How Much Can a Batch of Concrete Vary From the Mix Design?

General Purpose Concrete Mix

• The standard ratio of ingredients for concrete is one part cement, two parts sand and three parts nonsand aggregate. Aggregate is a general term for materials like sand, gravel, crushed brick, perlite, or clay balls. Gravel is the nonsand aggregate used for general-purpose concrete. To these ingredients are added enough water to make the concrete workable but not too stiff.

Water Variance

• The ratio of water to cement is one of the most important considerations in the quality of the final product. Water increases the workability of concrete but reduces its strength. Cement increases its stiffness and strength, but reduces the workability of the concrete in the wet stage. As a general rule, add 6 gallons of water to 1 bag of cement. This can produce concrete able to withstand pressures of up to 4,000 pounds per square inch, or psi. Using 8 gallons per bag of cement will cut the strength down to a maximum of 2,800 psi. When adding in water, you also have to keep in mind that the sand may contain up to 1 gallon of water per cubic foot of sand, if it is very wet sand.

Aggregate Variance

• Aggregates stiffen the concrete mix when it is wet. The greater the amount of aggregate, the stiffer the mix becomes. They also harden the final product. Hard stone as an aggregate makes for a concrete that is very resistant to abrasion. Additionally, aggregates reduce the amount that the concrete shrinks as it dries. The total percentage of aggregate material can vary between 60 and 75 percent of the volume of the mixture. A mix ratio of one part cement, three parts sand and four parts nonsand aggregate forms a very strong and stiff concrete. A mix of one part cement, two parts sand and two and a quarter parts nonsand aggregate forms a weaker, very workable concrete.

Testing

• If you worry that your mix is varying too much from the recipe you have determined that you need, than mix up a small test batch. Use it, let it dry, and watch how it performs. This will allow you to determine if your mixture is still usable.