Types of Application of External Prestressing
In total, then a three types of applications of external pre-stressing applicable to bridges which are mentioned as below:-
(i) In the first type, the unbounded pre-stressing tendons/cables comprise the total pre-stressing force. Segmentally constructed bridge and cast-in-situ box girder bridge superstructure using external cables are an example of this category.
(ii) In the second type, the unbounded elements act as part of total pre-stressing force, the other part being provided by bounded pre-stressing elements embedded in concrete. External pre-stressing provided for repair/rehabilitation/strengthening of existing PSC bridge is an example of this type. The external pre-stress may or may not contribute to the enhancement of ultimate strength.
(iii) In the third type, the external pre-stressing is used as a means of applying external load acting in the desired direction and quantum and the deformation of the structure due to subsequent live load-both in service and under ultimate condition do not affect the magnitude and direction significantly. The external pre-stressing is taken as equivalent load only without considering its contribution to the strength of the structure. This type of application is used in strengthening of existing RCC structure by using pre-stressing elements to relieve part of the dead load effects.
tendon placement in external post-tensioning
Disadvantages Of External Prestressing
The various disadvantages of external pre-stressing are described as below:
(i) Only straight profile of tendon is possible.
(ii) Additional cost for ducts and anchorages.
(iii) Diffusion of Post-Tensioning forces.
(iv) External tendons are more sensitive to fire and more easily attackable by the vandals.
(v) The pre-stressing force disappears over the overall length of the tendon once a section of tendon is broken.
(vi) External tendons are protected by HDPE ducts which are fully cement grouted or wax filled which result in a higher initial material cost for the pre-stress system.
(vii) Under ultimate bending conditions external tendons require more pre-stressing to generate the same moment of resistance.
(viii) Anchorage points and deviators are subjected to high concentrations of forces which need to be properly tied into the structure.
Advantages Of External Prestressing
The various advantages of external post-tensioning are described below:
(i) The casting of structure is simplified by eliminating the presence of ducts in the webs and flanges. The formwork, fixing of reinforcement and concreting are all made easier.
(ii) Reduced web thickness due to the absence of ducts result in a saving on superstructure weight and substructure costs.
(iii) Installation is made easier by having access to all parts of the duct. Placing of the strands is more straight forward and not prone to the blockage of the duct that can be associated with internal pre-stressing.
(iv) Tendon layouts are simplified.
(v) Grouting of tendon is easier due to the better access available and because individual tendon can be grouted without the leakage or blockage problems that can occur with internal tendons in some forms of construction.
(vi) Reduced friction losses within the tendon results in a higher effective force in the strand.
(vii) It is easier to inspect tendon during construction and for long term maintenance.
(viii) The tendon is replaceable in the future.
What is External Prestressing ?
External Pre-stressing is one of the latest developments in pre-stressed concrete technology. It refers to pre-stressing techniques where the pre-stressing tendons are places outside the concrete section and thus no continuous bond is provided between the pre-stressing elements and concrete by virtue of which concrete and pre-stressing tendons develop structural interaction. In these types of structures, the pre-stressing force is transferred to the concrete by means of end anchorages, deviators and saddles. The pre-stressing elements and the concrete elements behave as different component structures of the overall structure, acting in unison by virtue of connection at anchorages and deviators, and thereby contribute to overall strength. The external pre-stressing has been applied mainly in bridges and this technique is growing in popularity because of the speed of construction and minimal disruption to traffic flows. Because of its practice advantages, external pre-stressing becomes a widely used technique in construction and strengthening of bridge structures.
Figure below shows typical layout of an externally pre-stressed post-tensioned box girder bridge. Generally, the external tendons are placed in the hollow section of the box girder. The pre-stressing force is transferred to the beam through end anchorages and deviations.