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 - a persistent demand from civil engineers. The immediate short-term
objective was for bars of 400 N/mm2 with an ultimate goal of about 500 N/mm2. These high values were
demanded, but with adequate ductility. This was the basic global requirement. In steel industry it is common knowledge that
cold working increases strength and thus the first step was around 1970 – the cold twisted deformed (CTD) rebars. These
rebars were generally accepted in the yield strength range of around 400 N/mm2 with elongation values of 14-15%.
Since high strength was achieved at the cost of ductility, higher strength CTD bars did not gain acceptance as elongation
values dropped to 12 % or less. (As a matter of fact, the Indian Code IS 13920, till recently, barred the use of Fe 500 and
Fe 550 in seismic zones 3, 4 & 5.) Thus CTD bars could not fulfill the demand of civil engineers for rebars of yield strength
500 N/mm2 with good ductility & weldability. 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 mid-1970s, a few years after its development. But in India,
the story was different. Introduced in 1970-1972, 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. All that
we appeared to appreciate was the significant savings from use of CTD bars of 415 N/mm2 as against the earlier
ribbed bars of 250 N/mm2. 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”.
The global basic requirement of rebars toady is – low cost deformed bars
and a guaranteed minimum 500 N/mm2 yield strength with adequate ductility for seismic zones. The objective
is clearly cost savings through reduced quantity of steel used in RCC without compromise on safety.
This is where Thermex® rebars play a significant role. Since
nearly 55% of India falls under the seismic
zone 3, 4 & 5 Thermex rebars become very relevant.