ISO 15630-3 Testing of Prestressing Steel Strands Relaxation, Fatigue, Tensile testing

Information on the most widely used ASTM standards within the materials testing industry

Descriptions of the most prominent lSO standards within the materials testing industry

Standards including EN, DIN, JlS, and more

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ISO 15630-3 Steel for the reinforcement and prestressing of concrete — Test methods — Part 3: Prestressing steel

ISO 15630-3 specifies uniform, repeatable test methods for prestressing steel products: bars, wires, and strands used in prestressed concrete structures. Mainly include tensile test, bend test, reverse bending test, wrapping test, Axial force fatigue test etc.,

ISO 15630-3, ASTM A416 and ASTM A1061 test standards offer guidelines for testing steel strands. Manufacturers of steel strands are required, through product standards, to test their products using both static and dynamic conditions.

Complete List of Tests Stipulated in ISO 15630‑3:

Test	Nature
Tensile test	Core mechanical, (Rm, FP0.1/FP0.2, E, Agt, A, Z)
Bend test	Mechanical / formability
Reverse bend test	Mechanical / wire ductility
Wrapping test	Mechanical / wire surface/ductility
Isothermal stress relaxation test	Time‑dependent mechanical (loss of force at constant strain)
Axial force fatigue test	Cyclic mechanical durability
Stress corrosion test — thiocyanate solution	Environmental‑assisted cracking
Deflected tensile test (strand over mandrel, 20° deviation)	Specialized mechanical (local bending + tension)

Test stipulated in the ISO 15630-3 details introduction:

1, Tension Test (Mechanical Strength)	Determines the basic strength/elastic/ductility performance.
Test Principle	Modulus of elasticity (E); 0.1 % and 0.2 % proof force (Fp0.1, Fp0.2); Maximum force (Fm) → tensile strength (÷ nominal area Sn) Total extension at max force (Agt) — key ductilitymetric for prestressing; Optionally A (elongation after fracture) and Z (reduction of area) Performed generally per ISO 6892‑1 (tensile testing at room temp), with extra rules for prestressing products.
Test Sample Information	Free length sufficient for Agt measurement. Manual gauge marks: 20 mm / 10 mm / 5 mm depending on diameter. Gauge length for A: typically 8× nominal diameter unless product standard specifies otherwise..
Test Equipment	Universal testing machine (UTM); Electronic Universal Testing Machine; Extensometer (ISO 9513);
Test Standard	ISO 6892‑1, ISO 7500‑1, ISO 9513.
2, Bend Test	Evaluate ductility and bending performance under cold bending.
Principle	Bends the prestressing bar/wire over a mandrel to a specified angle and mandrel diameter; checks for unacceptable cracking.
Test Equipment	Universal Testing machine with bending fixtures.Or Bending device with mandrel + supports; may use apparatus per ISO 7438. Mandrel diameter and bend angle set according the product standard.
Test Procedure	Interpret per product standard; if none given → absence of cracks visible to normal/corrected vision = pass. A superficial ductile tear at rib/indentation base is nota failure if tear depth ≤ tear width.
Test standard	ISO 7438 (metallic materials — bend test)
3, Reverse Bend Test	Evaluate ductility and bending performance under cold bending.
Principle	Wire is bent alternately in opposite directions through a specified angle (cyclic bending ductility / surface integrity check).
Test Equipment	Steel wire repeat bending testing machine. (UnitedTest JWJ series)
Test Procedure	For 10 mm < d ≤ 12.5 mm: r = 30±1 mm, h = 125 mm.
Test standard	ISO 7801 (Metallic materials — Wire — Reverse bend test)
4, Wrapping Test	Check winding ductility of fine wire.
Principle	Wire is wrapped around a mandrel (sometimes under tension) to verify it can sustain tight curvature without cracking — a classic ductility/surface‑defect integrity check.
Test Equipment	Steel wire wrapping testing machine (UnitedTest NCR series)
Test standard	ISO 7802 (Metallic materials — Wire — Wrapping test)
5, Isothermal Stress Relaxation Test	Measure force loss over time at constant length (critical for long‑term prestress retention). Prestressed concrete relies on sustained tension. Over time, steel relaxes (force drops at nearly constant strain), causing loss of prestress in the structure. Relaxation class (Low‑Relaxation vs Stress‑Relieved) is a defining product property.
Principle	Measure force decay of a test piece held at constant length (L₀ + ΔL), isothermally (usually 20 °C) :
Test Sample	Must remain straight, free length between grips not mechanically deformed/treated.If F₀ is expressed as % of mean Fm (e.g. 70 % × Fm̄), then two adjacent pieces are reserved for tensile verification of Fm̄.
Test Equipment	Stiff frame loading machine; force device calibrated ISO 7500‑1, accuracy ±1% (≤1000 kN) / ±2% (>1000 kN). Extensometer: gauge length ≥200 mm (≥1000 mm preferred for strands), Class 1 ISO 9513.Anchors to prevent slip; smooth loading device.
Test Procedure	Loading schedule to F₀:Up to 20 % F₀: arbitrary20 → 80 % F₀: continuous or ≥3 uniform steps, ≤ 6 min80 → 100 % F₀: continuous, ≤ 2 minAt F₀: hold 2 min, then define t₀. Strain constancy: ΔL₀ variation ≤ 5×10⁻⁵ L₀ between consecutive force readings.Temperature: specimen at specified temp ±2 °C. Recording: continuous or at standardized intervals (1,2,4,8,15,30,60 min / 2,4,6,24,48,96,120 h… then ≥weekly). Duration: ≥ 120 h minimum; 120 h data may be extrapolated to 1000 h.
6, Axial Force Fatigue Test	Resistance to cyclic axial tensile loading in elastic range (wind, seismic cycles).
Principle	Subject specimen to sinusoidal axial tensile cyclic loading in the elastic range: Run until failure or until a target cycle count specified by the product standard is reached without failure.
Test Sample	Min free length per table; no surface treatment; aligned axially without bending. For strands: all wires equally gripped and loaded.
Test Equipment	Fatigue testing machine calibrated ISO 4965‑1/‑2 or ISO 7500‑1; accuracy ≥±1%.
Test Procedure	Force applied axially, free of bending; strands need equal loading of all wires.No planned interruptions (accidental stops allowed, must be reported). Cycles counted from first full force‑range cycle. Max frequency:Wire/bar: ≤ 120 HzStrand: ≤ 20 HzSpecimen temperature ≤ 40 °C; lab 10–35 °C. Invalid if failure in grip or within 2d of grip, or initiated at an exceptional feature.
7, Deflected Tensile Test	For strands ≥12.5 mm: measure max force reduction when bent 20° around a grooved mandrel (simulate curved‑duct performance).
Test Sample	≥ 12 test pieces2 end pieces → uniaxial tensile to get Fm≥ 10 pieces cut for deflected tensile (need 5 valid results)
Test Equipment	Tensile testing machine; fixed grooved mandrel (tool steel ISO 4957, 58–62 HRC, Ra ≤1.6 μm). Anchors: grip efficiency ≥95% of axial strength; wedge displacement limited.
Test Procedure	Clean mandrel groove before each test; Lay strand so natural curvature follows deviation directionVerify alignment; check no grip slip, Valid only if fracture at mandrel contact.
8, Stress Corrosion Tests	Mechanical‑corrosion interaction; critical for durability
Test details	8.1 Thiocyanate solution stress corrosion testConstant load; immersion in thiocyanate solution at 50±1 °C (wires/strands) or 50±2 °C (bars).Measure time‑to‑fracture; median lifetime from ≥6 specimens. 8.2, Thiocyanate + galvanostatic current (H‑induced cracking)For wires 6.0–12.5 mm; cathodic polarization to accelerate hydrogen uptake. 8.3, Distilled water stress corrosion testHalf‑immersed, circulating distilled water; evaluate pitting, SCC, mass loss, residual tensile properties.

