"Pollution is nothing but the resources we are not harvesting. "
R. Buckiminster Fuller
In may seem odd, or at best premature, to discuss opportunities for the processing and marketing of recycled construction and demolition debris. Along with all the other scrap markets, the market for recycled C&D materials crashed and burned last year along with the collapse of the financial sector. But what didn’t collapse was the desire of industrializing nations such as China, India, and a dozen other developing countries to improve their economic standing and achieve a level of material prosperity for their people on par with that of the West in general and America in particular.
The United States (307 million people), with only 4.5% of the world’s population, annually consumes approximately 25% of the world’s resources (energy, water, food, raw materials, etc.). Together the total populations of China (1,338 million) and India (1,166 million) alone represent 37% of the world’s population (6,790 million). To achieve the same levels of consumption as Americans, the peoples of China and India would have to consume the equivalent of over 200% of the world’s current production of resources. That won’t happen tomorrow, if it ever happens at all, but it remains the eventual goal of these newly industrialized powers. The factories that previously fed this demand and consumed those resources may have been idled by the world financial crisis, but they are still there. When the economy recovers, those factories will start up again. Needles to say, the continued growth of Asia’s emerging economies will create growing demand for raw materials of all kinds that can be used either to manufacture consumer goods or to build much needed infrastructure.
This brings us back to recycled construction and demolition debris. Traditional sources of raw materials (mines and forests) will not be able to keep up with demand. Demand-driven price increases will once again make recycling of all kinds of waste materials a lucrative enterprise. The recycling of construction and demolition debris will take on renewed importance as a source of raw materials. Everything from copper wire to crushed concrete to structural steel to fancy wood flooring can be reused for construction purposes.
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Photo: Diamond Z
Tub grinders and wood chippers handle material that typically must be screened for dirt, sand, rocks, and other abrasives before it can be processed. |
Will America be able to meet the demand for recycled goods? Don’t forget that America also has an infrastructure program: the Obama administration’s stimulus package. Hopefully we will be seeing its positive effects before the end of the year. It should be remembered that significant quantities of debris are produced by construction activities, not just by demolition. As for demolition, whole sections of traditional industrial cities in the Rust Belt that have been gutted by economic woes are slated to be razed under the proposed “shrink to survive” program. Beginning with Flint, MI (once the headquarters of General Motors), 50 cities will see empty neighborhoods and defunct business districts dozed and returned to nature as forests and meadows. With these and other programs (along with the debris generated by normal construction activity) there will be no shortage of recyclable C&D debris.
The only question remaining for the pending upturn in C&D debris recycling is how C&D debris managers can best capitalize on this market with the most efficient processes and cost effective equipment.
What Exactly Is C&D Debris?
When witnessing a pile of rubble created by either a natural disaster or a deliberate demolition effort, it is difficult to differentiate the mass of materials into individual components, let alone types of materials. C&D debris, by definition, is highly variable, consisting of multiple types of materials in variable quantities, depending on the type of structure being built or demolished. The state of Florida has an official (if lengthy) definition that illustrates the variety of materials that are defined under the heading of “construction and demolition debris”:
“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.”
Even relatively simple C&D projects—tearing up a concrete paved roadway for example—would result in several different types of materials being produced (steel rebar, aggregate, and broken concrete). More complicated building structures can involve the use of brickwork, cat-in-place concrete electrical cables, plastic conduits, metal pipes, asphalt roof shingles, marble facades, glass planes, wooden frames, aluminum studs, concrete slabs, floor tiles, brass doorknobs and expensive statuary. Even those items that are not easily recyclable or lack a market demand that makes their recycling worthwhile have to be dealt with in the course of extracting more valuable materials. Each of the deliberately recycled materials requires a unique effort involving individual processing and separation, often with equally varied types of equipment and processes.
Grouping these various kinds of materials into similar categories provides some simplification. Nationwide, on average, construction and demolition debris consists of the materials shown in Figure 1 and Figure 2 (with percentages by weight).
