TMT BARS

THERMO MECHANICALLY TREATED[TMT] BARS 2009-2010

1 INTRODUCTION

1.1 Emergence of steel (Dayarathnam P, 2004)

Concrete has been most extensively used in buildings ever since Joseph Aspdin invented and

patented Portland cement, about one hundred sixty years ago. At its face value, concrete is a

simple material close to the natural one. However, it is lot more complex and can be source of

strength or weakness to the user depending on how well the quality assurance is built into it. It is

robust and reasonably durable one, with compressive strength going up to 150 N/mm 2 under

controlled conditions in suitable combinations of the components. However, its tensile strength is

far below the compressive capacity, so reinforcement is required to resist tensile forces.

Laboratory practices have not been able to device a direct method of evaluating the tensile

capacity, even though indirect tests are available to extrapolate the tensile strength. Many

structures are subjected to bending or combined bending compressive forces. Members such as

ties, pipes and tanks that are subjected to direct tensile forces require reinforcement. Steel bars or

wires are embedded in concrete in the tensile zone, thus reinforcing the tension capacity of the

concrete. The reinforcement is not only used to resist tensile forces but also to resist even

compressive force as in the case of struts, columns etc. Steel is equally good in resisting tension

and compression and its strength is measured by yield or proof strength in uniaxial tension test.

Steel being ductile, it also helps in making the brittle concrete a ductile material to a limited

extent depending on detailing of the reinforcement. The mild steel has a percentage elongation of

the order of 24 percent with yield strength of 250 N/mm2. The deformed reinforcement bars

either cold twisted or hot rolled do not have a clear yield point like the one in mild steel,

therefore the strength of such high yield steel is specified by proof strength. The proof strength is

the stress that is measured at a preassigned permanent set, normally taken in the range of 0.2

percent. (Most commonly used high yield strength deformed bars have proof strengths of 415

N/mm2 and 500 N/mm2.)

Steel bars of proof strength 550 N/mm2 are also available in the market and used is limited

extent. Even though steel of strength more than 550 N/mm2 can be produced at reasonable cost,

but not used as reinforcement because of the strain limitations of the concrete. The ultimate

tensile strain in plain concrete is of the order of .00015 and crushing strain is of the order

of .0035.Where as the tensile strain in commonly used high yield strength steel is of the order of

Dept. of Civil Engg. 1


fifteen percent, corresponding to a strain of 0.15. Even though the high strength steel is ductile,

its ductility and strength can not be totally imparted to the brittle concrete without causing

apparent or real damage to the concrete structure as a whole. To avail the advantage of the high

strength of the steel, higher strains in concrete create wide cracks in tension zone and also lead to

crushing under compression. To make the reinforced concrete behave as reasonable

homogeneous material, the embedded reinforcement must be integrated to act along with the

concrete both in compression and tension. Therefore, the bond between the two materials is

equally important to ensure the achievement of the designed strength. That is how the plain bars

in construction were replaced by the deformed bars during sixties. The high strength or even

medium grade concrete must have the high yield deformed bars to ensure integrated action and

eliminate slip between the concrete and steel. The word deformed bar is a misnomer, because

there is really no deformity in the reinforcement in the real sense. The deformation introduced in

the production of the bars aids to enhance the bonding between the two materials and provides an

integrated reinforced concrete.

There are other important points one must pay attention to, regarding reinforcement besides its

strength and bond. The durability of a structure depends on the quality of steel, surface treatment,

and the exposure conditions. Even though concrete provides an alkaline environmental

protection, but in an aggressive exposure condition, the steel reinforcement must be protected

with proper surface treatment of the bars. The level of protection to the reinforcement depends

on the aggressiveness of the exposure. Surface treatment and galvanizing the bars are commonly

used methods besides using the stainless steel reinforcement bars in the most aggressive

exposure condition.

Detailing of reinforcement is another important parameter in establishing the strength and

durability of a structure. The location of the laps and splices, and curtailment of the bars are

important not only from the point of strength but also from durability and constructability angles.

In framed building construction, the beam-column joints are critical, and bar bender has

tendency to provide bar laps and splices at such locations, the percentage of reinforcement is

high and is on both the faces of the members. Providing laps at joints congests the reinforcement

thus resulting in voids or even honeycombing.

