study of crumb rubber materials as paving asphalt modifiers

Study of crumb rubber materials as pavingasphalt modifiers

Daryl MacLeod, Susanna Ho, Ryan Wirth, and Ludo Zanzotto

Abstract: Waste tire crumb rubber materials (CRM) were used to modify paving asphalts. The mixing time, hot-storagestability, Superpave grades, pumping and handling properties, phase separation tests, and repeated creep properties ofthe modified asphalts were studied using base asphalts of different hardness. Applying the Long-Term Pavement Performance(LTPP) program and the Transportation Association of Canada (TAC) model, optimal levels of CRM and suitable baseasphalts were selected for the climatic conditions of Lethbridge, Alberta, Canada. High-temperature grade bumpingprotocol, regarding traffic volume and speed, was also considered. With joint efforts from the Tire Recycling ManagementAssociation of Alberta (TRMA), Husky Energy, and the City of Lethbridge, three test sections in different Lethbridgelocations with various traffic volumes were paved from the years 2003 to 2005. So far, the City of Lethbridge is pleasedwith the initial performance of the test sections.

Key words: waste tire, crumb rubber materials (CRM), paving asphalt, recycled, modification, Superpave, repeated creep,field test.

Résumé : Des fragments de caoutchouc (« CRM ») provenant de pneus usés ont été utilisés pour modifier les bitumesde pavage. Le temps de mélange, la stabilité de l'entreposage à chaud, les classes de Superpavés, les propriétés de pompageet de manutention, les tests de séparation en phases ainsi que les propriétés de fluages successifs des bitumes modifiésont été étudiés en utilisant des bitumes de base de différentes duretés. En appliquant le programme « LTPP (Long-TermPavement Performance) » et le modèle de l'Association des transports du Canada (ATC), les niveaux optimaux de « CRM »et des bases asphaltiques adéquates ont été sélectionnés pour les conditions climatiques de Lethbridge, Alberta, Canada.Le protocole d'augmentation du niveau de classes de haute température, touchant au volume et à la vitesse de la circulation,a également été considéré. Des efforts conjoints entre la Tire Recycling Management Association of Alberta (TRMA),Husky Energy et la Ville de Lethbridge ont permis de paver, entre 2003 et 2005, trois sections tests dans la région deLethbridge présentant divers volumes de circulation. La ville de Lethbridge est satisfaite du rendement initial de cessections tests à ce jour.

Mots-clés : pneus usés, fragments de caoutchouc (« CRM »), bitumes de pavage, recyclé, modification, Superpavé, fluagesuccessif, essai sur le terrain.

[Traduit par la Rédaction] MacLeod et al. 1288

Introduction

There are slightly fewer than 300 million tires stockpiledin the United States, and approximately the same amount isgenerated annually (Rubber Manufacturer’s Association 2001).In 1990, there were 1 billion tires stockpiled in the UnitedStates. That number was reduced to 800 million in 1994,500 million in 1996, and further reduced to 300 million in2001 (Blumenthal 2005). In 1997, a U.S. federal government

mandate required all fifty states to use waste tire modifiedasphalt in 20% of their total asphalt tonnage laid each year(Ghaly 1999).

In Canada, the Canadian Association of Tire RecyclingAgencies was formed in 1999, with representation from allten provinces and from the Yukon Territory. In Alberta, Canada,3 million tires are discarded every year and approximatelythe same number is recycled annually, according to datagiven by the Tire Recycling Management Association ofAlberta (Tire Recycling Alberta 2006).

Asphalt modification with crumb rubber materials (CRM)from scrap tires has been studied quite extensively in recentyears. Of special interest are the studies of optimal CRMlevels, depolymerization of CRM materials, CRM sizes, typeof asphalt binders, and how CRM size and asphalt type affectthe performance of modified asphalt binders and asphaltconcrete mixtures (Zanzotto and Kennepohl 1996; Tayebaliet al. 1997; Ghaly 1999; Kim et al. 2001; Mehta et al. 2004;Kuennen 2005; Navarro et al. 2005). Field test experiencesand trial results have also been reported in published litera-ture (Way 1999; Huang et al. 2002; Soleymani et al. 2003;Bilawchuk 2005).

Can. J. Civ. Eng. 34: 1276–1288 (2007) doi:10.1139/L07-056 © 2007 NRC Canada

1276

Received 29 September 2006. Revision accepted 25 April2007. Published on the NRC Research Press Web site atcjce.nrc.ca on 27 October 2007.

D. MacLeod. Asphalt Marketing, Husky Energy, Calgary, ABT2P 3G7, Canada.S. Ho,1 R. Wirth, and L. Zanzotto. Schulich School ofEngineering, The University of Calgary, Calgary, AB T2N 1N4,Canada.

Written discussion of this article is welcomed and will bereceived by the Editor until 29 February 2008.

1Corresponding author (e-mail: [email protected]).

The literature reports that the possible benefits of usingCRM-modified asphalt include reduced reflective cracking,traffic noise, design thickness and life-cycle costs, and in-creased fatigue life and resistance to rutting (Huang et al.2002; Jorgenson 2003; Bilawchuk 2005). However, thestorage stability of CRM-modified asphalts was found todecrease with increasing particle size and storage tempera-ture (Navarro et al. 2004), leading to the recommendationsof using a CRM size of less than 0.35 mm and mixing athigh-shear rates (1200 rpm with a low-shear batch mixerwas used in their study) during manufacturing operations(Navarro et al. 2004).

The properties of asphalt concrete containing scrap tirerubber were recently reviewed (Siddique and Naik 2004).Asphalt materials from different crude sources and manufac-turing processes vary significantly in their quality and thesize and quality of CRM from different cutting methods maydiffer from one manufacturing operation to another. At thesame time, Alberta climatic conditions are usually differentfrom those reported in literature studies. Thus, caution shouldbe exercised when applying literature findings in the designof CRM-modified pavements for use in Alberta.

