Building A Concrete Patio
PROJECTS MADE EASY
Good To Know
Edging & Jointing Trowels
Lumber for Forms
RENNOVATION & REPAIR
YARD & GARDEN
1. Begin by marking off, and then excavating, the patio area. The depth of the excavation will equal the thickness of the slab plus a 4 to 6-in.-deep gravel base. Drive a stake, and measure up from the bottom of the excavation to mark the finished slab surface. Slabs should be pitched slightly to shed water. use a string and line level to adjust the slope.
2. Prepare the base by leveling the excavated area. Tamp the soil, and remove any large rocks with a pry bar. Small stones do not pose a problem, but over time rocks several inches in diameter or larger can be forced to the surface by groundwater or by freeze-thaw cycles in colder areas. This movement can gradually lift a slab and cause it to crack.
3. Place 4 to 6 in. of crushed stone or gravel over the entire patio area, and use a rake to level and smooth it. The more gravel under the slab, the more it will be protected from ground movement and water. Spaces within the gravel allow groundwater to infill and expand in freezing weather, reducing the buildup of hydrostatic pressure beneath the slab.
4. Concrete is heavy, and it exerts considerable pressure when poured. Build sturdy 2x4 forms to hold the wet concrete. This provides a slab depth of ½ in.,which is adequate for a small slab in most areas. For larger, thicker slabs, use 2x6 forms and pour to the depth desired. Set your stakes below the form board tops to allow clearance for screeding.
5. Use a straight 2x4 to screed the gravel. The gravel surface should be no higher than the bottom of the form boards to ensure an even, continuous depth for the concrete pour. Some gravel will extend under and past the forms, which provides a wide, firm footing for the slab and allows any groundwater to disperse from beneath the slab.
6. Tamp the gravel well to ensure a firm, solid base. Using a gas-powered tamper makes this work go quickly and results in an even, well-compacted surface. Crushed stone gravel, like that used here, compacts better than round, smooth pea or river gravels and provides a solid base for the heavy concrete.
7. After compacting the gravel, cover the entire slab area with wire reinforcing mesh. Several mesh types and sizes are available. For poured concrete slabs, masons typically specify 6 x 6-in. uncoated wire mesh. The mesh helps to prevent cracking as the concrete shrinks and expands. Overlap mesh sections by 6 in., and tie them together with wire.
8. Place boards for wheelbarrow ramps to prevent collapsing the forms as the concrete is delivered to the site. When you begin pouring the wet concrete mix, dump each load against the previous one. Place blocks under the mesh, or use a rake to lift the mesh into the middle of the slab. use a hoe to spread the mix evenly, and fill to just above the forms.
9. Slice into the concrete mix with a shovel or trowel to eliminate air pockets and voids, especially along the form edges and in corners. Shovel the concrete into low spots—screeding is easier if the forms are slightly overfull. With a helper at one end, push and pull a straight 2x4 screed board along the top of the forms to level the still-wet concrete.
10. Use a long wood darby or a bull float attached to an extension pole, as shown, to level the screeded concrete. Floating helps to force the aggregate in the mix below the concrete surface. As you work the darby or float across the surface with a swirling motion, keep the leading edges slightly raised to avoid digging into the wet “mud.”
11. Concrete surfaces can be easily finished to a putty-like smoothess, but outdoor patios need a more textured surface to prevent them from becoming slick when wet, making walking hazardous. Give the surface some “tooth” by going over the uncured slab with a stiff broom. one light pass of the broom in each direction is usually all that is needed.
12. While the concrete is still uncured—just as it begins to stiffen up—run an edging tool along the entire perimeter of the pour. Insert the tool between the form and slab, and use light pressure to pull it in a continuous sweep along the edges. This will round the edges, which helps to strengthen them and makes them less brittle or prone to chipping.
13. Use a jointing tool, which is similar to an edging tool, to create control joints in the slab. Concrete shrinks as its moisture dries and the mix hardens, and cracks may occur for a variety of reasons. Control joints spaced at recommended intervals may not prevent slabs from cracking, but they will stop cracks from spreading or continuing into adjacent areas.
14. When the concrete dries hard enough so that you can’t see an impression when you step on it, mist the entire surface with a fine spray of clean water, and cover the slab with burlap or polyethylene sheeting to keep it from drying out. Concrete takes about a month to fully harden. keeping it moist while it cures, or hydrates, is critical to the process.
15. As concrete hardens, it develops most of its strength within the first week or so. Cure the slab for at least five to seven days, or longer if possible, before walking on it or placing heavy objects on it. Mist the surface periodically to keep it damp, and keep a covering in place to help retain the moisture. Spray-on compounds to hasten curing are also available.
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