Reinforcement is necessary in all reinforced construction not only for strength, but also to

minimize the micro-cracking due to secondary effects such as shrinkage, creep, moisture and



thermal variations. The minor discontinuity in concrete at the reinforcement bar level has a

tendency to change the stress distribution pattern from the assumed simple beam, column and

plate theories. The size of the crack in concrete, either due to bending or tension or due to

secondary effects depends on the diameter of the bar and its location with reference to the

surface of the member. Smaller diameter bars spaced at closure workable spacing are preferred

to large diameters. However, one has to balance between the bar size and volume of the

reinforcement. Just because the reinforcement strengthens the concrete, one should not under the

impression that more reinforcement means more strength. There is always a maximum workable

volume of reinforcement for optimal construction, performance and even for cost. For durability

and good performance, design and construction must take precautions of protection and detailing

of reinforcement bars, in addition to the design considerations.

Concrete structures subjected to dynamic loads such as wind, cyclonic weather and earthquake

undergo repeated reversal stresses. Such loads cause micro cracking and increase the brittleness

of the concrete. Reinforcement is required for protection against cracking of concrete and also to

provide ductility to the structure. Lack of proper reinforcement detailing and enforcement of

quality assurance in buildings have lead to disasters in the recent earthquakes. Therefore the

design and detailing of the reinforcement must be looked into a holistic manner rather than

following simple strength consideration.



2 STEEL AS REINFORCEMENT (Mediratta S.R, 2004)

2.1 INTRODUCTION

The concept of reinforced concrete (RC) was first initiated by Jean Louis Lambot in 1850 and

subsequently, several engineers improvised this to bring about efficient RC members and

structures of today’s standard.

Steel is a versatile material in terms of strength, rigidity, workability, low cost and scope of

application. The per capita consumption of steel is strongly linked with economic development

of a country. The per capita consumption of steel is comparatively low in comparison with that

of the developed world. The average per capita annual consumption of steel is more than 400 kg

in developed countries and about 50 kg in developing countries. But in India, the per capita

consumption is only about 30 kg. Faster infrastructure development will consume more steel as

well as provide sustainable assets. Steel has major application in the areas of construction,

infrastructure building and rural development. Now by realizing the importance of steel

consumption in the national economy, all major countries in the world have been making large

investments for expanding steel capacity and the world steel production has been growing.

Recent statistics shown that in India total steel production is only 3.8 percent of the total world

steel production. However, even this small quantity is difficult to consume and we have to export

2-3 million tones to keep our mills running. It has been estimated that by 2006-07, the total

production of finished steel in India will be 48.7 million tones, whereas the apparent

consumption is expected to be around 42.6 million tones with the need to export excess

production. There is a need to increase steel intensity in structures without increasing the cost so

as to accelerate steel consumption in the country. Steel being the most reliable material, increase

in percentage of steel will increase durability of the structure.

Construction is one of the major steel consuming sectors of steel. There are three common types

of constructions materials-concrete, steel and composite. However, concrete construction uses

two techniques namely reinforced concrete (RC) and prestressed concrete (PSC). In both these

techniques, steel reinforcements or the so called rebars play a very important role, comparable to

the bones in the human body.

Pain concrete has very poor tensile and bending strength. Steel reinforcements in RC and PSC

make up this deficiency.



In spite of this, reinforced concrete, though not suitable, is used for beams, which are normally

subjected to tensile stresses. Often cold twisted deformed (CTD) bars are used as reinforcement,

which are prone to accelerated corrosion. With a view to improve durability and cost

effectiveness, steel-intensive structures are increasingly being built using load bearing beams of

steel with RC slab/floor for buildings or PSC deck for bridges/flyovers by adopting technology

of steel-concrete composite construction.

2.2 STEEL BARS/ RODS FOR REINFORCEMENT

Concrete has high compressive strength but low tensile strength. Therefore, steel reinforcement

used in these zones of a concrete structure that develop tensile stresses due to bending, shear or

even compressive loads as in beams, slabs and long columns. However reinforcement action is

efficient only to the extent of bond strength between the reinforcing steel bars and the

surrounding concrete as well as the end hook anchorages. Because of difference in modules

elasticity of steel and concrete, the latter develops tensile cracks. Major cracks can be avoided by

using ductile mild steel and by improving the local bond all along the bar. The bond is improved

by ensuring a non smooth surface, that is, by providing ribs of certain profile and depth on the

surface of the bars and by using deformed bars with ribs and ridges. Further, well distributed

steel reinforcements help in reducing temperature and shrinkage effects in the concrete.

