REQUIREMENT OF HIGH STRENGTH BARS

Reinforced cement concrete (RCC) today has become an integral part of every structure, be it

a multi-storeyed building, a tunnel, a flyover, a TV tower, a bridge, a nuclear reactor, an

industrial unit or a residential apartment.

With RCC so much coming under use, the engineers are constantly striving to devise ways

and means to reduce its cost and bring economy in construction. Among its constituent

materials, RCC has steel as the costliest material accounting for 30 to 40% of the cost of

RCC. Therefore, the focus has been more on steel than on any other constituent material.

The need for reduction in the steel used for concrete reinforcement has prompted most

countries to switch to reinforcement bars of higher yield strengths of say 500 and 550 MPa

(& even higher).

The direct benefits (to customers) of using high reinforcement bars are:

¶ Reduced total weight of reinforcement bars leading to reduced cost to the customer.

¶ Reduced manpower for reinforcement fabrication.

¶ Lower transportation costs

PROBLEMS ASSOCIATED WITH HIGH STRENGTH BARS

Reinforcement Steel can be in the form of mild steel, medium tensile steel, high tensile steel,

deformed bars, high tensile steel wires, hard-drawn steel wire fabric, twisted steel fabric and

so on but the mild steel and the deformed bars have remained under most common use.

In the last 15-20 years demand for higher strength deformed bars has been increasing. The

most common of these bars is of strength grade Fe 415. Strength grade means the value of

yield stress or the 0.2% proof stress.

You may here recall the definitions of yield stress & proof stress; which are as below :

Yield Stress: Stress (i.e load per unit cross-sectional area) at which elongation first occurs

in the test piece without increasing the load in tensile test. In case of steel with no definite

yield point, proof stress is applicable.

0.2% Proof Stress: The stress at which the non-proportional elongation equal to 0.2% of the

original gauge length takes place.

In India, deformed bars for concrete reinforcement  are being produced for many years.  The

main processes for their production are hot rolling or hot rolling followed by cold twisting.

The increased yield strength was being first achieved by raising carbon & manganese and to

a great extent by cold twisting.

These cold twisted bars, though in extensive use at present, continue to have inherent

problem of inferior ductility, weldability and increased rate of corrosion due to presence of

residual stresses and higher carbon content. Additionally, cold twisting being labour

intensive, enhances cost of production with limitation on production rates.

Increasingly varied & innovative applications have resulted in growing demand for larger

diameter bars with similar strength, elongation, weldability and bendability as the small size

bars. Along with this, there is also a need for these steel bars to be welded and fabricated on

the site easily. For this strength & ductility have to be achieved at the lowest possible carbon

content. The most challenging requirement is to achieve all these superior properties at

relatively lower cost.

TECHNOLOGICAL DEVELOPMENTS IN PRODUCTION

PROCESSES OF HIGH STRENGTH BARS

In recent past, there have been two major technological developments in processes for

production of High Strength Deformed Bars. These are:

(i) Micro-alloying &

(ii) (ii) TMT process; which are discussed below:

Micro-alloying with Nb, V, Ti & B, in combination or individually helps in meeting most of

these requirements and producing High strength reinforcement bars having yield strength of

500 and 550 MPa. Production of reinforcement bars by the addition of micro-alloys gives the

desired results of high strengths but at a cost, which is prohibitive. Moreover, the achieved

ductility is also low.

The need for cutting down the cost of production of high strength reinforcement bars

initiated the involvement of a more economical and competitive process -- the so called

Thermo Mechanical Treatment (TMT) Process. The Thermo Mechanically Treated bars are

generally called TMT bars. 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 use of Thermo Mechanical Treatment process has not only helped produce

reinforcement bars of high yield strength but also having superior ductility, weldability,

bendability & better corrosion resisting properties.

COMPARISON OF DIFFERENT   PROCESSES FOR PRODUCTION OF

HIGH STRENGTH REBARS

Comparative features of the three main processes for production of High Strength Deformed Bars viz

(i) Cold Twisting ,

(ii) Micro-alloying &

(iii) TMT process are tabulated below:

S. No. Production

process

Treatment Production

Costs

Mechanical Properties

Ductility Weld-ability

1 Cold Twisting Cold work

hardening by

means of:

¶Stretching

¶ Twisting

High (Due to

Manpower and

Equipment)

Poor Good

(deteriorating

due to heat

generated by

welding)

2 Micro- Alloying Addition of

alloying

elements as: C,

Mn, V, Nb etc.

High (Due to

alloying

elements and

equipment)

Good Poor (Due to

high carbon

equivalent)

3 TMT Process Rapid cooling

and controlled

cooling from

rolling heat

Low Excellent Excellent (Due

to low carbon

equivalent)

The TMT bars have virtually replaced the conventional cold twisted deformed bars as reinforcement

material all over the world. With the increasing availability of TMT bars here, India is also set to

change over to the use of TMT bars.

TMT BARS MANUFACTURING PROCESS

Under thermo mechanical treatment, the steel bars are passed through a specially designed

water-cooling system (TMT box) where they are kept till the outer surface of the bars

becomes colder while the core remains hot. This creates a temperature gradient in the bars.

