Steps
1. Draw your plan to scale on a sheet of paper. For backyard-sized projects, a scale of 1/4"=1 foot works well.
2. Estimate the surface area and volume of your planned pour. Figure your area by multiplying length (in feet) x width (feet) to get area (in square feet). Multiply area x average depth to get volume (in cubic feet.) For example, a patio that is 8' x10' will have an area of 80 square feet. If the patio slab is 4" deep, then multiply 80 sqft x 0.333 feet and you will have a volume of 26.64 cubic feet. Since there are 27 cubic feet in a cubic yard, this mean you will need almost exactly one cubic yard or concrete for your project. If you buy the concrete from a batch plant, they will quote the price on a per-yard basis.
Note that you should always buy a little extra, rather than coming up a little short on concrete pouring day.
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3. Lay out your project on the ground placing temporary wooden stakes at the corners. Verify that your layout is square to the building or fenceline against which you want the slab lined up. For rectangles, measure corner-to-corner in two directions to ensure the rectangle is square. Double-check your work - it is hard to rebuild your forms to correct mistake son pouring day. Do this step far in advance of pouring day, not the morning immediately before the truck shows up.
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3. Lay out your project on the ground placing temporary wooden stakes at the corners. Verify that your layout is square to the building or fenceline against which you want the slab lined up. For rectangles, measure corner-to-corner in two directions to ensure the rectangle is square. Double-check your work - it is hard to rebuild your forms to correct mistake son pouring day. Do this step far in advance of pouring day, not the morning immediately before.
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4. Since your corner stakes will probably be dug up at some point, build some batten boards at each corner of the slab out about a foot or 2 from the slab. The batten board will have three vertical 2x4s driven into the ground, and two horizontal 2x4s nailed to the verticals about 18" above grade. Tie a string line from batten to batten, and line up the string line exactly over the top of the corner stakes, along the outside of the slab. This way, when you dig out for your slab and disturb the corner stakes, you can drop a plumb bob line from the string and position the corners of your slab any time you need to.
Note that crooked forms will help you create a crooked slab, and will make you look stupid if your goal was a straight slab.
5. Dig out the area for your slab. This is the hard part. Ideally, it is nice to have a friend with a backhoe, skid-steer loader, or a couple of teenagers to help. Be sure to dig out any tree stumps, organic materials, trash, loose fill, mud or anything else that will eventually settle. Also, be sure to call your state's One call utility notification 48 hours before you dig. It's the law, and it may keep you from being electrocuted or blow'ed up if you hit a buried line of some sort. This step is important, even if you are working in your own backyard, and even if you think you know where all the utilities are. It's free and it may well save your life.
6. If the slab will eventually hold up a portion of your house or other building, you need to dig out footings at the edges of the slab. For typical slab-on-grade construction, the footing is at the edge of the slab and is about 12" to 20" wide, and should extend to the frostline or deeper. (If you're not sure, call you local building department and ask them what the frost line depth is. They will probably also remind you to submit for a building permit if you are building a building or addition to your house.) Be sure to clean out the loose dirt clods at the bottom of the footing. If the patio will not support any walls or structures, you can probably avoid digging out extensive footings.
7. Build your forms around the outside of the slab, immediately underneath the string lines you installed. For a 4" slab with no footings, you can use 2x4s and 2x6s for the forms. Use good, straight boards for your forms, not junk lumber. Nail the forms together well and reinforce the corners with additional boards and braces. Use one or more levels (4' and 6' levels work best) to adjust the top of your forms to the exact height you want the top of your slab to be. Use steel stakes driven well into solid ground on the outside of your forms to anchor your forms. If the forms come above adjacent grade more than 6" or so, nail additional wooden braces at 45 degrees, anchored by more stakes. Your forms should look and feel as if the would withstand the direct impact of a Mack truck without moving. (Because wet concrete can exert the same amount of pressure as a Mack truck once you start pouring!) Locate braces at least every 4' all around the forms. When in doubt, add more braces everywhere the forming feels weak or shaky. get your forms built well before pouring day! Plug any holes at the bottom of the forms where concrete might ooze through. After completing your forms, doublecheck that they are still lined up under the stringlines and that the corners are square. Doublecheck the top of your forms for levelness and elevation.
8. Place an undamaged 6-mil thick plastic sheet vapor barrier on top of the dirt where you want your slab. It is not necessary to extend the sheeting under your footings, but it is OK if you do. Make sure the top of the dirt is level and compacted, and loose dirt clods are removed before placing the vapor barrier. The purpose of the vapor barrier is to reduce the amount of water vapor coming up from below your slab and entering into your basement or house (where it can contribute to mold and flooring problems.) An additional benefit to the vapor barrier is that it prevents water from wicking out of your wet concrete into the dirt while the concrete is curing. So use a vapor barrier even if you are pouring an outdoor slab, and especially if you are pouring in summer and/or in a dry climate.