2.3 TYPES OF REINFORCING BARS (Vishwanatha C.S, 2004)

Fig.2.1 Fig.2.2

a. Plain mild steel bars (Fig.2.1): Have yield strength of 250 N/mm2 and it is used as

reinforcement for the concrete construction industry up to 1965.

b. Square Twisted bars (Fig.2.2): They were brought into market by IRC steels. As these bars

were not efficient enough, they were phased out from the Indian market in very short -period.



Fig.2.3 Fig.2.4

c. Cold twisted bars (Fig.2.3): In 1967, Tor Isteg Steel Corporation of Luxumberg introduced in

India, Cold twisted deformed circular bars of proof stress 420 N/mm2, which is subsequently

became popular as Tor 40 rebars.

d. Hot rolled bars (Fig.2.4): In this strength of about 420 N/mm2 from 250 N/mm2 is achieved

by incorporating alloys like nickel and vanadium in the molten metal, while producing the

billets. The billets are subsequently passed through stands resulting in high strength

deformed hot rolled rebars.

Fig.2.5 Fig.2.6

e. Torkari rebars (Fig.2.5): In contrast to the cold twisting, a new procedure for manufacturing

high strength deformed rebars was developed by Germans, branded as Torkari rebars. It is

possible for production of rebars of proof strength up to 550 N/mm2. The manufacturing

procedure essentially comprises of simultaneous cold reduction of and cold ribbing of wire

rods. This process restricts diameters up to 11 mm.



f. Special reinforcing bars (Fig.2.6): For durability consideration, there is a corrosion resistant

rebars (CRS) from Tata steel.

g. Coated reinforcing bars: The two main categories of factory produced rebars in the Indian

market are, galvanized and epoxy coated rebars. Ex:The galvanized rebars were used in

Bahai’s temple at New Delhi. Epoxy coated rebars are used in bridge structures in coastal

belt around the country.

Fig.2.7

h. Thermo processed (TMT) rebars (Fig.2.7): Between 1980 and 1985, new type of rebars

termed TMT, a Thermo Mechanically Treated bars were developed around the world, for the

benefit of RC constructions. Notable amongst them were Torsid from France, Tempcore

from Belgium and Thermex from East Germany. The first to produce TMT rebars in India

were Tata steel by about 1992 through Tempcore technology. Later Vizag steel came on line

followed by SAIL through technology. In the initial years, the production was restricted to

bars having diameter higher than about 16mm.



3 THERMO-MECHANICALLY TREATED BARS

3.1 GENERAL

Thermo mechanically treated steel known as TMT steel can be described as new-generation-

high-strength steel having superior properties such as weldability, strength, ductility and

bendability meeting highest quality standards at international level. TMT bars possess tough

surface providing high yield strength and a soft core providing excellent ductility. Strength,

weldability and ductility are such properties which declare TMT steel highly economical and

safe for use. An additional advantage of TMT steel is that a twisting operation is included in Tor

steel, which subjects the bars to torsional stresses making them less corrosion resistant while

TMT bars are free of such stresses thus having superior corrosion resistance.

3.2 PROPERTIES (SAIL, www.sail.co.in)

Fire resistance

Ability to resist action of high temperature without appreciable deformation or loss of strength is

termed fire resistance. Ideal structural material should be fire resistant. Because of its high

thermal stability TMT bars can withstand high temperature of the range 500ºC-600ºC, ensuring

fire safety and providing its advantage over HSD bars.

Ductility

Ductile materials accommodate large deformations before failure. This quality distinctly

separates them from materials that are brittle which cannot do so and hence fail suddenly.

Ductile failures are gradual. Structures and elements of ductile materials are amenable for

restoration and rehabilitation. High elongation and ductility has given Elegant TMT bar high

seismic resistance which makes it withstand even in earthquake zones 4&5. TMT bars guarantee

elongation well above 15%.

Corrosion resistance

This is a property by virtue, the material possess resistance to corrosion activities which depends

on the composition and the weathering conditions. With chemical composition as per IS 1786,

Elegant TMT bar is produced by thermo-mechanical treatment and not by cold twisting.

Therefore, there are no torsional residual stresses in the bar.

Bending



TMT Bars exhibit very high bendability and also re-bendability due to lower carbon content and

higher elongation. They can withstand bending and re-bending with internal diameters of 1D and

4D respectively, where D is the diameter of the Bar. It is easier to work with these TMT Bars

due to easy bendability.

