A mechanistic evaluation of modified asphalt paving mixtures

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  • A mechanistic evaluation of modified asphalt paving mixtures

    N. ALI Depnrrrrienr of Civil Engineering, Teclznical Ur2iver:sity of No~~cl Scotin, PO. Box 1000, H ~ l i f i ~ ~ , NS B3J 2x4, Canncl~l

    SHAHER ZAHRAN Department of Civil Engineering, King Abdulazi: University, BOI 9027, Jeddnlz, Saudi A r ~ b r a

    JIM TROGDON Pavernerzt Manngernenr Unit, Norrlz Cnrolinn Deparrnient of Tr~lnsportcrtiorz, Raleigh, NC 27611, U.S.A.

    A N D

    ART BERGAN Dep(lrtr?~enr ($Civil Engineering, University of Saskatchewan, Snskatoori, SK S7N OWO, Canada

    Received May 27, 1993 Revised ~nanuscript accepted March 9, 1994

    The main purpose of this study was to facilitate decisions concerning the effectiveness of modifiers in mitigating pavement distress and improving long-term overall pavement performance in actual field conditions, by utilizing short- term laboratory results and a mathematical prediction model. The modifiers investigated were carbon black, neoprenc latex, and polymer modified asphalt (STYRELF). The statistical general linear model (GLM) and the Fisher least significant difference (LSD) were used for the analysis of data. The results of the study indicate that the effect of the modifier on the paving mixture properties was insignificant at low temperatures (down to - 17C). but significant at high temperatures (up to 60C) where the synergistic effect of the modifier on the paving mixture was pronounced. The VESYS IIIA pavement performance prediction model was utilized to assess the effects, if any, of the modifier on the pavement's overall performance. All the modifiers improve, to some degree, the overall pavement performance.

    Key words: modifiers, asphalt, paving mixtures, pavements, polymer asphalt.

    Le but principal de cette Ctude est de faciliter la prise de dCcision en ce qui concerne I'efficacitC des modificateurs ii limiter la dCtCrioration des chaussCes et ii amCliorer la performance globale ii long terme des cha~lssCes dans les con- ditions actuelles, en recourant aux rCsultats d'Ctudes en laboratoire ainsi qu'ii un modkle predictif. Les lnodificateurs suivants ont fait I'objet d'une analyse : noir de carbone, latex nCopr2ne et bitume modifiC par polymkres. Le modkle gCnCral de IinCaritC (MGL) ainsi que la plus petite diffirence significative de Fisher (PPDS) ont CtC utilisCs pour I'analyse des donnCes. Les rCsultats de I'Ctude indiquent que I'effet du modificateur sur les propriCtCs du revctement Ctait nCgligeable j. basses tempkratures (jusqu'ii - 17C) mais important ii tempkratures ClevCes Cjusqu'ii 60C), I'effet synergique du modificateur sur le mClange Ctant plus marque. Le modkle predictif de la performance des chaussCes VESYS IIIA a CtC utilisC pour Cvaluer les effets, le cas CchCant, du modificateur sur la performance globale de la chaussCe. Dans une certaine mesure, tous les modificateurs ont amCliorC la performance globale de celle-ci.

    Mots c1P.s : modificateurs, bitume, revctement, chaussies, bitume modifiC par polyln2res. [Traduit par la rCdaction]

    C;II~. J. Civ. Eng. 21, 954-965 (1994)

    Introduction Today there a re many asphal t cement modif iers o n the

    market. Several of these modifiers are said to significantly affect both the properties of the paving mixtures and the overal l pavement performance by decreasing permanent deformation and increasing resistance t o low temperature cracking. T h e primary role of modifiers is to make paving mixtures less susceptible to temperature changes so that the stiffness of the mixture does not vary significantly as tem- peratures fluctuate.

    evaluate the effect of modified asphalt paving mixtures o n overall pavement performance as compared to the effect of using conventional paving mixture alone. This research con- sists mainly of two parts. T h e primary objective of the first part was to evaluate the interactive effect of modifiers and original asphalt grade on the mechanical properties of paving m i x t ~ ~ r e s over a wide range of temperatures . T h e second part of the study involves an evaluation of the influence of t h e m o d i f i e r s o n p a v e m e n t p e r f o r m a n c e , b a s e d o n t h e mechanical characterization determined in the first part.

