new basement slab with some cracks forming


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Old 05-02-12, 06:40 AM
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new basement slab with some cracks forming

Had the basement slab put down in Mid December just noticing a few hairline cracks. Are these normal.? I have had suspicions the pour was not the best and had the masons come back in and grind down and seal the top layer as it was "dusting"

I still think it seems a little weak and now cracks appearing. So far I just see the 1 but I already have rugs down on about 85% of the floor so hard to say if there are more

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Old 05-02-12, 07:26 AM
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No getting around it, concrete cracks.

What do you mean by "I still think it seems a little weak"?
 
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Old 05-02-12, 07:39 AM
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I am not an expert and only know what I have read about concrete so I hope I just dont know what im talking about BUT

we had a dusting issue that the masons came back and grinded down and sealed. I was hoping this was normal because we used a vapor barrier under the concrete so I know sometimes that can cause dusting as all the water must evaporate up through the concrete. The way I determined the dusting was bad enough to need repairs was I could scratch into the floor with a penny.

Now fixed and sealed I can still take say a nail and scratch into the floor. I dont think I could dig it up and I have not really tried to see how big a mark I can make because I dont think I want to know. but I can scratch an indent into it with a nail. and now the minor crack appearing have me worried

I know the old floor that was 65 years old I could toss a hand grenade on and it would not scratch. of course it sank 10 inches but it was strong as iron.
 
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Old 05-02-12, 07:47 AM
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Using a vapor barrier beneath the concrete means you stop the moisture from getting to the slab, thereby limiting what has to evaporate through it.

Regardless, being able to scratch the concrete does not sound good, I think I would have the contractor come back out.
 
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Old 05-02-12, 07:57 AM
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Thats not even an options now as I had a 50,000 basement renovation built on top of it this month. If the concrete is bad I would just bulldoze the house and go live under a bridge someplace.

What I read is with a vapor barrier the water does not get to evaporate through the top and leach into sand or soil when curing because the barrier prevents absorption from the bottom. I read this often cause dusting or a weak surface layer as all the moisture from the pour comes up through the slab.

but again I wouldn't be here if I had a good answer
 
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Old 05-02-12, 08:40 AM
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Oh, I get it now - you're not talking about ongoing moisture, you're talking about moisture in the curing process.

Hang tight, we have guys here with more concrete knowledge than I who should be along.
 
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Old 05-02-12, 08:57 AM
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Bingo!. I know I worry to much I actually think my crack is probably fine im more concerned that I can easily (well somewhat easily) scratch into it. my recent renovation on top of it the contractor had to cut into it for a drain and he said while dusty it seemed fine. Hes not a concrete guy but he is very good. Still I cant get over the fact that I can scratch into the surface with a nail and now its really to late to fix.


my hope is I just have dusting and not a total failure -> Concrete Dusting: chalking, powdering at the surface of a concrete slab
 

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Old 05-02-12, 12:50 PM
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Concrete is not super hard. Carpet layers can nail their tack strips into it with a hammer and you can shoot longer nails into it with Powder guns.

Was this from a ready mix plant or hand mixed?

I think you might be expecting too much.
 
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Old 05-02-12, 01:34 PM
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it was from a plant but unfortunately I did not do research on things to look out for until after I saw a problem so I dont know anything about the batch that was used and calling now seems pointless as its poured and built on.

I would love to be dead wrong trust me. What is a good self test I can do to give myself some reassurance that I am just nuts?
 
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Old 05-02-12, 04:12 PM
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If it was from a plant I would suspect the mix was good.

Easily scratched with nail or stiff bristle broom.
Highlighted a couple of important parts. Hopefully our concrete guy will swing by, but I think your nuts.
 
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Old 05-02-12, 11:41 PM
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I'm not a "concrete guy" (wonder who he is, anyway?), but I've poured a few yards in my day, and provided quality control inspection on a few thousand more yards. I don't believe a vapor barrier under the slab was totally responsible for the softness of your slab's finish, but it could have contributed to it. Your slab's water-cement ratio was probably compromised by the concrete being finished (i.e., overworked) before the bleed water had a chance to evaporate off. Contractors in a hurry to finish the job and send the boys home, before the overtime clock kicks in. I've seen it happen dozens of times, and unfortunately there's little that can be done about it at this point. Wouldn't hurt to give whatever is still exposed a good coating of Kure-N-Seal, and hope for the best.

On interior pours I've done myself, I've often used large towels to soak up the bleed water to enable me to get things finished. That, and even setting up fans to hasten the evaporation rate of the water as it comes to the surface. Trying to finish "soup" is one of the most common mistakes amateurs (and even so-called professionals) can make.
 
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Old 05-03-12, 06:33 AM
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So final question. If the slab was finished poorly and so not as strong as it should be does that mean that it will deteriorate completely.? It is a non load bearing interior floor so I dont need a bomb shelter just something that wont turn to dust that holds my rugs up.

This is also at grade level its a full walkout basement not below grade so probably less water issues the slab has to deal with. I also put french drains and a sump in to be sure.

If it stays as is for the next 50 years im fine it its gonna be dust in 5 years im selling my house
 
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Old 05-03-12, 09:53 AM
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As Bridgeman says, overfinishing or early finishing could contribute to dusting. However, my first question to you is how did they keep the area warm while they were pouring/finishing? Given where you live and the timeframe your basement was finished in, it's possible they used a kerosene, propane, or oil heater to heat the area. If they used any type of fossil fuel space heater, then THAT could be the cause of the dusting. The Carbon monoxide (or dioxide, I forget which) produced can react with the surface paste and cause dusting.
The crack in the photo is a no-brainer. It originates off a re-entrant corner (a corner pointing into the slab). Concrete (good or bad concrete) will almost ALWAYS crack off re-entrant corners because as the curing concrete shrinks, it can't shrink around the corner so it cracks there.
Dusting will continue down to a point, then stop. However, to stop it now, use a liquid floor hardener/densifier. Google search for brands. I would suggest one, but the site mods don't like advertising. Good luck.
 
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Old 05-03-12, 10:45 AM
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great news on the crack the more I look at it the more I hope its nothing

The house was built in 1962 I have the basement gutted, dug up and re poured because it had sank really bad. So it was inside of a finished home no need to heat. I would guess it was 50's ish down there the whole time
 
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Old 05-03-12, 02:12 PM
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not great but here are 3 pics one with no floor, one with rebar ready to pour and after remodel (2 months after the pour.)

I have no good pics of just the cement sorry
 
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Old 05-06-12, 09:19 PM
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The ball's in your court--you can continue to worry this thing to death, lose sleep over it and shorten your life in the process. Or you can heed my earlier suggestion, and then forget about the situation.

Your basement will not collapse and disintegrate. Trust me.


P.S. Do you ride a Ducati? Or possibly own the factory?
 
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Old 05-07-12, 09:55 AM
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I would not care about my basement floor if I owned Ducati

I Rode and raced them (amateur) for years. Sadly now the wife, kids, houses, work are no longer conducive to it so all I have left is good memories.
 
 

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