Characteristics and Performance of Asphalt-Rubber Material Containing a Blend of Reclaim and Crumb Rubber

Transportation Research Record 821 29

Test. Center for Highway Research, Univ. of and Bituminous Mixes. Proc., lst International Texas at Austin, Res. Rept. 98-10, 1971. Conference on Structural Design of Asphalt s. A.S. Adedimila and T.W. Kennedy. Fatigue and Pavements, Univ. of Michigan, Ann Arbor, Aug. ResiJ.ient Characteristics of Asphalt Mixtures 1962. by Repeated-toad Ind.irect Tensile Test. Center 13. J .A. Deacon. Fatigue of Asphalt Concrete. for Highway Research, Univ. of Texas at Austin, Transportation Engineering Division, Univ. of Res. Rept. 183-5, 1975. California, Berkeley, Ph.D. dissertation, 1965. 6. w.o. Hadley, T.W. Kennedy, and W.R. Hudson. A 14. J. McElvaney. Fatigue of a Bituminous Mixture Method of Estimating Tensile Properties of Ma Under Compound Loading. Univ. o f Nottingham, terials Tested in Indirect Tension. Center for Nottingham, England, Ph.D. thesis, 1972. Highway Research, Univ. of Texas at Austin, 15. C.L. Monismith, J.A. Epps, and D.A. Kasi- Res. Rept. 98-7, July 1970. anchuk. Asphalt Mixture Behavior in Repeated 7. R.K. Moore and T.W. Kennedy. Tensile Behavior Flexure. Institute of Transportation and Traf of Subbase Materials Under Repetitive Loading. fic Engineering, Univ. of California, Berkeley, Center for Highway Research , Univ. of Texas at Rept. TE 68-8, 1968. Austin, Res. Rept. 98-12, Oct. 1971. 16. R.A. Jimenez. Fatigue Testing of Asphal tic 8. T.W. Kennedy and J.A. Anagnos. Procedures for Concrete Slabs. AS'IM, Philadelphia, Special Conducting the Static and Repeated-toad Ind i Tech. Publ. 508, 1971, pp. 3-17. rect Tensile Tests. Center for Transportation 17. R.A. Jimenez and B.M. Gallaway. Behavior of Research, Univ. of Texas at Austin, Res. Rept. Asphaltic Concrete Diaphragms to Repetitive 183-14 (in preparation). Loadings. Proc., 1st International Conference 9. Manual of Testing Procedures: Test Method Tex- on Structural Design of A.sphalt Pavements , 109-E. Texas State Department of Highways and Univ. of Michigan, Ann Arbor, Aug. 1962. Public Transportation, Austin, Jan. 1, 1978 . 18. P.A. Pell. Fatigue of Asphalt Pavement Mixes. 10. T.W. Kennedy and R.K. Moore. Tensile Behavior Proc., 2nd International Conference on Struc of Aspha~t-Treated Materials Under Repetitive tural Design of Asphalt Pavements, Oniv. of Loading. Proc., 3rd International Conference Michigan, Ann Arbor, Aug. 1967. on Structural Design of Asphalt Pavements, 19. R.J. Schmidt. A Practical Method for Measuring London, England, Vol . 1, Jan. 1972. the Resilient Modul.us of Asphalt-Treated 11. J.A. Epps and C.L. Monismith. Influence of Mixes. HRB, Highway Research Record 404, 1972, Mixture Variables on the Flexural Fatigue Prop pp. 22-32. erties of Asphalt Concrete. Proc., AAPT, Vol. 38, 1969, pp. 423-464. P11blicutlo11 of this paper sponsored by Comml/lu 011 01arac1eristics of 12. P.S. Pell. Fatigue Characteristics of Bitumen m111111i1w11s Pavi11g Mixtures to Mee/' Stntct11ral Req11ire111eJ1ts.