In this study, local commercially available asphalt materialsof different softnesses and commercially available 40-meshcrumb rubber materials were used. The optimal compositionand mixing procedure of a CRM-modified asphalt that wouldbe suitable for Alberta locations, with different traffic volumesand speeds, were studied in terms of Superpave grades, hot-storage stability, microscopic studies, pumping and handlingproperties, phase separation tests, and repeated creep proper-ties of the modified asphalts. The repeated creep propertiesof modified asphalts were found to be important parametersin evaluating the rutting resistance of polymer-modifiedasphalts, as the Superpave high-temperature parameter, i.e.,the dynamic shear rheometer (DSR) of G*/sin�, where G* isthe complex shear modulus and � is the phase angle, wasfound to describe the rutting resistance of polymer-modifiedasphalts insufficiently (Bouldin et al. 2001; Kim et al. 2001;Shenoy 2001; Vacin et al. 2003; Bahia et al. 2001).

The operational management of the U.S. Strategic High-way Research Program’s Long-Term Pavement Performance(LTPP) started on 1 July 1992 (Teng 1993). The LTPP is aninternational in-service pavement performance monitoringprogram with the U.S. Federal Highway Administration(FHWA). The LTPP division is responsible for the manage-ment and coordination of the data collection efforts on morethan 2000 test sections located across the United States andCanada (Teng 1993). Since its inception, there have beennumerous publications regarding its development (Rabinow etal. 1993; Ali and Selezneva 1999), implementation (Saeed etal. 1994; Leahy and Briggs 2001), design (Owusu-Antwi etal. 1993; Haider and Chatti 2006), evaluation (Goodman2000; Tighe et al. 2000; Park et al. 2002; Tighe 2002), anddata analysis (Kerali et al. 1996; Roberts and Martin 1996;Bosscher et al. 1998; Moody 1998; Rada et al. 1998, 2004;Wu et al. 2000; Salem et al. 2004; Smith and Tighe 2004;Watson et al. 2004; Papagiannakis and Jackson 2006; Yin etal. 2006) in North America and abroad (Kanzaki et al. 1993;Kerali et al. 1996; Martin 2005; Agarwal et al. 2006).

The LTPP program and the Transportation Association ofCanada model were used in this study to determine the suit-

able performance graded (PG) grades of the CRM-modifiedasphalts to be used as pavement binder in Lethbridge, Alberta,at different locations with different traffic volumes and speed.

Experimental

The CRM used was commercially available from AlbertaEnvironmental Rubber Products (Edmonton, Alberta, CanadaT5V 1L3). The particle sizes of the 40-mesh CRM wereanalyzed and they were found to be quite similar to the 20-mesh sizes (Fig. 1). The majority of the crumb rubber wasbetween 1.18 and 0.3 mm. This is approximately equivalentto 16 mesh (less than 1.18 mm) or roughly similar to 20 mesh(less than 0.85 mm). This 40-mesh CRM was used in thisstudy. The base asphalts used in this study were commercialasphalts produced by Husky Energy and were of differentpen grades, i.e., 150/200A, 200/300A, and 300/400A. TheCRM were added to the asphalt, in different percentages,and low-shear mixed for either 3 or 6 h at 180 °C.

The modified asphalts were characterized by conventionaltests, such as penetration and softening points. They werealso characterized according to the Superpave binder specifi-cation of the American Association of State Highway andTransportation Officials (AASHTO) standard M320-05(AASHTO 2005).

Traditionally, since the early 1900s, asphalt was graded bythe penetration grading system to characterize the consis-tency of semi-solid asphalts. The penetration grading systemassumes that the penetration depth will give an indication ofthe softness of the asphalt. Softer asphalt of high penetrationnumbers are used for cold climates while harder asphaltbinders with low penetration numbers are used for warmclimates. This grading system does not directly relate toasphalt binder performance. The Superpave tests and speci-fications are specifically designed to address high-mix asphaltpavement performance, such as rutting (DSR G*/sin �

parameter), fatigue cracking [DSR G*sin � parameter on thepressurized aging vessel (PAV) residue], and thermal crack-ing (Wiscons DOT Pavement Guide 2006). The Superpavegrading system specifies criteria for the asphalt to pass orfail a high-temperature or low-temperature grade (AASHTO2005). However, the Superpave grades are set at 6 °C apart,i.e., PG52-34, PG58-40, and PG64-28, etc. At times whenthe Superpave grades are determined at the exact passingtemperature, i.e., PG65-38, we consider them to be true PGgrades.

The dynamic shear and repeated creep tests were carriedout on the original binder and the rolling thin film oven test(RTFOT) residues of the modified asphalts using a BohlinCVO-100 dynamic shear rheometer. The creep test tempera-ture was the same as the high-temperature grade temperatureof the CRM-modified asphalt. The creep portion of the testlasts for 1 s, followed by a 9 s recovery. The creep stresslevels were 100 Pa. Each material was tested for 95 cycles.Other Superpave test equipment included a Cannon Instru-ment bending beam rheometer (BBR), an Instron BTI-3direct tension tester, a James, Cox and Sons RTFOT oven,and a Prentex Model 9300 PAV.

The phase-separation test was performed by filling 4 cmdiameter, 25 mm long copper tubes with modified asphalt.The pipe was set up to stand vertically in a 140 or 180 °C

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MacLeod et al. 1277

oven for a test time of either 24 or 72 h. After the test time,the tube was allowed to cool in the vertical position to roomtemperature. It was then heated with a gentle flame to allowthe asphalt to slide out of the tube. The asphalt tube was cutin half, i.e., top and bottom sections. Each half was remixedand the percentages of the crumb rubber material were deter-mined in terms of the trichloroethylene insoluble contents ineach section.

