Forming and pouring a concrete slab can be intimidating. Your heart races because you know that any mistake, even a little one, can quickly turn your slab into a big mess, a mistake literally cast in stone.
In this article, we'll walk you through the slab-pouring process so you get it right the first time. We'll pay particular attention to the tough parts where you're most likely to goof.
Still, pouring a large concrete slab isn't a job for a beginner. If you haven't worked with concrete, start with a small sidewalk or garden shed floor before attempting a garage-size slab like this. Even if you've got a few small jobs under your belt, it's a good idea to find an experienced helper. In addition to standard carpentry tools, you'll need a number of special tools to finish a large slab (see the Tool List below).
The bulk of the work for a new slab is in the excavation and form building. If you have to level a sloped site or bring in a lot of fill, hire an excavator for a day to help prepare the site. Then figure on spending a day building the forms and another pouring the slab.
In our area, hiring a concrete contractor to pour a 16 x 20-ft. slab like this one would cost $3,000 to $4,000. The amount of money you'll save by doing the work yourself depends mostly on whether you have to hire an excavator. In most cases, you'll save 30 to 50 percent by doing your own work.
Before you get started, contact your local building department to see whether a permit is required and how close to the lot lines you can build. In most cases, you'll measure from the lot line to position the slab parallel to it. Then drive four stakes to roughly indicate the corners of the new slab. With the approximate size and location marked, use a line level and string or builder's level to see how much the ground slopes. Flattening a sloped site means moving tons of soil. You can build up the low side as we did, or dig the high side into the slope and add a low retaining wall to hold back the soil.
Your concrete slab will last longer, with less cracking and movement, if it's built on solid, well-drained soil. If you have sandy soil, you're in luck. Just scrape off the sod and topsoil and add gravel fill if needed. If you have clay or loam soil, you should remove enough to allow a 6- to 8-in. layer of compacted gravel under the new concrete.
If you have to remove more than a few inches of dirt, consider renting a skid loader or hiring an excavator. An excavator can also help you get rid of excess soil.
Note: Before you do any digging, call 811 or visit call811.com to arrange to have your local utilities locate and mark buried pipes and wires.
Start by choosing straight form boards. For a 5-in.-thick slab with thickened edges, which is perfect for most garages and sheds, 2x12 boards work best. For a driveway or other slab without thickened edges, use 2x6s. If you can't get long enough boards, splice them together by nailing a 4-ft. 2x12 cleat over the joint. Sight down the boards to make sure they're aligned and straight before nailing on the cleat. Cut the two side form boards 3 in. longer than the length of the slab. Then cut the end boards to the exact width of the slab. You'll nail the end boards between the side boards to create the correct size form. Use 16d duplex (double-headed) nails to connect the form boards and attach the bracing. Nail through the stakes into the forms.
Photos 1 – 3 show how to build the forms. Measure from the lot line to position the first side and level it at the desired height. For speed and accuracy, use a builder's level (Photo 1), a transit or a laser level to set the height of the forms.
Brace the forms to ensure straight sides Freshly poured concrete can push form boards outward, leaving your slab with a curved edge that's almost impossible to fix. The best way to avoid this is with extra strong bracing. Place 2x4 stakes and 2x4 kickers every 2 ft. along the form boards for support (Photo 2). Kickers slant down into the ground and keep the top of the stakes from bending outward.
Stretch a strong string (mason's line) along the top edge of the form board. As you set the braces, make sure the form board lines up with the string. Adjust the braces to keep the form board straight. Cut stakes long enough so that when they're driven at least 8 in. into the ground (4 in. more in loose, sandy soil), the tops will be slightly below the top of the forms. Cut points on the kickers and drive them into the ground at an angle. Then nail the top of the kickers to the stakes. If your soil is sandy or loose, cut both ends of the kickers square and drive a small stake to hold the lower end of the kicker in place.
Photo 3 shows measuring diagonally to set the second form board perfectly square with the first. Use the 3-4-5 method. Measure and mark a multiple of 3 ft. on one side. (In our case, this is 15 ft.) Then mark a multiple of 4 ft. on the adjacent side (20 ft. for our slab). Remember to measure from the same point where the two sides meet. Finally, adjust the position of the unbraced form board until the diagonal measurement is a multiple of 5 (25 ft. in this case).
