Mixture Design
Concrete is a combination of portland cement, aggregates (crushed stone, sand, shale, etc.) and water. Admixtures, which are ingredients added before or during the mixing of concrete, are also used. Admixtures are used to strengthen concrete, to speed up or slow down the set-up time, and to help protect concrete against the effects of temperature changes and exposure to chemicals such as deicers.
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The performance of the driveway is greatly influenced by the introduction, or entrainment, of microscopic air bubbles into the concrete. Air entrainment helps protect concrete that will be exposed to freezing and thawing and deicers. An air entrainment admixture causes microscopic air bubbles to form throughout the concrete. These tiny bubbles function as relief valves when water in the concrete freezes, helping to prevent surface deterioration. The typical air entrainment for a driveway is in the five to seven percent range. Please note: Excess water added to concrete will destroy the air entrainment bubble structure.
Slump is the term used to describe the consistency, stiffness and workability of fresh concrete. The results of a slump test are stated in inches. It is influenced by the amount of water in fresh concrete. More water means higher slump, but water is not the only influence. Admixtures can be used to increase slump without increasing the water in the concrete. The type of aggregate, the air content, the admixtures, temperature and the proportions of all the ingredients affect slump. The typical slump used for a driveway is five inches plus or minus one.
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A pound per square inch (psi) is the unit of measurement used to describe the compressive strength of concrete. The most desirable strength for driveways varies based on climatic conditions. In broad terms, the colder the climate the higher the desired psi. Areas which experience large numbers of freeze and thaw cycles also require a higher psi concrete mix design. The typical concrete strength used for a driveway is in the 3000 psi to 4000 psi range, although temperate areas of the country may use lower strengths.
Contractors, when ordering ready mixed concrete, should advise the producer of the intended use for the concrete. Quite often suppliers have standard mixes for specific applications. When the contractor specifies a particular strength level, the ready mixed concrete producer is responsible for proportioning and delivering a mixture that will yield the desired strength.
Contractor
Selecting an experienced and qualified contractor is the most important step in assuring a long service life for a concrete driveway. It is not unusual for a concrete driveway to have a useful life of 25 to 30 years, if installed properly. A good source of information on contractors is your local concrete producer. The producer will have had contact with dozens of contractors and will be glad to give you a list of the most qualified. It is best to receive bids from at least three contractors and to compare the bids item by item. Be sure that each bid is based on the same scope of work. Ask the contractor for a reference list of both recently completed jobs and jobs completed in years past. When checking these references, ask if the work was completed on schedule, within budget, was of good quality, and if the customer would recommend the contractor.
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Before concrete is placed, it is important to properly prepare the subgrade. The basic rule for preparing a subgrade is to keep it uniform in firmness, grade, and dampness. If the subgrade is not uniform the concrete will be under more stress in those locations and may develop cracks. Polypropylene fibers can also be used for additional reinforcement. Always slope a driveway so the water runs away from the house. The minimum slope recommended is 1/8 inch drop per foot. Grass slopes adjoining concrete driveways should have a minimum 1/2 inch drop per foot to ensure proper drainage.
The concrete should be placed as close as possible to its final position. Excessive horizontal moving of the concrete, as well as dropping it from more than 4 feet, can cause separation of the mix and result in future problems. The concrete should be leveled off as it is placed. If the driveway will be used by cars only, the concrete slab should be 3 1/2 to 4 inches thick. If it will be used by cars and light trucks, the slab should be 4 1/2 to 5 1/2 inches thick.
Finishing the concrete consists of floating and troweling the surface to the desired smoothness, density and flatness. Good finishing done at the right time gives the concrete a hard, dense surface, making it more durable and impermeable. Poor finishing can seriously impair the concrete surface. The most commonly used surface finish for driveways can also be colored and stamped to present decorative surfaces. Concrete expands and contracts slightly with changes in its moisture content and temperature. It is prone to crack if it is restrained from moving as its volume changes. The cracking can be controlled by proper jointing or use of reinforcement or both. Control joints are placed where stresses accumulate and are likely o cause cracks. The control joints should be spaced 10 to 12 feet apart, making concrete sections as nearly square as possible. The control joints should be one-fourth the depth of the concrete. Isolation joints should be installed between concrete sections that need to move relative to each other, such as where the driveway meets the garage slab.
After the finishing is completed the concrete should be left to cure. To properly cure the concrete cover with polyethylene and keep moist for a minimum of seven days. Curing is the process of allowing the cement to chemically react with water, thereby achieving the desired strength. It should be noted that fresh concrete should not come into contact with bare skin due to the possibility of cement burns. The concrete will reach the majority of its strength during the first few days, but will actually continue to gain strength for up to 30 days. Prior to the first winter, the new driveway should be sealed with a commercially available concrete sealer designed for use on exterior concrete flatwork.
Reprinted in part with permission from the National Ready-Mixed Concrete Association.
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