Performance of scrap tire rubber modified asphalt paving mixes.
The use of scrap tire rubber for paving applications is also desirable from a solid waste management point of view. Each year the U.S. discards approximately 285 million tires. Of that number, 55 million are reused or resold, another 42 million are diverted to various other alternative uses. The remaining 188 million are added to stockpiles, landfills and illegal dumps[ref. 4]. The number of discarded tires in Ontario is 11 million[ref. 5] and when the Ontario number is projected to the whole of Canada, it is estimated that approximately 30 million scrap tires are generated in Canada per year and the means of disposal is similar to that in the U.S. Among all the available methods for scrap tire recycling, the use of processed scrap tire rubber in asphalt pavements does have the potential for being both economically and ecologically sound, and it can at the same time consume a very large number of tires. The annual asphalt consumption for both paving and roofing applications amounts to over 30 million tons in North America[ref. 6]. If a fraction of this volume were to be modified with scrap tire rubber, it would go a long way towards reducing the scrap tire disposal problem.
Experimenting with scrap rubber for asphalt modification started in the 1920s[ref 4]. However, the introduction of the McDonald process by the Roads Department of the City of Phoenix, Arizona in the 1960s led to the development of rubber modified asphalt binders, as they are now most often used throughout North America and in many other countries[ref. 7]. Since then, various proprietary and generic technologies have evolved for the use of recycled rubber from scrap tires in asphalt binders and rubber modified asphalt concrete. The wet process (based on the McDonald process), in general, requires the use of at least 20% more liquid asphalt than is used in a conventional hot-mix pavement. In some cases 40-60% more asphalt is used, accounting for most of the increase in both cost and performance. The high initial cost combined with the uncertainty regarding the future benefits is probably a factor which has hindered the large scale acceptance of asphalt-rubber technology. The cost of such rubber modified pavement mix, in general, is currently anywhere from 60-150% above the cost of a conventional pavement mix[ref. 8]. However, if modest amounts (5-10% by wt.) of fine crumbs are used for asphalt modification, a pavement could be constructed with normal binder contents and aggregate gradations, which would result in only a slight overall increase in cost. This approach has already been taken in recent years. Paving trials in Florida[ref. 9] and Ontario[ref. 10] have used asphalt binders which contain only 7-9% fine crumb rubber (80 mesh, ~180 microns) directly blended into the asphalt cement. Initial laboratory and field results are quite promising, but it is too early to draw any firm conclusions.
The dry process which was developed in the late 1960s in Sweden under the trade name Rubit was patented for use in the United States in 1978 under the trade name PlusRide[ref. 11]. It differs from the wet process in that the crumb rubber is used as a portion of the aggregate and is directly mixed with The aggregate. This process uses crumbs of larger sizes (1/16-1/4 in.: ~1.58-6.35 nun) at a loading of 3-4 wt% of the aggregate. This process also requires 1.5-3% more liquid asphalt than a conventional hot-mix. The increased asphalt content is needed to achieve a void content of 3-5% in asphaltic concrete, in order to prevent premature ravelling of the pavement[refs. 12 and 13]. It is reported that the PlusRide technology has been effective in reducing the harmful effects of ice formation on roads[ref. 14]. Other dry process techniques include those developed by the Army Corps of Engineers at the Cold Regions Research and Engineering Laboratory (CRREL) and the Generic Dry Technology[ref. 15]. Early results on the use of the dry process in Ontario have not been very positive[ref. 16].
