Concrete primer: Just the basics

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If you are planning a spring project that involves the use of concrete, then you should know some of the basic principles about how concrete is formed and its basic characteristics, says Dr. George Grandle, a University of Tennessee Extension specialist and emeritus faculty member with the UT Department of Biosystems Engineering and Soil Science.

Grandle say cement and concrete are not the same things. “Concrete is a mixture of Portland cement, water, air and aggregates.  The cement and water form a paste that hardens and glues the aggregates together. Concrete quality is directly related to the binding qualities of this cement paste. The proportions of each ingredient will depend on the intended use and desired strength of the final product.”

Portland cement gets its name from the English Isle of Portland and is so named for its resemblance to a limestone found there. Portland cement is sold in bulk or in bags of one cubic foot (94 pounds). It is made by burning limestone and clay, adding gypsum, and grinding the result into a fine consistent powder.

Grandle says general purpose concrete is usually made with type I Portland cement, but several other formulations have been developed. Types II and IV are used for massive concrete structures because they produce less heat during curing. Type III is useful for slip form and cold weather construction because it develops a high-early-strength. Type V cement is used where resistance to sulfates is important. Air entrainment for increased resistance to weathering can be added to types I, II or III. The aggregates provide volume at low cost and compose two-thirds to three-fourths of the concrete.  Sand is generally used as the fine aggregate and gravel as the course aggregate. The strength and durability of concrete are greatly affected by the ratio of water to cement, Grandle adds.  “We tend to think of concrete as drying, but curing is a more accurate term.  When water is added to the mixture, a process called hydration takes place. Heat is given off during hydration, which is why concrete is sometimes referred to as "hot" or "cold," he said. “Many people add extra water to make their concrete more workable; however, this is not a good practice. The excess water evaporates leaving voids which weaken the concrete.” Grandle says ready-mix concrete is probably the best choice for most jobs. “For small projects you can buy the ingredients pre-mixed dry in bags where all you need add is water. Or you can buy the various ingredients yourself and mix the desired proportion,” he said.

He recommends this formula for a good general purpose mix for floors, walks and driveways: one part cement, 2-1/4 parts sand, three parts gravel and four to six gallons of water per cubic foot of cement used. The gravel for this mixture should be no larger than 1 inch, and the exact amount of water will depend on how dry or wet your sand is. “Be very careful to not add too much water,” he cautions.

“Concrete is strong in compression, but even the best concrete is weak in tension. In other words, if we try to "bend" concrete, it will break. Steel bars or wire are often embedded to increase the tensile or bending strength of concrete,” Grandle explains. Concrete is one of our most useful building materials. It can be formed into various shapes, and finished in many textures from smooth to rough. When properly formulated, mixed, finished and cured, concrete is very strong and durable.

For additional information about cement and concrete, Grandle recommends the "Frequently Asked Questions" section of the Portland Cement Association's website:

For more tips about spring building projects, visit the UT Extension publications website  or the Cannon County Extension Office.

Bruce Steelman

614 Lehman St.

Woodbury, Tn 37190

615-563-2554 O

615-542-1364 C

CannonCounty - UT Extension

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