Understanding the Role of Aggregates in Concrete

Learn about what are aggregates in concrete and their role in construction projects. Understand how to select quality materials that meet ASTM C 33 standards.

Understanding the Role of Aggregates in Concrete

Concrete is a composite material made up of a combination of geological materials such as gravel, sand, and crushed rock. The size of the particles determines whether it is a coarse-grained aggregate (e.g., gravel) or a fine aggregate (e.g., sand). The resulting concrete can be used in its natural state or crushed, depending on its intended use and application. The size of coarse-grained aggregates is the primary factor in determining the strength of concrete.

Generally, smaller coarse-grained aggregates are used for stronger concrete, with 20mm aggregates meeting the threshold for strong concrete and 40mm aggregates for normal strength concrete. Since fine aggregates are used to fill the voids of coarse-grained aggregates, the smaller the coarse-grained aggregates, the finer the fine aggregates should be. They increase the workability of concrete. Aggregate is an essential ingredient in making concrete used in construction.

The quality of the material greatly influences concrete performance, including how well it mixes and hardens, as well as its long-term durability. If you are considering exposed aggregate for your garden, you will need to pay more attention to the materials used to create the aggregate mix. Well-graded aggregate is best, however, aggregates rated at maximum density can give concrete a rough texture. Aggregates are a broad category of materials, such as sand, gravel, crushed stone, slag, and recycled concrete.

A specific aggregate (or combination of aggregates) is selected for its durability, strength, workability, and ability to receive finishes. Only 15-34% of the aggregates in zone 1 will pass through a 0.6 mm screen; 35-59% of the aggregates in zone 2 will; 60-79% of zone 3 will; and 80-100% of zone 4 will. It is important that your concrete producer purchases good quality aggregates, as verified by regular aggregate test results in accordance with ASTM C 33, Standard Specifications for Concrete Aggregates. Aggregates are used to create concrete slabs and form surfaces and substrates. The size distribution of fine to coarse-grained aggregates plays an important role in the workability and performance of concrete.

The aggregate is also very important for the strength, thermal and elastic properties of concrete, dimensional stability and volume stability. Crushed stone is preferred in pavement mixes since the higher paste-aggregate bond produces greater flexural strengths (Photo courtesy of PCA). When specifying void graded aggregates, certain particle sizes are omitted from the aggregate size continuum. Producing good quality, durable concrete containing a portion of recycled concrete aggregate often requires test concrete mixes and close control of the properties of old recycled concrete with mix adjustments made as needed.

The ratios between coarse-grained and fine aggregates will change depending on the unique characteristics of each aggregate, the method of placement and the desired finish. Aggregates are an essential component in making strong and durable concrete used in construction projects. It is important to select quality materials that meet ASTM C 33 standards for optimal performance. The size distribution between fine and coarse-grained aggregates plays an important role in workability and strength while crushed stone is preferred for pavement mixes due to its higher paste-aggregate bond. When using recycled concrete aggregate it is important to closely monitor properties such as mix adjustments to ensure good quality results.

Brock Cottew
Brock Cottew

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