Concrete is a mixture of Portland cement, water, sand and gravel ("aggregate"), and air. Portland cement comes in packaging of various weights, but is most commonly sold in 94 lb. bags. Aggregate is a mixture of gravel and sand, with the gravel ranging in size from dust to 2 1/2 inches in diameter. Generally speaking, round gravel does better than sharp fragments; and the larger the gravel the more cost-effective the concrete mix will be. However, the aggregate diameter should never be larger in size than 1/4th the thickness of the concrete you're pouring (i.e. if you are pouring a 6 inch slab, your aggregate should be no larger than 1 1/2 inches in diameter).

The strength of concrete increases as:

• The amount of cement in the mixture increases
• The coarser the aggregate is
• The amount of water (relative to the amount of cement) is decreased
• The density of the concrete is increased (by compacting the concrete with a concrete vibrator)

Other important factors that affect the quality of concrete are the mixing and curing of the concrete. The strongest concrete is produced by scrupulously thorough mixing; and durability is greatly increased by curing the concrete properly.

Curing of the concrete is necessary for the mixture to harden properly. The concrete is kept moist for at least seven days; the temperature should not drop below 50 degrees during the "cure" period. Concrete reaches it's full strength after 28 days or so, but doesn't completely cure for years.