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Questions about Concrete

Many builders and homeowners have questions about concrete. CEMEX has compiled and answered some of the most frequently asked questions about concrete below.


Basic Concrete FAQs

What is concrete?

Concrete consists of a mix of ingredients, including Portland cement, pozzolans, water, coarse aggregates, fine aggregates, and additives. Concrete may also contain mineral colors, granulated blast-furnace slag, and blended cements. When fresh, cement can be molded hours after it is produced. Once the initial set time is reached, the concrete continues to gain strength.

Why is concrete gray?

First of all, concrete can be ordered in many colors. The natural color of concrete is gray because the color of the cement used to make it is typically gray. Now, the reason the cement is gray has to do with the manufacture of Portland Cement. In cement manufacturing, Iron Ore is used as one of the main constituents, and Iron Ore is black in color, so when it is combined and melted with the other materials it tints the cement gray.

How is the strength of concrete measured?

Scientists use compression and flexural mechanical strength tests to determine the strength of concrete. The strength of concrete depends to a large degree on the water-cement ratio, and the quality of the aggregates and paste in the cement.

Are there quality standards for concrete?

Yes. Concrete must comply with local building codes, ACI, and ASTM.

What is the difference between cement and concrete?

It's simple. Concrete is used for the finished products, such as sidewalks, foundations, and the surface of many roads. Concrete contains sand, gravel and cement. Cement is the special hardening ingredient (the gray powder) that makes concrete harden. Cement is usually made of 60% lime (limestone), 25% silica, 5% alumina, and 10% other materials, such as gypsum and iron oxide. (Content provided by the Mineral Information Institute, © 2002 www.mii.org)

How is concrete measured?

Concrete is measured by the cubic yard - measuring three feet by three feet by three feet, or 27 cubic feet. One cubic yard of normal concrete will weigh about 4,000 pounds. (Content provided by the Mineral Information Institute, © 2002 www.mii.org)

Does concrete gain strength by drying out?

No. Concrete is made by mixing cement, aggregates and water together. When the water comes in contact with the cement a chemical reaction starts to take place. This chemical reaction is called hydration. Hydration is the reaction between the chemicals in water and the chemicals in cement. This reaction forms new compounds and crystals interlocking themselves and the aggregates together. A majority of this reaction takes place over the first month after placing the concrete. Small amounts of additional reaction and strength gains could take place for years as long as moisture is still present to cause more hydration. Actually, when the concrete does finally dry out, it stops gaining strength.

Industrial/Commercial Questions about Concrete

What types of concrete does CEMEX offer?

CEMEX offers a variety of high quality products such as Performance Concrete, Durable Concrete, High Strength Concrete, Pavement Concrete, Light Weight and Heavy Weight Concrete, Grouts, Architectural Concrete, and Customized Concrete.

Is there an average going price per cubic yard of concrete?

Not really; it varies from city to city and region to region. If you are estimating a project, call our local ready-mix concrete company, and they can give you an estimate.

What is fly ash?

Fly ash is a by product of coal combustion. Most commonly the material is produced by coal fired electrical generating facilities. Fly ash is a cementitious material, meaning it has certain properties that cause it to harden upon exposure to water. Typically, fly ash does not develop much compressive strength on its own. However, in the presence of Portland Cement, fly ash can develop strength characteristics very similar to cement. The fly ash reacts with a chemical by-product of the cement hydration reaction called calcium hydroxide -CaOH-. CaOH can cause deleterious effects in concrete such as increased porosity and efflorescence -the formation of calcium carbonate crystals on the concrete or mortar surface-. Because fly ash reacts with the calcium hydroxide to form more calcium silicate hydrate -the binder derivative of cement- fly ash actually adds strength to the concrete and helps to remove an agent that may be harmful. Fly ash is also particularly beneficial for use in hot weather as it tends to slow the generation of heat in the concrete.

Residential Questions About Concrete

What strength concrete is normally used for the slab under a home?

Typical mixes range from 2500 pounds per square inch (psi) to 3000 psi, depending on the geography of the country and the quality of the raw materials. Sometimes we are asked if a different weight should be used for the garage area. This is not really necessary. Usually the biggest load on a slab is the heaving from the sub grade and not the structure on top of it (or the car).

I pulled up the carpet (linoleum, tile) and now I have to get rid of the glue so I can stain my concrete. How do I get rid of the glue?

Unfortunately, this is a question for the flooring people. Determine what type of glue was used, then consult a flooring supplier to find out the "antidote" for that glue (Muriatic acid will not break down the glue). Once you get the glue off, you should be able to stain the concrete as long as the concrete wasn't previously sealed. Check with your local tile contractor for more information.

How do I stain/paint my concrete floor?

Any paint store will have concrete stain or paint. Discuss how you want the floor to look and what function it will perform with the paint store manager, and he/she should be able to recommend the appropriate stain/paint.

My concrete is cracking after only a short period. Is there something wrong with it and can it be repaired?

All concrete cracks. It has to crack because it contracts during the drying, curing, hardening process, and the bond between the cement paste and the aggregates is not strong enough to withstand that stress. The best way to prevent unsightly cracking is to put joints in your concrete at regular intervals. A good formula is to measure the depth of your structure and multiply the number by three. Use this number to determine the approximate number of feet between joints. (For example, a 4 inch slab of concrete should have joints every 10 to 12 feet.) Uneven shifting of the substructure or sub grade can also cause cracking. This is a structural failure, as opposed to improper curing or jointing as mentioned above. Before repairing any concrete cracking, determine the source of the cracking and remedy that first. Epoxy grout is an excellent crack repair agent. For more trouble shooting information, visit www.concretehelpline.com

My concrete has oil/grease stains. How can I get rid of them?

  • Commercial products are available in paint/hardware home centers.
  • Sprinkle with tri-sodium phosphate. Allow to stand for 30 minutes, then scrub with a stiff brush and hot water. Rinse with clean water.
  • Scrub stain with concentrated detergent using a stiff brush. Rinse well with water. Dry and repeat if necessary.
  • Sprinkle dishwasher detergent on wet concrete. Let it stand a few minutes, then pour boiling water on the area. Scrub and rinse.
  • Dissolve a cup tri-sodium phosphate in 1 gallon of hot water. Pour over stained concrete surface and allow soaking for 25 minutes. Scrub with stiff brush or broom. Rinse with clean water. Repeat if necessary.

How long should I cure (keep the plastic, straw, blankets, water, etc.) my concrete (driveway, sidewalk, slab, etc.)?

This question can only be answered by the professionals associated with the project. Concrete cures at different rates depending on the constituent ingredients and the ambient conditions it is exposed to. Your contractor (concrete supplier) should know what sort of curing is required for the particular mix being used. Also, by following their recommendation, you can maintain whatever warranty might be associated with the work performed.

If you have other questions about concrete or any other building material, contact CEMEX.