B.J. HUFF AND B.A. VALLERGA

Asphalt ccmont, rubber ex I.ender oil, and a mixture of ground reclaim and the blend as the perce ntage of rubber was incteased, crumb rubber, blended 10gothor at an eleva1ed 1emperature in spocific propor one could use only small percentages and still main tions and sequoncos. form a tough, durable, and adhosivc mombrano when hot tain a workable material. Obviously, this limited spray-applicd lo a surface and allowed to cool to ambiont tompera1uret. Th is the benefit that could be added by the rubber. cast-in-piece Mphalt-rubber mombrano has been found 10 be suitable for uso In add ition, the virgin polymers were susceptible in the construction of surface treatments for oxisting pavements (chip seals), stross-absorbing membrano interlayun (SAMls) in tho placing of asphalt con· to oxidation by the elements, and their beneficial croto overlays, and waterproofing membranes for bridge decks and hydraulic properties dissipated rather rapidly with time. lining!' (ponds, canals, and reservoin). When hot-poured Into pavement joints Their instability in relation to heat also created and cracks and allowed to cool, it also servos as an offectivo joint ond crack problems with their use. Overheating converted the filler. Tho concepts and proportions of tho formulation and prep-aration of rubber to an oil, which only served to soften the this materiel ure prnsen1ed together with Information and data on its prop asphalt. This severely l imited the production of erties and applications. A discwslon is presented ot the rosults of two the rubberized asphalt t o jacketed kettles or as a analytic studies on the applicability of asphalt-rubber membranes (a) in latex in a sphalt emulsions. minimizing rofloc1ion cracking when used as a SAM! and (b) in producing a With the advent of synthetic rubber, the same "multllayeled aggregate structure" when used as a slnglo-pas• chip seal. A exercises were repeated with essentially the same summary of the field performance observed to date on a numbor of innalla· results. The synthetic rubber was somewhat cheaper I.ions of 1he asphalt·rubber matoriul in its various applications is also included, than the natural rubber, but it a~so lacked some of 1ogether with observations on the efficacy of the material as a mambrano and as a filler. the elasticity and tackiness of the natural rubber. As the use of rubber increased, the 9 rowing pile of scrap tires was eyed as a cheap source of rubber Many attempts have been made to impart the desirable for preparing rubberized asphalt. Early experiments elastic and resilient properties of rubber to as showed that these tires could be ground and mixed phalt. The earliest of these involved the use of wit.h hot asphalt in large percentages to produce a natural rubber and were relatively successful. How material that had properties superior to those of ever, because of the rapid buildup in viscosity of the base asphalt. Since the rubber in these tires 30 Transportation Research Record 821 was synthetically compounded and vulcanized to 1. Carbon black--Scrap rubber contains more than resist heat and weathering, the problems encountered 2 0 percent carbon black, an element that has be en with the virgin polymers were eliminated. shown to add reinforcing properties to asphalt (1•!>. The synthetic vulcanized rubber, however, lacked 2. Antioxidants--For the tire to survive the solubility of the virgin polymers. The only weathering, it is necessary that antioxidants be solubility observed was a drawing of the oils out of added to the rubber compound. I t is believed that the asphalt by the rubber to produce swollen rubber thes e chemi cals contribute an additional measur e of particles with gellike surfaces. With time, this durability to the asphalt-rubber. swelling would progres.s to the point that the swol 3. Amines--Although they are added to the rubber len rubber parti cles would knit together within the compounds for other reasons, particularly during the asphalt matrix to form an asphalt-rubber sheet that devulcanizing process, amines are closely related to was more resistant to the stresses that produce the antistrip compounds and current laboratory fracture in pavements than the asvhalt itself. 11tudies indicate that they do aid in resistance to This was a simple way to improve the asphalt, but stripping. it was not without its shortcomings. The drawing of 4. Aromatic oils--Aromatic oils are the same the oils into the rubber particle adversely affected oils that are used in recycling to rejuvenate the the cohesive and adhesive properties of the asphalt old asphalt and in emulsion form to restore oxidized phase, thereby reducing its ability to bond to pave asphalt pavements . By their presence, t hese oils ment surfaces or to bind togethet aggregate parti will prolong the life of the asphalt-rubber material. cles. The loss in bond strength, which occurred primarily in the early stages of service on the road .By using the foregoing knowledge of the behavior when the rubber-asphalt matrix was being formed, of combinations of various types of ground rubber could be counteracted to some degree by the use of a and asphalt based on both laboratory a nd field very soft asphalt rich in oils, but the resulting e>Cperiments and observations, a formulation of !;ll~n

