Scrap tires are a serious disposal problem and continue to accumulate at increasing rates. If not managed properly, the scrap tires will present increasing environmental problems.

By Nongnard Sunthonpagasit and
H. Lanier Hickman Jr.

Utilization of scrap tires should minimize environmental impact and maximize conservation of natural resources. As a result, both government agencies and environmental groups have strongly supported scrap tire management programs. Each state has its own scrap tire laws and regulations regarding scrap tire storages, collection, processing, and use. The regulatory practices include landfill bans and scrap tire fees. Scrap tires can be shredded into raw materials for use in hundreds of crumb rubber products. The crumb rubber market has been one of the fastest-growing scrap tire markets over the last six years. One of the most interesting and potentially large-volume uses for crumb rubber is in rubberized asphalt pavement. Although the rubberized asphalt pavement indicate many advantages, many state highway departments have not been convinced of the merits of using rubberized asphalt in a major capacity. Deterrents for the use of crumb rubberized asphalt include high initial cost and occasional poor performance under certain conditions, when compared to conventional asphalt concrete. However, when one compares rubberized asphalt to conventional paving on a life cycle basic, the life cycle costs are less disparate.

INTRODUCTION

The purpose of this paper is to provide information that will expand the understanding of the opportunities for the use of crumb rubber manufactured from scrap tires. The information presented is current as of the year 2002. This paper is a summary of a large report prepared and presented to the Maryland Environmental Service and the Hickman Intern Program of SWANA. This paper:

• Identifies current utilization alternatives in the use of scrap tires.
• Reviews the current status of markets for scrap tires, uses of scrap tires as an energy source, civil engineering applications, and crumb rubber.
• Describes the current utilization and emerging alternatives in the use of crumb rubber.
• Identifies the current and emerging technologies and approaches to manufacture and market crumb rubber.
• Identifies the current and emerging crumb rubberized asphalt technologies.
• Reviews current status of markets for rubberized asphalt.
• Provides historical, current, and future crumb rubberized asphalt activities and obstacles to crumb rubberized asphalt in specific states. Reponses received from rubberized asphalt officials and state highway organizations to a survey aided in the accomplishment of this task.
• Identifies strong and weak points of using crumb rubber in rubberized asphalt.
• Recommends techniques and approaches to overcome the weak points.

SCRAP TIRES

About 280 million scrap tires (ST) were generated in the US in 2000 with an annual growth rate of about 26 percent. In addition, about 2 billion scrap tires have accumulated in stockpiles. A tire is a thermoset material that contains cross-link molecules of sulfur and other chemicals. This makes STs very stable and nearly impossible to degrade under ambient conditions. STs stored out in the open are potential breeding grounds for disease-carrying insects and rodents. In addition, tire fires can be difficult and expensive to extinguish and can cause environmental problems (Clark et al. 1992, Jang et al. 1998, Snyder 1998, and USEPA 1993).

SCRAP TIRE MARKETS AND PRICES  

In the U.S. there are six major markets for scrap tires:

1. tire derived fuel (TDF),
2. crumb rubber (CR),
3. civil engineering applications (CEA),
4. export,
5. agricultural uses, and 6) retreading. Figure 1 provides the estimated total U.S. scrap tire markets from 1994-2000.

Figure 1 shows that at the end of 2000, the markets for STs were consuming 66% of the annual generation. The other 34% of annual generation were either being legally disposed by placement in landfills or monofills, dumped illegally, or were going utilized in the retread market (Blumenthal and Serumgard 1999a). From the end of 1996 through the end of 2000, the total number of STs going to recovery or use markets actually decreased. Although the use of STs as CR and in CEA markets have been increasing, this increase is not enough to make up for the losses in the TDF market. However, one difficultly in estimating the decline in the TDF market from 1996 to 1998 is the change in calculation methods used to measure capacity to actual use. There appears to be an actual drop in usage of TDF from 1998 to 2000. The main reason for this decrease is an economic boom in construction and increased productivity requirements for cement kilns (Serumgard Email-Contact 2001).

SCRAP TIRE UTILIZATION ALTERNATIVES

Utilization of STs should minimize environmental impact and maximize conservation of natural resources. This ideally means reuse of retread first, followed by reuse of the rubber to make rubber products or paving, and then combustion and, ultimately disposal in landfill. Figure 2 summarizes scrap tire utilization alternatives.

Scrap Tire Utilization Alternatives

Tire Derived Fuel (TDF)

STs have a potential calorific value slightly greater than coal (> 12,000 Btu). STs can be burned either as a whole tire or processed into chunk materials (Jang et al. 1998 and Riggle 1994). TDF is considered to be environmentally safe and the emission products of combustion from burning tires can be adequately controlled with appropriate air emission control equipment (UNEP 2000). In fact, emissions from TDF combustion processes are lower in moisture content, sulfur content, fixed carbon, and more chemically homogeneous compared to most types of coal (STMC Document Undated). 

Agricultural Use

STs are regularly used in agriculture in a variety of ways; such as used to weigh down covers at animal feed lots, to protect fence posts, and used in erosion control and retention purposes (Blumenthal and Serumgard 1999a and Kearney 1990, 1997, 1999).