After the intensive cooling, the bar is exposed to air and the core re-heats the quenched

surface layer by conduction, therefore tempering the external martensite. When the bars are

taken out of the cooling system, the heat flows from the core to the outer surface, further

tempering of the bars, which helps them attain a higher yield strength. The resulting heat-

treated structure imparts superior strength and toughness to the bars. Cooling process is

illustrated below:

The pre-determined cooling of the bar periphery transforms the peripheral structure to

martensite and then annealed through the heat available at the core. The peripheral and core

temperature difference finally equalises at around 600 0C and the resultant bar structure is of

tempered martensite at the periphery and of fine-grained ferrite-pearlite at the core.

Generally speaking, the resultant soft core forms about 65-75 per cent of the area (depending

upon the desired minimum yield strength) and the rest is the hardened periphery. The

equalizing temperature together with the final rolling temperature is the most important

parameter to achieve the required mechanical properties.

Finally, when the bar is discharged on to the Cooling Beds, the remaining austenite

transforms into a very fine-grained pearlite structure.

The figure below illustrates a typical TMT bar manufacturing process:

After this process of thermo mechanical treatment, a dark etched peripheral rim of tempered

martensite and a grey core of ferrite pearlite get formed. The tempered martensite surface

layer is very hard while the microstructure of the core is a very fine-grained ferrite and

pearlite which is quite soft. The result is a structure with a high yield strength combined with

high ductility.

Microstructure of TMT bar showing peripheral rim of

tempered martensite & core of ferrite pearlite get

formed.

Tempered Martensite Rim

Ferrite-Pearlite core

TMT bars are also known as 'Quenched and Tempered rebars', because of the quenching and

tempering processes involved in making the bars.

The production of quality TMT bar depends on three major factors - quality of raw materials,

a properly designed and automated mill, and a well-designed quenching and tempering

technology.

TMT bars having uniform and concentrated hardened periphery and the softer core will have

the desired tensile strengths coupled with high elongation as required in seismic zones.

Depending on the size and grade, rebars with hardened periphery of about 15 to 30 per cent

of the cross sectional area of the bar are ideal for civil constructions (constructions of houses,

offices, etc.)

TMT Box

Pinion Gear Box

The pinion gear box used here are manufactured by "AMT". These are manufactured by

using the tested steel plates procured from main steel producers of the country and are joined

by automatic welding equipment and stress relieved. The gears and pinions used are

manufactured out of tested engineering quality forged steel and duly machined and tooth cut

and grounded and hardened .The bearings used are double spherical roller bearings and

double oil seals are used to avoid leakage of oil and inbuilt oil lubrication system is fitted for

oil bath of gears and bearings. Air breathers are also fitted to allow vent and intake of fresh

air.

Technical Specifications:

3 Hi and 2 Hi Pinion Gear Boxes, Low Speed and High speed type.

4 Hi Gear Boxes for Cold Rolling Mills.

Speed Increaser cum Pinion gear Boxes.

Gear Boxes suitable for 200 HP to 2000 HP Drives.

Gear Boxes suitable for 150mm PCD to 600mm PCD Mills.

Speed Range 50 R.P.M. to 750 R.P.M.

Salient features:

Torsionally Rigid Steel Fabricated Body designed to Dampen vibrations.

Double Hellical Gears made of En-9, En-19, or En-24 quality Steel Forgings. Duly

Hardened and grounded.

Amply Rated Bearings to work efficiently at Peak Loads.

Double Oil Seals fitted on Shaft extensions.

Lubrication lines for Bearings and Oil Splash system for gears are provided.

ADVANTAGES AND APPLICATIONS OF TMT BARS

Advantages

i. Better Safety of structures:  because of higher Strength combined with

higher Ductility.

ii. Easy working at site:  owing to better Ductility and Bendability. Pre-welded

meshes can be made to eliminate manual binding at site. Reduces

construction and fabrication time.

iii. Resists fire: Unlike Tor steel/ CTD Reinforcement bars, TMT bars have high

thermal stability. They are the preferred choice when elevated temperatures

of 400-6000 C may be encountered (Chimneys, fires).

iv. Resists corrosion: The TMT process gives the bar superior strength and

anti-corrosive properties. Controlled water-cooling prevents the formation of

coarse carbides, which has been cited as the main cause for the corrosive

nature of common bar.

Another reason for better corrosion resistance is the absence of surface

stresses caused by the cold twisting process.

Figure above shows view of Platform during rainy season which clearly shows the difference

between quality of CTD & TMT Bars

v. Formability: Due to very high elongation values and consistent properties

throughout the length of bar, TMT rebars have excellent workability and

bendability.

vi. Earthquake resistance: The soft ferrite-pearlite core enables the bar to bear

dynamic and seismic loading. TMT bars have high fatigue resistance to

Dynamic/ Seismic loads due to its higher ductility quality. This makes them

most suitable for use in earthquake prone areas.

vii. Malleability: TMT bars are most preferred because of their flexible nature

viii. Fine welding features:  TMT rebars (having low carbon content) can be

used for butt and other weld joints without reduction in strength at the weld

joints.

ix. Bonding strength: External ribs running across the entire length of the TMT

bar give superior bonding strength between the bar and the concrete. Fulfils

Bond requirements as per IS: 456/78 and IS: 1786/85.

x. Cost-effective: A high tensile strength and better elongation value gives you

great savings, Reduced Transportation Costs. .

Applications

TMT bars find wide applications in different spheres:

i. General purpose concrete re-enforcement structures

ii. Bridges

iii. Flyovers

iv. Dams

v. High rise buildings

vi. Industrial structures

vii. Concrete roads

viii. Underground structures