How thin is too thin for concrete

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Old 07-12-16, 06:08 PM
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How thin is too thin for concrete

We were going to have a truck come in to pour concrete this weekend coming but now I'm thinking of holding off.

The issue is that we are renovating our garage and it has a slope to the floor from around the center of it to where the door would be. This means it goes from a "0" grade to a negative depth of 2-1/4". I'm realizing that it would be impossible to hit the "0" grade as the concrete would be way too thin IMO that it would crack?

Once the floor is level we would be putting down the sub floor and then the laminate flooring. I jsut worry that if the concrete is too thin where it comes up to the zero grade it would crack just from people stepping on the floor or from curing.

Any suggestions?

Thank you
 
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Old 07-12-16, 07:27 PM
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A slab for a floor is usually around 4" thick. So your grade should not vary much at all from that.
 
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Old 07-12-16, 08:36 PM
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Welcome to the forums.

we would be putting down the sub floor and then the laminate flooring.
One of my worries would be about a moisture problem putting wood over a concrete garage floor. Have you checked for moisture coming up from the concrete ?
 
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Old 07-13-16, 04:08 AM
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Generally plastic is laid under the slab to help control moisture migrating up but many don't lay the plastic under the garage slab because it's not considered part of the living area. I'd check the slab for moisture first! A cheap simple test is to tape plastic [garbage bag will do] over several sq ft of the slab and then check it a few days later to see if it's still dry under the plastic.
 
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Old 07-13-16, 06:12 AM
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Thanks for the tip, I'll try the plastic trick and keep an eye on it for a few days. As for the 4" thickness, this would be in place for the entire floor already but with this slop in the one area we need to bring it up to being level with the rest of the garage.
 
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Old 07-13-16, 08:10 AM
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If going with concrete you'd be better off going higher so the thin end isn't so thin. What many do around here is to cut 2x runners/joists and lay them over the existing concrete so they can install a level plywood floor ...... but that's not my area of expertise although I'm sure some of the other can elaborate.
 
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Old 07-18-16, 02:09 PM
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I think you should be more worried about the concrete overlay debonding from the parent concrete than a few cracks showing up. If you don't know what you are doing, it's easy to screw up an overlay. Having to rip everything out and start over can be an expensive lesson.
 
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Old 07-27-16, 10:05 AM
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I've chipped up what was on the floor then I plan to scrub the floor clean and pickup a bonding agent to spread on the floor for the new concrete to adhere to. What others steps would you recommend?

Thank you!
 
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Old 07-27-16, 07:44 PM
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I've found that a neat (meaning no other additives) Portland cement slurry, about the consistency of heavy cream, is the best bonding agent available. It's cheap, easy to mix up and apply using a stiff-bristle street broom after being dumped onto the saturated-surface-dry floor. Just don't let the slurry dry before your concrete is placed, or it will act as a debonding agent. You want the bonding surface to be rough, so if the "chipping" you mentioned earlier didn't roughen the smooth floor, you should do so using either bush hammers or a scabbler. Removing enough depth of the existing floor will also alleviate your fears of too thin an overlay causing cracking--just make sure the removal depth is at least one-half inch larger than the coarsest rock in the mix. Strike-off, placement and finishing on the shallow end will be impossible if that's not the case.
 
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