*

3336 NORTH 32ND STREET, SUITE 106

PHOENIX, ARIZONA 8501 8 (602) 955-1 1 4

ASPHALT RUBBER PRODUCERS GROUP

ASPHALT-RUBBER LITERATURE REVIEW State of the Art

The Asphalt-Rubber Producers Group has summarized research to date, and has attempted to establish a state of the art of Asphalt-Rubber. The data presented here is a review of 33 papers and proceedings published. The data has been presented both orally and in written form at one or more technical sessions including ASTM, TRB, AAPT, and Seminars.

Asphalt-Rubber as defined here has been in use for approximately 20 years. Researchers have reported on laboratory results as well as demon- stration projects. Historical performance evaluations are included with cost advantages and durability factors.

Asphalt-Rubber is defined as a blend of asphalt cement, reclaimed tire rubber, and certain additives in which the rubber component is at least 15% by weight of the total blend and has reacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles. The Rubber gradation used for Asphalt-Rubber is a minus 10 mesh. Asphalt cement is in accordance with ASTM D 946 or ASTM D 3381.

The percentage of crumb rubber is normally between 15 and 25 percent of the blend depending upon the particle size of %he crumb rubber and its application. Diluents are added to improve sprayability and aggregate wetting. Asphalt, rubber, and diluents are field blended.

The use ;f Asphalt-Rubber as a binder for asphalt concrete mixtures results in mixtures which have increased durability due to the increased binder oxidation resistance resulting from the antioxidants and carbon black contained in rubber. The higher viscosity of the asphalt rubber binder results in increased resistance to permanent deformation while the elastic characteristics impart increased resistance to reflective and thermal cracking.

CHARLES MCDQNALD R E S W C H

Since 1964, Charles McDonald has performed and reported his results on 446 experiments. His evaluations were primarily physical observation based on trial and error. His comments are summarized for the sake of brevity herein. (Reference #l)

1965 Compression and Reco very test o n Asphalt - Rubbber , alone, being a pure plastic, has no recovery after loading. Visco- elastic properties of Asphalt-Rubber are the factors that enables it to resist cracking. "

"Asphalt

ittlm e g s Temperature Test on AsPhdt - Rubber ASTM D 746 "The Asphalt-Rubber specimen failed at 90 degree deflection. Control specimen could not be deflected without breaking."

1966 Stopping Reflectio ' n Cracking from Soil -Cement Base" "Asphalt- Rubber appears to eliminate reflection cracks entirely"

1967 Resistance to sub - zero cold "The test constituted one of the major criteria in setting rubber content and application rates." 25% to 33% rubber was used to apply 0 . 5 gal. per sq. yd. with no damage for the test duration.

Phalt - Rubber on Nati ve Soib for surfacing certain types of Earth roads and streets successfully without benefit of base course.''

"This material definitely has a place

1968 Bffe ct of Varvina Rubbe r Content and G radat ion "Test area showed any of the combinations with rubber were superior to conventional materials. "

fect of Vazyinra: Asphalt Source. Rubber Gradation and Content "The source of the asphalt or the gradation of the rubber in hot reacted Asphalt-Rubber did not make any significant difference in the test panels. "

1969 Check Crack Reflection Over Broken Concr ete "Reflective Cracks came up and stopped at the Asphalt-Rubber. This material provides satisfactory repair for broken and cracked concrete pavement."

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. McDonald Research Continued

Test For Domestic Water Contaminatios City of Phoenix Water and the Health Department for use in sealing concrete reservoirs."

"Asphalt-Rubber was approved by

Gkmbal Resistance tests".

"No effect on the Asphalt-Rubber from any of the

1970 U o l v e d Rubber Content of Asphalt - Rubber Com~ositbw "Tests for dissolved rubber in Asphalt-Rubber compositions generally yield less than 5% dissolved rubber. The older the composition becomes, the more erractic are the results indicating continuation of complex changes that are not yet understood".

Yater Proofness "Asphalt-Rubber waterproofed the surface preventing water from reaching the subgrade. Asphalt-Rubber prevents differential accumulation of water in the subgrade."

1971 Field use of kerosene as a Diluent in Asphalt - Rubber "High dilutions of kerosene are not detrimental to long range properties of Asphalt-Rubber. "

1972 Joint S ealinsa; "After six years Asphalt-Rubber joint seal is superior to any that had been previously used." This filler was placed seventeen years ago and has completely sealed the joint.

