There’s a debate in our office- do you place the vapor barrier under -or- over the granular fill when pouring a slab on grade? Why? Some of our structural engineers are locating it under the granular fill- stating that is provides better protection of the V.B. and allows the slab to cure more evenly- what do you guys think
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Replies
i like to put in some of the granular fill , then the VB, then some more granular fill or even say 2" of mason's sand , to protect the VB, stop the bitching of the cement finisher, and wind up with a better job...
if your are compacting the granular fill in lifts, just install it after the compaction of the 2d to last lift..
Mike Smith Rhode Island : Design / Build / Repair / Restore
I agree with Mike - the sand course allows for proper hydration at the bottom of the slab as well as offering protection from aggregate damage.T. Jeffery Clarke
Quidvis Recte Factum Quamvis Humile Praeclarum
Here's an answer that might throw a curve ball in the scenario.
We do NOT use compacted gravel or rock for a fill bed. Our concrete slabs are done a little different than most.
We start with virgin, undisturbed soil. Then we lay out the slab location and install form boards. Then we fill the area with a 60/40 mix (60% sand, 40% red clay) and spread it out. In some cases, the fill is put down first, then a bulldozer is run over it several times to pack it. THEN the forms are built.
Once the 60/40 is spread and packed, the beams are dug out. The beam areas must bottom out at the undisturbed virgin soil level. In most cases, the inspector must be able to see the grass at the bottom of the beam trench.
Next comes the rebar for the beams, and the plumbing/wiring/conduit (and whatever else is going in the slab). It is ok for the plumbing and such to be in the 60/40 mix, but none of it can penetrate the undisturbed virgin soil.
Next goes a layer of 6 or 8 mil plastic for a vapor barrier. Beam trenches are not covered in plastic.
Next is the wire mesh and the the little stands that hold the mesh up off the plastic.
Next is the concrete itself. James DuHamel
J & M Home Maintenance Service
Organic matter
Once the 60/40 is spread and packed, the beams are dug out. The beam areas must bottom out at the undisturbed virgin soil level. In most cases, the inspector must be able to see the grass at the bottom of the beam trench.
If an inspector here saw any organic matter (grass) beneath a slab, grade beam, or slab they would not allow a pour. Leaving grass beaneath a concrete pour is asking for trouble. It is going to rot and leave a void beneath the beam or slab. The concrete is then unsuported if only by a matter fractions of an inch. It is going to settle into that void unevenly and cracks will be a major issue.
VP goes directly under the slab. Nothing on top of it. There is no need for water to bleed or weep out of a slab toward the bottom. Concrete cures by hydration. That means that the correct amount of water will be used up in the reaction over the 28 day cure cycle. Most concrete is pour with way to much water because it makes it easy to place. To much water will weaken the concrete to well below its' design specification. It is best parctice to pour any concrete as stiff as possible and use a super plastizer add mix to attain workability durring the placement. From that point forward the focus should be on proper curring. The vp beneath the slab is a part of that cure control. It prevent the water from weeping or leaching out of the bottm of the slab, while a layer of poly is placed on top of the finished slab for the same reason. Control the cure cycle and you stand a much better chance of achieveing the desired spec. strenght of the mix. If done correctly a properly cured slab will exceed the mix specifications because the batch plant knows most builders and/or finisher will soup up thier mix and screw it up, so they start with a slightly richer mix than is really needed.
I wish Brownbag would visit here once in a while . He was and is the resident concrete expert on this forum and at BTC.
Where's the water table in this imaginary site?
The vb MUST be above the weeping tile's highest elevation.
Gabe
Let's bring this discussion back...
In California where I live, every set of plans that I have ever seen for slab on grade construction show the vapor barrier with sand on top. As one of the below comments said, a very frequent design is for 2-3" of sand, followed by the VB, followed by another 2-3" on sand.
Check out the below article, it suggests that a VB placed directly below the slab is absolutely the only right way to build:
http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems?topic=resources/flooring-probs
Would love to hear some of your thoughts on this. Also, is it just me or does it appear that Photo 1 (referenced as the "correct" way) shows the mesh under the plastic?!!