When you think of concrete, you think of an attractive, durable, versatile, functional and quality product. Concrete is the most widely used construction material in the world. The components of concrete are simple: gravel or limestone, sand, water, cement and certain chemical additives. The science surrounding concrete is complex.
When considering what type of material to use for a driveway, there are many considerations – initial price, life cycle costs, durability, and aesthetics. Concretes biggest competitor is asphalt. When comparing the two materials, you must compare apples to apples, not apples to oranges. A five-inch asphalt driveway is not equal to a five-inch concrete driveway. The Asphalt industry recommends using full depth asphalt for driveways. The equivalent full depth asphalt driveway would consist of two inches of wearing course and four and six tenths inch of asphalt base. Even a comparable asphalt section, not full depth, would consist of three inches of asphalt and eight and one half inches of crushed stone.
A concrete driveway will increase property values in most areas, that is, if it is done properly. Many homeowners do not have sufficient knowledge regarding proper construction practices and therefore, end up with a product that they are not happy with. Constructing a concrete driveway requires planning and dialog with your contractor. Issues that should be addressed are proper subgrade preparation, proper mix, good concrete finishing practices, jointing, curing and sealing the concrete.
Sub grade preparation is critical to a good concrete job. The subgrade should be uniform, smooth, compacted, free of frost and have positive drainage. The subgrade should have an even thickness, to insure that your concrete also has an even thickness. Uniformity is vital to proper sub grade compaction and is more important than subgrade strength. Never place concrete directly on a polyethylene vapor barrier or bone-dry subgrade. The subgrade should be damp, with no standing water. Polyethylene vapor barriers do not allow for proper drainage of the excess water in the mix, forcing all of the water to rise to the top. This can lead to problems such as surface peeling, cracking and possibly curling of the slab.
Concrete is sold based on compressive strength. In cold climates, subject to multiple freeze/thaw cycles, air entrainment should be added to the concrete to combat these cycles and protect the concrete. This entrained air allows any moisture, which does enter the concrete to expand in the microscopic air pockets during a freeze/thaw cycle instead of putting internal pressure on the concrete. The proper mix to order is a 4000 psi concrete with five to six percent air entrainment. The water to cement ratio should not exceed .45. The slump, or stiffness, of the concrete should not exceed five inches. Do not add excessive water at the jobsite as it will weaken the concrete and diminish the overall quality and durability of the concrete.
Concrete should be delivered and placed as quickly as possible. It is a perishable product. The concrete should be placed as near the finished position as possible, either directly off the mixer truck or by wheelbarrows. Do not use garden tools to move concrete or let concrete drop from a chute or bucket more than 3 feet. This may cause aggregate segregation. A square nose shovel or concrete rake should be used to move any concrete if necessary. You should also consider installing steel or fiber reinforcement to provide extra strength and evenness. Without it, there’s nothing to hold cracked slabs together or keep the cracks from getting bigger. Reinforcement also helps the slabs to stay level with each other. If steel is used, it should be installed no deeper than two inches from the concrete surface. Note that reinforcement does not eliminate cracks – it simply holds them together.
Finishing the concrete is a very important step in insuring a quality end product. Many short cuts taken during this stage can have disastrous end results. A concrete driveway can last you at least 30 years if it’s installed, finished and cured properly. First, screed (strike off) the concrete to the proper elevation. Next, use a bull float before bleed water accumulates. Do not bull float the concrete excessively or bleed water may be sealed in. After concrete is bull-floated, it should be left alone until all the bleed water on top of the concrete has evaporated. Starting the finishing operation too soon can trap surface water and create a weak surface. The addition of water to the surface while finishing the surface may cause scaling or crazing of the surface and should be avoided. Also, do not “hot shot” (add of raw cement to the surface). This may also cause scaling. If the weather is excessively hot, windy, or dry, the concrete should be protected by covers. This prevents rapid evaporation before finishing begins and avoids plastic shrinkage cracks. A rough bristle broom should be used to place the final finish on the concrete. This will provide skid resistance and traction when wet. Brooming the surface must await the evaporation of bleed water. Do not steel trowel exterior concrete, such as driveways or sidewalks.
Proper drainage should be addressed before the placement. The concrete should be sloped ¼ inch per running foot away from the structure. If proper drainage is prevented because of the area of the concrete being locked between two structures, installation of a drain will be required, with the concrete sloped toward the drain.
The concrete industry has done a very good job of hiding one of the most important facts about concrete – it cracks. These cracks are a result of the concrete shrinking. Yes, concrete shrinks. Typical concrete will shrink 5/8ths of an inch in 20 feet. The industry has addressed this little known fact by placing joints in concrete, telling it where to crack. Wise concrete finishers install contraction or crack control joints that encourage the concrete to crack at these locations. For the joints to work correctly, they need to be a minimum depth of a quarter of the thickness of the slab. Control joints should be placed at a maximum spacing of two and one half times the slab thickness. (EXAMPLE: A 4″ slab = 10 ft. joint spacing.) Control joints may be made after finishing by hand jointers or they may be sawed as soon as the concrete is hard enough that no raveling occurs. Isolation joints must be placed where the new concrete meets a fixed object such as a footing, wall, sidewalk, or porch. Isolation joint material must be placed before placing concrete. If joints are spaced too far apart, cracks will often occur where the joints should have been. The Joints Sealant should be used to give the concrete a neat appearance, keep dirt, small rocks, and other debris out of the joints, and minimizes water intrusion to the sub grade through the joint.
Once the concrete is finished, it needs to be cured. Curing is a simple step that is often neglected. Some of the water in the concrete is necessary for a chemical reaction called hydration that continues to happen for weeks and months. If this water evaporates too rapidly, the concrete might never achieve its design strength. Uncured concrete may develop only 50% of its ultimate strength. Liquid-spray curing compounds can be applied to the concrete as soon as the final finish is complete. You can also cover concrete with sheets of plastic, but beware; this may cause discoloration of the surface if the plastic is not uniformly spread over the concrete. If the plastic is in direct concrete with the slab, that area will be much darker than the concrete that is not in direct contact. Protect concrete against freezing temperatures for at least three days
Deicing products should not be used the first winter for any new driveway. Salt and other products increase the number of times the concrete will freeze and thaw, putting tremendous stress on concrete that may not have reached full strength. Use sand to help provide the necessary traction. Never use deicers containing fertilizer ingredients such as ammonium sulfate and ammonium nitrate. Concrete should also be protected from exposure to liquid fertilizer systems used on lawns.
Concrete that is specified, mixed, placed, finished and cured in accordance with industry guidelines will last and last. Although asphalt offers a lower initial cost, concrete offers lower long-term costs and valuable qualities such as better riding surface, skid resistance, nighttime visibility, and durability. No doubt about it, a concrete driveway is a good investment.
