A mechanistic evaluation of modified asphalt paving mixtures

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 - 17°C). but significant at high temperatures (up to 60°C) 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 conditions 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 - 17°C) mais important ii tempkratures ClevCes Cjusqu'ii 60°C), 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 temperatures 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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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 performance 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 carbon 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 modulus 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 second 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 decreasing 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 susceptibility to temperature, cohesion, adhesion to aggregate, and flexibility. Terrel and Walter (1986) reported that there are improvements in the physical properties (stiffness, cohesion, and adhesion of binders) of asphalt concrete mixes modified with polymers. Other repoi-ted improvements include the following: improved stiffness, rutting and stripping resistance, 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 engineering properties. They reported that the new binder's consistency and temperature and consistency and loading time susceptibility, as well as its cohesion and antistripping properties, 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 -30°C), and lower permanent strain at higher temperatures (as high as 45°C).

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 increasing 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 experiment 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, 40°C, and 60°C. 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 21°C 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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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 permanent strain due to the ith load pulse; E,,,, total accun~ulated permanent strain due to 11 pulses; and E,, strain amplitude measured at 0.03 s due to the ith load pulse.

Materials and sample preparation The MICROFIL 8, STYRELF, and latex were blended

with each of the base asphalts, AC-5, AC-10, and AC-20, which were supplied by American Oil Company (AMOCO). Table 1 shows the physical properties of all binders, including the conventional asphalts. The aggregate used, including the filler, was 100% c r ~ ~ s h e d granite. Table 2a shows the gradation and Table 2b shows the physical properties of the aggregates.

The mixtures were designed in accordance with the Marshall method of mix design (The Asphalt Institute 1984). A summary of the mix properties is given in Table 3. Due to the wide range of specific gravities associated with the chosen modifiers, it was necessary to vary the proportion of binder in order to maintain an equal volume of the binder and the air void content in the paving mixtures. Another reason for using an equal volume of the binder was that a high volume of binder content will result in a low air void in modified paving mixtures designed in accordance with the Marshall design method. This occurs primarily because the modified binder has a higher viscosity than does conventional asphalt binder. Table 4 presents the specific gravities and the binder contents of all the modified mixtures.

Specimens for resilient modulus and fatigue tests were 100 mm in diameter and 62.5 mm high and were fabricated using the mechanical Marshall compactor. Specimens for permanent deformation tests were 100 mm in diameter and 200 mm high and were prepared using a modified Rainhart mechanical compactor. This procedure was developed (Omer 1984) to provide uniform density within specimens and pro- duce specimens with equal density.

Test procedures Resilient rlzoclirl~~s testitlg

The resilient modulus tests were conducted on diametral specimens in indirect tension mode at test temperatures of - 17"C, 4"C, 21°C, 39"C, and 60°C. The diametral resilient modulus test developed by Schmidt (1972) and described in the ASTM Method D 4123-82 was used in this research. The test was conducted by applying a diametral vertical pulse load of short duration (0.05-0.1 s ) on a Marshall

TABLE 3. Summary of mixture properties

Binder

Property AC-5 AC-I0 AC-20

Percent asphalt by wt. aggregate 5.87 Percent asphalt by wt. mixture 5.55 Mix bulk specific gravity 2.30 Absorbed asphalt (percent) 1.10 Aggregate bulk specific gravity 2.57 Asphalt specific gravity 1.020 Voids in mineral aggregate (percent) 15.78 Air void (percent) 3.46 Marshall stability (N) 9740 Flow (0.25 mm) 1 1

sized specimen and recording the corresponding horizontal deformation. The resilient modulus, M,, was calculated as follows:

where P is the load in lbs ( I Ib = 0.454 kg); \!, the Poisson's ratio, has a value of 0.35; t is the thickness of the specimen in inches ( 1 in. = 25.4 mm); and A is the total horizontal resilient deformation in inches.

