How to Pour a Concrete Slab

What Do You Need to Do Before Pouring Concrete?

Site preparation determines whether the slab stays flat. Every step that follows depends on what is done before the concrete truck arrives.

How do you prepare the subgrade?

Strip all organic material, grass and topsoil down to stable mineral soil. Topsoil compresses under load and causes settlement. Excavate to a depth that accommodates both the base course and the slab thickness. For a 4-inch slab over a 4-inch gravel base, that is 8 inches below finished grade. Compact the native soil with a plate compactor to a minimum 95 percent standard Proctor density. Soft or organic subgrades require over-excavation and replacement with compacted fill. Check the surface with a level. Any low spot deeper than 1 inch needs correction before the base goes down.

How thick should the gravel base be?

A gravel base under a residential slab is 4 inches of crushed stone compacted in 2-inch lifts. A garage floor or light commercial slab benefits from 6 inches. The base serves two purposes: it distributes point loads across the subgrade and it improves drainage under the slab, reducing moisture-related heave. Use clean crushed limestone or gravel with a maximum 3/4-inch aggregate. Do not use sand alone as a base. Compact each 2-inch lift before adding the next.

How Do You Set Forms and Place Reinforcement?

Forms define the slab geometry. Reinforcement controls cracking when the concrete does move under thermal cycling or load.

What material and spacing do forms require?

Set 2x4 lumber forms for a 4-inch slab, 2x6 for a 6-inch slab. Stake forms every 2 feet with 18-inch wood or metal stakes driven outside the pour area. Check forms with a 4-foot level and a string line. The top of the form is the finished slab surface, so accuracy here matters. Coat the inside face of the forms with form-release oil so they pull cleanly without breaking the slab edge.

What rebar schedule does a residential slab use?

#3 rebar on 18-inch centres in both directions is typical for a residential patio or driveway. A garage floor carrying vehicle loads steps up to #4 rebar on 12-inch centres. Rebar sits at the middle of the slab depth, which means 2 inches from the bottom in a 4-inch pour. Use concrete chairs or dobies to hold the rebar at that height. Rebar resting on the subgrade provides zero structural benefit. Where wire mesh is used instead of rebar, the same mid-slab positioning rule applies, and the mesh must be lifted into position before, not during, the pour.

How Do You Calculate How Much Concrete to Order?

Concrete is ordered by the cubic yard. Ordering too little stops the pour mid-slab, which creates a cold joint that will crack. Order 5 to 8 percent extra to account for subgrade variation and spillage.

What is the formula for a slab pour?

Multiply length in feet by width in feet by thickness in feet, then divide by 27 to convert cubic feet to cubic yards. A 20-foot by 20-foot slab at 4 inches thick works as follows: 20 x 20 x 0.333 = 133.2 cubic feet, divided by 27 = 4.93 cubic yards. Order 5.3 cubic yards to include the 7 percent buffer. At typical 2026 ready-mix rates of $140 to $175 per cubic yard, that pour costs $740 to $930 in material alone, before delivery, pumping or finishing labour.

What PSI mix does a residential slab need?

A residential patio or walkway uses 3,000 PSI. A driveway or garage floor uses 3,500 PSI. A freeze-thaw climate or a slab exposed to deicing salts calls for 4,000 PSI with air entrainment of 5 to 7 percent. Order the mix with a slump of 4 to 5 inches for hand-placed pours. A slump above 6 inches means excess water in the mix, which reduces finished strength and causes surface dusting.

How Do You Pour and Screed a Concrete Slab?

Start pouring at the far corner of the form and work toward the truck. Do not drag or rake concrete long distances across the form, as aggregate segregation weakens the finished slab.

How do you consolidate and level the pour?

Use a concrete vibrator or a spud rod to consolidate the mix against the forms and around the rebar. Move the vibrator every 18 inches and withdraw it slowly. Screed the surface immediately after placing each section. A screed board spans the two form edges and is pulled in a sawing motion to strike off excess concrete and bring the surface to grade. Screeding a 20-foot span is a two-person job. Low spots that appear after screeding get filled with fresh concrete from the batch and rescreeded, not filled with water or dry mix.