Figure 1. Composition of asphalt·rubber·membrane system. satisfacto ry . The main physical properties of i nter est a nd applicability are consistency, dura 0% Powdered Reclaim bility , and adhesion . With appropriate modifica 20 ±2% Ground Rubber 60% Ground scrap tions to account f o r the small particles of rubber [ (High in natural dispersed i n the asphalt, the various viscosity rubber content) 4 ±2% Extender Oil measuring devices can be used as well as various tests for bonding and resistance to weathering.

Consis tency

76 ±4% Paving Grade Asphalt Table 1 gives data on consistency versus temperature AASHTO M226 for an AR-4000 asphalt and the same AR-4000 blended ASTM 0-3381 with extender oil and ground rubber in accordance with the formulation presented above. The tabulated results are typical of the change in viscosity char acteristics of an asphalt when the extender oil and rubber blend are incorporated into the asphalt at an elevated temperature in the manner and sequence specified Cil · Figure 2 shows a plot of these data on a special asphalt consistency versus temperature chart that provides for plotting together test data Teblo 1. Consistency-temperature characteristics of asphalt-rubber material from the various test methods commonly used for compared with base asphalt. determining the flow characteristics of asphalt. The consistency versus temperature data show that Asphalt- the influence of the rubber on the base asphalt is Test AR-4000 Rubber Property Method Asphalt Material• to decrease its susceptibility to temperature. In the high temperature range particularly, the im Penetration ASTM D-4 provement in viscosity is substantial : There is 39.2° F 9 17 approximately a 15-fold increase in viscosity at 77.0°F 76 IOI Softening point (ring and ball) (° F) ASTM D-36 120 132 140° F and above. Improvement of cold-weather prop Dynamic viscosity at 140° F (poises) ASTM D-2171 1800 75 80 erties is also indicated by the significant increase Kinematic viscosity (cSt) Brookfield in penetration value at 39.2°F. 165° F 165 000 110°F 45 600 Durability 190° F 6 200 2 12° F I 984 230° F 860 The r.esistanc e t o weathering o f the asphalt-rubber 250°F 450 mate rial was measured by exposing various blends to 265°F 3 400 direct sunlight in the Phoenix, Arizona, climate for 277° F 194 a two-year period . Specimens 0 . 25 in thick and 4 in 3!0°F 82 330° F 58 i n diamet er we re cased in metal pans and placed on 350°F 42 912 the roo f of the laborato ry . Kinematic viscosity was 400°F 720 measured at 140°F be fore a nd after exposure . Table 2 gives the results obtained . 8Con11lns 78.4 percent AR-4000 asphalt, 1.6 percent extender oil, and 20 percent rubber blond. The data i ndicate that the ratio of viscosity after exposure t o original viscosity was lowe r when e x tend e r oil was i nc l uded i n t he blend . This im provement is considered significant and attests to operating condition and the heating system can be the benefit of incorporating the oil extender in the adequately controlled. asphal t-rubber material. More sophistic ated durabilit y t e sting , including CHEMICAL AND PHYSICAL PROPERTIES c omp arisons with other mate rials of known durability c haracteristics, is c u rrently under way . These Determining the chemical properties of a blend of results will be published when they a re a vailable. asphal t , extender oil, and ground rubber particles is extremely difficult. Most of the conventional Adhe sion tests are not readily adaptable to the material. Research is currently under way at several govern Preliminary evaluation of the adhesion character men t and private laboratories , and it is expec ted istics of the a s phalt-rubber material was performed that p r ogress will be report ed in the literature in in a cold-bond test. In this case , a comparison was due c ourse . made with the base asphalt used in making the From the sta ndpoint of chemical prope rties, com asphalt-rubber material. positi o n can be a nalyzed a nd the s ol ubility charac The cold-bond t est was run i n a specially modi teristics of the separate ingredients can be mea fied ductility tester designed to pull a spec imen sured (§_, ASTM D-297 , a nd ASTM D-2007) . However , with lxl-in cross s ection and 2-in length between once the i ngredients are combined and formed i nt o a two brass blocks. The specimens were cast (hot product, t he interactions amo ng the asphalt , ex poured) in a mold so that the two ends adhered to tende.r o il , a nd rubber partic les a re complex and the brass blocks. After a specimen is conditioned dependen t on both time and t empe r a t u r e. Moreover, a in a refrigerated alcohol bath for 10 min, it is method o f determining the c hemical state of the quickly transferred to the modified ductility tester blend is not a vailable - Fo r example, t he f r acti o n and elongated O. 5 in (i.e., 25 percent) at a stan of rubber disso ;Lved i nto t he asphalt at any t ime dard rate. The specimen is removed from the duc would seem t o be a significant factor , but relia ble tility apparatus and again brought to temperature in laborat ory procedures and technique s f or making t his the alco ho l bath. and stret ched O. 5 in t o half again determination are not yet available. its o riginal length. This step is repeated until From the s tandp oint of physical properties, the the s pecimen has bee n elongated a full 2 in, or situation is more favorable although by no means twice its original length. 32 Transportation Research Record 821

