Aggregates production technology
More than 80 percent of the 1.33 billion tons of crushed stone consumed in 1996 was used as construction aggregates, mostly for highway construction and road maintenance. Crushed stone is also used in the manufacture of concrete for road, building, and bridge construction, and nonconstruction applications. Approximately 72 percent of the crushed stone produced came from limestone, about 15 percent from granite, 7 percent from basalt, and the remainder from a variety of other rock types.
Construction aggregates are hard materials suitable for forming concrete when a cementing or binding material is added, or used alone in other applications. Aggregates derived from sand and gravel, crushed stone, or recycled sources generally make up the bulk of the volume of the concrete being produced. Although deposits from which crushed stone can be produced are widespread in the United States, they are not available everywhere. Factors such as market availability, transportation distances, local environmental impact, and permitting factors must be considered in site selection. Deposits suitable for concrete aggregates production must meet strict technical specifications related to quality and quantity. Specifications for crushed stone are developed by organizations such as: The American Society for Testing and Materials (ASTM), American Association of State Highway and Transportation Officials (AASHTO) , U.S. Department of Transportation, Federal Highway Administration (FHWA), U.S. Army Corps of Engineers, and the various State departments of transportation (DOTs). Product specifications are often mandated for all State or Federal construction projects. Local transportation districts may select specifications suitable for their needs, generally based on State guidelines. Crushed stone specifications may be modified regionally to reflect local climatic conditions and availability of materials.
Stone suitable for producing a crushed stone product is most often recovered by standard quarrying techniques. Underground operations are occasionally used where suitable and in areas where community resistance to surface mining is high. For most surface operations, once a suitable deposit is selected and acquired, the stone is often recovered by removing any overburden, loosening of the rock by blasting, crushing the rock to the desired size, separation of deleterious material, stockpiling of marketable material of various sizes and grades, and transporting products to market. Depending upon nature of the deposit and rock type, not all of these steps may be required. The capacity of a crushed stone operation in the United States ranges from less than 25 thousand tons to over 5 million tons per year.
The relative layout of the primary and secondary crushers, screening plant, stockpiles, and ancillary equipment determines how efficiently materials can move throughout the operation. Because transportation among resource, processing plant, and markets is one of the most important factors in site profitability, efficient site location and design are critical.
Most crushed stone operations employ surface mining methods to recover the resource. Trucks and shovels are commonly used at larger operations or where material must be moved longer distances or over public roads. Front-end loaders or scrapers are used at smaller operations or where shorter haulage distances are necessary. Draglines may be used where material to be moved is under water and hydraulic shovels may be used where the rock surface is irregular.
The nature and geometry of the deposit determine the drilling and blasting requirements. Operation size may determine whether company or contract drilling is used. Environmental, regulatory, and social considerations also affect type of drilling and scheduling of blasting, particularly if the site is located in or near an urban area.
Crushed stone plants are generally classified as either wet or dry plants, depending upon stone classification, the types of contaminants present, availability of water and land area, zoning, and environmental considerations. In the wet process, clay and contaminants are removed by washing and waste water is sent to settling ponds. Wet plants do not have the dust problems often associated with dry plants, but require a larger site area in order to accommodate the space required for settling ponds. Dry plants often require dust collectors or other dust suppression systems to reduce the amount of dust generated in processing.
Both permanent and portable plants or a combination of the two types may be used, depending upon such things as plant capacity, deposit life, product mix, equipment and space availability, and time constraints. Portable plants are often used to supply stone for specific construction projects, and may be sited at the project rather than at the resource location. Such plants are designed to travel on public roads and are generally used for large projects or where no permanent operations exist. A portable plant may not be as efficient as a comparable permanent plant. A permanent plant is generally preferred when a wide range of products is desired. Crusher selection depends on a number of factors including rock characteristics, products desired, the quantity and type of deleterious material, screening capacity, and economic factors. Impact and compression crushers are used to reduce the size of stone particles. Impact-type crushers have greater capacity-to-cost ratios than do compression-type crushers, but abrasive stones may cause increased wear on impact crushers.
