Prestressed concrete is the concrete in which to counteract the stresses from external loads, internal stresses are introduced to the desired degree.
Prestressing in reinforced cement concrete is introduced by tensioning steel reinforcement
The following terminologies are used in prestressed concrete
A stretched element used in a concrete member of the structure to allow prestresses into the concrete.
High tensile steel bars, wires, strands or cables are used as tendons in general.
To permit tendon in concrete and to maintain prestress Anchorage device is used.
The Magnel Balton, Freyssinet, Lee-Macall, Leonhardt-Baur anchorage systems are commonly used.
The stage corresponding to prestress of concrete is described as a transfer.
Prestress is delivered from bulk-head. For pre-tensioned members, the transfer occurs at this stage.
The transfer takes place in post-tensioned members after the end of the tensioning process.
The Prevention of the bond between the surrounding concrete and the steel wire is termed as debonding.
2 Methods of prestressing
There are 2 methods of prestressing for prestressed concrete.
Before the concrete is placed in Pre-tensioning method, the tensioning of tendons is done.
The prestress is introduced to concrete by a bond between concrete and steel in this method.
Hoyer’s Longline system of pre-tensioning is used.
In this system, a minimum of M 40 grade concrete is utilised.
For small span members, this method is utilised.
The loss is more but the cost is small in this system.
Against hardened concrete tensioning of tendons is done in Post-tensioning method.
The prestress is introduced to concrete by bearing in this method.
Leonhardt, Fresyssinet, Gifford-Udall, Magnel , Lee-Mc call anchorage systems are used.
In this system, a minimum of M 30 grade concrete is utilised.
For lang span members this method is utilised.
In this method, the loss of prestressing is less but the cost is more than pre-tensioning.
Prestressing Systems of prestressed concrete
Main prestressing systems of prestressed concrete are:
- Freyssinet system
- Magnel-Blaton system
- Lee-Mc Call system
- Hoyer’s long time system
1. Freyssinet system
This is a post-tensioning system. Anchor unit is a female and male cone.
5 mm to 8 mm diametre tendons are used and they are arranged to form a cable of 12 wires.
This is a post-tensioning system. Steel sandwich plates are used as Anchorage unit.
Tendons of diameter 5mm to 8 mm diameter are used and organised in 16 layers, with each layer
consisting of 4 wires.
The number of wires varies from 2 to 64 wires in this magnel cable.
3. Lee-Mc call
Lee-Mc call is a post-tensioning system.
At ends diameter ranging from 12mm to 40 mm of High-tensile bars are threaded.
Anchoring is done by screwing a washer and nut tightly against the endplates in this system.
4. Hoyer’s long line system
Hoyer’s long line system is a pre-tensioning system. Jack and abutment are used as an Anchorage unit.
For mass production of small structural members in factories, this system is utilised.
What are Prestressed concrete materials?
Prestressed concrete Materials are:
M 30 to M 60 are the Grades of concrete used.
To develop greater bond stress high grade of concrete is utilised.
To overcome the shrinkage loss of concrete due to stress, the high strength of steel wires is utilised.
for pre-tension members is 40 N/mm2 and for post-tensioned members is 30 N/mm2.
Type of Losses in Prestressing
There will be a continuous reduction of effective stresses in prestressed concrete due to various causes with time from stage of a transfer.
Such losses are termed as the loss of prestress.
Losses in Pre-tensioning
Given below different type of losses :
- Elastic deformation of concrete
- Relaxation of stress in steel i.e creep in steel.
- Shrinkage of concrete
- Creep of concrete
Losses in Post-tensioning
Given below different types of losses:
- If there is simultaneous tensioning of all the wires, then no loss happens due to elastic deformation.
- Due to elastic deformation, there will be Loss of prestressing if successively bars are tensioned.
- Relaxation of stress in steel.
- Shrinkage of concrete
- Creep of concrete
- Frictional losses
- Anchorage slip.
1. Elastic deformation of concrete
When prestress is applied elastic shortening takes place in concrete which results in prestressed steel shortening.
In both post-tensioning and pre-tensioning members, this elastic deformation loss will be there.
This elastic deformation depends upon the stress in concrete at the level of steel and modular ratio (m).
Elastic deformation loss can be given by = m x Fc
where ,modular ratio (m )= Es/Ec
2.Loss due to shrinkage of concrete
Shrinkage is defined as a change in the volume of concrete members.
This shrinkage loss is dependent on the passage of time and humidity present in the atmosphere but is independent of the load’s applied.
Due to Shrinkage of concrete tensioned wire will get shortened and hence produces the prestressed loses.
In both post-tensioning and pre-tensioning members, this shrinkage loss will occur.
Factors affecting shrinkage
- Type of cement and aggregate
- Method of curing
There will be a reduction in shrinkage and consequent prestress loses if high strength concrete with low water-cement ratios is utilised.
Loss of prestress = ecs X Es
ecs = Total residual shrinkage strain
= 0.0003 (for members with post-tensioned )
= 0.0002/ log 10 (t +2) ( for members with pre-tensioned )
Es = Youngs Modulus of the elasticity of steel
3.Loss due to creep of concrete
Under sustained loading, the progressive increase in the plastic deformation of concrete is termed as Creep of concrete.
The transferred prestress effects in the creep of concrete in this method.
The stress in high tension steel is reduced due to this.
In both post-tensioning pre-tensioning members this creep of concrete occurs.
There are two methods for estimation of creep.
- Ultimate creep strain method
- Creep -coefficient method
4. Relaxation of stress in steel
Under constant strain at a constant temperature, the loss of stress in steel members are termed as relaxation of steel.
This loss is similar to creep in concrete
Loss due to relaxation differs uniquely for steels.
In both post-tensioning pre-tensioning members this loss of stress in steel members occurs.
In general, the loss is about 2% to 8 % of the initial stress and this loss will be higher at a greater temperature.
This loss is generally referred to at 1000 hours at 27-degree Celcius.
5.Friction loss due to post-tensioned member
In the members with post-tensioned only this friction loss occurs.
There will be small frictional losses in Jacking, equipment.
The friction is not small between tendons and the surrounding materials and so it is considered party wobble or length effect and partly a curvature effect.
Due to the wobble effect, friction loss occurs in straight length.
in curved members, the friction loss occurs due to wobbles effect and curvature.
6.Loss due to Anchorage slip
Due to the strain or due to the slip in anchorage this loss occurs.
In the members with post-tensioned this loss generally occurs.
The above mentioned prestressed concrete materials are used in high raised building constructions and prestressed concrete losses need to be considered during concrete design stages.