Figure 2. Effect of addition of reclaim and crumb-rubber blend on consistenc:-: 1 I\ temperature relation of AR-4000 asphalt. 5 AR-4000 Aspha 1 t 15 • l • c M\ Asphalt-Rubber 0 25 ----- ·;; '-.\ 5 78.4% AR-4000 ..."' 50 10 1.6% Extender Oil ... \\ 20.0% Rubber consisting of: QIc 100 25 40% powdered reclaim QI 0.. r'\ 50 60% ground scrap 200 \ 300 100 400 ,, ~ 200 ~,,

" 11, 4 ,. ~ 100~ " ', 0 " 0.. ', 400 '\. 200 "· 100 " '\. ", 8000 \. 4000 ' ' .... ~ ~~ 2000 -...... 1000 \. --..... -o 500 '\

"'QI ~ ...0 200 '\ "' ...c QI 100 \ u \, 50

>- '\ u z 30 w >- ~ 20 V") 0 ""u

10 0 32 39. 2 77 100 140 210 275 325 400 450 TEMPERATURE, of

Table 2. Results of durability study of asphalt rubber blends. Composition of Blend (%) Viscosity at Exposure 140' F (poises) Test Extender Ground. Time Viscosity No. Asphalt Oil" Rubbe..U (months) Before After Ratio

soc 20 15 5704 18 034 3.16 2 78.4c 1.6 20 15 5204 12 341 2.37 3 72.i'ld 7.4 20 15 5978 15 759 2.64

flSht1U Dulrex ?39 .. bU.S. Rubber Reclaiming Compu.ny G·'l74, coo1b:'ling of 40 peorcent powdered reclaim (i.e., devulcanized) rubber and 60 pt:rconl crumb o.e., ground 1onp)rubba.r hl5h In natural rnblH!l f conhrnl. CAR-4000. dAR-8000.