Pernzanent deforllzatiorz testing Creep testing was conducted under a static load of 138 kPa

for prescribed time increments (Fig. 2). Five temperature levels, -17"C, 4"C, 21°C, 39"C, and 60°C, were tested. Specimens made with AC-5 and AC-I0 were too soft to be tested at 60°C and thus no permanent deformation data were generated for these mixture specimens at this temperature.

Fatig~ie testing The fatigue response of each mixture was measured on

diametral specimens in the indirect tension test. All fatigue tests wereconducted at a test teinmrature of 21°C. The controlled stress mode of loading, which included 0.1 s loading and 2.9 s unloading, was used. Stresses in the range of 100 to 350 kPa were applied.

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958 CAN. J . CIV. ENG. VOL. 21, 1994

TABLE 4 . Binder specific gravity and binder content of modified mixtures

Binder

AC-5 + AC-5 + AC-5 + AC-10 + AC-10 + AC-10 + AC-20 + AC-20 + AC-20 + Property carbon latex polymer carbon latex polymer carbon latex polymer

Specific gravity 1.107 1.025 1.029 1.095 1.028 1.036 1.098 1.039 1.044 C/o asphalt by wt. 6.389 5.828 5.929 6.306 5.909 5.995 6.36 1 5.963 5.992

aggregate % asphalt by wt. 6.005 5.577 5.599 5.932 5.587 5.647 5.98 1 5.632 6.653

mixture

TABLE 5. Resilient modulus test results

Resilient modulus (MPa) Statistic

Binder parameter - 17°C 4°C 21°C 37°C 60°C

AC-5 Mean 1 5 7 0 0 8 1 0 0 1435 400 - Std. dev. 3 360 607 807 17 -

AC-5 + carbon Mean 17400 8 8 0 0 3 3 1 0 772 160 Std. dev. 2 030 707 140 2 9 7.3

AC-5 + latex Mean 16 400 9 350 3 330 835 176 Std. dev. 2 442 580 7 1 3 1 10.5

AC-5 + polymer Mean 17 000 10 050 3 350 940 231 Std. dev. 2 600 640 95 22 13.3

AC- 10 Mean 16 050 8 470 1 950 420 - Std. dev. 3 456 580 80 13 -

AC- 10 + carbon Mean 18250 8 9 7 0 3 3 5 0 780 151 Std. dev. 2 650 5 64 116 2 8 7.6

AC-10 + latex Mean 16 700 10 860 3 650 980 200 Std. dev. 2 000 494 135 43 10.7

AC- 10 + polymer Mean 18400 11650 4 1 3 0 1 1 1 0 268 Std. dev. 2 850 650 106 3 6 9.7

AC-20 Mean 18250 11200 3 5 6 7 860 160 Std. dev. 2 750 565 102 24 7.5

AC-20 + carbon Mean 18933 11750 4 9 1 0 1 1 0 0 225 Std. dev. 2 766 610 158 33 10.4

AC-20 + latex Mean 18 800 12 550 5 120 1 320 250 Std. dev. 3 300 730 169 28 11.4

AC-20 + polymer Mean 2 1 2 0 0 13950 5 8 8 5 1 4 3 0 313 Std. dev. 2 400 710 117 49 12.6

Fatigue analysis required the determination of both the [ 3 ] K, = 11s induced tensile strain in the paving mixture and the relation of tensile strain to the allowable number of load applications. [4] K , = 1001~' The following equation was used:

Test results

[2] N , = K , - ( : I K 2 Resilient rnod~ll~ls test results The effect of the original asphalt grade and modifier on the

where N, is the number of load applications to failure; E is modulus of resilience over a wide range of pavement service the induced tensile strain; and K , and K2 are the material temperatures was evaluated. The introduction of temperature constants which can be determined from the regression equa- as a factor in the experiment was intended to provide an tion. The number of applications to failure, N,, versus the ini- adequate assessment of the relative effect of the modifier tial tensile strain, E, were plotted on a log-log graph. Then on the modulus of resilience at each test temperature, and also the best-fit straight line was drawn through the points. The to provide a better understanding of the role of modifiers, and slope of the line was denoted as S and the strain value cor- their effect on the susceptibility to temperature. responding to load applications was denoted as I. The fatigue A summary of the mean value and standard deviation for parameters, K , and K?, were calculated as follows: each binder, as determined by the indirect tension testing

