Concrete Gurus: best Vapor Retarder

[304065]

I am wondering whether any of you fellas with experience with concrete can point me to the best vapor retarder. Small slab, about 500 square feet, Northeast USA, soil conditions are about average and the site is not particularly wet. However, I want to have a very good vapor retarder under the slab to prevent moisture from percolating up through it and rusting things.

Any advice would be greatly appreciated.

[URY914]

Visqueen. Lap and tape the joints. Two layers if you have enough left over on the roll. Any building supply store, HD, Lowes etc will have it.

[Dantilla]

6 mil visqueen. You want the black stuff. The clear visqueen will slowly deteriate, the black will last longer than you or I under a slab.

[911Rob]

6 mil poly.
3M tape for joints.

Use 15m Rebar at 2' centers, chaired up into the bottom 1/3 of the slab.
After pouring, 24 to 48 hrs, saw cut control joints 10' o/c in top of surface, about an inch deep to control the shrinkage cracks.

Job Done.
Good Luck.

[daepp]

Stego-wrap is the best product on the market today. Husky is a close second. Both are 15 mil, hard to work with in the cold, and are chemically much more than visqueen.

[dad911]

Make sure you have 3-4" of stone under the slab, with good drainage, and you won't have a problem. 6mil sheeting under slab, pver stone, seams sealed. Perforated 4" PVC pipe in the stone, stubbed outside if you are in a radon area.

If you're really anal, use rubber roofing/pond liner instead of plastic. But that is a ridiculous overkill.

Still, if you leave raw steel on concrete, it will rust. Concrete absorbs moisture/humidity. Perhaps sealing it will help.

[HarryD]

Originally Posted by 911Rob

6 mil poly.
3M tape for joints.

Use 15m Rebar at 2' centers, chaired up into the bottom 1/3 of the slab.
After pouring, 24 to 48 hrs, saw cut control joints 10' o/c in top of surface, about an inch deep to control the shrinkage cracks.

Job Done.
Good Luck.

I do not know the shape of your 500 sf slab, but when you cut the control joints, be sure you make the pattern as square (vs rectanglular) as you can make it. For what ever reason, concrete likes to be square not rectangle shaped. If you look at cracked concrete, you will often see that it is just "squaring itself up".

Also, it is very important the concrete prep of the subsurface. No voids, even surface, and well compacted.

As noted above, you will need rebar to help it stay together (spread the loads). The "chairs" he mentions are little blocks you can get (around here we call the "dobies") that you can tie to the rebar.

They look sort of like this:



They tie to the rebar like this:



When done it will look like this:



It keeps the rebar at the correct elevation. Many folks just drop the rebar grid into the hole, and attempt to pull it up to the correct height. I would say 90+% of the time it sinks back to the bottom where it serves no useful purpose other than cost you money.

[HarryD]

Originally Posted by dad911

Make sure you have 3-4" of stone under the slab, with good drainage, and you won't have a problem. 6mil sheeting under slab, pver stone, seams sealed. Perforated 4" PVC pipe in the stone, stubbed outside if you are in a radon area.

If you're really anal, use rubber roofing/pond liner instead of plastic. But that is a ridiculous overkill.

Still, if you leave raw steel on concrete, it will rust. Concrete absorbs moisture/humidity. Perhaps sealing it will help.

Rebar, if properly installed, will last a long time in concrete as it emtombed in the matrix with minimal access to air (oxygen).

He needs to avoid exposed ends or sides of the rebar, hence the need for chairs, properly spaced.

Also, once placed, the concrete mix needs to be vibrated to get the air bubbles out and not over worked on the surface.

[TimT]

John, if you are willing to toss a little extra coin into you project, your could use what NYSDOT calls 50 year pavement...

A well prepared subgrade.... then a few inches of what is called "popcorn" by the workers...it is "permeable base material" in the spec.. simply 5/8-3/4 stone, cement, and water.... no fine aggregates

Place a base of 3-4 in popcorn compact with a plate tamper... install some vapor barrier... 6 mil poly should be fine... then your slab with reinforcement on chairs..

Though a layer of plastic and a slab reinforced with wire mesh will outlast you and the next few owners..

[URY914]

You guys are really gold plating his 500 sf slab.

[TimT]

You guys are really gold plating his 500 sf slab.

No kidding....

However after reading Cramers numerous posts... He leans toward gold plating...

Me.. I wouldn't use rebar for a slab that is not structural.. I'd use mesh.. just as millions (billions) of sf of simple slabs in the US are reinforced..

For a simple, non structural slab on grade.... the only thing the rebar or mesh is providing is crack control...

[dad911]

Originally Posted by HarryD

Rebar, if properly installed, will last a long time in concrete as it emtombed in the matrix with minimal access to air (oxygen). ......

I meant steel on top of slab, not re-bar, as the original post said 'to prevent moisture from percolating up through it and rusting things'

Only time I've seen major rust failure of rebar, in a reasonable amount of time (20 years) has been 'salt environments' like bridges in NJ and condos in Florida by the water.

[dad911]

Originally Posted by TimT

.....
Me.. I wouldn't use rebar for a slab that is not structural.. I'd use mesh.. just as millions (billions) of sf of simple slabs in the US are reinforced..

For a simple, non structural slab on grade.... the only thing the rebar or mesh is providing is crack control...

I use rebar in sidewalks, two pieces running with the sidewalks. It prevents frost/root heaving / vertical movement at expansion/control joints.

[HarryD]

Originally Posted by TimT

No kidding....

However after reading Cramers numerous posts... He leans toward gold plating...

Me.. I wouldn't use rebar for a slab that is not structural.. I'd use mesh.. just as millions (billions) of sf of simple slabs in the US are reinforced..

For a simple, non structural slab on grade.... the only thing the rebar or mesh is providing is crack control...

Agreed, a good mesh would be ok and simpler but he still needs the chairs.

[URY914]

Here are some photos of the project I'm currently running. I'm the senior project manager. This is a Caterpillar heavy equipment dealer's new maintenance building in central Florida.

The love their concrete. The rear and sides of the building has, follow closely now:

10" thick
4000 psi
Granite aggregate
Double mats of #5, 12" OC both ways
The joints are protected with 1/4" x 4" x 4" angles with nelson studs and welded at the corners.
Smooth dowels between sections

Due to edge angles hanging on the forms we have to pour in a checker board pattern.

[URY914]

The squares are 20' a side.

[URY914]

All pump via boom truck. We pump about 30 squares at a time.

[URY914]

The building is tilt-up. Some panels are 10" thick and insulated.

On the right of the picture you can see the concrete squares. They are different colors because they are placed about a week apart so they are curing and bleaching out.

[Neilk]

Originally Posted by URY914

All pump via boom truck. We pump about 30 squares at a time.

Can you build the foundation for my next house? The expansive soil in Dallas is doing some odd things to our house...

[URY914]

If you gots the moneys - I gots the time.