There are several grades of TMT available as per Bureau of Indian Standards (IS 1786:2008) namely – 1) Fe-415/ Fe-415D, 2) Fe-500/ Fe-500D, 3) Fe-550/ Fe-550D & 4) Fe-600. Where D means Ductile, ie grades with D are having high ductility compared to normal grades. TMT rebars used in high seismic zones must also conform to IS 13920:2002.
The TMT rebars are manufactured through Thermo Mechanically Treatment process. These bars owe their advantage to their composite micro-structure, which is a soft and ductile ferrito-pearlitic core and a tough surface rim of tempered martensite. The outer hard core provides the requisite strength to the TMT bar whereas the soft inner core provides the requisite flexibility leading to a reinforcement that can withstand/ resist the devastating effect of earthquakes.
I) The below exhibit from IS 1786:2008 (Clause 8.1) mentions the Mechanical properties of various grades. You can observe that the strength and % elongation varies from one grade to other.
| S. No. | Property | Fe-415 | Fe-415D | Fe-500 | Fe-500D | Fe-550 | Fe-550D | Fe-600 |
| 1) | Yield Stress(YS), Min. N/mm² | 415 | 415 | 500 | 500 | 550 | 550 | 600 |
| 2) | Elongation % (Ductility) | 14.5 | 18.0 | 12.0 | 16.0 | 10.0 | 14.5 | 10.0 |
| 3) | Ultimate Tensile Strength(UTS), Min., N/mm² | 485 | 500 | 545 | 565 | 585 | 600 | 660 |
| 4) | UTS/YS Ratio, Min. | ≥ 1.10 | ≥ 1.12 | ≥ 1.08 | ≥ 1.10 | ≥ 1.06 | ≥ 1.08 | ≥ 1.06 |
II) As per the revised & amended clause of IS 13920:2002 (Specification on Ductile detailing of Reinforced Concrete Structures subjected to Seismic Forces ) ” The High strength deformed bars produced by the thermo mechanical treatment(TMT) process should have elongation more than 14.5% and this code shall be adopted in all reinforced concrete structures which are located in seismic zone III, IV or V.”
Based on the above two specifications we can be safely draw the inference that normal grades owing to their low % elongation (ductility) are not suitable for building construction in earthquake zones. The construction suitability of the D grades are discussed below –
| Properties/Grade | 415D | 500D | 550D | 600 |
| Flexibility | High | High | Medium | Low |
| Strength | Medium | Good | High | Very High |
| Bendability | High | High | Average | Low |
| Seismic Resistance | – | Excellent | Average | Low |
| Remarks | Discontinued from Market | Suitable for High Seismic Zones | Low Suitability for High Seismic Zones | Not Suitable for High Seismic Zones |
Conclusion: From the above discussion it can be concluded that TMT rebars with Fe500D grade is the most suitable for building construction because it has a balance mechanical property (strength & ductility) with consistent quality & is the best choice for earthquake resistance due to high flexibility.
