Application of CRM in asphaltic materials.
The result of the new testing systems, which were created during extensive research carried out acroks the country, is the ability to pinpoint the critical physical and chemical properties of asphalt in today's market. This also means determining probable long term performance when given factors such as traffic load, climate and temperature. The use of scrap materials in asphaltic mixtures will lie far beyond that of a mandate. If our roads fail to perform and meet the new criteria set by the SHRP test procedures and, more importantly, the various levels of the Superpave mixture tests, scrap tire rubber use in roads will be very short lived. Facts, not emotionalism, will be the determining factor as to whether rubber has a place, both economically and performance wise, in today's highway construction, rehabilitation and maintenance. There is no doubt, if rubber can be used in roads safely and economically, that a portion of today's waste tires can have a lasting benefit in America's road infrastructure and throughout the world.
To fully understand the effect of the new ISTEA mandate, this article is broken down into the following main considerations with corresponding main topic response:
* Why use rubber and modifiers in asphalt pavements? Highway traffic and environmental conditions;
* What is CRM [crumb rubber modifiers] and the general CRM technologies? CRM-hot mix asphalt (HMA) mixtures;
* What is Superpave) Binder and mixture testing developed by SHRP;
* What is the effect of ISTEA mandate on scrap tire availability? Legislation and tire facts.
Highway traffic, Figure 1 illustrates the reason why America's road infrastructure is deteriorating at an exponential rate. Vehicle traffic, or road miles traveled per vehicle, is nearly doubling each year, according to Federal Highway Administration statistics (ref. 2).
This amount of vehicular traffic puts undue stress on our roads to the point that new road construction and rehabilitation cannot keep pace under present funding for new construction and maintenance. In order for today's road engineers and technologists to meet the new demands being placed upon our highways, new hot mix asphalt designs and asphalt binder specifications need to be developed to meet this challenge. The binder qualities of conventional asphalts (asphalt cements) now produced by refineries need to be enhanced by adding chemical modifiers which improve performance under many types of loads and climactic stresses. An asphalt binder is defined as the substance used to hold the aggregate particles together or to a substrate. Hot mix asphalt (HMA) concrete is a designed aggregate and asphalt cement mixture produced in a hot mix plant (batch, drum, drum/batch or double barrel) where the aggregates are dried, heated, mixed with (liquid) asphalt cement (hot mix) and then transported, placed and compacted while still at an elevated temperature of 125[degrees]C (257[degrees]F) or 135[degrees]C (275[degress]F) to give a durable, definitions and fatigue resistant pavement course (ref. 3).
These definitions appear to present an ideal picture of how a pavement should perform. However, with the tremendous pounding to the surface and internal structure of the pavement by increased vehicular traffic, many of the designs incorporated into today's pavements cannot withstand the increased loads. This in turn causes pavement deterioration such as stress cracking, rutting, raveling and shoveling, which are the physical results of increased traffic loads (ref. 4). Environmental conditions, such as extreme temperature variations and moisture. also play an important role in the service life of pavements.
It is well known that pavements need relaxation time for recovery. This is generally found in the phenomena called healing (ref. 5). It is also known that during the off peak traffic hours, the small stress cracks within an asphalt pavement will tend to heal or close during resting hours. However, increased traffic loads around the clock prevent this healing and thus accelerating pavement failure (ref. 6). Most importantly, heavy tractor trailer axial weight loadings play a significant role in accelerating pavement deterioration on primary (interstate), secondary (state) and tertiary (county and city) road routes. It is the author's opinion that rubber, when properly reacted in the AC, provides memory to the binder matrix and thereby accelerates healing without loss of physical properties in the pavement.
The occurrence of pavement failures and the trend to improve the properties of asphalt binders began with the rubber industry in the early 1940s. Rubber reclaimers realized that scrap rubber had many inherent properties that could improve the asphalt binder's performance. The inherent properties in a tire include carbon black for ultraviolet resistance and strength, antioxidants for oxygen resistance, antiozonants for ozone resistance, sulfur for improved strength, and the rubber polymers, such as natural rubber, for improved tack or adhesion, and the styrene-butadiene and polybutadiene rubber for improved cohesion of the asphalt matrix (ref. 7). These ingredients found in scrap tires transfer the enhancing properties to asphalt binders. Additionally, in many types of HMA mixes, especially the large stone or stone matrix mixes, geotextiles are used to improve tensile strength as well as film thickness. The addition of the fibers from tires also offers a very exciting new development to improving the overall strength of the HMA design. However, the addition of rubber particulate increases the viscosity of the asphalt cement and, thereby, improves the asphalt's film thickness on the aggregates, allowing the hot mix to hold more binder content and at the same time reduce the absorbency of the aggregates. This, in turn, can help optimize the binder content to the advantage of the pavement design engineer. Therefore, the use of rubber as an economic alternative to virgin produced modifiers in asphalt pavements can be a major advance in solving the effects of increased highway traffic and environmental factors.