Weldability

The materials which do not suffer from loss of strength at the weld joints are said to have

excellent weldabilty. Surprisingly in TMT bars there is no loss of strength due to its low carbon

content and can be easily welded with CTD bars. Further no pre-heating or post-heating is

necessary during welding.

Others

The significance of TMT bars is that it shows a combination of toughness, hardness, excellent

straightness, high strength and high fatigue resistance on dynamic loading.

3.3 INDIAN STANDARD SPECIFICATIONS FOR TMT BARS

The IS-1786 (1985) stipulates the percentages of some of the constituents of TMT bars.

Table3.1 Composition of TMT bars (IS-1786)

Constituent Percent, Maximum TMT(Fe 415/FE

500/Fe 550)

TMT

415

Fe

415

TMT

500

Fe

500

TMT

550

Fe

550

Carbon 0.25 0.3 0.25 0.3 0.25 0.3 0.2

Sulphur 0.05 0.06 0.05 0.055 0.05 0.055 0.045

Phosphorus 0.050 0.06 0.05 0.055 0.05 0.05 0.045

Sulphur and Phosphorus 0.1 0.11 0.1 0.105 0.1 0.1 0.09

Corrosion Resistant

Elements

- - - - - - 0.75 min

Carbon Equivalent - - - - - - 0.53 max



Table3.2 Properties of TMT bars (IS-1786) (Mary land Metrics http.mdmetric.com)

Property Grade

Fe 415 TMT415

Fe500

TMT500

Fe550

TMT550

0.2 percent proof stress/yield stress Min N/mm.sq.

415 415 500 500 550 550

Elongation.percent ,Min,on gauge length 5.65.Square Root(A) where A is the cross-sectional area of the test piece

14.5 22(Up to 28mm dia) 20 (Above 28 mm dia)

12 20(Up to 28mm dia) 18 (Above 26 mm dia)

8 18(Upto 25 mm dia) 16(Above 28 mm dia)

Tensile Strength,Min

10% more than the actual 0.2% proof stress but not less than 48.5 N/mm.sq.

10% more than the actual 0.2% proof stress but not less than 500 N/mm.sq

8% more than actual 0.2% proof stress but not less than 545 N/mm.sq

8% more than actual 0.2% proof stress but not less than 580 N/mm.sq

(Upto 28mm dia)560 N/mm.sq.)(above 28 mm dia)

6% more than the actual0.2% proofstress but not less than 585 N/mm.sq.

(Upto 28mm dia) & 560 N/mm.sq.)(above 28 mm dia)

6% more than the actual0.2% proofstress but not less than 630 N/mm.sq. (Upto 28mm dia) & 610 N/mm.sq.

(above 28 mm dia)

Upto 28mm dia) & 560 N/mm.sq.)(above 28 mm dia)



3.4 THERMO-MECHANICAL TREATEMENT PROCESS

In this process advanced heat treatment techniques such as controlled water quenching are

applied on the red hot rebars as they come out of the rolling mill and the end product is known as

thermo mechanically treated (TMT) rebars. Though this is the standard practice in developed

countries, it has recently been introduced in India and TMT bars manufactured by SAIL, TISCO

and the Vizag Steel Plant are commercially available in the country. Thermo-mechanical

treatment has helped to produce reinforcement bars of high strength, superior ductility,

weldability, bendability and thermal resistance creating a virtual revolution in reinforcement

engineering.

The thermo-mechanical treatment process involves rapid quenching of hot bars through a series

of water jets after the bars come out of the last rolling mill stand Fig.3.1. The short residence

time in the water jacket provides intensive cooling of the surface layer, transforming it into a

hardened structure (Fig.9 & 10). The bars are then cooled in the atmosphere so that the

temperature between the core which is still hot and the cooled surface layer is equalized. The

heat extracted from the core tempers the peripheral hardened structure, while the rebar core cools

down slowly to turn into a ferrite-pearlite aggregate. The strength of the bars is carefully

controlled by optimizing the water pressure for their specific alloy chemistry and bar diameter.