When and how do you finish the surface?

Wait for the bleed water sheen to disappear from the surface before troweling or brooming. On a warm day that takes 30 to 60 minutes. On a cool or humid day it may take 2 to 3 hours. Working the surface while bleed water is still present drives water into the top layer, weakening it to depths of 1/4 to 1/2 inch. Float the surface with a bull float immediately after screeding to close the aggregate. Apply the final broom finish perpendicular to traffic direction so the texture provides traction. A steel trowel finish produces a denser, harder surface suited to garage floors.

When and How Do You Cut Control Joints?

Control joints give the slab a preferred location to crack, so visible cracking follows the joint rather than running randomly across the surface.

Cut control joints with a grooving tool during finishing, or with a wet or dry-cut saw within 12 hours of the pour before the slab develops random cracks on its own. Joint spacing is 2 to 2.5 times the slab thickness in feet. For a 4-inch slab, that is 8 to 10 feet. Joints must be one-quarter of the slab depth, so 1 inch deep on a 4-inch slab. Joints that are too shallow do not control cracking. Isolation joints separate the slab from fixed structures such as foundations and posts to allow independent movement.

How Do You Cure a Concrete Slab Properly?

Curing is the process of retaining moisture in the slab so the cement hydration reaction runs to completion. A slab that dries too fast loses 30 to 40 percent of its potential compressive strength.

What are the curing methods and timelines?

The three workable curing methods are wet curing with burlap kept continuously damp, plastic sheeting sealed at the edges, and liquid curing compound sprayed on immediately after finishing. Wet curing produces the strongest surface but requires daily attention. Curing compound is the most common site method. Cure for a minimum of 7 days in temperatures above 50 degrees Fahrenheit. At temperatures below 50 degrees, hydration slows significantly and the slab needs insulating blankets or heat to maintain the reaction. Do not allow foot traffic for 24 to 48 hours. Keep vehicles off the slab for 7 full days.

Is Pouring a Concrete Slab a DIY Job?

A small patio of 100 to 150 square feet in good ground conditions is within reach for a capable DIYer with one helper, rented forms, a rented plate compactor and a single ready-mix truck. Beyond 200 square feet, the window between pour and finishing closes faster than one person can manage. A standard ready-mix truck delivers 8 to 10 cubic yards, which covers roughly 640 to 800 square feet at 4 inches thick. That volume requires 3 to 4 finishers working without interruption. Structural slabs, slabs over problem subgrades, garage floors and driveways with grade changes belong with a concrete contractor. The cost of a failed DIY pour, including demolition and replacement, typically exceeds the cost of hiring out from the start.

Does Ground Condition in Greater New Orleans Change Any of These Steps?

Greater New Orleans sits on soft alluvial clay and organic silt with a high water table, conditions that do not apply to most of the how-to guides a reader finds online. In this region, a standard 4-inch gravel base is frequently not enough. Slabs in Metairie, Kenner and the east bank of Orleans Parish often require 6 inches of compacted limestone and a 6-mil polyethylene moisture barrier between the base and the slab to address upward moisture migration. Subgrade that appears stable in dry months can lose significant bearing capacity after rainfall. In Jefferson and St. Bernard Parish, slabs attached to structures may require a soil investigation before a thickness recommendation is reliable. These are not upsells. They are the reality of building on deltaic soil, and ignoring them is why so many slabs in this area have settled 2 to 3 inches within a decade of placement.

If a slab in Greater New Orleans is part of a permitted project or sits near a structure, consulting a local engineer on subgrade treatment is worth the cost before the pour, not after.

For a site assessment, subgrade evaluation or a priced scope on a residential or commercial slab in Greater New Orleans, contact New Orleans Concrete Contractors for a direct conversation about what the ground under your project actually requires.