What does it takes to weld rebar correctly?

Welding of reinforcement is inevitable in precast production. Whether in the pre-pour fabrication, installation of inserts, or making connections during installation, there are a myriad of scenarios where welding comes into play. In most cases, the quality of welds is crucial to the structural integrity of the final product. As such, PCI and NPCA quality control standards have specific requirements that govern the welding of rebar and it is therefore essential that producers understand the importance of welding rebar correctly. Both standards refer to AWS D1.1 as the standard for welding structural steel or AWS D1.4  for welding reinforcement.

The most commonly used reinforcement grades in the precast industry are ASTM A615 and ASTM A706. A706 is specifically formulated to be weldable while A615 is not. The main factor that differentiates these two grades is their carbon content, which must be considered, along with the tensile grade bar size when applying the proper welding practice. These factors will determine the minimum preheat and interpass temperatures for the base metal. A higher carbon content generally means susceptibility to marstenite formation and brittle failure/fracturing after the weld cools; producing a more ductile metal structure.
When specifying a weld, the starting point is always to check the carbon content of the base metal and calculate the CE value.
To calculate the CE (Carbon Equivalency) value, the mill certificate must be obtained from the supplier, and the following formula used:
C.E. = %C + % Mn/6

If information about the chemical composition of the steel is not available, the following procedure can be used:

A615/A615M Steel
1. For bars up to #6 inclusive, preheat to at least 300 degrees F (150 deg C)
2.For bars larger than #6, preheat to at least 400 degrees F (200 deg C)

A706/A706M Steel
1. For bars up to #6 inclusive, no preheat required
2. For bars #7 to #11 inclusive, preheat to at least 100 degrees F (40 deg C)
3. For bars larger than #11, preheat to 200 degrees F (90 deg C)
Regardless, when welding any metal in cold climates, the base metal should always be heated to a minimum temperature of 70 degrees F (21 deg C) and maintained at same temperature during welding.

For large elements, the material within at least 3" (75mm) of the weld should be heated.
This methodology certainly also applies to welds in structural steel plates and inserts as well as combination welds between reinforcement and structural components like plates and angles, which is very common in precast.

Some of the most difficult scenarios are presented in the form of welds on inserts and reinforcement in galvanized, stainless and epoxy coated finishes.

These types of welds are extremely difficult and should only be carried out by certified professionals under the correct conditions specified in the AWS code, as they have specific requirements for preheating and removal/reinstatement of coatings once welding is complete.

For further information it is recommended to consult PCI MNL 116 (Quality Control Manual) and PCI MNL 127 (Field Erection Manual) as well as the relevant AWS standards cited therein.

It is highly recommended that all staff engaged in welding at precast plants are certified to weld to AWS standards, and trained to follow the relevant precast quality control procedures which will include the calculation of carbon equivalency as a minimum.
Reference:
https://precast.org/2017/11/importance-calculating-carbon-equivalent/