Asphalt - Rubber Binder in Open Graded Mix "Performance has been outstanding in converting a street that required daily maintenance to one that required almost no maintenance fo r seven years."

1973 Three Laver SYS tem on Cracked an d Jointed Concrete (PCCP) "The cracked surface did not receive any preparation such a5 crack filling etc. The open graded leveling course was less than 1-inch followed by an Asphalt-Rubber chip seal. Then followed by a sand surface and later chip sealed with a conventional seal. This thin overlay is now 16 years old with little or no cracks."

'*The 1974 Comparison of Aspmts in Asphalt - Rubber Compoai- properties of the Asphalt-Rubber are not related in any way to the properties of the asphalt alone, indicating we have here an entirely different material. "

Ground Tire Rubber Versus P a r U Y D e n l a z e d Rubber indicates the partially devulcanized rubber formulation is not as effective in reducing temperature susceptibility as the ground tire formulation. Asphalt-Rubber made from a particular asphalt source cannot be assumed to have the same or similar, viscosity as Asphalt- Rubber made with asphalt of the same grade from different source. Likewise, low temperature susceptibility cannot be assumed, although it is generally true that the lower the viscosity of the asphalt, the

"Test

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McDonald Research Continued

lower the temperature susceptibility of Asphalt-Rubber made from asphalts derived from the same source.''

"Experiment at 7,200 feet evlevation with ze-thaw cycles of two or more per-day.

'*Success of this project should leave no doubt of the ability of Asphalt-Rubber under severe freezing and thawing conditions." 1975 -on of A s U t - R W e r b t e x . Partially D e w e d Rubber, Asphalt Source. Kerosene Con tent. Crvoaeaical IY Gro und Rubber m d Selected Rubber Gradations. "Test shows Asphalt-Rubber is superior to latex formulation. Asphalt-Rubber containing ground tire rubber is superior in low temperature susceptibility to that containing the partially devulcanized rubber under the same conditions."

"Tests prove the properties of an Asphalt-Rubber made from asphalt from a particular source cannot be accurately predicted but must be pre-tested to determine them. Kerosene diluent may be included to obtain viscosities and ease of handling up to 7 1/2% by volume of the Asphalt-Rubber."

"Fine cryogenically ground rubber did not increase the viscosity due to lower surface area exposed to reaction. It is not far from being comparable to the normal coarser ground composition in viscosity and reaches final reaction sooner."

Asphalt Rubber Amzrewate Mi-arison in Us e of L OW 1976 - Temperature "Mixes containing Asphalt-Rubber can tolerate as much as 100% more asphalt than ordinary mixes, resulting in much longer service life from the crack inhibiting benefits of the rubber."

"Dense grading in an Asphalt-Rubber mix interferes with the free play of elastic properties of the binder making the mix more susceptible to fracture as temperatures drop. The open graded mix is less inhibiting in this respect and should be the mix of choice when using an Asphalt-Rubber binder."

Bow Asphalt - Rubber Effects Stripping "Asphalt-Rubber has a better resistance to stripping than asphalt unreacted with rubber. The longer the curing period before rains etc, the greater the stripping resistance. "

**The - . * 1978 Effects of Awing on Various Compositions of Asphalt Rubber aged low temperature susceptibility is m o r e dependent upon the percentage of rubber in a fully reacted mix than on any other single variable. The best average for low temperature susceptibility after aging were Asphalt-Rubber mixes containing 25% rubber with small percentages of Kerosene "

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ARPG has reviewed more than seventy papers presented by researchers in the U.S.A., Europe, South Africa, and Australia. The review and the sited investigator and/or author will be subdivided for ease of understanding by properties of Asphalt-Rubber. These properties are compared by several investigations to a standard asphalt mixture or seal. The properties are physical and are based upon a standard test evaluation accepted by engineers and scientists.

Material Characteristics are similar to engineering properties of asphalt cement and asphalt concrete recognized by the asphalt industry and laboratories.

February 1973, Robert E. Olsen of the Implementation Division Office of Federal Highway Administration published an Implementation Package 73-1. "Rubber-Asphalt Binder for Seal Coat Construction." (Reference #2). The report was widely distributed dealing with a new elastomeric material called Rubber-Asphalt. Although earlier work done by Mr. McDonald dating back to 1959 existed, this report brought Asphalt- Rubber to the attention of the world.