Civil Engineering Applications (CEA)

STs are used in many CEA, but there are a number of technical, environmental, and economic constraints that need to be more fully evaluated (Blumenthal and Serumgard 1999a) than has occurred to date. While there are hypotheses that shredded tires can affect water quality, there is no evidences that the concentration of metals in surface and groundwaters in contact with STs will fail to meet primary drinking water standards (DWS). However, the steel belts exposed at the cut edges of the tire shreds might increase the levels of iron and manganese (Kearney. 1990. 1997. 1999).  

Artificial Reefs: Artificial reefs are built with STs by bundling them together, then dropping and anchoring them in coastal waters. The tires form artificial reefs attractive to many species of fish in the same way as natural coral reefs (Clark et al. 1992, Hershey et al. 1987, RCT Vol. 51, Snyder 1998).    

Breakwaters: Tires have excellent energy absorbing characteristics. As a result they have been used as breakwater barriers to protect a harbor or shore from wave impacts. Normally such barriers are built by filling STs with foam rubber and then lashing them together in modular bundles (Clark et al. 1992 and OECD 1981).

Backfill for Wall and Bridge Abutments: STs can be used as backfill for walls and bridge abutment. The weight of the tire shreds reduces horizontal pressures on the wall, allowing for construction of thinner and cheaper walls. In addition, tire shreds are free draining and good thermal insulation, which eliminates problems with water and frost buildup behind the walls (Kearney, 1990, 1997, 1999).  

Culverts: In this application, groups of three whole tires, held together with steel strapping, are placed side by side with other groups of three whole times in a trench to form a culvert. The bottoms of the tires are filled with sand, which serves as ballast to hold the tires in place (Epps 1994 and RRI WWW 1999):

Highway Crash Barriers: STs can be used as highway crash barriers by binding STs together with steel cable with a fill of fiberglass or sand. The ST crash barriers absorb impact of crash from automobiles traveling at high speeds, reducing the risk of fatality and injury (Clark et al. 1992 and Jang et al. 1998).

Landfill Construction and Operation: STs can be used beneficially in the construction and operation of landfills (Hershey 1999). STs have been used as drainage media in leachate collection and removal systems. STs have also been used in surface water run-on/run-off control structures. In some instances, STs have been used as alternative daily cover (ADC), but the combustibility and high permeability properties of tire shreds makes this application questionable. If used as ACD, it would be prudent to mix tire shreds with soil prior to placement over the working face. This should help mitigate the potential for fire hazard and help reduce rainfall infiltration (CIWMB, 1997, 1998a, 1998b, 1998c, 1998d).

Levee Reinforcement: In 1998, a reinforcing wall 1,400 feet long and 20 feet deep using 2-inch rubber chips made from 45,000 STs, was added to the levee of an irrigation canal in California adjacent to the Feather River. The levee is being monitored under carefully controlled water flow and pressure conditions to evaluate the device's performance on seepage previously occurring at the site (CIWMB 1999)

Membranes: The State of Arizona has studied the use of asphalt-rubber membranes as pond liners and controlling moisture content in swelling clay soil sub-grades. The study indicates that asphalt-rubber membranes are cost-effective solutions for reducing the effects of swelling clay sub-grades (Epps 1994).

Sludge Composting: Shredded tires can be used as a bulking agent in the composting of wastewater treatment sludges. The main advantage of using tire chips over wood chips in composting is that the tire chips are more nearly uniform in size and composition. This uniformity helps improve air circulation, thereby helping in odor control. Since tire chips do not degrade, they can be recovered from the compost mass and used once again for the composting. A major disadvantage of using tire chips for sludge composting is the initial cost of the chips (Clark et al. 1992).  

Temporary Roads: Whole tires can be used to build temporary roads by binding them together in overlapping patterns and then laying down directly on soft soils as a road-bed. They can also be used to build temporary fords at small stream crossings (Goldberg 1991).

Slope Stability, Erosion Control, Sub-Grade Fill, and Embankment: Tire shreds can be used as sub-grade fill, in the construction of highway embankments and other fill projects. The purpose of this use is to help stabilize road beds in crossing swamps and other soft soils where settlement is a critical factor. STs have also been used to retain forest roads, protect costal roads from erosion, enhance the stability of steep slopes along highways and reinforce shoulder areas (Kearney. 1990. 1997. 1999.). Cost savings over using conventional construction material can be impressive. For example, use of tire shreds for lightweight fill in the Portland, Maine highway embankment saved the Maine Turnpike Authority $300,000 (CIWMB 1999, Clark et al. 1992, and Jang et al. 1998). 

Domestic Reuse and Export

Used tires, which still have legal tread depths can be resold for use on other than new motor vehicles. Domestic reuse appears to be only a negligible percentage of the market (Clark et al. 1992 and CIWMB WWW 1994). Export of used tires constitutes a major market for used tires; more than one million used tires are exported per month (Blumenthal and Serumgard 1999a). 

Disposal in Municipal Solid Waste Landfills

Disposal of waste tires in municipal solid waste landfills is the least desirable method of management. STs or shredded tires have been banned in many states. Some of these states include California, Illinois, Iowa, Massachusetts, and Virginia. However, other states permit the disposal of shredded or split STs in landfills. Some of these states are Iowa, Louisiana, and Oklahoma (USEPA 1999 and RCT Vol.51). Advantages for this option are low capital investment and operating costs, ease of management, and the possibility for use of the discarded tires in landfill operations.

Disposal in Dedicated Monofills

Dedicated monofills have become more prominent in some locations as a means to manage STs. Currently shredded STs are acceptable in monofills in some states, such as Arkansas and Kansas (USEPA 1999).

