One of the most important parts to pouring a concrete slab is preparing the foundation below the slab. The foundation provides support and strength to your concrete slab, and helps it maintain its integrity for the life of the slab. Before you pour the concrete into their forms, consider the type of climate you live in to determine ground frost levels and the type of soil present in your yard, especially when you are pouring a concrete slab to support a garage or home foundation. Here are some tips to help you prepare your concrete's foundation.
Check Ground Frost Depth
First, it is important to figure out the depth of your area's soil frost level. This is the level where the moisture content inside the ground freezes during winter. Temperatures that drop low enough can freeze the moisture in the ground below your concrete slab. This causes freeze-thaw patterns within the ground, causing the soil to settle and your slab to move and crack. Check with a local building permit office for your area's frost level, or you can look online to find a average frost depth map.
Once you know the frost level of your area, it is recommended to remove this depth of soil from the site and replace it with crushed gravel. This removes the moisture-filled soil, replacing it with a stable base of crushed gravel that allows better drainage of any moisture below your concrete slab. For example, if your frost level reaches 35 inches down into the soil, remove 35 inches of soil and replace it with 35 inches of crushed gravel.
It can be helpful to rent contractor equipment like a Bobcat or skid-steer to remove soil and level the soil, especially when you need to move large amounts of soil. You can rent earth moving equipment for use in your own yard at most tool and industrial equipment rental shops.
Pour the gravel in layers no more than 12 inches thick, then use a compactor to compact each layer of gravel before applying any additional gravel. You can rent a compactor from most industrial tool rental stores or from your concrete supplier. Compacting the gravel is an important step, as the angular sides of the gravel pieces will "grip" each other when you compact them together and create a tighter layer. This creates a more stable foundation for your concrete slab.
Remove Poorly Draining Soil
If you live in a climate with shallow or no soil frost line, you will need to inspect the soil in the site where you plan to pour the concrete slab. Dig down several inches past the topsoil to see if the soil is sandy and well-draining, or if the soil is clay and does not provide adequate drainage. When you squeeze a handful of soil in your hand and it forms into a clump and does not crumble apart, the soil contains clay.
If your soil contains clay, you will need to remove the soil and replace it with crushed gravel to provide drainage below the concrete slab. Without proper drainage, your concrete slab is more likely to move and crack as the soil settles with its moisture movement. If you are dealing with sandy soil, you will only need to scrape off any vegetation and the topsoil, then pour a layer of crushed gravel for the concrete slab base, as needed.
When you don't need to worry about your soil's frost line, it is recommended to remove enough soil to allow for a layer of six to eight inches of gravel as a foundation for the concrete slab. As you pour the gravel onto the site, use a compactor to compress the gravel together to create a solid foundation.
Now your concrete's foundation is prepared, so you can install the concrete forms and order your pre-mixed concrete.
