Structural fill below slab on grade 5

[riverlot]

I have a daylight basement slab on grade to pour. The existing soils are compacted. I need to add about 8" of sand, gravel, ?, to reach sub-grade for my slab. Also, I have radiant tubing in the slab so will need to add insulation board. I am being told to go 4" of 5/8" gravel compacted, 4" pea gravel, 6 mil vapor barrier, 2" insul board, and topped with wire mesh. (My radiant flooring sub wants the wire mesh) My main concern is the drying of the slab. My concrete sub says that without sand or some other "absorbing" material below the concrete, the slab will only cure from above. (uneven curing) He suggests 4" of 5/8" compacted gravel, 2" insulation board, vapor barrier, 4" sand, then wire mesh. I should also mention that I have intermittent footings on the interior of the building so this makeup is "broken" at these locations. Is it ok to pour 4" directly over the top of these footings without a capillary break or insulation?

 

[fattdad]

Do you have a near-surface water table concern? What do you mean by "vapor barrier" - is that a plastic sheet?

Many folks have gone to excluding the plastic sheet altogether and I would also think that the insulation board would provide a similar function.

One item to consider when using plastic sheeting covered by sand: What happens if it rains. Where does the water go and how long is the contractor prepared to wait for the sand to dry out.

The primary purpose of a plastic sheet is to allow slippage as the concrete cures and minimize slab curl.

I see no value to the wire mesh within the sand layer. It'll likely rust. If you honestly believe that tension will develop within the subgrade (or subbase), use a woven geotextile or a geogrid.

Just some initial thoughts. Other's may provide further insight. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[riverlot]

Good point on the rain. We get a lot of weather here in the Northwest. I don't have a water table problem so would like to eliminate the sheeting (6 mil poly)if possible. Since it can prevent curling, will I have a problem with my slab over the top of the interior footings without this sheet?

 

[miecz]

What's a daylight basement? Is it outdoors?

 

[DaveAtkins]

Ahh, the old vapor barrier under slab-on-grade debate. The best article I have ever read on this is entitled "Where to place the vapor retarder" by Suprenant and Malisch (I can't remember the magazine, but I think it was Aberdeen's Magazine of Concrete Construction).

If you are going to put a vapor sensitive covering over the slab (vinyl tile, carpet, etc.), then the vapor retarder should be placed directly below the slab. If there will be no covering, then no vapor retarder is required. BUT PROPER CURING MUST BE DONE, TO PREVENT SLAB CURLING! The contractor MUST prevent the top of the slab from drying out too quickly.

Regarding pouring a slab directly on a footing, it is done sometimes, but I prefer seeing the top of the footing 4 or more inches below the bottom of the slab.

DaveAtkins

 

[concretemasonry]

If the radiant sub wants the wire mesh as part of the design, then, if it is eliminated what happens with a guarantee on the heating system or do you want to be 100% responsible?

It seems to me that he would have more experience with this type of intallation than a concrete contractor that probably has done very few by comparison.

Dick

 

[miecz]

riverlot

The vapor barrier doesn't help to prevent curling, it causes curling. It's the sand ovver the vapor barrier that helps prevent curling.

I'd have to agree with your concrete sub. If he suggested to put the sand over the vapor barrier, then he must be willing to deal with the consequences. You might suggest a 3/8" stone, in lieu of the sand, as it's more stable.

Pouring the slab on the footing will force the slab to move with the footing. You could get cracks in the slab at the perimeter of the footing, if the slab settles differently than the footing.

I live in a house with a damp basement and mold is a problem. I would never eliminate a vapor barrier under an enclosed space.

 

[riverlot]

Thanks guys for your help. Sounds like the consensus is to keep the vapor barrier whether it be below the slab or below the sand. There will be floor coverings added so I'd rather be safe than sorry. Any opinion on pea gravel in lieu of sand?

 

[dirtsqueezer]

This is an afterthought, but I'm pretty sure that vapor barrier is required by the IBC 2000 (or later publications).

Anyway, in the Northwest- Washington, to be exact, I've seen a lot of pea gravel placed beneath a SOG. The problem I have is that it yields so much even beneath foot traffic. It seems to me that poorly-sorted sand(no fines- to mimic the draining effect of your pea gravel) is a good option in that it can be compacted and therefore densified.

Also, I just hate wire mesh. The pulling-up seems so pointless, as the contractors just stand on it, pushing it back down. I imagine water migrating up from the lower mesh, into the higher mesh, cracking and spalling ect. Have you thought about fibermesh? Seems like it might fit in better with all your internal parts. The only thing, it's not entirely fireproof.

 

[riverlot]

Thanks dirtsqueezer. Makes sense with the sand. The main reason for the mesh is so the radiant tubing can be tied to this grid. I could upgrade to rebar but don't have that budgeted.

 

[engphila]

To address the where-do-I-locate-the-vapor-barrier question, please see ACI 360R where there is a convenient flowchart decision tree. The only more devisive topic do lather up a group of civil/structural engineers is probably guardrail design, but that's a different thread altogether.

engphila

 

[oldestguy]

Hi:

No one seems to have caught a dumb statement by your contractor.

"slab will only cure from above."

Hey, he apparently wants to "dry" the concrete, thinking that is how it cures.

With this dumb statement, I wonder if the guy really knows what he is doing.

In case you didn't know, curing is done by having water stay within the mix. You don't want to dry anything. However, after it cures, that may be OK.

 

[fattdad]

Just for the record, I don't agree with this statement, as quoted above:

"The vapor barrier doesn't help to prevent curling, it causes curling."

Improper curing causes curling. There is no reason to believe a vapor barrier, which acts as a slip sheet, when located immediately beneath the slab will "cause" curling.

f-d

¡papá gordo ain’t no madre flaca!

 

[miecz]

Just for the record, I don't agree with this statement, as quoted above:

"The vapor barrier doesn't help to prevent curling, it causes curling."

Okay, well, you're going to get a lot of opinions on this. I'm getting mine form an article published by Bruce Suprenant in Concrete Construction, April, 1992, and Article 11.11 of ACI 302, Guide for Concrete Floor and Slab Construction.

 

[fattdad]

Refer to Figure 1.7 at this link:

http://books.google.com/books?id=oQ...oLamp&sig=oTm-OwLFaMZUUFclaC6kF6tv4ao#PPT3,M1

Refer to current work by ACI 302

Refer to work by Eldon Tipping.

While the popular thought 15 years ago may have been as you state, the current thinking is different. Place the plastic sheet immediately below the concrete slab.

. . . . and the battle continues - ha.

f-d

¡papá gordo ain’t no madre flaca!

 

[fattdad]

This is a better link (more complete) for the ACI 302 citation. Actually, there is an interesting flow diagram for when you need a platic sheet and when there should be a sand cushion.

http://www.concrete.org/General/f302.1(04)Chap3.pdf

Enjoy!

f-d

¡papá gordo ain’t no madre flaca!

 

[engphila]

fattdad,

. . . yes, this is the same flowchart/decision-tree that I referenced above in ACI360.

engphila