Cut, Stamped, and Punched Products

STs without steel bead can be stamped or punched to achieve specific shapes. Examples of products created by this process are shoe soles, insulators, and fishing equipment (Clark et al. 1992 and Jang et al.1998). The main obstacles to the further growth of this industry might be the inferior quality and aesthetics of such products (Douglah 1995).

Pyrolysis

Pyrolysis is the chemical breakdown of organic compounds by heating in the absence of oxygen. Products of pyrolysis are less complex molecules than the fuel pyrolyzed. Examples of such products are an oil-like substance (not unlike bunker C fuel oil), char, carbon black, and in the case of waste tires, scrap steel (STMC and Jang et al. 1998). Carbon black is used to produce molded goods, inks, and pigments. The oil produced by pyrolyzing STs, after further refining, can be used as a gasoline additive to increase octane and as fuel (USEPA 1993). The scrap steel residue from the pyrolyzing of STs is both an operation and management problem. Pyrolyzing STs, as a management method, is totally dependent on the cost of crude oil. This barrier has limited the use of pyrolysis (Hershey et al.1987).  

Reclamation of Rubber From Waste Tires

Reclamation of the rubber in waster tires can be done using oil, water, and reclaiming agents. Reclaimed rubber can be blended with virgin compounds to produce rubber feedstocks. The combination of environmental regulations and the drop in blended rubber markets prices has resulted in the almost complete elimination of reclaimed rubber in the US (Baranwal and Klingensmith 1998 and UNEP 2000). 

Retreading

Retreading is a method by which worn tires with usable carcasses, are given a second life. In the retreading process, the remaining tread rubber is removed by a grinding process called buffing, and then replaced with a new tread. Properly retreaded tires are as structurally safe and effective as new tires (USEPA 1993). Due to the high cost of truck tires, the truck tire retreading business has shown steady growth over the years. Passenger tire retreading is decreasing because of low prices of new tires. The retreading tire market is limited to certain types of tires by the structure and quality of used tires and user preferences (Clark et al. 1992). Increased usage of retreaded tires can extend the life the life of tires, but eventually they will still become a waste tire requiring management. However, increased usage of retreaded tires would result in, over time, fewer waste tires.

CRUMB RUBBER

This section discusses the manufacture, uses and markets for crumb rubber (CR). CR is to describe shreds from scrap/waste tires that have been reduced to a particle size of 3/8-inch or less. CR comes from two principal sources - tire buffings, a retread by-product, and ST rubber (Blumenthal 1997a). In the general ST market, there is no significant difference between buffings and ST rubber (Blumenthal and Serumgard 1999a, 1999b). However, it appears that the ST rubber will be the source of most CR in the future since retreading is declining. Consequently, the focus of this section will be on the utilization of ST as a source for crumb rubber.

Crumb Rubber Manufacturing Technologies

The finer the particles size, the higher the surface area, the cleaner the CR, and the greater the capital investments for a production plant are general rules in CR production (Dufton 1995). This section describes current and emerging crumb rubber manufacturing technologies. The emerging technologies that are discussed are not currently shelf-item technology, but offer possibilities for the future

Crumb Rubber Manufacturing Technologies: STs can be reduced to crumb rubber by:

• ambient grinding - a mechanical grinding system that operates at room temperature and literally tears the tire material apart, or
• cryogenic processing - a freezing process where STs are frozen at very low temperatures by liquid nitrogen, and then shattered like breaking glass (Blumenthal and Serumgard 1999b and TNRCC 1999).

Emerging Crumb Rubber Production Technologies: Two emerging technologies to reduce STs to crumb rubber are:

• surface modification technology - size-reduced rubber is exposed to either fluoride or bromide gas. The gas causes a permanent chemical change to the outer layer molecules of rubber particles, which allows it to blend with urethane, and
• devulcanization technologies - devulcanization technologies break the carbon-sulfur bonds of rubber. These technologies include ultrasonic, chemical, bio, and ozone-knife devulcanization technologies. In ultrasonic devulcanization, STs are exposed to high intensity ultrasonic vibrations. The resulting energy absorbed by the rubber is theorized to fracture sulfur-sulfur bonds. Chemical devulcanization technology blends sulfur and chemical additives with ST pieces on a mill or in an internal mixer to scissor sulfur-sulfur bonds by the heat and shear. Bio-processing technology relates to the biological processes of microorganisms while ozone-knife technology deals with ozone-rich atmosphere to break the sulfur-sulfur bonds. Once these bonds are broken, the material can be recombined with polymers in a greater percentage than is currently possible. This recycled rubber can be used as or is compounded back into virgin rubber to give lower cost rubber products with good physical properties (Blumenthal Document Undated, Baranwal and Klingensmith 1998, CIWMB 2001, Kim and Lee 2000, Kim and Park 1999, M.F. 1998, RRI WWW 1999, Romine 1998, and ZYN WWW 2001).