Table 3 gives the results of this teat for ad reduction in cross-sectional area during testing hesion characteristics. Observations were made of than the base asphalt under the same conditions of adhesion, cohesion, and reduction in cross-sectionai temperature and elongation. area. After 100 percent elongation, the specimens were cut free from one of the blocks and returned to CHARACTERIZATION OF ASPHALT-RUBBER the low-temperature bath, and measurements were made of recovery in elongation at time intervals of 1 and Since the asphalt-rubber material is being widely 10 min. Test series were run on separate specimens used as an SAMI in pavement resurfacing operations at 40° and 60°F. and therefore becomes an integral part of the struc From a study of these data, it can be s·een that tural section of a pavement system, it became nec the aspha.lt-rubber maintains its ductility at the essary to determine the stizfness characteristics of lower temperature and has the ability to recover the asphalt-rubber membrane in terms of a stiffness elastically about half of the elongation imparted to modulus. This was done in a creep mode of load-ing. the specimen whereas the asphalt has essentially no The procedure used in this "tensile creep test" ductility at the lower temperature and no elastic was to cast test specimens in a mold that consisted recovery. The asphalt-i:ubber also exhibited less of the end pieces of the ASTM D-113 ductility test Transportation Research Record 821 33

Table 3. Results of tests for adhesive and cohesive Test Observations at Elongation of Recovery Time characteristics of asphalt-rubber material. Test Tern- Material perature (° F) Characteristic 25% 50% 75% 100% I min 10 min AR-4000 40 Adhesion Pass asphalt Cohesion Fail (brittle fracture) Reduction in 0 cross-sectional area (%) Recovery (%) 0 0 60 Adhesion Pass Pass Pass Pass Cohesion Pass Pass Pass Pass Reduction in 9 22 30 35 cross-sectional area(%) Recovery (%) 0 0 Asphalt· 40 Adhesion Pass Pass Pass Pass ru bber• Cohesion Pass Pass Pass Pass Reduction in 14 14 24 33 cross-sectional area (% ) Recovery (%) 30 55 60 Adhesion Pass Pass Pass Pass Cohesion Pass Pass Pa ss Pass Reduction in 5 14 19 24 cross-sectional area(%) Recovery (%) 45 55

acontains 7 8.4 percen t A R-4000 asphalt , 1.6 percent extender oil, and 20 percent rubber blend.

mold and 5.8-in long, stcaight side pieces in place APPLICATION CONSIDERATIONS of the conventional wedge-shaped ones. The side pieces were coated with a release agent of glycerine Immediately after mixing, the asphalt-rubber mate and kaolin to facilltate removal. The restricted rial described above is generally ready for applica portion of the test specimen cast in this mold had tion by either hot spray or hot pouring within a parallel straight sides 6. 2 in long and a uniform temperature range of 375°-425°F. If a delay occurs cross section of o. 3 in1 • Gage marks were placed when the material is ready to be applied, the heat 4 in apart on the center section of the restricted is turned off until the job resumes. length. The material may a l so be allowed to stand over The specimens were then subjected to creep load night and be applied the following day, provided the ing in tension under various constant loads at tem heat is turned off and restarted at a time interval peratures of 40°, 70°, and 100°F by placing them in prior to application suf f i cient to ensure that the a constant-temperature ductility bath modified so application temperature is again within the range o f that one end of each specimen was fixed whil e the 37S 0 -425°F. Mixing by recirc ulation or stirring, or other end was supported by a floating "raft" made of combinations thereof, must be maintained during re wood and polystyrt;!ne foam. A hanging weight wa·s heati ng to obtain uniformity of temperature and to used to apply the load to the specimen through a avoid localized overheating, which will damage tbe thread that passed over essentially frictionless asphalt-rubber material. pulleys and was attached to the raft. During the test, the 4-in distance between the gage marks on USES OF ASPHALT-RUBBER MATERIAL the specimen was followed with dividers, and read ings were taken at l~min intervals. Each specimen The principal uses of .the asphalt-rubber materia 1 was temperature-conditioned by placing it i n the described above are in connection with membrane sys constant-temperature bath for at least 90 min before tems designed for pavement maintenance and rehabili the start of the test. Figure 3 shows the test tation. The four asphai t-rubber membrane systems setup and a mea·surem.ent being taken with calipers most widely us ed are the f ollowing: between the gage marks. A separate specimen was creep-tested in tension 1. Surface treatment--A hot-spray, cast-i n-pl ace at each temperature, a.nd various loads were applied asphalt-rubber membrane i nto which clean, dry rock sequentially, in increasing order, to a single spec chips are embedded (i.e., chip seal); imen. For each loading, stress and elongation were 2. SAMI--A hot-spray, cast-in-pl ace as,phalt- calculated on the basis of the length and cross r ubber membra ne placed on an existing asphal t or sectioned area at the time of application of that portland cement conc-rete (PCC) pavement before re particular load (see Table 4). s urfacing to reduce the transfer of stresses to the An analysis of the test results given in Table 4 resurfacing layer from the underlying pavement was made in connection with an analytic study of the structure and hence minimize reflection cracking eff ects of the asphal t-rubber material, when used as (clean, dry chips or sand particles are spread on an SAMI, in the distribution of stress and strain i n the membrane while it i s hot to provide a working an asphalt concrete overlay (1) , Assuming that 70° F surface for placing the overlay); is a representative temperature £ or the asphal.t J . Waterproofing membrane-- A hot-spcay, cast-in rubber membrane layer, the sti ffness for the place a sphalt-rubber membrane used on such struc asphalt-rubber material at t imes of loading of 0.02- tures as bridge decks and hydraulic linings to 0.05 s, considered t"epres entative of moving traffic, prevent passage of water to the underlying regions; was estimated to range from 7500 to 3600 lbf /in:t . and In their analytic s tudy, Coetzee and Mon i smith (2_) 4. Joint and crack filling--A hot pour of chose to use a stiffness modulus of 5000 lbf/in1 • asphalt-rubber into joints and cracks to serve as a 34 Transportation Research Record 821