Modification of 200/300 pen grade asphalt with 0% to 15%40-mesh crumb rubber materials, mixing for either 3 or 6 h

Superpave characterization resultsHusky 200/300 pen grade asphalt was modified with 0%–

15% CRM with a mixing time of either 3 (Table 1) or 6 h(Table 2) at 180 °C with a low-shear mixer (~400 rpm). Ahigh-shear mixer (~1500 rpm) was not used because eventhough a high-shear mixer has the capability to cut CRMinto finer crumbs, the low-shear mixer is more readily avail-able in asphalt manufacturing plants and more cost effective.Also, in our study, the low-shear mixer was sufficient to mixthe CRM into the asphalt. Tables 1 and 2 show that the mod-ified asphalts had practically the same characteristics: themixing time of 3 or 6 h did not make a difference. Thus, the3 h mixing time should be sufficient for preparing the modi-fied asphalt, to save time and energy. Overall, we had thefollowing findings:

• The Superpave high-temperature grade increased by ~1.2 °Cper each 1% of CRM added.

• The low-temperature grade improved by approximately1 °C per each 6% of CRM added.

• The critical cracking temperatures were usually 1–2 °Clower than the BBR low-temperature grades.

• The flash point increased upon CRM modification andthere was no danger in terms of lowering the flash pointwith CRM modification.

• The RTFOT weight losses were all within the 1.0% criteriaand the CRM poses no problem of releasing volatile mate-rials during the mixing process, just before paving.

• The PAV DSR results G*sin� values decreased with in-creasing CRM content, indicating possible reduction offatigue cracking with increased CRM modification (Table 2).

• One area of concern was the high 135 °C viscosity, whichis an indicator of handling and pumping ability, at the 12%–15% CRM levels. This means 200/300 pen grade asphalt,when modified with 12%–15% CRM, was too thick to behandled and failed the 3000 MPa·s maximum viscosityrequired by the Superpave specification. Figure 2 showsthe plot of 135 °C viscosity versus the CRM levels. Itindicates that at approximately 11.7% CRM, the 135 °Cviscosity reaches the 3000 MPa·s limit.

• Thus, the 200/300 pen grade asphalt material with approx-imately 10.5% of the specific 40-mesh CRM could, atbest, produce PG64-37 modified asphalt and meet all theSuperpave requirements.

Separation test at 140 °C for 24 and 72 hThe separation test results allowed us to determine if CRM

separation occurred during hot storage at 140 °C for 24 h(Table 3). After 24 h in a 140 °C oven, we had the followingseparation results:

• There was no difference in the separation results betweenmaterials with either the 3 or 6 h mixing time.

• At 6% CRM content, most of the CRM settled to thebottom half of the tube.

• At 9% CRM content, the bottom half had ~80% moreCRM than the top half.

• At 10.5% CRM content and after 24 h, the bottom halfhad ~24% more CRM than the top half.

• At 10.5% CRM content and after 72 h, the bottom halfhad ~44% more CRM than the top half.

• At 12%–15% CRM content, there was almost no separa-tion, probably due to the high viscosity of the material.

Repeated creep test results of the base asphalt and theasphalt modified with crumb rubbers materials (originalbinder and rolling thin film oven test residue)

The repeated creep test results of the original binder andthe RTFOT residues of the 0%–10.5% CRM-modified 200/300

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1278 Can. J. Civ. Eng. Vol. 34, 2007

Fig. 1. Particle size distributions of 40-mesh crumb rubber materials.

pen grade asphalts with a 3 h mixing time are shown inFigs. 3a and 4, respectively. An illustration of percent recoverycalculation is also shown in Fig. 3b. The creep test tempera-tures were the same as the high-temperature grades of theCRM-modified materials. At these temperatures, all testedmaterials should have the same high-temperature performanceas measured by the DSR (G*/sin �) parameter, according tothe Superpave specification. In other words, they shouldaccumulate the same amount of permanent deformation inthe repeated creep test. An examination of the results inFig. 3a shows that this was not the case. We found accumu-lated deformation decreased with increased CRM content,with a significant decrease from 9.0% to 10.5%. Figure 4shows essentially the same trend. The reverse trend between6% and 9% crumb rubber could be due to the fact that, afterRTFOT, some crumb rubber could adhere to the RTFOTbottle and the RTFOT residue did not have the representa-tive amount of CRM.

Thus, the repeated creep test showed more improvementin high-temperature performance of CRM-modified asphaltsthan the one shown by the Superpave G*/sin� parameter.Upon repeated creep, the material with the highest elasticityhas the highest recovery, thus producing lower creep compli-

ance with an increasing amount of recovery cycles. Figures 3aand 4 confirm that the CRM in the asphalt served to increasethe elastic recovery of the material and not just to increasethe high-temperature rutting resistance.

Hot-storage test resultsA large batch (7 kg) of 200/300 pen grade asphalt was

modified by 10.5% CRM by low-shearing mixing at 180 °Cfor 3 h and stored at 150 °C in a laboratory hot-storage tank.The storage tank was insulated and equipped with heatingcoils and two paddle mixers that were engaged for the wholeexperiment to keep the CRM from separating from the asphalt.Samples were taken daily from the tank and tested forviscosity at 60 and 135 °C. These viscosities were checkedto monitor the changes in the modified asphalt, especiallyfor the tendency to gel or increase in viscosity to a point thatthe material could no longer be handled or pumped. Theresults are summarized in Table 4. It is clear from Table 4that the viscosities did not increase with hot-storage time.The variations or decreases in viscosities may be due to theinhomogeneous nature of the material and the sample takenmay have had slightly less CRM than the bulk. Thus, thesematerials held in long-term storage will have to be continu-