May of 1980 a user-producer conference on Asphalt-Rubber was held in Scottsdale Arizona "First Asphalt-Rubber User, Producer Workshop" (Reference 8 3 ) . Some major conclusions of the conference participants were: "Additional research, development, and implementation efforts need to be undertaken to improve the economy and predictability of performance of Asphalt-Rubber." More than 15 objectives were listed by the conference workshop. Most of the objectives have been accomplished as pointed out in the following references included herein.

Terminology was defined at the workshop to clarify the rubber used in Asphalt-Rubber.

B ecvolad Vulcanized Crumb Rubbs is simply scrap vulcanized rubber (with wire and fiber removed) that has been ground to pass a given screen. It retains all the properties of the orginal vulcanized scrap.

Reclaimed Rubber is defined as the product that results when vulcanized scrap rubber is treated with chemicals, plasticizers, pressure and heat to produce a plastic material which can be processed, compounded and vulcanized with or without the addition of either natural or synthetic rubbers. B.H. Huff (Reference # 3 f in 1980 at the Scottsdale Conference indicated "Probably the most critical and most difficult factor to control is the chemical nature of the asphalt". H i s research showed that the second acidafines fraction of the asphalt has the greatest effect on the solubility of the rubber. Aromatic asphalt or extender oil provide solubility.

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G. Morri; of ADOT (Reference $3) pointed out in the 1980 Conference that laboratory test procedures developed for asphalt and asphalt concrete are not applicable to Asphalt-Rubber, New test procedures for Asphalt-Rubber provide engineering characteristics that can directly apply to structural design and analysis.

Morris also pointed out the reaction phenomenon in a study by ADOT Materials Service Report ADOT-RS-14, "The Chemical and Physical Properties of Asphalt-Rubber" by E.L. Green and William J. Tolonen 1977. (Reference P4). This research developed test methods that could be used in the production control of hot Asphalt-Rubber. The rheological factors included effect of the swollen rubber particle on the viscosity of the hot Asphalt-Rubber. Some of the conclusions reached in this research were:

1. The viscoelastic properties differ greatly over asphalt that normal asphalt test procedures do not describe its behavior.

2. Empirical comparisons being fitted to swelling data with two constants such as the rate of swell and maximum swell.

3 . Test were developed to measure viscosity of hot Asphalt-Rubber using vacuum capillary viscometers.

4 . Viscoelastic measurements conducted on the Rheometrics Mechanical spectrometer has demonstrated the differences between asphalt and Asphalt-Rubber. Asphalt-Rubber has higher elastic modulus. Asphalt-Rubber responds both elastically and viscously over a large range of temperatures.

5. Test procedures have been developed which measure the viscoelastic nature of Asphalt-Rubber.

The 1980 conference refered to three pieces of laboratory equipment that had been developed to measure engineering characteristics. The Arizona Torque Fork for high temperature studies of reaction and spray range. The Schweyer Rheometer to study characteristics in high range of service temperatures and the force ductility apparatus for low temperature ranges.

In 1982, a summary was completed by Dr. J . C . Rosner and J. Chehovits for the Arizona Department of Transportation Report number FHWA/AZ-82/159 Summary "Chemical and Physical Properties of Asphalt- Rubber Mixtures". (Reference # 5 ) . This research looked at the properties of Asphalt-Rubber through the Schweyer Rheometer viscosity, forced ductility at load failure, elongation failure, engineering stress at failure and engineering strain at failure. Force - ductility evaluations for creep experiments using Asphalt-Rubber.

This is a 'large volume of data wherein the following conclusions were reached:

1 . Physical properties of Asphalt-Rubber mixtures vary and depend on the type of rubber and asphalt.

2. Lower viscosity asphalts in Asphalt-Rubber have lower viscosity and failure stresses and higher failure strains and creep compliance.

3 . Temperature significantly affects physical properties of Asphalt- Rubber.

J.W.H. Oliver reported in 1981 at the National Seminar on Asphalt-Rubber in San Antonio Texas, (Reference # 7 ) that "Morphology of the rubber particles has been shown to play an important role in determining the properties of Asphalt-Rubber'' (Reference #6). This morphology can be characterized by a simple bulk density test. Elastic recovery increases as the size of rubber particles decrease. The 1981 National Seminar on Asphalt- Rubber contained 17 papers that reported experiences of Asphalt- Rubber including SAMs, SAMIs, designs, equipment and specifications. (Reference t t7) .

