One of the most common ways to reinforce concrete is with steel rebar. When a product design or rebar layout requires welding, it is important to know if the steel you are using can be welded. ASTM A706 weldable-grade rebar is acceptable for welding under most circumstances. But unless special analysis and precautions are taken, it is not acceptable to weld ASTM A615 steel. For ASTM A615 steel, you must be aware of the carbon equivalent. This requires a simple calculation based on information available on the steel mill certificate.
Steel is an alloy of iron, carbon and often other elements. Differing amounts of alloyants will create different properties in the resulting steel. Carbon, for example, is a hardening agent that will make steel stronger as it is added, but can also make the steel more brittle when present in excessive amounts.
The American Welding Society D1.4 Structural Welding Code for Reinforcing Steel states that the carbon equivalent for welded bars must be less than 0.45% for bars larger than #7 and less than 0.55% for #6 bars and smaller. The equation for calculating the carbon equivalent for ASTM A615 reinforcing steel is:
CE = %C + (%Mn)/6
%C and %Mn are listed on every mill certificate, and the carbon equivalent should be calculated for every mill certificate. Sometimes, the carbon equivalent may even be calculated on the mill certificate for you. An example mill certificate is presented below. The blue boxes show the %C and %Mn.
If your steel has a carbon equivalent that exceeds 0.45% for bars larger than #7 and 0.55% for #6 bars and smaller, you can still weld them, but you must preheat according to the calculated carbon equivalent. Table 5.2 in AWS D1.4 lists the minimum preheating temperatures based on the rebar diameter and carbon equivalent.
Understanding your reinforcement goes beyond just strength. It also involves being aware of the ingredients in your steel. Calculating the carbon equivalent is simple, but it remains an important factor in creating quality reinforcement.
More information on welding rebar is available in the September-October 2017 issue of Precast Inc. magazine.
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