams etc. The is an added element of danger in this type of demolition work.
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