9. Place your reinforcing steel. For the footings, you will probably have two to four horizontal steel reinforcement bars (rebars) running the length of the footing, with at least two rebars in the bottom of the footing. If you are pouring a house foundation, you should be working with an engineer, who will trypically specify more or bigger rebars depending upon the loads that will be seen by the footing. For a patio slab with a thickener edge, having at least two rebars at the bottom of the thickened slab edge is typical. Use steel baling wire to tie the rebars into place. Keep them up off the dirt and away from the sides of the footing by at least 3 inches. For the slab itself, it is typical to use wire mesh. Typical reinforcement for a non-structural slab is a mesh using #10 guage wires at 6" on center each direction. You can get this wire in roll or sheet form at the lumber yard or home center. Be careful unrolling and cutting rolled-up wire mesh. It can pop up and put your eye out. Use a couple of helpers to straighten out the wire and hold it down while cutting it. Wear safety goggles.
Be sure that all you rebar is in place and securely tied down where you want it. Be sure that your wire mesh is tied down and lays flat - before the concrete truck shows up.
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10. Now that your forms, vapor barrier and reinforcement are in place, and secured, and that your dimensions with respect to the batten boards are verified, again!, and your tools required for finishing the concrete below are purchased and in place, now you are ready to order your concrete. For pours of about 1/2 yard or below, you can mix the concrete yourself in a plastic tub using pre-mixed concrete you can purchase by the sack. For pours between 1/2 yard and 2 1/2 yards, you should buy or rent a portable concrete mixer. Unless you have a bunch of hard-working teenagers at your disposal for a long weekend, pours of 3 yards and over are best done with a truckload of concrete from a batch plant. Their minimum purchase for a load is usually about 3 yards, and they will deliver up to about 7 yards in a single truck. The cost per yard is lower if you buy more. But the work involved in pouring and finishing multiple yards of concrete is higher, and you really need a bigger crew for bigger pours. Make sure you have a big, wide path from the road to your pour so the concrete truck can get in there without sinking in mud and without driving over existing driveways (if possible.) If possible, order your concrete for early morning. Otherwise, you be finishing late at night and in the dark.
11. Pour your concrete out of the truck. Have your crew stationed around the end of the chute where the concrete comes out and rake the concrete (with big concrete rakes) out across the slab, starting at the point farthest from the truck. Work quickly, because the concrete is setting up as you speak, and the concrete plant will charge you extra if you take over an hour or so. Have plenty of friends, teenagers, and/or paid workers to help. This is hard work, and you are working on a strict time limit. There is no time to be repairing shoddy forms or inaccurately placed reinforcement at this point.
Have two workers level the top of the slab using a 'screed' (a very straight 2x4 board slightly longer than the width of the slab.) Move the screed rapdily back and forth while you level the top of the concrete using the top of the forms as a guide. A third guy will use his rake to move concrete if it piles up in front of the screed. And he/she will throw an extra shovel full of concrete in back of the screed to fill in low spots. Make several passes with the screed as required to get the concrete as level as possible.
Concurrently with screeding, have a worker tamp down the concrete at the edge of the form to eliminate any air pockets. You can use a very big tamper called a jitterbug to tamp the edges, but it takes humungous forearm strength to use such a tool. A regular steel rake works, but you have to tamp down hard, multiple times to get out the air pockets. Rescreed the area after tamping to ensure that the top of concrete stays level.
12. For areas where screeding is inconvenient, you can use a trowell to level the concrete, but use the screed as much as possible because it keeps the overall slab much flatter. For minor ridges left in the screeding process, use the trowell to knock down the ridges. Work quickly and deliberately. As the concrete sets up, it becomes harder and harder to flatten out. Ideally, you should plan on having the concrete poured in place, tamped and levelled within 60 to 90 minutes of the time the truck arrives.
13. Float the slab. This step further flattens and consolidates the top layer of concrete while it is still wet and before it sets up. Although you can do this step with a hand float for very small slabs, you can get much better quality using a bull float with a long handle. The bullfloat is aluminum or magnesium and is about 6" x 26", with handles in sections that can extend out to 20' or more. Buy or rent one. It is worth the $80 on larger slabs. Push the float across the full length of the slab in one long motion, then pull it back across the slab. Use each pass to push down on the wet concrete and flatten it out. About four passes with the float across each section of slab is typical. use a hand trowell to flatten out any ridges.
14. Now sit and wait! (since you are likely dead tired, anyway, this will feel good at this point.) Resist the urge to tweak your handiwork and just sit down on a lawn chair and have a glass of iced tea. Alcoholic beverages are a bad idea on hot days when you are doing heavy manual labor, by the way. And they are an especially bad idea on a construction job site, even if its your own back yard. But you need to relax about 3-4 hours and let the concrete start setting up. You will notice a lot of water will "bleed" up to the surface. Just watch for a while, and do not try to finish the concrete at this point. (Working the water back into the concrete will cause future delamination problems.)