Crumb Rubber Markets

The overall consumption of CR increased (see Figure 1) about 400 percent during the 1994-2000 period. Although the CR market offers a greater return on investment than any other scrap tire market and continues to grow, the prices for the CR did not trend upward during the period 1994-200. Therefore, growth in production increased and an industry expected increase in prices did not. The main reason for this contradiction is overcapacity. There are many reasons for this overcapacity including (Blumenthal and Serumgard 1999a. 1999b, Blumenthal 1997b, Phillips 1996, and Serumgard Email-Contact 2001):

• new entrants: It takes up to three years for a newly formed company to develop an end-use market and become profitable. With such a short time to become competitive, new entrants have typically put tremendous downward pressure on CR pricing to secure some market share.
• failed enterprises: Since the beginning of 1999, at least four new CR generators have started operations and at the same time at least one major producer went out of business. Failed operations sell their liquidated remaining inventory at virtually any price just to move the material.
• imports: Imports undercut the local market to the detriment of pricing in the US market.
• paving markets: Many companies depend on paving applications with 30 percent of CR being diverted to this segment annually. It seems that there are not enough paving applications to go around. Many states highway departments are not yet convinced to process rubberized asphalt. Then, companies are looking elsewhere to sell their product with aggressive in pricing to secure their market share.
• buffings: Buffings come from tire retreading processes, are essentially a by-product, are more easily processed and are of high quality. As a result, the availability of tire buffings from retreading operations can depress the tire CR market.
• glut: Glut is a sub-standard CR. Some companies just throw every tire together and grind them up.

Crumb Rubber Utilization Alternatives

In 2000, the market place consumed 867 million pounds (23 million tires) of CR. Of the total 867 million pounds, asphalt modifications and molded products were almost equal in market share, and combined had approximately 60% of the total market, with the remaining 40% going into the manufacturing of the new products. This section summarizes the alternative utilization options for crumb rubber:

• agrimats:The low thermal conductivity of rubber can provide a warm bed for animals. This product is resilient, stays soft, provides a comfortable cushion and energy absorption characteristics, and does not compact in the way straw and sawdust does (Dufton 1995). In the case of dairy cattle, agrimats can increase milk production by 10% (Snyder 1998).
• automotive parts: The future of this market depends on better technical processes, better quality material, automotive industry influence, and change in image. Recently, the automotive industry has expressed strong interest in seeing rubber parts it purchases contain CR (Blumenthal and Serumgard 1999a). Examples of products in this market include hoses, friction materials, and pickup bed liners (RRI WWW 1999).
• filter medias: Small size CR can be used as a filter media for biofilters for volatile organic compound removal and it is cost competitive with other filter media (Shin 2001).
• footwear uses: Footwear is an emerging CR molded product that emphasizes development of both thermoplastic and thermoset compounds. Nike and Reebok have been examining the possibilities for producing shoes with recycled rubber content. With CR incorporation at only a 20% level, this would represent an annual usage of 150 million pounds per year (Baldwin et al. 1995).
• molded and extruded products: In molded and extruded products, CR is used as a processing aid and as an extender. An extender improves mold release, increases the sharpness of moldings, stiffens the compound, and makes for better control in extrusion and reduction of die-swell (Dufton 1995). Examples of the products that can be produced form these methods include tiles, mats, bumpers, soaker hoses, ramps, safety surfaces, erosion control, and replacement for rocks in gravel roads.
• mulch: Using CR as mulch retains soil moisture, inhibits weeds, insulates a plant's root structure, and reduces the amount of pesticides, water and fertilizes needed in landscaping and agricultural applications. Because tire material does not readily decompose, mulch made from tire shreds is not as prone to been replacing as is mulch from other mulch materials. Although tire mulch is more expensive initially, it is more economic over the long term (TNRCC 1999).
• new tires: CR can be used as a low volume filler material in the manufacture of several types of tires. Recently, tire manufactures have been trying to use approximately 5% CR in the production of new tires. That might not seem like a lot, but if every tire made in the U.S. had a 5% CR content, the market for CR in new tires would increase by more than 185 million pounds annually (about 6 million STs) (Baldwin et al. 1995). The State of North Carolina is currently studying the potential to use 25% CR in the manufacture of new tires. Safety and new product quality demands will probably restrain the growth of this market application (Blumenthal and Serumgard 1999a. 1999b).
• noise barriers: Molding CR in a state of loose granules with a polymer-based adhesive enables the production of noise barriers. Use of CR in noise barriers can increase the sound absorption capability compared to other materials used to make noise barriers. As an example, rubber panels have a better sound absorption capability than concrete blocks do (RPA WWW 2001, CIWMB 1997, RRI WWW 2001, and MDE 2000).
• parking lot surfaces: A porous durable surface made of fiber-reinforced rubber granules could replace mulch and sand in non-paved parking lots (ART WWW 2001). There are a number of benefits in using CR in parking lots including:

• provides constant traction with superb drainage,
• creates a resilient surfacing,
• reduces dust in dry conditions,
• eliminates puddles and mud problems in wet conditions,
• durable,
• economical long lasting ground covering, and
• heavier than water it will not float or blow away.

