Scrap Tires - Material Description

ORIGIN

Approximately 280 million tires are discarded each year by American motorists, approximately one tire for every person in the United States. Around 30 million of these tires are retreaded or reused, leaving roughly 250 million scrap tires to be managed annually. About 85 percent of these scrap tires are automobile tires, the remainder being truck tires. Besides the need to manage these scrap tires, it has been estimated that there may be as many as 2 to 3 billion tires that have accumulated over the years and are contained in numerous stockpiles.(1) Scrap tires can be managed as a whole tire, a slit tire, a shredded or chipped tire, as ground rubber, or as a crumb rubber product. Whole Tires A typical scrapped automobile tire weighs 9.1 kg (20 lb). Roughly 5.4 kg (12 lb) to 5.9 kg (13 lb) consists of recoverable rubber, composed of 35 percent natural rubber and 65 percent synthetic rubber. Steel-belted radial tires are the predominant type of tire currently produced in the United States.(2) A typical truck tire weighs 18.2 kg (40 lb) and also contains from 60 to 70 percent recoverable rubber. Truck tires typically contain 65 percent natural rubber and 35 percent synthetic rubber.(2) Although a majority of truck tires are steel-belted radials, there are still a number of bias ply truck tires, which contain either nylon or polyester belt material. Slit Tires Slit tires are produced in tire cutting machines. These cutting machines can slit the tire into two halves or can separate the sidewalls from the tread of the tire. Shredded or Chipped Tires In most cases the production of tire shreds or tire chips involves primary and secondary shredding. A tire shredder is a machine with a series of oscillating or reciprocating cutting edges, moving back and forth in opposite directions to create a shearing motion, that effectively cuts or shreds tires as they are fed into the machine. The size of the tire shreds produced in the primary shredding process can vary from as large as 300 to 460 mm (12 to 18 in) long by 100 to 230 mm (4 to 9 in) as wide, down to as small as 100 to 150 mm (4 to 6 in) in length, depending on the manufacturer, model, and condition of the cutting edges. The shredding process results in exposure of steel belt fragments along the edges of the tire shreds.(3)Production of tire chips, which are normally sized from 76 mm (3 in) down to 13 mm (1/2 in), requires two-stage processing of the tire shreds (i.e., primary and secondary shredding) to achieve adequate size reduction. Secondary shredding results in the production of chips that are more equidimensional than the larger size shreds that are generated by the primary shredder, but exposed steel fragments will still occur along the edges of the chips.(3) Ground Rubber Ground rubber may be sized from particles as large as 19 mm (3/4 in) to as fine as 0.15 mm (No. 100 sieve) depending on the type of size reduction equipment and the intended application. The production of ground rubber is achieved by granulators, hammermills, or fine grinding machines. Granulators typically produce particles that are regularly shaped and cubical with a comparatively low-surface area. The steel belt fragments are removed by a magnetic separator. Fiberglass belts or fibers are separated from the finer rubber particles, usually by an air separator. Ground rubber particles are subjected to a dual cycle of magnetic separation, then screened and recovered in various size fractions.(4) Crumb Rubber Crumb rubber usually consists of particles ranging in size from 4.75 mm (No. 4 sieve) to less than 0.075 mm (No. 200 sieve). Most processes that incorporate crumb rubber as an asphalt modifier use particles ranging in size from 0.6 mm to 0.15 mm (No. 30 to No. 100 sieve). Three methods are currently used to convert scrap tires to crumb rubber. The crackermill process is the most commonly used method. The crackermill process tears apart or reduces the size of tire rubber by passing the material between rotating corrugated steel drums. This process creates an irregularly shaped torn particle with a large surface area. These particles range in size from approximately 5 mm to 0.5 mm (No. 4 to No. 40 sieve) and are commonly referred to as ground crumb rubber. The second method is the granulator process, which shears apart the rubber with revolving steel plates that pass at close tolerance, producing granulated crumb rubber particles, ranging in size from 9.5 mm (3/8 in) to 0.5 mm (No. 40 sieve). The third process is the micro-mill process, which produces a very fine ground crumb rubber in the size range from 0.5 mm (No. 40 sieve) to as small as 0.075 mm (No. 200 sieve).(4) In some cases, cryogenic techniques are also used for size reduction. Essentially, this involves using liquid nitrogen to reduce the temperature of the rubber particles to minus 87oC (-125oF), making the particles quite brittle and easy to shatter into small particles. This technique is sometimes used before final grinding.(5) Additional information on the production and use of scrap tire products can be obtained from:
Scrap Tire Management Council 1400 K Street, N.W. Washington, D.C. 20005

CURRENT MANAGEMENT OPTIONS

Recycling About 7 percent of the 250 million scrap tires generated annually are exported to foreign countries, 8 percent are recycled into new products, and roughly 40 percent are used as tire-derived fuel, either in whole or chipped form.(1) Currently, the largest single use for scrap tires is as a fuel in power plants, cement plants, pulp and paper mill boilers, utility boilers, and other industrial boilers. At least 100 million scrap tires were used in 1994 as an alternative fuel either in whole or chipped form.(1) At least 9 million scrap tires are processed into ground rubber annually. Ground tire rubber is used in rubber products (such as floor mats, carpet padding, and vehicle mud guards), plastic products, and as a fine aggregate addition (dry process) in asphalt friction courses. Crumb rubber has been used as an asphalt binder modifier (wet process) in hot mix asphalt pavements.(1) As previously noted, of the roughly 30 million tires that are not discarded each year, most go to retreaders, who retread about one-third of the tires received. Automobile and truck tires that are retreaded are sold and returned to the marketplace. Currently there are roughly 1,500 retreaders operating in the United States, but the number is shrinking because there is a decline in the market for passenger car retreads. The truck tire retread business is increasing and truck tires can be retreaded three to seven times before they have to be discarded.(1) Disposal Approximately 45 percent of the 250 million tires generated annually are disposed of in landfills, stockpiles, or illegal dumps. As of 1994, at least 48 states have some type of legislation related to landfilling of tires, including 9 states that ban all tires from landfills. There are 16 states in which whole tires are banned from landfills. Thirteen other states require that tires be cut in order to be accepted at landfills.(6)

MARKET SOURCES

About 80 percent of all scrap tires are handled by retail tire vendors. The remaining 20 percent are handled by auto dismantlers. These two industrial groups, while not the generators of scrap tires, collect and store tires until they are picked up by transporters, sometimes referred to as "tire jockeys". These transporters take the tires to retreaders, reclaimers, and grinders or slitters or to tire disposal sites (landfills, tire stockpiles, or illegal dumps).(1) Figure 1 presents a graphical overview of the scrap tire industry.
Figure 1. Overview of the scrap tire industry.
Since tires are combustible, tire storage sites can be potential fire hazards. Care must be taken to safeguard against carelessness or accidental ignition, which can occur at tire storage facilities.(7) Tire shreds or chips would normally be available from tire shredder operators. Ground rubber or crumb rubber would normally be available from scrap tire processors There are probably 100 or more tire shredders in the United States, but there are only about 15 to 20 scrap tire processors.

HIGHWAY USES AND PROCESSING REQUIREMENTS

Embankment Construction - Shredded or Chipped Tires Shredded or chipped tires have been used as a lightweight fill material for construction of embankments. However, recent combustion problems at three locations have prompted a reevaluation of design techniques when shredded or chipped tires are used in embankment construction.(7) Aggregate Substitute - Ground Rubber Ground rubber has been used as a fine aggregate substitute in asphalt pavements. In this process, ground rubber particles are added into the hot mix as a fine aggregate in a gap-graded friction course type of mixture. This process, commonly referred to as the dry process, typically uses ground rubber particles ranging from approximately 6.4 mm (1/4 in) down to 0.85 mm (No. 20 sieve).(4) Asphalt mixes in which ground rubber particles are added as a portion of the fine aggregate are referred to as rubberized asphalt. Asphalt Modifier - Crumb Rubber Crumb rubber can be used to modify the asphalt binder (e.g., increase its viscosity) in a process in which the rubber is blended with asphalt binder (usually in the range of 18 to 25 percent rubber). This process, commonly referred to as the wet process, blends and partially reacts crumb rubber with asphalt cement at high temperatures to produce a rubberized asphalt binder. Most of the wet processes require crumb rubber particles between 0.6 mm (No. 30 sieve) and 0.15 mm (No. 100 sieve) in size. The modified binder is commonly referred to as asphalt-rubber. Asphalt-rubber binders are used primarily in hot mix asphalt paving, but are also used in seal coat applications as a stress absorbing membrane (SAM), a stress absorbing membrane interlayer (SAMI), or as a membrane sealant without any aggregate. Retaining Walls - Whole and Slit Tires Although not a direct highway application, whole tires have been used to construct retaining walls. They have also been used to stabilize roadside shoulder areas and provide channel slope protection. For each application, whole tires are stacked vertically on top of each other. Adjacent tires are then clipped together horizontally and metal posts are driven vertically through the tire openings and anchored into the underlying earth as necessary to provide lateral support and prevent later displacement. Each layer of tires is filled with compacted earth backfill.(8) This type of retaining wall construction was initially performed in California. Slit scrap tires can be used as reinforcement in embankments and tied-back anchor retaining walls. By placing tire sidewalls in interconnected strips or mats and taking advantage of the extremely high tensile strength of the sidewalls, embankments can be stabilized in accordance with the reinforced earth principles. Sidewalls are held together by means of metal clips when reinforcing embankments, or by a cross-arm anchor bar assembly when used to anchor retaining walls.(8)

