Everything You Wanted To Know About Concrete!
What is concrete?
Concrete is a mixture of gray powder called "Portland Cement", sand and gravel or crushed stone. The "Portland Cement" reacts chemically with water and hardens over time - a process called "hydration", (concrete technically doesn't "dry") There are generally one or two chemicals added to the mixture also. The most common are water-reducers that improve strength, and air-entraining mixtures that make the concrete resistant to cycles of freezing and thawing. Air-entrained concrete contains billions of microscopic air bubbles that reduce expansion and contraction-which is critical when installing a concrete driveway in Minnesota.
How strong is concrete?
The compression strength (resistance to downward force) of concrete is determined by a number of factors. The most important is the ratio of water to cement in the mixture - the lower the ratio the stronger the concrete. The easiest way to increase the strength of concrete is to increase the amount of cement used. The strength of concrete is measured in pounds per square inch (PSI) Concrete used in driveways is generally 3000 PSI, this strength is the result of using 5 bags of cement per cubic yard in the mix. If you increase the cement of 6.5 bags, the strength goes to 4000 PSI.
If concrete is so strong, why does it crack?
There are two kinds of concrete: the kind that is cracked and the kind that is going to crack. If you run in to a contractor that tells you his concrete driveways won't crack - run for the hills, because it's just not true! Although concrete has very high compression strength, it has very little tensile strength (resistance to being pulled apart). Because concrete shrinks as it cures at a rate of about 1/16th of an inch for every 10 feet, there is tremendous pulling pressure on the cement slab. Because the whole slab can't move over the ground, it pulls a part in the middle.
Why do you put steel reinforcement in the concrete?
As mentioned, concrete gets pulled apart easily. Steel on the other hand has a very high tensile strength and expands and contracts at rates very similar to concrete. When steel rods are set into the slab in a grid pattern they greatly improve the overall strength of the concrete and reduce its potential to crack.
What other factors can cause the concrete to crack?
In addition to shrinkage, there are four other main causes of cracking.
- Expansion and contraction due to temperature changes
- Concentrated heavy loads
- Poor un-compacted sub-grad conditions
- Rapid water loss prior to finishing the concrete when it is put down
All of these causes can also be minimized with proper care and installation. A minimum of 4 inches of aggregate type base should be put under the slab. The base must be completely compact prior to pouring the concrete because if the ground settles under the slab, the concrete will have to bridge the gap, putting additional stress on it. In cold climates, be sure the concrete is air entrained. Keep heavy vehicles off the slab, especially near the edges, Lastly, be sure your contractor is experienced and will know how to manage the curing process after the concrete is poured.
Why do you cut concrete?
"Control joints" as they are called weaken the concrete slightly at the cut, so that when the concrete cracks it will likely follow the cut and not be noticed. The cuts should be roughly ¼ of the depth of the slab. Hand-trowled control joint is another option.
The difference between cement and concrete
It's very common for homeowners to call their driveway a "cement" driveway. When in actuality the correct term would be concrete driveway. What's the difference between cement and concrete? The simplest way to understand it is to know that cement is actually the substance that binds with sand, gravel or crushed rock to form concrete. Concrete then, is much more then cement and is why it is a great material to be used for driveways.
Cement is made from limestone, calcium, silicon, iron and aluminum ... plus lesser amounts of other ingredients. This mixture is then heated to abut 2,700 degrees to form a product called "clinkers." These "clinkers" are ground to a powder and gypsum is added, which creates the gray powder substance known as cement. Gypsum is a mineral found underground and causes cement to harden when water is added.
Cement is a material that binds together sand, crushed stone, and rock. Cement makes up about 10%-15% of the total mass of concrete. And although concrete is very strong it needs reinforcement because of it's poor tensile strength or ability to stretch. Because of it's high rock content, concrete is extremely durable, attractive and is often used around pool areas, sidewalks, driveways, roads and buildings.
How To Choose The Right Contractor
Learn the most important question to insure you hire the right contractor