Concrete is the mixture that results from blending Portland cement, water, sand, stone, and admixtures.

The easiest way to add strength is to add cement. The factor that mostly 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. Every desirable physical property that you can measure will be adversely affected by adding more water.

It is concrete that is strong enough to carry a compressive stress of 3,000 PSI for 28 days.

Strength is usually determined by compression testing and is expressed in pounds per Square Inch (PSI). Here are some examples of common uses for different strength concretes. Note: These are not exclusive. Designers and structural engineers may specify different strengths for specialized construction projects.

• 2500 PSI can be used for fence posts and pole barn post holes.
• 3000 PSI is a standard multipurpose mixture for general use in construction. Commonly used for private sidewalks, patios, footings, and foundations.
• 3500 PSI can be used for footings, foundations, garages, driveways, hot tub pads.
• 4000 PSI is used for heavy-use floor slabs such as shops, warehouses, or RV slabs.
• 5000 PSI and higher concrete mixes are normally used for specialized construction projects where high-impact resistances, very low wear rates, or extreme conditions are expected.

Ready-mixed concrete is sold by volume, measured in cubic yards. A cubic yard is 3 feet long x 3 feet wide x 3 feet high.

One cubic yard will cover 162 square feet if spread 2 inches thick. One cubic yard will cover 108 square feet if spread 3 inches thick. One cubic yard will cover 81 square feet if spread 4 inches thick.

Rule 324

(Length in Feet X Width in Feet x Height in Inches) Divided by 324 = Cubic Yards of concrete

Example 8' x 12' x 4" = 384 Divided by 324 = 1.185 Cubic Yards

Or

Rule 27

(Length in Feet x Width in Feet x Height in Feet) Divided by 27 = Cubic Yards of Concrete

Example 8' x 12' x 0.333333' = 31.999 Divide by 27 = 1.185 Cubic Yards

Up to 10 yards.

14 feet

Slump is a measure of the consistency of fresh concrete.

Slump is measured using a right circular cone that is 12 inches high. The base of the cone is 8 inches in diameter and the top of the cone is 4 inches in diameter. The cone is filled with fresh concrete in three layers of equal volume. Each layer is stroked 25 times with a rod that is ¾ inch in diameter. The end of the rod is bullet-shaped. After the cone has been filled and the concrete has been cut off level with the top of the cone, the cone is raised vertically, allowing the concrete to fall or slump. The distance that the concrete falls or slumps from the original height is the slump of the concrete. Slump is measured in inches and is reported to the nearest ¼ inch.

2’x2’x3’ or 2’x2’6’

2’x2’x3’ = 1750 lbs.

2’x2’x6’ = 3500 lbs.