Pouring a Concrete Slab Over Poly Sheeting
I’m getting ready to pour my basement slab and the contractor is concerned about pouring over a poly membrane. He is questioning its position in the layers, and its usefulness. The footings are 10″ high, and they rest on moist sand. The water table is 18″ below that. After reading about basement slabs I have chosen the following layers, from the ground up: 10″ of gravel, 2″ of EPS foam, 6 mil poly sheeting, 4″ concrete slab. The sand has not dried out despite the hole being open for two weeks, and exposed to the 90 plus degree heat and low humidity of northern Utah, so I know there is plenty of water below the sand. Do I just need to stick to what I have learned on GBA and go with my system? Is there anything I can suggest to limit the potential curling or cracking that may happen because the slab can’t “dry below”? Thank you in advance for your advice!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
You want the poly. Don't let your contractor talk you out of it.
Slabs don't need to ever "dry" bellow. Concrete cures from hydrolyzing, you actually need to prevent it from getting dry and keep it moist to get proper strength.
All slabs will crack, if you care about where, make sure to put in control joints. These can be grouted later for a smooth look.
Nathan,
I don't want to sound like the old crank I may be, but I do hope that the "gravel" you are placing is actually washed rock for at least 2" of the planned depth. You do not want to have your EPS in contact with soil types that can transport moisture, especially with such a scarily close water table. If you can't dry out the surface after two weeks in summertime northern Utah, then you must prepare now to deal with a water table that will likely rise come spring thaw.
Are there neighbors with similar basements that you can speak with about their sump needs? Also, now would be a very good time to do all 10" under the EPS with washed stone and embed that with radon collection tubes. I am just east of where old uranium claims were made in the 50's , so it is always wise to anticipate the possible presence of radon bearing soils. Maybe northern Utah is different geology, but do check. I did mine for a few hundred dollars of pipe that admittedly are lying fallow. Still cheaper than a retrofit.
My gravel fanaticism is due to my childhood home experience. It was built in a former sand and gravel pit blessed with bands of different "gravel" deposited after the ice sheets melted last ice age. The finer gravel did provide truly excellent sandbox material for kid scale road projects. The most useful feature of the "gravel" was how long it held moisture after a rain fall. Made smooth compacted roads a cinch.
The coarse gravel also held moisture very well, which is why I still try to get people to specify washed stone for at minimum 2" under the foam layer. You do not want a damp sponge of soil in direct contact with the foam which can take up moisture over time.
Akos is quite right that cement needs moisture to cure correctly. Control joints will help direct the inevitable cracks, but for added quality control in my equally low humidity area, I came up to the site twice a day to mist walls and slab pours to keep a better balance during the cure. Only have two unexpected floor cracks.
Last thing to think about would be a capillary break between footing and wall. Given your current water table it may be prudent. In my situation I went all in with Xypex additive for the footing and the wall pours. Pricey, but my shop is so dry that I do not fear putting my lumber directly against floor or walls. Even boxes on the floor don't turn to mush the way they did in my former midwest location.
Thank you for the replies. Akos- I’ll make sure and get that poly in place. I will also make sure the gravel is washed. The load he dropped to go under my rough plumbing was certainly not the cleanest. I’ll make sure the rest goes in clean. (We had to stage it because of inspections and other events out of my control). Thankfully, the neighbors sump pits stay dry, but I’ll have one just in case, as well as a robust footing drain that connect with the old land drain. Radon piping is in for insurance as well. After researching on GBA, I did apply a capillary break atop the footings. The concrete guys thought I was crazy because they don’t do that around here. It was a sticky mess like no other, but I got it on with the help of my family. Super fun project! I’m sure I’ll have more questions down the road during my attempt to build a “Pretty good house” with subs who have never heard of such a thing. I’m looking forward to having a healthy, comfortable, and efficient home.
Haha Nathan - Your post made me laugh. I'm running through the same exact things with my subs. They look at me like I'm crazy. GBA has taught me so much.
> anything I can suggest to limit the potential curling or cracking
Use low water content concrete and use something to prevent the top side from drying (until the slab is cured). Control joints are the primary cracking solution, but more rebar and/or fibers will reduce it.
> 6 mil poly sheeting
"ACI 302.1, Guide for Concrete Floor and Slab Construction, states that the minimum thickness of an effective vapor barrier is 10 mils."
I'd bring this poly sheeting up the wall a few inches above the slab and lap it inside the basement wall EPS.