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MacLeod et al. 1279

Sample No. 1441 2166 2165 2200 2150 2147CRM content (%) in 200/300 asphalt 0.0 6.0 9.0 10.5 12.0 15.0180 °C mixing temp., mixing time (h) 0.0 3.0 3.0 3.0 3.0 3.0Standard tests

Penetration at 25 °C (dmm) (100 g/5 s) 260 162 140 125 114 94Softening point (°C) 37.2 44.0 47 49.3 52.2 56.9Flash point (°C) 261 290 285 271

Superpave testsOriginal binder properties

Viscosity at 135 °C (MPa·s) 200 600 996 1423 3943 7890Dynamic shear (G*/sin�), (min. 1.0 kPa), (kPa) 1.02 1.05 1.02 1.09 1.01 1.09Temperature (°C) 53 63 67 69 74 82

Rolling thin film oven test (RTFOT)RTFOT mass loss (%) –0.844 –0.880 –0.760 –0.660 –0.750 –0.470Dynamic shear (G*/sin�), (min. 2.20 kPa), (kPa) 2.20 2.46 2.25 2.35 2.21 2.34Temperature (°C) 54 60 65 65 69 73

Pressure aging vessel (PAV) residuePAV aging temperature (°C) 90 100 100 100 100 100Dynamic shear [G*(sin�)], (max. 5000 kPa), (kPa) 3490 4192 4331 3708 4372 4543Temperature (°C) 13 10 7 10 7 4Creep stiffness S, (max. 300 MPa) at 60 s 278 254 212 214 208 194M value, (min. 0.300) at 60 s 0.320 0.307 0.306 0.308 0.305 0.309Temperature (°C) –26.0 –27.0 –27.0 –28.0 –28.0 –29.0

Tcritical (°C) –38.0 –39.1 –39.2 –39.9 –39.2 –40.7Superpave grading PG52-34 PG58-34 PG64-34 PG64-34 PG64-34 PG70-34True Superpave grading PG53-36 PG60-37 PG65-37 PG65-38 PG69-38 PG73-39High–low temperature spread using bending-beam-

rheometer low-temperature parameter (°C)89 97 102 103 107 112

High–low temperature spread using critical crackingtemperature (°C)

91 99 104 105 108 113

Table 1. Superpave results of 200/300 asphalts modified with 0%–15% crumb rubber materials (CRM) with 3.0 h mixing time at 180 °C.

ally agitated to avoid the settling of CRM materials. How-ever, there is no danger of them gelling or increasing inviscosity to a point that they cannot be handled.

Modification of 150/200 pen grade asphalt with 0% to 15%of 40-mesh crumb rubber materials, mixing for 3 h

Superpave characterization resultsThe test results of the CRM-modified 200/300 pen grade

asphalt indicated that there was no difference between themodified asphalts mixed for 3 or 6 h. Thus, the 6 h mixingtime was eliminated from the experimental design in thestudy of the modification of 150/200 pen grade asphalts withCRM. Also, because the CRM-modified 200/300 pen gradeasphalt did not show a tendency to gel in the long-term hot-storage test, this test was also eliminated. The 150/200 pengrade asphalt was modified with 6%, 9%, 12%, and 15% 40-mesh CRM by low-shear mixing for 3 h at 180 °C.

The following points can be made regarding the character-ization results of the modified asphalts (Table 5):

• The high-temperature grade increased ~1.47 °C per each1% of CRM added.

• The low-temperature grade improved 0.2 °C per each 1%of CRM added or it improved 1 °C per each 5% of CRMadded.

• These high- and low-temperature improvements were slightlybetter than the grade improvements when CRM was usedto modify 200/300 pen grade asphalt base.

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1280 Can. J. Civ. Eng. Vol. 34, 2007

Sample No. 1441 2179 2172 2152 2151CRM content (%) in 200/300 asphalt 0.0 6.0 9.0 12.0 15.0180 °C mixing temp., mixing time (h) 0.0 6.0 6.0 6.0 6.0Standard tests

Penetration at 25 °C (dmm) (100 g/5 s) 260 178 145 118 94Softening point (°C) 37.2 42.7 46.2 49.9 58.8Flash point (°C) 261 283 287 283 283


Viscosity at 135 °C (MPa·s) 200 593 814 3257 8533Dynamic shear (G*/sin�), (min. 1.0 kPa), (kPa) 1.02 1.05 1.03 1.01 1.04Temperature (°C) 53 61 67 73 82

Rolling thin film oven test (RTFOT)RTFOT mass loss (%) –0.844 –0.630 –0.710 –0.810 –0.460Dynamic shear (G*/sin�), (min. 2.20 kPa), (kPa) 2.20 2.29 2.40 2.36 2.38Temperature (°C) 54 60 63 67 74

Pressure aging vessel (PAV) residuePAV aging temperature (°C) 90 100 100 100 100Dynamic shear [G*(sin�)], (max. 5000 kPa), (kPa) 3490 3727 4702 3486 2989Temperature (°C) 13 10 7 7 7Creep stiffness S, (max. 300 MPa) at 60 s 278 265 240 193 171M value, (min. 0.300) at 60 s 0.320 0.302 0.312 0.319 0.307Temperature (°C) –26.0 –28.0 –28.0 –27.0 –28.0

Tcritical (°C) –38.0 –39.2 –40.2 –39.5 –40.0Superpave grading PG52-34 PG58-34 PG58-34 PG64-34 PG70-34True Superpave grading PG53-36 PG60-38 PG63-38 PG67-37 PG74-38High–low temperature spread using bending-beam-

rheometer low-temperature parameter (°C)89 98 101 104 112


91 99 103 106 114


Fig. 2. Plots of 135 °C viscosity versus percent crumb rubbermaterials (CRM) in 200/300 asphalt.

• The DSR G*(sin�) values of the PAV residues decreasedwith increased CRM levels, indicating improvement infatigue cracking upon increased CRM modification.

