Not all rebar is created equally.
By Claude Goguen, P.E., LEED AP
Proper practice in the welding of reinforcement is of particular importance in the precast industry. Welding of reinforcement can serve both as a means of expediting the production process and of creating material savings. However, it is important to exercise caution to ensure safe practices and to produce quality structural welds that maintain both steel strength and concrete structural integrity. From time to time, reports from NPCA plant certification audits point out some deficiencies related to welding reinforcement that center around three major requirements.
The first is American Concrete Institute’s ACI 318-08, Building Code Requirements for Structural Concrete and Commentary. Section 3.5.2 of ACI 318-08 states that welding of reinforcing bars shall conform to AWS D1.4 and that type and location of welded splices and other required welding shall be indicated on the design drawings or in the project specs.
That takes us to our second standard, the American Welding Society’s AWS D1.4 – Structural Welding Code – Reinforcing Steel. This code requires that the carbon equivalent (C.E.) be calculated prior to welding. Metallurgists typically define the weldability of steel based on its carbon content or C.E. The lower the carbon equivalent, the more weldable the steel. Steels with carbon contents less than 0.35% are considered to be readily weldable. According to table 5.2 in AWS D1.4, the C.E. for #7 (22) bars and larger should be less than 0.45% and for #6 (19) and smaller bars, C.E. should be less than 0.55% in order to weld them. If the C.E. is not less than the value indicated, then the bars must be preheated as per AWS D1.4. The higher the strength and the carbon equivalent of the steel, the more susceptible it is to cold cracking (cracking that occurs after the metal has solidified) when welded. A brittle weld is unacceptable, especially in cases of fatigue or impact loadings. Brittle tack welds on cold-worked steel have caused rebar failures in reinforcement cages during the assembly process.
Rebar used in precast concrete products usually comply with either ASTM A615 / A615M-09, Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement, or ASTM A706 / A706M-09, Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement. And that takes us to our third and final standard from American Standards of Testing and Materials. The most common steel rebar we see in plants are manufactured in compliance with ASTM A615 / A615M.
Note 1 of this specification states that welding of A615 steel should be approached with caution since no specific provisions have been included to enhance its weldability. For this reason, the C.E. has to be considered if planning to weld this steel; AWS D 1.4D/D 1.4M is recommended for filler metals, temperatures and performance/procedure requirements. ASTM A706 / A706M rebar is manufactured with a chemical composition that is enhanced for welding; its C.E. is limited to a maximum of 0.55% and consequently requires little or no heat treatments. 706 steel is stamped with a “W” in the deformations of the reinforcing bar, indicating that the bar is weldable.
The formula for C.E. of A615 steel is given in AWS D1.4 as:
C.E. = %C + %Mn/6
The ASTM A706/A706M specification and the ANSI/AWS D1.4 Welding Code have the same formula for C.E.:
C.E. = %C + %Mn/6 + %Cu/40 +
%Ni/20 + %Cr/10 – %Mo/50 – %V/10>
where:
C.E. = Carbon Equivalency
C = Carbon Content
Mn = Manganese Content
Cr = Chromium
Mo = Molybdenum
Cu – Copper
Ni = Nickel
V = Vanadium
These values of carbon and manganese content are given on the mill certificates or Mill Test Report (MTR) you receive from your rebar supplier. Better yet, the C.E. is usually calculated for you and included on the mill certificate.
Recommended tack welding practices and C.E. values that are indicated in the specifications/codes listed in this article must be followed in the field to ensure that reinforcing cages are properly fabricated. The reason this is so important is because poor-quality tack welds on reinforcing cages not only can decrease steel yield strength, but can also decrease the concrete’s fatigue life and durability. So remember the next time you consider welding steel rebar without first confirming what type of steel it is and ensuring that it is truly weldable, the sparks that fly after a structural failure may make your torch look dim in comparison.
For more information on welding reinforcement, please refer to the recently revised Tech Note at www.precast.org.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.
Good article with good details .
In many instances the welding of re bar cannot be avoided .
Obviously testing needs to be done for the batch but far too often welding of this is rejected out of hand.
Testing is the key I believe & selecting bar that is weldeble
what is the weld reinforsment formula
Hi Kumar, thank you for the comment! This response comes from Claude Goguen, our Director of Technical Services:
If you’re referring to the carbon equivalency formula, it is C.E. = %C + %Mn/6 for A615 steel and C.E. = %C + %Mn/6 + %Cu/40 + %Ni/20 + %Cr/10 – %Mo/50 – %V/10 for A706 steel.
Where
C.E. = Carbon Equivalency
C = Carbon Content
Mn = Manganese Content
Cr = Chromium
Mo = Molybdenum
Cu – Copper
Ni = Nickel
V = Vanadium
I am tack welding a 25 mm rebar woith 10mm spirals in a concrete column. I have a rebar carbon content at 0.3% and CE at 0.52. It was preheated to 93 deg C for FCAW or SMAW process. Will this lead to rebar breaking at welding?
What is the correct welding procedure like preheating to avoid any cracks or brittlness?
