History of Prestressed Concrete
Prestressed concrete was built in 1886 by the San Franciscan engineer P.H. Jackson patented it, although it didn’t prove to be an accepted building material until 50 years later when a shortage of steel combined with technological advances was made prestressed concrete the preferred building material.
The early history of prestressed concrete is almost entirely associated with armament and the war effort, beginning in the early 1930s with Germany’s preparation for World War II.
The story is a bit bizarre. The idea behind it had been around for a long time, but it was Eugene Freyssinet who came up with the invention that made the idea possible. The French themselves did not show much interest in the invention. The Germans, on the other hand, wanted to save steel for war (they would of course also wage war against France), and the German company Wayss & Freytag received a license from Freyssinet – only to build their system almost exclusively for bunkers.
Eugene Freyssinet was commissioned to build three bridges over the Allier River at the beginning of the 20th century. He was convinced that these constructions are so economical that they are unparalleled in terms of price. Freyssinet distrusted calculations. He, therefore, ordered the construction of a test arch with a span of 50 m, the abutments of which were connected with very strong tension wires.
What is Prestressed Concrete
Prestressed concrete is a form of concrete in which prestressing is introduced into the concrete before the external load is applied in order to counteract the stresses from the external loads in the desired manner during the service life. This prestressing is provided by high-strength steel wires or strands (so-called “tendons”) in the concrete profile.
Or in other words
Prestressed concrete is, as the name suggests, a form of concrete in which internal stresses are introduced before it is applied in order to counteract the tensile stresses that arise in the concrete due to the external load.
Why Prestressed Concrete is required?
The question now is why do we need this complexity?
Also Read, Types of Non-destructive tests on concrete
Well, before you get started, let’s go back to some basics first. We know concrete can be compacted well, but it is very stress-free. Thus we see that after external loading, a compression zone has arisen in the lower part of the concrete. So try extending it and guess what? crack. This is why we add some steel bars in the lower section to help them withstand the most stresses and prevent the concrete from cracking. This is our traditional RC structure. But what about some massive structures with large beams? Think of the flyover, the famous Gateway Bridge in Australia, the Bridge in South Korea, or the nuclear reactor in Sweden, where the external load is very high.
The principle of prestressed concrete is that the compressive stresses induced in a concrete component by high-strength steel tendons are applied before loads are applied that compensate for the tensile stresses exerted in the component during operation.
Prestressing removes a number of design limitations of traditional concrete structures with regard to span and load-bearing and allows for the construction of roofs, floors, bridges, and walls with longer unsupported spans. This allows architects and engineers to design and build lighter, flatter concrete structures without sacrificing strength.
Uses of Prestressed Concrete
It is widely used today for roof beams, piles, and railroad ties, as well as structures such as bridges, water tanks, roofs, and walkways. Prestressed concrete is usually not required for columns and walls, but can be used economically for tall columns and high retaining walls with high bending loads.
As a rule, conventional reinforced concrete is the most economical way up to 6 meters. Prestressed concrete is more economical for spans over 9 meters. Between 6 and 9 meters, the two variants must be weighed according to the special requirements and the most suitable variant.
Materials used for Prestressed Concrete
According to AASHTO, high-strength sieve wire, high-strength steel wire, or alloys of grade and type (as specified by the designer) should be used in prestressed concrete. In addition, prestressed concrete requires stronger concrete than regular reinforced concrete. In general, a minimum of 28 days of concrete should be used with a strength of 5,000 psi.
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Source-
- Proceedings of the First International Congress on Construction History, Madrid, 20th-24th January 2003,ed. S. Huerta, Madrid: I. Juan de Herrera, SEdHC, ETSAM, A. E. Benvenuto, COAM, F. Dragados, 2003.
2.https://civiltoday.com/civil-engineering-materials/concrete/225-pre-stressed-concrete
3.https://www.civillead.com/prestressed-concrete/
4.https://www.cement.org/cement-concrete/products/prestressed-concrete
5.https://www.designingbuildings.co.uk/wiki/Prestressed_concrete
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