MATERIAL PROPERTIES

Physical Properties Shredded Tires Tire shreds are basically flat, irregularly shaped tire chunks with jagged edges that may or may not contain protruding, sharp pieces of metal, which are parts of steel belts or beads. As previously noted, the size of tire shreds may range from as large as 460 mm (18 in) to as small as 25 mm (1 in), with most particles within the 100 mm (4 in) to 200 mm (8 in) range. The average loose density of tire shreds varies according to the size of the shreds, but can be expected to be between 390 kg/m3 (24 lb/ft3) to 535 kg/m3 (33 lb/ft3). The average compacted density ranges from 650 kg/m3(40 lb/ft3) to 840 kg/m3 (52 lb/ft3).(3) Tire Chips Tire chips are more finely and uniformly sized than tire shreds, ranging from 76 mm (3 in) down to approximately 13 mm (1/2 in) in size. Although the size of tire chips, like tire shreds, varies with the make and condition of the processing equipment, nearly all tire chip particles can be gravel sized. The loose density of tire chips can be expected to range from 320 kg/m3 (20 lb/ft3) to 490 kg/m3 (30 lb/ft3). The compacted density of tire chips probably ranges from 570 kg/m3 (35 lb/ft3) to 730 kg/m3 (45 lb/ft3).(9) Tire chips have absorption values that range from 2.0 to 3.8 percent.(10) Ground Rubber Ground rubber particles are intermediate in size between tire chips and crumb rubber. The particle sizing of ground rubber ranges from 9.5 mm (3/8 in) to 0.85 mm (No. 20 sieve). Crumb Rubber Crumb rubber used in hot mix asphalt normally has 100 percent of the particles finer than 4.75 mm (No. 4 sieve). Although the majority of the particles used in the wet process are sized within the 1.2 mm (No. 16 sieve) to 0.42 mm (No. 40 sieve) range, some crumb rubber particles may be as fine as 0.075 mm (No. 200 sieve). The specific gravity of crumb rubber is approximately 1.15, and the product must be free of fabric, wire, or other contaminants.(4) Chemical Properties Tire chips and tire shreds are nonreactive under normal environmental conditions. The principal chemical component of tires is a blend of natural and synthetic rubber, but additional components include carbon black, sulfur, polymers, oil, paraffins, pigments, fabrics, and bead or belt materials.(2) Mechanical Properties Limited data are available on the shear strength of tire chips, while little or no such data are available on the shear strength of tire shreds. The wide variation in shred size makes it difficult, if not virtually impossible, to find a large enough apparatus to perform a meaningful shear test. Although the shear strength characteristics of tire chips vary according to the size and shape of the chips, internal friction angles were found to range from 19o to 26o, while cohesion values ranged from 4.3 kPa (90 lb/ft2) to 11.5 kPa (90 to 240 lb/ft2). Tire chips have a permeability coefficient ranging from 1.5 to 15 cm/sec.(10) Other Properties Scrap tires have a heating value ranging from 28,000 kJ/kg (12,000 Btu/lb) to 35,000 kJ/kg (15,000 Btu/lb).(2) As a result, given appropriate conditions, scrap tire combustion is possible and must be considered in any application. Tire chips can also be expected to exhibit high insulating properties. If tire chips are used as a fill material in subgrade applications, reduced depth of frost penetration compared with that of granular soil can be expected.(11)

REFERENCES

  1. Scrap Tire Management Council. Scrap Tire Use/Disposal Study 199 Update, Washington, DC, February, 1995.
  2. Schnormeier, Russell. "Recycled Tire Rubber in Asphalt," Presented at the 71st Annual Meeting of the Transportation Research Board, Washington, DC, 1992.
  3. Read, J., T. Dodson and J. Thomas. Experimental Project - Use of Shredded Tires for Lightweight Fill, Oregon Department of Transportation, Post-Construction Report for Project No. DTFH-71-90-501-OR-11, Salem, Oregon, 1991.
  4. Heitzman, Michael, "Design and Construction of Asphalt Paving Materials with Crumb Rubber," Transportation Research Record No. 1339,Transportation Research Board, Washington, DC, 1992.
  5. Spencer, Robert. "New Approaches to Recycling Tires," Biocycle, March 1991.
  6. Epps, Jon A. Use of Recycled Rubber Tires in Highways, National Cooperative Highway Research Program Synthesis of Highway Practice 198, Washington, DC, 1994.
  7. Humphrey, Dana N. Investigation of Exothermic Reaction in Tire Shred Fill Located on SR100 in Ilwaco, Washington, Prepared for the Federal Highway Administration, March 22, 1996.
  8. Forsyth, Raymond A. and Joseph P. Eagan, Jr. "Use of Waste Materials in Embankment Construction," Transportation Research Record No. 593, Transportation Research Board, Washington, DC, 1976, pp. 3-8.
  9. Bosscher, Peter J., Tuncer B. Edil, and Neil N. Eldin. "Construction and Performance of a Shredded Waste Tire Test Embankment," Presented at the 71st Annual Meeting of the Transportation Research Board, Washington, DC, January 1992.
  10. Humphrey, Dana. N., T. C. Sandford, M. M. Cribbs, and W. P. Manion. "Shear Strength and Compressibility of Tire Chips for Use as Retaining Wall Backfill," Presented at the 72nd Annual Meeting of the Transportation Research Board, Washington, DC, January 1993.
  11. Humphrey, Dana N. and Robert A. Eaton. "Tire Chips as Subgrade Insulation - Field Trial," Proceedings of the Symposium on Recovery and Effective Reuse of Discarded Materials and By-Products for Construction of Highway Facilities, Federal Highway Administration, Denver, Colorado, October, 1993.

Scrap Tires - Asphalt Concrete (Wet Process)

INTRODUCTION

Scrap tire rubber can be incorporated into asphalt paving mixes using two different methods referred to as the wet process and the dry process. In the wet process, crumb rubber acts as an asphalt cement modifier, while in the dry process, granulated or ground rubber and/or crumb rubber is used as a portion of the fine aggregate. In both cases, crumb rubber is sometimes referred to as crumb rubber modifier (CRM) because its use modifies the properties of the resultant hot mix asphalt concrete product. The wet process can be used for hot mix asphalt paving mixtures, as well as chip seals or surface treatments. The wet process can also be used to prepare rubberized joint and crack sealants, which are not included in the scope of this document. In the wet process, crumb rubber is blended with asphalt cement (usually in the range of 18 to 25 percent rubber) before the binder is added to the aggregate. When asphalt cement and CRM are blended together, the CRM reacting with the asphalt cement swells and softens. This reaction is influenced by the temperature at which the blending occurs, the length of time the temperature remains elevated, the type and amount of mechanical mixing, the size and texture of the CRM, and the aromatic component of the asphalt cement. The reaction itself involves the absorption of aromatic oils from the asphalt cement into the polymer chains that comprise the major structural components of natural and synthetic rubber in CRM. The rate of reaction between CRM and asphalt cement can be increased by enlarging the surface area of the CRM and increasing the temperature of the reaction. The viscosity of the asphalt-CRM blend is the primary parameter that is used to monitor the reaction.(1) The specified reaction time should be the minimum time, at a prescribed temperature, that is required to stabilize the binder viscosity. When CRM is blended with asphalt cement in the wet process, the modified binder is referred to as asphalt-rubber. To date, most of the experience with the use of CRM in asphalt paving has been with the wet process. Asphalt-rubber binders are used in chip-seal coats as well as hot mix asphalt paving. Chip-seal coat applications using asphalt-rubber binders have become known as stress-absorbing membranes (SAM). When an asphalt-rubber chip seal or SAM is overlaid with hot mix asphalt, the chip seal is referred to as a stress-absorbing membrane interlayer (SAMI). Early applications were batch wet processes and were based on the McDonald technology, which was developed in the early 1960's by Charles McDonald, a City of Phoenix engineer, and in the 1970's by Arizona Refining Company (ARCO). There are numerous patents related to the McDonald technology, some of which have expired and some of which have not.(1,2) A continuous blending technology was developed in Florida in the late 1980's and is frequently referred to as the Florida wet process. In this process, a fine 0.18 mm (No. 80 sieve) CRM is blended with asphalt cement in a continuous process. The Florida technology differs from the McDonald process in several respects: lower percentages of CRM (from 8 to 10 percent rubber), smaller CRM particle size, lower mixing temperature, and shorter reaction time. The Florida wet process has not as yet been patented.(1) Terminal blending is a wet process with the capability of blending or combining asphalt cement and CRM and holding the product for extended periods of time. This asphalt-rubber product has a shelf life and is blended at an asphalt cement terminal using either batch or continuous blending. Individual state highway agencies are now developing their own products with this technology, since it is not patented. At the present time, none of the terminal blending products have been fully evaluated in the field.(1)