Figure 3, Test setup for tensile creep test of asphalt-rubber material: (top) rial has been incorporated into many and varied overall view of equipment with pulley system and (bottom) calipers used to projects throughout the United States under the measure total strain over gage length. auspices of vari ous user agencies. 1\ chronological lis ng of these pro j ects has been maintained to gether with data sheets that recorcl construction details for use in subsequent evaluations of condi tion and performance. Periodically, conditi on sur veys have also been made and the result s have been recorded on forms specifically designed for the purpose. These are filed with the original con struction data sheets. Based on evaluations of the data obtained so far from field construction records and from surveys of present condition, the following general observa tions are made:

1. The handling characteristics of the asphalt rubber material from blending to application are uncomplicated . Mixing of the ingredients and appli cation o f the finished blend can be successfully accomplished in any tank that has adequate means for mixing and temperatur control. For example, a conventional pressure-a istributo r truck is suitable f or the purpos e if it is in good operating condition and equipped with appropriate heating equipment and good temperature -control devices. 2. The resulting asphalt-oil-rubber blend is smooth in texture and uniform in consistency. It is free of clusters of undispersed rubber particles or of nonuniformly blended ingredients. 3. Hot-spray applications are readily performed with conventional spray equipment, within the tem perature range of 375°-425°F. Hot-pour applications are similarly easy to make by using any of the standard pouring devices within the same range of application temperatures. 4. When the asphalt-rubber is hot-sprayed or hot-poured, it is sufficiently adhesive so that the

Figure 4. Effect of thickness of overlay on effective stress at crack tip with and without asphalt-rubber SAMI and on maximum stress in asphalt concrete ~nd SAMI.

