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|Federal Highway Administration > Publications > Public Roads > Vol. 58 · No. 2 > The Use of Recycled Materials in Highway Construction|
The Use of Recycled Materials in Highway Construction
by Robin L. Schroeder
As the world population grows, so do the amount and type of waste being generated. Many of the wastes produced today will remain in the environment for hundreds, perhaps thousands, of years. The creation of nondecaying waste materials, combined with a growing consumer population, has resulted in a waste disposal crisis. One solution to this crisis lies in recycling waste into useful products.
Research into new and innovative uses of waste materials is continually advancing. Many highway agencies, private organizations, and individuals have completed or are in the process of completing a wide variety of studies and research projects concerning the feasibility, environmental suitability, and performance of using recycled products in highway construction. (1) These studies try to match society's need for safe and economic disposal of waste materials with the highway industry's need for better and more cost-effective construction materials.
This article summarizes current research on those waste materials that have shown promise as a substitute for conventional materials. (See table 1.) It primarily focuses on new and innovative highway industry uses for waste materials and byproducts, rather than on more commonly followed practices.
Table 1 -- Summary of Known Uses in Waste Applications
Blast Furnace SlagBlast furnace slag is an industrial byproduct of iron produced in a blast furnace. This slag consists primarily of silicates and aluminosilicates of lime and other bases. (2) The Bureau of Mines reports that 14 million metric tons (t) of blast furnace slag was sold in the United States in 1992 with a value of $99.5 million. (3) Of this, 90 percent was air-cooled slag, which is primarily used in road construction. Table 2 summarizes current uses for blast furnace slag in the United States.
Table 2 -- Air-Cooled Blast Furnace Slag Sold or Used in the United States by Use
Source: Cheryl Solomon. Slag--Iron and Steel, annual report, U.S. Department of the Interior, Bureau of Mines, Washington, D.C., 1992.
aValue based on selling price at plant.
W = withheld to avoid disclosing company proprietary data; data are included with "other."
In some situations, blast furnace cement can provide equal or improved performance over conventional portland cement concrete. (4) Slag cement has low heat hydration, good long-term strength gain, and high chemical resistance. (5) Use of slag cement containing more than 80 percent granulated blast furnace slag can, however, increase the time needed to attain design strength. (4)
A recent innovative use study examined the effects of slag on roller-compacted, no-slump, lean concrete mixes. (6) Several agencies have also used air-cooled, nongranulated blast furnace slag as an aggregate in hot mix asphalt (HMA) pavements, as a base or sub-base material, as an embankment material, and in snow and ice control. (7) In asphalt pavements, it is primarily and successfully used as an aggregate in open-graded friction courses. (1, 2, 7, 8)
Steel slag, a byproduct of the steel-making process, contains fused mixtures of oxides and silicates -- primarily calcium, iron, unslaked lime, and magnesium. (1) Steel slag contains significant quantities of iron; its highly compressed void structure results in a very dense, hard material. In 1992, 6.9 million t of steel slag was sold in the United States at a total value of $21.9 million. (1) (The average selling price for steel slag at the plant was $3.02 per metric ton.) Table 3 summarizes current uses for steel slag in the United States.
Table 3 -- Steel Slag Sold or Used in the United States by Use
Source: Cheryl Solomon. Slag--Iron and Steel, annual report, U.S. Department of the Interior, Bureau of Mines, Washington, D.C., 1992.
aExcludes tonnage returned to furnace for charge material. Value based on selling price at plant.
bIncludes ice control, soil conditioning, and miscellaneous uses.
cData may not add to totals because of rounding.
Current research on steel slag in highway construction is focused on its use as an aggregate in HMA. A Pennsylvania Department of Transportation study found that bituminous mixtures containing steel slag exhibited high stability, high skid resistance, and longer heat retention resulting in easier compaction. (9) At this time, however, its use as an aggregate is not cost-effective given its high asphalt absorption rate.
