"Wet" set first course of Masonry 1

[lsmfse]

I have been asked by several masonry contractors to "wet" set the first course of hollow unit block for a retaining wall. According to the code we use(2001 Calif. BC), this is not permitted. I get the usual "we do this all the time" response from the contractor. However, what would be the problem of "wet" setting the first course if all the forces can be resolved?

 

[JAE]

You might explain what "wet setting" masonry means.

 

[boffintech]

The first course in set on TOF while concrete is still fresh? How is this mo better? How do they get layout correct? Never seen that here.

 

[youngstructural]

If the "wet" setting you're describing is what I think it is, ie: litterally setting the first course of masonry block into the plastic concrete of the footing, I think I can explain a few problens...

1. The masonry will absorb water from the concrete, reducing the local strength around the base setting of the wall. Mortar is mortar, concrete is concrete. Just because they both are cementitious does not mean they are interchageable. Would you accept concrete as a mortar bed material? Neither would I...
2. I would be very concerned about what has happened to the reinforcing bars in the footing. Have they shifted, lowered down and eliminated their required clear cover, etc, etc, etc. There is just no way of knowing, and that's a problem.
3. I have actually seen a contractor pushing the reinforcing bars into the footing through the hollows in wet-set masonry block. Kind of funny, but not very impressive, and certainly not acceptable.
4. And the number one problem (IMHO): The blocks actually sink slowly into the footing, preventing the concrete from setting up properly and grossly reducing the strength of the footing to wall interface.

I remember there was a list of five things when I was taught to watch for this, but for the life of me #5 escapes me at the moment... I'll post it if I can remember. I think it had to do with trapped air, or perhaps that air entrainment is underirable in a meterial bonding to CMUs. I'll scratch my head a bit for you...

Hope that helps,
Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[lsmfse]

Yes, the contractor literally installs the first course of block on fresh concrete. The vertical reinforcing is tied in place prior to the concrete pour, so "stabbing" the rebar isn't an issue. youngstructural, I agree with your points. In my case, your points 2&3 aren't an issue. The walls I designed are site retaining walls to 4' max retaining and are based on half stresses; however, the owner employs a full time special inspector so In my judgement felt this wouldn't be a big problem.