Concrete columns are key ingredients in the design of structures like parking garages and many other buildings. Often, columns are reinforced with steel rods that impart additional tensile strength to counteract some of the forces the column may experience. Whether they are reinforced or not, however, their design and installation involves planning, preparation and construction.
All concrete columns must rest atop a suitable footing or a foundation big enough to accommodate their weight -- never on loose earth. If you have separate columns, each must have a footing no less than 2 feet square and 12 inches thick, and the foundation must also be reinforced. Engineers also have to determine the load the column must sustain and how much strength it will need; based on these factors, they will choose the type of concrete and reinforcement needed to ensure structural integrity.
The footing is always poured before the column and contains metal "starter bars" that extend out of its surface. As their name implies, these starter bars provide a place to fix the rebar or steel bars that will reinforce the concrete itself. Next, the carpenter on the project makes up the formwork -- the wooden structure that will give the concrete in the column its shape and keep it in place while the concrete solidifies.
Once the formwork has been nailed down around the rebar, the construction crew can mix and pour the concrete. The concrete mix is very important because the ratio of water to mortar helps determine both the strength and workability of the concrete, so it must be done exactly according to specifications. The concrete is then poured into the formwork around the rebar where it begins to harden. The formwork generally contains movable "windows" through which the crew can pour the concrete, because if the concrete were allowed to fall from the top of the formwork, the components would begin to separate, thereby resulting in a much weaker structure.
Once the concrete has hardened sufficiently, the formwork is removed. Generally, this process takes a couple of days. Engineers also have a variety of tests they can use to determine whether the concrete is strong enough to remove the forms. Finally, the concrete is wrapped in plastic to prevent it from drying out while it cures. Once it's "cured" completely, the plastic is removed, and the construction crew can move onto the next phase of the project.