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Title_head
Tougher steel rebars to weather quakes
Business Line
Wednesday, 15 Jun, 2011

Why do 70 to 80 year old buildings continue to stand tall while relatively newer additions to the same structures are in need of repairs?

Architects and engineers said that the answer lies in the quality of steel used in reinforced concrete. Indian quality standards lag behind that of established norms globally. Quality standards need to be revised in line with international specifications.

Addressing a seminar on ‘Proper steel reinforcement', Mr Satish C Dhupelia consulting structural and civil engineer, and president American Concrete Institute (India Chapter) pointed out that some of the earliest examples of full frame, reinforced cement concrete construction of the 1930s can still be seen along the Marine Drive in Mumbai. These used mild steel rebars in ground plus 4 to 5 floor constructions and have withstood the test of time with minimal maintenance. But extensions to these same buildings done in the 70s and 80s have called for major rehabilitation work within a decade, he points out.

During this period use of steel has evolved from mild steel bars to CTD (cold twisted deformed) bars.

Long after the world had moved on from the CTD bars in concrete, India had continued. But now it is adapting to the latest in construction steel technology – QST (Quenching and Self Tempering) steel accounts for more than half the quantity of construction steel of about 20 million tonnes a year used in India.

Technologies
World over there are two technologies – Thermex and Tempcore for making QST construction steel. In India Thermex, a German technology

Mr RK Markan CMD of H&K Rolling Mill Engineers exclusive suppliers of Thermex equipment and technology at the seminar organised by the Thermex Rebar Manufacturers' Association said that steel quality is key to earthquake resistant structures. UN data show that globally three of the top eight most populous cities prone to earthquakes are in India Mumbai, New Delhi and Kolkata.

According to Mr Markan, CTD rebars in construction have fallen into disuse. Outdated standards led to the use of CTD while Europe stopped using them more than 40 years back. There is a need to expedite use of high tensile and high ductile rebars such as QST quality as nearly 60 per cent of the country is in the high hazard seismic zones ranging between 3 and 5.

Gaps in IS norms
There are loopholes in the Indian Standards (IS):1786-2008 that results in the use of less than optimal quality TMT steel reinforcement bars.

The IS has to specify the same high level of ductility for rebars irrespective of yield strength along the lines laid down in New Zealand and European countries. The quality standards in terms of elongation and stress ratio for rebars used in earthquake-prone zones also need to increased.

Yield strength of steel reinforcement bars and ductility are two key quality features. Ductility prevents the rebar from snapping outright resulting in building collapse during catastrophic events such as earthquakes.

The IS codes relating to these qualities need to be revised in line with global standards.

For instance, there are a number of variations and combinations of ductility and yield strength that are confusing even for engineers, apart from seven different rebar grades based on chemical composition in the IS code. These need to be simplified. Also, the elongation standards in the code give importance to elongation at fracture instead of uniform elongation of the rebar as is the norm globally.

(Sourced from BL)

 

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