TJ-144
Heavy Duty Hydraulic Grips
Heavy duty hydraulic wedge grips with serrated inserts bite into a ribbed bar and hold as it necks, which is where mechanical wedges tend to let go.
SpecificationsTesting standard
ISO 15630-1
Steel for the reinforcement and prestressing of concrete — Test methods — Part 1: Reinforcing bars, rods and wire
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 15630-1 is the ISO test method collection for reinforcing bars, rods and wire — tensile, bend, rebend and fatigue. Its most consequential rule is that stresses are calculated on the nominal cross-sectional area derived from the bar's mass per metre, never on the measured area of a ribbed section.
At a glance
- Test type
- Flexure & bend — the specimen is bent
- Published by
- ISO
- Edition
- ISO 15630-1:2019
- Material
- Metals, alloys & welds
- Runs on
- Series 7200 and Series 9000
What the test does
This is a collection of methods rather than a single test. The tensile method grips a full bar section in serrated wedges and loads it at a specified stress rate, reporting yield strength, tensile strength, their ratio and elongation. The bend method takes the bar over a mandrel of specified diameter and examines the outer surface. The rebend method bends it, holds it at an elevated temperature for a specified time, and then bends it back. An axial fatigue method completes the set. The values these are judged against come from a product standard such as ISO 6935, not from here.
What it measures, and why it matters
Strength, ductility and resistance to strain ageing, all on the bar as a product rather than on the steel as a material. The distinguishing convention is the cross-sectional area: it is calculated from the mass of a measured length divided by the density of steel, never measured with callipers. A ribbed bar has no single measurable section — the answer depends on whether the jaws land on a rib — and stresses can shift by several per cent for the same bar. Every specification's strength value assumes the mass-derived nominal area, so departing from it invalidates the comparison. At the mill, where every heat and every rolled size is verified against the product standard before despatch, and at the receiving end, where a contractor or a third-party laboratory tests bar as delivered to site. Reinforcement is one of the few construction materials that is routinely retested after purchase, because the consequence of an under-strength bar is buried in concrete and unrecoverable. The fatigue method is used less often but matters for bridge decks and any structure carrying repeated live load, where the bar sees millions of cycles rather than a single design load.
Bar and area
A ribbed bar has no meaningful measurable cross-section. The nominal area comes from the mass per metre and the density of steel, and every stress in the report rests on it.
- Nominal area
- From mass per metre ÷ 7,85 g/cm³Measuring a ribbed bar with callipers gives a figure that depends on where the jaws land.
- Test length
- Full section, ribs intactMachining a rebar to a smooth gauge section tests the steel, not the product.
- Bend test
- Over a mandrel of specified diameter
- Rebend test
- Bend, age, then bend backThe ageing step is what makes it a strain-ageing test rather than a second bend.
- Fatigue
- Axial, on the full bar section
- Weigh a measured length to get the area
- Rather than measuring the sectionDakIt is faster, more repeatable, and it is what the standard asks for.
Rebar is tested as the product, ribs and all. That is the difference between this and a general metals tensile standard, and it is why the nominal area convention exists.
Test speed
- Stress rate
- As specified, held through yield
- Reported
- Yield, tensile strength, ratio and elongationThe tensile-to-yield ratio matters for seismic design, so both figures are needed, not just strength.
- Rebend ageing
- At the specified temperature and time
- Check where the bar broke
- Away from the gripsDakA fracture in the jaws is a gripping result, not a material one, and the test is void.
Calculations
Nominal cross-sectional areaSn
Sn = m / (ρ × L)
- m
- mass of the sample, g
- ρ
- density of steel, 7,85 g/cm³
- L
- length of the sample, cm
This, not a calliper measurement, is the area every reported stress is divided by.
Tensile-to-yield ratioRm/Re
Rm / Re
- Rm
- tensile strength
- Re
- yield strength
A required figure for seismic grades, because a bar that yields and then immediately fails cannot redistribute load in a frame.
How the test runs
- 01Cut a measured length of bar and weigh it.
- 02Calculate the nominal cross-sectional area from mass, length and density.
- 03Fit serrated wedge grips sized for the bar diameter.
- 04Grip well clear of the intended fracture region.
- 05Fit the extensometer to the bar surface.
- 06Load at the specified stress rate, held through the yield region.
- 07Record yield strength, tensile strength and elongation.
- 08Confirm the fracture is clear of the grips.
- 09Calculate the tensile-to-yield ratio.
- 10For bend testing, bend over the specified mandrel and examine the outer surface.
- 11For rebend, age at the specified temperature and time before bending back.
