1. Elongation Requirements for Stainless Steel Rebar
Presented by:
Suresh Gupta
ASTM Committee Meeting A May 8, 2012

2. Purpose of Elongation
Elongation is a measure of ductility, which provides confidence that metals can be formed, without cracking or fracturing.
In rebar, it defines ductility so that bars can be bent without cracking.
If bars do not crack during fabrication, any excess elongation is irrelevant.

3. Alloy Characteristics
With all other variables remaining same,
Austenitic Alloys have highest elongation.
Duplex Alloys have elongation in mid range
Ferritic Alloys have lowest elongation

4. Measurement Methods and Facts
% Elongation, when measured in a fixed gage length, e.g. 8 inches, is lower than when measured in a gage length, as a multiple of diameter, e.g. 4 X diameter (4d).
(when 4d is less than fixed gage length).
Elongation is generally lower as diameter goes smaller.
This fact is clearly described in ASTM E8, Appendix XI

5. ASTM E8 Standard Test Method for Tension Testing of Metallic Materials Appendix XI – Factors Affecting Tensile Test Results
Comparison of elongation values determined using different specimens requires that the following ratio be controlled:
Lo/(Ao)1/ (XI.1)
Where:
Lo = original gage length of specimen, and
Ao= original cross-sectional area of specimen.
XI Specimens with smaller Lo/(Ao)1/2 ratios generally give greater elongation.
Note: (Ao)1/2 is proportional to diameter (d).
Therefore, for same ductility, larger diameter samples result in greater elongation values, when measured in fixed gage length.

9. ASTM A955-9a Until revision 10a, elongation was not required for
sizes # 3-5 Grade 75, probably because it was known that it is unrealistic or not necessary.
Grade 40 for sizes 7 and larger because, in any condition it will meet the requirement, anyway.

10. % Elongation Requirements for Rebar
Carbon and Alloy steels
ASTM A615 requires 6 to 7% for Grade 75
ASTM A706 requires 10 to 14% for all Grades.
Stainless Steels
BS 6744 requires 14% minimum in 5d gage length for Grade 500 (72.5 ksi).
ASTM A955 requires 20% minimum in 8 inch gage length for all grades, all sizes.

11. Manufacturing & Process Control
Even with precise chemistry and process control, duplex stainless steels meet the elongation requirement very marginally.
If rebar is supplied in coils, machine straightening will cause work hardening and elongation will go down even further.
This may cause unnecessary dispute.

12. Rationale It should be sufficient for us as well.
If 6% elongation in carbon steel is sufficient
If 14% in 5d is sufficient for stainless steel rebar per BS6744 in Europe,
It should be sufficient for us as well.
FYI
ASTM Specifications for plain smooth bars require elongation in 4d

13. Proposal It is therefore recommended that,
Using a foot note to Table 3, elongation requirements be lowered to 14% for Grade 75,
for all sizes of duplex grades and,
all grades of stainless steels under 1.00 inch.