Until the 1960s mostly plain mild steel rebars with yield strength of about 250 N/mm2 were used.
Around 1960, ribbed mild steel bars were introduced to allow for a better bond with concrete. Both the plain and the ribbed bars had very high ductility as indicated by the elongation values.
The next developmental thrust was on reduction in the quantity of steel used in RCC through the development of high strength rebars of 500 N/mm2 – a persistent demand from civil engineers.
Since high strength was achieved at the cost of ductility, higher strength CTD bars did not gain global acceptance as elongation values dropped to 12 % or less.
The demand of civil engineers for rebars of yield strength 500 N/mm2 with good ductility & weldability remained unfilled. The other drawback of CTD rebars was that the surface stresses due to twisting led to a high corrosion rate.
Europe, where the CTD process was developed, gave up its use in the 1970s, a few years after its development.
But in India, the story was different. Introduced in 1970, the CTD bars gained a strong foothold despite the findings in Europe.
The closed market conditions prevailing at that time helped matters in this regard – it appears that we only appreciated the significant savings from use of CTD bars of 415 N/mm2 and ignored the drawbacks.
Mr. R. N. Raikar, President of the India Chapter of the American Concrete Institute, at his opening remarks in the seminar on ‘Reinforcement – Today & Tomorrow’ held in Mumbai in June 2003, lamented that “fewer repairs were required in buildings prior to the use of CTD bars. Today, the repair of buildings has become a specialised industry”.
Two such processes were developed in Europe, Thermex and Tempcore, and both received world patents – and global acceptance amongst the civil engineers because it met all their requirements.
The steel mills all over have increasingly resorted to these unique technologies and demand for such rebars continues to increase.
“Latest developments on steel front” by Jagvir Goyal
“Thermex Quenching and Tempering Technology” by R.K. Markan, the Chairman and Managing Director of H&K India
Websites of SAIL, Rajuri steel & Evcon