Concrete provides the compressive strength required but not the tensile strength. To provide the necessary tensile strength, steel rods are used to re-enforce the concrete structure. TMT (thermo-mechanically treated) bars also known as ‘rebar’ or ‘Sariya‘ provide more strength and ductility to the structure than their counterparts i.e. cold twisted bars at a price that is not much higher.
TMT bars are made by heating and quenching (regulatory but suddenly cooled). After cooling down they are made stronger by mechanical processes. Steel accounts for more than 15% of the construction budget of the house. It is essential to put your money where it is worth.
Nomenclature and grading: ISO issues certifications if the bar meets a certain requirement. Fe 415 means than the yield strength of the bar is 415 N/mm2.
The composition of carbon is the most important factor in terms of strength. The more the carbon content, the more the tensile strength but more carbon leads to low ductility and that’s the beauty of TMT process. With just a little change in the treatment process, you can get different grades of steel with different properties. But each grade is different and how to choose which one fits your purpose the best?
Maximum carbon content allowed in the TMT bars (Indian Standard)
|Constituent||Fe 415||Fe 415D||Fe 500||Fe 500D|
The most common confusion arises in choosing between Fe 415 and Fe 500 grade TMT bars. But there are some parameters you should check to clear yourselves from the confusion-
|Property(Min value required)||Fe 415||Fe 415D||Fe 500||Fe 500D||Fe 550||Fe 550D||Fe 600|
Estimation of load and ductility
If the engineer underestimates the load, it becomes unsafe. If he overestimates, it costs more. So, estimation of the live load, dead load, earthquake load etc. and its sum total must be very accurate. Fe 500 provides more tensile strength. Thus, When the building is G or G+1, you should use Fe 415 as it provides the essential strength and everyone will suggest the same. The confusion arises in the case of high-rise buildings. In this case, the correct estimation of the load is of the utmost importance. If even after accounting the factor of safety(usually 1.2), Fe 415 can provide the necessary strength, then go for Fe415 over Fe 500. This is because there is always a compromise between tensile strength and ductility. Ductility is the ability of bar to deform without breaking, and under heavy and fluctuating loads ductility becomes very important. Fe 415 provides 14.5% elongation length, whereas Fe 500 ductility is only 12 %.
For high-rise structures, it is more economical to use Fe 500 grade because it would take a number of Fe 415s to give the same strength. For 10-15 story buildings, the difference in amount can go upto 20%. So, it would save on the amount and thus, save on the cost of steel. Another grade, Fe 415 D or Fe 500D should also be considered where D stands for ductile. It is a bit more costly but provides more ductility at the same tensile strength.
If the building you are considering lies in a seismic zone and is prone to earthquake, it is advisable to use Fe 415 or Fe 415 D due to the more elongation it can provide.
Fe-500 bar is bent very slowly and jerk-less. Bending done on a bar bending table/ block is always very sharp. Such bending reduces the tensile strength of the bar. Your engineer should provide the exact bends that will be required beforehand so that they can be done while procuring in the factory. Fe 415 is comparatively easier to bend and can be done at the construction site itself.
More carbon content in Fe 500 possesses a problem while welding. During welding, the steel is fused at high temperature and then quenched without regulation. This reduces the strength in case of higher carbon content. Thus, if welding is very frequent at the construction site, use of Fe415 is advisable.
The bottom line is if your structure requires tensile strength above 415N/mm2 (after safety factor) or when the structure is a very high rise and the difference in costs of 415 and 500 becomes very significant, then only use Fe 500.