PERFORMANCE RECORD

The reported performance of crumb rubber in asphalt pavement has varied widely in different sections of the United States. Several states have had fairly extensive experience in using crumb rubber, some with the wet process. A summary of the experiences of selected states are presented in the next few paragraphs. Over a 20-year period, starting in the early 1970's, over 3,000 lane-miles of streets in Phoenix, Arizona, have been seal coated using asphalt-rubber. In the early 1990's, the use of chip seals was discontinued in favor of 1-inch thick asphalt-rubber hot mix overlays. Nearly 600 lane-miles of streets have been paved using hot mix overlay. Both the chip seals and the hot mix overlays have reportedly been effective in retarding the reflection of alligator cracks and shrinkage cracks less than 6.3 mm (1/4 in) in width. Compared with the chip seals, the 25-cm (1- in) thick asphalt-rubber hot mix overlay reportedly provides a more improved riding surface and a marked decrease in traffic noise.(3) The California Department of Transportation (CalTrans) has been using rubber-modified asphalt concrete since 1978 and has constructed 17 wet-process overlay installations. CalTrans has placed rubberized overlays over asphalt as well as concrete pavements using dense-graded, open-graded, and gap-graded asphalt concrete mixes. Since 1987, these CRM overlays have been placed at a reduced thickness compared with conventional overlays. Overall, CalTrans has reported that wet-process overlays have usually out-performed thicker, dense-graded asphalt concrete by exhibiting less distress, requiring less maintenance, and being able to tolerate higher deflections.(4) The Florida Department of Transportation (DOT) constructed three asphalt-rubber demonstration projects between March 1989 and September 1990. These projects involved one dense-graded and two open-graded friction courses using the Florida wet-process technology. Although long-term performance has yet to be evaluated, data reviewed to date suggest that asphalt-rubber friction courses, particularly open-graded ores, will probably exhibit improved durability over conventional friction courses.(5) The Kansas DOT constructed five projects using asphalt-rubber interlayers during the 1980's. On two of the projects, the interlayer was able to somewhat reduce reflective cracking. On the other three projects, there was little difference between the pavements with an interlayer and the control sections, and, in some cases, sections with the interlayer had more reflective cracking. On all five projects, the Kansas DOT concluded that the additional cost of an asphalt-rubber interlayer did not justify its use.(6) The Minnesota DOT (MNDOT) has used CRM in asphalt paving on at least six different wet- process projects, beginning in 1979. The six projects involved two SAM's, three SAMI's, and one dense-graded overlay.(7) The MNDOT demonstration exhibited mixed results. Of the two stress- absorbing membranes, one was a success and one was a failure. The difference between success and failure depended on precoating of the aggregate chips that were used. Only minor problems were encountered with the installation of the three SAMI's. Reflective cracking was reduced, but not eliminated. Some improvement in crack reflection was observed in the asphalt-rubber dense-graded overlay, but the benefits were not deemed sufficient to offset the increase in cost.(7) In Texas, crumb rubber has been used in asphalt paving mixtures in at least three different products dating back as far as 1976. The most frequent use of CRM in Texas has been in the construction of asphalt-rubber chip seals. Over 2,000 lane-miles of asphalt-rubber chip seals (SAM's) have been constructed in Texas. After many years of experience, Texas DOT personnel have concluded that SAM's exhibit improved resistance to alligator cracking and raveling, but that resistance to shrinkage cracking is not improved by the chip seals.(8) Two other districts in Texas have experimented with the use of CRM in dense-graded asphalt-rubber hot mix overlays (wet process). Performance to date has been satisfactory for the two wet-process overlays.(8) Since 1977, the Washington State DOT (WSDOT) has used three types of paving products from the wet process. Wet-process products include SAM's, SAMI's, and open-graded asphalt-rubber friction courses. WSDOT concluded that the performance of asphalt-rubber SAM's and SAMI's did not justify the added expense of their construction.(9) All five of the open-graded friction course installations are exhibiting good to very good performance, with the exception of one bridge deck overlay, which is showing some distress in the wheel path areas.(9) In Ontario, Canada, three rubber-modified asphalt demonstration projects were evaluated in terms of pavement performance. The performance of the asphalt-rubber (wet process) projects was promising, insofar as the durability of these asphalt mixes appeared to be enhanced by the use of crumb rubber modifier.(10) There have reportedly been six projects in the United States where asphalt pavements with CRM have been recycled. Roughly half of these projects were wet process and the other half were dry process. Apparently, there are no physical problems with recycling reclaimed asphalt pavement containing CRM as a portion of the aggregate in a new asphalt paving mix. To summarize, the overall results of these performance investigations suggest the following:
  • Chip Seals (SAM's and SAMI's): Precoated chips must be used and are effective in reducing, but not eliminating, reflective cracking, especially in warmer climates.
  • Asphalt-Rubber Hot Mix Overlays: Improved performance was observed compared with conventional hot mix overlays in most climates. Dense-graded asphalt-rubber overlays may be effective at reduced thicknesses, compared with conventional overlays. Open-graded asphalt-rubber friction courses exhibit improved durability in warmer climates, compared with conventional friction courses.
  • Economic Comparisons: The overall consensus is that crumb rubber modified asphalt pavements may cost 1.5 to 2 times as much as conventional asphalt. Many states have questioned the cost effectiveness of CRM in hot mix asphalt.

MATERIAL PROCESSING REQUIREMENTS

Shredding The initial step in the production of ground or granulated scrap tire rubber is shredding. Scrap tire rubber is delivered to rubber processing plants either as whole tires, cut tires (treads or sidewalls), or shredded tires, with shredded tires being the preferred alternative. As scrap, the rubber is processed, the particle sizing is reduced, steel belting and fiber reinforcing are separated and removed from the tire, and further size reduction is then accomplished. Grinding and Granulation Crumb rubber can be produced by one of three processes. The granulator process produces cubical, uniformly shaped particles ranging in size from 9.5 mm (3/8 in) down to 0.4 mm (No. 40 sieve), which is called granulated CRM. The crackermill process, which is the most commonly used, produces irregularly shaped torn particles sized from 4.75 mm (No. 4 sieve) to 0.42 mm (No. 40 sieve), referred to as ground CRM. The micro-mill process produces a very fine ground CRM, usually ranging from 0.42 mm (No. 40 sieve) down to as small as 0.075 mm (No. 200 sieve).(1) In the wet process, ground CRM is normally used with the McDonald technology and very fine ground CRM is used with the Florida technology.

ENGINEERING PROPERTIES

Some of the engineering properties that are of particular interest when rubber is incorporated in asphalt concrete (wet process) include asphalt viscosity, asphalt softening point, resilient modulus, permanent deformation, thermal cracking, and resistance to aging. Viscosity: Adding crumb rubber to asphalt cement can dramatically increase the viscosity of the resultant asphalt-rubber binder. Various quantities of kerosene or other diluents can be used to adjust the viscosity. Viscosity increases can occur after the addition of diluents, but higher percentages of diluent usually result in lowered viscosity increases. Reaction temperatures also affect these relationships.(1) The benefit of increased viscosity of the asphalt-rubber binder is that additional binder can be used in the asphalt mix to reduce reflective cracking, stripping, and rutting, while improving the binder's response to temperature change and long-term durability, as well as its ability to adhere to the aggregate particles in the mix and to resist aging. Softening Point: In addition to modifying binder viscosity, asphalt-rubber binders used in seal coats and hot mix asphalt show an increase in the softening point of the binder by 11°C (20°F) to 14°C (25°F), resulting in reduced rutting or shoving of the asphalt-rubber products at elevated temperatures. Modification of asphalt cement with ground tire rubber greatly increases binder elasticity compared with unmodified asphalt cement, thus providing asphalt-rubber pavement systems with increased resistance to deformation and cracking. Resilient Modulus: Resilient modulus values for mixtures containing conventional aggregate and asphalt-rubber binder are generally lower than the resilient modulus values for similar mixtures in which conventional asphalt cement are used. The higher the temperature, the greater the difference between the resilient modulus of the conventional mix and the asphalt-rubber mix.(1) Permanent Deformation: The permanent deformation properties of dense-granded asphalt-rubber mixtures are within the range of properties normally associated with conventional hot mix asphalt paving mixtures, although asphalt-rubber mixtures may be somewhat less resistant to permanent deformation.(1) Thermal Cracking: Asphalt-rubber binders also exhibit reduced fracture temperatures compared with conventional asphalt cement, usually 5.5°C (10°F) to 8.3°C (15°F) lower, meaning that asphalt-rubber products are less brittle and more resistant to cracking at lower temperatures than conventional chip seals or hot mix asphalt paving. Isolated fatigue studies have also indicated greater resistance to low temperature thermal cracking. In summary, asphalt-rubber is more elastic than asphalt cement and remains elastic at lower temperatures. Resistance to Aging: Laboratory data also indicate that asphalt-rubber mixtures are somewhat more resistant to aging than normal asphalt mixtures. Aging studies performed in asphalt-rubber binders placed in northern and central Arizona pavements indicate that asphalt-rubber binders have an increased resistance to hardening.(1) When crumb rubber is added to asphalt cement, fatigue life is improved.

DESIGN CONSIDERATIONS

Mix Design Hot Mix Asphalt Variations of standard Marshall and Hveem mix design procedures for hot mix asphalt have been used to design dense-graded hot mixes using crumb rubber. Marshall or Hveem stability tests and weight-volume parameters are the basis for each of these designs. Lower unit weight and Marshall or Hveem stability values are obtained using CRM asphalt mixtures, while flow and voids in mineral aggregate (VMA) values are increased compared with conventional mixes.(1) Mixing and compaction temperatures for CRM mixtures are often higher than those for conventional paving mixes. Depending on the mix design method, samples should be heated to 149°C (300°F) to 190°C (375°F) before compaction. Design air voids and aggregate gradation depend on the CRM content. Low CRM content in the wet process has little or no effect on the mix design. As a rule of thumb, if 20 percent crumb rubber is used in the binder, then the CRM binder content will be 20 percent greater than a conventional binder.(1) Most wet processes use CRM particles ranging in size from 0.6 mm (No. 30 sieve) to 0.15 mm (No. 100 sieve). The CRM percentage by weight can range from 5 to 25 percent of the binder, but is typically 18 percent. The CRM and asphalt cement are blended at temperatures from 166°C (330°F) to 204°C (400°F). Reaction times may vary from 10 to 15 minutes up to 2 hours or more, with rubber type and gradation being the two most important variables. Chip Seals When asphalt-rubber is used in chip seals, the majority of SAM's and SAMI's have been designed and placed without predetermining the binder or the aggregate application rate. The most common approach has been to specify a fixed rate of asphalt-rubber binder, and then vary the aggregate application rate to achieve the desired product. The amount of asphalt-rubber binder suggested for use in chip seals is about 15 to 20 percent higher than that required for a typical asphalt cement binder without a temperature correction. The amount of asphalt-rubber binder suggested for use in interlayers is about 45 percent higher than that typically used in asphalt cement binder without a temperature correction.(1) Structural Design Conventional AASHTO design procedures for flexible pavements are typically used for pavements containing wet process CRM.(11) Most agencies elect to use the same thickness of asphalt-rubber pavement as the design thickness of a conventional hot mix asphalt pavement. Since 1987, CalTrans has placed and evaluated the performance of at least eight different projects in which all, or at least a portion, of the asphalt-rubber pavement was constructed at a reduced thickness, compared with the conventional pavement. Thickness reductions ranged from 20 to 50 percent in dense-graded asphalt mixes. In the majority of these projects, the thinner asphalt-rubber mixes reportedly performed at least as well as the thicker, conventional dense-graded asphalt mixes.(4)

