Use of Steel TMT Bars as Reinforcement | Different TMT Rebar Grades | Manufacturing & Testing

TMT Bars and Steel Cooling Bed - YouTube - Robin Yan

TMT Bars | TMT Reinforcement Steel Bars

Steel has been in the construction industry for a long time for its terrific properties such as Tensile strength, thermal conductivity, yield strength, and ductility. The role of steel in any building determines its structural strength and stability. Hence choosing the correct steel with desired properties is of utmost importance. In present days, TMT bars have gained an increased popularity in the construction industry.

TMT bar basically stands for Thermo Mechanically Treated bar. TMT is a process in which the steel bars are subjected to constant and repeated cooling to attain maximum yield strength. They are not just steel bars with enhanced strength but are also highly ductile and malleable in nature. They are specifically designed for earthquake resistant buildings and additionally possess high thermal resistance.

TMT Bars of various diameters

TMT Bars of various diameters

Grades of TMT Bars

There are various grades of TMT bars available in the market. The grade represents the strength and rigidity of the bar, specifically the yield strength of reinforcement. It is very necessary to choose the right grade, depending upon the type of structure. During the structural design of the structure, the designer selects the appropriate based on the material availability and cost considerations. Most widely used steel grades are Fe415 and Fe500 which provides sufficient strengths for most of the reinforced concrete applications.

TMT Bars and Steel Cooling Bed - YouTube - Robin Yan

TMT Bars and Steel Cooling Bed – YouTube – Robin Yan

These are even widely used for bridge construction projects. Unless there is a requirement to achieve lower rebar ratio, to prevent reinforcement congestion or special strength requirement in design, normal grades perform well in regular applications. The following TMT rebar grades are available:

Fe 415

TMT bars having the yield strength of 415 N/mm2 are commonly used for the construction of houses and residential structures. They have high uniform elongation which makes them earthquake resistant. Hence they are perfect to build homes in earthquake prone areas.

Chemical Composition

Carbon0.3
Sulphur0.06
Phosphorous0.06
Sulphur + Phosphorous0.11

Mechanical Properties

Yield strength (N/mm2)415
Ultimate Tensile Strength (N/mm2)485
Elongation (%)14.5
YS Ratio1.16

Fe 500

The highlighting property of a TMT bar having a yield strength of 500 N/mm2 is its resistance to corrosion. They are best suitable for multiple storey buildings and commercial buildings.

Chemical Composition

Carbon0.3
Sulphur0.055
Phosphorous0.055
Sulphur + Phosphorous0.105

Mechanical Properties

Yield strength (N/mm2)500
Ultimate Tensile Strength (N/mm2)545
Elongation (%)12
YS Ratio1.09

Fe 550

Their properties are pretty much similar to that of Fe 500. Except they have enhanced yield strength which is slightly more than Fe 500. They are most suitable for slightly large scale projects such as bridges and heavy underground structures.

Chemical Composition

Carbon0.3
Sulphur0.06
Phosphorous0.06
Sulphur + Phosphorous0.11

 

Mechanical Properties

Yield strength (N/mm2)550
Ultimate Tensile Strength (N/mm2)585
Elongation (%)10
YS Ratio1.06

Fe 600

TMT bars having the yield strength of 600 N/mm2 possess the highest strength available in that category. They are mostly used in structures which require high tensile and yield strength. Fe 600 TMT bars are used for large scale industrial construction projects.

Chemical Composition

Carbon0.25
Sulphur0.04
Phosphorous0.04
Sulphur + Phosphorous0.75

Mechanical Properties

Yield strength (N/mm2)600
Ultimate Tensile Strength (N/mm2)660
Elongation (%)10
YS Ratio1.1

Manufacturing process of TMT Bars

First, carefully selected high quality raw materials are charged into the furnace and heated up to 1600o C. The heated steel is then treated with an inert gas to release any dissolved gasses present in the metal.  The liquid steel is cast into steel billets which is apparently the most modern method of casting steel. These billets are then subjected to preheating and are re-sized according to the required diameter. The further manufacturing process of TMT bars involves 3 distinct steps which are as follows:

Quenching

In this step, the finished diameter bars are treated to give the desired properties. The red hot steel rod is sent to a chamber where water is continuously sprayed onto the incoming bar. This results a rapid change in temperature in the outer layer of the steel rod whereas the temperature on the inner core remains the same.

Quenching Process of TMT bars (Source- YouTube-Isteel Forever)

Quenching Process of TMT bars (Source- YouTube-Isteel Forever)

Tempering

The temperature of the outer layer suddenly decreases from 900oC to 280oC and the temperature on the inner core of the bar remains the same. Therefore the inner layer tempers the outer layer. The bar now reaches an equalization temperature of 575oC – 625oC depending upon the grade of steel chosen.

Cooling

The bars are then cut according to the desired length and sent to a cooling bed to attain the equalization temperature. This process results in the formation of a hard outer layer which keep the bar strong and a soft inner layer to provide ductility to the bar.

TMT bars being cooled on cooling bed (Source-YouTube-Isteel Forever)

TMT bars being cooled on cooling bed (Source-YouTube-Isteel Forever)

Testing of TMT Bars

Once the TMT bar of desired grade and diameter is manufactured, it is subjected to finishing to achieve a fine look. Various tests are conducted to check the quality of the bar. There are quite a number of TMT bar manufacturing companies out in the market. But not all of them provide the best quality steel. Hence testing the bar before buying it is of prime importance. The test are conducted to check the following properties:

Chemical composition

The chemical composition must be in accordance with the IS Specification. It must be right to get the desired properties and grade of steel. Spectrometer is the instrument used for testing the chemical composition of TMT Bar. It gives the chemical composition of the tested bar in terms of 32 elements and also ensuring the IS specifications are strictly maintained.

Spectrometer (Source- YouTube-Isteel Forever)

Spectrometer (Source- YouTube-Isteel Forever)

Strength

The bar strength is tested by the Universal Testing Machine, also known as UTM. TMT bar is placed in between the plates of the UTM and pressure is applied to it. If the bar breaks before the characteristic value for yield strength, it should be sent back.

Universal Testing Machine (Source- YouTube-Isteel Forever)

Universal Testing Machine (Source- YouTube-Isteel Forever)

Flexibility & Bending

The test conducted to check the flexibility of TMT bar is called the bending and re-bending test. In this test, the bar is bent to 1350 and kept in boiling water about 30 minutes. It is the re bent to approximately 157.50. If the bar is designed for right amount of strength and flexibility, it should bend easily without any appearance of surface cracks along the bends.

Bending & Re-bending Test (Source- YouTube-Isteel Forever)

Bending & Re-bending Test (Source- YouTube-Isteel Forever)