But these categories are somewhat artificial and can obscure real differences. “Wood” can include wooden doors, window frames, and wooden studs. “Plastics” can include electrical conduits, plumbing fixtures, light and outlet plates, and exterior siding. “Metal” can include electrical wire, air ducts, water pipes, and fire-control sprinklers. “Miscellaneous” can include everything from stored chemicals and carpeting to cork boards and rolled insulation. No matter how you try to simplify it, processing C&D debris is by its very nature a complicated process.
As the name suggests, debris is generated by demolition and construction activities alike. Another significant source of debris is the renovation of existing structures. Building and structural demolitions generate almost half of all the construction and demolition debris produced in the United States, about 48%, or 65 million tons per year. Renovations account for 44%, or 60 million tons per year.
Finally, construction sites generate 8%, or 11 million tons per year. How much of this is recoverable depends on the nature of the activity, its location, market demand for the potentially recycled materials, available equipment, the skill of the labor pool, and a dozen other factors. How each type of activity affects debris generation and recycling is examined below.
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Photo: Doppstadt
C&D debris is highly variable, often consisting of multiple types of materials in variable quantities, depending on the type of structure being demolished. |
Site-Clearing Operations
Clearing-and-grubbing operations are usually the first step in any new construction project. They differ from standard demolition tasks in that they mostly remove structures and objects created by Mother Nature instead of by men. By any measure, clearing-and-grubbing operations are the simplest forms of construction recycling, since they usually produce only one type of potentially recyclable material in significant quantities, green waste. Since they only produce one general type of waste, and need little if any manual separation, these operations can emphasize machines over labor.
Furthermore, onsite processing of the clearing-and-grubbing debris is preferable for economic reasons. Greenwaste has a relatively low density and many clearing-and-grubbing operations create large volumes of debris. So hauling it offsite is simply not practical for most locations. On the other hand, it makes perfect financial sense to bring greenwaste-processing equipment (tub grinders, wood chippers, shredders, and portable screeners) onto the site to process the debris. The resultant mulch or wood chips can be used for environmental (promoting plant growth) and aesthetic (landscaping) purposes. The processed material can be stockpiled and later used onsite or shipped offsite. Once reduced to small particles, greenwaste has much overall density and is easier to handle. This makes it economical to load it into trucks for hauling offsite.
Tub grinders are the primary means for processing bulk quantities of greenwaste and wood construction debris. These are largest machines used in clearing-and-grubbing debris processing, often requiring a large semi-trailer truck to haul them out to the site, and can process large tree trunks or other material having a material diameter of 8 feet or more. They use carbide-tipped flail hammers to pulverize wood rather than cut it, grinding it rather than slicing it.
Next in size are the smaller and more portable wood chippers. These are use to render smaller objects (brush, branches, and small trees) into mulch. Not as powerful as tub grinders, they still provide a great deal of operational flexibility. In fact, the ideal combination for most greenwaste-processing jobs would be one or two large tub grinders augmented by several smaller wood chippers.
Tub grinders and wood chippers manage material that typically needs to be screened before it can be processed. The purpose of the preprocessing screening is to remove dirt, sand, rock, and other small abrasive objects that could cause significant wear and tear to the grinders and chippers.
Screeners designed to work in conjunction with grinders and chippers are easily portable and specially designed (throughput capacity, screen opening sizes, etc.) to the task. The additional cost of a screener more than pays for itself in reduced maintenance and repair costs to the processing units. Screening units combine vibratory action with variable-opening screen separation. The vibratory action causes the smaller objects to fall through the screens and dissociates small objects that are adhering to larger objects. However, very large objects such as massive tree trunks would not normally be fed through a screening unit.
New Construction
As previously noted, demolition is not the only source of construction-related debris. During construction itself, significant quantities of waste materials are generated. This is due to the fact that it is difficult to impossible for project managers to calculate a perfectly accurate bill of materials for even relatively simple construction projects. Not every screw, foot of conduit, panel of insulation or ton of cement can be perfectly accounted for beforehand. Furthermore, the act of construction is never 100% efficient. Mistakes get made; materials are wasted, amounts and quantities get misread measured or measured wrong.