CONSTRUCTION PROCEDURES

There are a number of special construction procedures for hot mix asphalt pavements containing scrap tire rubber, as well as both chip-seal coats (SAM's) and interlayers (SAMI's). Hot Mix Asphalt Pavements The construction process normally used for hot mix asphalt pavements must be modified in order to produce a quality CRM hot mix. When using asphalt-rubber binders in either dense-graded, open-graded, or gap-graded mixtures, several changes in the normal construction process must be recognized. Material Handling and Storage Crumb rubber is most often shipped in 110 kg (50 lb) bags. The bags can be emptied directly into a reaction vessel for mixing with asphalt cement. The different viscosity of wet-process binders, particularly at the higher rubber content (in the 18 to 25 percent range), can cause problems with storage and/or pumping of the binder. Such problems are most likely to occur if the hot mix plant has been shut down for an extended time period. Mixing A blending and reacting unit should be added to ensure proper proportioning of the crumb rubber, base asphalt cement, and any other modifiers. Most wet processes use CRM particles ranging in size from 0.6 mm (No. 30 sieve) to 0.15 mm (No. 100 sieve). The CRM and asphalt cement are blended at temperatures from 166°C to 204°C (330° F to 400°F). Reaction times may vary from 10 to 15 minutes up to 2 hours or more, with rubber type and gradation being the two most important variables. The target temperature should be higher to allow for the greater viscosity of the binder at construction temperatures. Typical mixing temperatures for hot mix asphalt paving are from 149°C (300°F) to 177°C (350°F). Placing and Compacting Placement of hot mix asphalt paving material with wet-process CRM binder can be accomplished using standard paving machinery. Laydown temperature should be at least 121°C (250°F). Compaction must be completed as soon as possible.(1) Pneumatic tire rollers cannot be used because asphalt-rubber will build up on roller tires.(1) Quality Control To ensure proper quality control of the CRM binder, the crumb rubber particle size, the rate of addition of crumb rubber, the mixing temperature and the time of blending and reaction must all be carefully monitored. It is recommended that compacted mixes be sampled according to AASHTO T168,(12) and tested for specific gravity in accordance with ASTM D2726(13) and in-place density in accordance with ASTM D2950.(14) Chip-Seal Coats (SAM's) and Interlayers (SAMI's) Construction of asphalt-rubber chip seals and interlayers (SAMs and SAMIs) is nearly identical to the construction of conventional chip seals. The major differences include the preparation of the asphalt-rubber binder and the use of specialized spray equipment. Material Handling and Storage Crumb rubber is most often shipped in 110 kg (50 lb) bags. The bags can be emptied directly into a reaction vessel for mixing with asphalt cement. Mixing Preparation of asphalt-rubber binder is done in blending vessels, which are often either special tanks or specialized binder distributors. They must be capable of heating the base asphalt cement, mixing the crumb rubber and the asphalt cement, and keeping the crumb rubber in suspension to avoid separation. When the crumb rubber is introduced into the asphalt cement, it swells and physical-chemical reactions occur that alter the properties of the base asphalt. Diluents of various types (such as kerosene) may be introduced to adjust the viscosity for spraying purposes.(1) Placing and Compacting Binder distribution equipment must be able to maintain the temperature of the binder at the desired level, circulate the binder to avoid separation of the crumb rubber and base asphalt, and discharge the binder in a uniform manner. Special pumps and nozzles are required to handle some asphalt-rubber binders.(1) For chip-seal or interlayer applications, typical asphalt-rubber spray quantities are 2.5 L/m(0.55 gal/yd2) to 3.2 L/m2 (0.70 gal/yd2) for SAM's and 2.7 L/m2 (0.60 gal/yd2) to 3.6 L/m2 (0.80 gal/yd2) for SAMI's, compared with 1.6 L/m2 (0.35 gal/yd2) to 2.3 L/m2 (0.50 gal/yd2) for conventional chip seals. Typical aggregate application rates are in the range of 16 kg/m2 (30 lb/yd2) to 22 kg/m2 (40 lb/yd2) for SAM's and 8 kg/m2 (15 lb/yd2) to 14 kg/m2 (25 lb/yd2) for SAMI's, compared with conventional chip-seal application rates of 11 kg/m2 (20 lb/yd2) to 14 kg/m2 (25 lb/yd2).(1) As with all chip-seal construction, application of the chips should immediately follow application of the binder to ensure proper adhesion. Quality Control To ensure proper quality control of the CRM binder, the crumb rubber particle size, the rate of addition of crumb rubber, the mixing temperature, and the time of blending and reaction must all be carefully monitored. Quality assurance of the chip seal will require that the particle sizing and application rate of the stone chips be closely inspected to ensure compliance with applicable specifications.

UNRESOLVED ISSUES

There are several unresolved issues relative to the use of crumb rubber as an asphalt cement modifier in asphalt concrete using the wet process. Although only a limited amount of air emissions data from asphalt plants producing hot mix containing CRM are currently available, there is no evidence thus far that the use of an asphalt paving mix containing recycled crumb rubber exhibits any increased environmental impact when compared with that of emissions from the production of a conventional asphalt pavement.(10) Nevertheless, there is a need for additional studies on recyclability and worker health and safety issues for CRM asphalt paving mixes. Some of this work is presently underway and, as data become available, they should be incorporated into what is already known concerning these two aspects of using CRM in asphalt pavements. Because of fluctuations in the performance of CRM asphalt mixes in different locations and/or climatic conditions, there is a need for more carefully controlled experimental field sections in different climatic regions throughout the United States in order to obtain more reliable performance data. Binder and mixture properties in these different regions need to be more accurately determined and documented. Performance records of these test sections may need to be monitored over a long period of time, at least 5 years and possibly as long as 30 years.(1) Additional research is needed to define the properties of binders produced by the wet process. Desirable properties for chip seals, interlayers, and hot mix asphalt containing CRM need to be better defined using either existing or newly developed test methods.

REFERENCES

  1. Epps, Jon A. Uses of Recycled Rubber Tires in Highways, NCHRP Synthesis of Highway Practice No. 198, Transportation Research Board, Washington, DC, 1994.
  2. Charamia, Equbalali, Joe A. Cano, and Russell N. Schnormeier. "Twenty Year Study of Asphalt-Rubber Pavements in the City of Phoenix, Arizona." Presented at the 70th Annual Meeting of the Transportation Research Board, Washington, DC, January, 1991.
  3. Charamia, Equbalali, Joe A. Cano, and Russell N. Schnormeier. "Twenty Year Study of Asphalt-Rubber Pavements in the City of Phoenix, Arizona." Presented at the 70th Annual Meeting of the Transportation Research Board, Washington, DC, January, 1991.
  4. Van Kirk, Jack L. "CalTrans Experience with Rubberized Asphalt Concrete." Presented at the Technology Transfer Session of an Introduction to Rubberized Asphalt Concrete, Topeka, Kansas, January 23, 1991.
  5. Page, Gale C., Byron E. Ruth and Randy C. West. "Florida's Approach Using Ground Tire Rubber in Asphalt Concrete Mixtures."Transportation Research Record No. 1339, Transportation Research Board, Washington, DC, 1992, pp. 16-22.
  6. Parcells, W.H. Asphalt-Rubber for Stress Absorbing Membrane to Retard Reflective Cracking. Final Report. Kansas Department of Transportation, Topeka, Kansas, June, 1989.
  7. Turgeon, Curtis M. "The Use of Asphalt-Rubber Products in Minnesota." Presented at the National Seminar on Asphalt-Rubber, Kansas City, MO, October 30-31, 1989.
  8. Estakhri, Cindy K., Joe W. Button, and Emmanuel G. Fernando. "Use, Availability, and Cost-Effectiveness of Asphalt Rubber in Texas."Transportation Research Record No. 1339, Transportation Research Board, Washington, DC, 1992.
  9. Swearingen, David L., Newton C. Jackson, and Keith W. Anderson. Use of Recycled Materials in Highway Construction. Washington State Department of Transportation, Report No. WA-RD 252.1, Olympia, Washington, February 1992.
  10. Emery, John. "Evaluation of Rubber Modified Asphalt Demonstration Projects." Presented at the 74th Annual Meeting of the Transportation Research Board, Washington, DC, January 1995.
  11. AASHTO Guide for the Design of Pavement Structures, American Association of State Highway and Transportation Officials, Washington, DC, 1993.
  12. American Association of State Highway and Transportation Officials. Standard Method of Test, "Sampling Bituminous Paving Mixtures," AASHTO Designation: T168-82, Part II Tests, 14th Edition, 1986.
  13. American Society for Testing and Materials. Standard Specification D2726-96, "Bulk Specific Gravity and Density of non-Absorptive Compacted Bituminous Mixtures,"Annual Book of ASTM Standards, Volume 04.03, ASTM, West Conshohocken, Pennsylvania, 1996.
  14. American Society for Testing and Materials. Standard Specification D2950-96, "Density of Bituminous Concrete in Place by Nuclear Methods,"Annual Book of ASTM Standards, Volume 04.03, ASTM, West Conshohocken, Pennsylvania, 1996.