• The flash points stayed relatively constant upon CRM modi-fication and there was no danger of decreasing the flashpoint with increasing CRM levels.

• The 135 °C viscosity increased very quickly when theCRM content exceeded 10.5%, as shown in Fig. 5. At~10.9% CRM, the modified asphalt would have a 135 °Cviscosity of just over 3000 MPa·s. For practical applica-tions, we could only use up to 10.5% CRM in 150/200pen grade asphalt. In the literature, when using 60/70 pengrade asphalt as the base asphalt and approximately 60-

mesh CRM (0.29 mm mean particle size) with a similar low-shear addition, it was found that 9% CRM was the maximumlevel suitable for paving asphalts, based on thermo-rheological and storage stability tests (Navarro et al. 2005).This is not far from our results, considering the differentbase asphalt and different-sized CRM used.

• The true PG grade for the 10.5% CRM-modified asphaltwas PG75-33, or a Superpave grade of PG70-28. It wasalmost a PG76-34. This is probably the best grade thatcould be prepared using our present method, at least withthe largest high–low temperature spread.

• The critical cracking temperatures were slightly unpre-dictable, probably because of the non-representative amounts

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MacLeod et al. 1281

Laboratory No.

2166 2179 2165 2172 2200 2200 2150 2152 2147 2151

CRM content (%) 6.0 6.0 9.0 9.0 10.5 10.5 12.0 12.0 15.0 15.0Mixing time (h) 3.0 6.0 3.0 6.0 3.0 3.0 3.0 6.0 3.0 6.0Mixing temperature (°C) 180 180 180 180 180 180 180 180 180 180Storage time (h) 24 24 24 24 24 24 24 24 24 24

Insoluble in trichloroethyleneTop (%) 0.3 0.4 6.7 5.9 8.8 7.7 10.2 10.5 12.6 13.2Bottom (%) 10.3 10.2 11.8 10.9 10.9 11.1 10.6 11.8 13.9 13.5

Table 3. Crumb rubber content at the top and bottom portion of the tube after 24 and 72 h separation test at 140 °C for crumb rubbermaterials (CRM)-modified 200/300 asphalt.

Cre

ep

co

mp

lian

ce

(1/P

a)

Time (s)

0 100 200 300 400 500 600 700 800 900

00

.10

.20

.30

.40

.50

.60

.7

Cre

ep

co

mp

lian

ce

(1/P

a)

Time (s)

00

.10

.20

.30

.40

.50

.60

.7

Total

strain

Final

strain

Time

(b)

(a)

Fig. 3. (a) Repeated creep test results of the original binder of 0%–10.5% crumb rubber materials (CRM)-modified 200/300 asphalts;(b) illustration of percent recovery calculation [% recovery = (total strain – final strain) / (total strain)(100)].

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1282 Can. J. Civ. Eng. Vol. 34, 2007

Fig. 4. Repeated creep test results of the rolling thin film oven test residue of 0%–10.5% crumb rubber materials (CRM)-modified200/300 asphalts.

Days in hot storage

1 2 4 5 6 7 8 9 12 13 14 15

Storage temperature (°C) 140 145 150 152 152 152 152 152 152 152 152 152Average 60 °C viscosity (Pa·s) 561 461 379 454 359 387 363 252 330 290 617 313Average 135 °C viscosity (MPa·s) 1771 1720 1512 1550 1338 1254 1242 557 1001 899 1129 919

Table 4. Hot storage of 10.5% crumb rubber materials (CRM)-modified 200/300 asphalt, viscosities at 135 °C and 60 °C versus hot-storage time.

Sample No. 1505 2326 2329 2376 2348 2330 2331CRM content (%) in 200/300 asphalt 0.0 6.0 9.0 10.0 10.5 12.0 15.0180 °C mixing temp., mixing time (h) 0.0 3.0 3.0 3.0 3.0 3.0 3.0Standard tests

Penetration at 25 °C, (dmm) (100 g/5 s) 165 109 95 84 83 79 62Softening point (°C) 41.7 47.1 49.3 51.8 51.0 55.9 61.3Flash point (°C) 269 280 288 273 281 283 277


Viscosity at 135 °C (MPa·s) 270 798 1419 2049 2082 5293 16 417Dynamic shear (G*/sin�), (min. 1.0 kPa), (kPa) 1.06 1.03 1.02 1.06 1.01 1.05 1.23Temperature (°C) 58 67 73 77 78 79 88

Rolling thin film oven test (RTFOT)RTFOT mass loss, (%) –0.740 –0.760 –0.610 –0.570 –0.570 –0.530 –0.440Dynamic shear (G*/sin�), (min. 2.20 kPa), (kPa) 2.67 2.24 2.25 2.34 2.25 2.24 2.28Temperature (°C) 58 67 69 73 75 74 83

Pressure aging vessel (PAV) residuePAV aging temperature (°C) 100 100 100 100 100 100 100Dynamic shear [G*(sin�)], (max. 5000 kPa), (kPa) 4459 3360 4119 4512 4386 3021 3 634Temperature (°C) 16 16 13 13 13 13 7Creep stiffness S, (max. 300 MPa) at 60 s 274 196 196 168 187 212 167M value, (min. 0.300) at 60 s 0.308 0.308 0.308 0.306 0.314 0.300 0.303Temperature (°C) –22.0 –22.0 –23.0 –22.0 –23.0 –25.0 –25.0

Tcritical (°C) –34.1 –33.3 –35.4 –32.2 –33.0 –36.0 –36.8Superpave grading PG58-28 PG64-28 PG64-28 PG70-28 PG70-28 PG70-34 PG82-34True Superpave grading PG58-32 PG67-32 PG69-33 PG73-32 PG75-33 PG74-35 PG83-35High–low temperature spread using bending-beam-

rheometer low-temperature parameter (°C)90 99 102 105 108 109 118


92 100 104 105 108 110 119


of CRM materials in the PAV residues. Sometimes, afterRTFOT, especially for a thicker asphalt base, the CRMwould tend to stick more to the RTFOT bottle, resulting ina lower amount of CRM in the residue. Even thoughefforts were made to scrape the CRM out of the bottles,there may have been a difference in the actual amount ofCRM remaining in the PAV pan.

