Frequently Asked Questions

Cement is an ingredient used to make concrete. The cement is what reacts to the water in the mix and become the glue of the cement, holding it together and making it strong.

Materials that contain appropriate amounts of calcium compounds, silica, alumina andiron oxide are crushed and screened and placed in a rotating cement kiln.

Ingredients used in this process are typically materials such as limestone, marl, shale, iron ore, clay, and fly ash.

Although it may be tempting to roll up your sleeves, mix up some concrete, and tackle a concrete project yourself to save money, you will likely spend most of your time preparing for the project.

From planning a complete and realistic budget to considering help from professional concrete contractors for certain steps of the project to making sure your project is allowed by the local permitting office, there’s a lot to do before you start mixing up that gray matter!

Pouring and finishing concrete is hard work, and often the convenience of hiring a contractor outweighs the enticement of a challenge for most homeowners.

There are many options for upgrading concrete. Add color directly into the concrete mix.

Stamp the concrete to look like brick or stone. Stamping the concrete is not only pleasing to the eye it also helps disguise any future flaws form being quite as noticeable

Curing is one of the most important steps in concrete construction, because proper curing greatlyincreases 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.

Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes.

Air-entrained concrete is produced through the use of air-entraining portland cement, or by the introduction of air-entraining agents, under careful engineering supervision as the concrete is mixed on the job.

The amount of entrained air is usually between 4 percent and 7 percent of the volume of the concrete, but may be varied as required by special conditions.

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 (.4 cm in 3 meters). 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.

Slump, air content, unit weight and compressive strength tests are the most common tests.

Slump is a measure of consistency, or relative ability of the concrete to flow.

If the concrete can’t flow because the consistency or slump is too low, there are potential problems with proper consolidation. If the concrete won’t stop flowing because the slump is too high, there are potential problems with mortar loss through the formwork, excessive formwork pressures, finishing delays and segregation.

Air content measures the total air content in a sample of fresh concrete, but does not indicate what the final in-place air content will be, because a certain amount of air is lost in transportation, consolidating, placement and finishing.

Three field tests are widely specified: the pressure meter and volumetric method are ASTM standards and the Chace Indicator is an AASHTO procedure.

Unit weight measures the weight of a known volume of fresh concrete.

Compressive strength is tested by pouring cylinders of fresh concrete and measuring the force needed to break the concrete cylinders at proscribed intervals as they harden. According to Building Code Requirements for Reinforced Concrete (ACI 318), as long as no single test is more than 500 psi below the design strength and the average of three consecutive tests equals or exceed the design strength then the concrete is acceptable.

If the strength tests don’t meet these criteria, steps must be taken to raise the average.

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.

It is concrete that is strong enough to carry a compressive stress of 3,000 psi (20.7 MPa) 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.

Stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemical or water.

Common dry methods include sandblasting, flame cleaning and shotblasting, grinding, scabbing, planing and scouring. Steel-wire brushes should be used with care because they can leave metal particles on the surface that later may rust and stain the concrete.

Wet methods involve the application of water or specific chemicals according to the nature of the stain. The chemical treatment either dissolves the staining substance so it can be blotted up from the surface of the concrete or bleaches the staining substance so it will not show.

To remove blood stains, for example, wet the stains with water and cover them with a layer of sodium peroxide powder; let stand for a few minutes, rinse with water and scrub vigorously.

Follow with the application of a 5 percent solution of vinegar to neutralize any remaining sodium peroxide.

Each country has its own standard for portland cement, so there is no universal international standard.

The United States uses the specification prepared by the American Society for Testing and Materials-ASTM C-150 Standard Specification for Portland Cement.

There are a few other countries that also have adopted this as their standard, however, there are countless other specifications.

Unfortunately, they do not use the same criteria for measuring properties and defining physical characteristics so they are virtually “non-translatable.”

Though all portland cement is basically the same, eight types of cement are manufactured to meet different physical and chemical requirements for specific applications:

Type I is a general purpose cement suitable for most uses.

Type II is used for structures in water or soil containing moderate amounts of sulfate, or when heat build-up is a concern.

Type III cement provides high strength at an early state, usually in a week or less.

Type IV moderates heat generated by hydration that is used for massive concrete structures such as dams.

Type V cement resists chemical attack by soil and water high in sulfates.

Types IA, IIA and IIIA are cements used to make air-entrained concrete. They have the same properties as types I, II, and III, except that they have small quantities of air-entrained materials combined with them.