Scrap Tires - Asphalt Concrete (Dry Process)

INTRODUCTION

Scrap tire rubber can be incorporated into asphalt paving mixes using two different methods, which are referred to as the wet process and the dry process. In the wet process, crumb rubber acts as an asphalt cement modifier, while in the dry process, granulated or ground rubber and/or crumb rubber is used as a portion of the fine aggregate. In both cases, crumb rubber is sometimes referred to as crumb rubber modifier (CRM) because its use modifies the properties of the resultant hot mix asphalt concrete product. The dry process can be used for hot mix asphalt paving in dense-graded, open-graded, or gap-graded mixtures. It cannot be used in other asphalt paving applications, such as cold mix and chip seals or surface treatments. In the dry process, granulated or ground rubber and/or crumb rubber is used as a substitute for a small portion of the fine aggregate (usually 1 to 3 percent by weight of the total aggregate in the mix). The rubber particles are blended with the aggregate prior to the addition of the asphalt cement. When tire rubber is used as a portion of the aggregate in hot mix asphalt concrete, the resultant product is sometimes referred to as rubber-modified asphalt concrete (RUMAC). The dry process used most frequently in the United States was originally developed in the late 1960's in Sweden and is marketed in this country under the trade name PlusRide by EnviroTire. The PlusRide technology is a patented process. In this process, from 1 to 3 percent granulated crumb rubber by weight of the total mix is added to the paving mix. The granulated rubber consists of rubber particles ranging in size from 4.2 mm (1/4 in) to 2.0 mm (No 10 sieve). The target air voids content of the asphalt mix is 2 to 4 percent, which is usually attained at an asphalt binder content in the 7.5 to 9 percent range.(1) A generic dry process technology was developed in the late 1980's to early 1990's to produce dense-graded hot mixtures. This concept uses both coarse and fine crumb rubber to match aggregate gradings and to achieve improved binder modification. The crumb rubber may need a prereaction or pretreatment with a catalyst to achieve optimum particle swelling. In this system, rubber content does not exceed 2 percent by weight of total mixture for surface courses. Experimental pavement sections have been placed in Florida, New York, Oregon, and Ontario.(2) The U.S. Army Corps of Engineers Cold Regions Research Engineering Laboratory (CRREL) investigated dry process CRM mixtures for disbonding ice on pavements. This research resulted in a recommendation to place field sections with mixtures containing crumb rubber particles larger than 4.75 mm (No. 4 sieve), with a top size of 9.5 mm (3/8 in). The technology is referred to as the chunk rubber process.(2) Marshall properties, resilient modulus, and ice removal tests have been performed in the laboratory with crumb rubber concentrations of 3, 6, and 12 percent by weight of aggregate. Laboratory wheel testing indicates that the higher rubber content mixes can potentially increase the incidence of ice cracking.(3) The chunk rubber process has not as yet been field evaluated.(2)

PERFORMANCE RECORD

The reported performance of rubber-modified asphalt concrete pavements has varied widely in different sections of the United States. The following paragraphs summarize the experiences of selected states with the dry process. The California Department of Transportation (CalTrans) has constructed four projects using the PlusRide dry process technology. Some distress in the form of cracking or flushing in the wheel paths was observed in three of these projects, but overall, CalTrans has reported that two of the four dry process projects have out-performed conventional dense-graded asphalt, and a third project has performed comparably. A fourth project was not properly designed and required an overlay.(4) The Minnesota Department of Transportation (MNDOT) has used the dry process in asphalt paving on a least two different projects, beginning in 1979. The two dry process projects were both PlusRide installations, using granulated crumb rubber and a gap-graded aggregate in an attempt to create a self de-icing pavement.(5)The two PlusRide sections have performed well, but have not shown benefits to offset the increased cost, and have not demonstrated any significant de-icing benefits.(5) In New York, two experimental hot mix overlay projects using granulated rubber in the dry process were installed during 1989 to compare the construction characteristics and performance of rubber-modified asphalt concrete with a conventional top course paving mixture. All overlays were 37.5 mm (1-1/2 in) thick and placed over existing Portland cement concrete pavements, each with a leveling course of varying thickness. On both projects, the rubber-modified mixes consisted of PlusRide with 1, 2, or 3 percent granulated rubber aggregate.(6) After 3 years, the New York State DOT did not consider that these two overlay projects were either economical or successful. One district in Texas has used rubber-modified hot mix asphalt (dry process). The mix raveled severely and the district was forced to place a chip seal over the mix within 3 months. (7) Since 1977, the Washington State DOT (WSDOT) has undertaken a number of demonstrations with the dry process, using crumb rubber particles up to 6.3 mm (1/4 in) in size. The performance of the seven PlusRide sections has ranged from excellent to immediate failure. Construction problems have plagued several of these installations. WSDOT concluded that, overall, PlusRide did not appear to provide improved performance.(8) In Ontario, Canada, eight rubber-modified, dry process asphalt demonstration projects were evaluated in terms of pavement performance. They generally exhibited poor short-term performance.(9) Performance of rubber-modified asphalt using the dry process has been mixed, with some early failures. Installations in service for several years generally show little improvement over conventional overlays. Little to no evidence of ice disbonding has been observed, except in laboratory tests.

MATERIAL PROCESSING REQUIREMENTS

Shredding The initial step in the production of ground or granulated scrap tire rubber is shredding. Scrap tire rubber is delivered to rubber processing plants either as whole tires, cut tires (treads or sidewalls), or shredded tires, with shredded tires being the preferred alternative. As scrap the rubber is processed, the particle sizing is reduced, steel belting and fiber reinforcing are separated and removed from the tire, and further size reduction is then accomplished. Grinding Rubber used in the dry process is ground rubber that is generally produced in a granulator process. This process further reduces shredded tire rubber and generates cubical, uniformly shaped particles ranging in size from 9.5 mm (3/8 in) down to a 0.42 mm (No. 40 sieve). However, the dry process can also use coarse crumb rubber from the crackermill process, which results in irregularly shaped particles ranging in size from 4.75 mm (No. 4 sieve) to 0.92 mm (No. 40 sieve).

ENGINEERING PROPERTIES

Some of the engineering properties of granulated or ground rubber that are of particular interest when used in asphalt concrete (dry process) include its gradation, particle shape, and reaction time. Gradation: RUMAC paving mixes incorporate granulated or coarse crumb rubber particles that are most often processed to meet the gradation requirements shown in Table 1.(10)
Table 1. Gradation Requirements for RUMAC Mixes
Sieve Size Percent Passing by Weight
6.3 mm (1/4 in) 100
4.75 mm (No. 4) 76 - 100
2.0 mm (No. 10) 28 - 42
0.85 mm (No. 20) 16 - 24
However, a chunk-rubber asphalt process developed for disbonding ice on pavements contains particles larger than a No. 4 sieve with a dominant size of 9.5 mm (3/8 in). Particle Shape: Ground or granulated rubber particles produced from granulators, hammermill, or fine grinding machines have a cubical shape and a relatively low surface area. Coarse crumb rubber particles produced from the crackermill process have an irregularly torn shape and a relatively high surface area. A cubical particle shape with a relatively low surface area is characteristic of conventional aggregate materials and is desirable for rubber particles that will function as a gap-graded aggregate in the dry process. Particles from the crackermill process that have an irregular shape with a relatively high surface area are more likely to react with asphalt cement at elevated temperatures and are better suited for use in the wet process. By limiting the time that the asphalt cement and crumb rubber particles are maintained at reaction temperatures and specifying a coarse granulated product with a relatively low surface area, the rubber particles can retain the physical shape and rigidity needed for use in the dry process. The smooth, sheared surfaces of ground or granulated rubber particles are also less reactive than the surfaces of the particles produced from the crackermill process.() Reaction Time: In the Plus Ride process, there is a relatively short reaction time when the rubber particles and aggregate are mixed with the asphalt cement, so the rubber particles do not have much opportunity to blend with the binder. There is a generic dry process that was developed in New York State, which uses coarse and fine crumb rubber prereacted with a catalyst to achieve optimum particle swelling, and is added at a maximum of 2 percent by total mixture weight for surface courses.(2) In this process, the rubber particles may be able to react to a somewhat greater extent with the asphalt binder. Some of the properties of RUMAC paving mixtures that are of interest include stability, resilient modulus, permanent deformation, and reflective cracking. Stability: Paving mixtures produced by the dry process generally have reduced stability values, regardless of whether the Marshall or Hveem mix design procedures are used. Resilient Modulus: Mixes containing granulated or crumb rubber typically have lower resilient modulus values than conventional hot mix asphalt. RUMAC paving mixes have been found to have resilient modulus values that are 10 to 20 percent higher than those of asphalt-rubber (wet process) paving mixes. Permanent Deformation: Previous studies of granulated rubber paving mixtures indicate that resistance to permanent deformation of such mixes is reduced compared with that of conventional paving mixes. However, fatigue life is generally improved when crumb rubber is added by this process.(2) Reflective Cracking: Addition of rubber aggregate can influence pavement performance in terms of reflective cracking. To achieve the benefits of delayed reflective cracking, a minimum rubber content must be added to the paving mix. This minimum rubber content is probably between 1 and 2 percent by weight of aggregate. The reaction between the rubber and the asphalt cement does not play a significant role in the enhancement of pavement performance in dry process mixes.(2)

DESIGN CONSIDERATIONS

Mix Design Conventional Marshall and Hveem mix design methods have been used successfully for designing dense-graded mixtures with granulated rubber, but mixtures produced using the dry process typically do not follow the normal mix design procedures. Where stability is the primary design factor in most conventional mixes, the primary dry process design property is the percentage of air voids. The target air voids are between 2 and 4 percent. During the laboratory mixing process, the granulated rubber is dry mixed with the aggregate before adding the asphalt cement. The asphalt concrete mixture is cooled for 1 hour after mixing. After compaction, the sample is cooled to room temperature. The air void content is determined after extrusion.(2) Dry process paving mixes should be designed volumetrically to compensate for the lower specific gravity of the crumb rubber particles. Binder contents in dry process mixes are typically 10 to 20 percent higher than those of conventional mixes. Although the air voids content is the criterion for mix design, lower stability values and higher flow values can be expected, compared with conventional hot mix asphalt paving mixtures.(2) Structural Design The method used for the thickness design of rubber modified asphalt pavements, which incorporate between 1 and 3 percent by weight of granulated crumb rubber modifier (CRM) as fine aggregate, is essentially the same as that used for the thickness design of conventional hot mix asphalt pavements.(11) No adjustments are normally recommended in the design thickness of rubber modified asphalt pavements compared with that of conventional hot mix asphalt pavements. When designing asphalt pavements using the structural number (SN), the resilient modulus at 20° C (68° F) is the material property that is considered. Resilient modulus values for 18 percent coarse (2.0 mm (No. 10 sieve)) and fine (0.2 mm (No. 80 sieve)) CRM by weight of asphalt binder in dense-graded mixtures were found to be lower than dense-graded control mixtures at three temperatures ranging from 5° C (41° F) to 40° C (104° F).(12) Since the structural layer coefficient of a pavement is directly proportional to resilient modulus, this would suggest that dry process CRM mixtures should have a lower structural layer coefficient and require some increase in thickness.

