Concrete pad - moisture barrier?

[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.