CRM-HMA mixtures FHWA has termed the use of crumb rubber produced from scrap tires for use in highway pavements as crumb rubber modifiers (CRM). The use of the material CRM is incorporated into HMA by either a wet or dry process (ref. 8). Figure 2 describes the FHWA's standard CRM terminology, process and product terminology.
Interestingly, only the fine CRM tire rubber gradation meets the full range of process and product applications. In general, coarse CRM is considered to have particles larger than 40 mesh (420 microns) in diameter and fine CRM smaller than 40 mesh. Please refer to the FHWA recent Report to Congress entitled "A study of the use of recycled paving materials," June, 1993. Additionally, for asphalt rubber membranes, the use of either a coarse or fine CRM will have a dramatic effect on cost and ease of use by the asphalt paving contractor. This is illustrated in figure 3 whereby fine CRM, called UltraSAM and UltraSAMI, can be produced at the terminal or HMA plant sites continuously or in a batch mode.
Today's tire is designed for aesthetics, comfort, performance and control. The tire itself is geometrically a torus, structurally a high-performance composite, mechanically a flexible-membrane pneumatic container and chemically consists of long chains of crosslinking micromolecules. It is for these reasons the tire seems indestructible, and if properly broken down into CRM particles, can be a useful addition for today's highway pavement specialists (ref. 10).
The manufacture of CRM materials varies according to the quality of the end products desired by the pavement engineer. The higher the quality and fineness of the CRM product the greater the capital investment for processing such products (ref. 11). Additionally, the support services required to ensure product quality need to be included in the final cost of the CRM to a highway project or asphalt blending operation. The reasons recycled products, such as tire rubber, have not been met with greater enthusiasm by highway departments and industries, has been the lack of dedicated recycling professionals, financial commitments and documentation to ensure product quality and performance. Also, the lack of understanding and the commitment to achieve the highest quality products, services and cost competitiveness in relation to other recycled materials and other polymers has been the reason for slow growth of the recycling industry. Therefore, the following three parameters must be present in CRM products for them to be competitive and accepted by industry and highway officials:
* provide performance and quality;
* ensure environmental/safety concerns;
* provide cost effectiveness.
If these parameters are met, mandatory legislation is not required. It takes a commitment by industry and government to define and experiment with recycled materials, such as CRM, to determine their place in the road construction industry (ref 12).
Depending on the desired end product mesh size and configuration, there are several ways to economically and practically produce CRM materials. The most common ways are listed in table 1.
[TABULAR DATA 1 OMITTED]
Each of these processes must be evaluated in terms of economics, particle size distributions and morphologies. As a general rule, the surface area increases with decreasing particle size, especially with the ambient wet grinding technologies. The finer the particle size, the cleaner the CRM can become of foreign matter such as fiber, steel and other inerts. The cleaner the CRM, the greater the capital investment for a plant. The fine minus 80 mesh ambient ground rubber is very clean, handles like a mineral filler, is compatible in all types of asphalt equipment and reacts at normal HMA temperatures to produce a homogenous asphalt modified rubber mix. Preblended fine rubberized asphalt is now becoming available at tenninals because it has excellent shelf life and handles easily (ref. 13).
Each of the above processes requires a basic feed stock of tires, tire parts or other types of surplus rubber derived materials. This article focuses on scrap tires in the form of buffings, tread rubber pieces and tire chunks or shredded tire chips from 1/2" to several inches in size.
A great deal of confusion has centered around the various rubber technologies for hot mix asphalt (HMA) which are being presented to the asphalt industry. The "dry method" technology generally consists of replacing a portion of the aggregates with a similar rubber gradation as illustrated in figure 4. The "wet method" technologies call for the rubber being dispersed and reacted with the asphalt cement (AC) binder prior to addition to the aggregates.
