Concrete is a mixture of cement, aggregates, and water. The aggregate used in concrete is an inert material that accounts for 60 to 75% of the volume and 70 to 85% of the weight. It is imperative to use the right type of aggregate as it influences the concrete mix in the strength, durability, workability, hardening, and ultimately the cost.
Aggregates can be made from sand, gravel or crushed stones. Even recycled concrete is crushed and reused as an aggregate for sub-base layers in projects. Fine aggregate is generally defined as a mix of natural sand or crushed stones with a majority of particles smaller than 3/16th of an inch or 5mm (although some define it as less than 4.75mm). Coarse aggregate is generally crushed stones or gravel particles that are larger than 3/16th of an inch or 5mm (or as some define it as larger than 4.75mm.).
The aggregate used in a concrete mix comes from natural sand or gravel pits, rivers, seas or lake beds, hard-rock quarries, or underground mines. The type of materials used as an aggregate influence the concrete more when it is wet than once it has cured. When the gravel is really rough or irregularly shaped, it requires more water in the mix as well as more cement to give it the proper consistency.
To make good aggregate, the material is prepared by crushing, washing, and then grading it. The grading of an aggregate is important as it determines the paste requirements for the concrete. It is an important step in the process because the paste requirements regulate how much cement is needed, which is the most costly part of the concrete.
According to the Portland Cement Association, “Although some variation in aggregate properties is expected, characteristics that are considered include: grading, durability, particle shape and surface texture, abrasion and skid resistance, unit weights and voids, and absorption and surface moisture.”
Conco is a leading supplier of concrete services for the Western United States. As one of the leading commercial concrete contractors in the San Jose area, we have been involved with large-scale projects in the area including the Mineta Airport. Our concrete services include commercial, educational, parking and other construction development as well as public works projects and highways.