This technique is used to control cracks in large span constructions, it is mostly used for bridges.
What is Prestressed concrete?
prestressed concrete (in some places of pre-stressed concrete in LatinAmerica ) to the construction technology of concrete structural elements subjected intentionally to compression efforts prior to its commissioning. These efforts are achieved by bars, wires or steel wire cablesthat are tensioned and anchored to the concrete.
This technique is used to overcome the natural weakness of concrete against efforts traction , and was patented by Eugène Freyssinet in 1920.
The objective is to increase the tensile strength of the concrete, introducing an internal compression force that partially counteracts the tensile stress produced by the service loads in the structural element .
The tensile strength of conventional concrete is much lower than its compressive strength, on the order of 10 times less. Bearing this in mind, it is easy to notice that if we want to use concrete in elements, which under service loads, must resist tractions, it is necessary to find a way to supply this lack of tensile strength.
In conventional reinforced concrete , tensile strength is provided to the structural elements by placing reinforcing steel (passive) in the zones of the structural elements where tractions may appear. This way of providing tensile strength can guarantee an inadequate resistance to the element and has the disadvantage of not preventing cracking of concrete for certain load levels. One way to prevent cracks from appearing, and therefore eliminating the dead weight of cracked concrete, is to introduce additional forces that compensate for the effect of the external actions so that, when the external actions and these additional forces act together, their effects they compensate. In this way, prestressed concrete is reached . In the most usual way used today, the prestressing materializes by embedding in the concrete steel cables with a suitable layout that are put into traction prior to the commissioning of the structure.
Prestressed concrete has the following advantages:
- Since the prestressing technique removes cracks from the concrete at all stages of loading, the entire section of the structures takes part in the resistance to external load. In contrast to this, in reinforced concrete, only part of the concrete above the neutral axis is effective.
- As the concrete does not crack, the possibility of steel corrosion and deterioration of concrete is minimized.
- Absence of cracks results in a greater capacity of the structure to withstand the load of stress, impact, vibration and shock.
- In prestressed concrete beams, dead loads are virtually neutralized. The reactions are therefore required much smaller than that required in reinforced concrete. The dead weight of the reduced load of the structure results in savings in the costs of the foundations. The neutralization of dead load is important in large bridges.
- The use of the tendons and the curve before the compression of the concrete helps resist the cut.
- The amount of steel required for prestressing about 1/3 of that required for reinforced concrete, although the steel for prestressing must be of high strength.
- In pre-stressed concrete, prefabricated blocks and elements can be accepted and used as a unit. This saves on the cost of formwork and the centering of large structures.
- With the arrival of prestressed concrete, which has now been possible for the construction of large lights. These structures have low cost and are safe from cracks.
- Prestressed concrete can be used with advantage in all structures where stress develops, such as the tie and tie rods of a bow rope beam, railway sleepers, electric poles, the face upstream of the gravity dam, etc.
- The prestressed concrete beams the deviation is usually low.
- Even if cracking or deformation occurs due to a temporary overload, it will be almost restored after unloading.
Prestressed concrete construction has the following disadvantages:
- High quality heavy duty dense concrete is required. Perfect concrete quality in the production, laying and compaction required.
- High strength steel is required, which is 2.5 to 3.5 times more expensive than mild steel.
- It requires complicated tension equipment and anchoring devices, which are usually covered by the patented rights.In designing and constructing prestress, it is also necessary to use PC- specific technology that requires careful introduction / fixing of tension, injection of grout material, etc.
- The construction requires perfect supervision in all stages of construction.
- When parts are damaged seriously, their repair is difficult compared with reinforce concrete structure
- If the filling of grout is not sufficient in the post tension system, there is a possibility that problems such as deterioration of load bearing performance and durability due to PC steel material breaking and third party damage may occur.