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Recently, a coworker shared a CRSI (Concrete Reinforcing Steel Institute) document called &#;Frequently asked questions about reinforcing bars.&#; (link)
Lots of really good info in there. My favorite one is about A706 bars vs. A615 bars.
We hear it all the time (and see it on the drawings) that for welded rebar and seismic applications, use A706, not A615&#;, but why?
The CRSI article explains it pretty well (it&#;s under the section &#;What is the availability and application of ASTM A706 reinforcing bars?&#;)
I&#;ll give you a CliffsNotes version below.
(Estimated reading time: 2 minutes and 18 seconds.)
Tightly Controlled Chemistry
First, the major difference between the two ASTM standards is the chemical make-up of the bars.
For A706 bars, chemicals are more tightly controlled to enhance weldability. There are maximum limits on carbon, manganese, phosphorus, sulfur, and silicon.
A615, on the other hand, only has a limit on phosphorous.
This is hugely important, which I&#;ll explain next.
Carbon Equivalent (C.E.) & Preheat Temperatures
AWS (American Welding Society) has an equation for calculating this thing called the &#;carbon equivalent&#; (aka C.E.). It is based on the chemical analysis of seven elements:
For A706, the requirement is that the C.E. must be 0.55% or less. (I.e., if you specify A706, you can be sure that the bars you get will have C.E. <= 0.55%)
And since C.E. is linked to the &#;minimum preheat temperature required for welding,&#; A706 bars can be welded at much cooler temperatures. (For example, based on the AWS Table below, #3 to #11 A706 bars can be welded at 50 degrees or less.)
So basically, no preheat for A706 in most common cases.
A615, on the other hand, has no C.E. requirements &#; meaning, the C.E. of any given bar is pretty much unknown.
Because of that, you have to assume the worst case, which could be above 0.75%. So if you want to weld it, the bar must be preheated to 300 to 500 degrees &#;.
That explains the weldability. How about seismic applications?
Strength Limit
For A706, there is a limit that the yield strength can be no more than 18,000 psi above the minimum specified yield strength. In other words, if you specify Grade 60 steel, the highest yield strength you might get is 78 ksi.
Also, regardless of the specific yield strength, you can be sure that the tensile strength will be at least 1.25 times the actual yield strength.
A615 doesn&#;t have such requirements.
This means that A706 is more suitable for blast-resistant and seismic-resistant applications where yielding needs to be tightly controlled.
Availability & Cost
You might be wondering, with all its benefits, is A706 more expensive and harder to find?
Apparently, it only costs a small premium and is widely available.
This means, you probably wouldn&#;t want to specify A706 everywhere in most cases, but you also don&#;t need to worry about breaking the bank for instances where you need a lot of them.
Markings
Oh one last thing.
When you are at a job site, how do you know if you are looking at an A706 or A615?
Contact us to discuss your requirements of a706 rebar. Our experienced sales team can help you identify the options that best suit your needs.
By looking at the markings. The letter followed by the bar size identifies the type of steel:
W = A706, and S = A615.
And there you have it. Hopefully this helps you know a little bit more about A706 and A615 than before!
P.S. This is a rehash of an article I wrote in my weekly newsletter, &#;Back of the Envelope" &#; where I teach you SE-related things in 5 minutes (or less), once a week.
If you enjoy reading stuff like this, consider following me on Linkedin or subscribing to the actual newsletter at BackOfTheEnvelope.co.
Grade Yield (psi)
Tensile (psi)
40
40,000
60,000
60 60,000 90,000 80 80,000 105,000
Grade Yield (psi)
Tensile (psi)
60 60,000 80,000 80 80,000 100,000
Grade Yield (psi)
Tensile (psi)
100 100,000 120,000 120 120,000 150,000
In the United States, the size and grade designations of these carbon (or "mild") steel bars used to reinforce concrete are set by ASTM International. Besides mild steel, rebar is also produced in stainless steel and other specialty alloys. Included in ASTM bar specifications are carbon steel bars with epoxy and/or zinc coatings used to resist corrosion.
Rebar is available in Grades and specifications that vary in yield strength, tensile strength, chemical composition, and percentage of elongation.
The Grade designation is equal to the minimum yield strength of the bar in ksi ( psi) for example Grade 60 rebar has a minimum yield strength of 60 ksi. Rebar is typically manufactured in Grades 40, 60, 75, 80, and 100.
Current ASTM Bar Specifications include:
ASTM A615/A615M: Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement
ASTM A706/A706M: Deformed and Plain Low-Alloy Steel Bars for Concrete Reinforcement
ASTM A767/A767M: Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement
ASTM A775/A775M: for Epoxy-Coated Reinforcing Steel Bars
ASTM A934/A934M: Epoxy-Coated Prefabricated Steel Reinforcing Bars
ASTM A955/A995M: Deformed and Plain Stainless-Steel Bars for Concrete Reinforcement
ASTM A996/A996M: Rail-Steel and Axle-Steel Deformed Bars for Concrete Reinforcement
ASTM A/AM: Deformed and Plain, Low-carbon, Chromium, Steel Bars for Concrete Reinforcement
If you want to learn more, please visit our website astm a615 rebar.