15. After a few hours, depending upon temperature, the concrete will begin setting up and getting harder, and the water will eventually begin soaking back into the concrete. Once the concrete is stiff enough to walk on while only leaving an imprint of about 1/4" deep, then you can "finish" the concrete. You finish the concrete by using a steel trowell and scraping it flat against the concrete by going in a wide circular or figure 8 pattern. You should press hard into the concrete with the trowell to compact the top layer of concrete paste. Trowell over 100% of the concrete, regardless of whether it looks flat or not. Continue working back and forth in a circular pattern as you progress across the slab.
Ideally, it is best to kneel in the dirt outside of the slab and strech your arms to trowell the slab. But if the slab is wider than about 7', or if you have short arms, you will have to get out on the concrete to finish the middle. You can walk out on unfinished concrete by preparing a piece of plywood about 3'x3' to stand on. You should also buy some kneepads for doing this sort of work, as well. Start out in the middle and work your way back from it, trowelling off the marks left by the plywood as you go.
Knowing when to start the trowelling is an art. Hard at first, but quality comes with practice. And the act of trowelling to get a hard, flat smooth surface is also a learned art. Practice helps here as well.
16. For surfaces exposed to the weather, you should apply a 'broomed finish". Wait another couple of hours after you finish the concrete and use a wide, clean broom lightly dragged across the surface to apply a grooved finish to the concrete. Buy or select a good broom prior to pouring day, and make sure the handle is long enough to reach across the slab in one long motion. (Otherwise, your pattern will have breaks that will look bad when you are finished.) Practice helps here, as well. For a heavier broom finish, more appropriate for walkways that are exposed to ice, apply the broom when the concrete is wetter (soon after finishing). For a lighter pattern, more appropriate to a patio, apply the broom much later after the concrete has set for some hours after finishing.
17. Edges of concrete slabs should be smoothed or rounded rather than left sharp. (Sharp edges are safety hazards where kids can get pretty badly lacerated when the fall on the edges of the slab.) You can round the edge with a handheld edging tool that can be purchased at the hardware store for this purpose. Prepared the edge prior to the intial set, concurrent with screeding and tamping. Then further smooth the edge later in the day concurrent with trowelling and finishing.
18. Slabs larger than about 15'x15' should have construction joints poured in placed with the slab, or cut into place as the concrete cures. Since larger slabs will almost always crack, the construction joints tend to force the cracking to occur in a regular pattern that doesn't look as bad as random cracking of your slab. I assume if you are pouring slabs bigger than 15'x15', you probably have some practice on smaller slabs first, and will have a good feel for what is involved in bigger pours. If you need construction joints, you can cut them in green concrete with a skill saw using a diamond blade. The cut should penetrate at least a third of the way through the slab. And be sure to connect the saw to a circuit with a ground fault interrupt breaker. If you do not know what a GFI is, you ought not be using power tools in your backyard.
19. Curing the slab. Your slab needs a week or two, left alone by itself, to chemically cure and get really hard. You can probably walk on your slab the next day, but you probably shouldn't. It can leave scuff marks and cracks if you walk on it too soon. Curing a slab works best if you keep it damp for a week. Although chemicals are available to seal and cure the slab, the easiest method is to cover the slab as soon as it sets up with a 6 mil plastic cover. Use a perforated sprinkler hose to keep the slab continuously damp during the curing process. The longer you allow the slab to cure, the better.
Tips and Tactics
SAFETY FIRST!. Really, dude. Read this and internalize it.
1. Call the One-Call utility locate guys. If you don't, you could hit a high-voltage electrical line, or a gas line, and get someone killed. Maybe even a loved one.
2. If you use power tools in an outdoor environment, you must connect them to GFI-protected circuits. Ignore this and you can easily get electrocuted.
3. The cement used on concrete is a dangerous, hazardous material that will destroy your skin in a matter of hours, producing serious chemical burns, often requiring skin grafts and surgery to repair. Do not allow powdery or wet concrete to touch your skin for any significant amount of time. Use thigh-high rubber boots to keep concrete out of your shoes and off your legs. Use forearm-length rubber gloves to keep it off your hands. Use goggles to keep it off your eyes. Keep a garden hose around for clean water to wash off whatever gets on your eyes and skin. If you pour concrete in tennis shoes, and wait until the end of the job to wash the concrete off your socks, you will get nasty chemical burns around your ankles that will take months to heal. Trust me. Been there, done that, got the scars to prove it.
4. If you are a beginning concrete guy, start small. Get some practice. Get some friends to help. Don't try a 7-yard truck pour on a hot Sarturday afternoon without a lot of friends to help you out.