After crushing, the stone is separated to desired specifications by means of screening, classifying, and washing circuits. Factors considered in designing the circuits include the number and types of products required, the nature of the processed material, amount of wet or sticky material, particle shape, amount of oversize or fine material, the relative densities of each material, and economic factors.
Construction aggregates can be used with or without a binder, such as asphalt. Road base, macadam surfacing material, riprap, and railroad ballast are construction applications that do not require binders. Aggregates for cement and bituminous concrete in highway construction and repair, and residential and commercial construction applications require binder material. The binding agent is generally added at the concrete plant or construction site.
Products of crushed stone operations are often stockpiled prior to sale. Automated stockpile systems require less labor and less equipment for handling the stone, but are higher cost and generally not used at smaller operations. Other factors to consider include the amount of available storage space, transport requirements, plant capacity, and storage time.
Transportation is a major factor in the delivered price of crushed stone. The method of transportation is generally determined by cost and availability. Although trains and barges are used, truck haulage is the most common mode of transportation. Because of the high cost of transportation and the large quantities of material hauled, haulage distances seldom exceed 160 kilometers. Crushed stone is usually marketed locally, although increasing land values, land-use decisions, and environmental concerns are moving crushed stone quarries farther from end-use locations.
Crushed stone operations are subject to Occupational Safety and Health Administration (OSHA) and Mine Safety and Health Administration (MSHA) regulations and must conform to established environmental regulations pertaining to air, water, noise, and safety. The type of control program selected depends upon the amount and type of dust generated; available water and pond space; local zoning, and environmental regulations; the type of processing utilized, and economic considerations. Special regulations are imposed on siliceous dust, due to potential health risks if inhaled. Regulatory limits vary depending upon location, site design, and the enforcing agency.
Sand and Gravel
Most of the 914 million tons of construction sand and gravel produced in 1996 in the United States was used in construction applications, principally portland cement concrete, road base, asphaltic concrete, and as general fill. Of the material consumed in 1996 with a specified use, about 43 percent was used for concrete aggregates; 23 percent for road base, coverings, and stabilization; 13 percent as asphalt concrete aggregates and other bituminous mixtures; and 12 percent as construction fill. In the United States, sand and gravel production ranks second in the nonfuel minerals industry behind crushed stone and is the only mineral product recovered in all 50 States. The quality of aggregates depends not upon rock type, but rather on physical and chemical properties and subsequent changes related to weathering, tectonic history, or chemical alteration. Suitable material is often found in unconsolidated beach, stream, alluvial, and glacial sedimentary deposits.
The construction industry uses sand and gravel chiefly in concrete aggregates, asphalt, or road base. Concrete mixes commonly contain 15–20 percent water, 7–14 percent cement, and 66–78 percent aggregates. The physical properties of sand and gravel most significant for concrete use include abundance and nature of fractures and pores, particle shape and surface texture, and volume changes resulting from weathering, freezing, or thawing. Nonreactive mineral and rock particles that are strong and capable of resisting weathering without decomposition are suitable candidates for concrete. Asphalt concrete mixtures predominantly used for paving consist of sand, gravel, and mineral fines coated with asphalt derived from the refining of petroleum. Crushing is generally required in high-quality asphalt applications to provide freshly fractured faces that provide maximum adherence for the asphalt binder.
Concrete aggregates have to meet physical and chemical requirements and specifications similar to those for crushed stone. Materials used in construction and transportation applications must conform to appropriate Federal and State specifications related to characteristics of abrasion, soundness, specific gravity, size and grading, reactivity, absorption, durability, and sand equivalent. Sand and gravel specifications may be modified to reflect local climatic conditions.
As with crushed stone, sand and gravel are a high-volume, low-unit-value commodity with an industry characterized by thousands of operations serving local and regional markets. Resources are widespread but shortages exist locally, either because the resource doesn’t exist in an area, or because of land-use conflicts and environmental concerns associated with rapid urbanization. Deposits are recovered by standard surface mining techniques. Ideally a commercial deposit would contain about 60 percent gravel-sized particles and 40 percent sand-sized particles, where the gravel would be used for road base or bituminous aggregates and the sand would be used for making concrete. This ratio can vary significantly, however, depending upon deposit makeup.