It has long been thought that devulcanization, partial devulcanization or depolymerization of crumb rubber would provide additional benefits, in terms of storage stability of modified asphalts and performance improvements. Epps[ref. 17] has found in an informal survey that there have been relatively only a few pavements constructed with devulcanized rubber binder systems. The patent literature contains numerous claims on processes for devulcanizing waste tire rubber: In 1971. Nikolinski and Dobreva[ref. 18] used various process oils to produce devulcanized rubber from waste SBR, nitrile rubber, butyl rubber and 1,4 cis polybutadiene. This is one of the earlier patents which clearly describes the use of aromatic oils in the process. Patents of C.H. McDonald[ref. 19], Nielsen and Bagley[ref. 20], Sergeeva et al.[ref. 21], and Ulicke and Cerner[ref. 22] all describe the use of aromatic oil for effecting devulcanization of scrap tire rubber in an asphalt medium. The main drawback with regards to the use of aromatic oils for devulcanization of waste rubber comes from the health hazard associated with these oils. The use of process oil or hydrocarbon liquid under high temperature and shear to render vulcanized rubber into a fluid form is described by Wakefield et al. in 1975[ref. 23]. Applications of shearing energy with addition of an aromatic oil to produce bitumen-asphalt compositions is described by van Bochove[ref. 24]. According to this patent, better control over the material properties were made possible through the application of shearing forces. In 1992 and in 1994, Liang and Woodhams[refs. 25 and 26] described a process for devulcanization of scrap tire rubber in asphalt with the aid of aromatic oils and high shear and subsequently further stabilizing the devulcanized or disintegrated rubber particles by reacting the product with liquid polybutadiene and sulfur. The devulcanized system is mixed with a sterically-stabilized, polyethylene-modified asphalt binder as described in earlier patents by Hesp et al.[refs. 27 and 28]. Recent work by Zanzotto and Kennepoh[ref. 4] describes a high temperature, high shear process, for devulcanizing scrap tire rubber and rubber buffings in a low penetration grade (200/300 pen.) base asphalt. The authors report that the modified asphalt materials containing 15-20% scrap tire rubber are being tested for their performance in paving mixes. It is quite apparent that morphology in the asphalt-rubber composition plays a crucial role in determining the properties of the crumb rubber modified binder systems and in the performance of crumb rubber modified asphaltic concrete. The work described here is concerned with achieving improved high temperature and low temperature performance of rubber modified asphaltic concrete mixes, taking into consideration the compatibility, crumb rubber particle size and distribution of the asphalt-rubber composition.
Experimental design consists of using two grades of asphalt binders commonly used in Ontario, Canada. In addition, three types of crumb rubber; 30 mesh (~600 microns), 80 mesh (~180 microns) and partially devulcanized tire rubber were used to prepare the rubber modified asphalts. HL-3 Asphalt concrete mix samples were prepared using the tire rubber modified asphalts to evaluate the resistance to rutting and low temperature cracking. Rutting resistance was evaluated using wheel tracking machine (WTM) and the low temperature cracking, resistance was evaluated by TSRST. SHRP binder tests and fracture toughness tests were performed to determine the correlation between the binder properties and the simulated field performance.
The asphalts used in this study were a 150-200 and an 85-100 penetration grade both obtained from the Lake Ontario refinery of Petro-Canada in Clarkson, Ontario made with crude from the Bow River area in Alberta, Canada.
The 30 mesh (~600 microns), cryogenically round, passenger, tire rubber sample was obtained from Recovery Technologies of Mississauga, Ontario and the 80 mesh (~180 microns), ambiently ground, tire rubber sample was obtained from Rouse Rubber Industries of Vicksburg, Mississippi. Partially devulcanized grades of scrap tire rubber were prepared from the 30 mesh, cryogenically ground rubber by Ortech Corp., Ontario under a contract with Ministry of Transportation Ontario[ref. 29] and consisted of samples prepared by partial devulcanization at temperatures ranging from 125 [degrees] C to 220 [degrees] C in an extruder and a sample prepared by a digester process at 200 [degrees] C.
A dense-graded mix design meeting the Ontario HL-3 specification[ref. 30] for surface course mixtures, was used to prepare the asphalt rubber concrete beam samples. Limestone coarse aggregate, limestone screenings and natural sand used for simple preparation were supplied by Dibblee Construction of Westbrook, Ontario.
Sample preparation -- modified asphalts
Scrap tire rubber modified asphalt binders were prepared by slowly adding 10% by weight of crumb rubber (30 mesh, 80 mesh and partially devulcanized types) to the molten asphalt (85-100 and 150-200 penetration grades) at 170 [degrees] [+ or -] 10 [degrees] C with moderate shearing. Samples of modified asphalt containing a very fine dispersion of scrap tire rubber were prepared by shearing a dispersion of 10% by weight of 30 mesh crumb rubber in a 150-200 penetration grade asphalt. The high shearing force was produced with a Polytron mixer (Brinkmann Instruments) employing a type PTA45/2M generator. This mixing tool has been specifically designed for dispersing and disintegrating tough materials like rubber or fibers. The disintegration was done at temperatures of 200 and 220 [degrees] C, while stirring speeds were kept at either 4,000 or 5,000 rpm, depending on the temperature. Temperatures were controlled with a Digi-Sense temperature controller (Cole-Palmer Instruments). Average particle size, standard deviation and maximum particle size were determined with an optical image analysis system.
