The previous blog explained about the evolution of steel using different techniques and also how it changed the economic status of the world. Steel is mainly preferred as reinforcements due to their high tensile strength. In earlier times Iron bars was used as reinforcement but it became unsuitable due to formation of corrosion very quickly. It was replaced by Mild Steel (MS) reinforced bar. Even though Mild Steel rebars has good workability, ductility and anti corrosion properties, it has poor bonding with concrete since it’s outer surface is smooth.
Also the maximum tensile strength for Mild Steel reinforced bar is 250 MPa which is sufficiently less for building structures that require more strength. Therefore the engineers and steel makers needed to overcome this problem which resulted in the introduction of High Yield Strength Deformed (HYSD) reinforced bars. HYSD reinforced bars are produced by twisting of the steel bars either hot rolled or cold twisted. This process results in the formation of ribs on the outer surface of the reinforced steel, which provides sufficient bonding strength with concrete. HYSD reinforced bars are mainly of two types:
HYSD and CTD Reinforcement Bars
Hot rolled HYSD reinforced steel
Cold worked HYSD reinforced steel, also known as Cold Twisted Deformed (CTD) rebars
HYSD reinforced steel bars had better tensile strength than Mild Steel. HYSD rebars produced grades of Fe 415, Fe 500 etc. Even though HYSD steel reinforced bars was superior
than Mild Steel reinforced bars, they had their own disadvantages in terms of reinforcement. Like Mild Steel, HYSD steel reinforced bars had to be replaced because it was unable to meet the newer demands in terms of strength, flexibility etc.
TMT Reinforced Bars
A newer, more refined form of reinforced steel was introduced and it was called Thermo-mechanically Treated (TMT) reinforced bars. The introduction of TMT created a radical change in the field of construction. What distinguishes TMT reinforced steel bars from other types is the additional manufacturing process in it which are:
In quenching process the hot rolled steel from the mill is cooled by water jets resulting in the rapid cooling of the outer layer which is called “martensite”. In Self Tempering the heat in the still hot core flows to the outer layer further hardening it. This is called Tempered Martensite. In Annealing the inner core results in the formation of ferrite-pearlite composition which provides ductility.
TMT steel reinforced bars is mainly preferred for reinforcements because of it’s higher tensile strength, ductility, anti-corrosion properties which are more than HYSD reinforced steel bars. Also HYSD steel reinforced bars had surface flaws resulted from twisting process in its manufacture. TMT reinforced bars had less residual stress when compared to HYSD rebar. Unlike the HYSD steel reinforced bar, TMT steel rebars has grades namely Fe 415, Fe 500, Fe 550 and Fe 600.
TMT process is a well refined process that have improved the quality of construction drastically with the higher tensile strength it provides. But the truly distinguishing factor is the ductile property the TMT bars offered. These bars with astounding tensile strength has ductile properties that allows them to be bent for construction without compromising. But as the grades go higher, the ductile TMT core is compromised for higher tensile strength. So grades like Fe 415 is recommended for constructing homes and commercial buildings and higher grades must be used in situations only if the engineer specifies it. By using higher grades it compromises on critical factors like ductility.