Authors use a material flow analysis to determine more than 600 million tons generated annually, twice as much as originally thought

By Kimberly Cochran, Phd and Timothy Townsend, Phd

There have been several estimates conducted to determine how much C&D material is generated. All have used a weight-per-construction-area approximation; national estimates using alternative procedures such as those used for other segments of the solid waste stream have not been reported for C&D. A materials flow analysis (MFA) approach for estimating C&D debris generation and composition for the United States has not been attempted. The consumption of construction materials in the U.S. and typical waste factors used for construction materials purchasing were used to estimate the mass of solid waste generated as a result of construction activities. Using various historical construction materials consumption data and estimates of average service lives of the materials, the generation of demolition materials was estimated. The MFA approach estimated approximately 680 million to 860 million tons of C&D debris was generated in 2002. This prediction is more than double all previous estimates on the amount of C&D generated.

Past C&D debris generation estimates in the U.S. have focused on one sector of the generating industry or one region. Most regions in the U.S. do not track the amount of C&D generated or do not have sufficient knowledge of the waste composition. Other methods must be used to estimate C&D debris generation where the material is not tracked.

Franklin Associates (1998) and the U.S. Environmental Protection Agency (EPA) (2009) estimated the amount of C&D debris generated in the U.S. using an approach similar to the waste-weight-per-construction-area method reported by Yost and Halstead (1996) to calculate the amount of drywall generated in a specified region.

This method has been used to estimate C&D wastes in Florida (Cochran et al., 2006) and Massachusetts (Wang et al., 2004). These estimates investigate building-related C&D debris only.

Another methodology used to estimate C&D debris generation is to require facilities that receive C&D debris to report amounts. Florida requires all C&D debris disposal and recycling facilities to report the amount of material they accept, but they are not required to report the composition of the waste stream. It is difficult to use this method nationally, as Florida is one of only a few states that require such reporting.

Also, each state has its own regulations and definitions regarding C&D debris management facilities, the numbers reported may reflect different sources or types of debris. So collating the information and interpreting it to compare apples to apples will be difficult. The lead author attempted in 2006 to certify the Yost and Halstead method by comparing actual data collected from C&D debris management facilities to a Florida estimate using a variation of the Franklin Associates method. But Florida facilities accept other C&D materials not included in the other methods, such as land-clearing debris, pallets and debris from non-building-related sources such as roads and bridges. Since their estimate was made for building-related material in Florida only, one cannot compare the results of the prediction to the amounts in the regulatory reports.

Several states have estimated C&D composition and generation by waste facility sorts, visual characterizations and monitoring. This requires the examination of a large number of waste samples to gather a representative estimate, a time consuming process that presents difficulties for a waste stream with bulky, heavy materials, such as C&D. This approach is good for regional waste investigations, but is difficult to apply nationally.

MFAs examine the amount of materials that come into service in a given time range and predict when those materials will come out of service as waste. Adjustments are also made for exports and imports. The U.S. EPA has been using the materials flow method to characterize MSW in the U.S. since the late 1960s. The agency uses production data (by weight), average product lifetime and some waste composition studies to determine the amount of MSW generated in the U.S. and its composition. C&D debris generation and composition has not been estimated this way.

The U.S. EPA method has produced results acceptable to many, as evidenced by frequent citations but there are no other estimates or definitive numbers for comparison. Additionally, since this method quantifies waste based on the amount of materials consumed and not from waste composition data, this method was selected as it may be able to predict waste generated from other non-building structures, such as roads, bridges and other infrastructure, which has not yet been quantified nationally due to lack of composition data.

We evaluated the MFA approach for the C&D debris stream to determine if it is a reliable approach to estimating the entire C&D materials stream in the U.S. Our first step was to determine the historical consumption of building materials. We developed those figures using data from government agencies and industry groups that provide specific production data for various construction materials. We also took into account variables such as how much material was wasted during construction and disposed of then. While more complex than explained here, this summary provides the basis for our assumptions.

Once the historical consumption of construction materials is known, the amount of construction and demolition debris generated in 2002 can be calculated. First, Eq. (3) is used, multiplying the average percentages of each material that is disposed during construction by the total amount of construction materials consumed in 2002 to determine the amount of construction waste in 2002. Materials used in the past will become the demolition waste of 2002. The service life of each material is set by the year of consumption. The consumption data of that year, less the amount of material discarded during construction during that year, is used to calculate the amount of demolition waste, as shown in Eq. (5).

Eq. 3

 

Eq. 5

 

Results
Our results show the U.S. consumed about 124,968,000 million tons of construction materials in 2002. The majority at about 678,960,000 tons was portland cement concrete consumption. In fact, concrete was the most consumed construction material for the entire century examined.

Asphalt concrete (or pavement) became the second most consumed construction material in the mid-1920s, and the U.S. consumed 393,600,000 tons in 2002. Prior to the mid-1920s, wood was the second most consumed construction material, but has remained in third place ever since. In 2002, the U.S. consumed approximately 88.5 million tons of wood. However, the consumption of portland cement and asphalt concrete dwarfs the consumption of the other construction materials. Figure 2 shows the historical U.S. construction material consumption.