700

600

105 Eac 500 Epcc 2 x C-"' t 8" Vl pee ...,Vl Crack Width 0. 5° J-- "'Vl 400 L'-' > (A) Crack tip stress (no asphalt-rubber membrane) J-- u (B) Crack tip stress w1th .25" asphalt-rubber L'-' u. ...,u. (C) Maximum stress in asphalt concrete at asphalt concrete/asphalt rubber interface 300 (D) Maximum stress in asphalt-rubber at asphalt concrete/asphalt rubber interface

200

(B) :--~( C"'--) -~ 100

0 ~~~~~~~~~-'-~~~~.._~~~~~~~~...... ~~~~- a 5 6

toverlay (tasphalt concrete 1 tasphalt-rubber) - in.

scrap--and 60 percent powdered vulcanized scrap placements associated with loading and thermal rubber high in natural rubber content with a paving stresses. g rade asphalt supplemented with rubber extender ·oil to produce an asphalt-rubber material with the de Al though there has been much development work sirable properties of both the asphalt and the rub with this asphalt-rubber material since its incep ber is valid. Laboratory experiments, analytic tion in 1975, further studies are needed at both the studies, and field experience to date support the laboratory and field stages to develop better following conclusions: methods of determining its properties, characteriz ing its behavior, and documenting its performance in 1. The asphalt-rubber material is easily pre each of its many applications. pared from its three basic ingredients with the equipment and know-how currently available in the REFERENCES field of asphalt construction. 2. The resulting asphalt-rubber product is 1. C.H. McDonald. A New Patching Material for smooth in texture and uniform in consistency and can Pavement Failures. HRB, Highway Research be readily spray-applied or poured within a tempera Record 146, 1966, pp. 1-16. ture range of 375°-425°F by using conventional as 2. B.D. LaGrone and B.J. Huff. Utilization of phalt spreading or pouring equipment. Waste Rubber to Improve Highway Performance and 3. When hot-sprayed on a surface and allowed to Durability. Presented at 52nd Annual Meeting, cool, the material forms a tough, durable, and ad HRB, 1973. hesive asphalt-rubber membrane of low stiffness and 3. F.S. Rostler, R.M. White, and E.M. Dannenberg. high deformability that makes it useful for the Carbon Black as a Reinforcing Agent for As construction of (a) surface treatments with a multi phalt. Proc., AAPT, Vol. 46, 1977. layered aggregate structure, (b) SAMis, and (c) 4 . B.A. Vallerga and P.F. Gridley. Carbon Black waterproofing membranes for bridge decks and hy Reinforcement of Asphalts in Paving Mixtures. draulic linings. AS'IM, Philadelphia, Special Tech. Publ. 724, 4. When hot-poured into clean and debris-free 1979, pp. 110-128. pavement joints and cracks and allowed to cool, the 5. D.L. Nielsen and J.R. Bagley. Rubberized As material forms an effective filler that resists the phalt Paving Composition and Use Thereof. U.S. intrusion of water and debris but still has the low Patent 4,068,023, Jan. 10, 1978. stiffness and high deformability to accommodate dis- 6. F.S. Rostler and R.M. White. Composition and Transportation Research Record 821 37

Changes in Composition of Highway Asphalts, 85- 9. B.A. Vallerga, and others. Applicability of 100 Penetration Grade. Proc., AAPT, Vol. 31, Asphalt-Rubber Membranes in Reducing Reflection 1962. Cracking. Proc., AAPT, Vol. 49, 1980, pp. 7. N.F. Coetzee and C.L. Monismith. An Analytical 330-353. Study of the Applicability of a Rubber-Asphalt 10. Specification for ARM-R-SHIELD. Arizona Refin Membrane to Minimize Reflection Cracking in As ing Co., Phoenix, Spec. M-101, 1976. phalt Concrete Overlay Pavements. Department 11. Construction Specification for ARM-R-SHIELD of Civil Engineering, Univ. of California, Surface Treatment. Arizona Refining Co., Berkeley, Geotechnical Engineering Rept., June Phoenix, Spec. C-201, 1976. 1978. 12. Construction Specification for ARM-R-SHIELD 8 . B.A. Vallerga and J.R. Bagley. Design of Stress Absorbing Membrane Interlayer. Arizona Asphalt-Rubber Single Surface Treatments with Refining Co., Phoenix, Spec. C-202, 1976. Multilayered Aggregate Structure. ASTM, Phila delphia, Special Tech. Publ. 724, 1979, pp. P11b/icatlo11 of this f)a{J

