With
due apologies to Charles Dickens, one can state that the present period represents the best of times as also, rather unfortunately,
the worst of times in light of the TMT bars now available to structural engineers in the Indian Region for use in RCC works. The
advantages and the dangers of use of TMT bars in the Indian context are discussed here.
Background
Civil and structural engineers have over the past
few decades been demanding a deformed bar with high strength of yield strength of minimum 500 N/mm2, to effect
considerable savings in the reinforcement steel used in RCC works. One method used to satisfy the requirement was through
use of micro-alloys in steel. But this raised the basic cost of steel and hence the very fundamental objective of steel savings
in RCC works was defeated.
Around 1970, cold twisted deformed (CTD) was developed
in Europe and sold under the brand name Torsteel. Many mills in Europe adopted this process with a lot of automatic twisting
and bundling facilities. However, this had an extremely short life in Europe where it was developed. In this process, the
yield strength was raised to desired levels by cold working but at the cost of ductility. The resultant elongation
values were poor a minimum of only 14.5 % was specified for yield strength of 415 N/mm2 and still lower for the
desired level of 500 N/mm2. Thus Europe was forced to abandon use of CTD bars within a few years of its development
by about 1974! Consequently, the use of bars with a minimum YS of 500N/mm2 did not really pick up though
the demand existed.
The break-through was achieved in mid-eighties
when two rapid water-quenching processes were developed rather independently viz Thermex and Tempcore. Both were granted patents
and Europe reaped benefits straight away. These two water quenching processes met the basic demand of civil engineers for
high strength deformed bars with high yield strength, toughness and ductility. The floodgates literally opened and the use
of such bars in Germany rocketed as seen from fig. 1. Today, the German standard DIN 488
specifies for Grade 500 an A10 elongation of minimum 10 % (equivalent to about 20% A5 elongation as used
in India). In IS 1786:1985 the corresponding elongation value is a low 12% for Grade Fe 500.
In India, CTD bars had been introduced within
a year or two of its development in Europe, but sadly, due to excellent marketing and the visible savings, the same became
a norm for use in all RCC works despite information from Europe about the inherent drawbacks. Thus, India missed out on a
superior product and, more importantly, on thousands of crores of Rupees of savings available to us by the new globally proven
products available viz. Tempcore & Thermex rebars. At a nominal saving of 10% and a base of 4 million tonnes / year,
the wasteful expenditure in India over the past 15 years is a mind boggling Rs. 7,500 crores (Rs 75 billion)
at an average price of Rs 12,500 per tonne. This wastage does not take into account the high interest costs in India
during the nineties.