 

Figure 2: Mass of construction materials consumed in the U.S. from 1900 to 2000, compiled using amounts for drywall, iron and clay tile reported by USGS and amounts for concrete, wood, asphalt, shingles, brick and asphalt pavement estimated using or converting data on materials or components reported by governmental or industrial organizations.

Total amount of C&D generated in 2002 was an estimated 600 million, 620 million, or 767.5 million tons, depending on the assumptions for how long materials last in the built environment. If construction materials last longer, then construction activity would generate fewer waste materials because the amount of materials produced has increased over time from population and economic growth. Figure 3 shows the amount of waste each job type contributed to the total amount of waste. Bars in Figure 3 show the range of values, as estimated by using the range of material service lives. The diamond value shows the anticipated waste calculated using typical service life values for each material. Road and bridge demolition produced the largest amount of waste, while buildings and other structures accounted for comparable amounts. The spread in the total values reflects the variability in debris amounts from each waste stream.

Figure 3: Ranges of estimates for the amount of C&D debris generated in the U.S. produced using an MFA approach (by job type in 2002).

The estimated material composition of the total C&D stream in 2002 is shown in Figure 4. The three compositions represent varying service life assumptions. As expected from the construction materials consumption amounts, concrete represents the largest segment of the waste, followed by asphalt. Composition of course varies as material usage through time changes based on market conditions and construction styles. Economy fluctuations affect the marketability of some materials. For example, the long service life assumption for asphalt was 33 years. Thus, asphalt concrete consumption in 1969 was evaluated, when consumption of this material was on its way up (Figure 2). In contrast, the typical service life assumption for asphalt was two decades. Asphalt concrete consumption was evaluated for 1982 for this assumed service life, when consumption of this material had declined (Figure 2). This is reflected in Figure 4, where the composition when the long service lives are assumed has a larger proportion of asphalt than the composition when a typical service life is assumed, when the opposite is expected. Also affecting the debris compositions are building codes changes, which then changed construction styles. New construction techniques have been developed.

Figure 4: Estimated U.S. C&D debris composition in 2002 from all job types produced using an MFA approach (by assumed service life of material).

Discussion
How much C&D is actually generated in the United States is unknown. The U.S. EPA estimates calculated the amount of debris from building-related sources only for 2003. Thus, it is only possible to compare the amount of building-related debris calculated in both estimates. A comparison of the U.S. EPA and MFA estimates is presented in Table 1, which shows the large disparity between the U.S. EPA estimate (151 million tons) and the MFA estimate using short service life assumptions (216 million tons). The U.S. EPA estimate is halfway between the MFA estimate that uses typical service life assumptions (187 million tons) and the MFA estimate that long service life assumptions (108 million tons).

Table 1: Comparison of estimated building-related C&D debris
generation amounts calculated using two different methodologies.

Note: Totals may not add due to rounding.
* No independent renovation waste estimate was produced for the MFA approach. Instead, the construction (or installation) portion of the renovation waste was included in the construction amounts and the demolition (or removed) portion of the renovation was included in the demolition waste estimate.
** USEPA (2009).

There could be a number of reasons for this difference. Because the EPA study uses composition studies to estimate material generation, it could underestimate heavier materials such as concrete, if those compositions do not accurately reflect the materials actually used nationally. More composition studies more reflective of the variety of construction styles in the U.S. would help resolve this problem. On the other hand, the MFA may overestimate the amount of material demolished. The accuracy of the MFA estimates of when or how much material is taken out of service is as good as the service life assumptions used. Also, the MFA assumes all material is removed and enters the waste stream for disposal or recycling at the end of its service life. It is possible, however, some portion of the materials is abandoned in place rather than removed and discarded. Since the U.S. EPA study takes the amount of construction, renovation, or demolition activity into account, it would not take abandoned materials into consideration. For example, an abandoned house not demolished is not counted for the EPA study, while the MFA method assumes all houses will be demolished at a set time. Finally, the MFA results may include materials disposed during the final product manufacturing process. For example, some of the concrete produced may actually get discarded by the manufacturer it is off-specification or otherwise unusable.

Concrete is the most consumed construction material, three times that of asphalt and 16 times that of wood, the second and third most consumed construction materials. Reasons for this include the heavier density of concrete and its ubiquity in construction uses, whether under water or on land. So errors in the assumptions to calculate concrete waste have the most dramatic effect on the total amount of waste generated. The exact amount of concrete taken out of service each year is unknown. Without that information it is difficult to evaluate true service life, especially for the ‘‘other structure” category. Estimates of waste from structures that have well-studied service lives, such as buildings, roads and bridges will be more accurate than other structures that are not as studied.

The Construction Materials Re-cycling Association (CMRA) estimates approximately 177 million tons of concrete are generated each year from all sources of debris. Its estimate is based on surveys of demolition contractors and recyclers who claim that 50% to 57% of concrete is recycled and that they recycle approximately 89.5 million tons of concrete (Sandler, 2003). The MFA approach estimates that 255.8 million to 383.7 million tons of concrete waste are generated each year, with 19.6 million to 128 million tons arising from building sources, 128 million to 167 million tons from roads and bridges, and 68.8 million to 128 million tons from other structures. Thus, the MFA approach estimates the generation of waste concrete to be 1.4 to 2.2 times the amount that the CMRA estimated. The discrepancy could arise from the CMRA underestimating the amount of concrete actually generated and not recycled. Or, the discrepancy could result from overestimating of the amount of concrete removed from service, not simply abandoned or used longer than the assumed service life, in the MFA approach.

