Concrete Frequently Asked Questions

 

What are the ASTM testing standards for nondestructive evaluation of concrete?

 

ASTM C 597 Pulse Velocity through Hardened Concrete

ASTM C 803/803 M Penetration Resistance of Hardened Concrete

ASTM C 805 Rebound Number of Hardened Concrete

ASTM C 900 Pullout Strength of Hardened Concrete

ASTM C 1040 Density of Unhardened and Hardened Concrete in Place by Nuclear Test Methods

ASTM D 2950 Density of Bituminous Concrete in Place by Nuclear Methods

 

What are the ASTM standard practices for testing and inspection agencies?

 

ASTM C 802 Conducting an Inter-laboratory Test Program to Determine the Precision of Test Methods for Construction Materials

ASTM C 1077 Laboratories Testing Concrete and Concrete Aggregate for Use in Construction and Criteria for Laboratory Evaluation

What is the difference between cement and concrete?

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 cement. Cement comprises from 10 to 15 percent of the concrete mix, by volume. Over time the cement and water harden and bind the aggregates into a rocklike mass. This hardening process is called hydration. The proper term is “concrete” for all cement based materials.

What is portland cement?

Portland cement is not a company name but a generic term referring to the finely powdered cement material.

How is cement made?

Materials that contain appropriate amounts of calcium compounds, silica, alumina and iron oxide are crushed and screened and placed in a rotating cement kiln and heated to about 3000 Fahrenheit. Ingredients used in this process are typically materials such as limestone, marl, shale, iron ore, clay, and fly ash.  The pellets from the kiln are very finely ground to produce portland cement. A small amount of gypsum is added during the grinding process to control the cement's set or rate of hardening.

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. New concrete needs to be kept moist for five to seven days after placement to permit the hydration.

Can it be too hot or too cold to place new concrete?

On hot days, too much water is lost by evaporation for concrete to cure properly. If the concrete is too cold it ceases to gain strength during the curing period.

What is air-entrained concrete?

Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve internal pressure on the concrete and help it resist cracking.

What is the rule of 6’s?

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

What are the most common tests for fresh concrete?

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.

·         Air content measures the total air content in a sample of fresh concrete. 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

What is 3,000 pound concrete?

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.

How do you control the strength of concrete?

The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the ratio of water to cement in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely effected by adding more water.

Is there a universal international specification for portland cement?

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.

Are there different types of portland cement?

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 portland 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.