ISO 6934-4 Steel Strand Mechanical Test for the prestressing of concrete

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ISO 6934-4 — Steel for the prestressing of concrete — Part 4: Strand

ISO 6934-4 specifies mandatory requirements for stress-relieved steel strands used in prestressed concrete structures, test include Tensile / Strength & Ductility, Reverse Bend, Bend, Relaxation, Fatigue. It covers 10 grades of steel strands composed of 2, 3, 7 or 19 individual steel wires, including ordinary strands and compacted strands.

ISO 6934-4 Steel Strand Mechanical Test for the prestressing of concrete

Mechanical Tests Stipulated in ISO 6934-4 — The Complete List

Tensile / strength & proof stresses + elongation	Mandatory
Relaxation	Mandatory baseline (70%); 60%/80% if purchaser requests
Fatigue	Conditional — If required by purchaser
Stress corrosion	Conditional — If required
Deflected tensile test	Conditional — If required

Test stipulated in the ISO 6934-4 details introduction:

1, Tensile / strength & proof stress, elongation	Fundamental mechanical test for verifying basic structural and tensile performance of steel strands. Ductility test to evaluate the plastic deformation capacity of strands under ultimate load, critical for anti-brittle fracture performance.
Test Sample Information	Sampling: Full-length finished steel strand taken from production reels; samples shall be free from artificial damage, rust pits and visible cracks. Sample length: Sufficient to install on tensile testing machine (compliant with ISO 15630-3); cut ends shall be secured to prevent wire dispersion.
Test Equipment	Universal testing machine (UTM); Electronic Universal Testing Machine; with high precision force and displacement measurement functions. Special strand clamping fixtures (to prevent wire slippage and damage during tension). Extensometer (high-precision displacement sensor for elongation measurement).
Key Test Stipulations	Cover all strand types: 2-wire, 3-wire, ordinary 7-wire, compacted 7-wire and 19-wire strands. Fixed material parameters: Nominal elastic modulus = 195 GPa; nominal density = 7.86 kg/dm³ (used to calculate mass per length). Core index: Characteristic total elongation at maximum force (Agt). Mandatory requirement: The minimum Agt value for all specified strands shall be 3.5%. Purpose: Ensure the strand undergoes obvious plastic deformation before fracture, avoiding sudden brittle failure in engineering applications.
2, Stress Corrosion Test	Combined environmental-mechanical test to evaluate resistance to stress corrosion cracking (SCC) under tensile stress and corrosive media. This test is optional upon purchaser’s request.
Test Sample	Standard strand samples; surface condition shall be consistent with actual delivery status (no additional anti-rust coating). Samples are not allowed to have pre-existing longitudinal cracks exceeding the standard limit.
Test Equipment	UnitedTest Stress corrosion testing machine. Force measuring device, timing device and constant-temperature control system.
Test Procedure	Mount the sample on the test frame and apply a fixed tensile stress. Place the stressed sample into the sealed test cell filled with standard thiocyanate corrosive solution. Maintain constant temperature and observe continuously until fracture or reach the specified test duration. Record fracture time and crack morphology to evaluate stress corrosion resistance.
Key Stipulations	High-strength prestressing strands are susceptible to SCC and hydrogen embrittlement in corrosive environments (coastal areas, industrial zones, humid environments). Test implementation follows Annex A of ISO 6934-4 and detailed methods in ISO 15630-3 (thiocyanate solution stress corrosion test).
3, Deflected Tensile Test	Special composite mechanical test for evaluating strand performance under multiaxial stress, mainly used for post-tensioned systems and stay cable structures.(Optional)
Test Sample	Complete strand samples; ends are reinforced to prevent local damage during deflection and tension.
Test Equipment	UnitedTest Brand Universal testing machine (UTM) , with Deflected tensile testing fixtures, can apply combined bending and tensile load simultaneously
Test Procedure	Fix the sample to form a specified deflection state (simulate on-site bending conditions). Apply tensile load gradually while maintaining the deflection angle. Record the ultimate load when the strand fails, i.e., deflected tensile strength. Compare with the required indicators to evaluate service performance under complex stress.
4, Stress Relaxation Test	A long-term static mechanical test to measure stress loss of strands under constant sustained load, Key indicator for prestressed concrete (prestress loss directly affects structural bearing capacity).
Test Sample	Sample: Full-section finished strand, no bending or pre-damage. Sample length: Meets the installation requirements of relaxation testing machine (specified in ISO 15630-3). Pre-treatment: Samples shall be stress-relieved in accordance with the standard manufacturing process.
Test Equipment	UnitedTest Tensile stress relaxation testing machine: capable of maintaining constant tensile force for 1000 hours, with temperature control and automatic data logging. Force calibration device; constant-temperature laboratory (to eliminate temperature interference).
Test Procedure	Install the strand sample on the relaxation machine and pre-tension to eliminate assembly gaps. Apply the specified initial load (60%/70%/80% of maximum force) rapidly and stably. Keep the load constant for 1000 consecutive hours under stable ambient temperature. The equipment automatically records stress changes over time. Calculate the final relaxation rate: (Initial stress – Final stress) / Initial stress × 100%. Judge conformity against the limit values of Relax 1 or Relax 2.
Core Test Stipulations	Test condition: 1000-hour sustained load test at room temperature. Standard initial load levels (percentage of characteristic maximum force): 60%, 70%, 80%. The 70% load test is mandatory; 60% and 80% tests are performed when required.
5, Axial Force Fatigue Test	Dynamic mechanical test to evaluate anti-fatigue performance under cyclic alternating stress, applicable to strands in dynamic load structures (bridges, stay cables).
Key Test Stipulations	Load cycle: The strand shall withstand 2 × 10⁶ stress cycles without fracture. Stress parameters: Maximum stress: 70% of the characteristic maximum force of the strand. Fixed stress range for all strand types: 195 MPa. Acceptance rule: No wire breakage, overall fracture or visible damage after 2 million cycles.
Test Sample	Sample: Finished strand without surface defects, cracks or weld defects. Sample length: Adapted to axial fatigue test fixtures (per ISO 15630-3).
Test Equipment	UnitedTest High-frequency fatigue testing machine, high-frequency cyclic loading function, precise stress amplitude control. Dynamic force sensor; dedicated strand fatigue clamping fixtures.
Test Procedure	Fix the strand sample on the fatigue machine, ensuring axial force application (no eccentric load). Set test parameters: maximum stress = 70% of rated maximum force, stress range = 195 MPa. Start cyclic loading and run continuously for 2,000,000 cycles. After the test, inspect the sample for wire breakage, fracture or cracks. No failure means qualified.

