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How to Lay Concrete Slab

Learning how to lay a concrete slab-at-grade takes a lot of effort and attention to detail. It is more than just pouring the concrete into a form and letting it dry. Some very specific basics must be followed to successfully know how to lay a concrete slab-at- grade.

First, what is the difference between a regular slab of concrete and a slab-at-grade? A slab-at-grade is a concrete floor slab entirely supported directly by the ground.

Think of it as a layer of concrete on ground level. There are no established floors, columns, or beams for the concrete to be poured over that would be needed for a regular slab of concrete.

Slabs-at-grade are usually used for foundations and basement work. This article will only touch upon techniques required to learn how to lay a concrete slab-at-grade.

Preparing the Ground

The first and most important step in laying concrete slab-at-grade is ground preparation. The subgrade, or the ground below your slab, has to be prepared to support the slab and whatever weight it will carry. This means the ground should be levelled and flattened. All garbage and debris must be cleared from the work area.

The subgrade must then be graded, moistened, and compacted to make it as firm as possible. A properly constructed subgrade will always cause water to flow away from the foundation. Any spot in the subgrade that is too muddy or any spot where a puddle forms must be dug up and replaced so that the ground is damp but not wet.

Lay Down Gravel and Membrane

Now that the subgrade is ready, a layer of loose gravel is laid evenly over the whole area to open up a route for water to pass easily beneath the slab after it is put down.

A waterproof membrane, like polyethylene, is placed atop the gravel as a vapour barrier to protect the slab from any moisture seeping up from the ground. Follow this rule and you’ll be doing good as you learn how to lay a concrete slab.

Building a Wood Frame

The perimeter formwork of the floor slab built atop the vapour barrier uses a wooden frame to the specifications you require. A system of 2×4 wooden rails, known as screed rails, are added on opposite sides of this perimeter to serve as a levelling system after the concrete is poured.

Spanning the screed rails is a straightedge (this can be another piece of 2×4 or other flat long piece of material) used to slide along the rails to push and level off any uneven or excess concrete.

The screed rails are supported by metal screeds or stakes and are placed in such a way so that the straightedge’s bottom sits at the desired level of the slab.

Pouring and Finishing

Now, the concrete is ready to be poured. As the frame fills, the straightedge slides along the screed rails to level off the slab. This levelling process on how to lay a concrete slab is completed after all extra concrete is removed. The rails and straightedge are then dismantled.

A final finish is done either by hand trowels or power trowels as the slab dries to work out any imperfections which were left after the straightedge has done its job, such as filling any low areas. Now, the slab-at-grade can be left to solidify.

Remember, these steps are only the bare basics on how to lay a concrete slab-at-grade. In most cases, like foundation work to support a major structure, trained professionals should only lay concrete.