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Concrete Basics

Concrete and Cement are not the same thing. Cement is an ingredient in concrete. Cement, or more correctly Portland cement was developed by Bricklayer Joseph Aspdin of Leeds, England in the 19th century. Although other cement-like mixes have been used for thousands of years. Portland cement is the key ingredient to concrete, and is key to the chemical reaction that results in concrete's hardness and strength.

Portland cement is essentially a combination of calcium, silicon, aluminum and iron. These ingredients typically are derived from a mixture of lime and sand and then blended with gypsum. The ingredients are heated and ground into a fine gray powder; the final product being Portland cement.

Concrete is the product used to build footings, foundations, walls and more. Portland cement is mixed with sand and aggregate (small rocks). When water is added to the mix, a chemical reaction occurs. The cement forms a paste and binds with the sand and aggregate forming a solid. As the chemical reaction progresses the concrete stiffens and ultimately becomes very hard. A common misconception is that the concrete "sets" as the water evaporates. The concrete sets as a direct result of the chemical reaction. Some of the water evaporates while the rest of the water becomes chemically bound in the process.

Concrete Facts:

  • A maximum of 1 gallon of water should be mixed with 60 pounds of concrete mix. More importantly, the mixture should be thick and pasty, but able to flow to fill the form.

  • One cubic foot of cured concrete weighs roughly between 95 and 150 pounds (a cubic yard is about 2,565 - 4,050 lbs).

  • Standard concrete mix cures over a period of months or years although is usable after 5-7 days and substantially cured after a period of 28 days.

  • Concrete should be wetted during the curing process to facilitate the hydration process.

  • Concrete that will be subjected to freezing and thawing should have air mixed into it. The air bubbles give the concrete room to expand without cracking.

  • Different mixtures of cement, sand, aggregate and additives are called for depending upon the concrete's use. There is fast-setting, delayed setting, reinforced, high-strength, fence post and several other mixes for specialized applications.

  • A standard mix of one cubic yard of concrete calls for a minimum of 6 bags of portland cement, a maximum of 6 gallons of water per bag, 6% air (for concrete subjected to freezing & thawing) and 6 days of cure time. This is commonly referred to the rule of 6's.

  • Typical mixes of concrete contain about 10 to 15% cement, 60 to 75% aggregates and 15 to 20% water.

  • A basic mix contains 1 part cement, 2 parts sharp sand and 3 parts aggregate rock.

  • Concrete should be poured only when the temperature will remain above 50 degrees (F). If it gets too cold, the chemical reaction process will stall and the concrete will not cure properly.

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