Beyond the Basics: Best Practices for Installing a Concrete Foundation

Whether a general contractor or concrete contractor, having an edge over your competitors can make your company stand out, and educating your clients about the best processes and procedures in your industry is one way to differentiate yourself. If you're a DIY-er, this article may provide a few key takeaways when tackling your next concrete project, too.

The concrete foundation must support the structure without settling over the years. On average, a concrete foundation lasts 100-200 years. However, an extremely well-built foundation could last 400+ years, and a poorly built foundation might deteriorate after only 25 years. Typically, the foundation of a home is forgotten unless there is an unfinished basement, but a solid foundation can make all the difference. 

Let’s first look at seven fundamentals regarding the design and installation of a concrete foundation for a residential home in a cold weather climate like the Midwest. 

1. The size of the footing and walls can be determined by the weight of a building. Typically, the width of the footing is designed assuming a safe soil-bearing pressure of 2,500 PSF at design elevations. If the soil does not reach the design-safe soil pressure, the width of the footing may be increased to spread the weight of the building to more soil surface area. This is known as spread footings. 

2. Steel rebar is typically placed in the footing and walls before the concrete is poured. The steel reinforcement keeps the footings and walls together when cracks occur. There are two continuous bars at the footings and the top and bottom of the foundation walls. 

3. The strength of the concrete is important. It depends on multiple factors, but basic concrete is a mixture of approximately 18% Portland cement, 60-76% aggregate, 15-20% water when wet, and sand. Cement is the glue that holds the various components in concrete together. The more cement, the stronger the concrete mix. 3000 PSI at 28 days is typical for home foundations. 

4. Chemical admixtures may be added to the concrete mix to alter various properties. These include retarders, accelerators, water reducers, air-entrain agents, superplasticizers, and specialty admixtures: which include corrosion inhibitors, shrinkage control, alkali-silica reactivity inhibitors, and coloring. 

5. Once the foundation is installed, the next step is drain tile and sump crock installation. Damp proofing the outside of the wall may be done before backfilling. An asphalt-based coating is applied by spraying or rolling directly onto the walls. This applied coating is normally below the soil line.

6. Polystyrene insulation board is usually installed on the outside of the foundation wall in a basement design below the soil or installed on the inside with slab-on-grade frost walls. 

7. Concrete slabs, whether a slab-on-grade, a basement floor, or a garage floor, may have polystyrene insulation board installed before the concrete is poured. 

It is also important to consider how the foundation can impact the living environment within the home. You may not realize it, but indoor air quality and moisture control are vital aspects that can be significantly influenced by a concrete foundation. Here are five steps you can take to help maintain a healthy indoor air quality as it relates to a concrete foundation: 

1. Promote a clean, healthy concrete mix. Chemicals in concrete do off-gas. Avoid concrete additives whenever possible. If unavoidable, compare the products available to decide which are best-in-class. This includes footings, foundations, stem walls, and flatwork. To achievea healthy concrete mixture:

   1. Use only clean, natural mineral aggregate.  

   2. Use only portable water.

   3. No fly ash.  

   4. The fewer additives you use, the better.  

   5. Use a low water-to-cement ratio. 

2. Geopathic stress is a form of natural earth radiation from underground earth anomalies such as cracks, shifts, water veins, etc. They are detected in grid lines known as Harmann and Curry grids. Unlike radon, these grid zones create natural DC magnetic fields. Using steel rebar within concrete can become magnetized. Using fiberglass reinforcement rather than steel rebar is superior to resolving this problem.   

3. Concrete loves water. Even when cured over many years, it will act as a sponge and absorb liquid and vapor moisture. When concrete gets wet, it off-gasses. The key to controlling moisture when it comes to concrete is to stop moisture from entering the concrete. If it does, it needs a way to escape and dry out. Using a rubber membrane vs. asphalt damp proofing is key. Applying a rubber membrane on top of the footing before setting the wall forms will stop the capillary migration of moisture into the foundation wall. Applying this membrane completely over the outside of the foundation wall, over the top of the foundation walls, and below the insulation if slab-on-grade, will stop any vapor transmission. 

4. Whether it’s a slab-on-grade or a basement floor, creating a negative pressure under the floor slab has many benefits, including drying the floor slab from underneath. Like a methane collection system, five inches of wash pea gravel with perforated tile vented outside would be installed below the insulation board. A heavy 15-mil vapor barrier taped at the seams and foundation wall should be installed directly against the concrete floor slab. 

5. A concrete wall is a thermal bridge. That means cold temperatures on the outside of the wall will transfer to the inside surface of the wall. Moisture moves from warm to cold surfaces creating condensation. There are a variety of board insulation products that can be used below the surface, look great above ground on exposed foundations, and have superior insulation value. Using a double layer of insulation board and staggering the seams over a rubber membrane works well.        

By focusing on the structural and environmental aspects of foundation construction, contractors can ensure long-lasting homes that meet evolving needs and expectations.