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Thermex QST Rebars

SECTION 3: CODE FOR REBARS - contd.
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Since we are more conversant with total elongation, the proposed code for India may continue to use total elongation as defined presently. However the values should be increased in light of the properties obtained through the quenching and tempering process now increasingly used in the country and the vast regions of high intensity earthquake-prone areas in the country.

 

A summary of properties specified to Thermex Licensees and the typical properties as noted from Thermex rebars are given in Table 2 alongwith that specified in IS: 1786-1985.

 

Table 2: A comparison - IS Code 1786 and Properties of Thermex rebars.

The typical values of Thermex Grades 400 and 500 rebars mentioned in the Table-2 above do not include samples from the Durgapur and Bhilai Steel Plants of SAIL. Instead they are test results obtained in the period from August 2000 to April 2003 of 934 samples in the small mills of Thermex Licensees. Special importance is given in checking the Stress Ratio as this is a very significant factor when considering rebars for earthquake zones. Many Licensees get UTS / YS values in the range of 1.20 to 1.35 even for Grade 500 against the IS standard of only 1.08.

 

 

Suggested IS Code

Keeping in mind the above and that 50% of India falls are under seismic zones 3,4 &5 the basic changes which may be considered, are as follows:

 

  • The minimum elongation, A5, should not be less than 16% for any grade.
  • Since CTD rebars will continue to be made, a grade 350 with 17% elongation can be added.
  • Grade Fe 415 may be replaced by a grade of 450 to allow for savings in steel. Grade Fe 550 may be deleted as it is hardly ever used and does not exist in Codes of most countries.
  • The mechanical properties to be specified should be restricted to Yield strength, Stress Ratio and elongation. Tensile strength need not be specified as limits for the same will be restricted by the Rm/Re Stress ratio.
  • An upper and lower limit to be prescribed for both Yield Strength and Stress Ratio as is done in the new AS/NZS Code4671-2001.
  • Special identifying marks to be evolved for identifying the rebar Grade. This will make the site engineer’s job easier.
  • It should be made mandatory for all rebars to have distinguishing marks to identify the producer.

 

Other changes in line with the above could be incorporated. For example, B. I. S may consider reducing the C content to 0.25% as against the present 0.30%.

 

The basic properties can be as tabulated below in Table 3. Grade 350 will be mainly for CTD bars.

 

 

            Table 3: Suggested new IS Code.

Property

Grade

Future Seismic Grades

350

450

500

350E

450E

500E

YS (MPa)   min

                   max

350

450

500

350

450

500

430

530

580

430

530

580

Rm:Re        min

                   max

1.15

1.15

1.15

1.15

1.15

1.15

1.40

1.40

1.40

1.40

1.40

1.40

Min. Total Elongation %

17

17

16

18

18

18

 

Initially grades 350, 450 & 500 may be introduced and at a later date (after 2 years) the specific grades for seismic zones may be added – the change being mainly in ductility – with minimum 18% elongation.

THERMEX is the registered trademark of H&K in India and of HSE Germany in other countries.