• railroad crossings: Recently, rubber has been used to pave railroad crossing, providing a smooth, safe, and durable crossing for the vehicle. This technique also simplifies maintenance problems, such as removal crossing materials. The installment cost of this rubber is higher than other materials, such as dirt and gravel, wood timbers, and concrete, but for the long term the life cycle cost is lower than competing materials (Clark 1992 and Snyder 1998).
• rubber and plastic blends: CR can be used as a component in mixtures with thermoplastics to extend and modify the properties of polymeric materials. These blending materials can be molded or extruded into new products (Dufton 1995). There appears to be significant market potential for this application due to continuing research and development of products (Blumenthal and Serumgard 1999b and Liu et al. 2000).
• rubber in equestrian areas: Sand in equestrian areas, such as arenas, is dusty and normally requires an indoor sprinkler system to control dust. As substitute product, particulate CR has also found some use here, because it is soft, safe, and moisture resistant. This product also reduces injuries to horses and riders, provides better riding comfort, obtains higher performance, alleviates the dust problem and provides low maintenance (Snyder 1998, ART WWW 2001, RTI WWW 2001, McIntire 2000, and TRMA WWW 2001).
• rubber matting: This market segment is one of the fastest-growing markets in North America due to the low price and variety of available colors. 3/8" CR with all the wire removed can be used as playground surface cover to improve safety. Rubber mats can replace asphalt, concrete, stone, sand, wood chips, and packed earth playground surfaces (TNRCC 1999 and Snyder 1998). Rubber mats drain well and are cheap, easy to use, clean, very durable, resilient, nontoxic, and provide better cushion than other materials. Rubber mats also do not rot, decay, catch fire, leach, and attract animals, rodents or insects.
• soil amendments: CR can be used for soil conditioning for sport, leisure, and safety surfaces. CR acts as an aerator and promotes drainage of water, aids water retention, provides stabilization for the growing roots of plants and grasses (Dufton 1995), as well as prevents soil from compacting. CR also improves the moisture retaining properties of sandy soils and also the drainage of clay soils. Desert soils have been rendered more fertile by the use of a CR based additives.

Currently, there are two patented soil amendment products on the market that use CR - Rebound, marketed by American Tire Recyclers (ATR), and Crown III marketed by Jai Tire Industries (Rigger 1994 and Snyder 1998). Crown III is layered on top of the soil and Rebound is mixed into the soil (Phillips 1996. and 1998 and STMC 1995). All types of playing fields are candidates for using CR as soil amendments in the high use areas of the fields. An example of a high use area, are the areas in front of the goals where the grass does not grow well. Similarly, public parks with a high volume of foot traffic are also candidates for the addition of CR as a soil amendment, particularly because many of these parks require frequent resodding. CR has the potential to alter surface characteristics and increase wear tolerance of turf grass exposed to traffic (Crum et al.1998, Grunthal 1998, TNRCC 1999, and Blumenthal and Serumgard 1999a).

• traffic cones: In the USEPA Comprehensive Procurement Guidelines, safety cones are required to contain CR (50-100% by dry weight) when federal funds are used to purchase the cones. Cones containing CR are of high quality, widely available, and cost-competitive with virgin products (USEPA 1997).
• treated plywood roof sheathings: Treated plywood roof sheathing is a product of Elastomer Technologies Inc., used to reduce the slippage characteristics of plywood sheathing. This product, a latex emulsion containing CR, is applied to one side of the plywood at ambient temperature. The latex emulsion serves as a vapor barrier, waterproofing, and anti-skid surface (CIWMB 1997).

RUBBERIZED ASPHALT

Crumb Rubber Modifier (CRM) for asphalt is a general term for CR used as a modifier in the asphalt paving materials. In 2000, CRM consumed around 30 percent of all the CR sold. Although there is increasing acceptance of CRM in many states, the demand growth for this market segment is still uncertain (Dufton 1995 and STMC 1991). The use of CR in highway applications has a long history and has been attempted with varying degrees of success over the years in the USA.  

Rubberized Asphalt Technologies

CR may be used either as part of the asphalt binder called asphalt rubber (AR), or as an aggregate substitution called rubber modified asphalt concrete (RMA). There are two processes used to incorporate CR, as CRM, into asphalt:

• wet process - in this process, CR is incorporated into the asphalt mix with a liquid, such as kerosene to serve as a blender. The terms "Asphalt Rubber," "McDonald" and "Arizona" are interchangeable and used to describe the wet process.
• dry process - in this process, a cubical, uniformly shaped cut CR particle with low surface area in size from 1/2-1/8 inches is normally used and is blended dry into the asphalt mix. The terms "Rubber-Modified-Asphalt (RMA)" and "Rubber-Modified-Hot-Mix-Asphalt (RUMAC)" are interchangeable and used to describe the dry process.

The Federal Highway Administration (FHWA) has been promoting CR in asphalt paving as AR or RMA. Usually one mile of two-lane road with a three-inch-thick layer uses 1,600 tires for AR and 8,000-12,000 tires for RMA (USEPA 1993). Factors in determining whether the wet or dry process are source, type, size, and surface area of CR, asphaltic and traffic conditions, climate change, technology, and price (Dufton 1995 and AVP WWW 2001). The smaller the CR particles, the greater the flexibility there is for using CR in asphalt paving (Bloomquist et al. 1993).  

SuperPave

The Superpave (SUperior PERforming Asphalt PAVEments) system is a new emerging technology, developed by the Strategic Highway Research Program (SHRP). SHRP is a cooperative effort of the federal government, state highway agencies, industry, and academia (Halladay 1998). Superpave is a tool to design asphalt pavements that will perform better under extremes of temperature and heavy traffic loads (NCSC WWW 2000).

Superpave binders are designated with a PG (Performance-Grade) rating (NECEPT WWW 2001). The binder is evaluated in the laboratory for the entire range of temperatures that pavement is expected to be subjected to over its design life, including extreme high and low temperatures (KEI WWW 2001). The binder selection is based on the climate in which the pavement will serve and the traffic it will bear. The binder types can be selected from various types of additives, including CRM and other polymers. Some polymer modified asphalts have a rubber component and can be graded in a Superpave system. However, the rubber does not contribute to these mixtures in any significant degree (Carlson Email-Contact 2001).