Sand and gravel deposits are mined with power shovels, draglines, front-end loaders, or dredges. The choice of excavating equipment depends upon operation size, resource type, economic considerations, and whether the resource is mined by wet or dry methods. In a dry operation, shovels, loaders, or draglines load the sand and gravel into trucks or onto conveyor belts for transfer to the processing plant. Because the material is unconsolidated, drilling and blasting are generally not required. A wet operation recovers sand and gravel from deposits below the water table. The material is excavated by a land-based dragline, floating dredge, or hydraulic mining operation. Conveyors or pipelines transport the slurry to adjacent processing plants.
Commercial processing plants are most often located at the resource, where blending can be performed to produce a variety of products. As with crushed stone, processing consists of crushing, screening, and washing. Sized material is then stockpiled based on product requirements prior to transport to market. Plant capacities range from less than 23,000 tons for small, intermittent operations to more than 4.5 million tons per year. Most operations are small, turning out one product or a limited range of products, but the bulk of total U.S. production comes from large operations.
Most plants are designed to produce different products. There is usually a dry side, where material is crushed and screened for use as road base or bituminous aggregate, and a wet side, where sand and gravel are washed and screened for use as concrete aggregate. A jaw crusher is commonly used for primary size reduction. Gyratory, roll, or impact crushers further reduce the size of the gravel. Rod mills are used to manufacture sand to supplement natural sand that is deficient in fine sizes.
The sand fraction is ordinarily washed and classified in spiral classifiers. Cyclones may be used to recover fine sand from classifier overflow. Settling tanks may be used to separate sand into various sizes. The desired blend of sand is then drawn off and dewatered in a second series of spiral classifiers. Plants processing clay-rich material may use scrubbers or log washers to remove the clay. Mechanical vibrating screens separate the gravel into appropriate sizes. Heavy media separators are used where soft porous particles such as shale are present. Jigs may also be used in selected applications.
For large construction projects or in remote areas where no permanent operations exist, a semi-mobile plant may be set up, which will remain at the construction site until completion of a specific project, then relocated to the next project. In urban areas, however, most production is done from permanent facilities. It is common for asphalt concrete and ready-mix concrete plants to be located at the sand and gravel production site.
Because transportation makes up a significant portion of the overall cost of sand and gravel operations, most construction sand and gravel continue to be marketed locally. Truck haulage is the main form of transportation; approximately 79 percent of sand and gravel products is transported by truck. Only a small percentage is transported by either rail or barge. Approximately 16 percent of construction sand and gravel is not transported, but used at the production site.
Sand and gravel operations are also governed by OSHA and MSHA regulations. As with crushed stone, regulatory requirements vary depending upon location and the enforcing agency.
Recycled Aggregates from Concrete
As recycling of construction materials increases, aggregate producers, building contractors, and road paving contractors are recycling a greater amount of material each year. In 1996, a total of 1.2 million tons of cement concrete was reported to be recycled by 43 aggregate producers in 16 States. These figures do not include the numerous recyclers not associated with natural aggregates production. It is estimated that about 7.3 million tons of scrap concrete was recycled in 1996, principally as road base (T. D. Kelly, oral commun., 1997). Recycled aggregates presently represent a small fraction of the total amount of aggregates consumed, but recycling potential increases as the amount of material available from traditional sources is increasingly affected by regulation and land-use issues.
Recycled concrete has many applications in road construction. It is commonly used as road base; 44 States allow recycled concrete in road base applications. Growth in the use of recycled concrete for retaining wall backfill, portland cement concrete mix, landscaping rock, drainage aggregates, and erosion control is also occurring.
Sources and the nature of recycled material can vary over time from project to project: the “resource” is the material being recycled. Each construction project requires material that meets specific specifications; therefore, the recycler must be able to adjust material feed to meet those specifications. As with crushed stone and sand and gravel operations, specifications are developed by a variety of Federal and State agencies, and often vary considerably by location and climatic conditions.