Why ISO 15630‑3 Is Important — Engineering Perspective

Prestressing steel carries extreme sustained load

Typical working stress = 0.6–0.8 Rm. There is almost no ductility margin leftin the structure to absorb surprises — so every material property must be measured under strict, repeatable rules.

Relaxation defines long‑term structural serviceability

Clause 9 is not optional “research”; it directly controls camber loss, deflection growth, and force redistribution over decades. Without a standardized method, supplier data would not be comparable.

Fatigue & deviation loading are real site conditions

Railway bridges, crane girders, and cyclic live loads create millions of elastic‑range cycles (Clause 10); strand saddle deviations (Clause 12) are unavoidable in detailing.

Hydrogen‑assisted cracking is the “silent failure mode”

High‑strength steel + tensile stress + aggressive environment (chlorides, carbonation products, acidic conditions) → catastrophic brittle fracture. Clause 11 (and Annexes B/C) exist precisely to screen this susceptibility.

Global interchangeability

Because ISO 15630‑3 ties test conduct to calibrated machines (ISO 7500‑1), extensometers (ISO 9513), and tensile methodology (ISO 6892‑1), a buyer can compare certificates from labs in different countries on an apples‑to‑apples basis.

Related Standard:

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
GBT 21839	Test methods of steel for prestressing concrete
ISO 6934-3	Steel for the prestressing of concrete - Part 3: quenched and tempered wire
ISO 6934-4	Steel for the prestressing of concrete - Part 4: Strand
EN 10319-1	Metallic materials. Tensile stress relaxation testing. Procedure for testing machines
ASTM E328	Standard Test Methods for Stress Relaxation for Materials and Structures

Key words:

ISO 15630-3 - Mechanical Testing - Dimensional Measurement - Corrosion Tests - Prestressing Steels - Concretes - Reinforcing Materials - Stress Corrosion - Reinforcing Steels - Prestressed Concrete - Construction Materials - Steels - Reinforced Concrete - Modulus of Elasticity - Wires - Bars (Materials) - Bend Testing - Stress Relieving - Fatigue Testing - Tensile Testing

The Hydraulic universal testing machine can perform tension, compression, flexure/bending and shearing test for kinds of specimens. Suitable for metal and nonmetal material, such as iron, steel, steel bar, rebar, cement, concrete, rod and so on. Specially design for production, research, student training and industrial laboratory. Available in wide range of loadframe capacities (300kN to 2000kN)/(10Ton to 200Ton) and with combination of control panels (Digital, Computerized) makes United Test product range suitable for every customer requirement.

UnitedTest Steel strand tensile testing machine can perform tension test for the steel strand, 7-wire strand for the reinforcement and prestressing of concrete, reinforcing bars, wire rod and wires.

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.

Metal bending test fixture used measure the ductility of metallic materials, bent the test specimen to a predetermined angle or until the specimen fractures. Conform with ASTM E290, ISO 7438, and JIS Z2248 standard.

Metal Bending & Rebending Testing Machine is used for bending tests of metal materials such as rods, sheets and rebar for construction, and is loaded with two-way hydraulic cylinders, adjusting the roll spacing by hand wheels and being able to lock itself.

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.

JWJ series metallic materials wire reverse bend testing machine manly used to measure metal wire material, like steel wire, steel bar, steel wire rope, steel strand, nude electrical wire, copper alloy wire etc., rotate the provision radius jaws ±90 degree repeat the reverse bending test; standard machine for 3-10mm (15mm) diameter wire samples, add suitable device, can do 0.3 to 3mm wire.

NCR series metallic materials wire wrapping testing machine used for determining the ability of wire of diameter or thickness 0,1 to 10 mm inclusive to undergo plastic deformation during wrapping. The test consists of winding a wire to a specified number of turns around a mandrel of the diameter specified in the relevant standard to form a closely wrapped helix. It may also include a specified sequence of windings and unwinding, or even rewinding.

Q1: Why is ISO 15630-3 critical for prestressing steel manufacturers, labs and structural engineers?

A: 4 core reasons:

Global test consistency: Uniform specimen prep, equipment calibration, loading rates and failure judgment eliminate lab-to-lab result deviation, enabling cross-border material trading without conflicting test data.

