Placing and Finishing Concrete

Finishing concrete is a race against the clock, and you only get one chance. Once the truck starts to pour, the crew has a limited time to spread, level and finish the slab before it sets. A slab that was finished at the wrong moment can flake, dust or crack for the rest of its life, and the only full repair is a jackhammer.

This guide covers flatwork, meaning slabs, driveways, sidewalks and floors, and the placing basics that apply to footings and walls. Plans and specifications control the thickness, strength, joints and reinforcement for your job.

What You Will Be Able to Do

• Get the base, forms and steel ready before the truck arrives.
• Place and consolidate concrete without separating it.
• Follow the finishing sequence and read when the surface is ready.
• Cut control joints at the right depth, spacing and time.
• Cure concrete properly in hot and cold weather.

Wet Concrete Burns Skin

Warning: Wet cement is highly alkaline, with a pH of about 12 to 13. It can cause deep chemical burns that may not hurt until hours later. Wear waterproof gloves, tall rubber boots, long sleeves and eye protection. Tuck pants into boots so concrete cannot get inside. If concrete touches your skin, wash with clean water right away. If it gets in your eyes, flush them for at least 15 minutes and get medical care.

Before the Truck Arrives

A pour goes well or badly based on what happens first. Walk this list with the foreman.

Base

The ground under a slab, called the subgrade, carries the load. Soft spots always show up later as cracks.

1. Remove topsoil, roots and soft material.
2. Compact the subgrade.
3. Place stone base and compact it in thin lifts. Crews using a light plate compactor often work in lifts of 2 to 3 inches, and heavier compactors can handle thicker lifts where the spec allows.
4. Check grade against the plan. Exterior slabs usually slope about 1/4 inch per foot to drain.
5. Dampen the base just before placing if it is dry, with no standing water. A dry base pulls water out of the mix.

Interior slabs usually go over a vapor retarder, which is a heavy plastic sheet with taped seams.

Forms

Forms must be straight, level and strong enough for the pour. Wet concrete pushes like a heavy liquid, at up to about 150 pounds per square foot for each foot of depth. A 4-foot wall form therefore sees about 600 psf at the base. Slab forms are low, but they still need stakes every few feet, bracing and a form release.

Slabs and Thickness

A typical house slab on grade is 4 inches thick. The International Residential Code allows 3-1/2 inches as a minimum, but 4 inches is the usual practice, and driveways and garage floors are often thicker. Footings under the IRC must be at least 12 inches wide and 6 inches thick, and more where loads or soils call for it.

Steel

Steel only helps if it stays where the drawing puts it. Bars lying on the ground do nothing. Hold them up on chairs, bolsters or dobies (small concrete blocks). These keep the correct cover, which is the concrete between the steel and the outside surface. Cover protects steel from rust and fire.

Laps must be long enough for bars to act as one. Many house plans use about 24 inches for #4 bar, a conservative figure of 48 bar diameters. A common rule of thumb elsewhere is 40 bar diameters. The drawing always governs.

Welded wire reinforcement must also be held up, generally in the upper part of the slab, not left on the base.

Crew and Access

Have enough people, tools and a clear path for the truck. A loaded mixer can weigh 60,000 to 80,000 pounds, so check that the ground, curbs and buried lines can take it. Keep trucks well back from excavations. Use a spotter whenever a truck backs up.

Placing

• Place concrete close to where it will stay. Do not drag it long distances with a vibrator or rake. That separates the stone from the paste.
• Avoid long free drops. Many specifications limit the drop to about 5 feet. Use a chute, a hose or an elephant trunk for deep forms, especially around steel.
• Work from one end to the other. Place each load against the last so there is no cold joint.
• Fill walls in even layers of 12 to 18 inches, moving around the form so that one area is not higher than the rest.

Keep your distance from pump hoses. The end of a pump hose can whip hard if air gets into the line. Do not stand in the path of the hose, and do not kink it.

Consolidating

Consolidation removes trapped air so concrete fully surrounds the steel and fills the form. Without it you get honeycombing, which is a void where stones show at the surface.

Use an internal vibrator in walls, footings and deep sections:

1. Insert it straight down at spacing of about 18 inches, and let it sink under its own weight.
2. Let it reach into the layer below by a few inches.
3. Hold it for about 5 to 15 seconds, until the surface stops bubbling.
4. Pull it out slowly.

Do not use a vibrator to move concrete sideways, and do not hold it against the steel or form. Vibrating too long pushes the stones down and brings too much paste to the top.

Thin slabs are usually consolidated by the screed itself.

