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Setting a firm foundation

Pouring a concrete slab might be easy, but proper preparation is essential

By Burl Burlingame

POSTED:
LAST UPDATED: 12:49 a.m. HST, Jul 01, 2010


Pouring a basic concrete slab isn't quite as simple as making a cake, but close. "It's getting easier and easier, and people are getting smarter and smarter about how to do it," said Tom Stoddard, of Swan Builders.

We asked concrete-installation experts like Stoddard for some tips on making a simple concrete slab. Absolutely the first thing to consider is whether you can actually do it yourself or whether you need to hire helpers. Their answers agreed on one point: If the pour requires more than 1 cubic yard of concrete, you need help. That's a lot of concrete.

How much? If the pour is 6 inches thick, that's 54 square feet. A pour that's 4 inches thick: 81 square feet. And that doesn't include concrete for the "footer" around the edge that most slabs require, so subtract a dozen or more square feet.

"A slab should be at least 4 inches deep in the middle, with a deeper footing around the edges. A building foundation needs to have at least a 12-inch footing to meet building codes," said Daniel Sawicki, of Sawicki Concrete.

Can't I just whip up some bags of Quikrete?

CALCULATED POURING

Nothing is so worrisome as figuring out exactly how much concrete is needed, particularly when you're paying by the cubic yard. It's also better to overestimate than to underguess.

The pros do it this way: Multiply the length by the width by the depth (in feet), then multiply by 1.5 to add in the excess for footers and wastage, then divide by 27 to get the figure in cubic yardage, the way you'll order the concrete.

For example, to pour a slab that's 8 by 8 feet by 4 inches with a footer:

» Multiply 8 by 8 by .33 (4 inches) to get 21.2.

» Multiply by 1.5 to get 31.68.

» Divide by 27 to get 1.173 cubic yards.

FINISHING THOUGHTS

Although concrete can be decoratively polished and stained, that's not usually the case with outdoor applications, and you also want some tooth to the finish so it's nonskid when wet. There are three main types of textured concrete finishes.

» Float and trowel finishes: Patterns swirled in the damp, settling concrete with trowels and screed booms or "floats." Metal floats make smooth patterns; wood floats are a bit rougher.

» Brooming: The surface is scratched with a broom that imparts a texture just as the concrete sets. It's the best nonslip surface, and also provides a mastic grip if tile is applied later.

» Rock salt: Chunky bits of rock salt are scattered over the concrete and pressed down into the surface. After the concrete sets, water dissolves the salt, leaving irregular holes.

"Anything over a (cubic) yard, get a professional mix. An 80-pound bag of Quikrete is about two-thirds of a cubic foot. So a yard takes more than 40 bags of Quikrete," Stoddard said.

So planning matters. In Hawaii, where the ground rarely freezes, what we're mostly dealing with is a "slab-on-grade foundation," which is poured all at one time into a set of forms, and the edges have footers. The slabs almost always include rebar reinforcing rods and a wire mesh to prevent cracking, and the whole concoction rests atop a tamped-down subfoundation of crushed gravel that aids in drainage.

A lanai or driveway pour doesn't need as substantial a footing as a building foundation pour. This means the forms for the mold can be made of 2-by-4s or 2-by-6s that are braced by stakes.

"Making a simple outdoor slab isn't hard if you have some carpentry skills to make the forms and it isn't too big or complicated. You and your buddies can do it in a weekend," Sawicki said.

PREPARE YOUR SPACE

Make the foundation as square and level as possible. This requires a wooden form to be built to mold the liquid concrete. Some forms are built above ground; most are set into the ground. (The term "grade" essentially means ground level.)

"Look for plumbing and electrical conduits that are underground before you place it, at least just to identify them," Sawicki said.

Use temporary stakes driven in at the corners of your area to mark levels and angles, and clamp the form boards to them.

In a rectangular form, if the opposite corners are exactly the same distance apart, then the form makes true right angles at the corners. Use a bubble level to make sure the tops of the forms are level, then make minute adjustments for drainage. Make sure it drains away from the house.

Once everything is adjusted, hammer in permanent stakes and screw or nail the boards to them. Depending on the size of the forms and the denseness of the soil holding the stakes, hammer in enough so that the form doesn't shift. Think of concrete as liquid rock -- it's heavy, and the mass puts out a lot of sideways pressure.

The stakes should be sawed off at the top of the forms so the concrete can be leveled after it's poured.

Evacuate soil from inside the form. If the slab is to be 4 inches thick, dig out an extra couple of inches for a layer of crushed gravel. They call this area the subgrade, and it needs to be dampened and tamped to make it as dense as possible -- it's supporting all the weight of the slab.

"Make sure there's nothing organic under the pour, plants or anything that will rot away and cause the foundation to slump," said Stoddard.

The best material to support the slab is a couple of inches of crushed gravel.

"It's always good to have a gravel bed subfoundation, one that's clean and tamped down hard below the grade. It's a foundation for the foundation," Sawicki said. It also allows water to pass under the foundation.

"You can rent a tamping jack or jumping jack machine that will tamp down your subgrade," said Stoddard. "Walking on it with your feet is nothing. It needs to go down an inch or so."

According to Sawicki, "soil that has a lot of sand in it usually packs pretty hard."

A FORTIFIED BASE

Although small projects such as pathways, steppingstones and small shed floors generally don't require reinforcing, adding rebar and wire mesh inside the concrete makes it substantially stronger. (Sidewalks are poured without reinforcement so that they can be broken up easily.)

Even if the slab isn't required by building code to include metal reinforcement, throwing in some steel rod around the edge never hurts.