Structural safety guarantee: Prestressing steel operates under 60–75% of its ultimate tensile load long-term. The standard validates key safety metrics (tensile strength, relaxation loss, fatigue resistance, ductility).

Long-term durability verification: Standardized stress corrosion and bend tests screen microstructural weaknesses causing delayed cracking in bridges, dams, and high-rise buildings.

Regulatory & certification compliance: Third-party certification, tender qualification, and international project acceptance all require test reports fully following ISO 15630-3 procedures. Without this standard, design calculations would rely on unreliable, non-comparable test values.

Q2: What is the difference between ISO 15630 Part 1 / Part 2 and Part 3?

A: The ISO 15630 series splits concrete steel test methods by product category:

Part 1: Hot-rolled reinforcing bars (ordinary rebar for reinforced concrete)

Part 2: Welded reinforcing fabric

Part 3: Prestressing steel (high-strength wire, strand, bar for prestressed concrete).

All three share unified testing logic but contain specimen and procedure rules tailored to their respective steel forms.

Q3: When is a tensile test deemed invalid per ISO 15630-3?

A: Two invalid scenarios:

Fracture occurs within 3 mm of the machine grips (the test fails due to jaw stress concentration instead of material intrinsic failure). Exception allowed only if all measured tensile parameters fully meet product standard limits.

Extensometer slips or loses contact during the elastic modulus measurement phase, making E and proof force data untrustworthy. Retest is required for invalid specimens.

Q4: What is the purpose of the isothermal stress relaxation test, and what is the minimum required test duration?

A: Purpose: Measure permanent force loss of prestressing steel held at fixed constant length under standard temperature (default 20 °C). Relaxation loss directly reduces effective prestress force in structures over decades, a critical design input for bridge and long-span beam deflection control.

Minimum test duration: 120 hours (5 days). Extrapolation to 1000-hour relaxation values is permitted only if the lab documents validated extrapolation formulas (log ρ = m log t + n) in the final test report.

Q5: What strict frequency limits apply to the axial fatigue test? Why different limits for wire/bar vs strand?

A: Frequency cap rules:

Wires & bars: max 120 Hz

Strands: max 20 Hz

Reason: Strands consist of multiple twisted individual wires. High cyclic frequency generates excessive inter-wire friction heat, raising specimen temperature above the 40 °C temperature limit and artificially accelerating fatigue fracture, leading to unconservative, inaccurate fatigue life results. Single solid wire/bar has no interlayer friction risk, so higher frequency is allowed.

Q6: What is the deflected tensile test (Clause 12) for, and which product requires it?

A: Exclusive test for strands with nominal diameter ≥12.5 mm. It simulates prestressing strands bent around curved duct mandrels during construction tensioning. It calculates the strength reduction coefficient D (average of 5 valid specimens), which quantifies how much ultimate tensile force drops under 20° bending contact. A test is only valid if fracture initiates exactly at the mandrel contact zone; jaw fracture is discarded.

Q7: What risks occur if prestressing steel skips ISO 15630-3 standardized mechanical testing?

A: Severe engineering risks:

Underestimated relaxation loss leads to excessive long-term beam deflection or slab cracking.

Poor fatigue resistance causes wire fracture under repeated live loads (vehicle traffic, wind vibration).

Insufficient ductility (failed bend/reverse bend test) triggers brittle sudden rupture during tensioning on-site.

Unverified SCC susceptibility leads to delayed strand corrosion fracture years after construction, requiring costly structural repair or replacement.

Q8: For strand fatigue tests, most specimens break near grips—how to comply with ISO 15630-3 validity rules?

A: Follow standard anchorage requirements:

Use wedge grips with tooth length ≥2.5× strand nominal diameter to evenly clamp every individual wire of the strand.

Pre-bed wedges with low pre-load before formal cyclic testing to eliminate early slip.

Ensure pure axial loading with zero bending moment on the specimen free length; misalignment creates localized jaw stress and premature grip fracture. Tests breaking within 2d distance from jaws are marked invalid and retested.

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