Aggregate is a collective term for the mineral materials such as sand, gravel, and crushed stone that are used with a binding medium (such as water, bitumen, portland cement, lime, etc.) to form
compound materials (such as asphalt concrete and portland cement concrete). Aggregate is also used for base and subbase courses for both flexible and rigid pavements.
Aggregates can either be natural or manufactured. Natural aggregates are generally extracted from larger rock formations through an open excavation (quarry). Extracted rock is typically reduced to usable sizes by mechanical crushing. Manufactured aggregate is often the byproduct of other manufacturing industries.
The main characteristics of aggregate that affect the performance of fresh and hardened concrete are:
• Shape and texture
• Mineralogy and coatings
• Strength and stiffness
• Maximum size
• Specific gravity or relative density
Aggregates must be relatively clean. Vegetation, soft particles, clay lumps, excess dust, and vegetable matter may affect performance by quickly degrading, which causes a loss of structural support and/or prevents binder-aggregate bonding.
Determination of clay, silt, and dust in fine and coarse aggregate can be tested by the sedimentation method. The aggregate is carefully mixed with water in a volumetric cylinder and then let to settle. The clay particles will form layers with different colors and structures on the surface of the aggregate.
Decaying vegetation may result in aggregates being contaminated with organic matter. This material may have a retarding effect on the setting of cementitious material and may result in lower strengths of the hardened material at all ages. Organic impurities can be tested by colorimetric test. Tested aggregate is mixed with sodium hydroxide (NaOH) or potassium hydroxide (KOH) to prepare a colored solution. The color of the solution is compared with the color of the standard solution, prepared according to the standard. If the color of the test solution is darker than the standard solution then aggregate has to be rejected.