Sample preparation -- asphalt concrete
The asphalt concrete beams 381 mm length by 140 mm wide by 76 mm thick, used for rutting tests on a wheel tracking machine, consisted of two lifts, each 38 mm from selected mix designs. The bottom lift of each beam was made of a high stability (13,600 N) Durham conventional HL-4 mix. The top lift was made of HL-3 mixes containing six types of crumb rubber modified asphalt binders and the unmodified 150-200 and 85-100 penetration grade reference binders. The beams were prepared using a standard MTO procedure[ref. 31].
Fracture toughness testing
Fracture toughness testing of asphalt binder samples was recently developed in Ontario, by using a three point bending, beam method[ref. 32] based on ASTM E 399-90 procedures. The neat and modified binder beam samples were prepared using 25 mm wide by 12.5 mm deep by 175 mm long silicone rubber molds which have 90 degrees starter notches, 5 mm deep, at the center of the bottom surface. The molds were filled with asphalt binders and kept in a freezer at -20 [degrees] C for about two hours until they became solidified. The binder samples were then removed from the molds and were kept at the testing temperature for 18 hours. The starter notch in each sample was sharpened with a razor blade prior to testing. The notched beam was then placed on a three point bending apparatus with a span of 100 mm within an environmentally controlled chamber. The beam was then loaded until failure. From the output, fracture toughness was computed according to equation 1. Where: [K.sub.ic] is the fracture toughness; P is the failure load; S is the span; B is the specimen depth; W is the specimen width; and a is the crack length.
SHRP low temperature performance testing
An Applied Test Systems bending beam rheometer (BBR) was used to measure creep stiffness and m-value (logarithmic creep rate) of rubber modified binders and the control samples at -12 [degrees] C and at -18 [degrees] C. Unaged samples and samples that have been subjected to aging in a rolling thin oven (RFTO) and pressure aging vessel (PAV) were used. Test specimens of 101.60 mm x 12.70 mm x 6.35 mm were prepared by pouring the asphalt binder into a mold, moving from one end to the other end, slightly overfilling the mold. The mold was allowed to cool to room temperature for 45 to 60 minutes and then the excess flash at the top trimmed off. The sample was then cooled to about -5 [degrees] C before demolding. Then the specimen was conditioned at the testing temperature for one hour. The conditioned sample was mounted in the sample chamber of the BBR and a constant loading was applied for four minutes[ref. 33]. The deflection at the center of the beam was measured continuously throughout the four minute period. The creep stiffness and the m-value were calculated from the deflection measurements.
Failure strain of tire rubber modified binders and the control binders was measured at -12 and -18 [degrees] C. An Applied Test Systems (ATS) direct tension tester (DAT) provided with an Electronic instrument Research laser extensometer Model LET-01 and an ATS environmental chamber were used to perform this test. The testing machine is capable of applying a load of 400-500N at a rate of 1.0 me/min. Direct tension test specimens were prepared by pouring hot asphalt into a silicone rubber mold provided with plastic end inserts. The mold allows fabrication of four specimens which are used to produce one set of results. The test specimens are 100 mm long including the two inserts, each 30 mm long. Thus the binder test specimen is 40 mm long. The nominal cross section is 6 mm by 6 mm. The trimmed and demolded specimens were tested within 60 [+ or -] 10 minutes[ref. 33].
SHRP high temperature performance testing
A Rheometrics dynamic analyzer RDA II was used for rheological testing of unaged rubber modified binders and the base asphalts. Hot asphalt samples were poured into a combined melts and solids (CMS) test fixture and allowed to cool to room temperature prior to testing. The CMS fixture consists of a 42 me diameter cup and a bilevel plate which has an 8 me diameter serrated surface concentric with and projecting from a 25 me diameter plate. A temperature sweep was used to measure G*, G', G" and tan [Delta] at four temperatures between 52 [degrees] C and 70 [degrees] C in intervals of 6 [degrees] C. A frequency of 10 rad/s was used. Samples were conditioned for at least 11 minutes at each test temperature. A soak time of 180 s, during which time the temperature did not vary by more than 0.1 [degrees] C, was used prior to each measurement.
