ABOUT READY MIX CONCRETE
Description of Concrete
Concrete is one of the most used construction materials. A composite material consisting of aggregate (sand and gravel), cement and water, concrete is workable, durable and economical. Concrete can be cast in almost any shape desired and once hardened it can become a structural (load-bearing) element.
"Ready-Mix" is a type of concrete manufactured according to a set formula and then delivered to a site, typically by truck. This practice results in a precise mixture, allowing specialty concrete mixtures to be developed and implemented with consistency.
Concrete's history dates back to the Roman Empire, and while the basics of concrete have largely remained the same, technological advances of today have made the product into one of the most versatile building materials available. More than six billion tons of concrete are made each year, amounting to the equivalent of one ton per person on earth. Concrete powers a $35 billion industry that employees more than two million workers in the United States alone. More than 55,000 miles of freeways and highways in America are made of concrete.
What is the difference between cement and concrete?
Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and portland cement. Concrete gets stronger as it gets older. Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver. Cement comprises from 10 to 15 percent of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. This hardening process continues for years meaning that concrete gets stronger as it gets older.
So, there is no such thing as a cement sidewalk, or a cement mixer; the proper terms are concrete sidewalk and concrete mixer.
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.
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 50 Fahrenheit during the curing period.
GENERAL CONCRETE INFORMATION
SUBGRADE : A uniform subgrade is the key to preventing unwanted settlements. A sand layer is not needed for increased subgrade support, but if it is used as a leveling course, it must be compacted with a vibratory plate compactor.
CONCRETE SPECIFICATIONS : Durable, long lasting and low maintenance concrete begins with selecting the proper mix design. For exterior concrete to resist the affects of snow and ice, freezing and thawing, and deicing chemicals, the National Association of Home Builders recommends the use of an air entrained concrete mix with a minimum design strength of 3500 psi and a maximum slump of 5". Higher slump concrete can be placed with the addition of water-reducing chemical admixtures.
JOINTING : Joints are used to aid construction and control cracking. Best results are obtained when control joints are spaced twice the thickness of the slab in feet; i.e., 8'x8' joint spacing for 4" thick slab. Maximum spacing should be 2 1/2 times the slab thickness in feet, and never exceed 15 feet. Slab panels should be as square as possible. Joints should have a minimum depth of 1/4 the thickness of the slab. Expansion joint material should be used full depth to isolate slabs-on-grade from building and foundation walls, street connections or any fixed object. Sealing joints will extend the life of slabs-on-grade and prevent water runoff from weakening the subgrade.
CURING : The most important and economical step in assuring durable, long lasting concrete is curing. Curing concrete will nearly double its strength. Curing should begin as soon as finishing is completed. Use an accepted curing method. The most common method of curing is applying a spray-on curing compound. Concrete should have at least 30 days of air-drying prior to the onset of harsh winter weather and exposure to deicing chemicals.