How to Construct a Concrete Slab on the Ground?

A slab-on-ground structure requires a properly prepared ground sub-base. A stable and sturdy ground base support is necessary to keep the concrete slab level and haul for a longer period without settlement. Here is a quick guide on pouring a concrete slab on the ground.

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1. Determine the Depth of the Slab

Determine the depth of the slab required and decide whether the slab is to be laid in line or above ground level. Check the drainage direction and site rules to prepare the site without any water flooding issues due to improper sloping.

2. Start Excavating the Slab Depth

Prepare the location of the ground, in the dimension of the slab (length and breadth). If you need the slab in-line with the ground, dig the slab area to the required depth of the slab. Professional expertise here can help you plan properly for digging. Dig uniformly that the bottom area is the same as the top area of the slab pit. Use dirt or soil to fill any divots or holes present. 

3. Dampen the Earth and Compact the Subgrade

Dampen the soil by spraying it with water so that it holds together. Do it in intervals so that instead of becoming a pool, the loose earth holds together. Tamp it, compact it, and level the subgrade surface, until your footprints on it are almost invisible. 

4. Lay the Base

The best base for a concrete slab is a mix of coarse and fine aggregate to form a compactable base that prevents settlement and promotes drainage.

5. Compact the Base

Use a hand or mechanical tamper to compact the gravel base. Usually, the base layer is placed in layers of 2 inches (not more than that) and compacted, following the layer. The thickness of the base layer for a concrete slab can range between 4 to 6 inches. A base of 6 inches is used for concrete driveways subjected to heavy-duty loads. Above the base, vapor barriers or a damp-proofing layer is provided based on the requirement of the site. It depends on how high is the water-table level of the site. 

Concrete Slab on the Ground With Vapor Barrier
© 6DProjects

6. Prepare the Formwork for Concrete Slab

The formwork is prepared based on the dimension of the slab. Wood, foam boards, etc can be used for formwork. Place stakes in intervals and secure them to provide extra reinforcement for the formwork.

Stakes Support for Formwork

Apply release agent inside the formwork for easy removal from the concrete slab. Silicone rubber or cooking oil can be used for this purpose.

7. Place Reinforcement on Formwork

Reinforcement bars are not necessary for concrete slabs with thicknesses less than 4 inches. The required rebars are placed in grids. Bring together the rebars using rebar ties. Chairs can be used to keep the rebars above the height of the ground. 

 As per ACI 318-2019, 20.5.1.3 Specified Concrete Cover Requirements, a cover of 3 inches needs to be provided for concrete slabs directly in contact with the ground, by means of cover blocks. For slabs of small thicknesses, wire mesh can be placed as reinforcement instead of steel reinforcement bars.

8. Pour Fresh Concrete on Formwork

The concrete of required strength and properties is poured either by ready mix concrete source or as a DIY project. But with the increase in the area and depth of the slab, it is recommended to enlist professional help. The concrete slab is compacted, leveled, and finished. The slab is left for curing to gain strength and for use.

Curing Period of Concrete Slabs

American Concrete Institute (ACI) Committee recommends a minimum curing period corresponding to concrete attaining 70 percent of the specified compressive strength. The often specified seven-day curing commonly corresponds to approximately 70 percent of the specified compressive strengths. It is ideal to provide a 28 days curing period for concrete slabs with large thicknesses and for major strength gain.

What is the Cost of a Concrete Slab?

The cost of constructing a concrete slab is dependent on the area of the slab, the thickness of the slab, the extent of ground preparation, the type of concrete used, the cost of reinforcement, and labor costs.
As a rough estimate, the cost of a standard 4-inch-thick concrete slab for a typical residential driveway or patio can range from $4 to $8 per square foot. For a more durable and decorative finish, a stamped or stained concrete slab may cost anywhere from $8 to $18 per square foot.

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