Concrete slab foundations for your container project are the largest, and most expensive type of foundation that you can build. For many types of projects concrete footings or wooden beam footings are suitable, and due to cost and the amount of time involved there are few situations when you would need to create a container foundation this large.
The variable that has the greatest impact on cost is the concrete itself. Due to the volume of concrete required it's the best option is to hire a concrete crew to pour the redi-mix concrete. Hiring a crew will save you a great amount of time and energy. As we discussed in the previous concrete footings article, bagged concrete doesn't provide a great volume of concrete, and for your slab foundation to set properly you'll need to have it poured quickly.
One area where you do have the option of saving on costs is building the form. A slab foundation for a shipping container is designed differently than a normal slab foundation; due in part to where the weight being carried on the edges and corners of the foundation.
If you've decided to construct a slab, it's well advised to take some time and draw out your plan on paper. This can help you to better calculate the costs associated with building your slab foundation and reduce the number of surprises later on.
Begin by tieing off the area that will be used for the foundation. The size of the area depends on the size of your container, so take extra consideration when you measure it out making sure that all corners are square so that your foundation will fit your container. It's a good idea to post the area with stakes, then tie them off so you have a strong visual idea of the size and space required.
Once the area is tied off you can begin digging out the foundation. The size and depth of the foundation will depend on your needs, but you'll want to make sure that you have room for a 4-5" stone bed where the concrete will be poured. The weight of the container will be carried in each of the corners, so you'll need to clear an area that's 1'x1'x1' in all corners. After the area is excavated, and before the stone is laid, the ground will need to be compacted to reduce the chance of unwanted settling and cracking later on.
With the area cleared and stone bed in place you can begin building the concrete form. Start by building the entire perimiter with 2"x6" lumber. Using a 2"x6" will allow you to have a form that's 4' deep. Once the perimiter form is in place, construct the 1'x1'x1' forms for the corners.
With the footing form structure in place, you have one final step before you're ready to have the concrete delivered, and that final step is the is reinforcing your slab with rebar and welding mesh. Create the floating reinforcement by laying the mesh across the founding and tieing it together. Once it's in place, use the pieces of brick to prop, or float, the reinforcement 2" above the stone bed. Building the reinforcement provides extra strength once the concrete is dried making it an important part of the structure.
With the completed form and reinforcements in place, your measurements and calculations double checked, you're ready to have the concrete poured and smoothed prior to the delivery of your shipping/storage container delivered and set in place.
Part 1: Wood Beam Container Foundation