Processing of recycled material is a relatively simple process, but one that can require expensive, heavy-duty equipment, capable of handling a variety of materials. Technology basically involves crushing, sizing, and blending to meet the required product mix. Concrete and asphalt recycling plants can be used to process natural sand and gravel, but sand and gravel plants cannot process recyclable materials efficiently in most cases. Much construction concrete contains metal and waste materials that must be detected and removed at the start of processing by manual picking or magnetic separation. Feed for recycling may be non-uniform in size or composition, so equipment must be capable of handling variation in feed materials. Equipment must be versatile yet efficient for a variety of materials. Figure 2 pictures a typical construction site recycling operation.
|Capacity||180-360 tons per hour ||<90-360 tons per hour ||220-320 tons per hour |
|Versatility||Jaw crusher can handle|
rebar and wire mesh; cone
crusher cannot; wood a
problem for both
variety of feed material
|Accommodates rebar and|
|Amount of wear on|
equipment is low
|Wear higher than|
|Wear low for jaw crusher;|
wear for roll high if
aggregates are abrasive
|Primary feed||Accommodates concrete;|
less suitable for soft
|Mainly suitable for asphalt ||Accommodates both|
asphalt and concrete
|Dust control||Easy to control ||More difficult to control ||Easy to control |
|Capital investment||High ||About half of jaw/cone ||High |
|Labor requirement||Semi-skilled ||Skilled ||Semi-skilled |
|Other||Maintenance critical on|
|Wide variation in crusher|
|Not applicable |
|Adapted from Justice, 1993.|
Location, equipment selection, and plant layout are in many cases critical to the efficiency of a recycling operation. Equipment size and type impact project performance. Items that need to be considered for both stationary and portable plants include the amount of space the plant requires, potential for fines bypass, crusher discharge area considerations, magnetic separation requirements and effectiveness, debris removal, and dust control. Portable plants need to consider the ability to set up and relocate the plant easily and quickly and must be small enough to fit on existing roads and under overpasses.
Most recycling occurs in urban areas with access to adequate transportation routes and infrastructure. Whether a mobile or a fixed plant is used, the site of the recycling operation must be located close to sources of raw materials and product destinations. Permanent plant sites tend to be small in area, usually between 2 and 4 hectares. Even with mobile sites, a small quantity of land is usually required for resource and equipment storage.
Moving and setup for portable plant affect profitability. Such operations frequently move 4–20 times a year, and time taken for transport and setup results in lost production. A shorter transportation and setup time minimizes the impact on cash flow.
Recycled concrete can result in more wear on equipment than some forms of natural aggregates, depending upon the rock type from which it was derived. For example, a crushed stone producer may get a 10-year life out of a conveyor belt, whereas a recycler may only get a 6-month life out of a similar belt because of the physical characteristics (coarseness, angularity) of the processed material and the presence of deleterious material (such as rebar or wire). Recycling also requires more labor than natural stone production on a per unit basis, as pickers are required to extract debris from the concrete being reprocessed.
The principal step in processing recycled concrete is the crushing of the material, generally conducted in two stages. Several types of crushers are used in recycling; each type has advantages that must be considered. Table 1 outlines some combinations and considerations for this equipment. A two-stage crushing system is generally preferred unless the operator is doing multiple small projects. For small asphalt projects or where concrete does not contain rebar or other debris, a portable single trailer operation may be suitable. Feed material must be free from debris and the feed must be fairly uniform in composition.
Material to be recycled can be dumped directly into the primary crusher; however, a grizzly can be placed ahead of the crusher to increase production and reduce crusher wear. Dirt and fines generated from the grizzly may be separated by the loader operator prior to crushing, and stockpiled as waste, eliminating the necessity to process this material further. Spacing between the grizzly feeder and the crusher must be sufficient to allow long slabs of concrete to dip into the crusher. Careful inspection of feed for deleterious material by the loader and crusher operators can prevent work stoppages and prolong crusher life.
A crusher discharges onto an underlying belt conveyor. Clearance between the two pieces of equipment should be at least 122 centimeters; larger distances allow long pieces of rebar to fall free of the crusher without jamming the machine. A smaller clearance height may be necessary on portable plants to allow for transport and bridge clearance. Material is often hand picked at this point to remove waste material.
