SECTION 2: STEEL REINFORCEMENT

BASIC

World over, rebars are specified for use in RCC strictly by the Yield Strength, India being some sort of an exception if one goes by the scenario prevailing from 1970 to 2000. Till the start of this century nearly all structural designers in the country talked in terms of CTD (or Torsteel) rebars only. The dominance of CTD bars in the civil industry was so profound since early 1970s that most civil engineers had become complacent and guilty of assuming that these were of Grade Fe 415 of IS 1786-1985. Not many engineers, except when used for specific projects, bothered to check the yield strength, ultimate tensile strength or the elongation when using CTD rebars in RCC. It was nearly always taken for granted that these bars met the IS code specified. Even today, nearly 50% of the type of rebars available in the country and the manner in which they are produced, sold and used merely suggests that we are anything but a developed country. We continue to rely mainly on grade Fe 415 though higher strength rebars of international standards such as Grade 460 and 500 with excellent elongation values have been available since over a decade. Consequently, rebars that neither have details of the name of the producer nor the bar strength continue to be accepted in many job sites. The laws remain slack and many loop holes exist in the way we execute RCC work.

Today, everyone talks in terms of ‘TMT’ rebars without really understanding much about these rebars. And probably out of habit acquired over the past few decades, the gullible civil engineer has again merely assumed that these ‘TMT’ bars are of the specified Grade Fe 415 but in many ways superior to the old CTD bars. Many civil engineers are surprised to learn that the blind use of TMT bars as available from stockists and traders is not a guarantee of a superior product.

Over the years the Indian citizen has been kept in the dark about the importance of a good basic structure - and today he is not bothered. He looks for granite, marble, bay windows, chimneys in the kitchen, modular kitchen units, weather resistant paint, plaster of paris finish and a host of other such items of visible beauty in the house he is purchasing but not bothered about the basic strength – the RCC work – on which all these modern amenities rest. Lakhs of Rupees are willingly spent on external gloss without any thought about the steel reinforcement. The life of an Indian is precious and he has a right to know whether the house he stays in is a safe one built with proper materials. Indian life is as precious as those in other countries. We must do what is necessary to protect his life as far as possible.

TRENDS IN REBARS

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.

The steel industry first developed, in the late 1960s, the cold twisted deformed (CTD) rebars generally in the yield strength range of around 400 N/mm²with elongation values of 14-15%. 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/mm²with 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/mm²and 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”.

The objective of guaranteed minimum 500 N/mm²yield strength with adequate ductility for seismic zones was finally met through the development of the “Quenching & Tempering” technology in early 1980s. 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.