The composite structure of the ductile ferrite-pearlitie core and the tough surface rim of tempered

martensite provide an optimum combination of high strength, ductility and toughness. The

absence of any cold worked structural zone and the specific design of steel chemistry in the form

of tempered martensite layer on the rebar surface, imparts high thermal resistance to the rebars,

even at elevated temperatures of up to 600oC. These rebars are ideal for use in places prone to

fire hazards. They possess excellent bendability due to the unique feature of uniform elongation

and can withstand bending and rebending better than conventional CTD bars. These bars have

very good weldability and do not suffer from loss of strength at the weld joints and can be easily

welded with CTD bars. No pre-heating or post-heating is necessary during welding. TMT rebars

are commercially available in strengths of 415, 500 and 550MPa. Considerable care has to be

exercised in the application of water quenching as improper application can lead to brittle and

hard rebars.



Fig.3.1 Water cooling system

Fig.3.2 Cross-section of a TMT bar

Fig.3.3 TMT bar showing soft core of ferrite-pearlite with strong and tough case of martensite



3.5 COMPARISON AND MERITS OVER OTHER PROCESSES

3.5.1 PROCESSES (Kaushik,S.K. and Singh,B, 2002)

To decide the percentage of carbon content in steel has been a major challenge for the engineers.

While certain minimum carbon content in steel is essential to achieve the required strength, an

excess of carbon content threatens its property of weldability. In TMT bars, this problem has

been eliminated. In these bars, the carbon content can be restricted to 0.2% to attain weldability

and at the same time no strength is lost on this account. The joints can be welded by ordinary

electrodes and no extra precautions are required. SAIL and RINL (Rastriya Ispat Nigam Limited)

are doing a good job in producing TMT bars.

Table3.3 Relative comparison of processes (Hy-Tuf steels, www.hytuf.com)

Process

Result Achieved by Ductility Weldability

1 Hot rolling Alloying elements

used in steel

Ductility impaired at

high strengths

Reduced due to

higher carbon

equivalent

2 Cold Mechanical

Working

Cold twisting

Cold drawing

Greatly reduced due

to cold working

Inconsistent; high

strength affected at

high temperature at

welding

3 Thermo-processing Utilizing the heat of

bars by controlled

cooling

Excellent.

Well above

international

standards

Consistent



Fig.3.4 Stress-strain curve for MS bar

3.5.1.1 Hot rolling

In this process the rolling temperature, ideally should be 900ºC, hence hot rolling, and the

reduction ratio should be a minimum of 1:20 and the rib pattern must be in conformity with the

requirements of IS 1786 so as to provide adequate bond between steel and concrete. The end

result of this process is the production of mild steel (MS) bars with proof strength of 250-MPa.

Nowadays, plain MS bars are less commonly used in reinforced concrete because of their

relatively lower strength, though they cost almost the same as high strength deformed bars.

However, they are frequently employed in practice where nominal reinforcement is called for, as

for example the distribution reinforcement in the case of one way slabs. Low strength is also

preferred in cases where deflections and crack widths need to be controlled or where excessive

ductility is required as in earthquake-resistant design. The bars obtained through this process are

referred to be hot rolled bars (MS bars) and the typical stress-strain diagram of these bars is as

shown in Fig.3.4

3.5.1.2 Cold mechanical working

The process of cold working involves stretching and twisting of mild steel, beyond its yield

plateau, and subsequently releasing the load, as indicated by the thin line in Fig.3.5. The end

product is the familiar cold twisted deformed (CTD) bars. Although stretching and cold twisting

results in a residual strain in the steel, it also results in an increased proof strength. Upon

reloading, the steel follows a linear elastic path (with the same modulus of elasticity, ES as the

original mild steel) up to the point where the unloading started the new raised yield point.



Fig.3.5 Effect of cold working on MS bars

After the yield point, as can be seen from Fig.3.5, the material enters the strain hardening range,

following the path indicated by the thick line in Fig.3.5. It should be noted that although the

process of cold working effectively increases the proof strength of the steel, it also reduces the

ductility in the material. This is perhaps the price to be paid in exchange for higher strengths. It

will be easily appreciated that higher proof strengths for cold-worked bars can be achieved by

suitably selecting the point of unloading in the mild steel strain hardening range and also by

using higher grades of mild steel to begin with. Cold mechanical working is a simple process

involving minimum costs.

Fig.3.6 Stress-strain curve for CTD bar

The bars obtained through this process are referred to be hot rolled bars (MS bars) and the

typical stress-strain diagram of these bars is as shown in Fig.3.6.

3.5.1.3 Thermo processing



Fig.3.7 Stress-strain curve for TMT bar

This process has been discussed in the previous clause.

