Steel is one of those metals which have exceptional tensile strength and flexibility. Due to these properties, it finds a wide range of application in construction. The use of steel bars in concrete helps to improve the overall factor of safety of the structure. Though, the quality of steel plays a vital role in it. Due to a large number of reported cases of the earthquake, researchers found three types of steel bars ideal for its use in the construction industry. They are Mild steel, TOR steel and the most popular of the lot TMT bars.
The Standards Prevalent In India Market: The prescribed standard in Indian market states that MS steel has got a tensile strength of 250MPa and TOR steel comes in the range of 415MPa. During designing of a building, the architecture takes Fe 415 whose tensile strength comes in the range of 250MPa while calculating different stresses acting on the superstructure. But Fe 415 TOR steel though has good tensile strength, but the ductility of these bars is much less in comparison to Mild Steel Fe 415 bars. In other words, when forces act on a TOR bar, the structure of the bar will deform but won't rupture or break.
The use of TMT bars: The advancement in the manufacturing process of TMT bars helped in overcoming all the mentioned problems. The buildings nowadays commonly use TMT bars in their construction to secure the structure. During the manufacturing process, the steel bars are heated at a very high temperature and suddenly quenches for few minutes under a spray of cold water. As cold water comes in contact with the surface of the bar, the outer layer hardens and forms martensite structure with high tensile strength. The time duration for cooling the bar under jet stream plays a vital role in imparting the flexibility of the bar.
The bar should be removed from the cooling medium before it cools the core. Once removed from the water, the heart is still at a very high temperature and soft. The soft base provides the elasticity to the bar, and the hard outer layer provides tensile strength.
The ductility factor is quite essential as bars with better ductility can withstand stronger shock waves. The soft core also helps the manufacturer to build bars much thinner in diameter thereby making it more economical.
The choice of bars depends on the type of building; high rise buildings always prefer TMT steel bars of Fe 500 and above like Fe 600. The use of Fe 415 may not be an economical option when taken into consideration.
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