Separation test at 140 and 180 °C for 72 hFor the 150/200 pen grade asphalt, the length of the sepa-

ration test was increased to 72 h for all the samples and wasconducted at two temperatures, 140 and 180 °C. This wasdifferent from the separation tests on the CRM-modified200/300 pen grade asphalt, where most of the testing wascompleted after 24 h storage at 140 °C. The longer the timeand the higher the temperature, the more conservative is thetest. Table 6 shows the separation test results. A higher tem-perature and longer storage time were used because, with the150/200 pen grade asphalt being more viscous than the 200/300pen grade asphalt, the separation of CRM in 150/200 pengrade asphalt could be less severe than that in 200/300 pengrade asphalt. The following points could be made regardingthe results shown in Table 6:

• After 72 h at 140 °C hot storage, the 12% and 15% CRM-modified 150/200 pen grade asphalt (No. 2330 and No. 2331in Table 6) showed almost no separation.

• After 72 h at 140 °C hot storage, the 10.5% CRM-modified150/200 pen grade asphalt (No. 2348 in Table 6) hadnearly the same separation as the 10.5% CRM-modified200/300 pen grade asphalt (No. 2200 in Table 3).

• After 72 h at 180 °C hot storage, all the asphalt modifiedby 6%–12% CRM had some separation, with the crumbrubber settling to the bottom, whereas the 15% CRM-modified asphalt had the crumb rubber rising to the top.

• The results in Table 6 demonstrate that the separation ofthe crumb rubber from the asphalt needs to be addressed.Lowering storage temperatures (i.e., to 140 °C), decreas-ing storage time (using the modified asphalt right after

preparation) or agitating between production and consumptionmay help to alleviate the problem.

Repeated creep test results of base asphalt and asphaltmodified with crumb rubber materials (original binder androlling thin film oven test residue)

The repeated creep test results of the original binder andthe RTFOT residues of the 0%, 6%, 9%, and 10% CRM-modified 150/200 pen grade asphalts are shown in Figs. 6and 7, respectively. The test temperatures were the same asthe Superpave high-temperature grade temperature. Again,similar to what we saw previously with the CRM-modified200/300 pen grade asphalt, there was decreased creep com-pliance or higher recovery with increased CRM content,demonstrating higher elasticity in asphalts modified withhigher CRM levels. Basically, with either the 150/200 or200/300 pen grade as the base asphalt, we witnessed that thehigh-temperature PG grade was dramatically increased withincreased CRM content, whereas the low-temperature gradewas slightly improved. In addition to this improvement (rec-ognized by the present Superpave specification) was theincreased elasticity captured by the repeated creep.

Comparing the 9% and 10% crumb rubber concentrationsat 900 s, there is increasing creep compliance as we go tosofter bases. At 9%, the creep compliance on the originalbinder extended from less than 0.2 for the 150/200 pen gradeasphalt to 0.3 for the 200/300 pen grade asphalt. At 10%crumb rubber, the creep compliance extended from less than0.1 for the 150/200 pen grade asphalt to 0.2 for the 200/300pen grade asphalt.

Modification of 300/400 pen grade asphalt with 0% to 14%40-mesh crumb rubber materials, mixing for 3 h

From the previous results using 150/200 and 200/300 pengrade asphalts as the base, the highest CRM content was ap-proximately 10.5% because of the relatively high 135 °Cviscosity, which exceeded the 3000 MPa·s limit required bythe Superpave specification. Thus, a softer base, 300/400pen grade asphalt was used to prepare modified asphaltswith 8%, 9%, 10%, 11%, 12%, and 14% CRM. The samelow-shearing mixing method for 3 h at 180 °C was used.

Superpave characterization resultsTable 7 summarizes the Superpave characterization results.

The following points can be made regarding Table 7:

• The high-temperature grade improved ~1.5 °C per each1% of CRM added. This was slightly better than the 1.2 °Cimprovement per percent of CRM added to the 200/300pen grade asphalt and the 1.47 °C improvement whenCRM was added to the 150/200 pen grade asphalt.

• The low-temperature grade improved slightly at ~0.2 °Cper each 1% of CRM added, which means it improved1 °C for each 5% of CRM added.

• Once again, the DSR G*(sin �) values of the PAV residuesseemed to decrease with increased CRM levels, indicatingimprovement in fatigue cracking upon higher CRM modi-fication.

• The flash points initially increased with increasing CRMlevels and then stayed relatively constant at approximately270–280 °C.

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MacLeod et al. 1283


• The 135 °C viscosity increased to the 3000 MPa·s limit at11.5% CRM content, similar to the CRM content for the150/200 and 200/300 pen grade modified asphalts (Fig. 8).For comparison, the 150/200 asphalt could take up to10.9% CRM, whereas it took the 200/300 asphalt 11.7%CRM to reach the 3000 MPa·s viscosity limit. Thus, thesofter asphalt did not seem to allow us to use more CRM.

• At 11% CRM, the modified 300/400 pen grade asphalthad a true PG grade of PG66-41 or a Superpave grade ofPG64-40. It should be a suitable material for paving incolder climatic locations.

• The critical cracking temperatures were within ±1 °C ofthe BBR low-temperature grade. In the case of modifiedasphalts with 150/200 and 200/300 pen grade asphalts

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1284 Can. J. Civ. Eng. Vol. 34, 2007

Laboratory No.

