So you have brought down a building and torn up a foundation. What was once a complicated structure of brick, concrete, steel, drywall, wood, and a dozen other materials has been reduced to a pile of rubble by explosions, wrecking balls, and simple crowbars. Now what? Treat it as waste and haul it off for final disposal in a landfill? Or do you process it further and use it to create new building materials and other useful byproducts?
This article focuses on the last alternative and examines the equipment and the processes that are needed to properly recycle bulk quantities of construction and demolition (C&D) debris in a cost-effective and timely manner.
C&D debris, by its very nature, is a highly variable mix of often completely unlike materials. Materials as different as electrical cable, roof shingles, glass planes, wood frames, concrete slabs, floor tiles, and even the kitchen sink can be produced by demolition operations.
Each material presents a unique challenge for individual processing as well as the separation from the whole mass of debris. Processes and equipment are as varied as the debris they are handling.
What is C&D Debris?
Many states do not have a formal definition of what C&D debris actually is. One that does is Pennsylvania, which legally defines C&D debris as: “Solid waste resulting from the construction or demolition of buildings and other structures, including, but not limited to, wood, plaster, metals, asphaltic substances, bricks, block and unsegregated concrete.”
The state of Florida has a similar but more extensive definition: “Discarded materials generally considered to be not water-soluble and nonhazardous in nature, including but not limited to steel, glass, brick, concrete, asphalt material, pipe, gypsum wallboard, and lumber, from the construction or destruction of a structure as part of a construction or demolition project or from the renovation of a structure, including such debris from construction of structures at a site remote from the construction or demolition project site. The term includes rocks, soils, tree remains, trees, and other vegetative matter that normally results from land clearing or land development operations for a construction project; clean cardboard, paper, plastic, wood, and metal scraps from a construction project.”
As can be seen from these definitions, construction and demolition debris is a heterogeneous accumulation of highly variable materials. Depending on the source of the debris the percentage of the total taken up by any one type of materials can vary greatly. Building demolitions account for 48%, or 65 million tons (59 million tons) per year, of the C&D wastestream. Renovations account for 44%, or 60 million tons (54 million tons) per year. And construction sites generate 8%, or 11 million tons (10 million tons) per year. Nationwide, on average, construction and demolition debris consists of the following materials (by weight):
- Drywall—14.4%
- Roofing—12.0%
- Cement, asphalt, brick—23.3%
- Wood—27.4%
- Plastics—0.5%
- OCC/paper—2.7%
- Metal—8.8%
- Miscellaneous mixed—11.9%
It is important to remember that debris is produced by construction operations as well as demolition activities. Demolition is an obvious source of debris. Indeed, the whole point of demolition is to render buildings and other structures into smaller bits and pieces that can be easily removed from the demolition site. However, construction also produces its share of debris, though in this case it is primarily the result of over ordering construction materials. Most structures are very complicated with lengthy materials lists and difficult-to-estimate quantities. No architect, engineer, or contractor will ever know the exact number of bricks required to complete a façade, the length of cable to provide electrical power, or the number of tiles needed to line every floor. As a result, most contractors and suppliers estimate their material needs based on past practice, tending to err on the side of caution by ordering more materials then they estimate they will need. The final materials estimate is based on assumed field wastage rates. This materials overage is preferable to a work stoppage caused by the delivery of too few materials. At the end of the project, the unused excess materials of all kinds (except specialty items that make sense to salvage), end up as debris.
Specific debris items have unique characteristics and specific potentials for processing and reuse. Carpet generated by demolition operations amounts to approximately 2.6 million tons (2001 data). However, carpets and rugs are difficult to reuse and recycle. Price volatility in the used plastics markets (which depend on the demand for recovered nylon) limits the demand for processed carpeting to limited markets such as donations to home building charities such as Habitat for Humanity. On the other hand, aggregates generated by crushing concrete, brick, asphalt, and other rubble are in demand for road building and foundation construction activities. The processed aggregate can be used as drainage and filter material, as a component of Portland cement concrete or asphalt concrete, as a stable subbase for foundations, fill for utility trenches, or as the base layer in roadway construction. Given the ongoing demand just for roadway repair alone, the demand for recycled debris aggregate made from process debris remains consistently high. And some recycled materials (such as copper tubing, piping, and wiring) are so valuable they are the targets of criminal thefts.
The amount of C&D debris as a percentage of the total wastestream can vary regionally from 20% to 45%. The actual volume amounts and the resultant airspace demand for landfill disposal also vary because of the fact that the density of each debris type can also vary considerably. For example, construction debris consisting of loose asphalt and concrete can weigh about 2,400 pounds per cubic yard. However, construction debris consisting of wood and wood products can weigh only 400 pounds per cubic yard (800 pounds per cubic yard if compacted). By comparison, MSW typically weighs about 800 pounds per cubic yard. Of the total amount of debris generated in the United States, 30%–40% is sent to municipal landfills, 35%–45% is sent to specialty C&D debris landfills, and 20%–30% of C&D debris is recycled. The estimated potential for debris recycling is as high as 70%.
