Garage slabs take a lot of abuse. They have to support heavy cars and trucks and resist ice, dirt, salty water, de-icers, gasoline, motor oil, antifreeze, and other contaminants. Plus - since few garages are heated - they have to tolerate freeze/thaw conditions. But a garage slab that has been carefully planned, installed, and cured should have no trouble providing years of crack-free performance in any climate, under even the toughest conditions. Commercial concrete contractors do this kind of work every day; adopt their attention to detail and you'll meet their high standard of quality, without having to raise your prices.
Don't worry too much about your soil's bearing capacity; even poor soils like silt and soft clay have an allowable soil pressure of around 400 pounds per square foot (psf). A 6-inch-thick slab weighs only about 75 psf, and live loads - anything that is not part of the building itself, including vehicles - typically don't exceed 50 psf in a...
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