2326 2329 2376 2348 2330 2331

CRM content (%) 6.0 9.0 10.0 10.5 12.0 15.0Mixing time (h) 3.0 3.0 3.0 3.0 3.0 3.0Mixing temperature (°C) 180 180 180 180 180 180Insoluble in trichloroethylene before separation test (%) 5.9 7.8 — 10.3 10.4 13.7Storage time (h) 72 72 72 72 72 72After 180 °C hot storage

Top (%) 0.1 0.4 4.0 4.0 4.7 19.9Bottom (%) 7.1 10.7 12.8 12.3 11.5 6.8

After 140 °C hot storageTop (%) 0.1 4.1 7.6 7.8 10.8 14.5Bottom (%) 9.6 12.9 12.6 12.6 11.3 14.7

Table 6. Crumb rubber content at the top and bottom portion of the tube after 72 h separation test at140 or 180 °C for crumb rubber materials (CRM)-modified 150/200 asphalt.

Fig. 6. Repeated creep test results of the original binder of 0%–10% crumb rubber materials (CRM)-modified 150/200 asphalts.

Fig. 7. Repeated creep test results of the rolling thin film oven test residue of 0%–10% crumb rubber materials (CRM)-modified150/200 asphalts.

(Table 5 and Table 2, respectively), they all had better orat least equal PG high–low temperature spreads when usingthe critical cracking temperature criteria. With 300/400pen grade asphalts, the PG high–low temperature spreadwas the same within ±1 °C using either the critical crack-ing temperature criteria or the BBR creep stiffness andcreep rate criteria. This was probably due to the softnessof the 300/400 base asphalt. The handling of the directtension test (DTT) specimen with soft asphalt could be

quite difficult: any twisting of the sample could lead tolower DTT results.

Separation test at 140 °C for 72 hTable 8 summarizes the separation test results of 8%, 9%,

10%, and 11% CRM-modified asphalts with the 300/400 pengrade asphalt base. All of the modified asphalts had separa-tion after 72 h at 140 °C. Even the 11% CRM-modifiedasphalt had 37% more CRM in the bottom portion than thetop portion. The softer base asphalt, being less viscous, prob-ably allowed the CRM to settle to the bottom more easily.

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MacLeod et al. 1285

Sample No. 2445 2521 2551 2524 2571 2555 2525 2531CRM content (%) in 200/300 asphalt 0.0 8.0 9.0 10.0 11.0 11.0 12.0 14.0180 °C mixing temperature, mixing time (h) 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0Standard tests

Penetration at 25 °C, (dmm) (100 g/5 s) 344 174 151 141 144 140 123 109Softening point (°C) 33.0 41.6 45.5 45.7 46.2 47.2 49.3 52.2Flash point (°C) 246 243 277 265 281 279 277 271


Viscosity at 135 °C (MPa·s) 183 1471 1192 1990 1531 1348 5260 6267Dynamic shear (G*/sin�), (min. 1.0 kPa) 1.04 1.11 1.01 1.07 1.06 1.01 1.02 1.10Temperature (°C) 50 64 66 68 72 73 72 78

Rolling thin film oven test (RTFOT)RTFOT mass loss (%) –0.940 –1.120 –0.790 –0.910 –0.820 –0.790 –0.810 –0.610Dynamic shear (G*/sin�), (min. 2.20 kPa) 2.51 2.29 2.31 2.25 2.27 2.32 2.32 2.39Temperature (°C) 50 61 64 67 67 66 69 71

Pressure aging vessel (PAV) residuePAV aging temperature (°C) 90 100 100 100 100 100 100 100Dynamic shear [G*(sin�)], (max. 5000 kPa) 3337 4204 2966 4290 2926 4501 4284 2432Temperature (°C) 10 10 7 7 7 7 7 7Creep stiffness S, (max. 300 MPa) at 60 s 284 260 251 252 228 267 199 223M value, (min. 0.300) at 60 s 0.327 0.300 0.301 0.308 0.301 0.300 0.302 0.301Temperature (°C) –28.0 –29.0 –29.0 –30.0 –30.0 –31.0 –30.0 –31.0

Tcritical (°C) –39.4 –39.8 –38.9 –39.2 –40.0 –41.3 –41.1Superpave grading 46–34 58–34 64–34 64–40 64–40 64–40 64–40 70–40True Superpave grading 50–38 61–39 64–39 67–40 67–40 66–41 69–40 71–41High–low temperature spread using bending-beam-

rheometer low-temperature parameter (°C)88 100 103 107 107 109 112


89 100 103 106 106 110 112



Laboratory No.

2521 2551 2524 2571

CRM content (%) 8.0 9.0 10.0 11.0Mixing time (h) 3.0 3.0 3.0 3.0Mixing temperature (°C) 180 180 180 180Storage time (h) 72 72 72 72

Insoluble in trichloroethyleneTop (%) 2.3 5.6 7.6 9.0Bottom (%) 10.5 10.6 12.3 12.3

Table 8. Crumb rubber content at the top and bottom portion ofthe tube after 72 h separation test at 140 °C for crumb rubbermaterials (CRM)-modified 300/400 asphalt.

Summary and conclusions

Findings, discussions, and conclusions found in this studyare summarized below:

(1) When low-shear mixing CRM into 200/300 pen gradeasphalt at 180 °C, the mixing time of 3 or 6 h made nodifference to their performance, according to the Super-pave and separation characteristics of the modified asphalts.A 3 h mixing time should be sufficient.

(2) Long-term hot-storage stability had to be addressed. Ourstudy showed that a 10% CRM-modified asphalt can bestored at 150 °C for at least 15 days, without posing aproblem of increase viscosity or gelling.

(3) The Superpave performance grades of the asphalts wereimproved with increased CRM content. The improve-ment at the high-temperature grades was approximately1.2–1.5 °C per each 1% of CRM added, whereas thelow-temperature grades were improved by approximately0.2 °C per each 1% of CRM added.

