hot dip galvanized WWR 2

[samdamon]

My understanding is that for hot dip galvanized WWR, the wires are hot dip galvanized per ASTM A 641, but, its done before they are welded into fabric. The welds obviously effect the protection provided by the zinc. I would like to specify that the WWR is hot dip galvanized after the welding of the wires, but there is no data about this on the web that I can find, I don't know if its feasible or common to do this. Any one have any experience specifying WWR that is hot dip galv after fabrication?

The WWR is being used in architecturally exposed cip concrete and corrosion resistance is important.

 

[Ron]

Sam...IMHO, you're approaching this from the wrong angle.

First, I'm just not a fan of galvanized rebar, epoxy coated rebar or passive corrosion protection systems in concrete.

What happens to the cut edges of your hot-dipped galvanized wire (post-welding)? There will be field cuts and there's not a field-applied substitute for hot dipped galvanizing. So what happens at those locations? Concentrated corrosion.

Leave the wire uncoated. Allow generalized corrosion and proper passivation of the steel to occur within the concrete. Spend your effort making the concrete better by controlling the mix design, getting good densification/consolidation, making sure the wire fabric is in the right location, and that you have adequate cover.

Pay attention to the little things...where does the wire terminate? Does it touch embedments? Is it close to aluminum or other dissimilar metals?

 

[larsacious]

Why not specify epoxy coated WWF? If they cut it in the field they can touch it up with epoxy paint.

 

[cvg]

are you going to spend the time inspecting each and every one of those cut and touched up spots to make sure they didn't miss it? I have to agree with Ron on this one, there are better ways to reduce the corrosion potential than with epoxy or galvanizing. And WWF is also not high on my list of favorite products.

 

[TXStructural]

Specify post-galvanized welded wire reinforcing as ASTM A123.

Unless you have a severe exposure, either A1060 or WWR coated to A123 can be used. The galvanic mechanism does not require 100% coating, since the coating will protect adjacent portions. Cut ends are not really an issue with galvanized bar or mesh.

Epoxy coatings protect by providing a barrier between the steel and agents of corrosion (air, moisture, and chlorides). These do have to be touched up, since the protection only happens where there is coating.

As Ron says, also consider the concrete quality and cover depth (look at ACI 318 Chapter 4. CRSI is in the process of drafting a new chapter on durability for the next issue of the Manual of Standard Practice - the durability considerations are covered in various places of the Manual in the current edition.

 

[samdamon]

Thanks for these comments.

 

[RebarRyan]

Samdamon,

From a reply you did way back in 2004, you mentioned that you could copy pages from your Jack Singleton, 1957 book "Manual of Structural Design" in regards to the loading tables for clay tile filler rib slabs. I would be very interested in those. Please post if possible. Thanks in advance.

 

[Gumpmaster]

Ron,

As TXStructural said, the zinc is the sacraficial anode and will corrode preferentialy before the steel. Even small uncoated portions will benefit from the presence of zinc because the zinc will use up the corrosion potential in the general area (a similar concept to using a sacrificial zinc block on your outboard boat motor).

Conversely, scratches in epoxy coatings can increase the corrosion potential because the steel needs to sacrifice a large portion of itself out of just a very small area (hence the advent of epoxy coated galvanized bars).

 

[Ron]

Gumpmaster..I agree. I just don't see that much benefit to using galvanized wire mesh as the concrete will passivate the steel sufficiently to yield a generalized corrosion rather than a localized corrosion.

Yes, cut ends will be protected to some degree, but will eventually corrode and exfoliate.

I'm still convinced that attention to concrete mix design, the placement density of the concrete and appropriate cover do more to protect from corrosion than other methods. The use of other methods can become a crutch to subvert the quality of other parameters that should be there.