Prestressed concrete is a concrete in which internal stresses are introduced in a planned manner to counter the stresses resulting from the superimposed load to the desired strength.
Basically, concrete is strong in compression and weak in stress, the reinforcement provided in concrete helps the concrete to withstand the stress.
Here we will learn about prestressed concrete, advantages & disadvantages of prestressed concrete.
Introduction to prestressed concrete:
Prestressed concrete components are similar to normal concrete but it has high strength concrete and high tensile steel wires.
High-grade concrete is used in prestressed concrete because reinforcement imposes large pressure forces for members causing high stresses at the ends to be developed by anchoring equipment.
The stress generated at the end of the beam can only be resisted by high strength concrete.
Due to the high strength concrete, the cross-sectional area required for the member will decrease, reducing the dead load of the member.
Methods of prestressed concrete:
In this system, wires/cables are stressed before casting concrete.
One end of the reinforcement is secured to the abutment, while the other end of the reinforcement is pulled using a jack and then this end is fixed to the other abutment and concrete is poured.
After treatment and hardening, reinforcement intentions are revoked.
The reinforcement that bends to resume its original length would be compressed by bending the concrete around it, thus transmitting the prestress to the concrete.
In this system, reinforcement is tensioned after the concrete is hardened, leaving the ducts to first insert the cable to the beam.
The ducts are released into the concrete by providing a steel spiral to the corrugated steel tubes.
When the concrete is hardened and develops its strength, the cable is passed through the ducts, one end is fixed to the anchor, then the other end of the cable.
Jack pulls the cable and compresses the concrete at the same time.
Equipment used in prestressed concrete:
Prestressing steel may be tensioned by means of levers, screw jacks, hydraulic jacks or similar mechanical apparatus.
The type of tensioning apparatus shall be such that a controlled force can be applied.
A device should be attached to this apparatus for measuring applied force, this measuring device should not have an error of more than 5 %.
Temporary gripping devices:
Wedges, yokes, double cones are the gripping devices, the prestressing wires may be gripped signally or in. group.
Gripping device shall be such that in a tensile test, the wires fixed by them should bleep before the failure of grip.
It should be such that during the period between the tensioning and release, the tension in the prestressed concrete elements is fully maintained by positive means as external anchorage.
The anchorage may consist of any device, which will provide the following clauses:
- It should be strong enough to resist the breaking strength of tendons.
- Holding capacity should be more.
- It should be safe against dynamic, static and impact loads.
Advantages of Prestressed Concrete:
- The reinforcement of concrete using excessive tensile steel improves the effectivity of the material.
- The prestressing system works for a span greater than 35 m.
- Prestressing will increase the shear strength and exhaustion resistance of concrete.
- Dense concrete is provided by a prestressing system, thereby bettering durability.
- The best choice for constructing smooth and thin buildings.
- Prestressing helps reduce the useless load of concrete construction.
- Prestressed concrete remains uncontrolled even below service load conditions which proves structural effectivity.
- Composite construction using the prestressed concrete unit and cast-in-unit achieves economic structure.
Disadvantages of prestressed concrete:
- This concrete is high materials cost.
- Formwork is extra complicated than RC.
Uses of prestressed concrete:
- In bridges, pre-stressed concrete is used to increase the potential span of the bridge and to make the bridge more durable under moving traffic conditions.
- The pre-stressed concrete is used to reduce the overall weight of the building thus reducing seismic forces and making the building economical.
- These slabs can also span large distances which can reduce the number of columns in the building.
- They are used in construction work such as factories and warehouses.
- They are durable and can travel great distances.