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.