Concrete slabs are multi-purpose surfaces or foundations for homes and gardens. For walkways, patios, and floors, concrete slabs are inexpensive to install and durable enough to last for years. When you pour a concrete slab, you're giving yourself a design material that is adaptive to many of your outdoor design needs while saving money that would have been spent on a contractor.
Working With Ready-Mix Concrete
It's not too difficult to DIY a concrete slab. And, if you mix it yourself, it's the cheapest way to make your slab. For most do-it-yourselfers, the best material for building a concrete slab is a ready-mix, crack-resistant concrete product. The wet mix is poured into a prepared wood form, then left to cure. After the concrete has hardened, the sides of the form are knocked off, and the slab is ready to use. You don't want to do the project alone; recruit at least two assistants.
Ready-mix concrete is a blend of gravel, sand, cement, and other additives. Bagged and available in most home centers, ready-mix concrete contains all the materials to make concrete except for water. Buy a crack-resistant, ready-mix to avoid setting a reinforcing bar (rebar) for strength. Its synthetic fibers eliminate the need for rebar on small-scale concrete slabs.
Working concrete by hand in a wheelbarrow requires strength, organization, and speed. For a 3-foot by 3-foot slab, you will need two assistants. Two people will need to mix the concrete in the wheelbarrow, while a third person should spread the mixed concrete into the form.
When to Pour a Concrete Slab
Wait for dry, warm conditions to pour your concrete slab. For most ready-mixes, the temperature should be 70 degrees Fahrenheit or higher for five days after pouring. You can pour the slab in colder temperatures, down to 50 F, but extend the curing time to seven days.
Never pour concrete when temperatures are expected to dip below 40 F since it may cause the concrete to crack. Also, it's best to refrain from pouring concrete when rain is expected; although it's not a deal-breaker, it might slightly compromise the concrete's integrity.
Concrete Slab Costs
To calculate the materials for your concrete slab, you must determine the length and width of space and thickness of the slab. This table provides an average of the costs that you can expect.
| Average Cost by Square Feet | |
|---|---|
| Size (sq. feet) | Cost |
| 10 by 10 (100) | $400 to $800 |
| 12 by 12 (144) | $576 to $1,152 |
| 10 by 20 (200) | $800 to $1,600 |
| 20 by 20 (400) | $1,600 to $3,200 |
| 20 by 30 (600) | $2,400 to $4,800 |
| 30 by 30 (900) | $3,600 to $7,200 |
| 40 by 40 (1,600) | $6,400 to $12,800 |
| Costs by Slab Thickness | |
|---|---|
| Slab Thickness | Price Per Sq. Foot |
| 2 to 4 inches | $5 |
| 6 inches | $6 |
| 8 inches | $8 |
Safety Considerations
Always use breathing protection when working with dry concrete mix since it irritates breathing passages and lungs. Eighty-pound bags of concrete are very heavy, so have help when moving them to protect your back. Most home centers and hardware stores also carry 60-pound bags of concrete.
What You'll Need
Equipment / Tools
- Hammer
- Level
- Electric miter saw or circular saw
- Concrete hand float
- Mixing hoe
- Wheelbarrow
- Measuring tape
- Shovel
- Tamper
- Eye and breathing protection
- Latex or nitrile gloves
- Speed square or carpenter's square
Materials
- 3 80-pound bags of ready-mix crack-resistant concrete
- 8 50-pound bags of all-purpose gravel
- 3 2 x 4s, 8-ft each
- 4 Wood stakes
- Deck screws
- Twine or mason's line
- 16d galvanized nails, 3 1/2 in
- Vegetable oil or release agent
Instructions
Roughly Outline the Slab Location
In the desired location, use the tape measure and a rope or hose to lay out the general area of the concrete slab. Use the speed square or a carpenter's square to set the perpendicular lines.
Stake the Location
With the hammer, drive the four stakes into the ground at each of the four corners of the intended slab. Run the twine tightly between the stakes to define the slab area.
