Homeowners can save thousands of dollars by learning how to build a cement slab by doing the work themselves rather than hiring a concrete contractor. The cement slab is the foundation of the garage, or shed, and it needs a proper base prepared before pouring concrete to prevent foundation failure.
If the cement slab support, or base, is not properly prepared there will be stress where the concrete is not evenly supported and it will attempt to bend, which can cause cracking in the floor, frost wall, or the thickened slab perimeter. Concrete is very strong with compression, but not so much with tension. Tension stress is when the cement is pulled apart such as when concrete is bent (one side of the bend is being compressed while the other side is pulled apart) and this can occur if the base (stone/gravel) is not level or properly compacted.
The most important aspect of a proper slab base is that it is level and very well compacted, preferably with a 'jumping jack' rather than a plate tamper. The jumping jack, or 'wacker' is more effective at compacting the stone base. This is even more important than the strength of the base so special attention should be used to accurately level and compact the base. The old adage about the only guarantee with concrete is that it will crack, but following this information will minimize the likelihood.
To clarify the terminology the American Concrete Institute's definitions follow:
Subgrade - native soil (or improved soil), which is usually compacted.
Subbase - layer of gravel on top of the subgrade.
Base - layer of material on the subbase (directly under the cement slab)
For garage and shed cement slabs there is typically only one size gravel for a base. This depends on the soil conditions and any engineered design requirements. In cold climates a 24' x 24' garage slab on solid ground that is quite level may only require leveling with a Bobcat or mini-excavator and then adding one dump truck load of 3/4' gravel.
Compaction is achieved with a jumping jack tamper or plate tamper. Several passes over the entire base is required to reach maximum compaction to reduce the chances of cement slab cracks cause by future settling. This equipment can be rented from any local tool rental location.
In warm climates it may be acceptable to place the cement slab on top of the natural ground if it is stump-free and compactable.