How to Pour a Perfect Concrete Slab
A concrete driveway makes a beautiful gateway to your home that can last for decades if it’s installed correctly. But if you don’t follow the proper procedures, it could turn into a pile of rubble in half the time. The good news: Installing it the right way doesn’t take extra time or money. We tagged along with a longtime professional mason, who showed us the best ways to prevent water from pooling and stop unsightly cracking, spalling and scaling.
Protect adjoining concrete
Our expert uses duct tape to protect adjacent slabs and sidewalks. Concrete can splash when it runs from the chute onto the ground, so you’ll also want to lay plastic over doors, siding, brick, windows or anything else you want to keep clean. Smearing wet concrete into porous surfaces creates an even bigger mess, so if you do get a couple globs where you don’t want them, wait till they dry and then scrape them off.
Dampen the base to lengthen finish time
To extend your finish time on hot, sunny days, spray bone-dry ground with water to keep the base from sucking the water out of the concrete. A water spray also slows down curing, which makes for a stronger slab. If there’s no hose bib nearby, you can use the water and hose that are onboard the truck. If you don’t have water on site, also use the truck hose to fill a couple buckets of water for cleaning your tools after the truck leaves.
Pour the concrete in small sections
Spread the concrete by moving the chute back and forth and by having the driver pull forward as you go. Once the truck has reached the end of a section, spread the concrete out evenly, and a touch higher than the form, with a concrete placer/rake. Don’t fill the whole form or giant sections because the mound of extra concrete you’ll drag back with the screed board will get too heavy.
This rebar grid is sitting on chairs, a setup that keeps it suspended at the proper height. You won’t be able to use chairs if you have to distribute the concrete throughout the form with a wheelbarrow. If that’s the case, use the hook on the edge of the concrete placer to pull the rebar up into the center of the concrete as you pour. Rebar should be placed near the center of the slab for maximum strength, not near the ground or the surface.
Slide the screed board back and forth as you pull
Pull back the excess concrete with the screed board. As you pull, slide the screed board back and forth to help you prevent voids in the surface. Have a mucker (that’s what they’re really called) pull the excess back and fill in low spots during the screeding process.
Crown side up on the screed board
A slight crown (bow) in the screed board is not only OK; it’s preferred. Just make sure the crown side is facing up. That will create a slight hump down the middle of the slab, so water will drain off. If the crown faces down, you’ll end up creating a trough in the slab where water can pool.
Push the rocks down
Larger aggregate (chunks of gravel) near the surface may cause spalling (chipping). Our expert pushes the larger rocks deeper into the mix. He does this by making small stabbing motions with the float on the first return pass. Start floating the slab immediately after the pour is complete.
Start floating right away
In addition to pushing the aggregate down under the surface, a bull float helps level the slab, so start floating right after you screed, while the concrete is still wet enough to shape.
Whenever possible, run the bull float perpendicular to the direction you pulled the concrete with the screed board (this slab was too long to do that). That will help to smooth out the ridges, troughs and valleys created by screeding. Our expert likes to float in both directions when he can.
Clean your tools as you go
Don’t wait till the end of the day to clean your tools. Clean them immediately after use.
Don’t over-trowel concrete
Actually, exterior concrete surfaces don’t need to be troweled at all. But if you want to use a trowel to knock down the ridges left by the bull float, make as few passes as possible and wait until all the surface “bleed water” is gone. Overworking and troweling wet concrete can trap water just under the surface, making it weak and more prone to spalling and scaling (pitting and peeling), especially on slabs poured in cold climates.
Make long strokes with the edger
The concrete should be firm before you start edging. If the edger is leaving behind large wet grooves, wait awhile before you continue. Longer strokes will result in straighter lines.
Keep a hand float nearby when edging
Run a hand float over the ridges left behind by the edger. A hand float is also a good tool for dragging small amounts of material or moisture (cream) into any voids or dry spots near the edge.
Cut in control joints
A 100-ft. run of concrete can shrink as much as 1/2 in. as it hardens. Shrinking causes cracks. You can’t stop your slab from cracking, but you can control where it cracks. Cut in control joints to create individual sections no larger than 8 x 8 ft. for a 3-1/2-in.-thick slab, and no larger than 10 x 10 ft. for a 5-1/2-in.-thick slab. Cut the control joints at least one-fourth the depth of the concrete. You could use a groover like this one, which works similar to an edger, but many pros prefer to cut the control joints with a diamond blade saw the day after pouring.
Create traction with a broom
A broom finish creates a nonslip surface for wet conditions. The harder the concrete, the less rough the broom will leave the surface. Try to achieve a surface rough enough for traction but not so rough that it hurts to walk on barefoot. If the broom starts to bounce as you pull, lower the angle of the handle. If possible, broom perpendicular to the direction that the slab is most visible. Wavy, crooked broom lines are less noticeable that way.
Spray on a sealer
Concrete will become significantly weaker if the water in the mix evaporates before the chemical curing process is complete. Spray down the slab with water every day for at least a week to slow down the curing process. Another option is to spray on an acrylic cure and seal product. Sealing the surface also protects the concrete from scaling and spalling in cold climates. Apply the sealer right after you broom.
The slab can be walked on and the forms pulled in 24 hours. Wait at least 10 days to drive on it, and avoid spreading any ice melting chemicals for the first two years. Ask your neighbors to keep an eye on their pets before you pour, and use caution tape around the area to warn the pesky neighbor kids.
Concrete doesn’t care if you’re not organized or don’t have time to finish all the edging—it will start setting up right away regardless. Make sure all your tools are ready to go and there are at least three people on hand, one to muck and two to screed. Sometimes the truck driver will help, but don’t count on it.
How to Order
Calculate the volume you need in cubic yards. Multiply the length by the width by the depth, and divide that number by 27 (the number of cubic feet in a cubic yard). Then add 10 percent to allow for spillage and slab depth variations. If you’re not sure which slump (consistency) or psi (strength) to order, ask your concrete supplier to suggest the proper mix for your slab’s intended use. To see how to build a concrete walkway, click here.