IS 14268 : 1995Uncoated stress relieved low relaxation seven-ply strand for prestressed concrete -

IS 14268:1995 is the specification for uncoated stress-relieved low-relaxation seven-ply (7-wire) strand for prestressed concrete — the dominant prestressing tendon used in post-tensioned and pre-tensioned PSC: bridges, long-span floors, transfer girders, tanks and prestressed precast. 'Low relaxation' is the property that protects the long-term prestress force.

It sits in the prestressing stack:

• IS 1343 — prestressed concrete design (the design basis) · IS 8543 — bonded post-tensioning construction (stressing/grouting)
• IS 6003 indented wire / IS 1785 Part 1 wire (other tendon forms) · IS 10790 Part 1 — sampling of prestressing steel · IS 456 — concrete

A prestressing tendon is stressed to a very high proportion of its strength and must hold that force for the life of the structure. IS 14268 controls the properties that decide whether it does:

• Low relaxation — under sustained high stress, steel slowly loses force (stress relaxation); low-relaxation strand loses far less than normal-relaxation, so the long-term prestress loss is much smaller and more predictable (a primary input to the IS 1343 loss calculation)
• High tensile strength & specified 0.1%/0.2% proof stress, elongation/ductility, and a defined stress-strain curve (modulus) — the design relies on all of these
• Seven-wire geometry (six wires helically around a king wire) — bond, duct/anchorage compatibility
• Stress-relieved condition — dimensional/force stability

The engineering point: prestress losses (relaxation + creep + shrinkage + friction + anchorage) directly reduce the effective force the structure was designed for — and relaxation is the strand-property part of that loss. Using normal-relaxation strand where low-relaxation was assumed, or unverified strand, builds an under-prestressed structure whose deflection and cracking behaviour differ from design.

Scenario: tendons for a post-tensioned bridge/transfer girder designed to IS 1343.

Step 1 — design force & losses: IS 1343 sets the jacking force and the loss budget, assuming low-relaxation strand — relaxation loss is taken from the IS 14268 class.

Step 2 — specify IS 14268 strand: correct nominal diameter/grade, low-relaxation, with the specified breaking load, proof stress, elongation and relaxation class.

Step 3 — verify by test: sample per IS 10790 Part 1; test breaking load, proof stress, elongation (and relaxation as required) — *do not* accept on the coil tag alone.

Step 4 — handle & stress: protect from corrosion/nicks (a notched tendon fails at far below its strength); stress and grout per IS 8543 with elongation reconciliation.

Step 5 — accept the tendon system per IS 1343/IS 8543.

Low-relaxation, verified, undamaged strand delivers the design prestress for the structure's life; substituted or damaged strand silently under-prestresses it.

1. Substituting normal-relaxation for low-relaxation strand. Much larger long-term prestress loss than the IS 1343 design assumed — an under-prestressed structure.

2. Accepting on the coil tag, not by test. Prestressing steel is too critical — sample/test per IS 10790 Part 1.

3. Nicked/corroded/over-bent strand. High-stress tendons are notch- and corrosion-sensitive; surface damage causes failure far below breaking load.

4. Ignoring the relaxation class in loss calc. Relaxation is a defined input to prestress-loss — wrong class = wrong effective force.

5. Poor storage. Rust/pitting on the construction site degrades tendon capacity before it is even stressed.

• IS 1343 — prestressed concrete design (loss calc uses the relaxation class) · IS 8543 — bonded post-tensioning construction
• IS 6003 — indented PSC wire · IS 1785 Part 1 / Part 2 — plain hard-drawn PSC wire · IS 6006 — stress-relieved strand
• IS 10790 Part 1 — sampling of prestressing steel · IS 1608 Part 1 — tensile test · IS 456 — concrete

IS 14268 is reaffirmed and governs the most-used modern prestressing tendon — uncoated stress-relieved low-relaxation 7-ply strand — and the single concept that matters is that a prestressed structure is only as good as the prestress force it actually retains. Relaxation is the strand-property component of long-term prestress loss, and the IS 1343 design is built on the low-relaxation class; substituting normal-relaxation strand, or unverified/damaged strand, silently under-prestresses the structure and changes its deflection and cracking. Prestressing steel is the one material where coil-tag acceptance is unacceptable — sample and test per IS 10790 Part 1, protect it absolutely from nicks and corrosion (it is highly notch-sensitive at its working stress), and stress/grout it per IS 8543. Get the strand class, verification and handling right and the prestress lasts the structure's life; get any of them wrong and the deficiency is invisible until deflection or cracking reveals it.