3.5.2 COST SAVING (Hy-Tuf steels, www.hytuf.com)

The replacement of conventional CTD bars by TMT bars results in saving in terms of weight and

thereby cost. However, the percentage savings will depend upon the type of structural member

and grade of steel. For CTD bars, design strength is 0.87 times fy corresponding to 0.002 strain

as there is no definite yield point. The design strength is further lowered by dividing it by 1.15

and 1.1; the respective factors of safety for material (steel) and eccentricity. Therefore, strength

equal to 0.687fy i.e. 0.687x415=285.105 N/sq.mm only is utilized for CTD bars in axial

compression. In case of TMT grade steel by virtue of definite yield point, design strength in axial

compression is 400 N/sq. mm. This design strength further divided by factors of 1.15 and 1.1,

gives strength of 400/1.15x1.1=316.2 N/sq.mm in place of 285.105 N/sq.mm corresponding to

CTD bars. In other words, an additional strength of 31 N/sq.mm is available if CTD bars are

replaced by TMT bars. This, in turn increases the load carrying capacity of a column of the given

cross-section and concrete grade with TMT bars in place of CTD bars. The following typical

example illustrates that, there are savings of steel by weight in a doubly reinforced beam, as the

definite yield point in TMT bars results in saving of compressed steel as compared to CTD bars.



Table3.4 Economy in using Fe500 & 550 in lieu of Fe 415

Dimension of the section (mm)

Mu

(kN-m) FcuGrade steel

Area of steel

(mm2)No. & dia. of bars

% saving with

respect to Fe415Ast Asc Tension Compr.

300 15

Fe415 2063 960 4-20# + 4-16# 9-12# ---

Fe500 1710 788 4-20# + 3-16# 7-12# 17.3%

Fe550 1545 728 4-20# + 2-16# 7-12# 24.8%

20

Fe415 2130 555 4-20# + 5-16# 5-12# ---

Fe500 1763 465 4-20# + 3-16# 5-12# 17.0%

Fe550 1613 4204-20# + 3-16# 4-12# 24.2%

Where Mu = Ultimate moment of the sectionAst = Area of the tension reinforcement\P = Proportion of tension steel (Ast / bd)Fcu = Characteristic strength of concreteAsc = Area of compression reinforcementP1 = Proportion of compression steel (Asc / bd)

3.6 TEST TO IDENTIFY A GENUINE TMT BARS (Vishwanatha C.S, 2002)

This test helps us for better understanding of the characteristics of TMT bars. The test initially

involves cutting a TMT bar followed by smoothening of the cross section to a fine polished state

by grinder and emery paper. The smooth end of sample is then pickled in nitrol solution (10%

nitric acid with 90% ethyl alcohol). The result shows a uniform tempered martensite periphery

with a softer core. This is the uniqueness to identify a genuine TMT bars.

The figures below show that has been found in various tests.



Fig.3.8 X-section good Q & T rebar:

Fig.3.8 shows the uniform and concentric hardened periphery and the softer core. Such bars will

have desired tensile strengths coupled with high elongation as required in seismic zone.

Fig.3.9 Typical Thermex Bar

In this, uniform tempered martensite periphery is clearly visible. Depending on the size and

grade, the hardened periphery will be about 30% of the bar cross sectional area for good Q&T

rebars. Ideal rebar for civil construction.

Fig.3.10 Indian Thermex X-section



Here we can observe the uniform tempered martensite periphery. Elongation measured was 19%.

A good rebar for civil construction.

Fig.3.11 Over quenched Low Ductility bar

Figure illustrates a highly over-quenched rebar. The hardened periphery is about 60% of the total

cross sectional area. Produced by mill personnel who are not fully trained in quenching and

tempering technology. Such bars will have high yield strength and very poor ductility and should

never be used in civil construction.

Fig.3.12 Non-uniform Quenched bar:

This fig illustrates improper quenching treatment. There is a non-uniform hard periphery

signifying that the quenching has not taken place all round the periphery. Such bars are very

frequently noticed in India in mills that have made their quenching lines. Such bars should be

avoided by civil engineers.



Fig.3.13 Bar by improper Q&T system-Fake TMT

This figure illustrates a bar produced by bad quenching and tempering system. The quenching is

not uniform and the test results will be anybody’s guess. Such bars are produced mainly by ‘hit-

and-trial’ “TMT” technology and system. Such bars should never be used in civil construction as

properties will vary from bar to bar.

