Here in NE Minnesota, it gets down to 40 below or a little colder in an
average winter. For an unheated garage, it is almost impossible to prevent
some hairline cracks.
If you're going to heat the garage, consider floor slab heat, it puts the
heat just where you need it most and the concrete will last forever. For
those that don't know, floor slab heat has wires imbedded under the slab
and heats the slab up to 68 degrees or so. Often this kind of system can
be put on some kind of interruptable power and, with good building
insulation, costs a surprisingly small amount to run. You can also do this
with hot water (use anti freeze) and a furnace, but this gets a little
expensive to install, even with plastic pipe.
The main problem with freezing is frost heaves. These are caused by water
(surface or ground) under the slab welling up to the frost line and
freezing (expanding). They tend to lift up, and cause any cracks on the
surface to open up. A little water dripping off a truck and then freezing,
and pretty soon those hairline cracks are major faults. The only way to
avoid a frost heave on a cold slab is to have excellent drainage all the
way down to below the frost line. That's around 8 feet here. This means
digging a hole, putting in drain tiles, and backfilling it with gravel that
drains well.
Putting rebar near the bottom of the slab is good for keeping the top tight
if the soil under the garage settles and the slab sags. To keep it
together on top with the ground heaving from underneath, the bar should be
near the top of the slab. The only problem with this is that any water
perking down into any hairline cracks, especially with salt, causes the
rebar to rust and pops the concrete. To fight this, use galvenized rebar
or epoxy coated rebar.
When you pour the concrete, keep it wet (not just damp once in a while) for
at least a week or you will get lots of hairline cracks as it shrinks
before it sets fully. Small cracks only get bigger.
Hope this helps. Maybe some of the Canadian members have even more ideas.
With all the members on this list, I'm sure I'm not the only engineer.
Bruce Kettunen
57 3200
Mt. Iron, MN
If you want to separate the floor from the grade walls around the edge,
that's fine.
>How thick is your slab and do you get cold temps? Would really cold temps
>increase the likelyness of a crack happening? What state do you live in? I
>thought about the fiber stuff. I'll look into that tomorrow. I want the whole
>slab as one and not sectioned. thx jim
>
>"R. Welch" wrote:
>
>> I used 3500# concrete for a floor in my barn and did not use rebar.
>> Instead, the concrete mix had fiberglass mixed in with it - I think it cost
>> about $5/yard more, but they said the slab shouldn't crack. Its been 2
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