Thank you for the comment Ravi. If you are still in need of information, Claude Goguen, one of our technical engineers at NPCA, has provided the following response:
“According to the American Welding Society D1.4 – Structural Welding Code – Reinforcing Steel, if you’re welding a 25 mm bar with a 10 mm bar and you have a CE of 0.52%, then according to table 5.2, your preheat temperature should be at least 10 degrees C. I can’t say for certain that 95 degrees C is excessive enough to cause issues but keep in mind that preheat temperatures can exceed 250 degrees C for some cases. What I would be careful of is tack welding in this specific situation. Please make sure tack welding is permitted by the specifying authority. Tack welding can be detrimental in some cases and may reduce tensile strength and bendability.
Additional information and the correct welding procedure can be found in AWS D1.4. You can click here to view the resource.”
The best thing is to follow your wps and to avoid distortion yours tacks must be strong to avoid twists and turns of your parent metal and u have to post heat it after completing your welding
Hello Sir,
Why we avoid welding below 16 mm dia bar….pls tell
Hi Salman,
Phil Cutler, an engineer from our Technical Services team, offers this response:
Welding of 16 mm reinforcing steel is certainly permissible under proper guidance. We are not sure of the source that you have cited where it would not be allowed, given the proper procedures. Our experiences point to the following:
Cages of reinforcement may be welded if permitted by the applicable ASTM product standards. Welding of reinforcing steel may be also permitted in other situations as determined by the manufacturer where the steel is not used for structural purposes.
In all cases, care and discretion must be used to assure that the integrity of the precast product is maintained.
Reinforcing steel used for structural purposes may be welded as long as it is accomplished in compliance with standards set forth in the American Concrete Institute’s “Building Code Requirements for Reinforced Concrete” (ACI 318) and The American Welding Society’s “Structural Welding Code – Reinforcing Steel” (AWS D1.4).
Welding of ASTM A615 reinforcing steel is not generally an acceptable practice. According to the American Welding Society D1.4, “Structural Welding Code for Reinforcing Steel,” the carbon equivalent for bars to be welded should be less than 0.45 percent for bars larger than #7 and 0.55 percent for #6 bars and smaller. If ASTM A615 steel is to be welded, an approved welding procedure meeting the requirements of AWS D1.4/D1.4M shall be used and the carbon equivalent shall be calculated and the bars preheated if necessary.
Use of ASTM A706 weldable grade rebar for welding applications is acceptable.
As stated in the article:
The Carbon Equivalent (CE) for ASTM A615 reinforcing steel is calculated as follows:
CE = %C + %Mn/6
The Carbon Equivalent (CE) for ASTM A706 reinforcing steel is calculated as follows:
CE = %C + %Mn/6 + %Cu/40 + %Ni/20 + %Cr/10 – %Mo/50 – %V/10
Hello,
What happen in the case I am spot welding (pressure, heat and no filler material) the reinforcement for a precast element? Does table 5.2 of the AWS D1.4 applies for this ? example of this may be Bamtec machine (https://www.youtube.com/watch?v=z7hNMXlEChY ) or Mbk ( https://www.youtube.com/watch?v=G5oBO18IQ6w )
does even AWS 1.4 applies for those machines ?
Thanks,
Thank you for your comment Roberto. I forward your question to our Technical Services engineers. The following response is from Kayla Hanson.
We recommend inquiring with the American Welding Society for clarification of their requirements.
AWS D1.4, “Structural Welding Code-Reinforcing Steel,” applies to the welding of reinforcing steel to reinforcing steel and of reinforcing steel to carbon or low-alloy structural steel. This code shall be used in conjunction with the prescribed general building code specifications and is applicable to all welding of reinforcing steel using the processes listed in section 1.4 including shielded metal arc welding, gas metal arc welding or flux cored arc welding, and performed as a part of reinforced concrete construction. When reinforcing steel is welded to structural steel, the provisions of AWS D1.1 shall apply to the structural steel component.
As to the applicability of the specific equipment you mentioned, I would verify with the equipment manufacturer and consult AWS.
Great article.
How to avoid the risk of getting a notch in the longitudinal bar due to tack welding stirrups or any transverse reinforcement?
Thank you for your comment Carlos. I forwarded this to our technical services engineers. The following response is from Mitch Rector.
AWS A3.0, “Standard Welding Terms and Definitions,” defines a tack weld as: “A weld make to hold the parts of a weldment in proper alignment until the final welds are made.” ACI acknowledged in the abstract, “Fatigue Tests of Reinforcing Bars-Tack Welding of Stirrups,” that tack welding reinforcement can cause damage to the longitudinal bars and reduce the quality of the reinforcement. R26.6.3.1 in ACI 318, “Building Code Requirements for Structural Concrete and Commentary,” states, “Tack welding (welding crossing bars) can seriously weaken a bar at the point welded by creating a metallurgical notch effect. This operation can be performed safely only when the material welded and welding operations are under continuous competent control, as in the manufacture of welded wire reinforcement.”
If tack welding must be done, it is important that the welds follow AWS D1.4, “Structural Welding Code – Reinforcing Steel.”
Two other articles for reference are Tack Welding Reinforcement Bars and Welding Reinforcement Steel: AWS D1.4/D1.4M: 2011.