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CMU Disintegrating

CMU Disintegrating

CMU Disintegrating

Hi all, this is my first post here. I was asked to inspect the CMU foundation wall in a small single-family home (I'm a licensed PE, practice structural engineering for 14 years) due to some "deterioration" in the CMU. The wall was 6 courses tall, with about 4-5 courses below grade, with 1 course below the finished crawl grade. There were large portions of wall on 3 sides of the home where the block was "mushy" at the lowest course, but seemed to be fine above the crawl space grade. The face shells could be wiped away like they were just mud, and there were areas that have to be supported on not much more than the joints. Anyone ever seen anything like this? Thanks in advance!

RE: CMU Disintegrating

Looks wet. Definitely appears to be due to moisture. I've never seen freeze/thaw degradation look like this though, but that'd be my guess especially since it's at grade. Do you have closer photographs? Where is this located?

RE: CMU Disintegrating

Yay first post!

Never seen holes like that, but I've seen LOTS of mushy block. How old is the wall?

I highly recommend replacing the CMU (because, duh) but it will mean jacking the house. I'd also recommend getting some testing done on the existing CMU and the soil to figure out why it happened and to ensure it doesn't happen again. That's just weird.

Please remember: we're not all guys!

RE: CMU Disintegrating

Can you provide a bit more information?
1. Freezing region or non-freezing?
2. Near saltwater?
3. How old is the house?
4. Is the damaged block only on the inside of the crawlspace or both sides?
5. Is the mortar also mushy and damaged?
6. Are any of the block cells fully grouted or is everything hollow? If some are grouted - are they also mushy?

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RE: CMU Disintegrating

Mushy block can be caused by several things. The questions SLTA and JAE asked are relevant to the evaluation. There is some efflorescence and it appears you have relatively constant wetting below grade and a wetting/drying interface corresponding to the deterioration.

Masonry is made from a relatively low strength concrete. Wetting/drying causes severe leaching of calcium from the cement matrix, thus deteriorating the strength. Couple that with some soil reactivity (acidic, high sulphates, etc) and you have a problem.

Agree with SLTA about remediation. Masonry should also be coated with a good waterproofing below grade. My choice is asphatic or coal tar based.

RE: CMU Disintegrating

All, thanks so much for the comments and input. A little more info on the project: located in central Virginia, moderate freeze/ thaw, no salt water, circa 1992. Very common entry level home, not in an area I would consider wet at all. I didn't note any grouted cells, and compared to the block the mortar seems to be in a little better shape. The deterioration seems to be on both sides, I could push my screwdriver right into the exterior grade from in the crawl space. I've inspected many basement walls in the area over the years, and most problems stem from saturated clays and walls failing due to excessive pressures. I'm definitely leaning towards some reactivity with some pockets of something in the soils. If that's the case, it seems like replacing the block would only buy some time until it degrades again.

RE: CMU Disintegrating

I think the problem has much more to do with the individual conc. blk. themselves. Five, six or seven blocks all in the same course would very likely have come off the same pallet, from the CMU manuf’er. I’ve seen this problem before too, and usually it has to do with a fairly small number of blks. which likely all came from the same manuf’er. lot, were laid at the same time and most likely from the same lot of blks. Blks. immediately around the mushy blks. seem to be o.k., as is the case in your photo, which I assume is upside down as attached. I’m not discounting moisture as a deteriorating aspect, nor freeze-thaw or some reactive soils condition, or some salts of one sort or another, however, the next course up shows much the same moisture patterns (not full height) and would be more susceptible to freezing, but doesn’t show the same blk. deterioration. Some sort of soil reactivity would not likely be so well confined/defined as to start/stop at a horiz. mortar joint. It does seem that the face shells go first, and that cross shells last a little longer, and of course the mortar is a whole diff. animal. I think there was a small portion of a conc. batch which was bad, poorly mixed, reactive aggregate, missing some percentage of some ingredients, etc. or that as the raw batch mix materials were placed in the mold and compacted something went haywire, or that something about the curing process went wrong. It would be interesting to see some sampling and testing of the conc. blk., surrounding soils, ground water, etc. to see if anything stood out.

RE: CMU Disintegrating

I know I'm butting into areas which aren't my speciality, but to me they look like hollow blocks which someone has decided to fill with something rather softer than the main structural material.

If I bought a solid block I would expect a solid block all made of the same stuff. Maybe they got unlucky there and laid hollow blocks on the wrong side and then either filled them with something or perhaps the manufacturer was making his "solid" blocks by filling hollow blocks??

The blocks immediately above the crumbling ones have a similar filled in appearance to me

Either way they look like they don't give you the strength you need anymore.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: CMU Disintegrating

I see this a lot around here (Phoenix area) on the bottom row/rows of Dooley or Pilaster walls (the hollow 4" block ones that span horizontally b/w pilasters) where the blocks on the other side are constantly wet or below grade. Ron's explanation makes a lot of sense from what I've seen.

RE: CMU Disintegrating

soil type? High sulphates?


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