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TABLE 6. Summary of ANOVA result for resilient modulus for each test temperature

Temperature Independent Degrees of ("c) veriable freedom Sum of squares F-value P > F

- 17 Grade Modifier Grade X Modifier

4 Grade Modifier Grade X Modifier

2 1 Grade Modifier Gradc X Modifier

3 7 Grade Modifier Grade X Modifier

60 Grade Modifier Grade X Modifier

method, is presented in Table 5. Figure 3 shows the typical relationships of resilient modulus vs. temperature. It was noted that the resilient modulus decreased with an increase in temperature, decreasing from about 21 000 MPa at - 17.0°C to less than 150 MPa at 60.0°C. An examination of Table 5 shows that for all modified mixtures tested, the addition of the modifier increased the modulus of resilience to some degree, especially at high temperatures.

Because of both the pronounced effect of the testing temperature on the resilient modulus and the emphasis of the study on determining the relative effectiveness of both the modifier and the original asphalt grade on the resilient modulus at each test temperature (that is, measuring the effect of the modifier on the temperatllre susceptibility of the paving mixture), it was decided to evaluate the effect of the modifier and the grade of asphalt at each temperature level. The mathematical model for the response variable (resilient modulus) used in the analysis of variance is given below:

where Y,,, is the response variable, U is the overall mean, G, is the effect of the asphalt grade, R, is the effect of the modifier, GRij is the effect of the interaction of G with M, and El,,, is the random error. The subscripts have the following values: i = 1, 2, 3; j = 1 , 2, 3, 4; and li = 1, 2, 3. The i values of 1, 2, and 3 correspond to the normally graded conventional asphalt of AC-5, AC-10, and AC-20. The,; values of 1, 2, 3, and 4 correspond to modifier type: plain AC, carbon black, latex, and polymer. The above model was used to examine the general effect on original asphalt grade and modifier with emphasis on the effect of each factor at each test temperature.

The statistical general model (GLM) was used for the analysis of the data and, optionally, was used to analyze the data at each temperature level. The results of the analysis of the ANOVA and the t-test on the effect of asphalt grade and modifier are summarized in Tables 6-8.

The results in Table 6 indicate that at - 17'C the effect of the asphalt grade is significant at a = 0. I, while the effect of the modifier on the modulus of resistance is insignificant at a = 0.3 ( a here represents the significance level). This

LEGEND

. .. .. .. .. .. AC-5

- - - - - - - - . ----- AC-10

,#a,,,,,,,,,,,,,,,,~,~>,,,~~>r~ AC-10 + POLYMER

AC-20

- . - . - . - AC-20 + POLYMER

10' I I I I I I I I 0 20 40 60 80 100 120 140

TEMPERATURE ( O F )

FIG. 3. Typical relationship Ibr resilient modulus vs. temperature.

result draws attention to an important point that deals with the original grade (viscosity) of the binder used, whether it is modified or not; that is, the effect of the grade of asphalt is the most important factor in determining the mixture stiffness at very low temperatures. More importantly, the effect on the stiffness resulting from the addition of the modifier is not apparent at that very low temperature of - 17.0°C. However, the above result could be attributed to the large variation inherent in the data at that temperature.

The results of the t-test, based on the Fisher least significant difference (LSD) test, on the grade of the asphalt are presented in Table 7. The results of the test support the above conclusion that at - 17°C the asphalt grade has a

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C A N . J . CIV. ENG. VOL. 21, 199-1

TABLE 7. Mean comparison difference for resilient modulus to test the effect of asphalt grade

Resilient modulus (MPa)

- 17°C 4°C 21°C 37°C 60°C

Grade Mean Group*: Mean Group Mean Group Mean Group Mean Group

'"Mean values are significantly

'Missing data.

with different letters are significantly different at cx = 0.005. For example, groups with different letters different at cx = 0.005 (e.g., resilient ~nodulus value for AC-5 at - 17°C is different from AC-5 at 4°C).

