TMT Bars for Constructions

TMT Bars

TMT stands for Thermo Mechanically Treated. The TMT bars have a hard outer surface and a softer core. Their manufacturing process includes hot rolled steel wires passed through water. This makes the surface hard and keeps the core warmer and softer. This helps in making the steel corrosion resistant and also increases its weldability.

On the other hand, HYSD stands for High Yielding Strength Deformed bars. These bars have more yield strength than Mild Steel. Unlike TMT bars, HYSD bars are both hot and cold worked. They are graded as Fe 415 or Fe500.

The Indian Specification for High Strength Deformed Steel Bars and Wires for Concrete Reinforcement (IS1786) has indicated three strength grades Fe 415, Fe 500 & Fe 550 which is given below in table. The numbers after the prefix Fe indicate the specified minimum yield strengths in MPa unit.

Mechanical Properties:

SL No.	PROPERTY	GRADE
		Fe 415	Fe 500	Fe 550
i)	0.2 percent  proof stress/ yield stress, Min, MPa	415.0	500.0	550.0
ii)	Elongation, percent, Min, on gauge length 5.65 ÖA, where A is the cross-sectional area of the test piece.	14.5	12.0	8.0
iii)	Tensile strength, Min	10% more than the actual 0.2% proof stress but not below 485.0 MPa.	8% more than the actual 0.2% proof stress but not below 545.0 Mpa.	6% more than the actual 0.2% proof stress but not less than 585. MPa.

Carbon Content and Carbon Equivalent: The Indian Specification (IS 1786) limits the carbon content to 0.25% maximum and carbon equivalent to 0.42% maximum (0.53% for low alloy steel) for guaranteed weldability. Steel manufacturer in India maintains a carbon equivalent of 0.18% with a standard deviation of 0.03% across all sections including those rolled in a high-speed wire rod mill.

Sulphur and Phosphorus Content: The Indian Specification (IS 1786) calls for sulphur and phosphorus content of 0.060% Max. Steel manufacturer in India maintains at levels considerably below the specification.

Percentage Elongation: The Indian Specification (IS 1786) stipulates 14.5% elongation minimum. However, in India most of the integrated steel plant maintains the elongation values between 22 to 25%.

UTS / YS Ratio: In the case of UTS/YS ratio Indian Specification (IS 1786) stipulates an UTS / YS ratio of 1.10, 1.08 and 1.06 for the Fe415, Fe500, Fe550 grades respectively. But all the steel manufacture in India maintains this value more than 1.15 in India.

Chemical Composition of Steel for TMT and other grades:

Typical chemical compositions of steel for TMT adopted by a reputed steel manufacturers for different sizes are given below:

S.N	TMT size in mm	Carbon percentage	Manganese percentage
1	8-12	0.19 to 0.23	0.54 to 0.84
2	16	0.11 to 0.15	0.50 to 0.66
3	20	0.09 to 0.17	0.50 to 0.66
4	25	0.17 to 0.25	0.51 to 0.75
5	28	- do-	0.99 to 1.11
6	More than 28	- do-	0.89 to 1.01