Yield strength of Intermediate Grade (40 ksi yield) rebar

[ajk1]

We had a lab test 4 two-foot long specimens of rebar cut from a 60 year old structure. The test lab reported the following results:

specimen taken from wall vertical bar:

1st specimen:
#3 rebar
64 ksi yield
90 ksi UTS
17.5% elongation

2nd specimen:
#3 rebar
86 ksi yield
103 ksi UTS
15.0% elongation

3rd specimen
#4 rebar
52 ksi yield
83 ksi UTS
22% elongation

Speciman taken from floor:
#5 rebar
49 ksi yield
83 ksi UTS
20% elongtion

QUESTIONS:

1. I do not have the ASTM Standard for rebar. What are its requirements for intermediate (50 ksi yield strength) rebar?

2. What yield strength would we be allowed to use when checking the basement wall strength by LSD? Could we safely use 50 ksi?

3. Is it likely that #3 rebar in the structure would be a different (grade material than #4 and larger bars? The structure is in the Toronto area and was built in 1957.

 

[hokie66]

3. The smaller bars probably are the same material, but have a higher yield because they were worked more in the rolling process.

 

[wannabeSE]

See the file provided by SlideRuleEra in this thread for old reinforcement information.

http://www.eng-tips.com/viewthread.cfm?qid=379414

 

[ajk1]

Thanks Hokie66. I did ot know that. Thanks

Thanks wannabeSE. Much appreciated.

 

[ajk1]

Unfortunately I do not see the answers to my questions 1. and 2. in the link that wannabeSE sent me. Can anyone answer that? Does anyone have the appropriate ASTM reinforcing bar standard, to see what they say about the requirements that the steel must meet when tested, to meet grade 40 ksi steel and grade 50 ksi steel?

 

[boo1]

ASTM A615 / A615M
"1.2 Bars are of five minimum yield strength levels: namely, 40 000 psi [280 MPa], 60 000 psi [420 MPa], 75 000 psi [520 MPa], 80 000 psi [550 MPa], and 100 000 psi [690 MPa], designated as Grade 40 [280], Grade 60 [420], Grade 75 [520], Grade 80 [550], and Grade 100 [690], respectively."

Table 2 gives the tensile requirement of 60k (420) for grade 40

Rebar with a grade 40 designation offers a minimum yield strength of 40,000 pounds per square inch and conforms to ASTM A-615 performance standards. Its metric counterpart is grade 280 rebar, with an equivalent minimum yield strength of 280 megapascals.

 

[ajk1]

to boo1 - Thanks.

 

[ajk1]

When testing rebar cut from an existing structure, how should it be determined what strength can be used in the structural calculations of required rebar?

For example, if a specimen of rebar cut from a reinforced concrete wall is tested in a lab and found to have a yield strength of say 51 ksi, can I do my calculations based on that yield strength? Or is there a minimum number of specimens that must be cut out of the structure and tested, and then a statistical analysis carried out to determine the standard deviation? The yield strength to be used in calculations would then be the average less say 2 standard deviations? Is that procedure spelled out anywhere?

 

[wannabeSE]

Have you looked at chapter 20 of ACI 318. It leaves a lot to the judgement of the engineer. Also, this chapter allows the use of different Φ factors when the strength is determined through testing.

20.2.4 — If required, reinforcement or prestressing steel strength shall be based on tensile tests of representative samples of the material in the structure in question.

R20.2.4 — The number of tests required depends on the uniformity of the material and is best determined by the licensed design profession responsible for the evaluation.

 

[boo1]

I think your testing indicates you have grade 40 (#4,#5) and grade 60 (#3).
You need more tests to statically have higher confidence, or just use the lowest grade.