Research has also been conducted on the use of steel slag blended cement. (3) Results indicate that although steel slag has a mineral composition similar to that of ordinary portland cement clinker, slag could become unstable because of its free calcium oxide.
Plastics comprise more than 8 percent of the total weight of the municipal waste stream and about 12 to 20 percent of its volume. (1, 10, 11) In 1992, approximately 14.7 million t of plastics were discarded in the United States; only 0.3 million t -- or 2.2 percent -- were recycled.
Current research on the use of recycled plastics in highway construction is wide and varied. The use of virgin polyethylene as an additive to asphaltic concrete is not new; however, two new processes also use recycled plastic as an asphalt cement additive: NOVOPHALTR and PolyphaltR. (1, 11, 12) These latter two processes both use recycled low-density polyethylene resin which is generally obtained from plastic trash and sandwich bags. The recycled plastic is made into pellets and added to asphalt cement at a rate of 4 to 7 percent by weight of binder (0.25 percent to 0.50 percent by weight of total mix). (11, 12)
Michigan State University is looking into the use of recycled plastic in portland cement concrete. In this study, recycled high-density polyethylene (HDPE) was used to replace from 20 to 40 percent of fine aggregate by volume (7.5 to 15 percent by total volume) in a lightweight concrete mix. Compressive strengths were reduced when either level of HDPE was used. Overall flexural strengths remained fairly constant and the impact resistance of the concrete, which can be related to flexural toughness, increased.
Many agencies and private companies have been experimenting with the use of recycled plastic for items such as guardrail posts and block-outs, delineator posts, fence posts, noise barriers, sign posts, and snow poles.
Glass makes up approximately 7 percent -- approximately 12 million t -- of the total weight of U.S. municipal solid waste discarded annually. Approximately 20 percent of this glass is being recycled, primarily for cullet in glass manufacturing. (10) The ability to use glass in highway construction depends on the types of collection methods used, costs, and public factors. In general, the large quantities of waste glass needed for such application are found only in major metropolitan areas. (16)
Many agencies have experimented with glass in highway construction. Much current research in this area focuses on the use of glass as an aggregate in asphalt pavements. This research includes laboratory testing as well as field testing and experimentation. (1, 2, 6)
In addition, several agencies are routinely using recycled glass in the manufacture of glass beads for traffic control devices. (6)
Municipal Waste Combustion Ash
In 1980, 2.5 million t of municipal solid waste was burned in the United States, resulting in approximately 816,000 t of municipal waste combustion (MWC) ash or residue. By 1990, the amount burned had jumped to 29 million t, creating approximately 7.3 million t of MWC ash or residue. (10)
Controlled combustion of municipal solid waste produces two types of ash: fly ash and bottom ash. Most MWC ash (80 to 99 percent) is bottom ash, which typically meets the environmental standards for the toxicity characteristic leaching procedure (TCLP). (2) Fly ash, however, usually contains a high percentage of heavy metals (e.g., lead and cadmium), and the leachate may not meet some environmental standards. (18)
Concern over the environmental acceptability of MWC ash has severely curtailed the initiation of research on the beneficial uses of MWC ash. The Environmental Protection Agency (EPA) estimates that less than 10 percent of the MWC ash produced in the United States is being used in a limited number of beneficial projects.
Considerable research on crumb-rubber-modified asphalt has been conducted since the 1991 passage of the Intermodal Surface Transportation Efficiency Act. This research has addressed both performance and environmental issues; additional research is examining the use of scrap tire rubber in other highway-related applications.
Carpet Fiber Waste
The carpet industry in the United States produces about 1 billion square meters of carpet per year. Of this, approximately 70 percent is used to replace existing carpet; this translates into 1.2 million t of carpet waste produced annually. (23) Additional wastes produced by the carpet-making industry increase the total amount of waste fibers to an estimated 2 million t.
Several research efforts are addressing ways to include these waste fibers in both asphalt pavements and portland cement concrete.
