Concrete Mix Design — Concrete | License Contractor School

The four ingredients

Concrete is cement, water, fine aggregate (sand), and coarse aggregate (gravel). Cement reacts with water to bind the aggregate into a solid mass. Aggregate makes up about 70 percent of the volume and is what gives concrete its bulk and stability.

Water-cement ratio

The single most important factor in concrete strength is the water-cement ratio, often written as w/c. Lower w/c gives higher strength and durability. Typical structural concrete uses a w/c of about 0.45. Adding water on site to make the mix easier to place is the most common cause of weak concrete.

Strength and slump

Strength is rated in pounds per square inch at 28 days. Residential slabs are typically 2,500 to 3,000 psi; structural concrete is often 3,500 to 5,000 psi. Slump is a measure of consistency; 4 inches of slump is typical for slab work. Order by both strength and slump.