Winnipeg Free Press - PRINT EDITION
There's more to pouring a proper driveway than meets the eye
More than meets the eye, that's what, says Wally Rooke of the Manitoba Ready Mix Concrete Association.
"If you want your driveway to stand the test of time, you'd better know what you're doing. That applies to the builders who pour the driveways of new homes, contractors who come to re-do driveways, or private citizens who choose to pour their own," he says. "The bottom line is whether you do it on your own or get someone to do it for you, learn how to do it right."
Darryl Rempel, sales/service manager for Northland Ready Mix Concrete, agrees, saying that builders in particular are advised to pay strict attention to detail. That's why on this sultry July day, Remple and Rooke are putting on a demonstration for a gathering of supervisors from Qualico Homes.
"It's extremely important that builders follow the proper steps to the letter," he says. "And there are clear steps out there to follow. Problems occur because people have their own ideas about how to do it. While what they do might work, the driveway may look fine, but it might not be up to code. That's why the Manitoba Ready Mix Concrete Association is putting on demonstrations on how to do it properly. We want to make sure that everyone installing driveways follows all the proper steps. We want the whole industry on the same page."
Should a builder or contractor not know what the proper procedure is, heartache for both them and the homeowner will be the probable result, says Rooke.
"Reputations can be damaged," he says. "The durability aspect of driveways is critical. Do a poor job, and the number of call-backs a builder can get can become a real nuisance. We've actually seen people take builders or contractors to small claims court because their driveway started to deteriorate within a year of being installed. That being the case, home builders and contractors should want to ensure top quality."
As for why problems occur -- such as scaling, dusting and uncontrolled shrinkage cracking -- Rooke says they happen due to ignorance, not cutting corners.
"It's my belief that builders and contractors have to be more familiar with the national building code (of the Canadian Standards Association). It requires that concrete be 32 MPa in its consistency. Because they don't know, many builders might order concrete that's 25 MPa. That's why durability problems occur. Order the right grade, and the problems will disappear."
Here is how to pour a proper driveway or garage slab, courtesy of Rooke, a consulting engineer and 50-year industry veteran:
* All vegetation, bricks, large rocks and other loose material need to be removed so the site can be graded.
* The base for a driveway or garage must provide uniform support for the slab -- it should preferably be undisturbed native soil with a thin, granular base course.
* A base of coarse gravel (about 80 mm thick) or crushed stone may be used to give uniform support to the concrete. The material must be fine graded accurately, and all back-fill material must be compacted.
* Forms should be constructed with plastic or wood, and be well-anchored with stakes trimmed flush with the top of the form. Oil all forms to make removal easier.
* Slab thickness should be a minimum of 100 mm (or four inches) where the driveway will be used by passenger vehicles only. If larger vehicles (trucks, RVs) park on the driveway, a thickness of 125 mm (five inches) is recommended.
* Full-depth isolation joints must be placed wherever the slab abuts a wall, stairs, garage floor, pavement or curb.
* Do not tie slab to walls or grade beams with reinforcing bars. Let the slab float, breaking bond around the edges.
* Placing reinforcing steel or fibres into the concrete is recommended but not required, as it doesn't prevent cracking.
Choosing the Right Concrete Mix
* Specify a mix that meets CSA standards A23.1 and A438 for C-2 Exposure freeze thaw with de-icers.
* A good choice is DURAMIX, a special mix that's proven itself in Manitoba's freeze-thaw conditions for years. Ask for it by name; it can be supplied only by a Manitoba Ready Mix Concrete Association Member.
* If not using DURAMIX, choose a paving mix with a minimum strength of 32 MPa, a W/CM ration of 0.45, fly ash option up to 20 per cent cement replacement, slump 100 +/- 30 mm and an air content of five to eight per cent.
* Important: except for floors in heated garages, all pavement concrete must include entrained air.
Placing, Finishing & Jointing
* Do not place concrete unless soil below and air temperature is five degrees Celsius and forecast to rise higher.
