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RE: SLAB ON GRADE

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Andy,
I've heard the same argument, but it doesn't make much sense to me. I'm not
an expert here, but a rational conclusion would be to create enough air flow
above and below the slab to encourage it to dry evenly - and this can't be
done with a plastic vapor barrier attached to one side of the slab. I am
assuming that, as I was taught, the concrete dries from the center outward
because of the heat created by the chemical reaction of the components. It
would also seem reasonable that any moisture of significance will drain
downward. Two inches of compacted sand between the barrier and the slab
should be sufficient to wick away the moisture and allow it to drain to the
edges of the vapor barrier or evaporate by the heat of the chemical reaction
of the curing concrete. I think I would be more concerned with water trapped
back against the concrete if the barrier was placed without sufficient
wicking space.

I've found that concrete can take a month or more to dry properly enough to
place down a hard floor such as marble or tile. It's probably more the case
of timing or of using an appropriate adhesive material to secure the hard
surface without worrying about moisture. In my experience with Saltillo
tiles, it isn't the moisture in the concrete that causes the damage (pops
etc.). It's the moisture retained in the tile from the thinset and grout
that causes this.

Another think contractors fail to do is to properly cover hairline cracks
and fractures in concrete with a "slipsheet". Most use builders paper which
I find to be ineffective. There is a membrane which I used in my own home
that is two layers of a fabric separated by a rubber material. It is secured
at 18" on each side of the cracks. As the crack moves the rubber pulls and
the tile surface is not affected by the movement. I've had it down for four
years now and not one crack penetrated the tile or grout. However, I've been
out to three or four homes where the contractor did not want to spend the
price of the membrane and the cracks in the slab penetrated the floor.

Dennis S. Wish PE

-----Original Message-----
From: NDZ28(--nospam--at)aol.com [mailto:NDZ28(--nospam--at)aol.com]
Sent: Tuesday, April 25, 2000 9:36 AM
To: seaint(--nospam--at)seaint.org
Subject: SLAB ON GRADE


Just hoping I can get a basic question clarified. I  have a client  I'm
doing
a series of homes for, and he wants me to specify that the slab on grade be
placed directly over the vapor barrier, (no sand between) He claims that
when
sand over the vapor barrier is moistened prior to pouring concrete, sand
will
 never dry out,  the moisture from the sand will travel through the slab for
a very long time causing bubbleing in the vinyl flooring, prevents  wood
flooring from sticking to concrete, etc. I've always believed that the sand
aids the curing of the slab.  The caveat is that vapor barrier is installed
properly and not punctured during placement of concrete. Any help is
appreciated.
Andy Vidikan