Defective concrete slab with excessive dusting?
We had our slab poured just over 1 year ago, it was a race against time as temps were rapidly dropping. Over the past year throughout construction I’ve noticed the basement air is very hard to breathe and there has been excessive dusting on the slab. Now I’m wondering what this means…
Sweeping the basement slab brushes up huge amounts of fine concrete dust. We’ve vacuumed up piles of grey concrete dust several times, but there is always more.
I don’t believe this to be efflorescence because we have 4” of EPS and poly VB under the slab, it’s also not visible white spots but rather the concrete surface itself is turning to fine dust. In high traffic areas like at the stairs and door to mechanical room, there is even a bit of exposed aggregate now.
There was some drama with the slab pour. In summary, our PM got some random concrete contractor and they started the pour in late afternoon.
– The PM left them there unattended and they were finishing the slab in total darkness. At like 10pm I think they finally left.
– All I know is that they broke ties with this concrete guy and had someone else come back to finish the partially complete work which cost another $8k so must have been significant…
– One room in the basement was not poured, no clue why. I guess they ran out of concrete.
– The unpoured room is in a photo below, I do not understand why it looks like gravel on top of the EPS and rebar. What is going on, won’t this puncture?
– The rest of the basement was poured but left very very rough. I have a couple photos I finally got the PM to send me but I don’t know if there were worse spots not known in these photos.
– It is abundantly clear they had no clue what they were doing, the finish was left horrible.
– Someone else came and fixed the slab, finished pouring the room which was missing, but it’s definitely still far rougher than we anticipated.
– Pic with the stairs sitting on the floor shows the current finish, as you can see there is texture.
So here we are. I think this is a warranty issue and I want my GC to pay to fix this slab, if there is a problem. What should I do about this?
I don’t know how we find out if there is a bigger issue, and what that risk could mean. As far as the surface finish, I am under the impression from a bit of research that we could have a densifier applied following by a concrete sealer.
We are planning to have concrete floor in the basement for some time and eventually put down either cork or linoleum. But I want to know there is not some serious issue being covered up. I find the air quality extremely poor in the basement and thought it was due to the ICF but now I’m not so sure. If you push a broom on the slab, it produces a haze/cloud of very fine concrete dust.
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Gaslighting is the name of the game in construction! In a way it sort of makes sense, there's lots of financial risk, extreme complexity and various skills involved. Anyways..
I guess my main question is: have you completed the contract with the GC? Or in otherwords, is this a deficiency that is still going to get resolved? Or even more simply put, is the expectation the GC is eventually going to pour the rest of the slab and apply a densifier to the whole thing?
For the dusting concern: concrete dusts if you don't apply a densifier. It will dust. Just looking at the photos, I don't see any spalling or surface chipping or excessive cracking, so my best guess without looking at it in real life is that I dont think there's a deficiency with the concrete itself. Given the complications you've described of your GC and their subs, I can see how it probably didn't get done. Check your contract and see if densifier and sealer was included.
If not, ask the GC to do it to make good on it (along with pouring the rest of the slab)
If all negotiations fail, and you're picking up the slack. Get a densifier and spray it on yourself, it's pretty easy. I like Prosoco products. Consider sealing the slab as well for any future stain prevention, and further consolidation of the surface.
For the finish concern: ya that's a pretty bad finish. Depends on what your contract said for the level of finish, and if it doesn't say anything, they you might be SOL. I kind of like that rugged look as it's a reminder of how hard concrete is to do (I love concrete because it's hard in density and in effort)
If you were planning to have a sleeker look to your slab and was going to leave it exposed, you might personally want a smoother finish.
Best looking and most expensive option is to hire a concrete grinding contractor and have them grind it down to whichever level of finish you'd like.
Another more modern or euro artsy option is to do a microcement topcoat, it's still quite expensive to hire contractor to do it, if you can find one. I do microcement myself and I love it. My wife hates it.
The cheapest worst looking option is to apply a concrete self leveller over the whole thing, and you'll have a drab looking, uniform grey slab. But it'll be smooth.
Jamie
That looks to me like a mix of power-troweling after the concrete had set up and over-troweling which raises fine particles to the surface where they don't have enough cement to keep them there. It may or may not mean defective concrete below the surface.
To add to what Michael said, some of the "puddle-like" areas look to me like maybe extra water was added to try to make the concrete easier to work, which can end up making a sort of slurry over the top of the slab if not mixed in well. Note that adding extra water to the mix causes other problems (that's what "slump tests" check for), so if they did any of that, that could be another issue. My guess is the concrete was starting to set while they were working (maybe they worked too slow?), so they tried to "fix" it with water, then tried to finish it with their machine anyway (making those marks).