A report published by the Federal Highway Administration (FHWA, 1993), in cooperation with the U.S. EPA, estimated the United States generated  89.5 million tons of asphalt concrete waste, with 80% of this amount recycled. The MFA approach found that 167 million to 295 million tons were generated. In comparing value put-in-place data, the U.S. spent 1.26 times more on streets and highways in 2002 than in 1993 (in constant 1996 dollars). Thus, adjusting the USDOT/USEPA figure to 2002, the amount of asphalt waste generated in the U.S. can be estimated as 108.2 million tons. The USDOT/USEPA report, however, garners its results from a survey of state departments of transportation that likely did not report asphalt concrete generated from non-highway applications, such as parking lots. It is also possible the states did not report the amount of waste generated from county- and city-owned roads.

In addition, the MFA approach assumes that all asphalt paving will have the same service life as a highway. This assumption may not be valid as asphalt paving is used in parking lots and other applications, which may put less strain on the material and allow the material to last longer. Additional information on the proportion of asphalt paving that is used in those applications and their service lives are needed.

In general, data sources play a significant role in the accuracy of the results from using an MFA approach. The sources of much of the data rely on industry surveys. Therefore, many numbers rely on the accuracy supplied by the respondents to those surveys. The more accurate these numbers are, the better the results will be.

Conclusions
An MFA approach was used to calculate the total amount of C&D generated in the U.S. in 2002 using the total amount of construction materials consumed over time, the average service lives of these materials, and the portion of materials scrapped during construction. The total amount of C&D debris generated was estimated between 600 million to 767.5 million tons, depending on the assumptions for material service lives. The range of C&D debris composition was 42% to 59% portland cement concrete, 26% to 43% asphalt, 6% to 7% wood, 1% to 3% brick and clay tile, 2% to 3% asphalt shingles, 1% to 2% gypsum products, and less than 1% steel and iron.

Two of the MFA estimates for building-related C&D debris utilizing short and typical material service life assumptions (216 million tons and 186 million tons, respectively) are 1.4 and 1.2 times, respectively, previous estimates of building-related C&D debris generation in the U.S. (151.5 million tons). The MFA estimate for building-related C&D debris utilizing long material service life assumptions of 108 million tons was close to the EPA estimate for building-related C&D generation (151.5 million tons).

Assumptions used for the service life of construction materials, especially that of concrete and asphalt concrete, will have the largest impact on the total amount of debris generated. Thus, more studies are needed as to the average actual service life of most materials.

Additionally, the MFA approach would benefit from improved estimates for the portion of materials that are abandoned and those discarded during the product manufacturing stage.

C&D is a large waste stream of concern. Even if the MFA estimates are high, examination of the materials consumed in the U.S. show there is a great potential for future C&D waste generation. With additional adjustments, this MFA approach can be utilized for estimating the total amount and composition of C&D debris generated in the U.S., or any other region with material production and service life data.

Dr. Cochran is an environmental engineer with the U.S. EPA and Dr. Townsend is an associate professor with the University of Florida.

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Sustainability and Building Materials

During the early part of the 20th century, the U.S. used more renewable resources such as wood and fewer nonrenewable materials such as concrete than today. When only building-related C&D debris is examined, a better understanding can be gained of the impacts from the other materials beyond those used heavily
in roads, bridges and other infrastructure.

Figure 5 presents the composition of building-related C&D using three different structure life assumptions. This figure reflects the increase in use of nonrenewable resources (such as portland cement concrete and steel) in construction from 1900 to 1950, while the use of renewable resources (such as wood) in construction has decreased. Overall, portland cement concrete is the dominant material used throughout the 100 years examined.

Figure 5: Estimated composition of U.S. building-related C&D debris produced using an MFA approach (by assumed length of building life).

Figure 6 presents the historical consumption of total materials and forecasts future consumption based on current consumption trends. Despite not accounting for the recent downturn in the economy, the 100-year trend predicts a downturn in consumption based on historical trends. Figure 7 presents projections of waste generation from 2002 to 2052. Projections were made using consumption data for those materials that last 50 years or more. For those materials that last less than 50 years, consumption trends were used to determine their approximate value up to 50 years. The difference between the three waste estimates increases through time, reflecting the escalation of material consumption (See Figure 1). The shorter service life assumes waste from farther up on the curve of consumption. The longer service life assumes waste from farther back on the curve.

Figure 6: Projected mass of construction materials that will be consumed until 2050 in the U.S. using a trend of construction materials consumption in the U.S. from 1900 to 2000.

Figure 7: Projected C&D generation amounts in the U.S. using (1) historical construction materials consumption trends, (2) the MFA methodology to estimate debris generation, and (3) three difference assumptions for materials service lives.

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