Crushed Rock Specifications

Particle Size

• ASTM International determines if a substance qualifies as crushed rock using a variety of standards, including particle size. The organization measures rock particles by sifting them through sieves, which are mesh utensils with regular-size openings. According to the City of Lincoln, Nebraska, for a material to qualify as crushed rock for surfacing, between 94 and 100 percent of that material's particles must pass through a sieve with 1 1/2-inch wide openings. That same material can only have between 0 and 10 percent of its particles pass through a sieve with 1/2-inch wide openings.

Composition

• Even if an aggregate material meets ASTM International's particle size specifications for being crushed rock, the mineral composition of that material's particles could preclude it from the "crushed rock" classification. As the City of Lincoln mentions, crushed rock should consist of limestone or dolomite, both of which are sedimentary rocks that consist of calcium carbonate. More specifically, these limestone and dolomite particles should be hard, durable and free of oils, organic matter and other potentially corrosive substances.

Production

• The way in which workers process aggregate rock materials is also an important specification for crushed rock. According to Alaska's Department of Natural Resources, for a material to qualify as crushed rock, workers must crush larger rocks fragments using a powerful rock-crushing machine so that the resulting particles have freshly fractured surfaces on nearly all of their sides. In addition, all particles that result form the crushing process must be of a relatively uniform shape. Crushed rock cannot contain thin or elongated particles. To ensure that crushed rock does not contain organic materials like clay and dirt, workers must run crushed rock particles through a screening device, such as a mechanically-operated vibratory screen.

Abrasion

• Abrasion refers to the loss of particles comprising a material due to scraping and wear. ASTM International measures abrasion loss, as La Paz County mentions, using the Los Angeles Abrasion Machine. For a material to qualify as crushed rock, it cannot lose more than 10 percent of its weight after 100 revolutions in the machine. In addition, the material cannot lose more than 40 percent of its weight following 500 revolutions in the machine.