Aggregate
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). By volume, aggregate generally accounts for 92 to 96 percent of HMA and about 70 to 80 percent of portland cement concrete.
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Introduction to Aggregate
Toughness and Abrasion Resistance
Aggregates undergo substantial wear and tear throughout their life.
Durability and Soundness
The soundness test determines an aggregate’s resistance to disintegration by weathering and, in particular, freeze-thaw cycles.
ASTM Aggregate and Soil Terminology
The basic reference for the Unified Soil Classification System is ASTM D 2487. Terms include:
Aggregate Specific Gravity
Aggregate specific gravity is useful in making weight-volume conversions and in calculating the void content in compacted HMA (Roberts et al., 1996). AASHTO M 132 and ASTM E 12 define specific gravity as:
Aggregate Weight-Volume Relationships and Terms
Basic aggregate weight-volume relationships are important to understand aggregate structure and properties.
Cleanliness and Deleterious Materials
Aggregates must be relatively clean when used in HMA or PCC.
Gradation and Size
The particle size distribution, or gradation, of an aggregate is one of the most influential aggregate characteristics in determining how it will perform as a pavement material.
Moisture Content
Since aggregates are porous (to some extent) they can absorb moisture. Generally this is not a concern for HMA because the aggregate is dried before HMA production.
Particle Shape and Surface Texture
Aggregate particle shape and surface texture are important for proper compaction, deformation resistance, and workability. However, the ideal shape for HMA and PCC is different because aggregates serve different purposes in each material.