Squaring the second form board is easiest if you prop it level on a stack of 2x4s (Photo 3) and slide it back and forth until the diagonal measurement is correct. Then drive a stake behind the end of the form board and nail through the stake into the form. Complete the second side by leveling and bracing the form board.
Set the third form board parallel to the first one. Leave the fourth side off until you've hauled in and tamped the fill.
Tip: Leveling the forms is easier if you leave one end of the form board slightly high when you nail it to the stake. Then adjust the height by tapping the stake on the high end with a maul until the board is perfectly level.
The key to crack-resistant concrete is a firm base that drains well. Unless you have sandy soil, this means adding a layer of gravel. With the forms in place, you can estimate how much fill you need. To calculate the amount of fill needed, stretch a string across the top of the forms and measure down to the ground. Do this in three or four spots and average the results. Subtract the thickness of your slab. Then use this depth to calculate the cubic yards of fill needed. Look for a supplier under “Sand and Gravel” in the yellow pages. Ask what your supplier recommends for fill under slabs. We used crushed concrete, which compacts and drains well.
Spread the fill in layers no more than 3 in. thick and tamp each layer with a rented plate compactor (Photo 4A). Leave a 12-in.-deep by 12-in.-wide trench around the perimeter for the thickened edge. If you're building a heated structure on the slab, cover the ground inside the forms with 6-mil polyethylene sheeting. Otherwise you can leave it uncovered.
Concrete needs reinforcement for added strength and crack resistance. It's well worth the small additional cost and labor to install 1/2-in. rebar (steel reinforcing bar). You'll find rebar at home centers and at suppliers of concrete and masonry products (in 20-ft. lengths).You'll also need a bundle of tie wires and a tie-wire twisting tool to connect the rebar (Photo 5).
Use a metal-cutting blade or disc in a reciprocating saw, circular saw or grinder to cut the rebar. Cut and bend pieces of rebar to form the perimeter reinforcing. Splice the pieces together by overlapping them at least 6 in. and wrapping tie wire around the overlap. Wire the perimeter rebar to rebar stakes for support. Then cut and lay out pieces in a 4-ft.-on-center grid pattern. Wire the intersections together. You'll pull the grid up into the center of the concrete as you pour the slab.
If you've never poured a large slab or if the weather is hot and dry, which makes concrete harden quickly, divide this slab down the middle and fill the halves on different days to reduce the amount of concrete you'll have to finish at one time (Photo 6). Remove the divider before pouring the second half.
Mark the position of the door openings on the forms. Then mark the location of the anchor bolts on the forms. Place marks for anchor bolts 6 in. from each side of doors, 12 in. from corners and 6 ft. apart around the perimeter (Photo 10).
Pouring concrete is fast-paced work. To reduce stress and avoid mistakes, make sure everything is ready before the truck arrives.
Triple-check your forms to make sure they're square, level, straight and well braced. Have at least two contractor-grade wheelbarrows on hand and three or four strong helpers. Plan the route the truck will take. For large slabs, it's best if the truck can back up to the forms. Avoid hot, windy days if possible. This kind of weather accelerates the hardening process—a slab can turn hard before you have time to trowel a nice smooth finish. If the forecast calls for rain, reschedule the concrete delivery to a dry day. Rain will ruin the surface.
To figure the volume of concrete needed, multiply the length by the width by the depth (in feet) to arrive at the number of cubic feet. Don't forget to account for the trenched perimeter. Divide the total by 27 and add 5 percent to calculate the number of yards of concrete you'll need. Our slab required 7 yards. Call the ready mix company at least a day in advance and explain your project. Most dispatchers are quite helpful and can recommend the best mix. For a large slab like ours that may have occasional vehicle traffic, we ordered a 3,500-lb. mix with 5 percent air entrainment. The air entrainment traps microscopic bubbles that help concrete withstand freezing temperatures.
Push a bull float across the screeded concrete, keeping the leading edge slightly elevated. When you reach the far side, lift the handle enough to slightly elevate the edge closest to you and pull back to the starting point. Move over and repeat, overlapping the previously floated surface by about one-third.
