STRUCTURAL

Post-Tensioning / PT

Concrete reinforced with high-tensile steel tendons under tension

Also calledpost tensioningpost-tensioningptprestressingpre-stressing
Related on InfraLens
CODES
IS 1343IS 6006IS 14268
Definition

Post-tensioning (PT) is a method of pre-stressing concrete where steel tendons are tensioned AFTER concrete has hardened. The tendons are placed in ducts (corrugated metal or HDPE) embedded in the concrete; once concrete reaches design strength (usually 25-30 MPa at 7-10 days), the tendons are stressed using hydraulic jacks and locked at end anchorages. Indian Standard IS 1343:2012 governs prestressed concrete design; IS 14268:1995 specifies post-tensioning systems and tendons.

The construction sequence: (1) Form-work and reinforcement placed including PT ducts. (2) Concrete cast and cured. (3) Pre-stressing strand threaded through duct. (4) Hydraulic jack tensions strand to design stress (~70-75% of strand ultimate strength). (5) Anchorage at one or both ends locks the tension. (6) Grout the duct (cement grout) bonding strand to concrete and preventing corrosion. Tendons can be straight (slabs), parabolic (long-span beams), or harped (specialty).

Key design considerations per IS 1343:2012: (a) Initial prestressing force after immediate losses (anchor slip, friction). (b) Effective prestress after long-term losses (creep, shrinkage, strand relaxation) — typically 12-22% reduction over 5-10 years. (c) Flexural design for service and ultimate states. (d) Shear design including prestress contribution. Major Indian PT applications: long-span flat slabs (10-15 m spans), transfer girders, bridge box-girders, water-retaining structures. Specialised contractors: VSL India, BBR India, SLPL.

Where used
  • Long-span flat slabs (10-15 m) in office buildings
  • Transfer girders and beam-and-slab transfer floors
  • Bridge box-girders and segmental construction
  • Pre-stressed concrete water tanks (IS 3370)
  • Beam-and-slab parking-deck construction
Acceptance / threshold
Per IS 1343:2012 + IS 14268: strand certified to IS 6006; jacking force monitored to ±5%; anchor slip within tolerance; grout strength ≥ 25 MPa at 7 days; visual inspection of strand and duct before grouting.
Frequently asked
What is post-tensioning?
Post-tensioning (PT) is a method of pre-stressing concrete where steel tendons are tensioned AFTER concrete has hardened. Tendons placed in ducts embedded in concrete; tensioned via hydraulic jacks once concrete cures; anchored at ends. The concrete is in compression from prestress, allowing longer spans with less cracking. Indian code: IS 1343:2012.
What is the difference between pre-tensioning and post-tensioning?
Pre-tensioning: tendons stressed BEFORE concrete is cast (factory process for precast). Post-tensioning: tendons stressed AFTER concrete has hardened (cast-in-situ process). Pre-tensioning gives better bond and faster repetitive precast; post-tensioning gives more design flexibility for cast-in-situ work and unique geometries.
What are losses in post-tensioning?
Pre-stress losses are reductions from initial jacking force to long-term effective. Immediate: anchor slip 5-10 mm, friction 5-15%, elastic shortening 3-5%. Long-term: concrete creep 5-10%, shrinkage 3-7%, strand relaxation 5-10%. Total loss: 12-22% of initial prestress over 5-10 years. Design uses effective prestress = (1 − loss) × initial.
Related structural terms
Slab Design (RCC)
One-way (Lx/Ly>2) or two-way slab design per IS 456
Beam Design (RCC/Steel)
Flexure + shear design per IS 456 (RCC) or IS 800 (steel)
Column Design (RCC)
Axial + uniaxial/biaxial bending per IS 456
Footing / Foundation
Spread footing, combined, raft, pile per IS 456 + IS 1904
Shear Wall
RCC wall to resist lateral loads (wind/seismic)
Retaining Wall
Wall to retain earth pressure
Staircase Design
Staircase design per IS 456. Riser 150-180mm, tread 250-300mm.
Truss Design
Steel truss for roofs/bridges per IS 800