How to Pour a Concrete Slab
To pour a concrete slab, excavate about 8 inches, compact a 4-inch gravel base, build level 2x4 or 2x6 forms, add reinforcement on chairs, then pour, screed, float, edge, cut control joints and cure the concrete moist for 7 days. Below is each step in order with the thicknesses, tools and numbers that decide whether the slab lasts, plus a calculator that tells you exactly how much concrete to order.
Key takeaways
- Excavation depth equals slab thickness plus base, so a 4-inch patio over 4 inches of gravel needs about 8 inches dug out.
- Compact a minimum 4-inch gravel base (6 inches in clay soil) using 3/4-inch crushed stone in 2-inch lifts; skipping this is the top cause of slab failure.
- Pour to 4 inches for patios and shed pads, 5 to 6 inches for driveways; use 3,000 to 4,000 PSI mix.
- Cut control joints a quarter of the slab depth deep, spaced 2 to 3 times the thickness in inches (8 to 12 feet for a 4-inch slab).
- You can walk on the slab in 24 to 48 hours, drive on it after 7 days, and it reaches near full strength at 28 days.
Plan the slab thickness and excavate the site
Start by choosing a thickness, because it drives everything else. Pour 4 inches for patios, walkways and shed pads, and 5 to 6 inches for a residential driveway or garage floor carrying vehicles. In a freeze-thaw climate across the northern US and Canada, add an inch.
Mark the footprint with stakes and mason's string, then check that the two diagonals are equal within half an inch so the slab is square. Excavate deep enough for the concrete plus the gravel base: a 4-inch slab over 4 inches of gravel means digging about 8 inches.
Remove all topsoil, grass and roots, since organic material rots and leaves voids that crack the slab later. Slope the subgrade about 1/8 inch per foot away from any building for drainage. To find how deep to go for your dimensions, plug the numbers into the concrete slab calculator.
Build a compacted gravel base
The gravel base is the single most important step, and skipping or skimping on it is the number one cause of slab failure. It spreads the load, drains water away from the underside, and gives the concrete a stable, unmoving platform.
Lay a minimum of 4 inches of 3/4-inch crushed stone, or 6 inches over soft clay soils. Compact it in 2-inch lifts with a plate compactor rather than dumping the full depth at once, because uncompacted gravel settles and takes the slab down with it.
Set level forms and add reinforcement
Use 2x4 lumber for a 4-inch slab and 2x6 for a 6-inch slab, with the top edge of the boards at the finished slab height. Stake the outside of the forms every 2 to 3 feet, screw the boards to the stakes, and check level in every direction, because a form that is off now will set that way for good.
Add reinforcement for any slab larger than about 8 by 8 feet. It does not stop cracks forming, it holds the slab together across them. Use wire mesh for light-duty patios and walkways, or #3 rebar at 18 inches on center for driveways and anything bearing weight.
Set the steel on rebar chairs so it sits in the lower-middle third of the slab, never flat on the gravel where it does nothing. Match your mix to the job too: 3,000 to 4,000 PSI ready-mix covers almost all residential slabs.
Pour, screed and float the concrete
Have your full crew ready before the truck arrives, because once concrete starts placing the clock is running. Work the mix into the forms with a square shovel, filling corners first and building the surface about 2 to 3 inches proud of the forms before you level it.
Resist the urge to add water to make it flow easier. Every extra gallon per cubic yard cuts strength by 200 to 300 PSI and worsens shrinkage cracking.
Screed off the excess by working a straight 2x4 across the tops of the forms in a sawing motion, filling any low spots and re-striking until flat. Then pass a bull float to smooth the surface and press the aggregate down. Once the sheen leaves, float, run an edger along the forms for a rounded edge, and add a broom finish for grip. To order the right volume with waste included, use the slab calculator before delivery day.
Cut control joints to control cracking
Concrete will crack; control joints decide where. Cut them a minimum of a quarter of the slab depth deep, so 1 to 1.5 inches on a 4-inch slab. A third of the depth gives even better crack control, but going deeper than a third can sever reinforcement.
Space joints, in feet, at 2 to 3 times the slab thickness in inches. That is 8 to 12 feet apart for a 4-inch slab and 12 to 18 feet for a 6-inch slab, following ACI 360R guidance. Keep panels close to square and never let one run more than 1.5 times longer than it is wide.
| Slab thickness | Joint spacing | Joint depth |
|---|---|---|
| 4 in | 8 to 12 ft | 1 to 1.5 in |
| 5 in | 10 to 15 ft | 1.25 to 1.7 in |
| 6 in | 12 to 18 ft | 1.5 to 2 in |
Timing matters as much as depth. Soft-cut joints as soon as the surface bears foot pressure without marking, usually 4 to 12 hours after the pour depending on temperature. Missing that window is the top cause of random mid-panel cracks.
Cure the slab and know when it is ready
Curing is a chemical reaction that needs moisture, not just drying time. Keep the surface continuously damp for at least 7 days by covering with plastic sheeting, wet burlap or a misting hose. A slab cured properly for 7 days gains roughly 50% more strength than one left to dry out, per the American Concrete Institute.
Concrete reaches walk-on strength, about 500 to 700 PSI, in 24 to 48 hours at 70 degrees, and slower in cold weather. It hits about 70% of design strength in 7 days, which is when driveways can take regular vehicles, and near full strength at 28 days.
Frequently asked questions
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References
- ACI 360R: Guide to Design of Slabs-on-Ground
- Concrete slab (Wikipedia)
- QUIKRETE: Pouring and Finishing Concrete Slabs
- NRMCA: Ready Mixed Concrete