CONSTRUCTION PROCEDURES

Material Handling and Storage Both batch and drum-dryer plants have been used to produce RUMAC. The reclaimed granulated rubber is usually packed and stored in 110 kg (50 lb) plastic bags. Additional manual labor and conveying equipment, such as work platforms, are needed in order to introduce the granulated rubber into the paving mix, regardless of the type of mixing plant used. A batch plant has a quality control advantage over a drum-dryer plant because the number of preweighed bags of granulated rubber can be easily counted prior to their addition into each batch. The bags may be opened and the granulated rubber placed on a conveyor, or the bags may be put into the pugmill or cold feed bin if low melting point plastic bags are used. Control of the feeding of granulated rubber is necessary because the correct rubber content is critical to the performance of the paving mix when using the dry process. Such control is more difficult to maintain in a drum-dryer system, due to the nature of the feed operation. Some drum-dryer plants have used recycled asphalt concrete hoppers to feed the granulated rubber, although a number of agencies recommend that the rubber be introduced into the mix through a center feed system. The process can be automated by the addition of a conveyor and hopper, plus scales to accurately proportion the granulated rubber. Mixing For both batch and drum-dryer plants the addition of rubber normally requires that the mixing time and temperature be increased. Batch plants require a dry mix cycle to ensure that the heated aggregate is mixed with the crumb rubber before the asphalt cement application. Mixtures should be produced at 149° C to 177° C (300° F to 350° F). Placing and Compacting Laydown temperature should be at least 121° C (250° F). A finishing roller must continue to compact the mixture until it cools below 60° C (140° F). Otherwise, the continuing reaction between the asphalt and the crumb rubber at elevated temperatures will cause the mixture to swell.(2) Continued compaction until the mixture cools below 60° C (140° F) serves to contain the expansive pressure of the compressed rubber. Quality Control Parameters that must be monitored during mixing for dry process mixes include rubber gradation, rubber percent of total mixture weight, rubber prereaction or pretreatment, and time of plant mixing. Since dry process binder systems are partially reacted with the rubber, it is not possible to directly determine the properties of the binders. It is recommended that compacted mixes be sampled according to AASHTO T168(13), and tested for specific gravity in accordance with ASTM D2726(14) and in-place density in accordance with ASTM D2950.(15)

UNRESOLVED ISSUES

There are several unresolved issues relative to the use of rubber as fine aggregate in asphalt concrete using the dry process. The overwhelming majority of projects and data concerning crumb rubber use in asphalt paving are from installations using the wet process. As a result, there is a lack of field data to evaluate performance. There have been six projects in the United States where asphalt pavements with CRM have been recycled. Roughly half of these projects were wet process and the other half were dry process. Apparently, there are no physical problems with recycling reclaimed asphalt pavement containing CRM as a portion of the aggregate in a new asphalt paving mix; however, additional field trials are needed. Although only a limited amount of air emissions data from asphalt plants producing hot mix containing CRM are currently available, there is no evidence thus far that the use of an asphalt paving mix containing recycled crumb rubber exhibits any increased environmental impact when compared with that of emissions from the production of a conventional asphalt pavement.(16) Air emission data from a project in New Jersey in 1992 where dry process CRM was recycled as 20 percent of new aggregate in a drum mix plant showed that current air quality standards were not exceeded during the recycling.(16) Nevertheless, there is a need for additional studies on recyclability and worker health and safety issues for CRM asphalt paving mixes. Some of this work is presently underway and, as data become available, they should be incorporated into what is already known concerning these two aspects of using CRM in asphalt pavements. Because of fluctuations in the performance of CRM asphalt mixes in different locations and/or climatic conditions, there is a need for more carefully controlled experimental field sections in different climatic regions throughout the United States in order to obtain more reliable performance data. Binder and mixture properties in these different regions need to be more accurately determined and documented. Performance records of these test sections may need to be monitored over a long period of time, at least 5 years and possibly as long as 30 years.(2) Additional research is needed to define the properties of binders produced by the dry process. Desirable properties for dry process hot mix asphalt mixtures need to be better defined.

REFERENCES

  1. Heitzman, Michael. "Design and Construction of Asphalt Paving Materials with Crumb Rubber Modifier." Transportation Research Record No.1339, Transportation Research Board, Washington, DC, 1991, pp. 1-8.
  2. Epps, Jon A. Use of Recycled Rubber Tires in Highways. NCHRP Synthesis of Highway Practice No. 198, Transportation Research Board, Washington, DC, 1994.
  3. Oliver, J.W.H. "Modification of Paving Asphalts by Digestion with Scrap Rubber." Transportation Research Record No. 821, Transportation Research Board, Washington, DC, 1981.
  4. Van Kirk, Jack L. "Caltrans Experience with Rubberized Asphalt Concrete." Presented at the Technology Transfer Session of an Introduction to Rubberized Asphalt Concrete, Topeka, Kansas, January, 1991.
  5. Turgeon, Curtis M. "The Use of Asphalt-Rubber Products in Minnesota." Presented at the National Seminar on Asphalt-Rubber, Kansas City, Missouri, October, 1989.
  6. Shook, James F. Experimental Construction of Rubber-Modified Asphalt Mixtures for Asphalt Pavements in New York State. ARE Inc., Riverdale, Maryland, Report Submitted to the New York State Department of Transportation, May, 1990.
  7. Estakhri, Cindy K., Joe W. Button, and Emmanuel G. Fernando. "Use, Availability, and Cost-Effectiveness of Asphalt Rubber in Texas."Transportation Research Record No. 1339, Transportation Research Board, Washington, DC, 1992.
  8. Estakhri, Cindy K., Joe W. Button, and Emmanuel G. Fernando. "Use, Availability, and Cost-Effectiveness of Asphalt Rubber in Texas."Transportation Research Record No. 1339, Transportation Research Board, Washington, DC, 1992.
  9. Swearingen, David L., Newton C. Jackson, and Keith W. Anderson. Use of Recycled Materials in Highway Construction. Washington State Department of Transportation, Report No. WA-RD 252.1, Olympia, Washington, February, 1992.
  10. Emery, John. "Evaluation of Rubber Modified Asphalt Demonstration Projects." Presented at the 74th Annual Meeting of the Transportation Research Board, Washington, DC, January, 1995.
  11. Allen, Harvey S. and Curtis M. Turgeon. Evaluation of "PlusRide" (A Rubber Modified Plant Mixed Bituminous Surface Mixture). Minnesota Department of Transportation in cooperation with the Federal Highway Administration, St. Paul, Minnesota, January, 1990.
  12. AASHTO Guide for Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, DC, 1993.
  13. Rebola, Sekhar R. and Cindy K. Estakhri. "Laboratory Evaluation of Crumb Rubber Modified Mixtures Designed using Tx DOT Mix Design Method," Presented at 74th Annual Meeting of the Transportation Research Board, Washington, DC, January 1995.
  14. American Association of State Highway and Transportation Officials. Standard Method of Test, "Sampling Bituminous Paving Mixtures," AASHTO Designation: T168-82, Part II Tests, 14th Edition, 1986.
  15. American Society for Testing and Materials. Standard Specification D2726-96, "Bulk Specific Gravity and Density of non-Absorptive Compacted Bituminous Mixtures," Annual Book of ASTM Standards, Volume 04.03, ASTM, West Conshohocken, Pennsylvania, 1996.
  16. American Society for Testing and Materials. Standard Specification D2950-96, "Density of Bituminous Concrete in Place by Nuclear Methods,"Annual Book of ASTM Standards, ASTM, West Conshohocken, Pennsylvania, 1996.
  17. Federal Highway Administration and U.S. Environmental Protection Agency. A Study of the Use of Recycled Paving Material -- Report to Congress. Report No. FHWA-RD-93-147 and EPA/600/R-93/095, Washington, DC, June, 1993.

Scrap Tires - Embankment or Fill

INTRODUCTION

Shredding of scrap tires produces chunks of rubber ranging in size from large shreds to smaller chips. Tire shreds and tire chips have both been used as lightweight fill materials for roadway embankments and backfills behind retaining walls. The shreds or chips can both be used by themselves or blended with soil. Tire shreds normally range in size from 305 mm (12 in) to 76 mm (3 in), while tire chips are usually sized from a maximum of 76 mm (3 in) down to a minimum of 12 mm (1/2 in). Embankments containing tire shreds or chips are constructed by completely surrounding the shreds or chips with a geotextile fabric and placing at least 0.9 m (3 ft) of natural soil between the top of the scrap tires and the roadway.

