Concrete pad - moisture barrier?

[RichT]

For 24ftx36ft, 5in slab: I had planned to lay plastic (8-10mil) as moisture barrier over existing 2in of rock sublayer. Concrete contractor told me the moisture barrier should have been "under" the rock sublayer, therefore don't lay the plastic. Does this seem right to you guys? We pour on monday...

 

[LarryD]

I have seen it used directly on subgrade (my own work included) and covered with sand as detailed in this UFGS spec.;

3.4.1 Vapor Barrier
**************************************************************************
NOTE: Include taping of joints when waterproof
paper is specified.
**************************************************************************
Provide beneath the on-grade concrete floor slab. Use the greatest widths
and lengths practicable to eliminate joints wherever possible. Lap joints
a minimum of 300 mm (12 inch) 12 inches [and tape or cement joints].
SECTION 03300N Page 31
Remove torn, punctured, or damaged vapor barrier material and provide with
new vapor barrier prior to placing concrete. Concrete placement shall not
damage vapor barrier material. [Place a 50 mm (2 inch) 2 inch layer of
clean concrete sand on vapor barrier before placing concrete.]

The sand is there to protect the barrier from damage. I use good strong plastic like you said and have not had any problems with my slabs. I've even worked from drawings (stamped engineered) that that placed concrete right on the barrier.

The first thing this barrier does is slow the hydration of the crete and provide for better strengths. You should also keep the top moist for a couple days and I think you'll be pleased with your efforts.

 

[gsganzer]

When they did my slab, the vapor barrier is between the finished grade and the concrete (see attached). I'd put the vapor barrier down before Monday.

 

[Holzster]

mine was done just like gsganzer's

 

[Holzster]

I forgot to make my point - If the plastic is between the 2 layers, then that keeps the moister in the concrete longer SO, it makes the concrete cure slower which makes a better concrete.

 

[RichT]

</font><font color="blue" class="small">( it makes the concrete cure slower which makes a better concrete. )</font>

...I think thats why he doesn't want me to use it: slower cure= longer time on jobsite

 

[LarryD]

It shouldn't change his finish time if it's batched right.

Now if he wants to put in a self leveling (6" slump) then it would be quite a while before he can finish it. I wouldn't let him pour soup...greater water/cement ratio makes for weaker crete.

 

[Henro]

The bottom line is when it is all said and done, a moisture barrier keeps moisture from moving from under the slap to the top of the slab.

It really does not matter much if it is under the gravel or on top. Granted, MAYBE if it is under it may be a little more protected from punctures, but then again, maybe not if the stuff on top is sharp, as compared to sand...

If it were me...common sense says any barrier, even if it gets a few punctures in it, is better than absolutely no barrier to moisture. Apply common sense and put the barrier down. It certainly can't hurt, and I would venture to say it certainly will help... /forums/images/graemlins/smile.gif

 

[GerardC]

When mine was poured, the ground was compacted then came the vapor barrier. They poured a 28'x40' 6" floor right on top of the barrier.
As I see it, it's your project and your paying for it. Therefore, if YOU want the barrier there, put it down regardless of what the contractor said/says. As Henro said, it's there to keep the moisture from coming to the floor surface and when the contractor is gone, he's gone. It's YOUR floor.

 

[RichT]

I was leaning toward the moisture barrier, and the responses here added to my conviction. Thx everyone for your input.

 

[Dargo]

I'm no expert, but I've poured several hundred yards of concrete at my house. The only place I didn't use a moisture barrier is in my driveway. I began on the advice of a neighbor who didn't use a moisture barrier in his barn. He now has to run two 70 pint dehumidifiers in his barn year round or everything in his barn rusts and his wood rots.

I really don't know if it helps a lot, but I have heard it really does. Besides using a 6 mil moisture barrier, I also used fibercrete and the 6 gauge wire matting along with a truck load of number 10 rebar. That, and the pad in my barn is also 8" to 10" thick with 5000 psi concrete. On my porch I used the same thing, but went 6" thick.

The only place I didn't use all the steel is around my pool. Local codes require you to not use steel in the concrete that is around the pool. Therefore, I had them double the fiber in the concrete. I'm not sure that I'd advise that though. For about the first 6 months or so, the concrete had a pretty good fuzz to it. /forums/images/graemlins/blush.gif It looked like a teenager who could use a shave. Anyway, I even used a moisture barrier there as well.

