Code For Rebars

SECTION 3: CODE FOR REBARS

It is imperative that the Bureau of Indian Standards (B.I.S.) and the steel majors (TISCO, SAIL & RINL) combine with knowledgeable organisations to develop a code for rebars that fully benefits the country as a whole. If this is not done, the import of the Tempcore and Thermex technologies at high costs will really only mean a ‘tick’ on the list of jobs done instead of any meaningful benefit to the nation. We have already lost nearly 20 years on account of incorrect steps; we should not lose any more time.

It is heartening to note that the B.I.S. is seized of the matter and is considering a revision of the code for rebars and is even contemplating a separate code for what they still term as ‘TMT’ bars. The outcome is eagerly awaited. It is hoped that the correct phrase ‘quenching and tempering’ is used and that B. I. S genuinely take advantage of the global technological advances that have now been introduced in India by specifying properties as obtainable by quenched and tempered systems as per Thermex (used by Durgapur & Bhilai steel plants of SAIL and most secondary mills in India) and Tempcore (as used by TISCO & RINL). It must be noted that such rebars made by these two groups have a combined total market share close to 50% in the country. Surely, the interests of the common man and India as a whole should be the only consideration.

What should the Indian Code specify?

One basic fact about India is that 50-60% of the country falls under the seismic 3, 4 & 5 categories. (See Fig.13: Seismic Zone Map). So, safety of construction is of prime importance and there is no place for casualness or complacency with regard to selection of rebars. The code in use must therefore take into account this basic need for India. Many countries are preparing special codes for rebars to be used in such areas whereas a few such as New Zealand have already done so through AS/NZS 4671:2001 – Steel Reinforcing Materials. Most countries are contemplating specifying elongation of 20 % or more. In India, as per IS: 13920, rebars for seismic zones 3, 4 &5 should have an elongation of 14.5% or more. This, I feel, is an extremely low minimum value and needs to be raised immediately to at least 16 % if not 18%.

Let us take the joint Code AS/NZS 4671-2001 developed by New Zealand and Australia and introduced recently. It replaced the old Code NZS 3402.

In the new Code 4671, Grade 430 was withdrawn (adequate time was given for withdrawal of Grade 430) and replaced with Grade 500.
The new Code covered three ductility classes – L, low ductility; N, normal ductility and E, high ductility for earthquake prone areas. These three classes of ductility are basically applicable for Grade 500 only. In case of Grade 250 only class N and for Grade 300 only class E is covered by the Code AS/NZS 4671. Class L includes cold worked steel wires & wire meshes.
The concept of uniform elongation, A_gt, was introduced as it was considered a more meaningful measure of ductility against the total elongation in the earlier code. Uniform ductility is the strain developed in the bar at maximum load while total elongation includes strain involved in the deformation at the necked region which is of no structural value. Limits are set not only for the yield strength, but also for the tensile (Rm) to yield (Re) ratio in both directions. The minimum value of the ratio Rm/Re is to ensure that yielding will not be confined to where it first commences, thereby permitting greater elongation of the bar before fracture and hence greater ductility of the structural member. The maximum value is to ensure that when the steel commences to strain harden the stress in the bar does not lead to a significant over strength of the structural member.
Yield Strength, R_e, measure of the maximum load that the steel can carry in elastic manner. On exceeding this load steel is permanently elongated. The code has three grades – 250, 300 & 500.
Ultimate Tensile Strength, R_m – no limits are specified. Instead limits of Rm:Re ratio have been specified and is fundamental to the ductile design concept in New Zealand.

It may be noted here that New Zealand follows the ductile design philosophy - to design for structures to yield but not fail during an earthquake. This facilitates absorption of the immense seismic forces allowing buildings to move and distort without complete catastrophic failure. Limits are set not only for yield strength, but also for the tensile to yield strength ratio in both directions. The minimum value of the ratio Rm/Re ensures yielding will not be confined to specific area, thereby permitting greater elongation of the bar before fracture and hence greater ductility. The maximum value is to ensure that the stress in the bar does not lead to significant over- strength.

The brief properties of the old and new Grades of rebars in New Zealand are presented in Table-1:

Table-1: Properties as per old and new Codes of New Zealand

Property

AS/NZS 4671-2001

Old NZS 3402

250N

500L

500N

300E

500E

300

430

YS (MPa)   min

                   max

250

500

500

300

500

300

430

-

750

650

380

600

355

500

Rm:Re        min

                   max

1.08

1.03

1.08

1.15

1.15

1.15

1.15

-

-

1.50

1.40

1.50

1.40

Min. Uniform     Elongation,   %

5.0

1.5

5.0

15

10

-

-

Min. Total Elongation %

-

-

-

-

-

20

15