Home / Pouring, Finishing and Curing a Driveway Slab

Pouring, Finishing and Curing a Driveway Slab

By concrete contractor carlsbad notes

A day with a deadline

Pour day compresses weeks of preparation into a few hours. Once the concrete leaves the plant, the clock starts, and the crew has to place, level, finish and protect the slab before it stiffens beyond repair. Knowing how that day unfolds helps a homeowner stay out of the way, notice problems and understand why the days afterward matter as much as the pour itself.

Before the truck arrives

The crew arrives ahead of the delivery to run a last check on forms, base, reinforcement and access. Wheelbarrows, rakes, screeds, floats and edgers are laid out. If the driveway is long or the truck cannot reach, a pump or a chute may be arranged. The ground is often dampened lightly, so a dry base does not pull water out of the bottom of the slab.

Homeowners can help by moving cars, clearing the street, turning off sprinklers and keeping pets inside. Tell the neighbors, since a loaded mixer truck is heavy, noisy and has to park for a while.

When the truck arrives, the driver or the foreman may check the ticket and the consistency of the mix. Slump, a measure of how fluid the concrete is, tells whether the batch matches what was ordered. A contractor who asks the driver to add water at the site should be questioned, since extra water weakens the final product.

Placing and screeding

Concrete is placed into the forms starting at the far end and working back toward the truck, so the crew never has to walk through finished work. It is spread with shovels or rakes and pushed into corners. Placement should be steady and without long gaps, since fresh concrete that sits becomes a cold seam.

Screeding follows. A straight board or metal bar is dragged across the top of the forms in a sawing motion, striking off extra concrete and leaving a flat plane at the planned height. Low spots are filled and screeded again. Good screeding sets the tone for the whole finish, since the later steps refine what screeding creates.

Floating and the bleed water wait

After screeding, a bull float, a wide flat tool on a long handle, smooths the surface and pushes the larger stones down. A thin layer of paste rises to the top. Then comes a pause. Fresh concrete releases a film of water, called bleed water, which rises to the surface and slowly evaporates. Finishing while that water is present traps it under the surface, causing scaling, dusting and weak spots later. Experienced finishers wait until the sheen has gone and the surface will hold a thumbprint of a certain depth.

The length of the pause depends on weather. Warm, dry or windy conditions shorten it, while cool, humid mornings stretch it out. That is why start times vary, and why a crew may seem to be doing nothing for a while. The wait is deliberate.

Edging, jointing and texture

When the surface is ready, the crew runs an edging tool along the forms to round the corners, which makes edges less likely to chip. Control joints may be tooled in with a grooving tool, or the crew may plan to saw them later. The surface is then floated again with a hand float, and troweled if a smoother finish is intended. On a driveway, too much troweling creates a slick surface, so most crews finish with a light broom pass pulled across the slab for grip.

The longer project articles on concrete contractor carlsbad notes describe a few finishes and how they were chosen for the use of each slab.

Broom direction usually runs across the slope, so water can drain along the lines rather than being trapped. Heavy brooming leaves deep ridges that hold dirt, while a light pass leaves a fine texture that is easy to sweep.

Why curing matters

Concrete does not harden by drying. It hardens through a chemical reaction between cement and water called hydration, and that reaction needs moisture and reasonable temperature to continue. If the surface dries out too fast, the reaction stalls near the top, leaving a weak, dusty, cracked skin. Curing is the practice of keeping the slab moist and protected so hydration can proceed.

The most intense period is the first several days, though concrete keeps gaining strength for weeks. Most of its design strength is typically reached after about four weeks, which is why contractors talk about a gradual return to full use rather than a single date.

Curing methods

• Curing compound. A liquid sprayed onto the surface that forms a film and holds moisture in. It is quick and common on driveways.
• Wet covers. Burlap or similar fabric kept damp, or plastic sheeting laid over the slab, which keeps the surface from drying.
• Ponding or misting. Light water application over several days, used in some conditions.

Whatever the method, the surface needs protection from rain damage in the first hours, from direct sun and wind and from foot or tire marks. Sealers applied later are not a substitute for curing.

Crew size and pace

A small slab can be handled by two or three people, but a long driveway needs more hands, because the stages overlap. While one worker is still screeding the far end, another is floating the middle and a third is waiting to edge the section nearest the street. A crew that is too thin falls behind, and the last portion of the slab ends up finished too late. Ask how many workers will be on site and whether a second finisher is planned for larger areas.

Protecting the new slab

Keep people and pets off until the surface is firm. Keep vehicles off for the time the contractor advises, commonly a week for cars and longer for heavy trucks, and ask for a clear date rather than guessing. Keep sprinklers from soaking the slab edges and avoid dragging ladders, bikes or hoses across it. Cold nights and extreme heat call for extra care, and the contractor should say what to do in either case.

De-icing salts and harsh chemicals are not a concern in a mild climate, but fertilizer, oil, rust from plant pots and spilled paint can stain young concrete. Wipe up spills right away.

Reading the finished surface

A few days after the pour, walk the slab. Joints should be clean and straight. The surface should be even in texture, with no patches of powder or flaking. Water poured from a bucket should run toward the intended direction without standing. Minor color differences and small trowel marks are normal. Large, shallow depressions that hold water, blisters, scaling or wide cracks are worth discussing with the contractor early, when a fix is easiest.

A successful pour is the result of everything before it: base, forms, reinforcement, mix and timing. What happens afterward, particularly curing, determines how long the surface keeps its strength and appearance.