PERFORMANCE RECORD

At least 15 states have utilized scrap tire shreds or chips as a lightweight fill material for the construction of embankments or backfills. Some projects have used tire shreds or chips as embankment material, while other projects have blended tire chips with soil. The states that have used tire shreds or chips in embankments are California, Colorado, Indiana, Maine, Minnesota, New Jersey, North Carolina, Oregon, Pennsylvania, South Carolina, Vermont, Virginia, Washington, Wisconsin, and Wyoming. The largest known use to date is in Oregon, where 580,000 scrap tires were shredded and used in a landslide correction project. In Colorado, between 400,000 and 450,000 scrap tires were used to construct an embankment containing tire chips on a section of interstate highway.(1) To date, more than 70 successful projects have been constructed on state, local, or private roads.(2) Aside from problems with puncturing of rubber tires on haul vehicles by the exposed steel in tire chips or shreds, there have been no other construction-related problems on scrap tire embankment projects. Adequate compaction, which is always a prime concern on any embankment project, is of even greater concern on a tire shred or chip embankment project, where it is known that some consolidation will occur. Some cracking of the roadway above a tire shred or chip embankment is possible because of long-term settlement or differential settlement. Although at least 15 states have had scrap tire embankment projects, only six states (North Carolina, Oregon, Vermont, Virginia, Wisconsin, and Maine) have prepared specifications or some provisions for this use.(1) Scrap tire embankments that have been constructed to date have remained structurally stable; however in 1995, three shredded tire projects experienced combustion problems. Two of these projects were located in Washington State. The other project was located in Colorado. Preliminary assessments indicate that the combustion process may have been initiated by heat released either by the presence of organic soils and microbial degradation, the oxidation of exposed steel wires, or microbes consuming liquid petroleum products that could have been spilled on the tire shreds during construction.(2) The presence of both sufficient air and crumb rubber particles in the embankment may have played a role in the process. The crumb rubber in the presence of air may have ignited when exposed to the heat generated in the embankment, initiating the combustion process. The use of tire shreds or tire chips for the construction of embankments provides a number of advantages. The most obvious advantage is that of reduced unit weight, which is especially beneficial in situations where an embankment is to be constructed over an area with low bearing support. In addition to being a lightweight fill material, tire shreds or tire chips offer good thermal characteristics in resisting frost penetration and have good drainage characteristics, being as permeable as a coarse granular soil. Embankments present an excellent opportunity to utilize large volumes of scrap tires from one location and, under the proper logistical circumstances, can be a very economical alternative to imported earth borrow.(3)

MATERIAL PROCESSING REQUIREMENTS

Shredding The size and shape of tire shreds or chips from tire shredding can vary depending on the type of shredding machinery used. Tire shreds have a wide range of sizes, from 76 mm (3 in) up to 305 mm (12 in), which is ordinarily the largest size recommended. Chip sizes normally range from 12 mm (1/2 in) up to 76 mm (3 in). Usually, tire shreds are irregular in shape with the smaller dimension being the size specified by the manufacturer and the larger dimension possibly being two or more times as much. The chips, on the other hand, are cubical in shape. Some shreds or chips may have pieces of steel belt exposed along the edges. To minimize potential compaction problems (i.e., to reduce void space) it may be desirable to use smaller size tire chips of 50 mm (2 in) or less.(4)

ENGINEERING PROPERTIES

Some of the properties of tire chips or shreds that are of particular interest when they are planned for use in an embankment or backfill include particle size and shape, specific gravity, compacted unit weight, shear strength, compressibility, permeability, and combustibility. Due to the differences between tire shreds or chips and stone or soil-like embankment materials, physical characterization of tire shreds or chips represents a specific challenge to the tire user. Particle Size and Shape: Tire chips are normally somewhat uniformly sized and will most often range in size from 25 mm (1 in) to 50 mm (2 in). Tire shreds are more well-graded, usually ranging from 101 mm (4 in) to 202 mm (8 in) in size. Some particles may be 305 mm (12 in) or even larger, including some strip-shaped pieces. The most unusual properties of tire shreds are their flat and somewhat irregular particle shape and their relatively low unit weight. The flat shreds, especially the larger sizes, tend to lay on top of one another and develop some degree of particle interlock. They also tend to be oriented parallel to the horizontal shear plane. Particle size distribution can be determined by performing a standard sieve analysis using the procedures of ASTM D422. No modification of the standard test method is required, except that tire shreds larger than 76 mm (3 in) cannot be screened through standard sieves. A limited amount of geotechnical analysis has been performed on different sizes of tire chips. Grain size analyses have indicated that the tire chips can be classified as a well-graded, coarse-grained material, similar to an A-1-b sand and gravel (AASHTO M145) or as an SW well-graded sand with gravel (ASTM D2487).(4) Specific Gravity: The specific gravity of tire chips is expected to be in the 1.1 to 1.3 range, with higher specific gravity values for chips containing steel belts.(5) Compacted Unit Weight: Depending on the size of the chips, compacted unit weights can range from as low as 322 kg/m3 (20 lb/ft3) to as high as 725 kg/m3 (45 lb/ft3).(4) Tire shreds or chips have a maximum density that is approximately one-third to one-fourth that of typical earthen fill material. The coarser the size of the scrap tire particle, the lower the compacted unit weight. Determination of compacted density of air-dried tire chips is best made by an adaptation of ASTM D1557 (Modified Proctor test). A 254 mm (10 in) diameter by 254 mm (10 in) high mold is recommended instead of the usual 101 mm (4 in) diameter high mold. Since the level of energy applied is not critical, 60 percent of standard Proctor compaction effort is recommended.(6) Shear Strength: Limited direct shear testing of tire chips has been performed using a specially made large-scale direct shear testing apparatus. The friction angle of tire chips from these tests ranged from 19° to 25°. Cohesion values range from 7.6 kPa (160 lb/ft2 ) to 11.5 kPa (240 lb/ft2), although significant deformation was required to develop cohesion. Tire chips with a greater amount of exposed steel belts tend to have a higher angle of internal friction.(5) Typical granular soils have friction angles between 30° and 40° with little apparent cohesion. The shear strength of tire chips can be evaluated by performing direct shear tests using a 305 mm (12 in) square shear box. This is a modification of ASTM D3080 and is applicable to the testing of 76 mm (3 in) maximum size tire chips.(6) Compressibility: Tire shreds or chips are much more compressible during the initial stages of loading than conventional soils. Subsequent loading cycles normally result in significantly less compressibility of the tire shreds or chips. Higher amounts of exposed steel belts appear to result in higher compressibility, especially during the first loading cycle, probably because of less rebound.(5) Compressibility analysis of tire chips indicates that the Young's modulus of tire chips is 2 to 3 orders of magnitude less than the modulus of granular soil. The values of Young's modulus for tire chips range from 770 kPa (112 lb/in2) to 1250 kPa (181 lb/in2). Therefore, at least 0.9 m (3 ft) of conventional soil is required to be placed on top of a layer of tire chips in order to prevent or minimize surface deflections. Compressibility can be analyzed by applying a vertical load to compacted tire chips (5 layers, 60% of standard Proctor effort) within a 305 mm (12 in) diameter by 305 mm (12 in) long schedule 40 PVC pipe equipped with horizontal and vertical strain gauges. The strain gauges record horizontal and vertical stress and strain readings at 10 second intervals. Readings are plotted on stress-strain curves. The slopes of these curves are indicative of the compressibility of tire chips and the coefficient of lateral earth pressures. Permeability: The coefficient of permeability of tire chips was found to range from 1.5 to 15 cm/sec, depending on their void ratio. This is equivalent to the permeability of a clean gravel soil.(5) Permeability testing can be accomplished using a 305 mm (12 in) diameter by 0.96 meter (38 in) long PVC pipe and following the constant head testing procedures of the California Department of Transportation. A 38 mm (1.5 in) diameter water inlet was fixed to the center of the end cap. A 101 mm (4 in) wide by 50 mm (2 in) deep slot was cut into the top of the PVC pipe to allow water to flow out the top of the apparatus. The initial length of the tire chip sample is about 600 mm (24 in).(5) Combustibility: Although scrap tire particles (shreds or chips) are not in and of themselves capable of spontaneous combustion, it does appear to be possible that, under certain circumstances, an initial exothermic reaction may occur within a tire shred or tire chip embankment or backfill that could eventually raise the temperature within the fill to a point where ignition could possibly occur.

