Not all rebar is created equally
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>
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.