Table 4 -- Compressive and Flexural Test Results of Fiber-Reinforced Concrete
Source: Youjiang Wang, et al., "Fiber Reinforced Concrete Using Recycled Carpet Industrial Waste and Its Potential Use in Highway Construction," in Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
CV = coefficient of variance
Roofing Shingle WasteIt is estimated that between 8 million and 12 million t of roofing shingles are manufactured each year in the United States. Since approximately 65 percent of these shingles is used for re-roofing, between 5 million and 8 million t of old waste shingles is produced annually. (26, 27) In addition, between 400,000 and 900,000 t of waste are produced annually from the manufacture of roofing shingles.
Several studies have focused on the use of roofing shingle waste as an asphalt pavement material.
Coal Combustion ByproductsThere are 720 coal-fired power plants in 45 states. (1) When coal is burned in these power plants, two types of ash are produced--coal fly ash and bottom ash. Coal fly ash is the very fine ash carried in the flue gas; bottom ash (or slag) is the larger, heavier particles that fall to the bottom of the hopper after combustion. The physical and chemical characteristics of these ashes vary depending on the type of coal burned. An additional byproduct of the coal combustion process is produced from coal containing sulfur. When this coal is burned, sulfur dioxide is produced; scrubbers are used to limit the amount of sulfur dioxide released into the atmosphere. The resulting waste of this process is flue gas desulfurization (FGD) waste. An estimated 18 million t of this waste is produced annually in the United States; 136 million t of it is currently stockpiled. (29)
Coal fly ashThe primary components of coal fly ash are silicon dioxide, aluminum oxide, iron oxide, and calcium oxide. Approximately 45 million t of fly ash are produced annually in the United States. About 34 million t are disposed of either onsite or in state-regulated disposal areas; 11 million t are reclaimed. (30)
Extensive research has been conducted on the use of coal fly ash as a highway construction material. Though most of this research has looked at its use as a mineral admixture to portland cement concrete, research has also been conducted on a variety of other uses, including in soil stabilization, roller-compacted concrete, and road base stabilization.
In 1988, a study was undertaken to evaluate the use of "ponded fly ash" as a component in a stabilized aggregate base course. (31) Ponded fly ash is the fly ash portion of coal ash waste previously sluiced into a disposal pond. Laboratory investigations determined that the optimum mix was a composite of 84-percent dense-graded aggregate, 11-percent ponded fly ash, and 5-percent hydrated lime. A 230-m- (755-ft-) long, 20-cm- (8-in-) thick test section was constructed and overlaid with an asphalt base, binder, and surface course. After three years of service, the experimental section is outperforming the conventional section; the amount of rutting is significantly lower in the experimental section than in the control section. Aside from minor reflective cracking associated with base shrinkage base, the experimental section has performed excellently.
Bottom ashBottom ash has a similar chemical makeup to fly ash but has a much coarser gradation. A recent study on its use as a sub-base material showed that it had sufficient engineering properties to perform adequately. (32) Bottom ash has also been marketed as an aggregate for lightweight concrete; coal boiler slag has been used as an abrasive in pavement deicing products and as a sandblasting abrasive.
Combined ashWhen fly ash and bottom ash are placed in landfills, they are generally combined. Consequently, most current research has focused on the use of combined ash. The physical properties of combined ash -- including gradation, specific gravity, and loss on ignition -- can vary considerably depending on the type of plant and source of coal. Chemical properties, however, are similar to those found in typical fly ash.
Researchers at Clemson University recently completed a study on the use of combined coal ash as a partial fine aggregate replacement in asphalt concrete mixes. (33) The study examined the effects of the ash on indirect tensile strengths and tensile strength ratios of asphalt concrete mixes. Conclusions from a limited number of samples indicated that the addition of coal ash at 6 and 8 percent by weight of aggregate decreased the 24-hour tensile strengths of Marshall specimens compared to the control mix. A number of agencies are also conducting research into the use of combined ash as an embankment material.