* Do not add water to the concrete as delivered to raise the slump above that which is specified. Why? By addling just five litres of water to one cubic metre of either mix, you will increase the slump by about 25 mm. This increases potential shrinkage and cracking by 10 per cent, reduces strength significantly, reduces surface freeze-thaw resistance and makes salt damage more likely.
* If you need more workability in the mix, ask your supplier to adjust the mix properly at the plant.
* Discharge concrete down the chute as close as possible to its resting place; do not move the concrete laterally, and don't use a garden rake to move the concrete, as it may separate the paste from the stones. Do use a square-end shovel or come-along tool to move it.
* Level concrete with a straight edge and vibration. Darby or bull float the surface immediately, but do not tool the surface if any bleed or rainwater is present; just let it evaporate. Fail to wait, and a scaled (flaking, peeling) surface will result.
* Do not sprinkle cement powder to absorb surface water.
* If evaporation rates are high any time of year, use Con Film to minimize risks of plastic shrinkage cracks.
* Do not use a steel trowel or fresno on air-entrained concrete; such tools collapse the protective air content at the surface, which again can lead to scaling problems. Use such tools only on non air-entrained concrete for floors in heated garages.
* All air-entrained concrete must be given a skid resistant texture; use a broom or brush as soon as the texture will remain on the surface. In a garage, give a floated texture when the surface can be accessed with knee boards. Use an edging tool along the forms.
* To control random cracks, locate construction joints no further apart than 2.5 metres in both directions; joints must be made one-quarter the slab thickness, no deeper. One option is to place plastic crack inducers on the base and pour concrete over them -- straight-line cracks will appear above them. More commonly, control joints are either grooved into the surface during finishing or saw-cut within 16 hours. Late saw cuts will result in unsightly, random cracks. So, be sure to be in time on saw cuts!
Curing for Strength & Durability
* Above all, curing of concrete involves time, temperature and protection from moisture loss.
* Without proper curing, the internal chemistry within any concrete mix, especially one with fly ash, will be incomplete without good curing, resulting in loss of strength and durability.
* Time required: a minimum of seven days at 10 degrees Celsius.
* Temperature: if the concrete temperature drops below 10 degrees Celsius within three days, the concrete must be protected with insulating materials such as insulated blankets or straw; a single sheet of poly (thick, clear plastic) is not enough. At completion of the curing period, permit concrete to air-dry several days before subjecting to frost action.
* Protection from moisture loss: Several curing methods are recommended. First, you can spray curing compound on the concrete, coating the surface completely; you can also use continuous water spray, cover it with wet burlap or a wet blanket, cover with plastic sheets or insulated tarp, or pond water on it.
* In Manitoba's climate, the use of sprayed compounds is not recommended after Sept. 15. Use the wet blanket, insulated tarp or poly sheet option since they can be removed to let the surface air dry before freezing. If frost is forecast for the following nights, use insulated tarps or straw.
* To enhance the look of the surface and to make surface cleaning easier, a high-quality sealer (such as Dura Seal) may be applied later. However, such sealers do not replace the need for early curing by one of the above methods.
Caution: to avoid scaling damage, do not apply de-icing salts or fertilizer to concrete the first winter after installation. Also ensure that chemicals applied do not include magnesium chloride salt or sulphate fertilizers.
Follow all these steps, and Rooke says the result will be what everyone wants: a durable driveway.
"It doesn't matter what you do, doing something right is always time well spent," he says. "Whether you're a do-it-yourselfer, builder or contractor, learning how to do it properly will minimize any problems that might crop up down the road."
Qualico Homes' site supervisor George Wright, one of the interested onlookers at the demonstration, concurs.
"A demonstration like this shows you just how important doing the little things right is," he says. "It's important to do it right the first time, because you only get one chance at it."
For additional information on the proper way to pour a driveway, see the Manitoba Ready Mix Concrete Association's website at www.mrmca.com.
Republished from the Winnipeg Free Press print edition August 6, 2006 $sourceSection$sourcePage
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