31-07-2012, 04:43 PM
BOND IN PRESTRESSED CONCRETE
BOND IN PRESTRESSED CONCRETE.ppt (Size: 2.19 MB / Downloads: 67)
Bond between steel and concrete in prestressed concrete construction exist on two different basis.
1. Pre-tensioning system
Used as a means of transffering the prestressing force of steel to the concrete section.
2. Post-tensioning system
In this bond is necessary for two purposes,
-Protection against corrosion
-Increase in ultimate strength
Bond in Pre-tensioned construction
It is furnished by two factors,
-Reduction in area of cross section of steel
-Adhesive property
The phenomenon of recovery of lateral contraction develops a wedge action at the end of the cable by which prestressing force is transferred.
This property was discussed detaily by Hoyer and is called “HOYER EFFECT”.
Transverse reinforcement has to be provided to resist tensile force.
Development length is given by,
The bond needed to transmit the complete prestressing force is called transmission length(Lt)
The bond length (Lb) is the minimum length over which, the stress in the tendons can increase from the effective prestress to ultimate prestress at critical location
Bond in Post-tensioned construction
Effect of bond in post-tensioned construction has two distinct purposes;
1.Protection against stress corrosion
-Moisture enters into duct
-Cause corrosion to high tension steel
-Rusting reduces effective area of steel
-This causes splitting of wires called stress corrosion
2.Increase in ultimate strength
In unbonded construction
-Crack developed at maximum moment location
-Strain in steel increases
-Strain distributed along the length causes excessive deformation
-Due to this, cracks open much wider reducing compressive zone
In bonded construction
-Crack at the critical section does not affect the strain in steel
-Because of this, the compressive area is not reduced considerably
There are 2 layers of bonding media in post-tensioned construction:
-Bond between the steel and the sheath or duct
-Bond between the sheath and the concrete
Bond stress is obtained by,
Where d=distance of steel from center of compression
Assuming a uniform bond stress,
CONCLUSION
An allowable bond stress of about 100 N/cm 2 is generally permitted in static loading conditions.
In bridges, where arepitition of load comes more often, the permissible bond stress is about 50 N/cm 2 for M 35 concrete