Current Status of Rubberized Asphalt

CRM asphalt highways in the hot and dry southwestern states of the US have performed satisfactorily. These states have extensive experience with AR. Figure 3 shows photos of a four-inch-asphalt-concrete-overlay (Left) and a two-inch-AR-pavement over a two-inch-asphalt-concrete-leveling-course (Right). Both sections were placed over cracked and sealed concrete. Test sections were constructed in 1990 on I-40 near Flagstaff, Arizona. The photos indicate that the AR-pavement is outperforming the ConVentional Asphalt (CVA). The AR section depicts reduced cracking (Carlson and Zhu 1999 and RPA News 1998).

CRM asphalt highways in the wet and cold northern states of the US have performed poorly or equal when compared to the CVA. The main reason for these failures appears to be weather-affected construction problems, where ambient temperature is required for construction. However, the exact cause of various pavement failures is not known. Figure 4 shows photos of a CVA (left) and an AR-concrete containing 14 percent fine CR (right). Test sections were placed in 1993 on Route I-44, Phelps County, Missouri and an evaluation was conducted in 1998. Both photos show low severity longitudinal cracking in the center section of the driving lane. However, overall the AR test section performed as well as the CVA (Trautman and Williams 1999).  

Figure 3

Performance of CVA (Left) and AR Pavement (Right) in Arizona

Source: RPA News 1998 

Figure 4

Performance of CVA (Left) and AR Pavement (Right) in Missouri 

Source: Trautman and Williams 1999

The following data and information was gathered primarily through personal e-mail contact by the senior author with State Highway Agencies and organizations relating to the CRM asphalt.

• A number of states including Illinois, Maryland, Oklahoma, and Ohio have experienced problems using CRM asphalt. The problems include cost ineffectiveness, poor performance or equal performance compared to CVA, and poor mix qualities, when compared to CVA.
• North Carolina, Illinois, Indiana, Kentucky, Missouri, Montana, Nevada, Ohio, and Oklahoma have no plans to use CRM asphalt in the near future.
• Iowa, Maryland, Michigan, New York, Rhode Island, Virginia, and Washington allow the use of CRM asphalt if the mixture properties meet the requirements in Superpave. However, due to the higher costs to produce this binder, the use of CR in this technique is not being used extensively.
• A number of states including California, Connecticut, New York, and Texas have had good experiences using CRM asphalt. Results show excellent construction ability and performance, maintenance-free performance, and increasing life cycle. Both wet and dry processes have been used extensively in California. Texas is increasing its annual CRM asphalt usage due to CR availability at a competitive price. New York allows the use of CRM asphalt produced by the wet process at the contractor's option.

Comparing CRM Asphalt and Conventional Asphalt

CRM asphalt is comparable to CVA as a paving material. The initial cost of CRM asphalt is higher that CVA. However, when the two pavements are compared on a life cycle basic (LC), the LC costs for CRM asphalt are less costly. The lower LC costs can be attributed with the longer life span of CRM asphalt. LC cost factors that make this possible include reduced cost of construction, inspection, and maintenance, fewer traffic inconveniences, less construction noise, and other inconveniences to the public (RACTC WWW 2001). Noise studies in Western Europe and the US show that the use of AR can reduce traffic noise as much as 85%. The advantages and disadvantages of using CRM are summarized in Table 1.

Although the test results indicate many advantages, many state highway departments have been unwilling to use asphalt rubber paving. The primary states using CRM asphalt is still limited to California, Arizona, Texas, and Florida. Legislative, economic, technical, sociological, and environmental barriers limit the use of CRM asphalt elsewhere in the US.

 Legislative/Regulatory and Political Barriers: The US Congress passed Intermodal Surface Transportation Efficiency Act (ISTEA) in 1991, addressing the use of scrap tires in transportation facilities. Subsection 1038(d) of the act, "Use of Recycled Paving Materials," requires states to use scrap tire in federal funded highways. The requirement began in 1994 at the level of 5% to a maximum of 20% in 1997 and each year thereafter. If this act were implemented, it would have increased the use of scrap tires from 17 million in 1994 to 70 million in 1997 (USEPA 1993). However, in 1995 ISTEA was repealed, presumably as a matter of politics and lobbying efforts of the asphalt industry. As a result, the use of CRM asphalt now depends on the willingness of the state's DOT to begin their programs (Blumenthal and Serumgard 1999). 

 Economic Barriers: The economic barriers include high initial cost, high capital investment, LC analysis, funding sources, and patents:

