Prefabricating Bar Mat with Welds

[CUChalk]

My client is a concrete contractor that I work with frequently. He has an upcoming job with a very tight construction schedule and in order to save time he has suggested pre-fabricating the rebar mats in his shop using ASTM A706 Grade 60 rebar. The engineer involved with the project has stated that "welding rebar mats together goes against everything I've ever been taught". The contractor has asked me for help with references or anything that permit this. I personally haven't had any experience with this specific application so I went to the codes.

ACI 318-14 26.6.4.1(b): Welding of crossing bars shall not be used for assembly of reinforcement unless permitted by the licensed design professional.

The commentary talks about how tack welding can create a metallurgical notch effect that weakens the rebar, but indicates it's possible if welding operations are under continuous competent control.

Then I found ASTM A184 which seems to indicate a process that allows this to happen (indicates how often to weld and some other min. requirements). It doesn't specifically provide any welding procedures that are acceptable, nor does it rule any out. I've also seen that CRSI allows fusion welding to be done by special machinery, but that's not an option here.

What are your thoughts on allowing the contractor to weld the rebar solely for the purpose of saving time over field-tying? This reinforcing will be going in foundations/slabs supporting heavy industrial machinery.

Thanks,

Chalk

 

[Seppe]

Refer to CRSI ETN-M-9-16 for the rebar industry's position on welded rebar mats. They promote the use of pre-welded rebar mats provided they are (a) comprised of ASTM A706 rebar in one direction and ASTM A1064 non-structural holding wires in the other direction and (b) produced on machines that employ a controlled electric resistance welding process as is done with WWR. Be forewarned that there is no ASTM standard for a pre-welded bar-to-wire mat, so this approach has its own potential pitfalls when it comes to material certification and QA/QC, as the finished welded product is not covered by ASTM A706 or ASTM A1064.

As for pre-fabrication by traditional stick welding (SMAW, for example), you'll likely not find a whole lot of supporting technical documentation or reference standards for such a practice. AWS presents no guidance on a crossing bar "joint", so the acceptance/promotion of this welding method would likely just be at the discretion of the EOR, a potentially dicey place to be without support documentation.

Finally, ASTM A184 has long been a fairly ambiguous spec section, and this is reflected in its limited acceptance in ACI 318 Chapter 20. I'm still not real sure how A184 gets deployed in practice.

I'd personally be leery of getting on board with what the contractor is proposing in the scenario you've described.

 

[Deker]

It sounds like you're aware of the issues. See this article for more information: Link. If the rebar fabricator wants to take this on they would need to obtain a copy of ASTM 1064 and determine weather or not they could comply with the requirements. There are also companies that specialize if providing prefabricated deformed bar mats that comply with ASTM 1064, and that may be the more practical option. I have experience with this company and they were easy to work with: Link

 

[jittles]

How about half-way in between - a shop-tied mat?

 

[Seppe]

Seeing as ASTM A1064 is a specification that is limited to wire and welded wire reinforcement (both plain surface and deformed), just be careful in trying to use the spec as a benchmark for fabrication of welded rebar mats.

Deker's second link refers to a company that manufactures ASTM A1064 WWR, which is often substituted for rebar for the very reason mentioned by the contractor in the original post.

 

[Deker]

Seeing as ASTM A1064 is a specification that is limited to wire and welded wire reinforcement (both plain surface and deformed), just be careful in trying to use the spec as a benchmark for fabrication of welded rebar mats.

Seppe is right, thanks for clarifying.

 

[bones206]

I'm with jittles. A shop-tied mat would be much simpler. Then it just becomes a rigging design problem.

Here's an example:

https://www.hanessupply.com/feature...er-on-a-project-with-a-complicated-lift-plan/

 

[bob33]

A large number of precast bridge beams were replaced during construction of the Herb Gray Parkway in Windsor, ON in 2013 because tack welds had been used,