The Finishing Sequence

Each step has a right moment. Too early is usually worse than too late.

1. Screed

Screeding strikes off the surface to the right height. Two people pull a straight board or power screed across the form tops in a sawing motion. A third person keeps a small roll of concrete ahead of it to fill low spots.

2. Bull Float

Right after screeding, push a bull float across the surface. Raise the leading edge slightly so it does not dig in. This embeds the stone and smooths out ridges. Use magnesium or aluminum on air-entrained concrete, since wood can tear the surface.

3. Wait for Bleed Water

Now stop and wait. As solids settle, a sheen of water rises to the surface. This is bleed water. Do not work it back in.

Finishing while bleed water is present traps it in the top layer. That layer ends up weak and watery, which leads to scaling (flaking), dusting (powdery surface) and crazing (a fine web of surface cracks).

Wait until the sheen is gone and the slab can take your weight, leaving about a 1/4-inch footprint. Heat, sun and wind speed this up. Cold and humidity slow it down. Never sprinkle water or dry cement on the surface to help finishing.

4. Edge and Joint

Run an edging tool along the forms for a rounded edge that resists chipping. If you are hand tooling joints, do it now with a groover.

5. Final Finish

A steel trowel produces the smoothest and densest surface. Use it for interior floors. Do not steel trowel air-entrained concrete, because that can seal the surface and cause blisters and scaling.

Joints

Concrete shrinks as it dries. If you do not tell it where to crack, it will choose for itself.

• Control joints (contraction joints) create a planned weak line. Cracks form in the joint, in a straight line, instead of wandering across the slab.
• Isolation joints (expansion joints) are full-depth gaps filled with compressible material. Use them where a slab meets a fixed structure such as a foundation wall, column or existing slab.
• Construction joints go where one pour stops and the next begins.

Control joint rules:

• Depth: at least one-quarter of the slab thickness. A 4-inch slab needs a 1-inch joint.
• Spacing: in feet, about 2 to 3 times the slab thickness in inches. A 4-inch slab gets joints every 8 to 12 feet.
• Layout: keep panels close to square, with no long narrow shapes. Start at inside corners.
• Timing: cut joints early, usually within 4 to 12 hours after finishing, and sooner in hot weather. Wait too long and the slab has already cracked on its own.

Sawing concrete makes silica dust. Use a saw with water or a vacuum, and wear the right protection.

Curing

Concrete needs moisture and warmth to keep hydrating. Curing means keeping those conditions in place after finishing. Concrete allowed to dry out early can lose a large part of its potential strength and durability.

Keep concrete moist for at least 7 days at moderate temperatures. Concrete usually reaches about 70 percent of its design strength at 7 days, and reaches design strength at 28 days.

Hot Weather

Above about 90°F, concrete sets faster and loses surface water fast.

• Have everything ready before the truck comes. Wet the subgrade and forms.
• Use evaporation retarder, windbreaks, shade or fog sprays.
• Start curing immediately.

Cold Weather

Below about 50°F, setting slows down. Concrete that freezes before reaching 500 psi is permanently damaged and will not reach design strength.

• Never pour on frozen ground.
• Use warm mix water, Type III cement or a non-chloride accelerator.
• Cover with insulated blankets, and leave them on.
• Vent any fuel heaters to outside the enclosure. Unvented heaters can harm a fresh concrete surface and fill a tent with carbon monoxide.

Troubleshooting

Check Your Understanding

1. A sheen of water is on the slab and the finisher wants to start troweling. What should you say?

Wait. Troweling now traps bleed water and weakens the top layer. Wait for the sheen to go.

2. Where do joints go on a 5-inch slab, and how deep?

Spacing of about 10 to 15 feet, and at least 1-1/4 inches deep.

3. You are placing a slab against a house foundation. What kind of joint do you need?

An isolation joint with compressible filler, full depth. This lets the slab move on its own.

4. It is 30°F at night and the slab was poured this afternoon. What must you do?

Protect it with insulated blankets until it has reached at least 500 psi. If it freezes earlier, it is permanently damaged.

5. Why not lay rebar on the stone base and pull it up during the pour?

It is hard to control, and bars may stay too low. Set it on chairs, bolsters or dobies to hold the cover.

Key Takeaways

• Protect your skin. Wet concrete causes chemical burns.
• Compact the base, support the steel, and check forms before the truck comes.
• Do not finish until bleed water is gone.
• Control joints go at a depth of one-quarter of the slab, within 4 to 12 hours.
• Cure at least 7 days. Concrete hits design strength at 28 days.
• Protect it from freezing until it reaches 500 psi.