• high initial cost - There is an increased cost associated with using CRM asphalt over CVA. The factors affecting this include increased asphalt content, energy consumption, and cost of CRM (Dufton 1995). The price for a ton of A-R hot mix is typically $10 more per ton than CVA (Carlson Email Contact 2001). In general, the cost per mile of laid pavement or road maintenance increases by 50-100% over CVA, even though the road surface life is extended by 150-300% (Secretariat's Statistician 2000). Therefore, even though initial cost is higher, CRM asphalt roads cost less when considered on a LC basis.
• high initial capital investment costs - The main reason for high initial capital investment costs are associated with necessary modifications to mixing/batching plants, paving design, and compacting equipment to ensure proper crumb rubber mixing. The capital investment for A-R binding equipment is not unreasonable for a paving contractor. However, it is unreasonable if specifications allow CRM contractors to have to bid against non-CRM contractors (Carlson Email-Contact 2001).
• life cycle cost analysis - There is no standard LC cost analysis available to evaluate the LC costs of CRM compared to CVA. Many study results are unclear, are not relevant to all applications, lack long-term performance data, and have different input variables, making comparisons difficult.
• funding sources -State subsidies or grants to encourage the use of CRM asphalt are limited. Due to the high initial cost, it is difficult for state or local governments to increase the highway investment or make increased lifecycle cost to be their main goal. Instead, many set goals of a certain number of road miles to be paved per year. Since cost is often the critical factor, the long-term advantages of CRM asphalt are usually suppressed in favor of short-term savings (Secretariat's Statiscian 2000).
• patents - Some techniques are patented, which add costs to the process. It is estimated that cost increases by 35% and 27% by using McDonald and PlusRide process, respectively. As a result of expiration of the McDonald patent in 1992, since then, more CRM application contractors are becoming involved (Baldwin 1995 and Gauff Email Contact 2001). However, this decrease in costs is still not competitive when compared with CVA costs.

 Technical Barriers: The American Society for Testing and Materials (ASTM, the national voluntary setting standard organization) has not established standards for CRM asphalt. In addition, there is a lack of data, presented in a comparative and standard form, of long-term tests to demonstrate the efficacy of CRM asphalt as a paving material. As a result, other materials with ASTM standards and long-term performance/test data histories have a competitive and technical advantage over CRM asphalt (Kearney 1990, 1997, 1999).

 Sociological Barriers: Many states have conducted research on CRM asphalt during the ISTEA mandate era. Many tests were unsuccessful. As a result, many state DOTs are reluctant to try CRM regardless of the benefits it might provide (Carlson and Zhu 1999). Moreover, contractors or engineers are often reluctant to change from materials that are proven and in common use.

 Health and Environmental Barriers: Health and environmental studies of the effects of CRM have been conducted by many organizations. This section summarizes the conclusions from those studies.

• air emissions - Emission tests were conducted in California and Canada (CIWMB 1997 and Sainton and Takallou 1992). Testing results reported through 1993, have shown no obvious trend of significantly increased or decreased emissions that can be attributed to the use of CRM pavement production (FHWA and USEPA 1993). Tests have also indicate that there is no evidence that workers involved in the production and construction of CRM pavement are exposed to increased risk when compared to workers involved in the production and construction of CVA pavement.
• recycling - California conducted a study of the recyclability of CRM pavement in 1994. The results indicate that the CRM asphalt can be recycled using either microwave technology or conventional mix design. Additionally, the report indicates that during paving recycling, the employee exposures to air contaminants were below the Occupational Safety and Health Administration (OSHA) permissible exposure limits (PEL) (Carlson and Zhu 1999). Moreover, in six projects in the US, CRM pavement material has been recycled as a portion of the aggregate in a new asphalt paving (RMRC WWW 2001). The results from the use of the aggregate have not been reported in the literature at the time this paper was prepared.

 Overcoming Barriers: There are many barriers that deter the use of CRM asphalt. The future of CRM asphalt will depend on how successfully those barriers are overcome. Proponents of CRM asphalt have proposed a number of initiatives to help reduce, or at least minimize the impact of these barriers:

• provide potential funding mechanisms to help address the initial cost difference between CRMA and CVA including (Douglah 1995):
• tire disposal fees assessed on new tire purchases,
• tipping fees collected at the point of tire disposal, or
• vehicle title registration/transfer fees.

• support, promote, and fund studies regarding CR and CRM. The following studies are needed to develop criteria for increasing the use of CRM:
• LC cost analysis to determine cost effectiveness,
• quality control programs to allow continuous monitoring of the uniformity of the product for chemical composition and gradation (Heitzman 1992),
• environmental studies related to production, use and recycling of RMA asphalt,
• ASTM standards to provide technical information on product quality, process information, and production specifications,
• CRM pavement technology development to determine the optimum mix for the spectrum of anticipated environmental and road conditions that CRM asphalt would be used and the optimum method with the lowest cost of CRM asphalt (Broughton 1994), and
• mixture design method to accommodate the use of CRM. The combined influence of CRM particle size, amount, and aggregate gradation needs to be evaluated for each application (Bloomquist et al. 1993).

• educate engineers, the academic community, the federal government, state agencies, and public and private contractors about the advantages and disadvantages of CRM asphalt. Various options are available for such education:
• public educational workshops, seminars, and conferences on CRM asphalt,
• provide training and consultation services, or
• publish a comprehensive CRM asphalt technology guideline report.

• stimulate corporation among the industries, EPA, FHWA, States, and other government agencies to share information and to study the feasibility of using CRM asphalt,
• convince the FHWA to provide state DOTs with more details and guidance regarding the use of CRM pavement in their states,
• find a way to address the price differences between CRM asphalt and CVA,
• develop more markets for CRM asphalt use,
• provide research grant support for CRM asphalt R&D,
• assist manufacturers to develop equipment and methods to produce CRM asphalt at cost effective prices, and
• enact legislation, guidelines, and regulations at the federal and state levels, and ordinances at local government levels to encourage CRM asphalt usage where practical.

SUMMARY

Uses for Scrap Tires

Approximately 280 million ST were generated in the US in 2000. Approximately 66% of these tires were consumed in tire derived fuel (TDF), civil engineering applications (CEAs), crumb rubber (CR), exports, agricultural and other miscellaneous uses. The three applications that have the most potential for increased use are TDF, CEA, and CR.

