How to Form and Pour a Concrete Slab
A step-by-step walkthrough for forming, reinforcing, placing, and finishing a concrete slab on grade — from subgrade prep to final cure.
Step-by-Step
- 1
Prepare and compact the subgrade
Remove all organic material, topsoil, and soft spots. Proof-roll the subgrade with a loaded truck — any area that deflects more than 1 inch must be overexcavated and replaced with compacted fill. Compact to 95% Proctor.
- 2
Place and compact the gravel base
Install a minimum 4-inch compacted gravel base (6 inches for driveways). Compact in 4-inch lifts. The base should not deflect under foot traffic. Grade the base to within 3/4 inch of finish grade before forming.
- 3
Install vapor barrier
For interior slabs, lay 10-mil poly sheeting directly on the gravel base. Overlap seams by 12 inches and tape them. Run the vapor barrier up the inside of the forms and trim after the pour. A torn barrier is as bad as no barrier.
- 4
Set forms
Set 2×6 lumber forms to the finished slab elevation. Stake every 2 feet and brace corners well. Check that forms are level or at the correct slope (minimum 1/8 inch per foot for drainage). Double-check diagonal measurements for square.
- 5
Place and support rebar
Install rebar or welded wire mesh per the structural plan. Support rebar on plastic chairs to maintain 1.5-inch cover from the bottom. Tie intersections with wire ties. Do not use bricks or rocks as chairs.
- 6
Order and place concrete
Order concrete with the correct mix design (typically 3,000–4,000 psi for slabs). Test slump at the truck before placement. Discharge concrete close to final position — do not drag it long distances with a rake, which segregates the mix. Vibrate around all edges and rebar.
- 7
Screed to grade
Pull a screed board across the forms immediately after placement to strike off excess concrete to the correct elevation. Work in a sawing motion. Follow immediately with a bull float to close the surface and embed aggregate.
- 8
Finish the surface
Wait for bleed water to fully evaporate before any finishing — the surface should have a matte look, not a sheen. Perform initial float pass, then edge and groove. Final trowel passes are done as the slab stiffens — broomed finish for exterior slabs.
- 9
Cut control joints
Saw-cut control joints within 4–12 hours of finishing, at a depth of 1/4 the slab thickness. Space joints no more than 2–3 times the slab thickness in feet. A 4-inch slab gets joints every 8–12 feet.
- 10
Cure the slab
Apply curing compound immediately after finishing, or cover with wet burlap and poly sheeting. Maintain moisture for a minimum of 7 days. In hot or windy weather, increase curing efforts — the surface dries out faster than you think.
Pro Tips
- → Order 10% more concrete than calculated — running short mid-pour creates cold joints.
- → Have the finishers on site before the truck arrives. Concrete won't wait for your crew to show up.
- → On hot days, wet the forms and subbase with water just before the pour to reduce heat absorption.
- → Edge the slab corners with a 3/4-inch edger to prevent chipping.
- → Broom finish perpendicular to the direction of travel for maximum slip resistance.
Watch Out
- ! Never finish over bleed water — it creates a weak, flaky surface layer.
- ! Do not add water to the mix at the site to make it more workable.
- ! Avoid pouring in temperatures below 40°F or above 90°F without cold- or hot-weather precautions.