Selecting the Right Aggregates for Concrete: Key Characteristics to Consider

To get the best concrete mix, it is essential to use aggregates that are clean, hard, and strong particles, free of any absorbed chemicals or coatings of clay and other fine materials that could cause deterioration. Concrete is a combination of cementitious material, aggregates, and water. Aggregates are usually considered inert fillers, making up 60 to 80 percent of the volume and 70 to 85 percent of the weight of concrete. Even though they are considered inert, they are an essential component that defines the thermal and elastic properties of concrete and its dimensional stability. Aggregates can be divided into two categories: coarse-grained and fine.

Coarse-grained aggregates are usually larger than 4.75 mm (retained at No. 200 sieve). The compressive strength of the aggregate is an important factor when selecting it. For normal strength concrete, most concrete aggregates are several times stronger than the other components of concrete, so they do not affect its strength.

However, for lightweight aggregate concrete, the compressive strength of aggregates can have a greater influence. In addition to strength, other characteristics should be taken into account when selecting aggregates for concrete. They should not be reactive with cement alkalis. The unit weight with dry rod is the mass (weight) of one cubic meter (foot) of dry coarse-grained aggregate that is compacted by forming rods in three equal layers in a standard container. The density of the aggregates is necessary to determine weight-volume ratios in the mixture.

Fine aggregates are usually sand, crushed stone, or crushed slag, while coarse-grained aggregates include gravel, pebbles, broken stone fragments, slag, and other coarse-grained substances. According to Bryan Perrie, CEO of Cement & Concrete South Africa (CCSA), aggregate selection is very important for producing quality concrete because it plays a major role in determining the strength, thermal and elastic properties, as well as stability of concrete. Aggregates containing natural shale or shale-like particles, soft and porous particles and certain types of chert should be avoided since they have poor weather resistance. The specific gravity of an aggregate is the ratio of its weight to the weight of an equal volume of water at a given temperature. Most natural sands and gravel found in riverbeds or seashores are smooth and rounded and make excellent aggregates. Since the weight of the aggregate depends on its moisture content, it is important to maintain a constant moisture content.

The main natural resource for aggregates is volcanic material; synthetic aggregates are produced by heat or heat treatment of materials with expansive properties. Aggregates can be extracted from natural sand or sand and gravel pits, hard rock quarries, dredging submerged deposits, or mining underground sediment. All aggregates contain some moisture depending on their porosity and the moisture state of their storage area. Although they are known to be inert fillers in concrete, their different properties have a major impact on the strength, durability, workability and economy of concrete. Other physical and mineralogical properties must also be known before mixing the concrete to obtain a desirable mix.

Leveling affects the amount of aggregate used as well as cement and water requirements, workability, pumpability, and durability. Coarse-grained aggregates with a size of 20 mm or less are commonly used for most concrete constructions.

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