Scrap Tires as an Energy Source (TDF)

From the end of 1996 through the end of 2000, the total market for scrap tires to be used as TDF market decreased. However, the economic advantage of using scrap tires as TDF might increase in the future.

Civil Engineering Applications (CEA)

The CEA market increased over about 250% during the 1994-2000 period. STs can be used in many CEA. Because of the economic advantage that STs offer over competing materials, the CEA market is expected to increase in the future.  

The CR market has been one of the fastest-growing since 1995. While the use of CR increased, the price of CR did not trend upward as the industry expected. The main reason for this contradiction is the overcapacity. The CR market is expected to expand in the future, but continue to have the same high degree of risk. The pattern for CR manufacturing is normally that companies enter the business and are forced out of business over a relatively short time period due to the low market prices.

In 2000, asphalt modifications and molded products had approximately 60% of the total CR market, with the remaining 40% going into the manufacturing of the new products.

Market availability is a function of cost and product quality or characteristics. Currently, applications for CR are becoming more complex and the product needed necessitates production of smaller particle size material. The lack of national standards for CR product hinders the growth in what has become a quality driven market.

The common processes for manufacturing CR are ambient and cryogenic grinding. In the general scrap tire market, there is no significant difference in the product produced by either process. However, these processes differ in both techniques and product characteristics. The general rule in the manufacture of CR is that the finer the particle size, and the cleaner the crumb rubber, then the greater the required capital investment for a plant. Surface modification and devulcanization technologies are emerging CR technologies that offer possibilities for the future.

Rubberized Asphalt Pavement

One of the most interesting and potentially large-volume uses for CR is in rubberized asphalt pavement. Since the 1960s, many states have conducted research on crumb rubber modifier (CRM) as an additive to asphalt since1960's. Research has studied both wet and dry processes with neither process emerging as the process of choice. Test results show many advantages to use CRM asphalt including:

• improving fatigue and skid resistance,
• improving durability,
• reducing temperature susceptibility,
• reducing traffic noise,
• reducing temperature stiffness and cracking, and
• increasing the life cycle (LC) of asphalt pavement.

However, doubts still remain about life expectancy, recyclability, emission safety related to the production and construction of asphalt pavement, and application techniques for different climates. These doubts are important and must be addressed when considering CRM asphalt pavement. Furthermore, the greatest deterrents for the use of CRM are high initial cost and occasional poor performance under certain conditions, when compared to conventional asphalt (CVA). As a result, many state highway departments are not yet prepared to include CRM asphalt as an accepted alternative in highway paving.

Additionally, a survey of the states indicates that many states are using the Superpave method to design asphalt pavements. CR may be used in this method as a rubber modified mixture, depending on the paving contractor and price. However, the deterrent for the use of CRM in this method is high production cost. As a result, there has not been extensive use of CR using the Superpave design method even though it is an accepted alternative in highway paving.

The future of rubberized asphalt will depend on how successfully the technical, economic, and attitudinal barriers are overcome. Several actions can be taken by state and local governments to help reduce, or at least minimize the barriers that impede the use of CRM asphalt. These actions include:

• create funding sources to encourage use of CRM asphalt,
• finding financial means to support various CRM R&D activities,
• educating designers and paving project buyers in the use of CRM asphalt, and
• finding means to overcome the price difference between CRM asphalt and CVA roads.

Crumb rubber is a new emerging material, which has been suggested as a solution for a large portion of scrap tires. Crumb rubber markets might be sufficiently large enough to ensure long-term scrap tire management capacity. However, crumb rubber markets are in the adolescent evolutionary stage and further research is needed to expand this market. Rubberized asphalt is comparable to conventional asphalt paving. It is more costly to use when initial costs are compared but when one compares on a life cycle basic, the life cycle costs are less costly. The cost per cubic yard CRM asphalt can be lower by as much as 13 to 33 percent over its life in an asphalt paved road. The future of rubberized asphalt will depend on the willingness of state highway departments to include rubberized asphalt in their road paving programs. If these products can demonstrate cost effective performance, production and application techniques, and environmentally acceptability, then CRM could become a major method for the management of great quantities of rubber from scrap tires.

AERPI (Alberta Environmental Rubber Products) World Wide Web. 2001. Link: AERP Photo Gallery Home Page: www.aerpi.com/index1.htm.

Allen, G. 1993. A review of Different Methods of Recycling Tyres. Presented at Rubber Europe '93 Conference, The Congress Centre, Hague, Netherlands. 214-221.

Amirkhanian, S.N. 2000. Waste Tires Cut Costs of Building New Highways. BioCycle. 46-47, December.

Amirkhanian, S.N. and Burati, J.L. 1996. Utilization of Waste Tires in Asphaltic Materials, FHWA-SC-96-02. South Carolina Department of Transportation. Columbia, SC. June.

ART (American Rubber Technologies, Inc. World Wide Web). 2001. Link: State of the ART Rubber Recycling Facility Home Page: www.americantire.com.

AVP (American Virtual Production Co.) World Wide Web. 2001. Link: Crumb Rubber Modifier Technology Home Page: http://www.bei-emulsions.com/rubber.htm.

Baldwin, S.D., Cornell, D.D., Rader, C.P., Sadler, G.D., and Stockel, R.F. 1995. Plasti