If you're worried about the integrity of the slab, take a core and see how it looks through it's depth. This isn't difficult to do. If you have any electrical or plumbing guys on site, ask if any of them would be willing to do maybe a 3" core in the slab in a suspicious spot for some extra beer money on the side. Shine a flashlight into the hole, see if you see anything other than a good solid slab on the edges of the cored hole. If it looks OK, you just have surface issues. If it shows lots of voids/cracks/etc, then you might have problems with the slab itself.
If you end up only having surface problems you could have a concrete polishing company come and grind down the surface to make it look flat (and even shiny if you want), or use some floor leveler to fill in the worst of it to get a flat surface and use an epoxy to fill in the small stuff for a good finished look. Note that epoxy floors and polished floors can sometimes end up costing about the same amount of money, so it might be cheaper to just have the floor polishing guys finish the entire slab for you.
Bill
I’ll take note of these suggestions, thanks guys!
I will update my post to add that my GC did have another concrete sub out to finish pouring the missing room and do whatever they did to the rest of the slab. I believe they poured an inch or 2 on top of the rest. Of course I don’t think we had anything in our contract about the concrete finish… I will have to go back and dig to see an invoice for the concrete but I feel like it probably said barely anything.
There is one long fracture developing and there is definitely chipping around the cut lines.
We are still in construction, finishing stage now getting final fixtures and such in. We have a cost plus contract and will have Tarion coverage (specific to Ontario Canada).
I’m confused why there was gravel on top of the EPS in the originally unfinished section. I was not expecting to see gravel or such chunky concrete if that’s what it is, in the photo you can see with the rebar.
The core sample seems like a good idea… as for the finish, I don’t want to spend too much $ on it because we are planning to put flooring down eventually. But we don’t want this gross dust everywhere and we definitely don’t want the basement to have poor air quality like it does now.
That rebar isn't placed correctly. Rebar should be held up on chairs (little metal stands), or other suitable supports, so that it ends up in the approximate middle of the slab after the concrete is poured. If the rebar is UNDER the slab it doesn't do anything. I see this all the time. Putting rebar in the gravel bed isn't doing anything.
If you are primarily concerned with dust, just seal the slab and that should solve that problem. Wash, seal, wash, done. The concrete sealer should keep it from "dusting" more in the future.
Bill
Thanks Bill. About the rebar - you’re joking, right?
I’ve never even seen a basement built so had no idea what it should look like.. that one photo is all I’ve seen and it looked wrong to me on many levels. I assumed our PM knew what he was doing because he has a structural engineering degree…
So the rebar on the bottom/foam is a problem? And then what about all that crap on top of the rebar? What is it? I thought concrete would look different and I don’t see why it would be gravel because the gravel is below the foam.
It's not the engineer that messed up the reinforcement in the concrete, it's the installers. I've seen this MANY MANY times! Reinforcement, be it rebar or remesh, is supposed to be INSIDE the concrete, and that usually means about halfway deep in a slab. What I see all the time is the reinforcement is either not supported at all, which means it ends up UNDER the slab during the pour, or it's not supported enough, which means the weight of the concrete during the pour pushes it down in most areas, so it's under the slab in many places and up in the slab a little where there are supports.
Ideally, you support the reinforcement sufficiently that it stays in place during the pour, and ends up in about the middle of the finished slab. It looks to me like they just laid the reinforcing materials on the gravel bed and then presumably poured over the top, which means the reinforcing materials aren't really INSIDE the concrete, so they aren't really doing anything. This is a problem, but it's suprisingly common -- even on commercial projects!
Luckily, a basement slab isn't super critical, so you'll just end up with a slab more prone to cracking.
What rebar/remesh is supposed to do is to take tensile stress inside the concrete, since concrete is weak in tension (concrete is very strong in compression). The combination of the steel and the concrete makes a stronger overall structure, but that only works when the reinforcing steel is placed correctly within the concrete.
If they put down the rebar and then put the gravel bed over that, then they really have no business working with concrete.
Bill
Very interesting. Good to know that the slab is not that important, as far as critical errors go…
Ok so what I’m struggling to understand most is why that gravel is there in the photo as you can see! We have EPS under the slab, and is it common to then put gravel on TOP of the EPS under the concrete and rebar?! There was already a gravel bed under the EPS.
Concrete contractors like you have sand or gravel directly under the concrete as they get a better cure and it’s easier to work. This doesn’t mean it’s the right way to build it, just that this has been something that has been in practice for a long time. https://www.greenbuildingadvisor.com/article/polyethylene-under-concrete-slabs
It's also a cheaper way of getting the same thickness rather than using concrete.
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