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New CMU on top of exist CMU

New CMU on top of exist CMU

New CMU on top of exist CMU

Interior cmu wall, non-load bearing, probably 6" cmu x12' high but still need to verify.

For access purposes they have to demo about half the height and then re-install matching masonry. Since most likely there isn't going to be any rebar in the wall, I am just going to let them install the new cmu on top of exising with a mortar bed ofcourse. Would that be enough for the tie between the two or do I need to dowel the two halves together. I am going to have them brace the wall at top, if it wasn't already.

RE: New CMU on top of exist CMU

Also I am going to them tooth the new and old cmu.

RE: New CMU on top of exist CMU

I'd check the wall for the 5 psf minimum and seismic too, depending on your area.

I am in a high seismic zone, so I would feel better if I installed verticals at 48" minimum spacing, but you might not need it in your area.   

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: New CMU on top of exist CMU

I agree w/ MM

A few rebars and some grout is rather cheap

RE: New CMU on top of exist CMU

Mike & Mike, gracias. I am in Chicagoland so seismic is not an issue, and I have seen notes on drawings from a couple of engineers where they say no reinf required for 6" interior wall upto 14' as long as its braced on top.

RE: New CMU on top of exist CMU

But I guess I have another question now, since it was suggested that there should be some minimal rebar in the wall, how easily can the place new rebar in an existing wall? They won't be able to dowel it into the slab but I guess that would be OK, since shear at the base of the interior wall would be minimal. How would they grout the existing cores though?

RE: New CMU on top of exist CMU

I assume you wish to make the new section to match the existing for structural and appearance purposes. How high is the total wall after construction?

First thing is to get accurate information of the size, dimensions and configuration of the existing wall units, including the number of cores and webs.  The 12" height is extremely rare. Are you sure are not 8" high?

If it is actually concrete masonry, you can use normal mortar to lay the new units on the existing portion.

When you remove the old section and there is rebar, do not cut it off, but retain enough to allow the new rebar and be lapped and then you can grount those cores. Use 8" to 11" slump grout and not mortar. Depending on the height, it is up to you, as the engineer, to determine the placement methods based on the height of the new masonry. I would avoid clean outs if at all possible since they can decrease the quality of the construction.

Finding 12" high units, especially in a 6" thickness, will be a challenge since they are extremely rare in the world.


Engineer and international traveler interested in construction techniques, problems and proper design.

RE: New CMU on top of exist CMU


Pardon my ignorance but I am assuming that by cleanouts you mean shell openings knocked into the side of a cmu block, right? And thanks for the detailed response, like I mentioned I still need to go out in the field and verify the dimensions.

And while I have your attention here, what do you think about a 4" interior partition wall, 10' tall, braced at the top. Ignore any seismic loads, the wall is for mechanical chases and some toilets.

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