Prestressing-Principles-Types

1. PRINCIPLES OF PRESTRESSING
1. Example of prestressing
2. Plain concrete beam
3. Cracks/Fails due to tensile stress
4. Steel reinforcement
5.Cracks near supports- shear forces
6. Prestressed beam – best solution

2. PRINCIPLES OF PRESTRESSING

3. PRINCIPLES OF PRE- TENSIONING

4. PRINCIPLES OF POST TENSIONING

5. FOR CONSTRUCTION OF :
 BRIDGES
 FLYOVERS
INDUSTRIAL STRUCTURES (Cement silos, Digesters etc.)
 WATER TANKS
 NUCLEAR REACTORS
BUILDINGS & FACTORIES (Post tensioned slabs and/or beams)
APPLICATION OF PRESTRESSING

6. ADVANATAGES & BENEFITS OF PRESTRESSING:
Post-tensioned concrete is a very efficient and cost-effective
structural system that has become especially important with the
escalation of steel prices. The other key advantages include:
 Longer spans/reduced structural depth
 Lighter structure weight
 Deflection & vibration control
 Crack control
 Functional flexibility
 Cost Saving
APPLICATIONS OF PRESTRESSING:
PT is used for a wide range of applications including bridges,
flyovers, office buildings, hotels, parking structures, shopping
malls, ground anchors, storage tanks, barriers, nuclear
containment structures, stadiums, silos and cable stays.

7. TYPES OF PRESTRESSING
PRE-TENSIONING – Stressing is done before concreting
POST-TENSIONING – Stressing is done after concreting

8. POST-TENSIONING:
 Concrete is placed around sheaths/ducts containing unstressed
tendons (HTS strands).
 Load is applied after concrete attains required strength.
 The stressed tendons are locked off by anchor grips.
 Force is directly transmitted to the concrete and conventional
formwork is used.
 After stressing, grout mixture is pumped into the duct/
sheathing in bonded system (mostly used in India).

9. PRE-TENSIONING:
 Concrete is placed around previously stressed LRPC/HTS
strands or wires (tendons).
 Ducts or sheathing are not required in this method.
 Principally used for pre-cast concrete, where the forces can be
restrained by fixed abutments.
 Once concrete hardens, it grips the stressed strands/wires.
 When concrete attains sufficient strength, the tendons are
released from the abutments.

10. TYPES OF POST TENSIONING SYSTEMS
BONDED SYSTEM:
 Grout (mixture of cement and water) is
pumped into the duct.
 Provides a continuous bond between strand
and the duct.
 Increases protection against corrosion.
UNBONDED SYSTEM:
 The tendon is not grouted and remains free
to move independently of the concrete.
 Tendons used are typically coated in grease
within a protective cover (HDPE).

11. Anchorage & sheathing for
bridges/flyovers etc.
Flat anchorage & sheathing
for buildings
Hydraulic pumps and multipull jacks
LRPC strand and Sheathing pipes
PRESTRESSING PRODUCTS.
LRPC Strands

12. Round Sheathing Pipes
Flat Sheathing Pipes
Grout Agitator
Grout Pump
PRESTRESSING PRODUCTS.

13. Profiling of duct in a box girder

14. Strands inserted in ducts

15. Strands inserted in ducts of a box girder – before stressing

16. Stressing operation –
in situ construction

17. Stressing a cable having
19 strands with multipull jack

18. LRPC strands in use for construction of
post-tensioned slab in a building

19. POST – TENSIONING IN PROGRESS
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