Flue gas desulfurization wasteResearch on the use of FGD waste has focused on its use in stabilized road bases and as an embankment material. Recent research by the Texas Transportation Institute addressed the use of cement-stabilized FGD waste in roadbase construction. (29) The research consisted of placing two 91.4 m (300 ft) experimental sections containing FGD waste stabilized with 7 percent by dry weight of high early strength, high sulfate-resistant portland cement. To date, no distress related to the FGD waste in either pavement section has been identified. It was also found that the strength of the cement-stabilized FGD increased when mixed with coal bottom ash. Additionally, surface water and soil leachate were analyzed for both sections; the material constituents were compared with EPA drinking water standards and TCLP concentrations. The results showed that none of the EPA heavy metal concentrations were exceeded. However, the drinking water standards were exceeded for sulfates; TCLP standards do not contain values for sulfate levels.
In 1992, Ohio State University researched the use of FGD waste as an embankment material for a garbage truck ramp. (34) After one year of service, there is no evidence of physical deterioration resulting from the FGD waste embankment. Tests of the leachate showed no heavy metal concentrations above drinking water standards.
ConclusionThe problems associated with the environmentally safe and efficient disposal of waste continue to grow. In many areas, existing landfills are beginning to fill up, and a "not-in-my-backyard" philosophy has made the establishment of new landfills very difficult. The cost of disposal continues to increase while the types of wastes accepted at municipal solid waste landfills is becoming more and more restricted. One answer to all of these problems lies in the ability of society to develop beneficial uses for these waste products.
The highway construction industry can effectively use large quantities of diverse materials. The use of waste byproducts in lieu of virgin materials, for instance, would relieve some
of the burden associated with disposal and may provide an inexpensive and advantageous construction product. Current research on the beneficial use of waste byproducts as highway construction materials has identified several promising uses for these materials. Some of these materials include:
Much of this research has been conducted primarily in the laboratory. The next step will be to put these ideas into action by initiating a systematic program to determine the viability and long-term performance of these materials in actual highway construction projects.
(1) R.J. Collins and S.K. Ciesielski. Recycling and Use of Waste Materials and Byproducts in Highway Construction, Volumes 1 & 2, 1993.
(2) Imtiaz Ahmed. Use of Waste Materials in Highway Construction, Report No. FHWA/IN/JHRP-91/3, 1991.
(3) Cheryl Solomon. Slag--Iron and Steel, Annual Report, U.S. Department of the Interior, Bureau of Mines, Washington, D.C., 1992.
(4) K. Sakai, et al. "Properties of Granulated Blast-Furnace Slag Cement Concrete," Proceedings of the Fourth International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Istanbul, Turkey, May 1992.
(5) S. Nagataki, et al. "Properties of Concrete Using Newly Developed Low-Heat Cements and Experiments with Mass Concrete Model," Proceedings of the Fourth International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Istanbul, Turkey, May 1992.
(6) K. Togawa, et al. "Study on the Effects of Blast-Furnace Slag on Properties of No-Slump Concrete Mixtures," Proceedings of the Fourth International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Istanbul, Turkey, May 1992.
(7) American Association of State Highway and Transportation Officials, Subcommittee on Construction, Quality Construction Task Force. "Use of Waste Materials in Highway Construction," unpublished report, August 1993.
(8) Availability of Mining Wastes and Their Potential for Use as a Highway Material--Executive Summary, Publication No. FHWA-RD-78-28, Federal Highway Administration, Washington D.C., September 1977.
(9) Tim Ramirez. 79-12 Steel Slag Aggregate in Bituminous Mixtures--Final Report, Pennsylvania Department of Transportation, 1992.
(10) Characterization of Municipal Solid Waste in the United States: 1992 Update, Executive Summary. Report No. EPA/530-S-92-019, Environmental Protection Agency, Washington, D.C., 1992.
