We're having a house built in a rural mountain area where the
predominant composition of the soil is decomposed granite and a little
clay. Our house is a passive solar design and uses a rock floor (about
3 feet high, 2" rocks or gravel) over a water barrier. The floor of the
house is 4" thick reinforced concrete on top of the rock floor. Because
of concerns about ground water, a French drain was installed around the
perimeter of the house. In one room of the house, the concrete floor
shows signs of cracking (about 1/16" cracks with about 1/8" uplifting).
The design of the rock and floor and been approved by engineers and
been used many times by this builder with no structural problems. At
certain times of the year, a hole at one side of the house that is
deeper than the rock floor seems to have water in it which makes me
think that the ground water can rise and fall.
The contractor speculates that areas with a little more clay may have
swollen a bit and caused the uplifting. Does this seem likely? Is there
a way to control this type of uplifting? I realize it is hard to make
an assessment without seeing the site. What sort of professional should
we look for for advice?
This is exactly what happened to the slab in my p/s house. The soil had a
lot of clay in it so there was a lot of movement between winter (wet & clay
expanded) and summer (dry & clay shrunk). The slab was laid according to
council regulations with extra cement pourings needed (which meant doubling
the cost) because the clay went deeper than was first thought (of course!!).
There was no long term effect on the p/s performance but tiles laid on the
cement in the living areas had hairline cracks in them from the seasonal
movement. The tiles were laid within 2 months of the slab being poured and
I was told (much later) that in order to avoid the hairline cracks I should
have waited 12 months to put them down.
The bedroom floors were not tiled but were painted with (outdoor) cement
paint and the cracks were much less apparent there. Cement paint on the
slab or epoxy sealers will be the way for me next time. No cracked tiles
and much less expense to finish off the floor.
Cracks in concrete are normal but what would concern me is the uplift.
You are probably looking for some type of reassurance, get that from
your builder for he is the one responsible for the finished product.
Clay is the best foundation for concrete slabs because the superfine
particles compact into a solid mass. In fact clay is hauled in and used
as a stabilizer over looser soils. *Any* soil will expand & contract
with moisture variations, but clay will do it less. That is why the
old saying is: "the only sure things in this life are death, taxes, and
cracks in concrete".
Concrete takes a while to shed it's water and cure to size & strength.
In fact the cureing curve of concrete extends upwards to 30 years. So
your waiting for 12 months would have had little effect on hairline
cracks. Those are characteristic of concrete and *cannot* be avoided.
Larger cracks are avoided by wire and rebar in the concrete - it cracks
but doesn't separate into large cracks.
I waited 10 years to install ceramic tiles on a floating slab (several
inches of SAND under it to isolate it from the gumbo [ultra heavy clay]
soil underneath it), and I still have several tiles with hairline cracks
You should hire yourself the best structural engineer you can find and
make sure he/she represents your interests. Don't rely on the
builder's or his own expert's explanations, which may be designed to
lull you into a false sense of security until all warranty deadlines
I don't know about the state you're in but well over half the states
have one version or another of a "right to repair law" now, and
you/your lawyer will need to be familiar with it. It's a builder
protection law and will waste more of your time, but if you have it,
it's not an option, you will have to follow its steps.
Good luck. Both vertical and horizontal displacement are, shall we
say, not good.
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