Figure 5 illustrates the reactivity of various rubber particle sizes in an AC for a theoretical condition called the "wet method." The particle size, AC type and temperature, rubber dispersion completeness, and CRM mofphology all play a critical role in the reactivity of the rubber particles in an AC binder (ref. 14). This reactivity will result in very distinct and individual characteristics in the final HMA design and pavement performance. In figure 4, the "dry method" shows coarse rubber particles as discrete unreacted chunks of rubber in a HMA. The Arizona "wet method" process results in partially reacted and unreacted rubber in the mix design. Only the UltraFine GF-80A (-80 mesh rubber with a 200 mesh mean particle size and high surface area of [nearly equal to]2 [meter.sup.2]/gram) appears to fully dissolve into a wide range of HMAs. Thus, an ambient ground minus 80 mesh tire rubber incorporates the inherent properties of the tire rubber into the asphalt cement binder, and generally requires 25% to 35% less rubber versus coarser rubber particles to achieve modified asphalt cement properties. The use of fine rubber results in minimal changes in mix designs or the need to modify testing procedures. In general, conventional HMA equipment can be used to prepare, handle, distribute and apply HMAs using the Florida Design or UltraFine process (ref. 15).
Reactivity is a direct function of surface area and particle size. The smaller the particle size, the greater the reactivity, and thus, the less the capital and operating costs. Figure 5 gives a general indication of the time required to react various sized rubber particles, adjusted to their mean size after passing a particular screen opening (ref. 16). The larger the particle, the greater the time required to react. Reactivity or solvating can also be hampered by high heating temperatures which can lead to environmental and polymer degradation problems. Extender oils, which are generally aromatics, can greatly affect the performance of the final rubber modified asphalt cement and should be avoided or used with caution.
The preceding curve was generated by applying the formula, [Mathematical Expression Omitted] where T = time to react the rubber particle and D = average particle diameter. A -20 mesh rubber with a mean particle diameter of 50 mesh (297 [mu]m) may require 20 minutes to react. Therefore, to determine the reaction time for a -80 mesh rubber particle with a 200 mesh (74 [mu]m) mean particle size over a -50 mesh particle would be determined by substituting in the preceding formula, [T.sub.50]=[T.sub.200]([297.sup.2]/[74.sup.2]), resulting in a -80 mesh ambient ground rubber reacting 16.1 times faster than a -20 mesh rubber in 75 seconds versus 20 minutes (ref. 17).
The reactivity and dispersion of rubber in the HMA will have a profound outcome on the performance of the applied HMA pavement. Coarse or chunky rubbers, in addition to continually absorbing the light ends in the asphalt binder, can also absorb chemicals leaked, spilled or emitted from vehicles. This is not to mention the civil engineering considerations rising from the new mechanical dynamics presented by using coarser rubber particles in HMA pavement. Absorption, thermal expansion and contraction may be the chief reasons why so many aggregate or gap rubber gradation projects have failed. This is generally evidenced by rubber granules being found along the roadside of these projects.
In selecting a CRM for any type of asphaltic application, particle size, gradation, source and types of rubber, technology, testing procedures (existing or modified) and the asphalt itself all need to be taken into consideration. However, the bottom line will be the actual cost effectiveness and performance of scrap tire rubber in pavement designs. Asphalt pavements have to compete with other road building materials such as Portland concrete cement (PCC). In order for rubber to have a future, it must be an asset and benefit for the asphalt industry.
Figure 6 summarizes the current understanding of interaction between the asphalt and rubber (ref. 19). The implementation of the SHRP binder tests and more in depth laboratory work, a greater understanding of how rubber interacts with specific asphalt grades and crude oil sources will be required.
The foregoing was intended to give a general overview of CRM's role in hot mix asphalt mixtures. Today, as paving technologists search for better ways to enhance road performance through the use of secondary materials, such as scrap rubber, a good understanding of how rubber interacts with the binder and the aggregates will be essential for optimum pavement design and construction.
Binder and mixture testing Assuming that scrap tires will be used in asphaltic pavements. any rubber modified asphalt will have to meet the new criteria set up by the American Association of Highway Transportation Officials (AASHTO)-Federal Highway program, which began in October 1987 and continued through March 1993. FHWA invested $50 million in SHRP to develop new ways to test and specify asphalt binders and asphaltic mixtures.
In 1987 the SHP began developing new tests for measuring the physical properties of asphalt. One of the results of this research is the new provisional performance grade binder specifications that were written with a set of tests to match these criteria. The resulting document is called the binder specification because it is intended to function equally well for all modified as well as unmodified asphalts.