Be prepared to hustle when the truck arrives. Start by placing concrete in the forms farthest from the truck. Use wheelbarrows where necessary.
Concrete is too heavy to shovel or push more than a few feet. Place the concrete close to its final spot and roughly level it with a rake. Try to leave it just slightly over the top of the forms. Lift the rebar to position it in the middle of the slab as you go. As soon as the concrete is placed in the forms, start striking it off even with the top of the form boards with a straight, smooth 2x4 screed board (Photo 7). Tip the top of the screed board back slightly as you drag it toward you in a back-and-forth sawing motion.
The trick to easy screeding is to have a helper with a rake moving the concrete in front of the screed board. You want enough concrete to fill all voids, but not so much that it's difficult to pull the board. About 1/2 to 1 in. deep in front of the screed board is about right. It's better to make several passes with the screed board, moving a little concrete each time, than to try to pull a lot of concrete at once.
Start bull-floating the concrete as soon as possible after screeding (Photo 8). The goal is to remove marks left by screeding and fill in low spots to create a flat, level surface. Bull-floating also forces larger aggregate below the surface. Keep the leading edge of the float just slightly above the surface by raising or lowering the float handle. If the float angle is too steep, you'll plow the wet concrete and create low spots. Three or four passes with the bull float is usually sufficient. Too much floating can weaken the surface by drawing up too much water and cement.
After you smooth the slab with the bull float, water will “bleed” out of the concrete and sit on the surface. Wait for the water to disappear and for the slab to harden slightly before you resume finishing. When the slab is firm enough to resist an imprint from your thumb, start hand-floating. On cool days, you may have to wait an hour or two to start floating and troweling. On hot, dry days, you have to hustle.
You can edge the slab before it gets firm since you don't have to kneel on the slab (Photo 9). If the edger sinks in and leaves a track that's more than 1/8 in. deep, wait for the slab to harden slightly before proceeding.
You'll have to wait until the concrete can support your weight to start grooving the slab. Cut 2-ft. squares of 1-1/2-in.-thick foam insulation for use as kneeling boards. The kneeling board distributes your weight, allowing you to get an earlier start.
Grooving creates a weakened spot in the concrete that allows the inevitable shrinkage cracking to take place at the groove rather than at some random spot. Cut grooves about every 10 ft. in large slabs.
When you're done grooving, smooth the concrete (Photo 12) with a magnesium float. Hand floating removes imperfections and pushes pebbles below the surface. Use the float to remove the marks left by edging and smooth out humps and dips left by the bull float. You may have to bear down on the float if the concrete is starting to harden. The goal is to bring a slurry of cement to the surface to aid in troweling.
For a smoother, denser finish, follow the magnesium float with a steel trowel. Troweling is one of the trickier steps in concrete finishing. You'll have to practice to develop a feel for it. For a really smooth finish, repeat the troweling step two or three times, letting the concrete harden a bit between each pass. At first, hold the trowel almost flat, elevating the leading edge just enough to avoid gouging the surface. On each successive pass, lift the leading edge of the trowel a little more. If you want a rougher, nonslip surface, you can skip the steel trowel altogether. Instead, drag a push broom over the surface to create a “broom finish.”
Keep concrete moist after it's poured so it cures slowly and develops maximum strength. The easiest way to ensure proper curing is to spray the finished concrete with curing compound. Curing compound is available at home centers. Follow the instructions on the label. Use a regular garden sprayer to apply the compound. You can lay plastic over the concrete instead, although this can lead to discoloration of the surface.
Let the finished slab harden overnight before you carefully remove the form boards. Pull the duplex nails from the corners and kickers and pry up on the stakes with a shovel to loosen and remove the forms. Since the concrete surface will be soft and easy to chip or scratch, wait for a day or two before building on the slab.
Working Safely with Concrete
Wet concrete on skin can cause everything from mild redness to third-degree, permanently disfiguring chemical burns. You and your helpers should take these steps:
- Wear eye protection. Sandy concrete splashes can wreak havoc with your eyes.
- Wear long pants and long sleeves to protect your skin.
- Wear tall rubber boots if you have to wade in wet concrete.
- Wear gloves (rubber gloves are the safest bet).