Slab-on-Grade Foundation

A slab-on-grade foundation is is a type of foundation where the concrete slab is placed directly on the ground. This concrete slab act as a foundation from a mold set in the ground. This is used in various construction projects, such as residential buildings, commercial buildings, factories and distribution centers. This blog will provide the details that can help in understanding the design, manufacturing, and benefits of slab-on-grade foundations.

What is a slab-on-grade Foundation?

A slab-on-grade foundation can also be called a slab foundation. It is a solid piece of concrete poured on the ground directly. This foundation helps in supporting the building without the need for a basement or crawl space. It often includes thickened edges or footings to prevent shifting in cold weather.

When compared to other types of foundations like basements or crawl spaces, slab-on-grade foundations are simpler and inexpensive as they need minimal digging. Crawl spaces will have gaps in between the ground and the building, while the basements are the closed spaces present below the ground level. Slabs on grade foundations are more common in warm climate areas where there are fewer issues regarding cold.

There are mainly three types of slab-on-grade foundations, they are:

Traditional Slab-on-Grade Foundations: 

In this type, thick concrete is poured directly on the ground. These types can be used in warm climates where there is no frost, as they do not have frost protection properties.

Slab Foundations with Shallow Footings: 

These foundations are sometimes also known as T-shaped slab foundations. It mainly consists of concrete slabs that are poured directly into the ground, and they have additional footage at the edges or corners.  They give extra stability as the footings go deep into the soil and prevent heavy frost even if the top layer of the soil is frozen.

Frost-Protected Slab Foundations: 

These slabs use insulating foam sheets for protection from heavy frost. They also have shallow footing to reduce the risk of damage from freezing the soil under the slab.

Cost for Installing Slab-On-Grade Foundation

​The average cost for installing a slab-on-grade foundation ranges from approximately ₹330 to ₹660 per square foot, with a typical cost of around ₹495 per square foot.​ For a home with a 1,000-square-foot area, the complete installation cost would exceed ₹495,000.

Slab foundations are relatively low-cost since they require little or no excavation. In contrast, a basement foundation may cost nearly ₹1,650 per square foot. This significant price difference can amount to approximately ₹1,320,000 in savings for a 1,000-square-foot area.

Construction Process

Preparation of the Site:

The process starts by clearing the area of trees, rocks, and debris. The area is leveled and compacted to make a solid base. Sometimes, gravel can be added to improve stability and drainage.

Layout and Excavation: 

By using the stakes and builders lines the foundation outline can be marked. Excavation can be carried out according to the size of the slab, local codes, and the frost line.

Installing Formwork: 

For holding the concrete in particular, wooden boards are placed around the excavation area. The formwork must be leveled and secured to ensure an even slab.

Placing Reinforcements: 

For strengthening the slab a steel mesh or rebar can be placed that can also prevent cracking.

Concrete Pouring: 

Concrete is then forced into the formwork, and it is spread evenly with the help of a screed tool.

Surface Finishing: 

After pouring the concrete, a trowel is used to smooth it. For making visible slabs, a broom can be used to add non-slip surface texture.

Concrete Curing: 

The concrete needs to be cured for several weeks to get the desired strength. The concrete is covered using a plastic sheet or it is sprayed by a curing component for preventing rapid drying or cracking.

Advantages of Slab-on-Grade Foundations

Some of the advantages of slab-on-grade foundations are:

Cost Efficiency: 

The main advantages of slab-on-grade foundations are cost-effectiveness and affordability. They are less costly to construct compared to other types of foundations. This is because they don’t need any additional material, such as floor joists or a crawl space. This lack of extra components also means reduced heating and cooling costs.

Simple Construction: 

The process of building slabs on graded foundations is simple. Concrete can be poured in one day. And with proper preparation, the construction process ends in 4 days, including curing time, mainly if weather conditions allow it. This quick turnaround is faster compared to more complex foundation types.

Low Maintenance: 

Slab-on-grade foundations have high levels of durability, and they can last until 80 to 200 years if made correctly. They are solid and provide a robust base that requires minimal upkeep. In non-freezing areas, slabs on graded foundations are stable and have fewer issues.

Energy Efficiency: 

Homes that are built using slab-on-grade foundations have lower utility consumption. They have efficient heating and cooling properties in the home. 

Reduced Risk of Mold and Pest Issues: 

Molds and termites can be prevented when using slab-on-grade foundations. As there is no space between ground and structure, the growth rates of molds are reduced and also protected from pests. 

Best for Warmer Climates: 

In areas that have no ground-freezing problems, slab-on-grade foundations can be used. They were designed for handling non-freezing soil conditions. This helps in avoiding the complications related to frost.

Suitable for Clay Soil: 

A slab-on-grade foundation can be used in clay soils, as clay soils have the properties of expansion and shifting. The flat, solid nature of a slab foundation helps to stabilize the structure against these soil movements.

Good for Low-Load Requirements: 

Even if the slab-on-grade foundation is the strongest option, it can be used in lighter load-bearing structures. They can be used in garages or storage sheets where there is a constraint in budget.

Disadvantages of Slab-on-grade foundation

Cracking: 

In freezing temperatures, slab-on-grade foundations are prone to cracking. Even small cracks can let moisture enter the building, and the repair costs will be high. Fixing these cracks usually requires professional help.

Lack of Crawl Space: 

As slabs on grade foundations do not include crawl space, there will be no extra storage in the house. So the heating and cooling systems need to be installed inside walls that can limit their accessibility.

Plumbing Issues: 

As plumbing pipes were attached inside the concrete, it may be difficult to repair the leaks. The repair cost will be high.

Uneven Flooring: 

If there is soil shifting, then the slabs may be unevenly settled, which can lead to uneven flooring in the house.

Earthquake and Flood Problems: 

Slab-on-grade foundations give less protection from earthquakes and floods.

Moisture Problems: 

The concrete is porous and has no moisture barrier. So, moisture can get inside the slab. This may lead to the growth of mold and rotting problems.

Conclusion

Slab-on-grade foundations provide a practical and economical foundation solution for many types of buildings. Their ability to reduce excavation and material costs aligns with environmentally friendly building practices. Despite some potential issues, such as uneven flooring and moisture problems, the overall efficiency and reduced environmental impact make slab-on-grade foundations a compelling choice for modern construction projects.

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