Thermo-mechanically treated (TMT) bars are the backbone of reinforced concrete construction in India. The standard that governs high-strength deformed steel bars for concrete reinforcement is IS 1786. Steel producers test bars to certify each heat, and construction materials labs test samples from site to confirm that the steel delivered matches the grade ordered. This guide explains what is tested and how the tests are run on a universal testing machine.
What IS 1786 covers
IS 1786 specifies requirements for deformed steel bars and wires, including chemical composition, mass per metre, surface geometry (ribs), and mechanical properties for each strength grade. Grades are named by their minimum characteristic yield or 0.2% proof strength, such as Fe 415, Fe 500, Fe 550 and Fe 600, with D and S variants that carry additional ductility requirements. The required values differ by grade, so always check them against the current edition of the standard rather than relying on memory.
Tensile test
The tensile test of reinforcement bar is carried out to the method of IS 1608, usually on the full bar section without machining. The results checked against IS 1786 typically include:
- 0.2% proof stress or yield stress
- Tensile strength
- Ratio of tensile strength to proof stress, an indicator of the bar's capacity to strain-harden
- Percentage elongation on the specified gauge length
- Total elongation at maximum force for grades where it is specified
Because the bar is tested in its as-rolled form, the cross-sectional area is not measured directly from the ribbed surface. It is usually calculated from the mass per metre of the bar, which is weighed on a measured length. This is an important step: an error in area translates directly into an error in strength.
Gripping ribbed bars
Ribbed TMT bars are hard on the surface and the ribs make gripping more demanding than for smooth round specimens. Labs commonly use wedge grips with serrated jaw inserts sized for the bar diameter range. Hydraulic grips help by applying a consistent clamping force, reducing slippage and jaw breaks, and they make testing faster when many bars are tested in a day. See UTM with hydraulic grips for this kind of set-up.
Other practical points:
- Cut bars to a length that gives enough free length between the grips for the gauge length and grip engagement.
- Check that jaw inserts are clean and not worn; worn serrations cause slip.
- Machine capacity must exceed the maximum force expected for the largest diameter and highest grade you test, with a margin. Share your bar sizes and grades with the supplier when sizing a machine.
A front-open crosshead UTM makes loading long bars quicker because the specimen can be placed into the grips from the front rather than threaded from above.
Bend and rebend tests
Ductility of the bar is also checked by bending:
- Bend test: the bar is bent around a mandrel of specified diameter (a multiple of the bar diameter, depending on grade and size) through a specified angle. It must show no transverse cracks on the outside of the bend.
- Rebend test: the bar is bent to a specified angle, aged (for example by heating in boiling water for a set time and cooling), then bent back by a specified angle. Again no cracks are allowed. The rebend test checks that bars can be straightened after bending on site without becoming brittle.
These tests can be carried out on a UTM with a bend fixture or on dedicated bend test equipment. Mandrels for each diameter and grade combination are needed.
Other checks
| Check | Purpose |
|---|---|
| Mass per metre | Confirms nominal size and is used to calculate the effective area |
| Rib geometry | Confirms bond characteristics with concrete |
| Chemical analysis | Confirms composition limits for the grade |
Lab set-up and record keeping
A rebar testing lab typically needs a UTM of suitable capacity, grips for the full diameter range, a bend and rebend fixture with mandrels, a weighing balance and length measurement for mass per metre, and a water bath for ageing in rebend tests. The machine should be calibrated to IS 1828 (Part 1) at the interval your quality system requires.
Records should identify the heat or batch, the bar diameter and grade, the mass per metre and calculated area, all tensile results, the bend and rebend outcomes, the date and the operator. Keeping the force-extension curve from a computerized machine is valuable when a result is disputed.
Browse the machines for testing metals for related equipment.
Retests
When a test result fails, the standard sets out the conditions for retesting, such as testing additional samples from the same lot, and how the lot is judged on the retest results. Check whether the failure was caused by a testing fault, such as a jaw break or slip, before treating it as a material failure.
Frequently asked questions
What UTM capacity do I need for TMT bars?
It depends on your largest bar diameter and highest grade. Calculate the maximum expected force from the area and the tensile strength, then add a margin. Send us your bar range and we can suggest a suitable capacity.
Why is area calculated from mass per metre?
The ribbed surface makes direct measurement of diameter unreliable. Weighing a measured length and dividing by the density of steel gives the effective cross-sectional area.
Why do bars slip in the grips?
Worn or dirty jaw inserts, the wrong insert size or insufficient grip length are the usual causes. Hydraulic grips reduce slippage by keeping clamping pressure consistent.
Request a quote
Akuracy (Ratnakar Enterprises) can advise on a configuration for your materials, standards and test volume. Share your specimen details and the standard you test to, and our team will reply with a suitable option. Request a quote online or message us on WhatsApp at +91 94038 91579.