In 1982, Chen-Divito and Morris presented to the Association of Asphalt Paving Technologists, "Finite Element Analysis of Arizona Three-layer System of Rigid Pavements to prevent Reflective Cracking" (Reference # 8 , 2 9 , 30 & 31). This is 1979 construction that utilized Asphalt-Rubber and open graded asphalt concrete mixtures to prevent reflective cracking using thin overlays. The research development is based upon early 1973 work by McDonald and Morris on Madison Ave (Reference t t l ) . Conclusions reached were that the ratio of stress in the lower layer to the stresses in the upper layer were ten to one or higher. This research describes a method of prevention of crack reflection with thin overlays.

Newcomb and McKeen reported t o the New Mexico Engineering Research Institute 1983 "Development of Criteria for the Use of Asphalt-Rubber as a Stress Absorbing Membrane, Interlayer (SAMI)" (Reference # 9 ) .

1 . The value of true Strain at maximum true stress increased with increasing mixing temperature.

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2. Modif ieg <softening point temperatures achieved "peak" values at the 375 F mixing temperature for all the rubber types.

3 . Visgosities at a constant power also achieved "peak" values at 375 F mixing temperature.

4 . True strain at maximum true stress increased with increasing mixing time.

5. Multiple linear regression of the stress-strain diagrams at different deformation rates and temperatures produced predictable stress at a given level of strain.

In 1979, Dr. R.A. Jimenez, Gene Morris and D.D. DeDeppo presented a paper to the Association of Asphalt Paving Technologists "Tests for a Strain-Attenuating Asphaltic Material" (Reference #lo). Dr. Jimenez also presented a paper to TRB in 1985 related to the 1979 work "Laboratory Evaluation of An Asphalt-Rubber SAL" (Reference #ll). SAL is defined as a strain-attenuating layer which is a SAM1 without aggregate cover.

The purpose of the research was to test Asphalt-Rubber as an interlayer to minimize reflection cracking. A major portion of the study was devoted tu developing equipment and test procedures for Asphalt-Rubber, Such t e s t s as horizontal shear, vertical skiear and viscosity using coaxial cyclinder viscometer. The study summarized the following:

1.

2.

3 .

4 .

5.

6 .

7.

Swelling of the rubber particles was primarily a physical effect.

The falling coaxial cylinder viscometer produced acceptable repeatability measurements.

Viscosity of Agphalt-Rubber was 200 times greater than base asghalt at 14OWF but about six times smaller than base asphalt at 32 F.

Variations of ductility indicated the Asphslt-Rubber were not affected by temperature changes from 77-33 F for the specific blend tested.

Asphalt-Rubber may have lower values of cohesion and adhesion than straight asphalt.

Tests from the horizontal shear (simulating thermal stresses) indicate the Asphalt-Rubber does serve as a strain- attenuating layer.

Results from the vertical shear test (simulating repeated wheel load shear) suggested that Asphalt-Rubber as an interlayer came from its pliability at low temperatures.

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M.R. Pig&tt and R.T. Woodhams (Reference #12) in 1979 made some observations in a report for Environment Canada. Asphalt-Rubber increased toughness, improved low temperature flexibility, improved retention strength when wet, resisted flow or creep at elevated temperatures and traffic, and had great resistance to oxidation hardening due to the presence of rubber antioxidants.

The effect of Asphalt-Rubber as a membrane to protect asphalt concrete from age hardening due to oxidation was shown in the 1985 Fifteen-year pavement condtion survey of Asphalt-Rubber Membranes in Phoenix, Arizona by Russell €3. Schnormeier (Reference #13). The tests show a 20 fold difference in absolute viscosity of the asphalt binder with and without Asphalt-Rubber seal.

George Way in 1979 ADOT report "Prevention at Reflective cracking Minnetonka - East" (Reference #14) also reported the reduction in oxidation hardening of the Asphalt-Rubber as compared to asphalt. Heerkins and Von Meier presented Asphalt-Rubber rolling thin film results at the 1989 Asphalt-Rubber Seminar. Asphalt-Rubber does retard age hardening.

Robert Doty presented a comprehensive report for the 67th meeting of TRB in 1988. "Flexible Pavement Rehabilitation using Asphalt-Rubber combinations" (Reference # 15). This is a field demonstration of several rubber combinations with several thicknesses to measure crack reflection or development along with inplace testing. The results indicate the overlay design thickness can be reduced by 25% to 50% with the addition of rubber. Caltran ha5 increased allowable deflection by 25% in overlay designs.

