basement floor cracks

[Ken Meyer]

Got a call last night from someone whose home warranty expires in 2 days. So I went out this morning to have a look.

She is very concerned about a crack that she & her husband noticed in a finished basement room. It has patio doors opening to the back of the house. Just inside the patio doors, there is a noticeable dip in the floor. There is a retaining wall behind the house, and the lot is sloped. Only a small crawl space under the front entry.

The crack runs the length of the room, all along the back of the house.

No signs of movement from the outside. This seems like more than the normal amount of cracking. My guess would be either a poorly compacted base or a wet mix, since the foundation walls look okay.

The downspout drain runs into the wall and through the crawl under the front entry and exits the foundation at the front of the house.

There had been some cracks in the drywall around the upper corners of the patio doors and windows in the floors directly above the basement patio doors, but they have already been patched and painted.

The homeowner is nervous about it, so for her peace of mind, I recommended a structural engineer. I did tell her that at the very least, the builder needs to remove all the loose material and fix the crack.

I'd like to know if you think this may be anything serious.

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[Chad Fabry]

The crack likely indicates where the footing stops and the floor starts

[kurt]

Yep. And a lousy mix for the bsmt. floor.

[randynavarro]

I concur - line is between slab and stem wall.

How can you assess a bad mix Kurt? The slab is just shrinking at a different rate than the stem wall. I see that frequently on slabs poured like this.

[Ken Meyer]

Agreed, so the solution seems to be remove all loose material and and fill it in to level the floor. How much more can it be expected to shrink?

[AHI in AR]

It shouldn't shrink anymore since it's through curing. Also, I suspect that the dip in the floor has likely been there all along unless there are other cracks running basically front-to-back, indicating settlement.

[Brandon Whitmore]

I suspect that if you pulled up carpeting at areas such as this in other homes, you'd see the same cracks more often than not.

[Jim Katen]

Ken,

What you're seeing is typical.

The perimeter footing & stemwall are first placed on undisturbed soil or engineered fill. Then the builder adds fill to the inner side of the stemwall but he never manages to compact it properly. Most fail to compact it because they just can't be bothered. But some are afraid to run the compactor over it because they fear that they'll damage the green stemwall.

After the slab is placed, the fill settles and, eventually, the slab cracks.

Some builders are aware of this problem and address it by placing rebar pins across this stress point. This actually works for a while -- at least until the building is out of warranty.

How much it will eventually move will depand on how "fluffy" the fill was.

When it's under carpet, the fix is easy. Just clean out the loose stuff and use some leveling compound to level it out. If the slab continues to settle, you can add more leveling compound. Sounds cheesy, but it works just fine.

When this becomes a big stinking deal is when the floor is covered with tile. That's a real mess because you never know when you're done settling.

- Jim Katen, Oregon

[Ken Meyer]

It shouldn't shrink anymore since it's through curing. Also, I suspect that the dip in the floor has likely been there all along unless there are other cracks running basically front-to-back, indicating settlement.

They just recently noticed the dip, maybe it's because they haven't used that patio door much lately, or perhaps they went around looking for things since the warranty is expiring. There are other cracks perpendicular to this one, she had pulled up the glued down carpet pad about a foot or so.

[Bill Kibbel]

I'd like to know if you think this may be anything serious.

Many homes around here, with walk-out basements, have the same cracks as what's shown in your pics.

[kurt]

How can you assess a bad mix Kurt?

Easy. They're all bad. That's only half kidding.......

Crap mixes aren't that hard to identify. They usually dust up easy, they're *whiter*, and they splay feather edge shards of mix at cracks like these, they soak up water like a sponge. Really crappy, you can hear the water suck down through the mix if you pour a bucket full on the slab.

Honestly, that's totally subjective, but I've seen enough good concrete to know what bad concrete looks like. It just isn't right looking. Good stuff just has a texture and look to it.

[Ken Meyer]

Thanks for all your replies. The client is anxious about it, and I wanted to make sure it is not a serious concern. Her husband was putting his arm around her and offering reassurance. At least I think that's what was going on, they spoke to each other in Chinese. Maybe some of her anxiety rubbed off on me.