Prestressed concrete is way of treating concrete to reinforce its strength and resistance to tension and erosion. Concrete that is not Prestressed is naturally weak to tension, and as more weight is placed on it, more cracks begin to form and the weaker it becomes. Pre-stressing uses a series of steel cables and rods to compress the concrete. As the concrete is compressed, its tension resistance becomes stronger. Essentially, pre-stressing concrete can be likened to pressure treated wood. By subjecting it to certain elements, it makes the structure stronger and more durable. Concrete is also reinforced with steel bars, called rebar, to further strengthen its load-bearing capabilities. Prestressed concrete is generally able to withstand cracks and is more cost effective for reinforcing large slabs of concrete. Because there is a lesser number of joints within the concrete, it cuts down on maintenance costs during the overall life of the building.
There are three ways concrete is Prestressed: pre-tension, bonded post-tension, or un-bonded post-tension.
When the concrete is first poured, it is cast over tensed bars. Because the bars are already under maximum tension, the concrete bonds to the bars and is compressed when the tension is released. The concrete then becomes as compressed as possible, and it cannot be compressed any further. Therefore, any extra tension on the concrete is transferred back to the steel bars inside. Pre-tensioned concrete is usually prefabricated, because it takes incredibly strong anchors to “stretch” the metal bars inside the concrete. Usually, this type of pretension only yields small sections of concrete at a time.
Bonded Post-Tensioned Concrete
This type of concrete is compressed after it is poured, not during the casting process. During casting, the concrete is poured around ducts made of steel or aluminum. The ducts are placed in areas where the concrete is expected to experience tension. Afterwards, a series of rods are passed through the ducts before pouring to concrete. The concrete is then allowed to set, and once it has hardened, hydraulic jacks are used to create pressure on the concrete and stretch the bars inside. Once the bars are stretched, the jacks are removed and the ducts are sealed with grout to prevent water from seeping in and causing rust or corrosion.
In this way, the concrete is rigid and reinforced, and the entire slab of concrete, not just the tension rods inside, absorbs fluctuations due to temperature. Concrete Prestressed in this method can be made on a larger scale and is usually used for foundation slab for buildings and in parking garages. In the past, bridges were also made out of this type of concrete, but recent studies have found that the ductwork can sometimes be sealed improperly, which leads to corrosion and in severe cases, bridge collapse.
Un-bonded Post-tensioned Concrete
This type of concrete is made similarly to the bonded version; however, the rods inside are greased and provide flexibility. Cables within the concrete can be individually adjusted after the concrete has already set and the need for grouting inside the ductwork is eliminated.