The new SHRP tests measure the physical properties that can be related directly to field performance by engineering principles. In other words, unlike the past tests, pavement sections needed to be laid and evaluated over very long periods of time, usually 15 years, in order to determine whether a new additive or modifier was effective in improving their life cycles. The new test methods are designed to produce long term performance data in the laboratory within days. Each of these tests are identified in table 2.
[TABULAR DATA 2 OMITTED]
In addition to the new SHRP binder tests, there will be a set of new equipment designed around the new binder-mixture specifications. That is, once the asphalt binder has been modified and added to an aggregate mix, the binder, as well as the new mixture will be tested. Fortunately, this will allow the addition of ground rubber in a dry form to the hot mixes to be analyzed and to determine exactly what the performance would be forthcoming. For the "wet method" technologies, such as the Arizona, Bitumar and Rouse UltraFine, most of the SHRP binder specifications can be used. For the present time, only the Rouse and Bitumar processes can apply all the SHRP binder specifications with minimal modifications. The main difficulties in handling the coarser, digested rubber in the "wet process" is the rolling thin film oven test. The challenge for the "wet" technologies is to show that their methods can meet the new SHRP-PG (performance grade) specifications. To date, the only technology that is shown to be able to use all the new SHRP test equipment previously described, is the continuous or fine rubber technology developed by Rouse. Therefore, the challenge in the recycling industry is to develop those methods that economically allow one to incorporate ground rubber into the binder and improve the properties of the asphalt (ref. 21).
The "dry method" technologies, such as the Generic, Plus-Ride and the fine ambient ground rubber will require the application of the new SHRP Superpave mixture tests. Unfortunately, many of the proposed Superpave tests will require several more years of development and optimization. Therefore, the real effective determination of optimum binder contents for both the asphalt binder and the CRM will not be known for some time. Normally the Superpave mixture tests will be conducted at three levels of hierarchy called level 1 - volumetric design using gryatory compactor; level 2 - gryatory compactor plus simple shear and indirect tensile tests; and level 3 - level 1 plus level 2 with enhanced tests (ref 22).
The new SHRP asphalt binder specifications are intended to control permanent deformation, low temperature cracking and fatigue cracking in asphalt pavements. The SHRP specifications accomplish this by controlling various physical properties measured with the equipment described previously. One important difference between the currently used asphalt specifications and the new SHRP specifications is the way the specifications work. The most important difference is that the physical properties remain constant for all grades, but the temperatures at which these properties must be achieved vary depending on the climate which the binder is expected to serve.
There are two forms of testing associated with the new SHRP specifications - performance testing and classification testing.
In both cases, series of tests are performed on the binder sample and a decision is made based on the result of these tests. Performance testing answers the question "Does the material meet all the requirements of a PG specification?" In other words, specification performance testing is an accept or reject form of testing where the properties of the binder sample are compared to the required properties of a single grade.
On the other hand, classification testing answers the question "What specification grade or grades does the sample meet?" In this case, a coordinated series of tests must be formed to classify the unknown material according to the SHRP binder specification.
In general, purchasing agencies, such as state department of transportation purchasing agents, are usually concerned with performance testing. During a paving project, an agency laboratory will often collect and test an asphalt sample to make sure it conforms to the appropriate specification. If the tests do not achieve the specified values, the sample is reported to be out of spec. No further testing is necessary. However, research and development laboratories usually perform classification testing. That is, if an asphalt being tested does not meet the requirements of a specific grade, the laboratory continues testing to see whether the asphalt might meet another grade. While classification and performance testing share all the same tests, they vary in the decisions of each test result. Classification testing is a trial and error process. Thus, when performing SHRP binder tests on an unknown performance grade asphalt sample, the technician observes the results and decides what direction the tests will proceed. He will usually perform the same test at another temperature. The most important difference between the SHRP binder specifications and those currently used under the new system, is the binder sample will often meet several performance grades. Therefore, the development of SHRP binder tests in compliment with the new Superpave mixture test procedures will be the true test for CRM in pavement materials (ref. 23).
Caution should be used with the preceding values and used only as an indicator until full implementation and enforcement of the ISTEA bill takes place. Enforcement of ISTEA has been delayed until this year and many states did not meet the 5% mandate for 1994 based on the amount of federally funded HMA allocated for the given fiscal year. There is some concern that the mandate will never see implementation during the years to come due to political intercession. It is for this reason that we fully understand the nature of the scrap tire rubber dilemma and how it can be properly incorporated into HMA pavements for rehabilitation materials, such as crack sealants, for improving the life of road surfaces ref. 24).
