The Use of Typical Recycled Aggregates
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 (Tepordei, 1997a). 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 (TxDOT, 1996). 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.
Technology of Typical Recycled Aggregates
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 13 pictures a typical construction site recycling operation.
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 5 outlines some combinations and considerations for this equipment.
Crusher combinations commonly used in concrete and asphalt recycling
Category Jaw/cone combination Horizontal-shaft impactor Jaw/roll combination
Capacity 180 - 360 tons per hour. reba r a nd wire mesh; cone
crusher cannot; wood a
problem for both. Accommodates wide
variety of feed material.
Accommodates rebar and
wire mesh Wear on
equipment Amount of wear on
equipment is low. Wear higher than
jaw/cone combination. Wear low for jaw crusher;
wear for roll high if
aggregates are abrasive. Primary feed Accommodates concrete;
less suitable for soft
asphalt concrete Mainly suitable for
asphalt. Accommodates both
asphalt and concrete
materials. 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
cone crusher. Wide variation in crusher
design. Not applicable.
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 stone 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 (Busse, 1993). 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.
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 (Tepordei, 1997a). 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 (TxDOT, 1996). 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.
Technology of Typical Recycled Aggregates
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 13 pictures a typical construction site recycling operation.
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 5 outlines some combinations and considerations for this equipment.
Crusher combinations commonly used in concrete and asphalt recycling
Category Jaw/cone combination Horizontal-shaft impactor Jaw/roll combination
Capacity 180 - 360 tons per hour. reba r a nd wire mesh; cone
crusher cannot; wood a
problem for both. Accommodates wide
variety of feed material.
Accommodates rebar and
wire mesh Wear on
equipment Amount of wear on
equipment is low. Wear higher than
jaw/cone combination. Wear low for jaw crusher;
wear for roll high if
aggregates are abrasive. Primary feed Accommodates concrete;
less suitable for soft
asphalt concrete Mainly suitable for
asphalt. Accommodates both
asphalt and concrete
materials. 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
cone crusher. Wide variation in crusher
design. Not applicable.
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 stone 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 (Busse, 1993). 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.
SHARE