As such, the typical project planner errs on the side of caution and orders somewhat more material then he initially calculates that he will need. This materials overage is preferable to halting work and awaiting the delivery of additional construction materials. A work stoppage is far more expensive than the purchase of more material than will actually be needed. The amount of extra material is based on established field wastage rates based on the results of previous projects of similar size and scope. Given the variable skills and experience of workers and contractors and the unique site-specific characteristics of any construction effort, more or less wastage may occur than was planned on. At the end of the project, the unused excess materials of all kinds end up as debris or individual items for salvage.
In contrast with the recycling of greenwaste, the recycling of construction debris requires far more labor. Construction waste, unlike greenwaste, is highly variable, dispersed, and is generated in relatively smaller quantities over time. Clearing-and-grubbing, on the other hand, is usually accomplished in a short time frame, generating a great deal of waste during that short span. Debris from construction activities, however, is generated in dribs and drabs over a more extended period. These factors make it impractical to utilize large equipment to recycling construction debris.
For such relatively small amounts of material, it is typically not necessary to have a separate subcontractor who is primarily responsible for recycling. Manual removal from a job site is typically the only practical means of handling construction debris. Equipment removal of debris from often-enclosed work areas is simply not practical or desirable given the anticipated quantities. Two practical approaches are available to the contractor. First, merely dump all the leftover construction debris of all kinds into a single dumpster and then haul the dumpster loads to a nearby materials recovery facility, which will then separate the debris into different types for recycling and resale. The second option is to have individual tubs for different types of debris, and a work force trained to differentiate the debris when it is thrown out. Of the two, the first is preferable for its simplicity. Furthermore, the time and training needed for the work force to self-recycle will detract from its overall productivity and primary goal of completing the construction effort. The value of the lost labor time will simply not be justified by the amount earned through recycling the debris.
Debris From Infrastructure Improvements
Infrastructure improvements can include rebuilding a bridge, widening a road, expanding a sewer system, and tearing up old utilities, replacing them with new lines. This effort involves both the demolition of existing infrastructure and the construction of new infrastructure. Of the possible debris-creating scenarios, that of infrastructural improvements is the most variable in terms of quantities of debris generated and the proper means of handling it.
Demolition work associated with infrastructure improvements and expansions will for the most part be done with heavy equipment since this activity typically involves the movement of large masses of concrete and earth. The ripping up of old pavement requires heavy equipment; the excavation and removal of sewers and other utilities requires backhoes; the removal of large chunks of concrete requires front-end loaders and trucks; and the separation of steel rebar from reinforced concrete requires crusher and magnetic separators.
However, once the dust settles and the contractor is left with piles of debris that have been broken off of the original structures and hauled to a stockpile, manual labor can become the predominant activity. Some mixing of different types of materials is almost inevitable, and that may require hand-sorting prior to further processing. Once unrecyclable materials are removed and special items separated, a second round of machine processing is usually necessary to completely process the debris. Concrete (pavement, piping, retaining walls, etc.) would be processed through a screening operation to remove earth and then passed through another crusher to render it down to aggregate sized particles that could later be used for road beds, drainage materials and foundations.
The important factor is the amount of recyclable material expected to be generated by any particular infrastructure improvement project. There is a certain minimum quantity of material (determined by such factors as market price of the recyclables, transportation costs, equipment operating and rental costs, etc.) that will justify the use of material-processing equipment and sorting operations. Should a proposed infrastructure improvement or expansion project result in insignificant quantities of debris (for example, the plan may call for most of the existing infrastructure to be abandoned in place) attempts to recycle the debris may not be economically justified.
Demolition Debris
Demolition is the “big one,” the activity that typically generates the greatest quantity and widest variety of materials suitable for recycling. Since (unlike construction activities) demolition generates significantly large amounts of potentially recyclable materials, and (unlike clearing and grubbing) it generates a wide variety of materials, what makes an effective recycling effort for demolition activities needs to be reexamined. Basically there are two recycling methods to choose from: source separation or commingled.
