Frost is the enemy
When water in the soil freezes, it expands and lifts whatever is on top. Garage and barn slabs crack and heave when they sit on wet, frost-susceptible soil without a proper base, drainage or insulation.
What a good slab sits on
- Topsoil and organics removed down to firm ground.
- A compacted gravel base, typically several inches or more, to drain water away.
- Rigid foam insulation under heated slabs, and often at the edges.
- Reinforcement such as wire mesh or rebar, as the job requires.
- Frost walls or a frost-protected shallow foundation for structures that can't move.
When to pour
Late spring through early fall is easiest. Concrete can be poured in cold weather with heated water, blankets and accelerators, but it must not freeze while curing, so cold-weather pours cost more and need more care.
Questions
Do I need a frost wall for a garage?
For an attached garage or any structure that must not move, usually yes, or an engineered frost-protected shallow foundation. Your local code office and contractor will say what's required.
How long before I can drive on a new slab?
Usually about a week for cars, longer for heavy equipment. Your contractor will give you the timing for your mix and weather.