The fixture this method needs
What the report has to contain
- Reference to ISO 15630-1 and the edition
- Bar designation, grade and nominal diameter
- Mass per metre and the nominal area derived from it
- Stress rate used
- Yield strength and how it was determined
- Tensile strength
- Tensile-to-yield ratio
- Percentage elongation and the gauge length
- Fracture position relative to the grips
- Mandrel diameter and result for any bend or rebend test
What the machine must be capable of
Considerable force — a 32 mm bar in a 600 MPa grade needs well over five hundred kilonewtons, and larger diameters proportionally more. Hydraulic wedge grips with serrated inserts sized for the diameter are effectively required, because the clamping force must hold as the bar necks and mechanical wedges tend to release at exactly the load being recorded. A rebend jig with mandrels of the specified diameters, and a conditioning oven for the ageing step, complete the requirement.
What goes wrong in practice
Measuring the section instead of weighing the bar is the error that quietly shifts every stress in the report. Gripping too close to the expected fracture voids the test and reads low. Reporting tensile strength without yield strength makes the tensile-to-yield ratio uncomputable, which matters because seismic grades specify a minimum for it — a bar that yields and immediately fails cannot redistribute load through a frame. And treating the rebend as a second bend, without the ageing hold, removes the only thing it was measuring.
ISO 15630-1 or ASTM A370
| ISO 15630-1 | ASTM A370 | |
|---|---|---|
| Scope | Reinforcing bar, rod and wire specifically | Steel products generally, with a rebar annex |
| Area | Nominal, from mass per metre | Nominal, from mass per metre |
| Includes | Tensile, bend, rebend, fatigue | Tensile, bend, hardness, impact |
| Paired with | ISO 6935 product standards | ASTM A615 and A706 |
Both use the mass-derived nominal area, which is the convention that matters most. The difference is scope: ISO 15630-1 is written for reinforcement alone and carries the rebend and fatigue methods with it.
Questions we are asked about this test
What is ISO 15630-1?
It is the ISO collection of test methods for steel reinforcing bars, rods and wire — tensile, bend, rebend and axial fatigue. It supplies the methods; the acceptance values come from the product standards such as ISO 6935 or a national specification.
Why is the area calculated from the bar's weight?
Because a ribbed bar has no meaningful measurable cross-section. Callipers give a different answer depending on whether they land on a rib, between ribs, or across the longitudinal rib, and the resulting stress can vary by several per cent for the same bar. Weighing a measured length and dividing by the density of steel gives a nominal area that is repeatable and is what every specification's strength values assume.
Is the bar machined before testing?
No, and this is a deliberate difference from general metals tensile testing. The bar is tested at full section with its ribs intact, because the product being qualified is the bar as it will be cast into concrete. Machining a smooth gauge section would test the steel rather than the reinforcement, and would remove the rib geometry that governs both bond and where cracks start.
What is the rebend test for?
Strain ageing. The bar is bent over a mandrel, held at an elevated temperature for a specified time, and then bent back. The ageing step is the whole point — some steels become brittle after being strained and then aged, which is exactly what happens to a bar that is bent on site and later loaded. A plain bend test cannot reveal that.
Why does the tensile-to-yield ratio matter?
Because it describes what happens after first yield. A bar that yields and then fails almost immediately cannot redistribute load, so a frame designed to shed energy through plastic hinging has nowhere to shed it. Seismic grades therefore specify a minimum ratio, which means both the yield and the tensile figure must be reported, not the strength alone.
What if the bar breaks in the grips?
The test is void. A fracture in the jaws reflects the damage the serrations did to the surface, not the strength of the bar, and it almost always reads low. Gripping well clear of the intended fracture region, with inserts matched to the bar diameter, is what prevents it — and checking the fracture position is part of validating the result, not an afterthought.
What does the machine need to be capable of?
Considerable force. A 32 mm bar in a 600 MPa grade needs well over five hundred kilonewtons, and larger diameters more. It also needs hydraulic grips that hold their clamping force as the bar necks, since mechanical wedges tend to release at exactly the point where the maximum load is being recorded.
Running ISO 15630-1 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | High — a 32 mm bar in a 600 MPa grade needs well over 500 kN | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ISO 7500-1 Class 1 over the working range | ISO 7500-1 Class 0.5 — a class tighter than the method asks |
| Strain measurement | An extensometer of the class the method specifies | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Serrated wedge grips sized for the bar diameter, plus a rebend jig for the bend and rebend tests | Our self-tightening serrated wedge grips, with V-jaws for round specimens, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.