    Modified binders are making inroads in routine paving Modifiers overview operations in North America. Consequently, it is essential that they are tested in both the laboratory and field before the T h e fo l lowing offers a n overv iew of carbon black and

    additional cost of their implementation is undertaken. polymer modifiers. Carbon black is considered because it This research was predicated upon the knowledge that is a pyrolytic by-product of discarded rubber tires. If carbon

    deficiencies in the properties of asphalt binding material black is a suitable modifier, then we will have a method have a negative econolnic impact; however, a number of for recycling scrap tires. The polymers were selected to rep- modifiers are available which have the potential to mitigate resent S B R and S B S ~ o l ~ l n e r groups. - these deficiencies. The main objective of the research is to c a r b o n black

    NOTE: written discussion of this paper is welcomed and will be T h e idea of using carbon black as a reinforcing agent for received by the Editor until April 30, 1995 (address inside front asphalt was initiated by Martin (1962). Martin reported that cover). the dispersion of 3% carbon black by the weight of asphalt Pr~nlutl In C.hn.td.( I Iolpriln2 ;lo C;kn:wJ~

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  • ALI ET A L

    appears to have no significant effect on the susceptibility of asphalt binder to temperature. One of the major subsequent research works in the use of carbon black was conducted by Rostler et al. (1977). They coinmented that the poor per- formance of carbon black in Martin's test results was caused by poor dispersion of carbon black into the asphalt, low carbon black concentration, and the addition of oil fluxing to obtain workable viscosity. They also performed photo- micrographs of carbon black dispersion in asphalt and reported that the carbon black particles should have a mean particle diameter of less than 70 X microns in order to be integrated into the internal phase of the asphalt cement. Tei~el and Rimsritong (1980) reported on their research at the University of Washington concerning the properties of lignin and the use of carbon black as a reinforcing agent. The car- bon black used in that research was MICROFIL 8. In general, they found that the addition of small amounts of carbon black to mixt~ires inade of lignin and asphalt significantly improved the mixture's stability, increased the resilient mod- ulus of the mixture, and improved its tensile strength. Their work indicated that treating asphalt cement with carbon black could improve the binder's temperature susceptibility and increase the moisture resistance of the mixes. Yao and Monismith (1986) found that carbon-black-modified paving mixtures possess much greater stiffness, especially at high temperatures, than inixtures with conventional asphalt. Their tests on the effects of carbon black on the fatigue life of the paving mixture were, however, not conclusive, since no test was conducted on conventional paving mixtures made from the same aggregate source.

    Recently, various types of polymers have been used as additives to alter the asphalt binder and, in turn, the properties of the mixtures. The idea of using polymers in asphalt to improve its properties is not new. The first patent for a polymer-modified bitumen was granted in 1823 and the sec- ond in 1844 (Zanzotto et al. (1987) offer a literature review).

    The amount and degree of polymer dispersion into asphalt binder depends mainly on the desired effect the polymer is to achieve in reinforcing the binder. Polymers can be either fully or partially integrated into asphalt binders where they form a network between the binder and the aggregate.

    Polymers are made up of repeating units formed from monomers, which are crosslinked together through a chemical process known as polymerization. Although many of these polymer systems are inappropriate for asphalt modification, there are numerous systems - SBR, SBS (Collins and Mikols 1985), SIS, EPDM, neoprene, other styrene-based materials, block copolymer, and various plastics such as polyethylene and polypropylene - which are compatible with asphalt. Polymers are produced as a fine dispersion or emulsion in water (latex), as a fine dispersion or solution in an organic solvent (resin), or as dry powder or crumb.

    Gregg and Alcoke (1954) reported that the addition of polymers reduced temperature susceptibility as determined by viscosity and penetration. Thompson (1964) reported that the addition of neoprene to asphalt cement increased toughness and improved aging characteristics by decreas- ing the softening point and penetration. Beagle (1967) added latex to sand asphalt inixture and used it in field experiments as an overlay. He reported that a mixture with latex had the best resistance to reflection cracking. Styrene-butadiene- sytrene (SBS) copolymers have also been added to asphalt

    FIG. I . Factorial design. Note: ( I ) there are three replications in each cell; (2) two sizes of specimens were used.