Application Industry & Usage Scenarios

The standard is exclusively applied to the civil engineering and construction industry, specifically for prestressed concrete engineering:

Building structures (high-rise buildings, large-span floors)

Bridge engineering (prestressed girder bridges, cable-stayed bridges)

Hydraulic engineering (dams, water pipelines)

Special structures (tunnels, storage tanks, offshore concrete facilities).

Importance of This Standard for Steel Strand Materials

Prestressing steel strand is a high-strength load-bearing component that directly determines the safety, durability and service life of prestressed concrete. ISO 6934-4 establishes unified global technical and testing rules for strand production, inspection and delivery:

Guarantees consistent mechanical performance (strength, ductility, relaxation) to prevent structural collapse under long-term load.

Regulates manufacturing quality (welds, cracks, surface condition, strand configuration) to eliminate hidden defects.

Standardizes test methods to ensure reliable and comparable test results worldwide.

Addresses service risks such as stress corrosion and fatigue failure, adapting strands to complex on-site service environments.

Unifies product designation and delivery requirements for international trade and engineering procurement.

Related Test Standard:

ASTM E328	Standard Test Methods for Stress Relaxation for Materials and Structures
EN 10319-1	Metallic materials. Tensile stress relaxation testing. Procedure for testing machines
ISO 6892-1	Tensile test method (Rm, proof stress, elongation)
ISO 7801	Metallic materials — Wire — Reverse bend test (for ductility evaluation of small-diameter wires).
ISO 10065	Steel bars for reinforcement of concrete — Bend and rebend tests (for bend tests of large-diameter wires).
ASTM A416	Standard Specification for Low-Relaxation, Seven-Wire Steel Strand for Prestressed Concrete
ASTM A1061	Standard Test Methods for Testing Multi-Wire Steel Prestressing Strand
EN 10138-3	Prestressing steels. - Part 3: Bars
XP A 35-045	PRESTRESSING STEELS - PART 1: GENERAL REQUIREMENTS
ISO 6934-3	Steel for the prestressing of concrete - Part 3: quenched and tempered wire
GBT 21839	Test methods of steel for prestressing concrete
GB/T 5224	Steel strand for prestressed concrete
JIS G 3536	Steel wires and strands for prestressed concrete
BS 5896	High tensile steel wire and strand for the prestressing of concrete. Specification
ISO 15630-3	Steel for the reinforcement and prestressing of concrete. Test methods - Part 3: Prestressing steel
AS/NZS 4672.2	Steel prestressing materials - Testing requirements
EN 10138-3	Prestressing steels - Part 3. Bars

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High temperature furnace chamber for tensile testing machine, furnace have a columnar split structure, three stage control function. Mainly consists of furnace, temperature controller, high temperature pull rod, high temperature clamp, high temperature extensometer and supporting device.

Steel Strand Extensometer is a high-precision electronic instrument specifically designed for measuring the deformation (strain) of steel strands, prestressed tendons, and similar elongated specimens during tensile tests.