Texas Transportation Institute at Texas A&M University completed a Department of Transportation, Federal Aviation Administration report: "Criteria for Asphalt-Rubber Concrete in Civil Airport Pavements", by Denise Hoyt, Robert Lytton and Freddy Roberts, (Reference #16). Two reports by TTI evaluated the Asphalt-Rubber hot mix under laboratory conditions and then constructed a field project. Evaluations were done under four climatic zones to represent most all conditions. Crack indexes were made for 20 year traffic. Crack index in the field showed Asphalt-Rubber out performed the AC-10 control mix by 4 to 7 times. Material properties were compared via compaction, air voids, stability, resilient modules, fatigue, fracture and crack propagation, creep and permanent deformation. (Reference #16)

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In 1989, <F. Hugo and R. Nachenius presented to the AAPT "Some Properties of Bitumen-Rubber Asphalt and Binders" (Reference tt.17). They discussed test methods used for bitumen-rubber binder and asphalt. It was concluded the methods used could be successfully applied to evaluate properties of Asphalt-Rubber. Testing included shear stiffness and viscosity by sliding plate rheometer. Another test is the indirect tensile strength test. Stripping susceptability was investigated. Hugo's indirect tensile test results show percentage strain of maximum stress to be 2.5 times higher than the standard asphalt.

Several states have studied the use of Asphalt-Rubber and have built demonstration projects. (Reference $#lo, 19, 20, 21, 22, 27, & 28 ) . Most all of the evaluations were based upon inplace performance compared to a standard. Conclusion reached were based upon performance in the field and not on laboratory evaluations.

In the past few years, there have been tremendous strides in both the engineering technology and the production equipment. Blends of Asphalt-Rubber can be designed for specific climatic and traffic: conditions and can be constructed with a very high degree of confidence.

The combining of ground rubber with asphalt does provide a material with substantially increased stiffness in the high temperature ranges and has flatened the temperature viscosity susceptability curve. This has improved properties of binders to permit softer asphalt grades and has improved the temperature properties for cold climates. The elastic properties contributed by the rubber provide improved fatigue behavior over a wide temperature range and increases ductility, tensile creep thus reduces low temperature tensile fracture (Reference #23, 24, 25, 26, & 32). The reinforcement contributed by rubber allows softer asphalt which yield lower tensile fracture.

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. Comparisons performed by research and industry quality control

are shown here:

Penetration @ 770Fo Penetration Q 38.2 F Penetration (3 OoF Resilience 0 77 F

& Rebound Low Temperature Flex

F Pass OF Fail

Viscosity @35Q°F, 8p Softening Point, F

35 56 10 18 1 5

1 -2

54 40 52 38

126 120

Test

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AC Asphalt Cement Grade AR Asphalt Rubber EXT Extender Oil

32 18 2

37

38 36

2600 152

123 56 31 22 10 4

-15 23

32 32 30 30

2900 108 148 -

255 9n 61 42 15 12

- 60 9

22 14 20 12

3000) 97 126 -

The modification of asphalt is necessary to meet the demands made on our highway systems. The choice of modification should be based upon the ability to alleviate cracking, rutting, raveling and aging. Asphalt-Rubber has made major improvements in the binder that time and performance have proven.

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. Reference # 1:

Reference # 2:

Reference # 3:

Reference # 4:

Reference $t 5:

Reference # 6 :

Reference # 7:

Reference # 8:

Reference # 9:

Reference $10:

Reference #ll:

Charles H. McDonald "McDonald Research Library on Asphalt-Rubber" 1984 AFtPG

Robert E. Olsen "Rubber-Asphalt Binder for Seal Coat Construction" Implementation Package 73-1 1973

"First Asphalt-Rubber User-Producer Workshop" Scottsdale 1980

E.L. Green - William J. Tolonen "The Chemical and Physical Properties of Asphalt-Rubber Mixtures" A2 DOT - RS-14 (162) 1977

J.C. Rosner - J.G. Chehovits "Chemical and Physical Properties of Asphalt- Rubber Mixtures " FHWA - D O T 82-159 1982 John W.H. Oliver "Modification of Paving Asphalt by Digestion With Scrap Rubber" TRB - 1981 "National Seminar on Asphalt-Rubber" San Antonio 1981

Nan Jim Chen - Joseph A. DiVito - Gene R. Morris "Finite Element Analysis of Arizona's Three-Layer Overlay System of Rigid Pavements to prevent Reflective Cracking" AAPT 1982

D.E. Newcomb - R.G. McKeen "Development of Criteria for the Use of Asphalt- Rubber as a Stress-Absorbing Membrane Interlayer (SAM1 1 '* New Mexico Engineering Research Institute ESL - TR - 83 - 50 1983 R.A. Jimenez - Gene Morris - D.A. DeDeppo "Tests for a Strain/Attenuating Asphaltic Material " AAPT 1979