Magnets are an important piece of equipment when recycling concrete, as they aid in the removal of rebar and wire mesh commonly found in concrete demolition debris. Separator design and layout are important; separators commonly used in other mining applications often have features (pulley design, metal belt, for example) that are costly in recycling. For optimum efficiency, the conveyor beneath the magnetic separator should be running at the same speed as the separator belt.
Once the material has undergone primary crushing, it generally is screened to separate usable sizes of material from waste. Screens that maximize open area are generally the most efficient but wear out rapidly in recycling operations. Screened material is either sent to a secondary crusher, conveyed to stockpiles, or sent directly to the construction project as feed.
Debris removal at a recycling facility can be minimized but not eliminated. Operators at permanent plants can be selective in the materials accepted, but portable operations accept most of what is available for reprocessing on-site. For both plant types, manual picking stations located both prior to crushing and during screening separate out rags, paper, wood, and other debris. At sites that process various materials, the loader and crusher operators can also serve to sort, blend, and keep the feeder properly filled, improving the productivity of the operation.
Because recycling operations are often located near construction sites in urban areas, the need for good dust control becomes increasingly important. An engineered water spray system with a wetting agent can meet most regulatory agency requirements for dust control. For dusty crushers, a baghouse may be used. Small baghouses designed for portable crushers and smaller stationary operations have been shown to meet regulatory requirements.
Recycled Aggregates from Reclaimed Asphalt Pavement
In 1995, a total of 1.6 million tons of asphalt concrete was reported as being recycled by 62 companies in 26 States. Although only a small fraction of the total material available is reported to be recycled, it represented a 92 percent increase over the amount recycled in 1994. An estimated 45.4 million tons of scrap asphalt pavement was recycled in 1996 (T.D. Kelly, oral commun., 1997). Asphalt plants allow up to 45 percent of the product to contain recycled material from reclaimed asphalt pavement; recycled material typically makes up 20–25 percent of the asphalt concrete mix in most U.S. locations. Parking lots may utilize up to 100 percent of recycled asphalt material in selected hot-mix applications.
Applications for recycled asphalt concrete in road construction include pavement hot-mixes, road base, parking lot and residential driveway surfacing, and road shoulder work. Technical specifications for recycling asphalt concrete are similar to those of primary asphalt concrete; local or State specifications must be met for each construction project. Often, specifications are different depending upon location or application.
Site location and equipment selection criteria are similar to those reported for recycled concrete operations. Much of the equipment has been adapted from rock crushing applications, with modifications for efficiently handling the oilbased asphalt mixture. As with concrete recycling, technology basically involves crushing, sizing, and blending to meet the required product mix. Crusher types used for asphalt concrete recycling are reported in table 1. As with concrete recycling, much feed material is not uniform in characteristic and composition, so the equipment must be able to treat a wide variety of materials and remove nonrecyclable debris. Although smaller jaw crushers have been used at some operations, it is generally agreed that unless the operation is a portable one, the bigger units provide better wear and productivity than smaller units. For portable operations, the size of the unit should generally be as large as local movement restrictions allow. When recycling asphalt concrete, operators make provisions for removing dirt from the feed material. A grizzly feeder located prior to the crusher can accomplish this task. Dirt removal can also reduce the moisture content of the asphalt concrete and reduce the amount of asphalt that adheres to the machinery.
Asphalt is often cleaner and easier to separate than cement concrete debris. In many operations, loads of asphalt are processed separately from loads of cement concrete debris, using the same equipment.
- American Society for Testing and Materials (ASTM)
- American Association of State Highway and Transportation Officials (AASHTO)
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- Busse, R., 1993. Tips for recycling concrete: Rock Products, v. 96, no. 9, p. 51-56.
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- ———1997b, Natural aggregates—Foundation of America’s future: U.S. Geological Survey Fact Sheet FS-144-97, 4 p.
- Texas Department of Transportation, 1996, TxDOT recycles: Austin, Tex., University of Texas, CD-ROM.
- U.S. Army Corps of Engineers
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