STRUCTURAL

Post-Tensioning (PT)

Steel tendons stressed AFTER concrete hardens via hydraulic jacks. Used for long-span slabs and bridges.

Also calledpost tensioningpt slabunbonded ptbonded pt
Related on InfraLens
CODES
IS 1343IS 14268
Definition

Post-tensioning (PT) is a method of pre-stressing concrete where steel tendons are tensioned AFTER the concrete has hardened, using hydraulic jacks to stress the tendons against the concrete itself. Distinguished from pre-tensioning (where tendons are stressed before concrete is cast — used for precast members). PT is the dominant prestressing method for cast-in-situ concrete in India, especially for long-span slabs, transfer girders, and bridge superstructures. Indian Standard IS 1343:2012 governs prestressed concrete design; IS 14268:1995 specifies post-tensioning systems and tendons; IS 6006 covers prestressing strand specifications.

The construction sequence: (1) Form-work and reinforcement placed including PT ducts (corrugated metal or HDPE tubing) embedded along the desired tendon profile; (2) Concrete cast and cured to ≥ 25 MPa (typically 7-10 days for M40 mix); (3) Pre-stressing strand threaded through the duct; (4) Hydraulic jack tensions the strand to design stress (~70-75% of strand ultimate strength = 1300-1450 MPa for 7-wire 12.7 mm strand); (5) Anchorage at one or both ends locks the tension; (6) Grout or wax-fill the duct to bond the strand and prevent corrosion. Tendons can be arranged with parabolic profiles (typical for long-span beams), straight profiles (typical for slabs), or harped profiles (for special applications).

Design per IS 1343:2012 covers: (a) initial pre-stressing force after immediate losses (anchor slip, friction); (b) effective pre-stress after long-term losses (creep, shrinkage, relaxation) — typically 12-22% reduction over 5-10 years; (c) flexural design for service and ultimate states; (d) shear design including the contribution of pre-stress tendons to shear capacity. Major Indian PT applications: long-span flat slabs in office buildings (10-15 m spans without intermediate columns), transfer girders and pile caps, bridge box-girders, water-retaining structures, pre-stressed concrete tank walls. Specialised contractors: VSL India, BBR India, SLPL.

Typical values
Strand diameter (7-wire)9.5 mm, 12.7 mm, 15.2 mm common
Tendon ultimate strength fpu1860 MPa (Grade 270)
Initial pre-stress (jacking)0.70-0.75 fpu = 1300-1450 MPa
Effective long-term pre-stress0.55-0.65 fpu after losses
Friction loss (parabolic profile)5-15% depending on span and curvature
Anchor slip loss5-10 mm typical → 30-50 MPa for short beams
Creep + shrinkage loss100-150 MPa over service life
Where used
  • Long-span flat slabs in office buildings (10-15 m spans)
  • 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 + IS 14268: strand certified to IS 6006 + supplier MTC; jacking force monitored to ±5%; anchor slip measured and accepted within design tolerance; grout strength ≥ 25 MPa at 7 days; visual inspection of strand and duct before grouting.
Site example
Site reality: a Hyderabad office tower used PT in 12 m span flat slabs. The grouting contractor used over-thinned grout (water-cement ratio 0.55 instead of 0.40), causing voids in the duct. Within 3 years, corrosion was detected on extracted tendon samples. Remediation: extensive grout-injection of the existing voids, plus monitoring program. ₹1.2 cr cost of remediation; original grouting properly done would have cost ₹4 lakh. Grouting quality is the most-overlooked aspect of PT construction; specialist grouting crews are essential.
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, using hydraulic jacks. Tendons are placed in ducts during casting; after concrete cures, the tendons are stressed and anchored at the ends. The concrete is in compression from the pre-stress, allowing it to span longer distances with less cracking. Indian code: IS 1343:2012.
What is the difference between pre-tensioning and post-tensioning?
Pre-tensioning: tendons are stressed BEFORE concrete is cast (factory process for precast elements). Post-tensioning: tendons are stressed AFTER concrete has hardened (cast-in-situ process). Pre-tensioning gives better bond and is faster for repetitive precast; post-tensioning gives more design flexibility for cast-in-situ work and unique geometries. Pre-tensioning is dominant for precast pre-stressed members (sleepers, beams); post-tensioning is dominant for cast-in-situ slabs, bridges, and long-span structures.
What are losses in post-tensioning?
Pre-stress losses are reductions in tendon force from initial jacking to long-term effective. (1) Immediate: anchor slip 5-10 mm, friction 5-15% (depending on profile and span), elastic shortening of concrete 3-5%. (2) Long-term: creep of concrete 5-10%, shrinkage of concrete 3-7%, relaxation of strand 5-10%. Total loss: 12-22% of initial pre-stress over 5-10 years. Design accounts for losses by using effective pre-stress = (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