16-04-2013, 04:13 PM
Prestressed Concrete
Prestressed Concrete.pdf (Size: 1.35 MB / Downloads: 209)
Introduction
Background
The idea of prestressed concrete has been around since the latter decades of the 19th
century, but its use was limited by the quality of the materials at the time. It took until
the 1920s and ‘30s for its materials development to progress to a level where
prestressed concrete could be used with confidence. Freyssinet in France, Magnel in
Belgium and Hoyer in Germany were the principle developers.
The idea of prestressing has also been applied to many other forms, such as:
• Wagon wheels;
• Riveting;
• Barrels, i.e. the coopers trade;
In these cases heated metal is made to just fit an object. When the metal cools it
contracts inducing prestress into the object.
Durability
Since the entire section remains in compression, no cracking of the concrete can
occur and hence there is little penetration of the cover. This greatly improves the
long-term durability of structures, especially bridges and also means that concrete
tanks can be made as watertight as steel tanks, with far greater durability.
Materials
Concrete
The main factors for concrete used in PSC are:
• Ordinary portland cement-based concrete is used but strength usually greater
than 50 N/mm2;
• A high early strength is required to enable quicker application of prestress;
• A larger elastic modulus is needed to reduce the shortening of the member;
• A mix that reduces creep of the concrete to minimize losses of prestress;
Steel
The steel used for prestressing has a nominal yield strength of between 1550 to 1800
N/mm2. The different forms the steel may take are:
• Wires: individually drawn wires of 7 mm diameter;
• Strands: a collection of wires (usually 7) wound together and thus having a
diameter that is different to its area;
• Tendon: A collection of strands encased in a duct – only used in posttensioning;
• Bar: a specially formed bar of high strength steel of greater than 20 mm
diameter.