A615 Rebar

[BigH]

Ron/SRE/Jim etc.

Two questions about A615.

1. On chemical analyses, A615 requires tests to be run for carbon, manganese, phosphorus and sulfur. However, only the phosphorus content as a target maximum that must be met. Why not the others? We must test for them but what to do with the results.

2. On strength, we have, on a recent batch of bars, compliant yield strength (>420 MPa - 63.22 kgf/mm2). However, 50% of the heats have low tensile strengths - no more than 10MPa low (1.6%) and typically 5% low (0.8%). A615 gives no "tolerance" - so if I have 418 - is this non-compliant? Do I reject the bars for tensile strength due to the 1.6% low value?

I don't particularly want to reject for tensile - but we do have many "spy eyes" - this is Indonesia and tongues wag . . . but still, I would think that there is a small tolerance - 620 MPa minimum 600 MPa (drop dead bottom).

Comments, please. Regards, BigH

 

[BigH]

p.s. relatively low strength concrete - 20Mpa at 90d and 30 MPa at 90d although 25 should be used. Around penstock, pier nose and intake.

 

[BigHarvey]

BigH

Difficult one but good sense should prevail. I recently had a problem with a concrete supplier for an ACP rigid inclusions jobsite in Mayotte ( Comores islands ). Since the concrete was pumped, I needed 400 kg of fines and it is usual to get the fines content with cement. Hence a specification with a typical resistance of 25 MPa at 28 days. 75% of the tests where not complying : results around 12 MPa ( I don't want to know what kind of results they get on a standard civil engineering concrete with less cement ! )but my rigid inclusions had stresses around 10 MPa. We reviewed our specs afterwards to match with what was done and was working !
Good sense is always better than sticking to specs stricter than necessary !

 

[BigH]

BigHarvey - totally agree. We've got some great Area Superintendents (lots of experience including one being the construction manager for the contractor on the new Vancouver rapid transit tunnel from the airport. However, client seems to be contract contract. Also, lots of spy eyes as one can imagine - outsiders looking for angle - we've already had a "rumour" on good steel - so I want to be totally above board and hard to nail. Still, my thought is to find out the tensile strength of the Canadian equivalent. While the spec says to follow ASTM A615 for 420 MPa, the spec allows 400 MPa (Canadian . . .) - thanks for the comments though. We'll have to chat sometime about the cement. We've got a max 50degC heat of hydration for our structure!

 

[BigHarvey]

BigH

I have been involved in the grouting of Piedra del Aguila dam in Argentina ( 3 million m3 concrete dam). Concrete was prepared with iced water and there was a tremendous amount of pipeworks in the concrete to control heat of hydration. It is amazing the temperatures you can obtaine just with large diameter piles.

 

[SlideRuleEra]

BigH - My favorite type of problem... and believe that I have an "outside-the-box" solution for you. You have already answered my first question: "Prefer to accept the rebar?" Answer appears to be yes.

1. Strength of each rebar is dependent in the product of the crossectional area times the strength of the steel.

2. It is my understanding that A615 allows rebar weight to to 6% less than the nominal value. Assume that variations in weight are directly proportional to variations in crossectional area (i.e. rebar that is 94% the nominal weight has 94% of the crossectional area). I realize that I am ASSUMING the unit weight of the steel is a constant.

3. Weigh some samples of your rebar and calculate the crossectional area, based on the above assumption.

4. Make the following comparison:

Calculated crossectional area (from your tests) x measured strength of steel (from your tests)
vs.
Minimum acceptable crossectional area (94% of nominal) x minimum strength of steel stated in A615 (420 MPa).

5. If your results are higher, then IMHO, the steel meets the INTENT of A615.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Ron]

BigH...

To answer your first question, the chemical properties are taken for the purposes of troubleshooting failures of the mechanical properties. With the exception of Phosphorous, there are no rejection criteria, so they are not used for acceptance or rejection. An example of where this might be used...suppose you have a failure of the tensile testing in elongation. The chemistries show the carbon content to be high and the manganese content to be low...those can often lead to poor elongation results.

For your second question, the strengths given in the table are minimum required strengths. There is a provision for retests if they fall below the strength. Here is paragraph 14.1 which shows the retest provision:

[highlight]14.1 If the results of an original tension specimen fail to meet the specified minimum requirements and are within 2000 psi [14 MPa] of the required tensile strength, within 1000 psi[7 MPa] of the required yield strength, or within two percentage units of the required elongation, a retest shall be permitted on two random specimens for each original tension specimen failure from the lot. Both retest specimens shall meet the requirements of this specification.[/highlight]

Ron

 

[BigH]

Ron = thanks - the contractor is sending out for retesting. This was a new supplier - and the steel is needed for the draft tube! Do you know the CDN Spec for tensile? Yield is 400 MPa, not 420. ??