Steel, the most versatile, strong and durable material for reinforcing concrete for constructing buildings. No doubt, it is the only material so far that has proven successful to bond with concrete for reinforcement. The main reasons are, One, steel has high tensile strength and is able to withstand the tensile forces in a structure which the concrete lacks. Second, the thermal coefficient of expansion of steel is similar to that of concrete.
The current innovation in steel reinforcement is Thermo Mechanically Treated rebars, also called TMT steel bars. TMT steel bars is unlike regular steel undergoes a particular 3 stage process called temp core process. This unique process makes TMT steel bars stronger and ductile. There are different grades available for TMT steel bars namely Fe 415, Fe 500, Fe 550 and Fe 600.
What do these numbers specify? Why are there these many grades? Well, it is described below
What’s with the grades
The grades are important in determining the strength of the reinforced steel bars. As I mentioned the different grades above, there are different numbers for each grade like Fe 415, Fe 500 etc. These are not just numbers. They denote the yield strength of the particular rebar. Alright, what is this yield strength?
It is defined as the stress at which the reinforced bar starts to exhibit plastic deformation. Therefore, the higher the numbers, the higher the yield strength. The construction of structures depends on the type of grade. Now let us see the characteristics of each grade and how does it help in the construction of structures.
In TMT steel reinforced bar, even though the tensile strength is important, an equally important factor is its ductility ratio. Ductility is what defines the ability of the steel bar to handle forces more effectively. More ductile the bar is, more capable it becomes in handling stress. In order to achieve higher tensile strength, changes are made during the Quenching process, which results in less ductility or brittleness of the reinforced steel bar. The TMT steel reinforced bars consist of the following grades namely:
Fe-415: The TMT steel reinforced bars started with this grade. It is often described as the true reinforced steel. Well, there’s a reason for that. It has a yield strength of 415 N/mm² and ultimate tensile strength of 485 N/mm² and a percent elongation of 14.5%. Here percent elongation is as the measure of ductility Fe 415 is the only grade that has the optimum tensile strength and ductility ratio, making it recommended for building homes. Also, the cost of Fe 415 grade steel is lower than it’s higher counterparts which makes it economically feasible for the common people.
Fe-500: A higher TMT steel rebar version of the Fe 415 steel. Fe 500 has a yield strength of 500 N/mm² and ultimate tensile strength of 545 N/mm², which is superior to the Fe 415 grade. Due to it’s higher tensile strength, it requires less number of bars which results in approximately 17% lesser consumption of steel in construction. However, it has a percent elongation of 12%. This is because the grade is less ductile than Fe 415 grade. Mainly Fe 500 is used for the construction of structures that demand more strength such as bridges, large buildings etc.
Fe-550: A technologically advanced and higher variation of the grades Fe 415 and Fe 500. This grade has higher tensile strength than its predecessor Fe 500, with a yield strength of 550 N/mm² and ultimate tensile strength of 585 N/mm². It is primarily preferred for RCC structures that require huge tensile strength such as skyscrapers, large bridges etc. Even though it has superior tensile strength, the rebar is more brittle than it’s predecessors, with a percent elongation of 10%, and also it is expensive which makes it economically infeasible for the common man.
Fe-600: These grade TMT Bars were introduced which further increased the tensile strength than its predecessors. With its superior tensile strength, the consumption of these steel rebars in the construction projects significantly got reduced and load-bearing capabilities of these bars were increased. It has a yield strength of 600 N/mm² and ultimate tensile strength of 660 N/mm². Similar to its predecessors Fe 500 and Fe 550, it is used for the construction of structures requiring huge tensile strength. However, it is more brittle, with a percent elongation of 10%. Also, due to high costs, Fe 600 is not used commonly as others.
One thing that we can say is that in terms of tensile strength, higher is the better which means Fe 600 is the strongest but, the tensile strength is not the only factor to consider such as ductility, cost etc. Therefore, for the common man, the recommended steel grade for constructing homes is Fe 415. What makes Fe 415 distinct from others is its uncompromised ductility ratio and lower cost.