A concrete slab can serve a number of purposes in a house. Poured as a floating slab on gravel, it can support the house without the need for frost walls or a full foundation. Poured in conjunction with frost walls, it can provide added heat retention and rigidity. In either application, water tubes can be integrated with the concrete slab for radiant heating.
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A concrete slab should be poured on top of material with low water content, in order to avoid the risk of heaving. If the ground you're working on is water-retentive clay, it needs to be excavated either down to bedrock or below the frost line, and replaced with crushed stone or gravel. Build a sturdy form made of reinforced 2x10s that are held in place with stakes driven into the ground. These are what will give the slab its shape, so they need to be placed accurately. If you are building an insulated slab, lay rigid foam sheets on the gravel and tape them together with siding tape. This will prevent the heat of the slab from escaping into the ground.
Concrete has extremely high compressive strength, but relatively low shear strength. This means that it needs to be reinforced with steel rebar, which helps to prevent cracking. Lay the rebar pieces 12 inches apart in both directions, creating a grid of 12-inch squares. Prop the rebar up using bricks or commercially bought plastic rebar holders, so that it is suspended roughly in the middle of the slab. If your slab is 6 inches thick, the rebar should be 3 inches off the ground. Wire the pieces of rebar together at each point where they cross, creating a rigid grid that will bind your concrete together.
After the concrete is poured into the form, it will dry and harden over a period of hours. During this time, it is important to repeatedly work the surface of the concrete to make it as smooth as possible. Much of the moisture in the concrete will rise to the top, where it can be shunted off with a long board. Use a board that is long enough that you can rest it on the forms on both sides of the slab. You will then be able to move it back and forth in a sawing motion across the slab, simultaneously smoothing the surface and removing excess water. For professional results, once the concrete is hard enough to walk on, go over it with a power trowel, which functions like a large horizontal fan and gives the concrete a flat and shiny finish.
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