Concrete FAQ's

What is concrete?

Concrete is a building material made from cement, aggregates (rock and sand), water and admixtures (chemical that enhances or modifies the properties of concrete). Concrete is one if the oldest, most widely used and versatile building products known to man. 

What is cement?

Cement, or Portland cement, is a dry powder made from limestone and other materials. It is burned in a kiln and then finely ground.  Cement reacts chemically with water to cause concrete to harden. There are several different types of Portland cement, designated as Type I, Type II, etc.

 

What is ready mix?

Ready mix, also known as ready mixed concrete, is concrete that is delivered in trucks that agitate and/or mix the concrete on the way to the job or at the job site.  The concrete is delivered in a plastic or unhardened state.

What is the difference between cement and concrete?

Cement is an ingredient in concrete.  It is the ingredient that forms a paste with the water, sand, and admixtures and fills the space between the coarse aggregates (rock) and binds the rocks together. The term "cement" is commonly misused to refer to concrete.

What is fly ash?

Flyash is a byproduct of coal­burning power plants. When used properly in concrete, flyash is an affordable mineral admixture that improves the quality of the mix. Concrete with flyash will typically have a higher ultimate strength, although early strength may be lower than with straight cement mixes. Sometimes users will complain about "too much flyash in the mix."   That is rarely the problem.  More often the problem is not enough cement in the mix.

What are recommended mix proportions for good concrete?

Good concrete can be obtained by using a wide variety of mix proportions if proper mix design procedures are used. A good general rule to use is the rule of 6's:

  • A minimum cement content of six bags per cubic yard of concrete,

  • A maximum water content of 6 gallons per bag of cement,

  • A curing period (keeping concrete moist) a minimum of six days, and

  • An air content of 6 percent (if concrete will be subject to freezing and thawing). 

What is 3,000 pound concrete?

  •  It is concrete that is strong enough to carry a compressive stress of 3,000 psi at 28 days. Concrete may be specified at other strengths as well. Conventional concrete has strengths of 7,000 psi or less; concrete with strengths between 7,000 and 14,500 psi is considered high-strength concrete. 

Why do concrete surfaces flake and spall?

Concrete surfaces can flake or spall for one or more of the following reasons:

  • In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface

  • The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface.

  • The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable. 

Why does concrete crack?

Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16 of an inch in 10 feet. The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage.