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Concrete is an age-old pillar of civilization, on a par with beer, steel and the plow. Master concrete (we're talking about simple backyard cement and water here, not engineer-grade flexible concrete.) and you can build a backyard empire--starting with a place to fire up your grill.
For small jobs, you can buy bags of cement that already contain sand and gravel--you just add water. On larger jobs, it's more economical to buy cement and aggregate, and combine the ingredients in a motorized mixer. These rent for about $50 a day and can be towed home. Cement comes in 94-pound bags; about six will make a cubic yard. The sand-and-gravel aggregate should include a range of sizes, from grains to 3/4-in. stones. If the aggregate in your area is too fine, add 3/8- to 3/4-in. gravel.
As for time commitment, you can easily pour a small slab in an afternoon. Here are the steps to get the job done.
Step 1: Tools of the Trade
Float: A wooden or magnesium surfacing tool (wood is slightly coarser) used to smooth a screeded, or leveled, surface.
Edger: To round the corners of a pad, run the edger back and forth while pressing down.
Trowel: A steel surfacing tool for an extra-fine finish. It's optional for outdoor work.
Groover: To make contraction and expansion joints, which help control cracking, use the groover to section off the concrete.
Step 2: Create Form
Build the form out of 2 x 4s, nailed at the corners with 16d nails. We chose duplex nails, because their double heads make them easy to remove. Long drywall screws can also make form stripping easier. Then use a shovel to slice the sod into manageable squares.
Step 3: Excavate Soil
Rough out the rest of the form bed, but be careful to not dig too deeply. Backfilled soil settles, so over-excavation will require compacted sand, which adds work and expense to the project.
Set the form in place and shave away soil from high spots and check with a level. A slight slope of about 1/8 in. per foot helps shed water.
Step 4: Stakes
When you have the bed roughed out, set the form in place. Drive 1-in. stakes around the perimeter to just below the top of the form. This prevents the concrete from pushing the form apart.
Step 5: Screw in Stakes
Fasten the stakes with 2-in. drywall screws, leveling the form in both directions as you go. After you're done, recheck the form bed for high spots. A slight slope of about 1/8 in. per foot helps shed water.
Step 6: Mixing Ingredients
Pour about 2 gal. of water into the mixer drum and then add the aggregate. If you have a sand-and-gravel combination, throw in one shovel of cement for every five shovels of aggregate.
Step 7: Coarse Aggregate
If you have a separate pile of coarse aggregate, use 2 1/2 shovels of sand, 2 1/2 shovels of coarse gravel and one shovel of cement. We find it works best when you keep the shovel topped off so the quantities are uniform.
When the drum is about three-quarters full, gradually add more water--the exact amount will vary, depending on the moisture content of the aggregate.
When the mixture reaches a gray/green color with hardly any of the rock color showing through, stop the mixer and pull out a small handful. Form it into a 3-in. ball and toss it from one hand to the other. If it crumbles, it's too dry. If it splatters, it's too wet. Cautiously add a little water or more aggregate and cement to correct the consistency, then run the mixer for 3 more minutes.
Concrete from a relatively dry mix will always be stronger than that from a wet mix, so it's best to err on the dry side. After touching the concrete, immediately give your hands a thorough wash.
Step 8: Prepare to Pour
After you're satisfied with the consistency, load the mix into a wheelbarrow and deliver it to the form.
Wash your hands immediately after handling wet concrete. It contains alkaline compounds like calcium oxide that can irritate skin, can cause up to third-degree burns.
Step 9: Fill Form
Dump the mix into the form, being careful to avoid skin contact with the wet concrete.
Step 10: Spread With a Screed
Spread the concrete around with a shovel and press it against the sides. Then mix another batch. When you have about half the form filled, use a 2 x 4 screed board to level the surface, but leave it slightly high. Fill the other half of the form in the same way. A larger slab requires rebar or steel mesh to help control cracking, but a small slab can do without that kind of reinforcement.
Then, steadily saw the screed board side to side while moving it along the form. If you find a low spot, fill it and saw through that area again.
Step 11: Smooth With Float
Finishing is all about timing--and surface water is the timer. Each pass with a tool brings up water and cement, but overworking the surface can weaken it. When the water from the previous pass is no longer visible, it's okay to move to the next operation.
After screeding, wait a few minutes before using the float. When the surface appears dull, hold the float on the concrete with extra pressure on the trailing edge. Run it along the pad edges and across the surface in wide arcs.
Step 12: Edge the Slab
We recommend you round the edges of the slab, as that prevents chipping. Use an edging tool, running it back and forth along the form with downward pressure until the edge is neatly rounded.
Step 13: Trowel Finish
A rougher finish may be good for outdoor slabs, as they provide more traction. But using a trowel will give you a smoother surface, if that's what you're looking for. Again, wait for the surface water to disappear. Trowel the edges, then sweep the surface in wide arcs. If you can't avoid sweep lines, wait 15 minutes and trowel again.
Concrete cures best between 50 F and 70 F. It achieves most of its strength--60 to 75 percent--during the first week, and about 95 percent over the first three weeks. To make sure the concrete dries correctly, keep the job damp for five days by covering it evenly with a plastic vapor barrier or by using a sprinkler. Alternatively, you can also apply a curing-and-sealing compound.
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