Modification of Paving Asphalts by Digestion with Scrap Rubber

JOHN W.H. OLIVER

The service performance of sprayed surface treatments laid where traffic binder and aggregate (stress-absorbing membrane induced stress is severe or where tho existing pavement is cracked can be signif· interlayer) overlaid by a thin course of asphaltic icantly improved if comminuted scrap rubber is digested in the asphaltic ce· concrete. ment before spraying. Work performed to identify the factors that are of im In Australia, the main use of the scrap rubber portance in optimizing asphalt performance is reported. Measurement of the asphalt digestions has been in sprayed surface d-Oformation response of the rubber digestions to sinusoidal loading indicated treatments where the advantages are considered to be thnt 1ho modified binder had Improved response under the loading produced by trnffic at high pavement temperatures and that produced by thermal con· traction at low pavement temperatures. A simple and rapid tort procedure was 1. The sealing of cracked pavements to prevent, developed to measure the response under loading conditions similar to !hose. or delay the onset of, reflection cracking, and The procedure is to subject a prism of the binder at 60° C to a shear strain of 2. The retention of chips in hot weather under 1.0-in creep and then determine the elastic recovery when the stress is removed. severe traffic stress conditions (such conditions The most important factor affecting elastic recovery was found to be the occur when seals are used to provide surface texture morphology of the rubber particles as determined by the comminution process on high-speed roads where a conventional a sphalt used in their manufacture. A simple bulk·density test was used to charactorizo seal is unable to provide satisfactory stone reten this morphology. OlgeSlions of natural (truck·tiro) rubbor are generally superior tion at bends or in acceleration and deceleration to those that incorporate synthetic (car-tirol rubber but· are more affected by areas). changes in the time or tomporaturo of digc.slion. Whore a cryogenic comrnlnu· ti on process hed boon used, only digostions of natural-rubber particles produced a significant improvement In asphalt properties. Elastic recovery of strain is DEFORMATION TESTING OF RUBBER-MODIFIED ASPHALTS linearly rolated to rubber concentration. Sinusoidal Loading

Rubber modifiers have been used in bituminous A useful means of evaluating the deformation be materials for many years. They have usually been havior of asphalts and rubber-modified asphalts is specially prepared natural and synthetic rubbers, by their response to sinusoidal loading in simple and their concentration has been limited to less shear. This provides basic information on the be than 5 percent by mass of the asphaltic cement havior of the materials , but specialized equipment because greater concentrations have caused problems is required and testing is slow. The sinusoidal in handling (pumping and spraying) and because of loading procedure was therefore used only to deter their high cost. At these concentrations, the mine the appropriate test conditions (temperature, improvement in binder performance obtained in pave rate of strain, etc;: .) for the rubber-asphalt diges ment service has been marginal unless a satisfactory tions, and a simpler apparatus, operated near the polymer network has been formed in the asphalt. required conditions, was then used for routine A major improvement in this situation occurred as testing. a result of the work of McDonald (1) and Morris and In the sinusoidal loading procedure, a force McDonald (_£), who introduced the use of digestions applied to the material produces a sinusoidal dis of scrap rubber in asphalt that contained up to 25 placement that lags behind the force (see Figure percent by mass of comminuted tire-tread rubber. A 1). The magnitude of the phase-angle difference sprayed layer of this material has been widely used between the force and the displacement (,p) is an in the United States to prevent cracks in the sub indication of the partitioning of the response strate from being reflected through overlays. It is between viscous and elastic behavior. The phase normally applied as either a chip-seal surface angle is zero for purely elastic behavior and 90° treatment with approximately 20 percent rubber added for purely viscous behavior. The ratio of the to the asphalt, commonly known as a stress-absorbing maximum value of the force to the maximum value of membrane, or as an interlayer of rubber-asphalt the displacement is proportional to the complex