    used for seal coats (Marvillet and Verschave 1979). It was reported that the addition of SBS improved the binder's sus- ceptibility to temperature, cohesion, adhesion to aggregate, and flexibility. Terrel and Walter (1986) reported that there are improvements in the physical properties (stiffness, cohe- sion, and adhesion of binders) of asphalt concrete mixes modified with polymers. Other repoi-ted improvements include the following: improved stiffness, rutting and stripping resis- tance, and adhesion (King et al. 1986); improved ductility and fracture toughness of the bitumen at low temperatures and rutting and distortion resistance at high temperatures (Jew and Woodhams 1986); and improved fatigue life and tensile stresses (Little et al. 1986). Further, Zanzotto et al. (1987) developed polymer-modified asphalt with enhanced engineer- ing properties. They reported that the new binder's consis- tency and temperature and consistency and loading time susceptibility, as well as its cohesion and antistripping prop- erties, are significantly improved. Finally, Anderson et al. (1989) reported that polymer-modified mixes, when compared to conventional mixes, exhibited higher stress and strain failure at low temperatures (as low as -30C), and lower permanent strain at higher temperatures (as high as 45C).

    It can be concluded that all researchers, to one degree or another, agree that the addition of polymer to the binder enhances its elastic response at high temperatures by increas- ing the toughness and adhesion of the mixture, and by decreasing the susceptibility of the binder to temperature fluctuations and changes.

    Part I. Mechanical properties The behaviour of paving mixtures is complex and depends

    upon the relationships among many variables. Since we only examined the effect of binders on paving mixtures and not the interaction effect of these variables, an effort was inade to keep these variables constant for the purpose of this s t ~ ~ d y . Those variables known to have a significant effect on the mechanical properties of paving mixtures - aggregate source, gradation, specimen compaction, air void, and curing time - were held constant throughout the research.

    Two sets of experiments were designed to st~tdy the effect of modifiers on the mechanical properties of paving mixtures. The first experiment measured the relative effect of modifiers on the elastic and viscoelastic properties of paving mixtures; the second experiment determined the effect of modifiers on the fatigue life of the paving mixture.

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  • CAN. J . CIV. ENG. VOL. 11, 1994

    TABLE 2a. Gradation of aggregates

    N O * " E D 2

    Sieve size Passing (%)

    19 mm (74 in.) 12.5 mm ('12 in.) 9.5 mm (31s in.) 4.76 mm (#4) 2.38 mm (#8) 1.19 mm (#16) 420 pm (#40) 177 pm (#30) 74 pm (#200)

    TABLE 2b. Physical properties of aggregates

    Specific gravity

    Size fraction Bulk Apparent Absorption (%)

    Course aggregate 2.549 2.616 1.4 1 (ASTM C 127-84)

    File aggregate 2.579 2.660 1.21 (ASTM C 128-84)

    Filler (ASTM D854-83) - 2.65 1 -

    A full factorial experiment was designed and analysis of variance (ANOVA) was used to evaluate the effect of each parameter (Fig. 1 ).

    The independent variables considered for the first exper- iment are the following:

    1. Asphalt grade: The three most commonly used levels of asphalt grade, AC-5, AC- 10, and AC-20, have been employed in this experiment.

    2. Modifier: Four levels were used. In the first level, no modifiers were used; the subsequent three levels represented the use of carbon black, latex, and polymer, respectively. The percent of binder was kept constant based on the man- ufacturer's instructions.

    3. Temperature: Temperature is the most important variable in determining both the elastic and viscoelastic properties of the paving mixture. Asphalt paving mixtures are known to be very susceptible to temperature changes because of their viscoelastic behaviour. Therefore, in evaluating the inodulus of resilicence and the viscoelastic behaviour of the paving mixture, compacted specimens were tested at five temperatures: - 17"C, -7"C, 5"C, 2 1 "C, 40C, and 60C. The temperatures chosen are random; our only concern was to have intervals wide enough to generate an adequate curve.

    The response variables measured and analyzed were the following:

    I . Modulus of resilience, M,, defined as the ratio of applied stress to the resilient (recoverable) strain, is the dynamic elastic modulus of viscoelastic material; it is used here to calculate the stress, strain, and deflection response of the pavement.

    2. Perinanent strain parameter is used to measure the accu- mulation of permanent strain related to pavement rutting.

    The second experiment studied the effects of the three modifiers on the fatigue life of the test specimens. The test was run at a constant temperature of 21C which is standard test protocol. The only independent variables were the asphalt grade and the modifier type. The response variables evaluated were the fatigue parameters.

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  • ALI ET A L

    STRESS

    + 1000 second _, creep test

    TlME t

    STRAIN

    TlME t FIG. 2. Stress and strain of incremental static te$t series. d,, duration of the ith load pulse; dy, rest period after the ith load pulse;

    E,,,, increment in perm...

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