(11) Engineering and Environmental Aspects of Recycled Materials for Highway Construction, Volume I: Final Report. FHWA Contract No. DTFH61-92-C-00060, Federal Highway Administration, Washington D.C., 1993.
(12) "Recycled plastic finds home in asphalt binder," Roads & Bridges, March 1993, pp. 41-47.
(13) Engineering News Record, February 22, 1993, p. 42.
(14) Anton Horner. "Use of Scrap Tire Rubber in Carsonite Noise Barrier," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(15) Peter Eng. Alberta Transportation and Utilities, Research and Development Report No. ABTR/RD/TM-92/09, Project No. 91021, June 1993.
(16) "'Glasphalt' utilization dependent upon availability." Roads & Bridges, February 1993, pp. 59-61.
(17) Clean Washington Center. Using Recycled Glass as a Construction Aggregate, a Summary of the Glass Feedstock Evaluation Project. Seattle, 1993.
(18) George Dewey, et al. "Municipal Waste Combustor Ash as an Aggregate Substitute in Bituminous Mixture," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(19) Three-Year Results on the Performance of Incinerator Residue in a Bituminous Base, Publication No. FHWA-RD-78-144, Federal Highway Administration, Washington, D.C., 1978.
(20) Macro Pasetto. "Cement Stabilization of Urban Solid Waste Slags and Ashes in Road Base and Sub-Base Layers," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(21) North Carolina Department of Transportation Materials and Tests Unit. "A Laboratory Evaluation on the Effects of Ground Tire Rubber on Strength Performance of Concrete," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(22) Dana N. Humphrey and Robert A. Eaton. "Tire Chips as a Subgrade Insulation--Field Trial," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(23) Youjiang Wang, et al. "Fiber Reinforced Concrete Using Recycled Carpet Industrial Waste and Its Potential Use in Highway Construction," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(24) Jeffery Groom, et al. "Use of Waste Nylon Fibers in Portland Cement Concrete to Reduce Plastic Shrinkage Cracking," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(25) Gregory Scot Gordon, et al. "Use of Waste Fibers in Asphalt Concrete," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(26) J.D. Brock. "From Roofing Shingles to Roads," Technical Paper T-20, 1990.
(27) Mary Stroup-Gardiner, et al. "Permanent Deformation and Low Temperature Behavior of Roofing-Modified HMA," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(28) "Shingle Scrap in Asphalt Concrete," unpublished report, Study No. 9PR1010, Minnesota Department of Transportation, 1991.
(29) Donald Saylak, et al. "Applications for FGD Byproduct Gypsum," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(30) "Fly ash sets standard for recycled material use," Roads & Bridges, November 1992, pp. 50-56.
(31) David Q. Hunsucker, et al. "Evaluating the Use of Ponded Fly Ash in Roadway Base Course," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(32) A.R. Dawson, et al. "Some British Experience of the Behavior of Furnace Bottom Ash and Slate Waste for Pavement Foundations," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(33) E. Eleni Vassiladou, et al. "Utilization of Fly and Bottom Ash as a Partial Fine Aggregate Replacement in Asphalt Concrete Mixtures," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
(34) William Wolfe, et al. "Truck Ramp Construction from Clean Coal Technology Waste Products," Symposium Proceedings - Recovery and Effective Reuse of Discarded Materials and Byproducts for Construction of Highway Facilities, October 1993.
Robin L. Schroeder has been with the Federal Highway Administration for 15 years. For the past three years, he has worked in the Materials Branch of the Construction and Maintenance Division in Washington, D.C., where he is extensively involved with the use of recycled materials in highway construction. In that capacity, he was the primary coordinator of a symposium that was held last year in Denver, Colo., on the use of recycled materials in highway construction. He also helped to develop a report to Congress concerning recycled materials as required by Section 1038 of ISTEA. He earned a bachelor's degree in civil engineering from Oregon State University, and he is a registered professional engineer in the state of Washington.
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