Fig.3.14 Typical Thermex Bar-Blue

Figure shows a very uniform cross section of hardened periphery with a soft core. An excellent

rebar for civil construction.

3.7 ADVANTAGES (Mediratta S.R, 2004)

The main advantages of TMT bars are

i. They contain lower carbon content and thus exhibit better ductility and weldability (can be butt

or lap welded).

ii. TMT bars have better yield strength, tensile strength and percent elongation, when compare

to CTD bars of the same grade.



iii. TMT bars display easy bendability and thus require less energy for bending and rebending

along with superior reverse bending properties.

iv. They possess inbuilt ability to resist loss of strength at higher temperatures.

v. TMT bars also available in higher strength levels than those listed in the Indian standard. Use

of Fe 500 grade TMT bars can result in saving of more than 15 percent in steel consumption

when compared to CTD bars.

vi. They display better corrosion resistance than CTD bars due to the absence of cold twisting

process.

vii. Superior product with consistent properties

viii. Easy manufacture of different strengths of rebars from nearly the same steel grades

ix. Better fatigue resistance

x. Easy and less construction time

xi. It is an ideal choice for seismic zones due to excellent ductility properties.

Lastly and most importantly, the Thermex Cooling System is an in-line process and the high

strength Thermex rebars are ready for dispatch to the customer almost immediately after rolling

is complete – unlike the few days taken in the case of CTD rebars which results in large

inventory levels in the mill.

3.8 APPLICATIONS (Mediratta S.R, 2004)

1. TMT Bars are much superior to conventional TOR Steel by virtue of their various engineering

properties and can be used for Residential Buildings, Bridges, and Industrial Establishment

2. In all types of concrete reinforcement purposes like Steel-concrete composite construction

which involves one or more of steel concrete composite elements, that is, composite beam,

composite slab and composite columns.

3. Because of corrosion resistance TMT bars are employed in construction exposed to coastal,

marine or underground environment.eering

properties and can be



4 PRODUCTION

4.1 QUALITY

Basically the desired properties of a thermo processed bar are

i. Minimum Yield strength 500 N/mm2 (or more)

ii. Minimum Tensile strength 10% more than yield strength subject to minimum 560 N/mm2.

iii. Stress Ratio (TS/YS) 1.10(generally 1.15 to 1.25)

iv. Minimum A5 Elongation 16 (generally 18 to 22).

v. Weldability

Basically, TMT is a good quality material if it is manufactured properly. (Dr.Vishwanatha,

2002). According to him the concern is the mills, capability to control the temperature of the bar

that gets into the cooling tube .The temperature of the bar that gets into the cooling tube must be

about 850-900 degrees, plus or minus 30-40 degrees. But if any particular mill cannot control the

temperature of the bar that gets into the cooling tube, then we don’t get a good product. Most of

the secondary sector rerolling mills in the country come in the category of cross country mills,

which are not that sophisticated and that refined as the major plants. In most of these mills, they

are not able to control the temperature of reinforcing bars when it gets into the cooling tube, if at

all they have installed that technology. When the temperature can not be controlled properly, the

product that comes out invariably is non uniform, and it can not satisfy the requirements. So, the

market is flooded with substandard material.

4.2 AVAILABILITY

TMT rebars are commercially available in strengths of 500 and 550 MPa. The brands from the

renowned manufactures like SAIL, TISCO, IISCO, RINL, VIZAG STEEL PLANT etc are

within the purview of desired limits on the chemical composition of steel given in IS 1786.

Various brands of TMT bars available in the market are SAIL TMT, SAIL TMT HCR, and

VIZAG TMT etc. The TMT reinforcing bars are available in almost all sizes: 8mm, 10mm,

12mm, 14mm, 16mm, 18mm, 20mm, 22mm, 25mm, 28mm, 32mm, 36mm, 40mm in standard

lengths & specific lengths up to a length of 20 meters.

As per today’s scenario there is an availability of some international brands namely THERMEX,

TEMPCORE. These bars are being manufactured to have greater strengths and good ductility so



that there is saving in steel coupled with safety. These are by far stiffer than that specified by IS:

1786-1985.

4.3 MARKETING

Production of TMT bars is not simple. It requires huge investments, expertise and stringent

process controls apart from quality inputs. Actual quality of steel gets deteriorated, unless proper

treatments are done. We find untreated re-bars are available in the market and being offered as

TMT bars made by integrated steel plants. These products are having very low physical

properties due to the absence of thermo-mechanical treatment and hence it very dangerous to use

these materials in construction activities.

