All finished concrete may look pretty much the same to the naked eye, but the ratio of ingredients used to make it can differ dramatically between batches. In the United States, manufacturers rate different types or grades of concrete by strength, measured in pounds per square inch. Choosing the right grade of concrete means adequate support for your project, while the wrong grade can lead to cracks, flaking or failure.
Concrete Grade Overview
The grade of concrete is determined by its compressive strength at 28 days after pouring, measured in psi. The higher the psi rating of a given concrete mixture, the greater its strength. Most residential applications require at least 2,500 psi, according to the National Ready Mixed Concrete Association, while commercial work requires 4,000 psi or greater. Concrete rated at 6,000 psi or higher is considered high-performance concrete, and is used for heavy-duty commercial applications. Some specialty projects use 10,000 psi concrete, and two skyscrapers in Seattle, Washington, incorporate high-performance concrete rated at 19,000 psi, according to the Portland Cement Association.
Grade and Applications
Each municipality sets its own requirements for minimum concrete strength or grade, based on the application. Protected basement walls and slabs require at least 2,500 psi in New York State, for example, while exposed slabs and walls require 3,000 psi or more. The New York State Building Code also requires at least 3,000 psi for residential driveways, patios, curbs and walkways.
How Strength Is Determined
Manufacturers use just five basic ingredients to make concrete, but the exact ratios of these ingredients determine the strength of the mixture. These ingredients are sand, gravel, water, cement and additives. Additives may include things like fly ash, which makes cement easier to mix and pump, or chemicals that speed up set time or improve freeze resistance. Even small variations in the amount of these ingredients can significantly affect the strength of the concrete, completely altering its grade and applications.
Contractors consider many other factors when purchasing concrete, along with its basic grade or strength. Depending on the application, buyers need to consider properties such as slump, or consistency, as well as shrinkage, drying time, exposure to salt and other chemicals, and whether the concrete will be subject to freezing and thawing.
Outside of the U.S., concrete manufacturers typically rely on megapascals rather than psi -- strength and grade are measured in Newtons per square millimeter rather than pounds per square inch. A standard 3,000 psi concrete is equivalent to roughly 20 MPa. Some countries also define concrete types by compressive strength, using a standard nomenclature not found in the U.S. In India, for example, concrete grades are rated using the letter M, followed by a number, where the number represents the compressive strength in MPa. Parts of Europe use a similar system, where the letter C is followed by a number representing compressive strength in MPa.