Care has to be taken during the handling and processing of C&D debris due to the presence of “universal wastes.” Universal wastes are defined as “common hazardous wastes that are generated by a variety of people and businesses, and are generally not allowed to be disposed of in solid waste landfills.” These include electrical equipment (fluorescent tubes, switches and thermostats that contain mercury, and batteries), lead piping, and paint coatings containing lead, as well as friable asbestos in the form of fire control insulation and shingles. Whether the debris is destined for disposal in a landfill or as the raw material for recycling operations, these hazardous waste components have to be separated (by hand if necessary) from the overall wastestream.
When does it make sense to process C&D debris? For the processing and recycling of C&D debris to occur, two things have to be present. First, the equipment and facilities necessary to physically process the debris have to be locally available. These can be collectively referred to as the debris recycling infrastructure. Second, there has to be a market demand for the recycled debris materials, a demand that supports a sales price of the processed debris that (combined with the beneficial cost avoidance of not having to transport and dispose of the debris in a landfill) offsets the operational costs of processing the debris and makes the whole effort profitable.
Like any other recycling effort, the processing of C&D debris has many potential direct and indirect benefits (job creation, resource conservation, reduction of greenhouse gas emissions, and avoiding the cost of building new or of expanding old landfills). While these indirect benefits will be of interest to local governments looking to spur economic activity via regulations mandating the recycling C&D debris, it is the immediate and direct financial benefits that will be most appealing to individual contractors.
C&D Processing Methods
The costs and benefits of C&D debris recycling will directly depend upon the method utilized. There are two primary recycling methods: source separation and commingled recycling. Source separation is defined as a single kind of recyclable C&D waste material that has been separated from other C&D waste materials at the site of remodeling, repair, construction, demolition, or land clearing before it is transported to a receiving facility. The resulting stockpile(s) consist of pure loads that contain at least 90% recyclable C&D waste materials by volume. Commingled recycling, on the other hand, consists of processing recyclable C&D waste that contains mixed types of recyclable materials stored in one onsite container that is taken to a sorting facility where materials are separated for recycling. Commingled waste can also include significant quantities of nonrecyclable materials. The resulting load consists of mixed waste that is made up of more than 10% but less than 90% recyclable C&D waste by volume.
| Nationwide C&D Composition |
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| Wood remains a chief component of construction and demolition debris. |
Of the two, source separation by onsite equipment and personnel is generally the most cost effective and efficient approach. Source separation results in both higher rates of recycling and higher-quality recycled materials. This does not imply that the recycling is done exclusively or even primarily by hand (though there is always a great deal of manual work during demolition recycling operations). Source separation relies on the use of individual containers for receiving unique material types (bricks, tiles, piping, shingles, or dry wall). This is much easier to accomplish than it seems at first glance. If the intent of the demolition project is to recycling in the first place, the demolition process itself will proceed in stages, with each type of material being removed from the old building like layers on an onion. In this case, such mass-demolition tools as the wrecking ball or explosive charges are avoided (at least until the valuable recyclable materials have been removed). In fact, such a recycling effort resembles a movie of building construction that is run backwards.
However, when job-site space is at a premium, there may not be enough elbow room for onsite source separation. Almost every urban demolition job is cramped for space, so this situation describes the norm for many large-scale demolition operations. In this situation, individual containers dedicated to different recovered materials must be done away with and replaced with general-purpose dumpsters. Unfortunately, separating out commingled is far more difficult and time consuming than source separation. This results in lower rates of recycling, poorer quality of the recycled materials and higher operating costs that reduce the potential profit margins and the incentive to recycle in the first place.
C&D Debris Cost Estimates
No matter which method is chosen, the central question remains: Will the contractor make (or at least save) enough money to justify the effort? The first step in this cost/benefit analysis is the determination of the types and quantities expected to be encountered during the demolition project. This can be done by using previous engineering estimates or the actual bill of materials for the building’s original construction. However, for purposes of both recycling and disposal, these quantities need to be converted into tons. For example, a contractor demolishing a high-rise apartment whose original construction required 10,000 liner feet of 1-inch diameter copper water lines will need to have this quantity converted into tons to determine both the potential tipping fee at the landfill or the current value at scrap metal market prices. It is always a good idea to keep accurate records of both disposal costs and scrap sales to provide benchmarks for this estimate (though less weight should be given to past scrap metal sales prices as the scarp metal market is notoriously volatile). A contractor should not neglect less obvious recyclables such as carpet, plastic, or tiles, as these can also have a decent market value.
Choosing the proper salvage company is the next step. In most cases, a subcontractor should do the actual salvage operation, since most C&D contractors lack the experience, personnel and equipment to perform this task properly. A salvage company that specializes in source separation will bring multiple containers to the job site to handle each type of recycled debris as it is generated. A salvage company that performs commingled recycling will bring out one container for repeated loading and hauling to a sorting facility.