The main steel plants produce about 1.5 million tones each year of what they strangely call TMT

rebars - this includes the Thermex bar production of SAIL at Durgapur and Bhilai steel plants.

Presumably we are talking Q&T rebars such as Thermex-as explained earlier, every mill in India

is legally making TMT bars. The Q&T rebars will become the preferred choice and it is expected

that by 2010 they will constitute 80 to 85% of the rebars made in the country against the present

50 to 55%. The CTD bars are definitely being phased out.

Some builders because of the lack of knowledge insist on CTD where inconsistency is very high,

because ultimately the strength is imparted by twisting the bar, which is a manual process.

Awareness of TMT bars is the order of the day.

4.4 NEW DEVELOPMENTS

The smaller is the diameter of steel, the greater are the chances of its getting corroded. It is a well

known fact that embedded steel corrodes faster than exposed steel and corrosion occurs mostly

due to chloride ion effect or carbonation. Instead of conventional method of providing metallic

coatings to steel, a change in the metallurgy of steel itself is finding its way now. A new

development on steel front is production of corrosion resistant steel, called CRS. Carbon content

in this steel is restricted to 0.18%, Manganese is absent, silicon is 0.45% and the percentage of

corrosion resistant elements such as chromium is as high as 1.5%... SAIL and TISCO have taken

great strides in this direction.



Further research towards production of fusion bound epoxy coated reinforcement steel and

corrosion resistant low alloy steel is under way. To meet with the increasing demand of electrical

industry SAIL has intensified its efforts to produce sophisticated silicon steel. Interesting results

in this direction are expected soon.

5 DISCUSSIONSA versatile material, steel has played a vital role in the construction industry. With its multitude

of features like superior strength, toughness and ductility, high strength to cost ratio, improved

safety in construction and operation and greater flexibility, it has become the preferred material

in construction. However, the non-availability of the right quality, cost effective steel and

absence of corrosion resistance technology has been one of the major hurdles in its growing

popularity.

The Cold Twisted Deformed (CTD) bars did meet the demand for low-cost, high-strength

reinforced steel bars and were cheap alternatives to costly alloying elements such as Cr, Ni, Ti

etc. But these bars sacrificed two essential properties of weldability and elongation. The search

for a process better than CTD led to the development of TMT bars. With their high-strength,

high-ductility properties, TMT bars overtook the popularity of CTD bars.

However, there is a great concern on non-availability of quality TMT bars. The factors, as per

the TMT bars are considered, are raw materials, rolling mill and treatment process. Bar which is

manufactured without sacrificing any one of the above factors undoubtedly ensures a quality

TMT bar. Moreover, there is hardly any difference in appearance between standard and sub-

standard bars. Due to this there is a misguidance and confusion among clients and users. Hence it

is the responsibility of civil engineers to adopt a detailed examination for requisite properties and

verification of source which carries a paramount importance as per today’s scenario.



REFERENCES1 C.S.Vishwanatha, “History in prospective- A journey through Indian reinforcing bars”,

The Indian concrete journal, Jan 2004, vol.78, pp14-18

2 S.R.Mediratta, “Steel reinforcement-Demand, quality and new application in India”, The

Indian concrete journal, Jan 2004, vol.78, pp.9-13.

3 P.Dayaratnam, “Guest editorial comments”, The Indian concrete journal, Jan 2004,

vol.78, pp.3-4.

4 C.S.Vishwanatha, “The ABC of TMT bars”, The Master builder, Oct-Nov 2004, vol.6,

pp.41

5 HY-TUF STEELS PVT LTD, www.hytuf.com

6 Jagvir Gaoyal, “Latest Developments on steel front”, www.tribuneindia.com

7 Kaushik.S.K. and Singh.B. “Influence of steel-making processes on the quality of

reinforcement”, The Indian concrete journal, July 2002, vol.76, pp.407-412.

8 Prabir C, Basu, Shylamoni P and Roshan A.D, “Characterization of steel reinforcement

for RC structures, An overview and related issues”, The Indian concrete journal, Jan

2004, vol.78, pp.22

9 Mary Land Metrics, http.mdmetric.com

10 SAIL, www.sail.co.in


TMT BARS

Documents

thermo mechanically treated

aggressive exposure condition

epoxy coated rebars

cold twisted deformed

lower carbon content

definite yield point

upto 28mm dia

resist tensile forces