slab vapor barrier under rock?

[ericm]

The concrete guy I've been talking to about putting a slab in my contracted but yet to be built 40x80 pole barn shop suggested that I get the guy prepping the pad to put the vapor barrier under the base rock. The excavation guy agrees. I have a question though- how do you deal with the edges of the pad so that water does not get into the base rock between the vapor barrier and the slab? It seems like if it does, it will have a hard time getting out.

 

[Rusted Nut]

I’ve never seen vapor barrier under rock. The rock is usually known as “ capillary break”, the vapor barrier at the top seals anything from getting wicked up by the capillary action of the concrete slab.

 

[bb29510]

vapor barrier goes under concrete so the bottom of the slab doesnt dry out, put vapor barrier under rock your slab will crack like crazy

 

[ConCretin]

Disregard the prior post as it is incorrect. A vapor barrier's sole purpose is to prevent the migration of water vapor through the slab from the soils below. The presence or lack of a vapor barrier will have no bearing on whether or how often a slab cracks.

While it's desirable to limit moisture intrusion into finished spaces, a vapor barrier is really only necessary to protect adhered floor coverings such as epoxy and/or if soil conditions are extremely wet.

There is one potential downside to a vapor barrier other than cost. Since a vapor barrier will block bleed water from going into the soil, you may see more of it on the surface. Not a big deal unless your finisher doesn't allow it to dissipate before subsequent finishing operations. Some finishers dislike vapor barriers for this reason, which might explain their recommendation.

Vapor barriers are usually placed directly beneath the slab. Not only would it be subject to damage placing it below a course of crushed rock, if water did get trapped in there it would have no place to go but up through the slab defeating the entire purpose.

Take a look at my Guide to Floor Slabs in the link below for some additional thoughts on vapor barriers and other slab related matters.

 

[pcmeiners]

"There is one potential downside to a vapor barrier other than cost. Since a vapor barrier will block bleed water from going into the soil, you may see more of it on the surface."

Actually there are other downsides. it affects the curing process, uneven curing through out the slab.

ACI 302.1R-96
Guide for Concrete Floor and Slab
Construction
Reported by ACI Committee 302

"Concrete placed in direct contact with a vapor barrier or
vapor retarder exhibits significantly larger longitudinal di-
mensional changes in the first hour after casting than does
concrete placed on a granular base6 ; there is also more verti-
cal settlement. Where reinforcing steel is present, settlement
cracking over the steel is more likely because of the in-
creased vertical settlement resulting from a longer bleeding
period. If the concrete is restrained by connecting members,
base friction, or reinforcement, shrinkage cracking is more
likely because the concrete placed directly on a vapor barrier
or vapor retarder retains more mixing water and thus shrinks
more. In one study, high-slump concrete placed directly on
plastic sheets exhibited significantly more cracking than
concrete placed on a granular base. 7
Surface crusting is also more likely for slabs placed direct-
ly on a vapor barrier or vapor retarder. Concrete that doesn’t
lose water to the base won’t stiffen as rapidly as concrete that
does. If the surface crusts over due to drying or to faster set-
ting caused by solar heat gain, the weight of a power float or
trowel could crack the crusted surface covering a softer layer
of concrete that hasn’t lost water. On-site conditions such as
low humidity, moderate-to-high winds, use of embedded
mineral-aggregate or dry-shake surface hardeners, or a com-
bination of these can aggravate the problem and increase the
likelihood of cracking.6,8"

It is extra work but a thin layer of sand or crushed stone over a thick vapor barrier, wetted just before the pour is the best practice. The vapor barrier must be designed/installed to exclude the entrance of ground water/moisture between the barrier and concrete after the pour or curing process. This was required on large building foundations I worked on in Manhattan NYC.

 

[ConCretin]

Those are all factors resulting from the same condition, which is increased bleed water. It's also true that since that guidance was published it has been largely ignored in the real world. Blotter courses over a vapor barrier have been abandoned as impractical and problematic in their own right. I haven't seen one spec'd in 20 years. We've placed hundreds of thousands or square feet of slab directly on vapor barriers without issue largely due to the use of water reducers that have all but eliminated bleed water.

In any case, it's certainly not something the average homeowner needs to worry about as long as they control slump. If you need a vapor barrier, it's just something you need to be aware of.

 

[Beemer]

Our Geotechnical Engineer consultant generally recommended that the vapor barrier be placed under a shallow layer of compacted granular fill (sand or gravel). That was regarding professionally designed buildings; every building.

If there is any plan to have a floor finish moisture from below can cause a long term nightmare and that would be a motivating reason for a vapor barrier.

I don't fully understand the OP's situation noting concerns for the edges of the slab.
We never produced a building design that did not have a full perimeter foundation, that's over 40 years of buildings.

 

[pcmeiners]

"In any case, it's certainly not something the average homeowner needs to worry about as long as they control slump. "

What contractor pours concrete properly? On this forum everyone constantly complains about cracks and curing issues; correct slump or reducing bleed water does not cure everything. What is so difficult of adding a water permeable layer as I mentioned between slab and barrier? It is not that difficult.

 

[ConCretin]

"In any case, it's certainly not something the average homeowner needs to worry about as long as they control slump. "

What contractor pours concrete properly? On this forum everyone constantly complains about cracks and curing issues; correct slump or reducing bleed water does not cure everything. What is so difficult of adding a water permeable layer as I mentioned between slab and barrier? It is not that difficult.

Actually it is pretty difficult to spread and grade a couple inches of sand over large areas on a vapor barrier that you don't want damaged and then keep construction traffic from tearing it all up again. Coupled with the fact that the benefits are pretty esoteric, they just aren't used anymore. If you control mix water, bleed water simply isn't that big a problem whether it goes up or down.

I daresay 99 percent of the complaints on the GJ involve finish, surface defects and cracking. A sand blotter course over a vapor barrier will have little to no real life effect on these problems. Other than finding a competent finisher, the single most important thing a homeowner can do to ensure the integrity of his slab is to control mix water, which is easy to do with a mid range water reducer.

You're unfortunately correct about the competence of many concrete contractors and even those are hard to find these days. I just think you need to pick your battles. I can already see the guy from Fred's Concrete Floors eyes glaze over as you tell him you want a permeable layer over your vapor barrier.

 

[ConCretin]

I don't fully understand the OP's situation noting concerns for the edges of the slab.
We never produced a building design that did not have a full perimeter foundation, that's over 40 years of buildings.

He's building a pole barn.

 

[Beemer]

He's building a pole barn.

So no foundation walls to separate the outside and inside environments?

 

[ConCretin]

So no foundation walls to separate the outside and inside environments?

Correct.

 

[ericm]

Our Geotechnical Engineer consultant generally recommended that the vapor barrier be placed under a shallow layer of compacted granular fill (sand or gravel). That was regarding professionally designed buildings; every building.

If there is any plan to have a floor finish moisture from below can cause a long term nightmare and that would be a motivating reason for a vapor barrier.

I plan to finish the floor with something that will keep oil and other automotive liquids from soaking in. Probably not epoxy. Maybe a clear polyaspartic coating.

I don't fully understand the OP's situation noting concerns for the edges of the slab.
We never produced a building design that did not have a full perimeter foundation, that's over 40 years of buildings.

It's the slab for a pole barn. I don't know how far the vapor barrier would extend past the building perimiter. Maybe it doesn't at all. I don't know how it works when contractor A does the dirt work and places the vapor barrier under rock, and contractor B drills the holes for the poles and erects the building.