If you have decided to add on a garage floor to your home, you will need to install a correctly drained foundation before you start on anything else.
Mark out the area that will become the garage floor using marking spray paint. It is a very good idea to draw out the floor plan to scale on some graph paper, and keep this as reference during the construction project.
The ground that the foundation will be laid over needs to be level and very well drained. The easiest way to do this is to dig 3 inches down into the soil, and fill this with abc or pea gravel. Tamp this down very well.
Dig down at least 8 inches for the footings. A footing is the trench that runs on the inside perimeter of the slab. When the slab is poured, it anchors the concrete pad in place. Each trench for the footing should be at least 4" wide .
Lay 1/2" rebar in the trench, held up off the bottom with small rocks or chunks of cinderblock. The bars need to be at least 2" off of the bottom of the footing trench.
Around the rim of the footing trench, build a 2x6" frame. Hold the frame in place with stakes pounded into the ground, or use heavy block to keep forms from moving when the concrete is poured or pumped in.
Pouring the Slab
On the day that you are going to pour the slab, lay down crib wire (4"x4") and more rebar. It is best to wire the rebar and crib together using ties. This helps keep the slab from cracking as the ground moves over the years. If the temperature is more than 90 degrees, you may need to use a "retardant" to keep the cement from curing too fast. It is also a good idea to have a number of assistants to help with the pour and finishing.
Pour the footings first. Agitate the concrete as it is poured or pumped in with an electric concrete vibrator or a wooden tamp. Try to get all of the air bubbles out of the footing mix.
As soon as the footings have been poured and tamped, pour the main slab. It is easiest to have it pumped in, but you can also have assistants with shovels maneuver the wet cement mix around the area with flat shovels. Keep from making large lumps of wet cement, it's best to keep the concrete as level as you can.
The concrete, once completely poured, will need to be leveled. A length of 2"x4" lumber can be used to screed the cement down to the level of the forms. This is done by moving the lumber back and forth quickly, and tamping down the mixture. Low spots may need to be filled in with more cement. The rough finish needs to be done as fast as is possible, depending on the temperature, the concrete may start to stiffen in 40 minutes or less.
With the concrete slab leveled out, it is time to begin the finishing process. A steel pole trowel is used to float the surface of the slab. Keep on going over the concrete until it appears to have no voids or bumps at all. A thin milky film will rise to the top as you work the cement into place. Once the slab is shiny and smooth, let it set until it begins to dry out. If you want rounded edges, run an edging trowel around the perimeter. Do not place any heavy weight on the slab for a minimum of three days.