Uncoated stress relieved low relaxation seven-ply strand for prestressed concrete -
1995 Edition

The standard mandates that when the strands are initially stressed to 70% of their specified minimum breaking strength (SMBS), the relaxation losses must not exceed 1.8% after 100 hours and 2.5% after 1000 hours. These limits ensure minimal loss of prestress over time, vital for structural longevity. Testing conditions and durations are prescribed in the relevant clauses to verify compliance.

Low relaxation in strands is attained through a hot stretching stabilization process where the strand undergoes tension during stress-relieving heat treatment, inducing linear hardening that enhances creep resistance and reduces relaxation. The relaxation is then measured over a test duration of up to 1000 hours or extrapolated from shorter tests, ensuring relaxation losses remain within defined limits.

The steel wires must conform to maximum limits of 0.050% for sulphur and 0.050% for phosphorus, ensuring the use of high-quality plain carbon steel. The wires are cold drawn and must be free from surface defects. Chemical analyses follow IS 228 parts related to sulfur and phosphorus determination to maintain these standards.

Welding is allowed only in individual wires prior to or during the final heat treatment stage, with a restriction of one butt weld per 45 meters of wire length. No joints or splices are permitted in the finished strand unless explicitly authorized by the purchaser, preserving strand integrity and performance.

Strands must be delivered in reels or reelless packs with a minimum core diameter of 600 mm, as agreed between supplier and purchaser. Each unit must be clearly labeled with the manufacturer’s identification, coil number, nominal strand diameter, and strand class if applicable. Additionally, coils may bear the BIS Standard Mark to certify compliance, facilitating traceability and quality assurance.