Thermal stress restrained specimen test (TSRST)
A thermal stress restrained specimen test was conducted according to the procedure described in the literature[ref. 34]. In this test, rectangular asphalt concrete specimens were mounted in an MTS/Sintech 2G load frame. The tests were done with linearly variable displacement transducers (LVDTs) located on opposite sides of the specimen. The LVDTs were fixed to the top plates while both the LVDT cores were connected with Invar steel rods to the bottom plate. The load frame was equipped with a liquid nitrogen controlled temperature chamber. As the specimen was cooled at -10 [degrees] C/hour it was prevented from shrinking by the test software. The cooling was continued until the sample failed. The data were recorded by operating the software as force-time curves. A failure stress and a failure temperature were obtained for each test.
The rutting tests, carried out using a WTM, consist of three parts; a constant temperature reservoir, a wheel carriage assembly and a drive linkage assembly. All of these parts work in unison to produce a back and forth movement of a tire along the lengths of the beam sample[ref. 31]. The prepared beam samples were conditioned at 60 [degrees] C for six hours and tested at 60 [degrees] C after allowing the sample to equilibrate in the constant temperature reservoir. Samples were subjected to 8,000 passes (4,000 cycles) using a treaded tire (pressurized at 550 kPa) at 60 [degrees] C to assess the rutting performance of asphaltic concrete samples. Profilometers were used to obtain the rutting profiles.
Results and discussion
Low temperature fracture toughness
Fracture toughness is a material parameter that measures the resistance of the material to crack. Fracture toughness is directly related to the energy released during crack propagation. As such, fracture toughness is a better parameter for measuring the ability of asphalt to relieve internal stresses before they build up and lead to catastrophic failure[ref. 35]. Recent studies[ref. 32] indicate that the fracture toughness can be effectively used to evaluate low temperature cracking resistance of polymer modified binders. The results of fracture toughness testing performed at -20 [degrees] C with rubber modified binders based on 85/100 pen asphalt are given in figure 1. The results presented are the mean values of 12-20 samples tested. The results show that the rubber modification improves the fracture toughness of the binder. The modified binder systems containing 80 mesh fine crumb rubber showed the highest improvements, when compared to the partially devulcanized and 30 mesh rubber modified systems.
[FIGURE 1 GRAPH OMITTED]
SHRP low temperature binder tests
The creep stiffness (S) and m-value as measured by BBR and strain at failure as determined by DTT on rubber modified binders and the base asphalts are presented in table 1. The results are from unaged samples and samples that have been subjected to RTFO and PAV aging. Stiffness values of the rubber modified samples are lower than those of the control sample. Among the rubber modified samples, the sample modified with 30 mesh fine crumb rubber had the lowest values indicating that this binder would be more flexible and therefore less susceptible to brittle fracture than the other samples. However, the m-value of the PAV aged samples do not meet the SHRP low temperature requirements at -18 [degrees] C. The failure strain values as determined by DAT, for rubber modified asphalt based on 80 mesh fine crumbs were above 3% at a testing temperature of -12 [degrees] C, even after PAV aging. However, the failure strain values of this system at-18 [degrees] C are similar to those of the base asphalt.
[TABULAR DATA 1 NOT REPRODUCIBLE IN ASCII]
Dynamic shear testing
The results of dynamic shear testing of tire rubber modified and the corresponding 150/200 base asphalt are given in figure 2. The results show small improvements in performance grade (PG) by the incorporation of tire rubber. However, the results appear to be insensitive to the particle size of the rubber modifier in the binder. The binder containing very fine colloidal dispersion of rubber particles with an average particle size of 0.4 microns and the modifier with coarser 30 mesh crumb rubber had similar results in this testing.
[FIGURE 2 GRAPH OMITTED]
Thermal stress restrained specimen test (TSRST) results
The failure stress and the failure temperature of asphaltic concrete samples, as determined by TSRSTs are given in table 2. The failure temperature of the asphaltic concrete sample prepared with fine crumb rubber (80 mesh, 180 microns) modified 85/ 1,000 pen asphalt binder was about 3-6 [degrees] C lower (colder) than the control. Also included in table 2 are the results of samples prepared with a thermo-mechanically processed (high shear mixing) crumb rubber modifier (average particle size of 0.4 microns) and the 150/200 pen base asphalt. Here again only a moderate improvement was obtained when compared to the control sample. These low temperature performance results are not as good as one would have expected to see. The fracture toughness of the rubber modified binder containing 10% wt. of 80 mesh fine crumbs was three fold to that of the control sample (figure 1). Failure stress of some of the mix samples was determined at -20 [degrees] C (S.A.M. Hesp - unpublished). The results of these experiments showed that the failure stress for the rubber modified mix containing very fine rubber particles is approximately 40% higher than that of the control. This improvement is quite significant, but it is unclear at present what it means in terms of long term performance.