Remove the Turf
Use a shovel or a turf-cutting tool to remove turf or lawn about 6 inches beyond your marked area. The intent is to provide yourself with extra working room on all sides.
Lay the Sub-Base
Open the bags of all-purpose gravel and pour them into the slab construction site. Use the tamper tool to compact and flatten the gravel.
Tip
After it is first poured out, the gravel sub-base will be about 1 inch too high. However, tamping will bring down the level to its correct height.
Set the Concrete Slab Form
If you're building the slab forms yourself, mark off four sections of the two-by-fours, each measuring 3 feet, 3 inches, and use the circular saw or electric miter saw to cut them. Drive the 16d galvanized nails into the boards to build the form. Place the form on the sub-base.
Cut four scrap pieces of two-by-fours to about 12 inches long and sharpen each one to a point (one end only). Pound each stake into the ground near a corner of the form. With the cordless drill, screw each stake into a side of the form to help stabilize it.
Depending on the price of wood, renting slab forms can be less expensive than building your own, and renting can also save you time.
Coat the Concrete Form
Coat the inside of the form with vegetable oil or a concrete-form release agent, such as Kleen Kote's water-based release agent.
Wet the Sub-Base
With the garden hose, lightly wet down the sub-base.
Mix and Pour the Concrete
Mix the ready-mix concrete according to the manufacturer's specifications with the wheelbarrow, hoe, and hose. When it has a peanut butter-like consistency, pour it into the form. Push the concrete around the form with the hoe and with gloved hands.
Using slightly more water than specified makes the concrete easier to work. Too much water will reduce the cured strength of the concrete. Only add more water if necessary. Add the water in 2-ounce increments; a little goes a long way.
Screed the Wet Concrete
With a scrap 4-foot-long piece of two-by-four, screed the top of the concrete slab. This is done by moving the piece of lumber (the screed) back and forth in a sawing motion to level the concrete and remove excess. Let the excess drop off the side for later removal.
After screeding, tap all around the outside of the form with a hammer. This will remove air and spaces on the edges that would give a "honeycomb" look.
Float the Wet Concrete
Let the concrete settle until the surface water evaporates. Then, using a concrete hand float, smooth and compact the slab's surface.
A metal concrete trowel can be used 10 to 20 minutes after floating for a smoother finish. It can be left smooth or textured by lightly dragging a broom across the surface. An edging trowel can be used for rounding the perimeter edges for a stronger and more finished look.
Let the Concrete Cure
Let the concrete cure for about 48 hours. Keep the slab continually wet by misting it with the garden hose and keep it covered with plastic during this period. Full strength is typically achieved after 28 days, though it can be walked on after two days. It's best to wait 7 to 10 days before putting patio furniture on the slab.
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Tips For Pouring a Concrete Slab
- Be mindful of the concrete hardening time and work quickly.
- Adjacent concrete slabs can be laid—even using the same forms—to build an overall larger slab.
- Letting the concrete dry out too fast is the main cause of cracking, so be sure to keep it moist.
- Clean your tools quickly after use to keep the concrete from ruining them.
FAQ
Can you pour concrete straight on dirt?
You can pour concrete on dirt, but it must first be prepared by compacting the soil. You might need to add a gravel layer if the ground is clay.
How thick does a concrete slab need to be to not crack?
For walkways, patios, garages, and driveways, pour a concrete slab at least 4 inches thick. Make the slab at least 5 to 6 inches to accommodate slabs for trucks or heavier vehicles.
What are the disadvantages of a concrete slab?
Concrete slabs are durable and versatile, but their most significant disadvantage is they can crack if not maintained well. They are prone to more cracks when exposed to harsh weather conditions. You will need to seal and repair it if a crack appears.
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Zeleke ZK, Moen BE, Bråtveit M. Cement Dust Exposure and Acute Lung Function: A Cross Shift Study.BMC Pulm Med. 2010;10:19. doi:10.1186/1471-2466-10-19