I still have almost 900 feet of driveway to pour, so I have several hundred yards more to pour. I still don't think I need a moisture barrier there, but I'm goint to stay with the fibercrete along with my 6" 5000 psi concrete and 6 gauge wire, tied together with plenty of rebar. From one mistake the concrete contractor made, I can testify that it takes them almost 2 full days to remove just one section. /forums/images/graemlins/grin.gif

 

[Inspector507]

<font color="blue"> Local codes require you to not use steel in the concrete that is around the pool. </font>

Wow, the new 2005 NEC® REQUIRES steel or a bonding grid be installed prior to pouring the deck around a pool.
How things change /forums/images/graemlins/grin.gif

 

[beenthere]

I recommeend the moisture barrier.
I recall the fall of 1959 when I had a big argument with my father, as to whether to put plastic down under the concrete in a 10-stall hog-farrowing house we were building. He said the sand was dry and there was no moisture to come out.
We had the sand down, and I had the plastic down. We went to lunch and came back. The sun had shone through the windows on the south side and found a large collection of moisture at each place the sun was shining on the plastic. He didn't say another word.

When I put plastic under my basement concrete floor in 1968, the concrete guy was not happy as he was trying to stretch the 3 loads of concrete to cover the whole floor. He added water, and he was up practically all night waiting for the water to 'leave'. He used my fiberglass insulation as a 'sponge' to try to mop up the water. Not a real good job from it, but no cracks.

When I poured my garage floor 3 years ago, the contractor said "fine" to the plastic, but that he wanted it down before he added a final 2" layer of sand on top. He said the sand on top would help set up the concrete and prevent the 'water' problem. I think he was right as that floor troweled out well and finished off perfectly, in the right amount of time.

Also, be around to watch as the contractors (if they don't like the plastic) will slice it up just before pouring the concrete, so it doesn't hold any water. If not there, you may not know and lose the protection you are looking for in the future.

Good luck.

 

[drm]

There has been some real good recommendations and I think everyone agrees that any interior slab must have the vapor retarder (as it is now called) installed. The location of the vapor retarder will impact the curing of the concrete in that short and long term.

The effect on the short term, setting up, will be minimal if the contractor uses a low water to cement ratio concrete with a plasticizer admixture to help workability. If he just uses a lot of water to get the workability then the retarder will have a greater impact causing and extended time to set up. This is the contractors fault.

The long term effect on curing will cause the bottom to cure slower initially but not hold as much overall moisture in the slab. This can be good if you wet cure the top for SEVEN days or use a good curing compound applied at the recommended quantity. Do not skimp on curing! If you do not cure it properly you may get curling – the edges of the slab will rise relative to the interior of the slab. You may also get an increase in the curing cracks some of which may be quite wide. Always provide control joints spaced at a maximum of 2 ½ times the slab thickness (12.5’ for a 5” slab).

The American Concrete Institute and Portland Cement Association recommend both locations for the vapor retarder depending on the flooring being installed. For an adhesive applied flooring that is susceptible to the water vapor given off by the slab, the over gravel method is preferred since the overall water vapor will decrease quicker since the gravel will not hold any water. For all other uses the retarder should be placed under the gravel (sand preferred).

Definitely keep an eye on the contractor if he does not want to use the retarder. And do not allow him to add water to get workability.

…Derek

 

[LarryD]

drm nailed it pretty good. There are additives that make for good workability AND strong concrete. They do cost extra. Slow curring is the last step but just as important.

I don't know if you've spec'd out a strength but local codes probably require 2500psi minimum. I would make a copy of each batch ticket the delivery drivers have. It details all the ingredients used and weights. Adding water to a single batch is not always a bad thing, as long as the water/cement ratio is not exceeded. Now if every truck "needs" water, then either the batch plant is not working right or your contractor doesn't like a stiff mix, ie more work. Since most batch plants are tested on commercial work, you can be relatively certain they are functioing properly. It's no fun buying a building foundation because your concrete failed to meet strength.

If the driver adds water at your end, there is a guage on the truck that tells how many gallons he is fixing to add. Make him mark his ticket. If there is any question or dispute later on regarding your slab, a testing lab can look at a batch ticket and determine if the ratios are appropriate for the strength required.