Concrete slabs are used to build guest cottages, outdoor hot tub decks, garden sheds or greenhouses, garages, additions, offices, pottery studios, and patios. Creating a concrete slab is pretty much a straightforward process. Its most basic steps simply require one to build a form, pour the concrete, level it, and let it cure. However, there are some finer points that one should consider in order to build a concrete slab that would last for years.
Basic Steps in Pouring a Concrete Slab
- Clear the area of vegetation.
- Remove the top soil down to a few inches and compact it.
- Build a form by using 2 x4 wood planks or plywood. Place stakes outside the forms to prevent the poured concrete from pushing it apart. Stakes should be lower than the form as the edges can be used to level the concrete.
- Spread crushed gravel over the area enclosed by the form. This will serve as a base for the slab.
- Place concrete reinforcing wire several inches above the gravel.
- Pour concrete inside the form.
- Level the concrete.
- Allow the concrete to cure.
Types of Foundations
It is important to differentiate the types of foundations to guide you in choosing the appropriate foundation for your project.
- T-shaped Foundations
This type of foundation is ideal for grounds that freeze during winter. Its basic steps require the footing to be placed before the walls are built and poured. The slab is then placed as a last step.
- Slab-on-grade Foundation
This foundation is often used in areas with ground that doesn’t freeze. It often has edges thicker than its interior in order to form an integral footing. These thickened edges are often strengthened with reinforcing rods. The slab is poured all at one time on crushed gravel to improve drainage.
- Frost Protected Foundation
This type of foundation only works with a heated structure. It makes use of two sheets of rigid polystyrene insulation. These are placed on the outside and at the base of the foundation wall. The insulation holds heat from the structure and prevents heat loss from the slabs.
Calculating Water to Cement Ratios
The calculation for the water to cement ratio is done by dividing the weight of water (lbs.) by the weight of cement (lbs.) for every one cubic yard of the mix.
The following are a few basic guidelines to determine the appropriate water to cement ratios:
- Use a maximum water to cement ratio of .50 when the concrete is subject to freezing and thawing conditions.
- Use a maximum water to cement ratio of .40 for concrete with extreme sulfate conditions.
- A water-cement ratio greater than .050 increases the concrete’s permeability exponentially. Durability increases as the permeability decreases.
- Strength increases with lower water to cement ratios.