[TABULAR DATA 2 NOT REPRODUCIBLE IN ASCII]
Effect of crumb rubber modifier on rutting performance
The rut depth results obtained for individual HL-3 asphalt concrete beams and the average values for each mix are presented in table 3. The results show that the resistance to rutting of mixes based on crumb rubber modified binders is significantly better than that of the reference samples. Improvements over the 150/200 pen control sample were about 37%, 51% and 60% for modified binders containing 30 mesh, 80 mesh and 0.4 [Mu]m crumb rubber, respectively. It is to be noted that the variation in rut depths between the duplicate samples are reasonable (except for the control mix prepared with an 85-100 pen asphalt), considering the fact that the preparation of the concrete beams for these tests involves a series of steps which have to be carefully performed. Table 3 also gives the rut depth values obtained for an 85-100 pen grade asphalt which is commonly used in high temperature service conditions such as in southern Ontario and the rubber modified systems containing devulcanized types and 80 mesh fine crumbs. The fine crumb rubber (80 mesh) modified mix shows an improvement over 20% when compared to the control 85/100 asphalt based sample. These results also indicate that, by crumb rubber modification of a softer grade (150-200 pen) asphalt, it may be possible to obtain comparable rutting resistance to that of an 85-100 pen grade asphalt.
[TABULAR DATA 3 NOT REPRODUCIBLE IN ASCII]
A comparison was made between normalized average rut depths and normalized performance grades of the three crumb rubber modified binders based on 85/100 asphalt and the reference binder. But this comparison does not seem to reveal any trend correlating the performance grade (PG) of a binder, obtained by dynamic mechanical analysis, with the rutting performance. The work of Hanson and Duncan[ref. 36] on crumb rubber binders also indicates that although the stiffness increases with concentration, there is little variation in G*/sin[Delta] for different gradations of rubber. The gradations of rubber used in their work were GF 16, 40, 80 and 120 mesh sizes.
The mean particle size for the thermo-mechanically processed sample was found to be 0.4 microns with a standard deviation of 0.4 microns. This represents a decrease of 1,500 times compared to the 30 mesh (590 microns) crumb rubber used as the starting material for high shear mixing. In terms of rutting performance, about a 35% decrease in rut depth was obtained for a particle size reduction from 30 mesh (590 microns) to 0.4 microns. The difference in rut depth between the sample containing 0.4 micron particle and the sample with 80 mesh (177 microns) is about 22%. These results clearly indicate that the particle size reduction of crumb rubber in asphalt results in improvement in performance, particularly the high temperature performance. Further experimentation on thermo-mechanical processing is necessary to determine the optimum particle size in terms of cost-performance for asphalt modification. Cost of thermo-mechanical processing (high shear mixing) for the production of fine colloidal dispersion of scrap tire rubber in asphalt also has to be worked out to perform a detailed cost-benefit analysis on various rubber modified systems.
A thermo-mechanical process employed for the preparation of fine colloidal dispersion of crumb rubber from 30 mesh crumb rubber in molten asphalt has been found to be an effective method for the preparation of crumb rubber modified binder with significantly improved high temperature properties. The low temperature TSRST revealed that there is moderate improvement in thermal cracking resistance of mixes containing rubber modified binders, with systems containing smaller particles performing better than the others. WTM rutting tests conducted on modified mixes revealed that the resistance to rutting improves with a reduction in the particle size of the crumb rubber modifier.
[1.] Epps, J., Petersen, J.C, Kennedy T.W., Anderson, D. and Haas, R. TRR 1096, Washington, D.C (1986).
[2.] H. Saywatzky, "Temperature dependence and complexation process in asphalt and possible influence on temperature susceptibility," Conference on hydrocarbon residues and wastes, Edmonton, Alberta, (Sept. 4-5, 1991).
[3.] L. Flynn, Roads & Bridges, 12, 42 (1992).
[4.] L. Zanzotto and G. Kennepohl, presented at the 75th annual meeting the Transportation Board, Washington, D.C (1996).
[5.] G.J. Kennepohl, Status Report PAV-92-08 "Scrap tire applications in Ontario's transportation industry," Ministry, of Transportation Ontario (April, 1993).
