The construction of a building is virtually incomplete without reinforcement. In construction, Steel is the primary alloy used for reinforcement. There is a reason for that because as we know that concrete is used for the construction of the building, even though it provides the compressive strength, it has weak tensile strength for withstanding induced forces. Therefore by combining reinforced bars, it provides the tensile strength for concrete to withstand induced forces. Also, The properties of thermal expansion for both steel and concrete are approximately the same which is why steel is used as reinforcement.
Timeline of Reinforced steel
In 1874, the first attempt on reviving the steel industry was made in India. In India, constructing building structures using reinforced steel started around 1907 when a hostel was constructed for Victoria Jubilee Technical Institute in Mumbai. It was in the same year which led to the formation of Tata Iron and Steel Company (TISCO). Mild steel was used as the reinforcement. The mild steel (MS) rebars of Grade Fe-250 were used widely till the 1960s. In 1961- The Bureau of Indian Standards (BIS) introduced the Indian Standard (IS) which created a standardization for the steel reinforced bars. This standard was revised four times in the years 1966, 1979,1985 and 2008.
The rise of TMT reinforcement
Until the 1960s, Mild steel bars were used as reinforcement to concrete. These reinforced bars had a tensile strength of 250 MPa. With the introduction of newer and better technologies in the field of construction, the demand for higher yield strength rose. One such innovative technological process was the Thermo Mechanically Treated (TMT) process on hot-rolled reinforced bars. The TMT reinforced bars were of Grade Fe 415, Fe 500 was introduced in 1979 as conformed to IS 1785:1979. Since its introduction, there has been a huge demand for these reinforced steel bars. In 1985, a higher grade Fe 550 TMT steel bar was introduced as conformed to IS 1786:1985. Another higher grade variant, Fe 600 was introduced in 2008 conforming to IS 1786:2008.
The TMT process mainly involved three steps which are Quenching, Self Tempering and Annealing. In the quenching stage, the hot rolled steel bars are cooled rapidly by a series of water jets operating at different pressure ranges. The rapid cooling hardens the outer layer. In the self-tempering stage, it results in the heat flow from the inner core to the outer layer further hardening it, thus making it a tempered martensite layer. The annealing stage the inner results in the formation of a ferrite-pearlite composition in the inner core that provides ductility.
The True grade Steel
As the title suggests, Fe 415 is considered the true grade steel despite the fact there are higher grade variations. There is a reason for that. In the higher grades, their tensile strength is achieved by compromising the sole important factor, ductility. By reducing the ductility content, the bars become brittle but will have more tensile strength, whereas, in Fe 415, there is no compromising in ductility. This is what makes Fe 415 different from other grades.