TMT steel:
grades and checks.

Reinforcement is the part of a building nobody sees once the concrete is poured, which is exactly why it is worth understanding before it is bought. Nearly all reinforcement used in India today is TMT bar, and the grade marked on it carries two separate promises: how strong it is, and how far it can stretch before it breaks.

What TMT means

Thermo-mechanically treated bar is quenched with water as it leaves the rolling mill, which hardens the outer layer while the hot core tempers it from inside. The result is a hard outer case around a softer, more ductile core. That structure gives a TMT bar its mix of strength and bendability - and it is why a TMT bar should never be heated on site to make it easier to bend. Heating undoes the treatment.

Reading the grade

Reinforcement bars in India are made to IS 1786. The number in the grade is the minimum yield strength in newtons per square millimetre: Fe 415, Fe 500, Fe 550 and Fe 600. Fe 500 is the most common grade in building work today.

1. The plain grade Fe 500 guarantees the yield strength and a minimum elongation - how far the bar stretches before it breaks.
2. The D grade Fe 500D has the same minimum strength but tighter limits on carbon, sulphur and phosphorus, a higher minimum elongation, and a larger margin between the stress at which it yields and the stress at which it breaks. In plain terms, it bends and stretches further before it snaps.
3. The S grade IS 1786 also defines S grades, with tighter requirements still, for structures where the designer calls for them.

Higher strength is not automatically better. Fe 550 and Fe 600 are stronger but less ductile, and a structure is designed around the grade it specifies - substituting another grade on site is a design change.

Why the D matters

In an earthquake, a well-designed concrete building is meant to bend and crack in a controlled way rather than collapse, and it is the reinforcement that does the bending. Steel that stretches further before breaking gives the structure more warning and more capacity to absorb the movement.

Surat lies in seismic zone III under IS 1893. Earthquake-resistant detailing of concrete buildings is set out in IS 13920, and structural engineers commonly specify a D grade for this reason. Whatever grade is on the structural drawings is the grade that should arrive on site.

Checks when steel arrives

1. The BIS mark Bars made under IS 1786 carry the ISI mark, and the documents should show the manufacturer's BIS licence number. Steel without it has no standard behind it.
2. Rolled-in markings Branded bars carry the maker's mark and the grade rolled into the bar at intervals. The grade on the bar should match the grade on the bill and on the drawing.
3. The test certificate Each lot comes with a mill test certificate giving its chemical composition and its tested strength and elongation. It should belong to the lot delivered, not be a general sheet.
4. Weight per metre A quick check needing only a scale and a tape: the standard weight of a bar in kilograms per metre is its diameter in millimetres squared, divided by 162. Bars noticeably lighter than that are thinner than their label, and the difference is steel the structure was designed to have.
5. Surface condition A thin, tight layer of mill scale or light surface rust is normal. Loose, flaking rust or pitting is not, and that steel should not go into the work.
6. A bend test A short sample bent around a pin should show no cracks on the outside of the bend. It is a simple test that sorts out brittle steel.

Storing and working it

• Stack steel off the ground on supports, separated by diameter and grade, and covered from rain. Steel lying in mud through the monsoon is steel being prepared for corrosion.
• Keep each lot identified with its certificate until it is used, so a test result can be traced to the bars it describes.
• Bend bars cold, around a mandrel of the right diameter. Never heat TMT bars to bend them, and never straighten and re-bend the same bar.
• Do not mix grades in the same member. If two grades are on site, keep them apart and labelled.
• Cut and bend to the bar bending schedule, and check cut lengths and laps against it before the cage is tied.

Very little of this is technical. It is the discipline of receiving steel as carefully as it was specified, and it decides whether the grade on the drawing is the grade in the building.