"It's a lot stronger if you put rebar in, which locks in the concrete," Sawicki said.

"Make sure the foundation will hold the weight, and use steel rebar and netting, and if it's attached to the house, use (steel) dowels. The more steel you use, the stronger it stays," said Stoddard.

If your job is big enough to merit reinforcement, it also never hurts to document their use with plans and photographs for building inspectors. If the slab has a conduit inserted for electricity, a large-gauge copper grounding wire needs to be clamped to the rebar.

The steel mesh is essentially fencing material; rebar is mild steel rods with a knobby finish to grip the concrete. It needs to be suspended inside the concrete. Usually it's placed atop clean rocks or rubble. Rebar is generally placed in the footer, and steel mesh over the entire pour.

"Steel rebar and the mesh need to be at least two inches from the edge. If moisture gets to the steel, it will rust and cause spalling or cracking," said Stoddard.

Whew. All that prep, and we haven't even gotten to the concrete yet.

PICK YOUR CONCRETE

Concrete is a cake mix of various ingredients: water, Portland cement, sand and crushed rock. The sand and rock together are called aggregate and make up the bulk of the mix. The end use determines the proportions.

For small jobs, buy the ingredients separately and mix in a wheelbarrow; for larger jobs, you can rent a portable mixer to tow home. Add water, then cement and aggregate, keeping it going until the soup is a uniform shiny gray, about the consistency of runny pudding.

But again: "If you're going to pour a cubic yard of concrete or more, it's better to have professionals mix and dump it for you. If you mix smaller amounts and add together, the drying times are different for each batch," Sawicki said.

"It can actually be cheaper -- and a lot easier -- to have Ameron come in and deliver a batch of mixed concrete," Stoddard said.

Ameron, housed on Sand Island, is the big dog among Hawaii concrete companies. (Once you estimate the amount of concrete you'll need, you can solicit competitive bids.)

"Our mixer trucks can deliver anything from one yard to 10 yards. There are literally hundreds of different kinds of mixes, but the standard for building foundations and lanai pours is usually 3,000-psi strength, with three-quarter-inch rock mix," said Ameron dispatcher Noa Castro.

"The average cost is about $170 per cubic yard, plus a delivery fee that varies," mostly depending on how far away you are from the Sand Island facility.

"The truck has a chute that can pour the concrete directly into the form, or if it's around back of the house, it can be pumped into place. The homeowner or contractor arranges for the pump from a third party," Castro said.

Dampen everything with a garden hose. Using a bucket, wheelbarrow, rented mixer or barrel-truck chute, the mixed concrete is plooped into the forms. (There is likely a more professional verb than "plooped," but it sounds about right.) Start at the far edge, moosh it down to prevent air bubbles around the forms; don't waste any time at this stage. Get it in there, and many hands makes the work lighter. Tapping the forms with a hammer can also break off air bubbles.

A board wider than the forms, called a screed, is sawed back and forth atop the forms to level it off and push off excess concrete. When it's smooth and level, make another pass with a metal float for the final finish. If this is a building foundation, insert J-shaped bolts around the edge of the framing.

As the concrete sets, at some point shimmering water will appear on the surface. This is called the "bleed," and is a sign the concrete mix is kicking.

How much work time do you have?

"It usually takes three to four hours for the concrete to kick, to start getting hard, but we can put in admixtures that can slow it down or do other things," said Castro. The reason cement truck hoppers are always rotating is because moving cement doesn't set as quickly.

"If you're just dumping it yourself using one wheelbarrow, the mixer can use retarder so you have six to 10 hours before it gets hard. Gives you a little relief before it goes off," Stoddard said.

The bleed will vanish just as mysteriously as it appeared, and this is when you apply control joints -- grooves in the surface so that if the slab does crack, it does so where you want it to -- and surface texturing.

Concrete needs to cure at least 24 hours or more before removing the forms, and keeping the slab moist for a couple of weeks increases the strength of the concrete. Laying plastic sheeting over it helps trap moisture.

It's at this exact moment the wife starts bugging you about new lanai furniture. Count on it.

SOME TIPS SET IN CONCRETE

» Keep it on the level. If the slab is a foundation for a building, even small leveling variations can cause wall construction to need shimming or trimming during framing.

» If the slab will be covered eventually with vinyl flooring, make sure the concrete mix isn't too permeable, allowing moisture to collect beneath the flooring.

» Control joints allow the concrete to crack exactly where you want it to. If your slab is on landfill or just above the water table, the slab's own weight might cause it to crack.

» Wire mesh and rebar ties need to be supported above the subfoundation so that the metal is completely encased in concrete. This not only strengthens the slab, it prevents the rebar from rusting.

» Make sure the subgrade is tamped down, dampened and compacted so there are no collapses beneath the slab, leaving concrete with little support.

» Concrete takes a while to cure properly, and keeping it moist for at least a week is not only a good idea, it makes the slab substantially stronger and resistant to cracking.

» If the ground around your home is always wet, consider a waterproof polyethylene membrane placed atop the subfoundation gravel as a vapor barrier.

» Check first to see whether you need a building permit, and also what the building codes in your neighborhood demand. Generally, a simple lanai or walkway pour does not need a permit; a slab that will have a building placed upon it does, and also needs to have deeper and stronger footings.

» Hot sunny days make concrete set up quickly. Time your pours so they take place during early morning or in late afternoon.

» Wear protective gear and keep raw concrete away from bare skin. It burns.

CORRECTION: In a previous version of this story an informational graphic on how to pour a concrete slab included a cutaway view that indicated steel rebar should be placed below a layer of crushed gravel. The rebar is actually supported above the gravel.






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