A great deal of confusion has been generated as to what impact the transportation bill will have on the disposal of scrap tires as recycled paving material. This public law (102-240, December 18, 1991) mandates a minimum 20% utilization requirement containing "recycled rubber as a percentage of the total tons of asphalt laid in such state and financed in whole or part" by our federal government by 1997. It should be noted that this bill has the provision that 5% of the 20% can be designated for other recycled materials such as glass and plastics. Therefore, the projected amount of scrap tires utilized for road work may be slightly optimistic (ref. 25).
The real question should be "What is the number of potential scrap tire units that could be consumed for federal road work while taking into account the many different types of tires and wide variance in tire weights?" According to Modern Tire Dealer's "1993 Facts/Directory," 259.4 million truck and passenger tire units were produced in 1993 (ref. 26). To this amount, another 1,956 million pounds of off-the-road type tires need to be included. In reviewing tire statistics since 1986, the units produced each year, both as original equipment and replacement tires, has increased only slightly. This is quite understandable since tires are made to last longer, but vehicle miles traveled annually is increasing dramatically. This is one reason why our road system is deteriorating at a faster rate than they can be rehabilitated. New tire shipment figures should be indicative of the annual number of tires being discarded to our nations environment. While retreading definitely leads the list of "highest and best use" for worn out tires, this activity does not affect these figures unless retreading increases substantially over historical performance.
To arrive at a fairly accurate determination of potential tires to be utilized in road work, we can assign a discard weight value to the various tire type categories based on the percentage of tire type units produced and weight of tires by tire type as noted in table 3.
[TABULAR DATA 3 OMITTED]
According to the National Asphalt Paving Association (NAPA), approximately 480 million tons of HMA are produced annually and 45% of this amount goes to federally funded projects. Assuming 10%, 15% and 20% of this HMA amount is applied to federally funded road projects over the next three years, and using the maximum of 20 pounds of CRM per ton of federally funded HMA, we can arrive at an anticipated demand for CRM for the years 1995 to 1997 while the Surface Transportation Act is in effect (table 4). Note, these figures may be lower if the 5% exclusion for other recyclable materials is exercised by State Departments of Transportation.
[TABULAR DATA 4 OMITTED]
In order to produce 864 million pounds of CRM in 1997, 33 million scrap tire units at an average weight of 37.4 pounds per tire will be required while using an optimistic 70% yield of clean CRM per scrap tire unit for 1997 and beyond. The remaining 30% or 370 million pounds of tire residuals such as steel, cords and other inerts, will have to be disposed of in a landfill until other suitable uses for these biproduct materials of CRM can be found. Thus, using 20 pounds of rubber per tire equivalent is misleading and results in a higher value of 63 million scrap tire units versus 33 million units as noted in table 4 for 1997 and onward. The latter value is more indicative of the number of scrap tires that could be diverted to road pavements. The 37.4 pounds per tire equivalent reflects truck, bus and off-the-road (OTR) units that are and will be used in HMA pavements.
Numerous studies have been conducted on the disposition of scrap tires. Studies by Keep America Beautiful, Roy E. Weston and others have noted that 2% to 3% of the scrap tires generated in 1993 went to road pavement applications. A 3% value is probably more indicative for 1994. In other words, if the ISTEA mandate becomes a reality, road work with CRM will increase from 2% to 8%. No doubt, road work is a major step forward for recycling and utilizing scrap tires as a resource. However, we all must strive to find new environmentally sound and economical opportunities for scrap rubber including increased business activity in retreading, new tire manufacture, composites, molded products, and reuse now and in the future. Figure 7 illustrates the potential impact of the ISTEA mandate on scrap tire demand in roads. The graphs assume all other non-road applications will appreciate as well, such as molded goods, athletic surfaces, used tire export and energy applications.
The 8% estimated value only reflects the ISTEA mandate. This value could be significantly higher if the State and Federal Departments of Transportation followed the research, evaluation and actions of the Florida and Arizona Departments of Transportation in using CRM in their road networks. These states have spent years of diligent investigation and evaluation on how CRM could potentially improve the life cycle of their roads in a cost effective, safe and environmentally sound manner. Their experience should be viewed as an opportunity and a challenge to other state and federal transportation agencies for improving pavement performance and taking advantage of a valuable, strategic material - crumb rubber modifiers.