R.A. Jimenez - W.R. Meier 3r. "Laboratory Evaluation of An Asphalt-Rubber SAL" TRB 1985

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Reference $12:

Reference $13:

Reference #14:

Reference #15 :

Reference #le:

Reference #17:

Reference #18:

Reference $19:

Reference #20:

Reference #21:

M . R . Piggott - R.T. Woodhams "Recycling of Rubber Tires in Asphalt Paving Materials" Environment Canada 1979

R.H. Schnormeier "Fifteen-Year Pavement Condition History of Asphalt- Rubber Membrane in Phoenix Arizona" TRB 1986 1096

George B. Way "Prevention of Reflective Cracking Minnetonka-East" Arizona DOT #1979 GW1

Robert N. Doty "Flexible Pavement Rehabilitation using Asphalt-Rubber Combinations " TRB 1988 Denise M. Hoyt - Robert L. Lytton - Freddy L. Roberts "Criteria for Asphalt-Rubber Concrete in Civil Airport Pavements", "Evaluation of Asphalt-Rubber Concrete"

F. Hugo - R. Nachenius "Some Properties of Bitumen-Rubber Asphalt and Binders" AAPT 1989

Harvey F. Allen - Carl P. Keel "Evaluation of an Asphalt-Rubber Interlayer" Minnesota DOT Special Study #388 1985

M.P. Strong "The Evaluation of Rubber Asphalt Surface Treatment in Preventing Fatigue Crack Reflection in Bituminous Overlay Construction" FHWA Demonstration Project #37 1983

R.H. Schnormeier "Asphalt-Rubber Approach to Low-Cost, Low Volume Streets" 1st Low Volume Roads Conference, TRB 1975

Gene R. Morris - Charles H. McDonald "Asphalt-Rubber Stress AbtrbZag Membranes Field Performance and State-of-the-Art" TRB #595 1976

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. Reference $22: Scott Schuler - Bob M Galloway - Jon A. Epps

"Evaluation of Asphalt-Rubber Membrane Field Performance" Texas Transportation Institute #287-2 1982

Reference tt23: Gene R. Morris "Asphalt-Rubber Overlay Systems" 1988 International Road and Bridge 4 R Conference, Atlanta, Georgia 1988

Reference #24: Russell H. Schnormeier "Waste Tire Utilization for Improvement of Asphalt Pavements " 1988 International Road and Bridge 4 R Conference, Atlanta, Georgia 1988

Reference #25: John L. McRae - B.D. LaGrone "Effect of a Modified Reclaimed Rubber and Ground Vulcanized Rubber on the Physical Properties of Bituminous Pavements as Evaluated by the Gyratory Testing Machines" TRB 1971

Reference #26: R . K . Frobel - R.A. Jimenez - C.B. Cluff "Laboratory and Field Development of Asphalt-Rubber for use as a Waterproof Membrane" A2 DOT RS-14 (167) 1977

Reference #27: Denise M. Hogt, Robert Lytton, Freddy Roberts "Performance, Prediction and Cost Effectiveness of Asphalt-Rubber Concrete in Airport Pavements" DOT/FAA/PM 86/39-11 and TRB 1988

Reference #28: Gottfried Nievelt - Gerhard Stehno - Helmut Stickler - Johann Ertl "Noise Level Reduction Through Silent Asphalt" Federal Ministry of Public Works and Technology Belgium 1984

Reference #29: Jamal B. Sarsam - Gene R. Morris "The Three Layer System on Arizona Highways Development and Analysis" 21st Idaho Asphalt Conference 1982

Reference #30: Gene R. Morris - Nan Jim Chen - Joseph A. DiVito "Application of Asphalt-Rubber on New Highway Pavement Construction" TRB 1982

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L .

Reference #31: Frank R. McCullagh A Five Year Evaluation of Arizona’s “Three Layer System on the Durango Curve in Phoenix” AAPT 1985

Reference #32: B.J. Huff - B.A. VAllerga “Characteristics and Performance of Asphalt- Rubber Material Containing a Blend of Reclaim and Crumb Rubber. “ TRB 1981

Reference #33: J.C.P. Heerkens - Ira Von Meier “Open Graded Rubberized Asphalt for Traffic Noise Reduction in Urban Areas.” 1989 Asphalt-Rubber Seminar, Kansas City, MO