Computer controlled steel strand tensile stress relaxation testing machine mainly used to check the prestressing steel material relaxation performance. This machine is use mechanical mode loading, computer control to complete the test. PC with control card to realize real time data collection.

UnitedTest Fatigue Testing Machine mainly used at a variety of material and components, parts for both dynamic and static mechanical property testing, include tension, compression, low cycling test, match with high-low temperature chamber can execute the temperature test.

Slow strain rate (SSRT) stress corrosion (SCC) testing machine is used to conduct corrosion tensile tests on materials in corrosive environments, mainly to study the stress corrosion sensitivity of metal materials in corrosive solutions.

Q1: Why is ISO 6934-4 and its associated tests so important for steel strand and prestressed concrete projects?

A: Prestressing steel strand is a core load-bearing component for prestressed concrete structures (bridges, high-rises, dams, tunnels). The standard and its tests are critical for three key reasons:

Structural safety: Unified mechanical test rules ensure strands have stable strength, ductility and fatigue resistance, preventing sudden structural failure under static or dynamic loads.

Long-term durability: Tests for relaxation, stress corrosion and deflection performance control long-term prestress loss and environmental aging risks, extending the service life of concrete structures.

Global consistency & trade: Standardized test methods, product designation and acceptance criteria enable uniform quality evaluation across countries, supporting international engineering procurement and cross-border material trade.

Quality control: Clear manufacturing and inspection rules eliminate inherent defects (unqualified welds, excessive cracks, poor strand straightness) during production.

Q2: What are the core referenced standards of ISO 6934-4:2020? What are their roles?

A: Two mandatory referenced standards support all tests and rules:

ISO 6934-1: Steel for the prestressing of concrete – Part 1: General requirements. It provides universal definitions, basic manufacturing requirements and general acceptance rules for all prestressing steel products.

ISO 15630-3: Steel for the reinforcement and prestressing of concrete – Test methods – Part 3: Prestressing steel. It defines the detailed test procedures, equipment specifications, sample preparation and operation steps for all mechanical, fatigue, stress corrosion and deflected tensile tests required by ISO 6934-4. All test operations must comply with ISO 15630-3.

Q3: Are welds allowed on steel strands per ISO 6934-4?

A: Rules differ by strand type:

2-wire & 3-wire strands: The raw steel wires used to make the strand cannot have any welds at all.

7-wire & 19-wire strands: No full-length strand joints/splices are permitted unless the purchaser explicitly agrees. For individual wires: Welding is only allowed before the final heat treatment. After wire drawing through the first die, no welds are permitted. If agreed by the buyer, one butt weld is allowed per 45 meters of finished strand at maximum.

Q4: What cracks on steel strands are acceptable and what are defects?

A: Longitudinal cracks on single wires with a depth less than 4% of the wire diameter are regarded as normal and not classified as defects. Cracks deeper than this limit or transverse cracks are unqualified.

Q5: What is the minimum elongation requirement for steel strands? Why is elongation test essential?

A: The characteristic total elongation at maximum force (Agt) for all strands shall be no less than 3.5%. The elongation test evaluates ductility: strands with insufficient elongation are prone to brittle fracture under extreme load, which is extremely dangerous for concrete structures. This test is conducted together with the static tensile test on one sample.

Q6: What is stress relaxation test? What are the test conditions and classification rules?

A: Stress relaxation refers to the gradual loss of stress of a material under constant sustained load, which directly causes prestress loss in concrete structures.

Test duration: 1000 hours of continuous constant-load test at room temperature.

Standard load levels: 70% of the characteristic maximum force (mandatory test); 60% and 80% load tests are only conducted when the purchaser requests them.

Two relaxation classes and maximum allowable relaxation rate:

Initial Load	Relax 1 (Max Relaxation)	Relax 2 (Max Relaxation)
60%	4.5%	1.0%
70%	8.0%	2.5%
80%	12.0%	4.5%

Relaxation class must be clearly marked in the product designation.

Q7: What is the purpose of the stress corrosion test? When is it required?

A: High-strength steel strands are vulnerable to stress corrosion cracking (SCC) and hydrogen embrittlement in corrosive environments (coastal areas, industrial zones, humid environments). This test evaluates the strand’s resistance to combined tensile stress and corrosive media. It is an optional test upon the purchaser’s request, and the test methods follow Annex A of ISO 6934-4 and ISO 15630-3. Buyers can also put forward additional anti-corrosion requirements based on actual service environments.

Q8: What is the deflected tensile test? Which scenarios need this test?

A: It is a composite mechanical test that simulates the multiaxial stress state (bending + tension) of strands in actual engineering. It mainly applies to post-tensioned concrete systems, bridge stay cables and other components where strands bear both tension and bending. This test is also optional, with test methods specified in Annex B of standard and ISO 15630-3.

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