(4) With our experimental conditions and mixing process,the amount of CRM allowable in the asphalt was limitedby the 135 °C viscosity. Regardless of whether a 150/200,200/300, or 300/400 pen grade asphalt was used as thebase, the practical amount of CRM that could be addedwas approximately 10% if the 135 °C viscosity was to bemaintained at 2000 MPa·s, which is comfortably belowthe 3000 MPa·s limit.

(5) With 10% CRM, we should be able to make the follow-ing grades of asphalts: PG73-32 using 150/200 pen gradeasphalt as the base (PG58-32), PG65-37 using 200/300pen grade asphalt as the base (PG52-36), and PG67-40using 300/400 pen grade asphalt as the base (PG50-38)

(6) All the blends failed the separation test at the elevatedtemperature of 180 °C. However, when using 150/200pen grade asphalt as the base, we did not observe anyseparation at 12% CRM at 140 °C, even after 72 h. Butwe did observe separation for 10% CRM content, whetherin 200/300 or 150/200 pen grade asphalts as the base(140 °C and 24 or 72 h for the CRM modified with200/300 pen grade asphalt base; 72 h for the CRM mod-ified with 150/200 pen grade asphalt base). There wasseparation for 8%–11% CRM-modified 300/400 pen gradeasphalt. This demonstrates that separation of the CRMfrom the asphalt needs to be addressed. Loweringstorage temperatures and decreasing storage times,using the modified asphalts right after preparation oragitating between production and consumption, willdecrease the tendency of the CRM to separate.

(7) In general, for all the CRM-modified asphalts in thisstudy, we witnessed improvement in the creep compli-ance with increased CRM levels. Comparing asphaltswith the same CRM levels, harder asphalts had better(lower) creep compliance than softer asphalts.

Application

In our application of CRM in asphalt pavements, we usedthe wet process in which the CRM was low-shear mixed intothe asphalt first. In the literature, it was reported that the wetprocess showed some promise while the dry process, in whichCRM was added to the aggregate, was not as desirable becauseof poor short-term performance (Emery 1995). There havebeen three test sections laid in the City of Lethbridge, oneevery year starting in 2003. The asphalt was modified at aHusky subsidiary, Pounder Emulsions. By mixing and storing

© 2007 NRC Canada

1286 Can. J. Civ. Eng. Vol. 34, 2007

Stafford Dr. North

3rd Avenue Southfrom Mayor Dr. W.to 19th Street

24th Avenue fromHwy. No. 4 to Hwy.No. 5 eastbound lanes

Paving season 2003 2004 2005Design traffic condition Slow (20–70 km/h) Slow (20–70 km/h) Standing <20 km/hDesign ESAL 0.3 to <3 million 0.3 to <3 million 3 to 10 millionTonnes of CRM 47 60 110Base asphalt 200/300 200/300 150/200CRM material 10% 20 mesh 8% 20 mesh 10% 20 meshMilling time at 190 °C 10 min 4 h 4 hCirculation time 4 h 2 h 2 hDesign PG gradea PG64-34 PG64-34 PG70-31DesignTrue PG grade PG65-38 PG65-37 PG72-32Quality control PG grade PG64-34 PG58-34 PG64-31Quality control true PG gradeb PG67-37 PG62-37 PG68-33Site inspection One reflective crack in a

two-block sectionPerforming well Performing well

Note: Stafford Dr. North and 3rd Avenue South have a 98% high temperature reliability with one grade bump and 24thAvenue has a 98% high temperature reliability with two grade bumps. ESAL, equivalent single-axle load [the ESAL unit ex-presses the amount of pavement stress caused by an 18 000 lb (1 lb = 0.4535 kg) axle]; PG, performance graded.

aThe design PG grade for the high-temperature grade utilized the Long-Term Pavement Performance program, whereas theTransporation Association of Canada model was used for the low-temperature grade.

bThe low temperature reliabilities for the quality control true PG grades are over 99% for PG 67-37 and PG 62-37 and97% for PG-68-33.

Table 9. Crumb rubber materials (CRM)-modified asphalts field trials in Lethbridge, Alberta, Canada.

with agitation close to the project sites, separation was notan issue when the modified asphalt was transported in atrailer. The proper PG grades were selected and then com-pared with the quality control PG grade when the field testmaterial was placed in the test sections. The CRM-modifiedasphalt was supplied blind to the contractor for paving, justlike any other asphalt binder.

The Long-Term Pavement Performance (LTTP) programand the Transportation Association of Canada model wereused to select the proper PG grade, suiting the Lethbridgeclimate and traffic condition, for the CRM-modified testsections in the City of Lethbridge (Table 9). As explained inthe footnote of Table 9, the design PG grade for the high-temperature grade utilized the LTPP program while the TACmodel was used for the low-temperature grade. The 2003and 2004 test sections had one grade bump with slow traffic,whereas the 2005 test section had two grade bumps withstanding traffic. To date, the City of Lethbridge has beenpleased with the performance of the test sections.

Over the years, there have been studies regarding the costeffectiveness of CRM asphalt pavement compared with con-ventional asphalt pavement. It was estimated that CRMasphalt mix costs 50%–100% more than an asphalt only mix(Fager 1996; Maupin 1996; Trimbath 2006). However, takinginto account the longer life cycle and possibility of pave-ment thickness reduction with CRM asphalt mix, most liter-ature found CRM asphalt pavement to be cost effective (Singhand Athay 1983; Morris 1993; Gawel and Slusarski 1999).Our present field trial was at a pilot scale and less than5 years of pavement performance data were collected; itwould not be realistic to estimate cost effectiveness at thispoint in time.

Acknowledgment

The authors wish to express their gratitude to the TireRecycling Management Association of Alberta for financialsupport of this project.

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