Construction Aggregate

Natural construction aggregate is one of the most abundant natural resources and one of the most widely used. Construction aggregate is the sized, or crushed and sized, rock material used in a variety of construction products. If selected for its color or appearance, it is decorative aggregate. Construction aggregates are used in concrete and asphalt, which make up most of our streets and highways, bridges, houses and other buildings, roofing, and other structural components. Aggregates range in size from large boulders (rip rap) used as fill in large construction projects to finely-ground flour-sized particles used in paint, glass, plastic, medicine, agricultural feed and soil conditioners, and many other industrial and household products. Construction aggregates are also used in water purification, emissions control, soil erosion control, and other environmental improvement products.

More than 90% of asphalt pavement and 80% of concrete consist of construction aggregate. The remainder is a binder such as asphalt or cement. About 52% of all construction aggregate is crushed stone, while 48% of the remaining is sand and gravel. Construction aggregate, comprising more than half the volume of all mining in the United States, totaled more than 3 billion tons in 2004 (Bolen, 2005; Tepordei, 2005).

Clinker (baked shale) quarry near Wright, 1978.

Aggregates consist of crushed and size-sorted rock (either quarried stone or crushed gravel) or natural sand and gravel, which are not crushed, but sized-sorted. Sand and gravel aggregate is less expensive than crushed stone aggregate, but crushed stone has the advantage of consistency in mineral makeup, hardness, angularity, and density. A few projects use other aggregate materials such as expanded shale or factory by-products such as mill slag.

Construction aggregates are the least expensive of all mined products. Because of their low price, transportation costs from the mine to the point of use often are the major part of their cost to the consumer. Even at very short distances, transportation costs may exceed the cost of the product at the mine mouth. Therefore, it is imperative that aggregate sources be located as close as possible to the point of use. This fact usually creates conflicts between aggregate producers and people who live in proximity to aggregate sources, since economical aggregate sources must be located close to population. People want an inexpensive aggregate, but generally don’t want the mining operation close to their dwellings.

Construction aggregate was produced in all of the 50 states in 2002. The most populous states usually mine most of this product. Only four states, Delaware, Hawaii, North Dakota, and Rhode Island produce less construction aggregate than Wyoming. However, construction aggregate is the fourth most important mineral product produced in Wyoming (by value) after oil and gas, coal, and trona. The value of construction aggregate produced in Wyoming exceeds the value of materials such as bentonite, uranium, and gypsum when all uses of aggregate including limestone produced for cement production are included. According to the State Inspector of Mines, Wyoming produced 21,323,600 short tons of construction aggregate during 2006. This included railroad ballast, power plant emissions rock, sand and gravel, and crushed stone. Limestone was the primary crushed stone produced in Wyoming, followed by various types of granite, clinker (baked and fused shale), and shale. Wyoming State Geological Survey Map Series MS-47 (Harris, 2004) shows the location of construction aggregate deposits in Wyoming at a scale of 1:500,000 (see Recommended Reference Material below).

Potential sources of aggregate in Wyoming.

Crushed stone
Crushed stone resources occur where suitable material is located close to transportation and end users. A few large quarries are located on suitable rock next to rail transportation, such as the Martin-Marietta Materials quarry west of Cheyenne. The rock type quarried can be any material that exhibits sufficient strength, resistance to wear, high production potential, and low amount of waste. Limestone is the preferred rock for highway construction, and can be found throughout Wyoming, especially on the flanks of the mountain uplifts. Granite, gneiss, basalt, sandstone, quartzite, scoria (clinker, baked and fused shale), and other rock types have been used for construction aggregate in Wyoming. Quartzofeldspathic gneiss at the Martin Marietta Materials quarry was originally developed by the Union Pacific Railroad for railroad ballast, which is still one of the quarry’s main products. The foliation in the gneiss causes the rock to break into pieces with the size dimensional ratios of 2:4:6, which is a specification for certain types of railroad ballast.

Martin-Marietta quarry west of Cheyenne. Photo by R.E. Harris.

Natural gravel sources
Natural gravel sources occur in a variety of different geologic environments. They consist of unconsolidated gravel, or loosely to partially cemented gravel that can be dug out of a pit without blasting or cutting.

Alluvial sand and gravel deposits
Stream channel and flood plain deposits occur along Wyoming’s larger streams and rivers. The largest historical natural gravel production in Wyoming for many years was from gravels of the Bear River at Evanston. Many permanent gravel sources are located in stream channel deposits, including those along the Snake River at Jackson; the Green River at Cora; and along the North Platte River at Saratoga, Fort Steele, and downstream from Casper. Other productive alluvial deposits include the Tongue River at Sheridan; the Wind-Bighorn Rivers at Dubois, Riverton, Worland, Greybull, and Lovell; the Shoshone River at Cody; the Laramie River at Laramie; and Lodgepole and Crow Creeks at Cheyenne.

