Steel is an indispensable part of construction. The possibilities presented by steel in the field of construction is not just limited to steel sections, but also as steel bars as reinforcement to cement concrete in construction of structures. The story of steel as a reinforcement presents a picture of innovation in the field of construction. Starting as iron as reinforcement to structures, to highly ductile mild steel to deformed bars (High yield strength deformed bars) and finally to thermo mechanically treated (TMT) steel bars, showcase an evolution of steel reinforcements to keep up with the needs presented by humans in the field of construction.
So what makes the current technology of TMT reinforcement bars the most suitable as a reinforcement material for cement concrete in construction. The answer lies with the high tensile strength and ductility provided by TMT technology. Unlike previous technologies like HYSD that had torsional forces in them that proved to be too rigid and possessed challenges of failure in case of quakes TMT bars are resistant to the seismic forces because of its ductile properties and absence twisting or torsional forces in their manufacturing process that leads to defects.
Grades of TMT bars
TMT bars are of mainly of grades – Fe 415, Fe 500, Fe 550, Fe 600 and Fe 700 with each grade finding its use in different areas of construction. Denominations in grades which denote the tensile strength of TMT bars, the higher the grades, having higher tensile strength. So what about the ductile properties of the grades? Grade Fe 415 has a minimum ductility of 14.5%, Fe 500 – 12% & Fe 550 having 8%. One factor that is easily overlooked by the advertisement media hooking onto marketing propaganda is the easy swallow up of the fact that, it is in return for the compromise in ductility of TMT bars, the higher tensile strength achieved. The question of the hour is why ductility matters.
Why Ductility matters?
One factor that demarcates HYSD to TMT rebar technology is the ductility provided by TMT bars. With more than half of the geographical area of India prone to earthquakes, it is a must to take precautions against it. During earthquakes, a large amount of seismic energies are suddenly thrust upon the buildings, which if too rigid can cause plastic failures in them, ie, a sudden collapse of the structure as a whole. This is where reinforcement bars come to play by providing ductility and higher elasticity to structures, which helps it to dissipate the energy by helping them from rather than being a rigid mass, allowing them to sway slightly to dissipate the energy. Understanding the need for the ductility, special grades have been developed to help withstand the earthquake activities like Fe 500 S. The grade have higher ductile and tensile properties, which is achieved by a technology called micro-alloying which makes use of rare-earth metals like niobium, vanadium, titanium, molybdenum, zirconium, boron in very precise and small quantities during the alloying process. The technology can be used to impart higher ductility to the TMT produced. But, the process is not only expensive, but possess chances of failure. It takes a precise quenching process and micro-alloying to impart the necessary qualities to the steel bars and in case of failure to meet this, TMT bars possess the risk of failure which can be catastrophic to the safety of the buildings.
It was observed during micro-alloying and subsequent quenching that precipitation hardening or age hardening imparted the higher strength to the TMT bars. But the process has to be precisely controlled to ensure that the higher strength imparted to the outer martensite layer do not compromise the elasticity of the steel bars. If this is left unchecked, it leads to failures in presence of seismic forces. There are studies studies contradicting the controlled precipitation hardening in respect to the temperature requirement for the controlled quenching process to ensure the right proportion of hardened martensite and softer ductile inner ferrite pearlite layer. Walking on a thin line of theoretical calculations presents a risk, that seems unworthy to take for the benefits of a space efficiency negligible to be noticed, especially so in the smaller scale of structures for commercial and residential purposes.
Fe 415, a grade with right proportion of Strength and Ductility to withstand Earthquakes
Higher grades of TMT like Fe 500 and Fe 550 are adopted to bring down the cost, space efficiency and resources required in construction. But in taking such shortcuts, one must take educated decision to ensure the geographical and seismic activities of the region do not possess a threat to the construction undertaken. This is a reason why Fe 415 grade is preferred, as they provide the right proportion of strength and ductility along with the benefit of a controlled process with minimal threats of failure in its manufacturing process. These reasons present a major role in choosing the right TMT grade for construction, especially in a country like India with a large part of it under the threat of earthquakes. Current technology and innovation in a pace to keep up with requirements and challenges of construction is still in a road of progress. With the technology we currently leverage a grade like Fe 415 is still the optimal choice for construction.