Terrible dusting on concrete slab

[dangti6]

I had a concrete slab laid a couple of months ago and the laitance is really bad.

This is the surface as it was drying.





I am sweeping up a load of dust.



Areas like this are very gritty underneath. If you run it, the grit loosens
separates



This area is probably the better part with a solid base near the top.



Before I hire grinders, is it beyond saving? I’m wondering if there was so much water added that it’s weak quite far down.

 

[ConCretin]

Wow! That's not laitance. Laitance is dissolved solids being brought to surface by moisture. I'd characterize what you are observing as a horrible case of dusting.

This occurs when water - rain, bleed water or water applied during finishing - destroys the water cement ratio in the concrete surface. I'm not sure which one it was but I'm betting it was one of them.

The only potential solution is to remove the weak surface and grind the substate smooth to make it serviceable. If however, the concrete was placed at a very high slump and is itself compromised, replacement is the only option.

I hope you haven't paid these guys. Sorry you are having to deal with this but that looks bad.

 

[dangti6]

Thanks LLWillysfan

I dug out this image. This was taken just after the pour. They then later used a vibrating trowel and bull float on it.



The pour was undercover so no rain was involved and they didn’t add water when finishing.

The truck was a ‘mix on site’ type where they pour what you require. Suspect they added a lot of water to make it flow easier.

Intention is to epoxy eventually but wondering whether it’s ever going to suitable post grind.

 

[JoeMcGov]

Wow! That's not laitance. Laitance is dissolved solids being brought to surface by moisture. I'd characterize what you are observing as a horrible case of dusting.

This occurs when water - rain, bleed water or water applied during finishing - destroys the water cement ratio in the concrete surface. I'm not sure which one it was but I'm betting it was one of them.

The only potential solution is to remove the weak surface and grind the substate smooth to make it serviceable. If however, the concrete was placed at a very high slump and is itself compromised, replacement is the only option.

I hope you haven't paid these guys. Sorry you are having to deal with this but that looks bad.

Eggzactly this ^^^.

There's no soft selling it. That looks very very bad. Also hoping you haven't paid for the work.

 

[dangti6]

Invoice has been paid

 

[Shea]

No, a coating is not going to adhere to that. It will just deteriorate underneath and peel up. The resulting mess on your hands is unfortunate. I would be calling the contractor back to ask what they are going to do to rectify the situation.

 

[LegacyIndustrial]

Diamond-Grind until you get a reasonable surface. As others have mentioned there may not be a reasonable surface to be had.
If you can get to the good stuff put down a full-flake system.

Good luck.

 

[ConCretin]

dangti6, there does appear to be an awful lot of bleed water on the surface, especially if the picture was taken before, as you say they used a 'vibrating trowel' and bull float. I'm assuming you mean a power trowel(?) and hopeful you don't mean a vibrascreed. If either operation was performed with that water present, you have the cause of your problem.

You don't generally see bleed water until after the floor is screeded and bull floated and you want the bleed water gone before you put a power trowel on the slab. if that amount of bleed water was present that early, it suggest excessive slump. If subsequent operations were performed with that much bleed water present, it almost guarantees dusting.

The situation may have been exacerbated by what appears to be a vapor barrier that prevents any of the mix water from going down and causing all of it to rise to the surface.

I'm afraid I don't have much hope this slab will be serviceable much less suitable for epoxy.

 

[dangti6]

Thanks guys.

They didn't use a power trowel or a vibrascreed. I looked up what I recall the vibrating bullfloat looked like, and it was very similar to a magvibe. Essentially a battery powered vibrating bracket on a bull float. Then a manual bullfloat was used.

Yeah a damp proof membrane is underneath which retained the water.

 

[Rural53]

I would be calling the contractor back to ask what they are going to do to rectify the situation.

And their answer had better be "Rip it out and start again".

 

[Armorpoxy]

Attempting to coat this floor could be very difficult. Once you have established that the floor is structurally ok we would suggest covering over it with a garage tile such as our www.suprtile.com industrial grade interlocking tiles, or other tiles recommended on the GJ which are designed to go over poor substrate floors and will make short order of getting a nice surface.

 

[dangti6]

I have power washed the surface and gone over it very quickly with a small 4” concrete grinding disc in one pass, purely as a test before I look to hire in a machine.







Obviously now exposing aggregate but the water does run clear after a scrub and the surface does feel OK.

 

[ConCretin]

How long has it been since the slab was placed? It might be counterintuitive but you generally want to wait until the concrete achieves full strength before grinding if you want a smooth surface. Otherwise the cement paste will abrade easier than the stone and leave a uneven surface, which is what you appear to have.

If the cement paste in your slab is still 'weak', the question is why. Inadequate time to reach full strength or so much water, it never will? Depending on the answer, you may have to wait or keep grinding until you reach sound concrete.

Hoping you can salvage your slab.

 

[dangti6]

Appreciate the support. I’m hopeful also, but not holding my breath.

It was poured on May 7th, so 8 weeks yesterday.

 

[SALIV8]

I would stop and call the company. Send them an email with pics. I’d at least give them a chance to fix it.
If they tell you to pound sand open an inquiry for consumer fraud with your states attorney general it’s free online.

 

[Nthill93]

That’s way too much water. I hope you called the contractor if he thinks it’s an issue he’ll go after the cement plant since they mixed it, but he should have caught this

 

[pepi]

OP what's the hold up, calling the folks that got your $$$$?

 

[dangti6]

Hi Guys

After burying my head in the sand hoping the issue would go away if I didn't think about it, keeping the door closed, I sent a mail to the contractor this week with some images and he is sending someone out to have a look.

Why didn't I do this earlier? Not wanting the confrontation if I am totally honest. Doing nothing felt easier than facing up to it. I naively believed I could simply make the issue go away by trying to sort it myself to save the stress, but in reality that itself was stressing me out. After washing more of the dusting away on an off over the last few weeks and thinking about proceeding to hire a grinder and possibly throw good money after bad if there wasn't a good surface beneath, I finally decided to raise the complaint.

Not sure what they will tell me. Perhaps reassure me it will be find after a grind. Maybe it will be, maybe it wont. Give them the benefit of the doubt.

Yes I should have written to them sooner. Yes an email should have been easy to write. Mentally I struggled to face up to it.

 

[SALIV8]

No prob OP it can be a rough pill to swallow when you get **** work after you paid good money. BTDT

Hopefully they fix it. Keep us in the loop were rooting for ya!

 

[The Cobbler]

good luck for a good outcome.