The Process of Concrete Mix Design

Ingredients in the Mix

• The three basic components of any concrete mix are Portland cement, water and aggregates such as sand and gravel. Portland cement reacts with water in a chemical process called hydration. The cement bonds the mix components together and forms a rock-hard material. Water reacts with the cement and allows the concrete mixture to be worked more easily. Aggregates can range in size, from sand to gravel, or crushed stone. These additions add more strength to the concrete mixture.

Water/Cement Ratio

• The amount of water required for the cement is the W/C ratio. This is the controlling factor for obtaining the concrete strength, durability, shrinkage and permeability. Most mixes require a W/C ratio of 0.5 or less, which means a mix with 100 lbs. of concrete should not have more than 50 lbs. of water. It is important not to add excess water to the mix because it weakens the concrete strength. The lower the W/C ratio, the higher the concrete strength, and the permeability is reduced.

Aggregates

• In designing concrete mixes, sand and stone are the aggregates commonly used. The rule of thumb suggests adding as much aggregate to the mix as possible. By decreasing the amount of Portland cement required, the cost is reduced and the shrinkage factor that causes cracks in concrete is also reduced. Most mixes specify the largest possible coarse aggregate (crushed rock and gravel) to be used.

Admixtures

• Chemical additions to the mix are admixtures that perform various functions in altering the concrete design. Water-reducing admixtures lessen the amount of water required by lowering the W/C ratio and increasing the concrete strength. Accelerating admixtures reduce the setting time of the concrete. Usually concrete requires a 28-day curing period to complete the hydration process and develop its maximum strength. The accelerator can cut this time down and provide early strength to the concrete. Reducing admixtures increase the curing time and acts as a water-reducer in hot weather climates. Fly ash, a byproduct of coal-burning industries, can replace 15 to 30 percent of the cement required in the mix and lowers the cost. Air entraining admixtures cause the formation of microscopic air bubbles that accommodate frozen water in cold climates.