Block Party

Several composition studies have been performed on residential construction debris. All of these studies have involved wood frame houses. None involved concrete-block-frame houses. The purpose of this study is to assist those interested in recycling C&D waste that might deal with concrete block homes. It is also essential information for current work in Florida on characterizing C&D debris composition for the state.

The results of the composition studies performed on wood-frame homes show that wood is the largest component of waste (by mass), ranging between 42 percent and 67 percent of the total waste amount. Drywall was the second largest component, ranging from 20 to 27 percent of the total waste amount (NAHB 1995, Metro 1995). Composition studies on construction debris from concrete homes were performed to determine if this would hold true for concrete-block-framed homes as well.

This study was conducted on two houses in Citrus County, Fla. Both houses are one-story homes with stucco exteriors and asphalt shingle roofs. The framing on the houses consisted of an external concrete-block frame, internal wood framing for load bearing walls, and internal metal framing for non-load bearing walls. House I and House II have areas of 2,859 square feet and 3,015 square feet, respectively, which are about the average size for this builder. These areas are larger than the average size home in Florida in 2000, which was about 2,000 square feet (U.S. Census Bureau, 2000).

All of the waste from the construction was disposed in 20 cubic-yard containers. Once they were full, Superior Waste Services hauled the containers to the Citrus County Central Landfill (CCCL). At the landfill, the waste was dumped onto a plastic tarp. The project team put down a tarp to prevent mixing of dirt from the landfill with the dirt in the waste. This would ensure that all of the waste would be weighed and recorded. Once the waste was dumped, four to five people would sort the material by component. These components were then weighed and recorded. Table 1 is a list of all of the component categories.

Visual characterizations of the loads were also made. The visual characterizations are volume-based composition studies and are done by simply visually estimating the composition of the waste. The same categories were used for this method of characterization as for the mass-based method. Four or five people made visual characterizations for each load of waste.

The construction of both of the houses had generated a large amount of concrete waste, as expected. From the mass-based composition study, the project team found that 51 percent of the total waste amount from House I and 40 percent of the waste from House II is concrete. Wood represents only 14 percent and 12 percent of the waste from House I and House II, respectively. The other large component of waste in the houses, drywall, represents 13 percent of the waste from House I and 22 percent of the waste from House II.

The construction of the two houses lasted approximately four months long each, starting near the very end of August and completing in the end of December for House II and the beginning of January for House I. The approximate total volume that was collected throughout this time period was about 70 cubic yards from each house.

When the components were disposed during the construction time period for both House I and House II were tracked. They show that concrete, which is the largest component, is mostly thrown away at or near the start of construction. This is when the foundation is poured and the concrete block frame is put up. Extra concrete not used for the foundation is disposed of as are extra concrete blocks not used for framing. Wood is primarily thrown away around the time of internal framing and the construction of the roof, which follows the construction of the external frame. The roofing materials are then put up. Then, drywall is installed. Finally, the house is painted, the flooring is installed, and the appliances and cabinets are installed. This results in a lot of cardboard and flooring materials waste. The total amount of waste was 10.4 and 19.5 tons of waste from House I and II, respectively. Averaging the two homes’ waste generation rates together gives an average waste generation rate of 10.2 pounds per square foot for concrete block homes. 

The average total amount of waste generated in a 2,600 square-foot wood-frame house construction is approximately 5.7 tons, resulting in a waste generation of approximately 4.4 pounds per square foot (NAHB, 1995). This amount is significantly less than the average 10.2 pounds per square foot produced during the construction of concrete-block-frame homes.

Most of the difference in generation comes from the disposal of concrete and dirt. There was also a significant increase in drywall disposal for one of the concrete-block-frame homes. Trends in residential design (such as tray and cathedral ceilings or plant shelves) have called for more drywall cutouts and, therefore, caused more drywall waste.

The construction of concrete-block-frame homes generates more waste per square foot by mass than wood-frame homes. This will cause the generation rate to increase significantly in comparison with the nation’s generation rate. This will have a significant impact on generation estimates for Florida, proving that the method that was used to estimate the nation’s generation amount cannot be applied to Florida. This study could also impact the generation amounts for other states that have large amounts of concrete-block-homes.

C&D references

Metro Solid Waste Department. February 1995. Metro 1993-1994 Waste Characterization Study.

National Association of Home Builders Research Center (NAHB). March 1995.

Residential Construction Waste: from Disposal to Management.

U.S. Census Bureau. Building Permit. December 2000. Housing Units Authorized by Building Permits. http://www.census.gov/const/www/C40/table2.html#annual (accessed March 1, 2001).