TMT Bars

TMT Bars vs “Quenched & Tempered bars”

TMT Bar

Requirement of High Strength Rebars Problems associated with High Strength Rebars Technological developments in production processes of High Strength Rebars
Comparison of different processes for production of High Strength Rebars TMT Bars Manufacturing process TMT bars vs “Quenched & Tempered” bars
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TMT Bars vs “Quenched & Tempered bars”

TMT is an acronym for the phrase ‘thermo-mechanical treatment’.

The Bureau of Indian Standards while issuing the code IS: 1786-1985 (reaffirmed 2004) used this phrase while mentioning that

‘Technological advances during the last few years in the field of deformed bar production have helped in meeting all the above requirements together. Microalloying with Nb, V, Ti and B in combination or individually, and thermomechanical treatment process are worth mentioning in this field’

IS 1786 (reaffirmed 2004) in its clause 3.1.3 specifies that “ The steel bars/wires for concrete reinforcement shall be manufactured by the process of hot-rolling. It may be followed by a suitable method of cooling and/or cold working.”

The technological advances referred to in the IS code must have been (most probably) the two patented “quenching & tempering” processes (viz. ‘Tempcore’ and ‘Thermex’) developed in Europe.

It should be pointed out that these processes are post-rolling operations and no mechanical treatment is involved whatsoever. Instead they obtain the unique properties in the rebars by “quenching and tempering” as explained earlier.

Thus, no mechanical treatment is involved in the technological advances referred to by B.I.S.

However, many major steel firms started vigorous publicity of their “TMT” rebars even though they were NOT manufactured from “Quenching and Tempering” technology which does not involve any mechanical treatment .

Provisions of new IS 1786: 2008 are applicable to “ Hot-rolled steel without subsequent treatment, or to hot-rolled steel with controlled cooling and tempering and to cold-worked steel”. The production process is at the discretion of manufacturer.

Thus the new IS 1786: 2008 is now referring to technology of “Quenching and Tempering” in a relatively more explicit way.

Every rolling mill involves thermal and mechanical treatment. So, even as-rolled bars, which do not undergo any quenching and tempering process, can legally be termed as “TMT” bars. Nothing stops them from claiming this and selling their products as ‘TMT’ bars even when they do not employ any “quenching and tempering” process.

No action can be taken against such mills even though they produce bars that have Yield Strengths of only about 250 N/mm2. A few mills take care not to state the yield strength of their bars and instead merely market them as “TMT” rebars. So, they cannot be held responsible if the civil engineers buy such bars under the mistaken belief that they are purchasing a superior product.

The discerning customer must not blindly just ask for ‘TMT’ bars merely because it is fashionable to do so today or because it is in vogue, so to speak. He cannot and should not assume that he is buying a product superior than the old rusty CTD rebars. All rebars must be purchased based on the properties of yield strength, tensile strength and elongation values.

Many civil engineers, even today, assume that ‘TMT’ bars have yield strength of 415 N/mm2 but better elongation than CTD bars. He should know that nothing in the current laws or regulations prevents the rolling mill to just sell untreated and untwisted deformed bars as TMT barseven though the strength can be as low as only 300 N/mm2.

The basic objective of this major technological advance would be defeated if India takes to the ‘TMT’ rebars as produced in the country today. No major advantage would accrue to the civil engineer if they continue to use Fe 415 grade rebars as defined by IS 1786-1985 by merely choosing ‘TMT’ Fe 415 bars in place of the old CTD Fe 415 bars.

Therefore all civil engineers need to be cautioned about blindly specifying ‘TMT’ bars. They should instead specifically ask for bars in terms of Yield Strength, Stress Ratio, and Elongation.

Enough damage has already been done by use of the label ‘TMT’ and there is now an urgent need to use the correct phrase “Quenching & Tempering” as used globally – or any other suitable phrase which cannot be exploited by persons who do not have the proper technology – if we are to limit further damage.

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References:
Latest developments on steel front” by Jagvir Goyal
Thermex Quenching and Tempering Technology” by R.K. Markan, the Chairman and Managing Director of H&K India
Websites of SAIL, Rajuri steel & Evcon

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