Summary and conclusions
Scrap tire rubber, as a marketable material, must meet the same scrutiny as any other asphalt additive. As experience is gained and information is shared by suppliers, pavement engineers and end-users, greater confidence and increased usage of scrap rubber will occur. Development of new and better test methods and procedures in accordance with the SHRP Superpave and improved process blending and application techniques will also enhance the use of CRM materials and its cost effectiveness. The selection of crumb rubber modified material should be based on those applications requiring a premium asphalt for high traffic conditions or extreme climate changes. The tire was designed and manufactured to ride on America's pavement. It seems logical that, at the end of a tire's useful life, it could be transformed into a CRM which takes advantage of its inherent properties for enhancing the pavement. Scrap tires, if properly processed and applied as a crumb rubber modifier, will improve paving performance and safety by being an excellent and cost effective modifier for today's highway pavement industry. All these benefits can be gained while helping to partially solve an environmental concern and utilize a strategic resource, the scrap tire itself.
[1.] Better pavements are the mother of invention, " The Asphalt Contractor, January 1994, pp. 11-14. [2.] Discussion with Michael Heitzman, FHWA, January 1993, Washington, D.C. [3.] "Evaluation of rubber modified asphalt demonstration projects," Ministry of Environment and Energy, February 1994 (PIBS 2857). [4.] Report to Congress "A study of the use of recycled paving material," June 1993, FHWA-RD-93-147 or EPA16001 R931095. [5.] Waller, H. Fred, "Use of waste materials in hot-mix asphalt," ASTM, Philadelphia, PA, June 1993. [6.] Wardlaw/Schuller, "Polymer modified asphalt binders, STP 1108, Philadelphia, PA, May 1992. [7.] "UltraFine asphalt cements applications, " Rouse Rubber Industries Bulletin No. 629. [8.] Report to Congress "A study of the use of recycled paving material," June 1993, FHWA-RD-93-147 or EPA16001 R931095. Rouse, Michael Wm., "Production, identification and application of crumb rubber modifiers (CRM) for asphalt pavements," SASHTO Meeting, August, 1993. [9.] Report to Congress "A study of the use of recycled paving material," June 1993, FHWA-RD-93-147 or EPA/600/R93/095. [10.] Morton, Maurice, "Rubber technology, " Second Edition, Van Nostrand Reinhold Company, New York, NY, 1973. [11.] Rouse, Michael Wm., "Development and application of superfine tire powders for rubber compounding, " Rubber World, June 1992. [12.] Discussion with Michael Heitzman, FHWA, January 1993, Washington, D.C. [13.] "UltraFine asphalt cements, " Rouse Rubber Industries Bulletin No. 628. [14.] "Florida likes fine ground rubber " (Contractors can buy it at the terminal), Asphalt Contractor, March 1992. [15.] "HMA design consideration, " Rouse Rubber Industries Bulletin No. 630. [16.] "UltraFine asphalt cement applications, " Rouse Rubber Industries Bulletin No. 629. [17.] J. Don Brock, "Asphalt rubber," Astec Industries, technical paper T-124, p. 9. [18.] "HMA rubber particle dynamics, " Rouse Rubber Industries Bulletin No. 631. [19.] "UltraFine rubber modified asphalt cement projects," Rouse Rubber Industries Bulletin No. 634. [20.] "Background of SHRP asphalt binder test methods," Asphalt Institute Research Center, Lexington, Kentucky, September 1993. [21.] "Polymer modifiers for improved performance of asphalt mixtures," Sheraton Colony Square Hotel, Atlanta, Georgia, November 2, 1993. [22.] Performance based asphalt for information, courtesy of U. S. Oil and Refining, Tacoma, WA. [23.] "SHRP binder grading system based on pavement design temperature, " Asphalt Technology News, Vol. 5, Number 2, pp. 1-5, Fall 1993. [24.] Bloomquest, N., et al, Engineering and environmental aspects of recycled materials for highway contractor, FHWD-RD-93-088, June 1993. [25.] "Impact of the 1991 Intermodal Surface Transportation Act on scrap tire disposal" by Michael Wm. Rouse, April 1992 issue Scrap Tire News, Suffield, CT. [26.] "Facts/Directory" 28th Edition, Modem Tire Dealer, Vol. 75, No. 1, January, 1994, pp. 19-47.
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|Title Annotation:||crumb rubber modifiers|
|Author:||Rouse, Michael W.|
|Date:||May 1, 1995|
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