[6.] Asphalt Institute, Introduction to Asphalt, Asphalt Manual Series No. 5 (MS-5), 8th Ed. (1990).
[7.] G.R. Morrison and S.A.M. Hesp, Journal of Material Science 30, 2584 (1995).
[8.] P. Tarricone. Civil Eng., April, 1993, 46.
[9.] C.G. Page, B.E. Ruth and R.C. West, Preprint 920452, TRB 71st annual meeting, Washington, D. C. (January 1992).
[10.] P.E. Joseph and G. Kennepohl, Report PAV-91-03, Ministry of Transportation Ontario, Downsview, Ontario (June, 1991).
[11.] A. Bjorklund, proceedings of the VII World Road Conference, Vienna (1979).
[12.] H.B. Takallou and R.G. Hicks, TRR 1171, 113. TRB, NRC, Washington, D.C (1988).
[13.] J.L. McQuillen, J.F. Member, H.B. Takallou, R.G. Hicks and D. Esch. Transp. Eng. 114, 259, (1988).
[14.] D. Esch, TRR 860, 5-13, TRB, NRC, Washington, D.C (1992).
[15.] J.A. Epps, NCHRP Synthesis 198, TRB, NRC, Washington, D.C. (1994).
[16.] V. Arillio. D.F. Lynch and R.P. Northwood, ASTM Special Publication 1193 (1993).
[17.] J.A. Epps, Ministry of Transportation Ontario meeting oil the use of crumb rubber modified binders, Downsview, Ontario (April 10t, 1995).
[18.] P. Nikolinski and R. Dobreva, Polim. Simp. 1971, vol. 3, 254-9 (Pub. 1972).
[19.] McDonald (1974), J.R. Bagley, U.S. Patent 4,068,023, filed on May 20, 1976 (1978).
[21.] M. Sergeeva, I.L. Zhailovich, P.I. Tumashchik and I.A. Oreklov, USSR Patent SU1 289872 Al, (issued on Feb. 15, 1987).
[22.] Chemical Abstracts, Vol. 116 (8) AN 61360C.
[23.] L.B. Wakefield, G. Crane and E.L. Kay, U.S. Patent 3,896,059, Filed Feb. 5, 1974 (1975).
[24.] Van. Bochove European Patent 0439232 A1 (Filed Jan. 22, 1991).
[25.] Z.Z. Liang and R.T. Woodhams, World Patent Application 94/14896, Priority date: December 29, 1992 - GB 92 27035.4 (1994).
[27.] S.A.M. Hesp, Z.Z Liang and R.T Woodhams, World Patent Application 93/07219, Priority, date: September 30, 1991 - US 863,734 (1993).
[28.] S.A.M. Hesp, Z.Z. Liang and R. T Woodhams, U. S. Patent 5,280,064, Filed on September 30, 1991 (1994).
[29.] A.M. Sinclair, Preparation of Reclaimed Rubber Samples, Report No. 94-T21-B003458, prepared for and submitted to Ministry of Transportation Ontario ( May, 1994).
[30.] R. Yacyshvn, K.K. Tam and D.F. Lynch, Report EM-93 Engineering Material Office, Ministry of Transportation Ontario, Downsview, Ontario (1990).
[31.] Ministry of Transportation Ontario, Laboratory Series LS Manual.
[32.] J.E. Ponniah, R.A. Cullen and S.A.M. Hesp, Fracture Energy Specifications for Modified Asphalts, presented at the 212th ACS National Meeting, Orlando, FL, August 25-29, 1996. Division of Fuel Chemistry, 41 (4) 1317 (1996).
[33.] Bituminous Section, Internal Report, Ministry of Transportation Ontario (1997).
[34.] D. Jung and E.S. Vinson. Thermal Restrained Specimen Test to Evaluate Low-Temperature Cracking of Asphalt-Aggregate Mixtures. TRR. No. 1417, 12 NRC, Washington D. C. (1993).
[35.] N.K. Lee and S.A.M. Hesp, Low Temperature Fracture Toughness of Polyethylene-Modified Asphalt Binders," Transportation Research Record (in press).
[36.] Hanson, D. and Duncan, G.M. (1995), TRR 1488, TRB, Washington , D.C.
"Performance of scrap tire rubber modified asphalt paving mixes," is based on a paper given at the October, 1997 meeting of the Rubber Division. "6-QDI - a review of a multifunctional chemical for the rubber industry," is based on a paper given at the October, 1997 meeting of the Rubber Division.
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