The density of aggregates can be used to create concrete with a wide range of unit weights, as seen in Table. Generally, aggregates are classified into three categories based on their specific gravity: light, normal weight, and heavy weight. To determine the density of an aggregate, its relative density (specific gravity) is multiplied by the density of water. Aggregates have a major impact on the stiffness, unit weight, strength, thermal properties, bonding and wear resistance of concrete. Aggregates can be divided into several groups based on various criteria such as size, font and unit weight.
The type of aggregate used will determine whether the concrete is structural or non-structural. Examples of lightweight aggregates include ash, blast furnace slag, volcanic pumice and expanded clay. The flowability, cohesiveness and rheological behavior of fresh concrete are largely determined by the quantity, type, surface texture and size gradation of the aggregate. In addition to reducing cost, aggregate in concrete can reduce shrinkage and creep of cement paste. The relative density (specific gravity) of an aggregate is the ratio between its mass and the mass of an equal volume of water.
For normal concrete used for structural elements such as beams and columns, the maximum size of the coarse-grained aggregate is approximately 25 mm. The bulk density or unit weight of an aggregate is the mass or weight of the aggregate that is required to fill a container of a specific unit volume.