DESIGN CONSIDERATIONS

Mix Design Tire chips can be mixed or blended with soil. As the percentage of soil is increased, the unit weight of the blend increases. To simplify blending in the field, mix ratios are usually prepared on a volumetric basis. A maximum 50:50 tire chip to soil ratio is suggested so that tire chip usage is not reduced too greatly. However, if the unit weight of the fill is not a concern, then even small percentages (10 to 25 percent) of tire chips can be blended into the soil. This could improve the compactibility of the fill. Structural Design Since tire chips and shreds are unlike conventional materials, special empirical design procedures must be considered. The principal design considerations include shred or chip containment, shred or chip particle size distribution, particle shape, type of belt, compacted density of the tire chips, and whether soil will be mixed with the chips. To contain tire shreds or chips, a geotextile fabric should be placed beneath the shreds or chips and wrapped around and above them. The geotextile must completely enclose the tire shreds or chips in order to provide the necessary containment. Although smaller size tire chips have an angle of repose of around 50°, 2:1 side slopes (horizontal to vertical) are recommended.(4) At least a 0.9 m (3 ft) soil cover should be placed between the top of the enclosed tire chip fill and the base of a pavement to reduce deflections and to minimize differential settlement within the fill. If heavy wheel loadings are anticipated, an additional 0.6 m (2 ft) soil surcharge can be placed, which can be removed following appropriate settlement prior to pavement construction. A major concern in the use of tire shreds or chips in an embankment are the comparatively large settlements (about 10 to 15 percent of the height of the tire layer) that have been observed in various field studies. There is little information available on the tolerable settlements of highway embankments. Postconstruction settlements of 0.3 to 0.6 m (1 to 2 ft) over the life of an embankment may be considered tolerable provided they are reasonably uniform, do not occur adjacent to a pile-support structure, and occur slowly over a long period of time. The detrimental effects of settlements in this range can be reduced by using flexible pavement over scrap tire fills, by inducing some of the postconstruction settlement during construction by placing a thicker soil cap or a surcharge earth loading over the embankment, or by using stage construction.(7) Another possible means of mitigating scrap tire embankment settlements is to use a rubber-soil mix to construct the embankment, instead of using tire shreds or tire chips alone. It has been found that a ratio of about 40 percent tire chips by weight of soil may be an optimum value for the quantity of chips in a rubber-soil mix, although this may vary depending on the size of the tire chips and the type of soil. The optimum ratio of tire chips to soil is likely to yield a compacted dry unit weight of rubber-soil mix that is roughly two-thirds the dry unit weight of soil alone. Data on the stress-strain and strength behavior of rubber-soil mixtures are not widely available, but are necessary for the design and prediction of performance of scrap tire embankments that contain such mixtures.(7)

CONSTRUCTION PROCEDURES

Material Handling At the tire processing facility, the number of tires to be processed into shreds or chips is directly related to the intended volume of the tire chip portion of the embankment. It is estimated that every cubic yard of volume will require about 75 automobile tires that have been shredded into shreds or chips and compacted into an embankment.(5) Site Preparation The site of the embankment should be prepared in essentially the same manner as though common earth were being used for fill material. If there is a high water table or swampy area that will be at the base of the embankment, it may be advisable to construct a drainage blanket. If there is a natural flow of runoff through the area where the embankment is to be constructed, provisions should be made to pipe the runoff beneath the embankment. Mixing When tire shreds or chips are to be blended or mixed with soil, the mixing should be performed volumetrically, using bucket loads from a front end loader and blending the materials together as well as possible with the bucket. As another option to the mixing of tire shreds or chips and soil, alternate layers of the tire shreds or chips and the soil can be constructed. Placing and Compaction Once the base of the embankment has been prepared, the geotextile that will enclose the tire chips should be placed. A nonwoven geotextile fabric is recommended. Sufficient length should be provided to completely wrap around the tire chips once they have been placed and compacted. Tire chips should be spread across a geotextile blanket using a tracked bulldozer. A minimum 0.6 m (2 ft) layer or lift should be spread out over the geotextile. A recommended maximum 1 m (3 ft) lift thickness can still be spread and compacted. Compaction may be achieved by at least three passes of the tracked bulldozer over the layer of tire chips.(8) The chip particles align themselves with each other and settle fairly readily. The weight of the bulldozer passing over the tire chips is enough to readily compact the layer of chips. For larger chips or thicker layers of chips, as many as 15 passes of a bulldozer may be required to achieve compaction.(9) Once the bulldozer is able to pass over a layer of tire chips with little to no noticeable deflection or movement under the tracks of the bulldozer, the next layer or lift of tire chips can be placed. There is really no practical method at this time for performing an in-place density test on a layer of compacted tire chips. The best way to ensure that the layer has been sufficiently compacted before placement of the next layer is to continue passes of the bulldozer over the tire chips until there is no more movement of the tire chips when the bulldozer passes over them. The top layer of a tire chip embankment should be kept at least 1 m (3 ft) below the base or subbase layer of the pavement that will be on top of the embankment. Each layer of a tire chip embankment must be fully compacted before the next layer is placed. When the top layer of tire chips has been fully compacted, the sides and top of the tire chips should be fully wrapped and enclosed by the geotextile. A minimum of 0.9 m (3 ft) of compacted soil (preferably granular soil) should be placed on top of the geotextile and tire chips. The soil should be compacted in thinner layers 15 mm (6 in) to 305 mm (12 in) in thickness. The tire chip embankment will experience further deflection during placement and compaction of the soil cover. At least 0.6 m (2 ft) of soil should also be placed on the side slopes of the embankment to cover the geotextile wrap. The soil on the slopes should be compacted to the extent possible, covered with topsoil, and seeded to establish erosion control protection. The additional soil cover on the side slopes will also help minimize the potential of exothermic reactions occurring within the scrap tire embankment. Quality Control There is little in the way of field quality control testing that can be done during the construction of a tire chip embankment, other than to very closely inspect the compaction of each tire chip layer to ensure that there is very little to no movement under the passage of a bulldozer before proceeding to install the next layer of chips. However, the overall settlement or deflection of a tire chip embankment can be monitored over time by the installation of settlement plates or platforms, slope indicator devices, and bench marks along the slopes of the embankment and within or adjacent to the roadway. Periodic readings should be taken using these devices in order to keep track of the extent and rate of settlement and to compare actual settlements with predicted settlements. To minimize the potential for an exothermic reaction to occur within a portion of a tire shred or tire chip embankment or backfill, a number of preventive measures should be taken. Contact with oxygen within the scrap tire fill should be reduced as much as possible by covering the fill with at least 1.3 m (4 ft) of well-compacted natural nonorganic soil. The amount of exposed steel belts at the edges of the shred or chip particles should be limited by using magnetic separation or using large-size particles. No crumb rubber should be allowed to be used in a scrap tire fill. Tire shreds or chips that have been contaminated by liquid petroleum products should be removed from a scrap tire fill and disposed of in an environmentally acceptable manner. There should be no contact between tire shreds or chips and either topsoil or fertilizer.(2)

UNRESOLVED ISSUES

There are several unresolved issues pertaining to the preparation and use of shredded scrap tires in fills and embankments. The first and most pressing unresolved issue is to determine the cause or causes of the exothermic reactions that resulted in three scrap tire embankment fires that occurred during 1995. Other tire shred or tire chip embankment projects, especially the thick fills, including those that have caught on fire, should be more closely monitored, possibly by installing temperature probes and gas sampling wells. Gas from such wells should be periodically sampled and analyzed for oxygen level, hydrogen sulfide, carbon dioxide, carbon monoxide, and hydrocarbons. The pH of any water leaching from scrap tire fills should be measured. Laboratory investigations should also be undertaken under varying conditions and temperatures to pinpoint under which conditions exothermic reactions may be initiated.(2) One of the principal questions concerning such use of shredded scrap tires is that of an optimum particle size and shape of the tire shreds or chips. More information is needed on the basic types of tire shredding machinery currently in use and their effect on particle shape and size. The effects of mixing or blending various size shreds or chips within an embankment also need to be further evaluated in terms of resultant engineering properties, optimum gradation of shreds or chips, compaction and settlement behavior, as well as potential combustibility. Another consideration that warrants further investigation concerns the blending of soil and tire chips or shreds. Among the variables that need to be further investigated are the effect of various proportions of tire chips and soil on the engineering properties of the resultant composites, especially the bulk density and compaction characteristics. The type of soil is another variable that will influence the bulk density and compaction characteristics of the tire chip-soil blends. If possible, optimum proportions of tire chips and soil should be identified for different tire particle sizes and/or soil types. There is currently very little in the way of field quality control testing that is now being done during the construction of a tire chip or tire shred embankment, other than visual inspection of movement or settlement of the tire chip or shred layers under compaction machinery. Some rational methods of in-place density and/or compaction percentage measurement need to be developed and field tested to help minimize settlement of tire chip or tire shred fills under traffic loading.

REFERENCES

  1. Collins, Robert J. and Stanley K. Ciesielski. Recycling and Use of Waste Materials and By-Products in Highway Construction. National Cooperative Highway Research Program Synthesis of Highway Practice No. 199, Transportation Research Board, Washington, DC, 1994.
  2. Humphrey, Dana N. Investigation of Exothermic Reaction in Tire Shred Fill Located on SR100 in Ilwaco, Washington. Report prepared for Federal Highway Administration, Washington, DC, March, 1996.
  3. Epps, Jon A. Use of Recycled Rubber Tires in Highways. National Cooperative Highway Research Program Synthesis of Highway Practice No. 198, Transportation Research Board, Washington, DC, 1994.
  4. Bosscher, Peter J., Tuncer B. Edil, and Neil N. Eldin. "Construction and Performance of a Shredded Waste-Tire Test Embankment." Presented at the 71st Annual Meeting of the Transportation Research Board, Washington, DC, January, 1992.
  5. Humphrey, Dana N. and Thomas C. Sandford. "Tire Chips as Lightweight Subgrade Fill and Retaining Wall Backfill." Proceedings of the Symposium on Recovery and Effective Reuse of Discarded Materials and By-Products for Construction of Highway Facilities, Federal Highway Administration, Denver, Colorado, October 1993.
  6. Humphrey, D.N., T.C. Sandford, M.M. Cribbs, and W.P. Manion. "Shear Strength and Compressibility of Tire Chips for Use as Retaining Wall Backfill." Presented at the 72nd Annual Meeting of the Transportation Research Board, Washington, DC, January, 1993.
  7. Ahmed, Imtiaz and C.W. Lovell. "Rubber-Soils as Lightweight Geomaterials." Presented at the 72nd Annual Meeting of the Transportation Research Board, Washington, DC, January, 1993.
  8. Upton, Richard J. and George Machan. "Use of Shredded Tires for Lightweight Fill." Presented at the 72nd Annual Meeting of the Transportation Research Board, Washington, DC, January, 1993.
  9. Newcomb, David E. and Andrew Drescher. "Engineering Properties of Shredded Tires in Lightweight Fill Applications." Presented at the 73rd Annual Meeting of the Transportation Research Board, Washington, DC, January, 1994.