How do you pour a concrete slab?

Typical slab

4 inpatios and sidewalks

Compacted base

4 ingranular fill, typical

Coverage

81 ft² / yd³at 4 in thick

Joint spacing

8–12 ft2–3× thickness in inches

Moist cure

3–7 daysNRMCA CIP 11

Short version

• Most of the work happens before the truck arrives — subgrade, base, forms, and steel.
• Order enough concrete the first time; a cubic yard covers 81 ft² at 4 inches.
• Screed and bull float immediately, then stop and wait for the bleed water to leave.
• Cut control joints at 2 to 3 feet of spacing per inch of thickness, a quarter of the slab depth.
• Keep it damp for 3 to 7 days. Curing is what turns a finished slab into a strong one.

How do you prepare the subgrade and base for a slab?

Subgrade preparation decides whether the slab cracks, and it is the step homeowners skip. Strip the topsoil and any organic material, then cut the excavation deep enough for the slab thickness plus the base. A 4-inch patio over a 4-inch base needs 8 inches of excavation below finished grade.

Compact the exposed soil, then place a granular base — crushed stone or well-graded gravel — in lifts and compact each lift with a plate compactor. Four inches of compacted granular fill is the common residential figure. The base does two jobs: it gives the slab uniform support so it does not span a soft spot, and it drains water away from the underside.

Uniformity matters more than absolute strength. A slab sitting half on undisturbed clay and half on loose backfill will crack along the transition no matter how strong the concrete is. Slope the base to match the finished slope you want, generally about ¼ inch per foot away from any structure.

Check for buried utilities before you dig, and call 811 first.

What forms and reinforcement does a slab need?

Forms set the edges and the elevation, so build them from straight 2× lumber staked every 3 to 4 feet and braced against the pressure of wet concrete. Fasten the stakes so screw heads stay accessible from outside — you will strip these forms the next day. Set the top of the form at finished grade and string a line to check that the slope runs the way you intend.

Reinforcement depends on the job. Welded wire reinforcement or reinforcing bar on chairs, positioned near mid-depth or in the upper third, holds cracks tight after they form; it does not prevent them. Bar size and spacing in a structural slab are engineer-specified under ACI 318, and residential slab requirements are covered by ACI 332. Fiber reinforcement helps with plastic shrinkage cracking but does not replace steel where steel is specified.

Isolation joint material goes anywhere the slab meets a wall, column, or existing slab, so the two can move independently.

General information, not engineering advice. Structural work should be designed by a licensed engineer.

Do you need a vapor retarder under the slab?

Slabs inside a heated building need a vapor retarder; exterior flatwork generally does not. Ground moisture travels up through a slab as vapor and destroys flooring adhesive, cups wood, and leaves dark patches under vinyl. NRMCA’s guidance on vapor retarders under slabs on grade covers when and how to use one.

Specify a minimum 10 mil sheet with a permeance of 0.1 perms or less, tested under ASTM E1745. Lap the seams generously, tape them, and seal around pipe penetrations and at the perimeter. A punctured or poorly lapped sheet is close to no sheet at all.

Place the slab directly on the vapor retarder for interior work. The older practice of putting a sand blotter layer over the sheet is now generally discouraged for slabs that will receive flooring, because that sand traps water and releases it into the slab for years.

Outside, skip it. A patio or driveway wants to drain downward, and a sheet under it just holds water against the underside.

How much concrete do you order and how does it get placed?

Order by volume with a waste allowance. A cubic yard covers 81 ft² at 4 inches, 65 ft² at 5 inches, and 54 ft² at 6 inches, so multiply your area, divide, then add roughly 5 to 10 percent for form deflection, uneven subgrade, and spillage. Running short mid-pour creates a cold joint you cannot undo.

Tell the supplier the strength you need, the slump you want, and whether the mix needs air entrainment. Exterior flatwork exposed to freezing or deicing salts is governed by ACI 332, which requires a 4,000 psi (27.6 MPa) minimum with air entrainment. Typical flatwork slump is 3 to 5 inches.

Plan access before the truck arrives. Know where it will park, how far the chute reaches, and whether you need wheelbarrows, a buggy, or a pump. Have enough people on site — a small residential slab needs three or four, and two of them should be on finishing tools rather than moving material.

Place the concrete in one continuous operation, working from the far corner back toward the truck.

What is the order of operations on pour day?

Screed first, immediately behind placement. Pull a straightedge across the top of the forms in a sawing motion to strike off the surface to grade, then fill any low spots and screed again. Bull float right away, while the surface is still wet, to knock down ridges and embed the coarse aggregate.

Then stop. This is the step that ruins slabs. Bleed water rises to the surface after screeding, and any troweling or floating done while that water is standing works it back into the top layer, creating a weak, dusty, scale-prone surface. Wait until the sheen is gone and the surface will support a footprint about a quarter inch deep.

Edge the perimeter, cut the control joints, then float and trowel. A hand float opens the surface and a steel trowel closes it — the more you trowel, the denser and slicker the finish. Exterior slabs get a broom finish for traction instead of a hard steel trowel finish, which is dangerous when wet.

NRMCA’s finishing guidance covers the sequence and the common defects in detail.

Where do the control joints go and how deep?

Control joints go in at 2 to 3 feet of spacing per inch of slab thickness, which puts a 4-inch slab at 8 to 12 feet and a 6-inch slab at 12 to 18 feet. Cut them a quarter of the slab depth: one inch deep in a 4-inch slab. Anything shallower will not reliably force the crack into the joint.

Aim for panels that are roughly square. Long, narrow panels crack across the middle regardless of spacing, so keep the length-to-width ratio under about 1.5 to 1. Joints should also run through re-entrant corners — the inside corner of an L-shaped slab is where a crack will start if you do not give it somewhere to go.

Timing depends on method. Tooled joints go in during finishing. Saw-cut joints go in as soon as the slab will support the saw without raveling, usually within 4 to 12 hours, and waiting past that means the slab cracks on its own schedule instead of yours. Dry-cutting releases respirable silica regulated by OSHA under 29 CFR 1926.1153, so use a wet saw or dust collection.

How do you cure a new slab?

Start curing as soon as finishing is done, and keep it up for 3 to 7 days minimum per NRMCA’s curing guidance. Curing means keeping water in the slab so hydration can continue, not letting the surface dry. A slab that dries out in the first day gains a fraction of the strength it was designed for at the surface, which is exactly where wear happens.

Methods that work: wet burlap kept continuously damp, curing blankets, plastic sheeting weighted at the edges, ponding on a flat slab, or a liquid membrane-forming curing compound. Do not use plastic sheeting on a slab that will be stained or given a decorative finish — it prints a mottled pattern into the surface.

Cold weather changes the plan. Under ACI 306, protection protocols apply when the air temperature falls below 40 °F (4 °C) for three or more consecutive days, and the concrete has to be kept warm enough to keep hydrating. Hot, dry, windy weather is the opposite risk: cover the slab or fog it to stop plastic shrinkage cracks from opening within hours.

Strip the forms after about 24 hours, then keep curing.

Common mistakes

• Placing concrete on soft or uncompacted subgrade — the slab settles differentially and cracks along the transition, and no amount of steel fixes it.
• Finishing while bleed water is on the surface — working that water into the top produces a weak, dusty layer that scales off within a year or two.
• Cutting control joints too shallow or too late — under a quarter of the slab depth, or past the first several hours, and the slab picks its own crack line.
• Ordering exactly the calculated volume — form deflection and an uneven base eat more concrete than the math predicts, and running short creates a cold joint.
• Walking away after the finish — with no curing, the surface loses most of what the mix design paid for.