A combination of both approaches is often preferred. In this case a three-container system is established, one each dedicated to valuable recyclables being removed during the current stage of demolition, commingled waste, general waste, and garbage. The first goes directly to sale, the second to a sorting facility, and the third gets deposited in a landfill.
Once the method and materials have been determined, the contractor can determine the costs of recycling versus the costs of disposal. First, the contractor breaks down the various types of recyclable materials and determines their quantities.
Each type will have a per-ton tipping fee cost or sales price. Additionally, each type will have a hauling cost charged per truckload to the contractor by the salvage operation to remove the material from the site.
Finally, each type of material will have a container rental fee for the duration of the demolition phase that the particular material will be removed. The overall cost equation is as follows:
Cost to Recycle = (tipping fee x tons)
+ (hauling fee x loads)
+ (container rent x months)
Note that the tipping fee could be replaced by the sales price of the material if the salvage operation finds it to be a valuable material. However, it is a rare material whose sales price to the salvage operation is so high that it completely offsets the hauling and container rental fees. It is important to remember that unless the contractor is performing the salvage operation itself, the contractor will still be paying the salvage company to remove the material. The cost however, will be defrayed by the subsequent sales earned by the salvage company on the scrap market. As such, the comparison isn’t usually between making money by recycling construction and demolition debris so much as comparing lower recycling costs with disposal costs.
The disposal costs are easier to calculate. The contractor first sums up the quantities of all the potential recyclable material and then applies the landfill disposal tipping fee for the gross tonnage, the hauling cost per load, and the dumpster rental for the duration of the entire project. Between the cost defrayed by subsequent scrap market sales and the lesser duration for container rentals, the salvage operation will tend to cost considerably less than the straight to the landfill disposal option. Note that only the potential recyclable materials are considered in both equations to allow for an apples-to-apples comparison. Nonrecyclable materials will have to be disposed of in a landfill in any case, and should not be part of the comparison.
C&D Processing Equipment
These financial incentives ensure that as much of the building or structure that can be recycled will be recycled. Recycling begins with the demolition equipment that makes recycling of the various building parts possible. Of these, first and foremost are the attachments that can be fitted to the end of booms on standard excavators. These include concrete pulverizers, steel cutters and brick pickers. Pulverizers are attachments with giant jaws that can literally crush chunks of concrete into manageable sizes. The paired jaws consist of a movable front jaw and a static back jaw that allow pulverizers literally to bite through the concrete. This operation efficiently separates reinforcing steel rebar from the encasing concrete. Steel cutters resemble giant scissors that are used to shear through steel girders and rebar. Specialized plate shears are designed to demolish and cut up steel storage tanks. Brick picks resemble pointed hooks. They poke through brick walls, grab and then pull down the brickwork for separate recycling from other materials.
Bricks are usually sorted by hand. Workers manually stack the bricks on pallets after having knocked off any residual concrete or mortar clinging to the brick. Even after all of that manual labor, recycled bricks are still inexpensive enough to market them to regions that lack significant sources of clay. Recycled brick also has an aged patina that many architects and builders find attractive.
Concrete chunks are sent to a portable crusher unit. Steel is fed into the crusher along with the concrete and is freed when the concrete is shattered. Once freed, magnets remove rebar. Larger crusher machines can process over 600 tons per hour and smaller machines can handle 150 tons per hour.
From the crusher, the reduced concrete particles are sent through a screener and sorter. Usually consisting of three raised discharge belts, the sorter separates the crushed concrete via a series of screens into three general sizes (cobbles and larger, aggregate, and pebbles to finer particles).
The cobbles can be used for everything from erosion control riprap to decorative stone.
Aggregate can be used as pipe bedding, foundation course, filter and drainage stone, or in the manufacture of new concrete.
Certain types of construction and demolition debris require multiple stages of processing. The combination of crusher and magnetic separator used to process reinforced concrete is only one example. Asphalt processing is also a multi stage process. It starts with a grinder to reduce its overall size of the torn up asphalt chunks. Further polishing is provided by a second stage pug mill. This reduces the particle sizes further or allows for onsite mixing of the asphalt with other materials.
Picking the correct construction and demolition debris recycling process requires the weighing of several factors.
First is the composition and quantity of the materials. The amount of material that has to be processed over the duration of the project will determine the equipment’s throughput capacity. The material’s composition can vary considerably from masonry to wood to metal to shingles to tile, and each requires a different approach to processing.
Second, the amount of ferrous material will determine the use of magnetic separators.
Third, the physical size of the material will drive the processing. Large concrete slabs, which have to be processed by crushers, will be handled differently from individual bricks that can be dressed and palletized manually.
Fourth, the market specifications as determined by the resale market will affect the type, size, and affect the color of the recycled material.
Finally, the material’s average density and anticipated variance from the mean will affect the processing unit’s productivity rate.