The prestressed concrete is a concrete in which internal stresses are induced artificially so that the stresses resulting from the external forces (loads) are counteracted or minimized to some extent. Let us understand the concept of pre-stressing through daily life examples.
Metal bands are used around the wooden barrel. These metal bands induce compression into the barrel which will then minimize to some extent the tension created by the liquid inside the barrel.
Your bicycle wheel has pretension spokes which will counteract the external compressive forces induces while driving the bicycle.
By inducing these internal stresses, load-carrying capacity or the capacity to take external stresses will increase e.g. say if a beam’s load-carrying capacity is 20 kN. If say internal stress of magnitude 2 kN (tension) is introduced now the beam will have to take a load of 20-2 = 18 kN. Thus, an additional 2 kN load can still be placed over it.
Concrete is weak in tension and its tensile strength is about 8 to 14% of the total compressive strength it possesses. Cracks start appearing in concrete members subjected to tension and to prevent it, the compressive force can be applied in this zone. If we stretch the steel and leave it, it will automatically apply the compressive force in the system. This prestressing increases the bending, shear, and torsional capacities of the concrete structure.
In prestressed concrete, zone below neutral axis is also effective in taking compression.
Uses of Prestressed Concrete
It is widely being used for the construction of long span bridges, industrial roofs, water retaining structure, structure under water, transmission pole, nuclear vessels, in railway sleepers etc
Materials for Prestressed Concrete.
High Strength Concrete
In this type of concrete, high-strength concrete is required to withstand the high prestressing force. Basically prestressing force which we apply depends on the strength of concrete, if low strength of concrete is used, we will have to apply a lower prestress consequently results/benefits of prestressing will be lower. Thus, high-strength concrete is required for the prestressing.
As per IS 1343-1980, the minimum compressive strength of concrete shall be 40 N/mm2 for pre-tensioned members and 30 N/mm2 for the post-tensioned members. In the pretension method, the tension in the steel reinforcement/tendons is applied before the casting of the member, and in the post-tension prestress tension is applied after casting or placement of concrete. Minimum 300 to 360 kg /m3 of cement is required and maximum cement content should not be more than 530 kg /m3.
High Tensile Steel
In reinforced concrete, reinforcement in the form of bars is used while in prestressed concrete reinforcement in the form of high strength wires, bars, or stands are used. High tensile strength is achieved by increasing the carbon content in the steel by approximately 0.6 to 0.85 percent.
Wires are available in the size of 2.5, 3,4,5,7, and 8 mm diameter. These are used in stands (group). Bars are available in the size of 10,12,16,20,22,25,28 and 32 mm.
Reinforced Cement Concrete
As we already know that concrete is very weak in tension and its tensile strength is approximately 10 % of its compressive strength. To make concrete strong in tension, reinforcement in the form of bars is introduced which will take the tensile forces. Thus in ordinary Reinforced Cement concrete, reinforcement is provided to withstand the shear bending and torsional forces.
In comparison to the prestressed concrete, the size of the RCC section is bigger, and also load carrying capacity will be lower. Prestressed concrete is an improvement over reinforced cement concrete to enhance some of its properties.
In traditional RCC design, the zone below the neutral axis is neglected in terms of taking compressional forces. But, in a prestressed concrete whole section can resist external loads due to the prestressing.
Due to the whole effectiveness of taking load by the whole area, and also usage of high strength concrete and high tension steel, sections required to resist the load will be considerably smaller than the RCC.
Due to thinner section, dead load of the structure reduces and also more economy can be achieved.
Traditional concrete shows appearance of cracks in tension zone, while in prestressed concrete cracks do not appear thus steel will not be corroded.
Due to the lighter section and higher load carrying capacity, important structures like bridges and water retaining structures can be made with the prestress which will take the higher load and will be more economical.
It will deflect less due the already applied prestressing forces.
Can be produced in a industrial scale and time to built a structure will be considerably reduced because it can be produced in factories can assembled at site. Large buildings can be built in 2-3 days which will take 2-3 months for traditional RCC structure.
Some technical know how and good quality control is required in this type of construction.
Since higher strength steel and concrete area used thus price of prestressed concrete will be more than the Reinforced cement concrete.
Sometimes members are made in factory thus additional cost of transportation of these concrete sections to the site account for increasing the cost of the project.
Inducing prestressing requires some type of equipment to be used, be it pre-tensioned or post-tensioned prestress concrete. These equipments are costly thus application of prestress concrete to the wider extent cannot be possible in the present-day scenario.
Also with time effect of prestressing reduces due to creep and shrinkage in this type of concrete.
Difference between RCC and Prestressed Concrete.
|economy||Low cost||More costly|
|deflection||Deflection is more.||Less deflection, approx. 25% of the RCC.|
|Steel||Mild steel is used, which is not that costly.||High strength steel is used in the form of wire and bars.|
|concrete||Concrete strength lower than 30 MPa can be used.||High strength concrete is required min 30 MPa.|
|Cracks||Tensile cracks can appear in the tension zone.||Tensile cracks will not appear.|
|Self weight||Bulky sections thus self-weight of the structure is considerable.||Lighter sections can be made which will take up the same load as RCC.|
|Quality control||Less quality control required.||In comparison, more quality control required.|
|corrosion||More chances of steel get corroded.||Less chances of corrosion due to less cracks.|
|brittleness||Steel fails first in RCC thus not brittle.||Since the whole section is under compression also elongation of steel reduces due to high carbon content. Thus it is more brittle.|
|Usefulness of section||Concrete in the tension zone is ignored.||Concrete in tension zone is taken into the consideration thus it can take more load|
|time||It gains strength slowly||It gains strength rapidly|