When done correctly, pouring a concrete slab can save as much as half the price it would cost to have a contractor do it, according to The Family Handyman. But the job can be stressful: Because concrete dries so quickly, everything has to be ready to go before you start pouring, and even minor mistakes -- often invisible at first -- can result in cracking later. Follow these tips and you should be able to pour an even, crack-free slab that will serve as a sturdy foundation for your building project.
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Preparing the Site
Pouring a smooth slab starts with the way you prepare the ground. Soil that doesn't drain well will eventually lead to cracks in the slab. Sandy soil, which is naturally well draining, will need little preparation beyond scraping off the topsoil. But with clay and loam soils, you will have to remove enough to allow for a 6- to 8-inch layer of gravel under the slab, according to the Family Handyman.
Form Joints and Reinforcements
Forgetting to properly joint and reinforce the form will result in an unusable slab. Slabs larger than 6 meters by 6 meters, according to pavingexpert.com, will require joints, which the National Ready Mix Concrete Association describes as preplanned cracks. Joints are easier to maintain than the irregular cracks that would naturally occur in any large concrete slab as it dries and shrinks. Cooper Concrete Company, a Texas-based ready mix concrete company, recommends placing the joints 10 to 15 feet apart and at least a fourth the depth of the concrete. You will also need to reinforce the form, or its sides will bulge outward when you pour the concrete. The Family Handyman recommends bracing every 2 feet with stakes and kickers, a special tool that slants into the ground to keep the top of the stakes from bending outward. Cooper Concrete recommends reinforcing every 3 to 4 feet and at every joint.
Install a rebar to reinforce the slab and further ensure against cracking. Rebar, a steel reinforcement bar, is available at most home improvement stores. Put it around the perimeter of your form, at least 2 inches away from the form and 2 inches from the top, according to buildeasy.com. Tie rebar pieces together with wire, leaving at least 16 inches of overlap.
Concrete dries very quickly, so time for adjustments will be limited once you start to pour. High temperatures speed drying times, so if it is hot or if this is the first time you have poured a slab, divide the slab in half and pour one part at a time. Shovel the wet concrete roughly where you will need it, then have a partner rake it evenly throughout the form as you pull a long 2 by 4, called a screed board, across the top to smooth it out. Buildeasy.com recommends pulling the screed board back and forth in a sawing motion until the concrete is level.
Once the concrete is poured and level, run a trowel or a float over the top to smooth it out. Buildeasy.com recommends sweeping the trowel or float back and forth in wide arcs. If you don't get it right the first time, leave it until the excess water has evaporated from the surface and re-trowel. You can repeat this step as many as three times.
- Build Easy: How to Make a Concrete Slab
- Concrete Network: How to Build High Quality Slabs on Grade
- The Family Handyman: Form and Pour a Concrete Slab
- Paving Expert: Joints for Concrete
- National Ready Mixed Concrete Association: Concrete in Practice: What, Why and How
- Cooper Concrete: Common Concrete Misperceptions