Aggregate

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. In HMA, gradation helps determine almost every important property including stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance and moisture susceptibility (Roberts et al., 1996[1]). In PCC, … Read more »

“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 … Read more »

Overview The soundness test determines an aggregate’s resistance to disintegration by weathering and, in particular, freeze-thaw cycles. Aggregates that are durable (resistant to weathering) are less likely to degrade in the field and cause premature HMA pavement distress and potentially, failure. The soundness test repeatedly submerges an aggregate sample in a sodium sulfate or magnesium … Read more »

Aggregates undergo substantial wear and tear throughout their life. In general, they should be hard and tough enough to resist crushing, degradation and disintegration from any associated activities including manufacturing, stockpiling, production, placing, compaction (in the case of HMA) and consolidation (in the case of PCC) (Roberts et al., 1996[1]). Furthermore, they must be able … Read more »

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. In HMA, since aggregates are relied upon to provide stiffness and strength by interlocking with one another, cubic angular-shaped particles with a … Read more »

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. However, this is a concern for PCC because aggregate is generally not dried and therefore the aggregate moisture content will affect the water content (and thus the water-cement … Read more »

Aggregates must be relatively clean when used in HMA or PCC. Vegetation, soft particles, clay lumps, excess dust and vegetable matter are not desirable because they generally affect performance by quickly degrading, which causes a loss of structural support and/or prevents binder-aggregate bonding. To test for clay lumps or friable particles, a sample is first … Read more »

Basic aggregate weight-volume relationships are important to understand aggregate structure and properties.  Fundamentally, aggregate volume properties are desired, but since mass measurements are typically much easier, masses are typically taken then converted to volume by using specific gravities.   The following is a brief discussion of the more important mass-volume relationships of aggregates. Gsa = Apparent … Read more »

Aggregate specific gravity is useful in making weight-volume conversions and in calculating the void content in compacted HMA (Roberts et al., 1996[1]). AASHTO M 132 and ASTM E 12 define specific gravity as: “…the ratio of the mass of a unit volume of a material at a stated temperature to the mass of the same … Read more »

Terms The basic reference for the Unified Soil Classification System is ASTM D 2487. Terms include: Coarse-Grained Soils: More than 50 percent retained on a 0.075 mm (No. 200) sieve Fine-Grained Soils: 50 percent or more passes a 0.075 mm (No. 200) sieve Gravel: Material passing a 75-mm (3-inch) sieve and retained on a 4.75-mm … Read more »