Better Roads Staff
Crushed stone, sand and gravel constitute the major types of construction aggregates. Crushed stone is an angular rock of plutonic or sedimentary origin, crushed and screened to spec in quarries, while gravel has a more rounded shape, produced by weathering and erosion (unless crushed), and is extracted from fluvial or glacial deposits or “pits”. Sand also is extracted from pits, but “manufactured sand” is produced in quarries.
Plutonic aggregates include the igneous rock granite, on the average by weight made up of 72 percent silica (SiO2), 14.4 percent alumina (Al2O3), with the remainder oxides of potassium, sodium, calcium, iron and magnesium; and varieties of basalt, known widely as trap rock, on average made up of 45 to 55 percent silica, 14 percent or more alumina, and some 10 percent CaO, 5 to 12 percent MgO, and 5 to 14 percent FeO.
Sedimentary aggregates include limestone, made up almost entirely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3); and dolomite, composed of calcium magnesium carbonate (CaMg(CO3)2), in which magnesium replaces calcium in amounts over 50 percent. Lesser replacements result in a stone called dolomitic limestone. A useful sedimentary or evaporative stone for use on unpaved roads is the abrasive anhydrite, a hard variety of normally soft gypsum (calcium sulfate, CaSO4·2H2O), but lacking the water component.
The popular image of a quarry is an open-cast site from which tombstones, countertops and other types of dimension stone are extracted. In reality dimension stone accounts for the barest fraction of stone in terms of raw tonnage; shot-and-crushed stone accounts for the vast portion of stone produced.
“After the blast, quarry material comes in size gradations unsuitable for product sale,” says Florian Festge, president, W.S. Tyler, Inc. “To make a final product, producers must first break larger rocks into smaller ones and limit the appropriate top size. This process is conducted by crushing using a close side setting equal to the largest rock size desired in the final product. Crushing therefore provides the foundation for the proper sizing of aggregate stone.”
As important as crushing is, the final product is refined by correct screening. “Screening is the separation of material by size,” Festge tells Better Roads. “Vibrating screens continuously convey and propel rocks into flight in order to allow for a comparison between rock size and screen opening once the material hits the screen again. Because of the significantly higher investments required into crushing equipment, producers often focus their attention solely to the crushing equipment, when in fact the determination of the product purity and quality – which is critical to the customer – is determined on the vibrating screen during the screening process. It’s essential for any aggregate operation to place equal attention on both crushing and screening to obtain the highest product quality, as well as operating profit.”
In the meantime, the chemical/mineral makeup of each aggregate type lends varying degrees of compressive strengths, water content and skid resistance. In the past – due to the cost of shipping of aggregate – local road agencies had to be content with whatever aggregates were locally available. But since the advent of the Strategic Highway Research Program (1987-1993) and the resulting Superpave system of performance-graded liquid asphalts and mix designs, the higher costs of premium aggregates shipped in from out of state – along with stone processed to boost angularity – have been absorbed by state road agencies and ultimately, highway users.
“Crushed stone and crushed gravel are the major sources of most pavement aggregates,” say Richard C. Meininger, P.E., highway research civil engineer at the Federal Highway Administration, and Steven J. Stokowski, P.G., aggregate technologist and petrographic laboratory expert with SES Group & Associates, in a presentation during the Transportation Research Board meeting in Washington, D.C., in January 2011.
On average, every lane-mile of Interstate uses 38,000 tons of aggregate, compared to an average 400 tons of aggregate used for a new home.
“Their angular shapes perform well in applications where interparticle friction adds to pavement strength, such as granular bases and asphalt layers,” Meininger and Stokowski says. “For portland cement concrete, natural sand, gravel, and crushed stone are widely used in pavements and structures as well. Natural sand, as the fine aggregate for concrete, is entrenched in highway agencies’ specifications because its rounded shape contributes to concrete workability. Using crushed, angular, and manufactured fine aggregates in concrete, mortar, and grout applications is more difficult, but may be necessary in some areas.”
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