Overview
ISO 6934-4:2020 - "Steel for the prestressing of concrete - Part 4: Strand" specifies technical requirements for steel strand used in prestressed concrete. It covers strand that has undergone a stress‑relieving heat treatment in accordance with ISO 6934-1 and defines strand types, dimensions, mechanical properties, manufacturing conditions and delivery requirements. The standard addresses 2-, 3-, 7- and 19-wire strand configurations and covers ten grades of strand for prestressed concrete construction.
Key Topics
- Scope and manufacture
- Applies to stress‑relieved steel strand produced by continuous linear heat treatment and rewound into reels.
- Requirements for compacted strand (7‑wire), finishing, appearance and protection against mechanical damage.
- Strand configurations and lay
- 2‑ and 3‑wire, 7‑wire (including compacted) and 19‑wire (Seale or Warrington patterns) with specified pitch ranges (e.g., 2–3 wire: 12–22 × nominal diameter; 7‑wire: 12–18 × nominal diameter).
- Welds and joints
- No welds permitted in 2‑ or 3‑wire strand; limited welding of individual wires during manufacture of 7‑ and 19‑wire strand only by agreement (butt welds limited to specified frequency).
- Mechanical properties
- Dimensions, mass per length and characteristic tensile forces are tabulated (see Table 1 in the standard).
- Minimum total elongation (Agt) of 3.5%.
- Nominal modulus of elasticity may be taken as 195 GPa.
- Relaxation and durability
- Two relaxation classes (Relax 1 and Relax 2) with maximum relaxation values specified (example: at 70% initial force - Relax 1: 8.0%; Relax 2: 2.5%; at 80% - Relax 1: 12.0%; Relax 2: 4.5%).
- Optional stress corrosion testing and guidance (Annex A) and purchaser can require corrosion‑resistance measures.
- Testing
- Optional fatigue and deflected tensile tests (Annex B) can be required by the purchaser; the standard specifies a stress range of 195 MPa for fatigue testing when requested.
- Delivery
- Reel dimensions, curvature limits to ensure strand is reasonably straight on unreeled, packing and protection per ISO 6934-1.
Applications and Users
- Relevant for manufacturers and suppliers of prestressing strand, precast concrete producers, bridge and building contractors, structural engineers, specifiers and quality assurance teams.
- Used to specify material for tendons, post-tensioning and pre-tensioning systems in bridges, buildings, parking structures and other prestressed concrete works.
- Helpful for procurement, production acceptance testing, design verification and durability assessment in corrosive or fatigue‑sensitive environments.
Related Standards
- ISO 6934-1 - General requirements for steel for prestressing of concrete (normative reference).
- Other parts of the ISO 6934 series detail wires, ropes and related requirements for prestressing steel.
Keywords: ISO 6934-4, steel strand, prestressing of concrete, stress‑relieving heat treatment, 7‑wire strand, 19‑wire strand, relaxation class, fatigue test, stress corrosion, strand configuration.