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CMU Foundation Walls

CMU Foundation Walls

CMU Foundation Walls

I have a slab on grade building with masonry foundation walls bearing on concrete footing.  The masonry walls have #4 dowels at 48" o.c..  The masonry walls will be below grade 36" for frost protection and not subject to lateral forces.  The rebar is in there to just tie the masonry to the concrete footing.

My question is:  Do the masonry walls have to be grouted solid or is it ok to only fill the cores that have the rebar?

RE: CMU Foundation Walls

We specify any CMU below grade to be grouted solid.

RE: CMU Foundation Walls

No good reason to grout the walls solid.

There is no structural reason, but it is a good place to dump the excess if you used grout and not concrete.

If you are really worried about uplift it is a plus, but with verticals at 48" you still have the footing and associated loads.

RE: CMU Foundation Walls

You will get a variety of answers. I like them filled with grout to prevent the cores from filling up with water and freezing.

RE: CMU Foundation Walls

jike is correct. Your wall does not have strength concerns but practices. Ever think to fill the voids with insulation material to prevent cold floor around the perimeter?

RE: CMU Foundation Walls

The plant site that I am at has CMU walls with every 3 block filled with concrete/grout and rebar in it.

RE: CMU Foundation Walls

Grouting the cells below grade prevents water ponding and freeze/thaw issues.  Definitely a sound construction technique that I've seen used many times.

RE: CMU Foundation Walls

There is not much worse than partially filled grouted walls that are grouted without any controls. The results give no benefit and possible problems. - Sort of like "belt and suspenders" not as good as a belt or suspenders.

For controlled, specified grouting, smart engineers will use blocks with cores that align to facilitate proper, complete filling with 8" - 11" slump grout. Normal three core or two core block (even with flush ends) do not align well enough. Engineers/inspectors make a big deal out of mortar projections into the core space when grouting, but permit units that cause the same or worse situations. Conventional 2 core block with open ends (half cores) will leak leak grout, creating a semi-solid pyramid around the rebar if it ends up having good bond. In any case the rare problem of water accululation still existes. If concrete is used instesd of grout, you are just permitting a location to dump unconsolidated left over concrete.

You can do it, but it is not professional or correct.

In 40 years I have not seen too many problems (except for remodeling/alterations) and later utility access.

Forgive the tiraid, but I have seen too many good uses of masonry internationally and hate to see U.S. engineers habitually misuse it.


RE: CMU Foundation Walls

In our office, we always call out fully grouted masonry walls below ground.  

RE: CMU Foundation Walls

I would also recommend fill solid with 3000 psi pea gravel. It does not have to be grout/concrete. There are so many reasons why you want to have CMU wall inside the soil to be filled solid.

RE: CMU Foundation Walls

The prudent thing is to grout all cells below grade, preferably to a foot above grade. You are leaving yourself open to too many problems to list here just to save the client a few bucks.

RE: CMU Foundation Walls

An added benefit of solidly grouting the masonry below grade is that it will act to a certain extend compositely with the footing.  This deep beam will be more able to resist soil movements, sinkholes, depressions, expansions, etc...

Also I think type S (maybe M) mortar should be used when CMU is below grade.  I'm at the house can't remember which.

I'm not sure but in Florida it might be code that CMU must be solidly grouted.

RE: CMU Foundation Walls

Definitely always solid grout the CMU wall below grade, and above the wall wherever vertical or horizontal rebar is present.

An additional consideration is that sometimes CMU walls have to be grouted solid full height for fire separation resons, i.e., too close to another building, an interior fire wall, etc.

Mike McCann
MMC Engineering

RE: CMU Foundation Walls


You say the walls extend 36' below grade for frost protection??  I am not sure I understand how this helps you.  Can you elaborate on the logic behind this?  I guess we don't have to worry about this in the South.

RE: CMU Foundation Walls

You want to have the bottom of the footings below the theoretical frost depth (36" in this case, about 48" where I am) that is specified for the local area. It is a way of eliminating any heaving from frost lenses under the footing.

It has nothing to do with whether the walls are brick block or concrete.

From a practical point, once a building is constructed and becomes part of a "heat island" frost to the design is quite rare unless this is a north wall with less sun exposure.

there are other ways to minimize the possibility of frost heaves, but they often rely on keeping the moisture in the soil below at a low point, which is always a challenge,

RE: CMU Foundation Walls

Concretemasonry - you are absolutley correct about the frost protection.

It seems the general consensus is to fill the cmu walls below grade solid regardless is there is rebar or not.  Can anyone tell me if this is a code requirement.  If so, what code and where is it.


RE: CMU Foundation Walls

FJ88 -

It is not a code requirement. Many 20 story loadbearing high rise masonry structures are built using partially grouted walls. Many codes (IBC, IRC, etc.) permit the use of hollow CMU construction, since there are cases where it is preferred. - It is just a "if does not work use a bigger hammer" or belt and supenders easy approach without looking further.

If the cores are filled, it should be required and filled according to code specification, not just dumping concrete since there are different specifications for grout and concrete.

A good deceased engineer friend and author always said "Do not count on what your are not sure about, able to observe or specify" - He said this in regard to his books on CMU structural seismic design and general masonry construction, but also applies to engineers attempting to do additions and modifications. We ran into this both above grade and below grade after the Northridge quake.

If the CMUs are filled as an afterthought or plan omissions, there should be a revision to the drawings or as-builts. Nothing is worse than an owner not having proper drawings and contracting with someone (GC, utility contractor, etc.) for revisions and run into sloppy drawings that do not reflect what was built.

RE: CMU Foundation Walls

I agree as-built drawings are important but it seems that most owners do not understand the value of these documents.  I remember working for a large insurance company that sold a 120,000 sf or so office building outside Pittsburg in the early 90's, and the finding out who to ship the original construction documents to (from the 1970's) was a bear.  I would think the buyer would ask for them during due diligence but like 6 months after the closing, I was still asking for a name and address.  I think I just sent them to the building.

Don Phillips

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