Replies

1. 
   MarkH | Mar 15, 2015 08:32pm | #1

   You may want to talk to another concrete finisher.

2. 
   AndyEngel | Mar 16, 2015 08:49am | #2

   I second the suggestion to look for a new concrete guy. In this case, the plastic is probably more inmportant than the concrete in terms of moisture control. A lot of concrete guys don't like plastic under slabs because they say it slows the bleedwater drainage and can make it take longer before the concrete is ready to be finished. For a rat slab in crawlspace, that doesn't even seem like an issue.

3. 
   junkhound | Mar 16, 2015 10:09am | #3

   +3, 'nuther vote for 'new concrete guy' - or- diy, just by asking you already probably know more than the original concrete guy.

   Own house as 1500 sq ft of 3" basement floor over 6 mil poly, over 4" of pea gravel.  'Been that way since 1973, no cracks, no mesh or fiber either.  8 ft by 20 ft sections, IIRC poured at 4" slump. 

   Poured 3" slab in son's new addition crawl space last year, poly over dirt, no cracks yet and that is one big single 28 ft by 36 ft slab.  Poured at 3" slump.

4. 
   sunsen | Mar 17, 2015 10:53pm | #4

   I'm not so quick to dismiss my finisher. He knows his way around concrete. He's been at it a long, long time. 

   I'm just wondering what the reason might be. Personally, I always like to use plastic under a slab because it helps retain water which gives a better cure thereby minimizing cracking. However, I've never poured anything less than 4" thick in the way of a slab or hardscape or whatever. Of course, there's always a bunch of rebar in that stuff too. I have some ideas about why a thin pour with no reinforcement over plastic might be more susceptable to cracking than without the plastic but I'm just speculating. That's why I figued I'd ask on here what the deal is.

   I think what we're going to do is spread a couple of inches of sand over the vapor barrier then pour on top of that. My finisher has a lot more confidence in this approach.  I've got a guy with one of those high speed conveyor delivery trucks so placing the sand is easy and relatively cheap. 

1. 
   AndyEngel | Mar 18, 2015 10:19am | #5

   Check with ACI

   Or the Portland Cement Association. I don't think placing sand under the slab is recommended, although I'm not sure why I think that.

   In a foundation slab, the plastic is a critical component in moisture control. I'd never skip it. As to cracking, the only reason I can imagine plastic having an effect is if you're using too wet a mix. Anytime you add more water to concrete than it it needs to set, you've bulked up the concrete. If the water can drain down or evaporate quickly, okay. But otherwise, it will evaporate over time, leaving a less dense concrete that may be more prone to cracking.

   Slab curl is a related issue, but I think that mainly happens when there's a lot of evaporative drying happening at the top of the slab. Then, as it cures, the top surface shrinks more than the bottom, causing curl.

1. 
   DanH | Mar 18, 2015 01:37pm | #6

   Sand has been placed under concrete slabs pretty much since concrete was invented.

1. 
   AndyEngel | Mar 18, 2015 02:00pm | #7

   True

   Yeah, and I'm not sure anymore what the objections I've heard are based on. Sand does allow the slab to move, which should help avoid cracks. But clean stone below does the same thing. The only thing I can think of is that there's some slight chance of water getting trapped between the plastic and the concrete. Really though, I think the main objection is that it's un-necessary work and material, and the benefits are questionable. 

1. 
   sunsen | Mar 18, 2015 09:56pm | #10

   To my mind, sand provides a 100% compacted subsurface and decouples also decouples the concrete from the plastic, which may be bridging areas and consequently may not fully support the thin slab. Somehow I can imagine that the fact that the plastic doesn't fully conform to the underlying surface could be a problem. It's the only reason I can think of as to why there might be more cracking than is typical. That and the thickness of the pour.

   I'm always mindful of not pouring too wet a mix so that isn't an issue. 

5. 
   junkhound | Mar 18, 2015 07:05pm | #8

   kto bad brownbag is not around here any more!

   e.g. Andy comment:  "finisher"   terminology and 'wet mix"  -  noticed 'finisher'  is used vs. concrete contractor .

   Everybody knows them there 'finishers' only care about how smooth it looks 2 weeks after pour, so they add so much water the slump is such that without forms the concrete, in brownbags terms, " runs down the gutter and into the drains".

   Cracks easy then, ensuring another job in a few years. 

1. 
   DanH | Mar 18, 2015 07:33pm | #9

   Of course, it should be pointed out that it doesn't really matter if a rat slab cracks or not.  So long as it doesn't turn to rubble it will fulfill it's job.

6. 
   calvin | Mar 19, 2015 07:45am | #11

   Junk

   that supposed joke took a lot of thought.  

   think you might want to pull that down?

   edit later to add:

   it has nothing to do with being politically correct unless you plan to run for public office.