The imposition of internal stresses into a structure that is of opposite character to those that will be caused by the working or the service loads is known as prestressed concrete. Prestressing has been used for many years in various kinds of structures. Prestressed concrete was 1st invented by Eugene Frevssinet in 1928 is a method for overcoming concrete is very weak to resisting tension forces.
It is a form of RCC that builds in compressive stresses during construction to oppose those found when in use is termed as prestressed concrete. Prestressed concrete is a combination of concrete and steel that takes advantages of the strength of each material.
Prestressing has a simple theory and has been used for many years in different types of structures. Prestressing is used to counteract the tension stresses in the concrete beam are caused by the weight of members and imposed loads and these loads cause a positive bending moment in a beam, so prestressing is used to introduce a negative bending moment that should counteract the all of the positive bending moment.
Beams can carry compressive stresses as long as the downward loads from squeezing at the bottom of the beam is higher than the tensile stress, so there the moment is produced by the weight of working and the superimposed loads. Such beams having no tensile strength and no bending moment until squeezed or prestressed together.
Flexural member of concrete can be prestressed so their entire cross sections are kept in compression, so these entire sections are available to resist the applied forces. Prestressed concrete producing the permanent compression in the areas subjected to the tension. The tensile stress portion is counteracted by reducing the cross-sectional area of the reinforcing steel.
It is more durable than RCC construction because it does not crack under loading condition. Prestressed concrete reduces the formation of cracks. Freezing and thawing action is durable than conventional concrete.
Application of Prestressed Concrete:
With the use of PSC reduction in structural members like slab and beam element.
Prestressed concrete also reduces the overall weight of the structure. Prestressed concrete is used in bridges to increase the possible span of the bridge and to make the structure more durable under moving traffic condition.
Prestressed concrete is widely used for residential and commercial products on the interior and exterior of building construction.
This concrete is used in large span distance between which are not possible through the normal RCC construction.
High yield strength steel alloys are used to producing permanent pre-compression in areas subjected to tension.
The area of tensile stresses forces is counteracting by reducing the cross-sectional area of the steel rebar.
Forms of the prestressed steel:
It is a single unit made of steel.
Tendons are wires and a group of strands is wound to form a prestressing strand.
Strands are two, three or seven wires that are wound to form a prestressing strand.
Cables are the series or group of tendons.
The tendons are simply made up of a single steel bar and diameter of this steel bar us larger than that of wire.
Methods of prestressing:
Two methods of prestressing are as follows:
Placement of concrete around the steel tendons which have been stressed to the desired degree.
Pre-tensioning is a type of prestressed concrete in which the tendons or wires are stretched before the pouring of concrete.
After the concrete hardens, the tendons were cut and the prestressed concrete force was transmitted to the concrete by the bond.
In pre-tensioning, the tension is applied to the tendons before the pouring of concrete.
The placement of concrete around the tendons that have been stressed to the desired degree.
Pre-tensioning concrete gives a good bond between the tendons and concrete.
Mostly the pre-tensioning concrete elements are precast in a factory and must be transported to the construction site.
Prestressed concrete produces a good bond combination between the concrete and tendons.
Pretensioning is very difficult to perform at the site, its structure is only done in precast yards.
Tendons used in pre-tensioning should only be straight and circular.
In the post-tensioning method, the tension is applied to the tendons after the hardening of the concrete.
The conduits, sleeves and the metal tubes with unstressed tendons inside located in the form and the tendons are stretched.
The precompression is transmitted from the steel to concrete by using the anchorage device.
Post-tensioning is an easier process than pre-tensioning because it can be done at the site.
Post tensioning requires specialized knowledge and person to fabricated, assemble and install.
After the proper curing of concrete, tendons are stretched by using jacks on the sides and grouts filled with as per mix design.
Post-tensioning requires specialized knowledge, experienced engineers and skilled labor.
Post-tensioning is used in structural member like a beam, in bridge decks, roof slab, etc.
Benefits of post-tensioning:
Concrete is good in resisting compression but weak in tension.
This fact will cause the bottom section of the beam to elongate and causes cracking.
Pre-tensioned tendons are considered as active reinforcement.
On the other hand, rebar is considered as passive reinforcement.
It can be designed to minimize deflection and cracking.
Materials used for prestressed concrete:
Concrete and high strength steel are used for the prestressed concrete.
The elastic modulus of prestressed concrete is much higher and the elastic strain is smaller when the tendons are cut.
The high strength concrete should be used for pre-tensioning to permit higher bond stresses between the concrete and the cables.
For pre-tensioning and post-tensioning the higher strength concrete is used for RCC members.
Full, Limited and partial prestressing:
1. Limited prestressing:
When the prestressing level is such that, the tensile stress under the working load within the cracking stress of concrete is termed as limited prestressing.
2. Full prestressing:
When the prestressing level is such that, no tensile stress under the working load within the cracking stress of concrete is termed as limited prestressing.
3. Partial prestressing:
When the prestressing level is such that, the tensile stresses due to service loads, the cracking stress is allowing within the limit is termed as limited prestressing.
Limitation of prestressing:
It requires skilled and experienced technicians.
The cost of high strength materials is high.
It requires good quality control and inspection.
Mix design of prestressed concrete:
Generally, we use a minimum of M30 grade of concrete for post-tensioning and M40 for pre-tensioning members.
If you use M30 and mild steel than w/c ration control on 0.55.
If use M35 grade than w/c ratio control on 0.45.
If use M40 grade than w/c ratio control on 0.40.
The lightweight aggregates are used in the mix design of prestressed concrete.
Advantages of prestressed concrete:
The prestressed concrete members are free from the cracks under compression.
Prestressed concrete members are more durable and strengthen then reinforced concrete.
The sections of prestressed concrete members are smaller than the RCC member sections so result can be used for the longer span.
The member of prestressing concrete has offered more resistance against shear force.
It improves the ability of a concrete member to resist the impacts loads more carefully.
Prestressed concrete members have good ability to resist the working load as compared to the reinforced concrete.
It is more economical than reinforced concrete for longer span structure.
It is more durable than reinforced concrete.
By the use of higher strength steel and concrete result in the reduction of the cross-section.
Disadvantages of prestressed concrete:
It requires experienced engineers, supervisor, and skilled labor.
The initial cost of prestressed concrete is very high.
Prestressed concrete requires high strength of concrete and steel.
It requires special tools like jacks, anchors, wedges, etc.
High reinforcement bars are needed which is too expansive than mild steel reinforcement.