CHANGING REBAR TRENDS

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 - 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.