Source separation occurs onsite and is performed by the construction crew itself or by a subcontractor specifically hired to perform this task. Individual types of recyclable materials are separated and temporarily stockpiled or stored onsite prior to transport offsite to the recycling market. Each individual stockpile constitutes a near-pure load of similar materials free from cross-contaminants, nonrecyclable materials, or other types of recyclables. Source separation usually results in higher quality of recycled materials and higher rates of recycling.
Commingled recycling, however, simply lumps all the debris together into a single stockpile or wastestream that is then hauled offsite for processing, separation, and sale. This presents greater difficulties resulting from the presence of nonrecyclables and contaminants. Furthermore, the contractor will be paying to haul a portion of nonrecyclable material, footing the bill for the transportation of material without any market value. The ratio of valuable-to-worthless materials can vary greatly from project to project and even from truckload to truckload. The subsequent separation of recyclable materials from a commingled pile of debris is often more difficult and time-consuming than upfront source separation.
It is for these reasons that source separation recycling of C&D debris is the preferred method for most projects. This does not necessarily mean that demolition debris recycling should be done exclusively, or even primarily, by hand. Labor is a key element in these operations, but there is an important place for appropriately sized machines as well. Still, source-separation recycling of construction debris represents a building effort performed in reverse order. For many construction projects, the most valuable building materials are those that go in after the bulk structural materials are constructed (metal plumbing pipes and fixtures, glass and lighting treatments, or exterior facades and brickwork). These can be removed and sorted manually prior to the use of the wrecking ball or explosives to demolish the bulk of the structure.
Each type of debris material has its own unique method of source separation and recycling. Concrete is usually processed with specialized attachments to the booms of standardized demolition equipment. Pulverizers are attachments that literally bite through concrete and its reinforcing steel. Concrete is also broken up with standard wrecking balls and vibratory drill attachments.
Once crushed, concrete chunks are further processed by portable crushers. The crushing process can also free the rebar embedded in the concrete. Rebar can be removed with overhead magnets working with the crusher unit. The crushed concrete then proceeds to a screener and sorter, which separates the concrete into stockpiles of uniformly sized materials (either cobbles, aggregate or pebbles)
Steel is chopped up by steel cutters that resemble large scissors designed to shear through rebar, plate, and structural steel. Steel storage tanks are cut up with specialized shears prior to removal.
Bricks are typically handled manually, with labor used to sort and stack the bricks onto pallets. The removal of brick from the demolished structure, on the other hand, is usually done with a “brick-picker” attachment. These are sharp points with hooks that can puncture a brick wall with a forward motion and catch the wall with its hook on the reverse motion. As the brick wall falls, the resultant impacts usually shake individual bricks loose from their confining mortar. Residual mortar usually has to be chipped off by hand prior to stacking, palletizing, and shipping.
Special items (expensive statuary, artistic renderings, fountains, marble facades, or stained glass) are almost always removed by hand. These items typically have a market resale value that justifies the labor-intense effort and associated high shipping costs. Care must always be taken to ensure that potentially hazardous materials are handled safely and appropriately.
Summary
Clearing-and-grubbing operations produce large quantities of a homogenous waste (greenwaste) that can be easily and almost exclusively handled by machinery.
Construction operations generate relatively small quantities of a heterogeneous waste that has to be handled almost completely by labor.
Infrastructure improvement operations can result in a wide range of quantities and debris types that can either be handled by equipment, or manually, or both, depending on the kind of project.
Demolition operations produce large quantities of heterogeneous waste that should (at least initially) be processed manually.
In general, recycling of construction and demolition debris makes financial sense only if two factors are present: a strong market demand for the recycled materials that will ensure high and stable prices over the duration of the project, and easily obtainable equipment and facilities to process the debris. Cost avoidance (not having to send the debris to a licensed landfill and paying the tipping fee) is also a factor, as is transportation cost (either of recyclables to the market or debris to the landfill). An apples-to-apples comparison requires that all these cost factors be examined beforehand.
In addition to the obvious upfront financial benefits recycling brings to the demolition company or building contractor, the processing and recycling of construction and demolition debris has many indirect social, economic, and environmental benefits. These include job creation, resource conservation, avoidance of landfilling, and spurring economic activities through increased operational efficiencies.