Terrace sand and gravel deposits
Terrace deposits in Wyoming formed mostly during the Pleistocene in basin areas near the present mountain ranges. Braided streams and sheet floods from the mountains provided material derived from various sources. The changing climate of the Pleistocene (glacial and non-glacial intervals in the higher mountains, dry and wetter cycles in the lower parts of Wyoming) resulted in terrace gravel deposition along mountain flanks. These occur on the tops of benches and terraces and frequently form a surface layer more resistant to erosion than surrounding deposits. The quality of such deposits varies greatly. Terraces may contain areas of caliche, a calcium carbonate particle coating that can be detrimental. Caliche-coated gravels will not bind adequately with asphalt or cement. Terrace gravel sources are used for construction aggregate in the Bighorn Basin, and near Casper, Lander, Cheyenne, Wheatland, and Laramie.

Pleistocene gravels of uncertain origin
Localized thick gravel deposits occur in areas closely related to Pleistocene faulting in several parts of Wyoming. Typically these deposits are very thick, often interbedded with sand lenses, contain a wide variety of rock types, and derive in part from ancestral stream channels. Rocks in these gravels can be traced to both local and distant sources. These deposits have been used extensively in central and eastern Wyoming for construction projects.

Glacial gravels
Glacial deposits containing large amounts of unsorted gravel occur in and adjacent to the higher mountain ranges in northern and western Wyoming, and in the higher parts of the Medicine Bow Mountains and Sierra Madre in south-central Wyoming. Glacial sources for aggregate are not extensively used due to surface disturbance restrictions or prohibitions in scenic areas such as National Parks and parts of National Forests. However, they are occasionally used for local construction projects such as roads and highways.

Older gravel deposits
In parts of Wyoming, rock units from Late Cretaceous to Pleistocene in age contain unconsolidated or poorly consolidated conglomerate that may be suitable for the production of natural gravel. These are usable if they can be quarried without additional disaggregation processes. These types of gravels have been used in the eastern Green River Basin, in western Wyoming, and south of Sheridan along the east flank of the Bighorn Mountains.

Windblown and other sand deposits
Sand-sized material is used as aggregate in certain types of construction projects such as sidewalk concrete, some concrete finishes, railroad traction sand, highway sand, and others. Wyoming has extensive deposits of windblown sand. Areas of active and stabilized dunes extend from the western Green River Basin across central and east-central Wyoming into western Nebraska. However, most sand used in Wyoming is a by-product or co-product of gravel operations.

Production of construction aggregate in Wyoming
After 1995, construction aggregate production in Wyoming increased each year, except for slight declines from 1996 to 1997 and 2002 to 2003. In 2008, a dramatic downturn in production reflected the downward plunge of the economy that year. However with the economy beginning to recover, construction aggregate production is starting to show improvement as well.

New sources of construction aggregate are needed to keep up with shifting and increasing demands related to Wyoming’s energy development and local urban expansion. A developed construction aggregate source is typically smaller than five acres. Wyoming has a ten acre permit exemption designed primarily for aggregate production (this exempts a producer from some costly permit application requirements, but not from reclamation requirements). However, recent interpretations of the permit requirements include the acreage of roads, stockpiles, and plant facilities in the ten acres allowed, which often limits quarry size to near or below the minimum size for economical operation of aggregate production. Wyoming has adequate deposits of raw materials for construction aggregate in most areas of growth, with the notable exception of the Powder River Basin. Continued production is necessary for sustained prosperity and economic growth.

References

Bolen, W.P., 2005, Sand and gravel (construction): U.S. Geological Survey, Mineral Commodity Surveys, 2 p.

Harris, R.E. 2004, Digital Industrial minerals and construction materials map of Wyoming: Wyoming State Geological Survey Map Series MS-47, scale 1:500,000 (color).

Holmes, D., 2000, Construction material: West Coast markets still going (abstract.): U.S. Bureau of Indian Affairs, Program with Abstracts, Tenth Annual National Indian Energy & Minerals Conference, Colorado School of Mines, Golden, Colorado, p.3.

Manydeeds, S., 2000, Crushed rock on the Annette Island Reserve, Alaska (abstract.): U.S. Bureau of Indian Affairs, Program with Abstracts, Tenth Annual National Indian Energy & Minerals Conference, Colorado School of Mines, Golden, Colorado, p.5.

State Inspector of Mines of Wyoming, 1996 – 2005, Annual reports of the State Inspector of Mines of Wyoming for the years ending December 31, 1995 – December 31, 2005: State Inspector of Mines of Wyoming, Rock Springs, Wyoming.

Teipordei, V.V., 2005, Stone (crushed): U.S. Geological Survey, Mineral Commodity Surveys, 2 p.

Recommended Reference Material

WSGS Map Series 47, Industrial minerals and construction materials map of Wyoming, by R.E. Harris, 2004, scale 1:500,000.