Concrete Basics

Concrete is one of the most durable, economical and versatile building materials, used on everything from driveways to lighthouses.

Concrete is composed of a binding medium (usually Portland cement), aggregate and water. Portland cement is a mixture of minerals fired in a kiln, combined with gypsum ground fine. It's sold in 1-cubic-foot bags weighing 94 pounds each. A combination of fine and coarse aggregate makes up 60 percent to 75 percent of a given volume of concrete. Fine aggregate is sand that will pass through a 1/4-inch screen. Coarse aggregate consists of gravel and crushed stone between 1/4 inch and 1-1/2 inch in diameter.

To determine how much concrete is needed to fill a rectangular or square surface, use either of the following formulas:

• [Width in feet x length in feet x thickness in inches] divided by 324 equals cubic yards of concrete needed
  
• [Width in feet x length in feet x thickness in inches] divided by 12 equals cubic feet of concrete needed

When working with concrete, always wear a mask or respirator and gloves for protection. Mix the concrete in a large bucket or wheelbarrow until you get a mass of uniform color without brown or gray streaks. For surfaces that will be exposed to moderate weather and wear, a mix of 1 part cement, 2-1/2 parts sand and 3 parts gravel is recommended. For projects not subject to wear and weather, use 1 part cement, 2-3/4 parts sand and 4 parts gravel. Use a metal trowel to spread the concrete, eliminating all air pockets.