Frequently Asked Questions
What is the difference between concrete and cement?
Concrete is made up of several ingredients: 41% course aggregate (gravel/crushed stone), 26% fine aggregate (sand), 16% water, 11% cement and 6% air. Cement is the key ingredient in concrete, even though only a small amount is used to produce the end product. Cement and concrete go hand in hand like yeast and bread.
Why does concrete crack?
Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16 of an inch in 10 feet (.4 cm in 3 meters). The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage.
What does it mean to "cure" concrete?
Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete's temperature stays within a suitable range. During the curing period - from five to seven days after placement for conventional concrete - the concrete surface needs to be kept moist to permit the hydration process. New concrete can be wet with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture.
Why should I use concrete for my driveway if I can get asphalt more cheaply?
Begin by considering the cost of the driveway over its lifetime. A good quality concrete driveway will last more than 30 years with little or no maintenance. Asphalt driveways need periodic sealing coats to retard-age related cracking. Even properly constructed residential asphalt driveways will deteriorate more quickly due to environmental influences than due to vehicle traffic. If you consider the cost of surface and crack sealers and the shorter life-span of the asphalt, concrete will cost much less.
How thick does a concrete driveway need to be?
Throughout North America it is conventional to form residential driveways with 2"x4" lumber. This results in a pavement thickness of approximately 3.5 inches or 90mm. This thickness is adequate for cars, pick-ups and SUVs. If heavier commercial vehicles will use the driveway more frequently then a greater thickness of concrete is needed.
What kind of joints should be used in concrete paving (such as a driveway)?
The most common joints used in concrete paving are called 'control joints'. These are to accommodate the shrinkage that is normal in concrete. 'Isolation joints' may also be required to separate the paving from posts, drains, or adjacent walls.
Can it be too hot or too cold to place new concrete?
Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. In general, the temperature of new concrete should not be allowed to fall below 10˚ Celsius during the curing period.
How is the strength of concrete measured?
The strength of concrete generally refers to its compressive strength. This is measured by casting cylinders while the concrete is still plastic and crushing them after 28 days. Most concrete has a specified 28-day strength between 20 MPa (3000 psi) and 35 MPa (5000 psi). If cylinders are not made cores can be drilled from the hardened concrete at any age and tested in the same way.
What determines how strong concrete will become?
The most important factor influencing the strength of concrete is the amount of water to cement (W/C ratio) in the mixture. The lower this ratio, the stronger the concrete will be. Therefore the easiest way to increase the strength of concrete is to use more cement in the mixture; the easiest way to weaken concrete is to add more water.
What else is made of concrete?
Structures made with concrete are everywhere, just look around. Some of the endless examples include bridges, condominiums, and commercial buildings. Landmarks like the CN Tower, the Hibernia oil rig, Confederation Bridge to PEI, even major highways such as the new 407 outside of Toronto are made with concrete. Around the home, you will see concrete driveways, patios, floors, roofs and landscaping materials.