TABLE 8. Mean comparison difference for resilient modulus to test the effect of modifiers

ResiIient modulus (MPa)

Modifier - 17°C 4°C 2 1 "C 37°C 60°C

Conventional asphalt 16 670 9 250 2300 550 -

Carbon 18 200 (NS) 9 850 (S) 3850 (S) 880 (S) -

Latex 17 300 (NS) 10 900 (S) 4050 (S) 1050 (S) -

PoIymer 18 850 (NS) l l 900 (S) 4450 (S) 1150 (S) -

NOTE: S indicates that comparison between the modified mix and the conventional mix is significant at cx = 0.05; NS indicates that the comparison is not significant at cx = 0.05.

TABLE 9. t-test for mean values of resilient modulus for each binder:':

Binder -17°C 4°C 21°C 37°C 60°C

AC-5 C G N T AC-5 + carbon C G M S U AC-5 + latex C G L S U AC-5 + polymer C G L R V

AC- 10 C G L T AC-10 + carbon C G L S U AC- 10 + latex C G K R V AC-10 + polymer D F K Q W

AC-20 B F K R U AC-20 + carbon B F J Q V AC-20 + latex B F I P V AC-20 + polymer A E H 0 X

":Mean values with the same letter are not significantly different at cx = 0.005. For example, groups with the same letter are not significantly different at a = 0.005 (e.g., resilient modulus value for AC-5 at - 17°C is not significantly different from the value for AC-5 + carbon at - 17°C); groups with different letters are significantly different at cx = 0.005 (e.g., resilient modulus value for AC-I0 at 4°C is different from the value for AC- I0 at 2 1 "C).

significant effect in determining the value of the resilient modulus of the paving mixture. For a mixture made with a base of AC-20, the mean value of the resilient modulus is significantly higher than it is for mixtures made with AC-5. However, the results of the t-test on the effect of the modifier, presented in Table 8, show that none of the modifiers have a statistically significant effect on the mean value of the resilient modulus of the paving mixture at - 17°C.

As Table 6 indicates, at a temperature of 4°C both the original asphalt grade and the modifier have a significant effect on the paving mixture's modulus of resilience at ci =

0.0001, whereas the asphalt grade has a more dominant effect as indicated by the F-values. The t-test for the mean value of the resilient modulus at 4°C shows that each asphalt grade has a statistically distinct mean value as shown in Table 7. In Table 8, it may be seen that, statistically, each modifier has a significantly higher mean modulus value than the conventional mixture.

The same trend exists at 21°C: however. the effect of both the modifier and the original asphalt grade became e q ~ ~ a l l y important; the interaction between the modifier and the asphalt grade became significant, indicating that a combination of the modifier and the grade of the original asphalt had a significant effect on the mean modulus value. The t-test for the modulus of resilience base of the original asphalt shows that each asphalt grade has a distinct modulus mean value. The same is true for the modifier, where each modifier has a statistically greater mean modulus value than the conventional mixture.

At high testing temperatures of 37°C and 60°C, the effect of the modifier became more dominant than that of the original asphalt grade, as indicated by the F-values in Table 6. Here. ilso. the t-test for the modulus in Table 6 shows that each asphalt grade has a statistically distinct modulus value, and the addition of the modifiers has significantly increased the modulus value.

A summary of the t-test results on the modulus v a l ~ ~ e for each binder is presented in Table 9. The major conclusion from Tables 6 and 9 is that the addition of modifier shows no significant increase in the resilient modulus of the paving mixture at low temperatures; for example, the modulus of AC-5 is not significantly different from the modulus of the modified AC-5. However, as the temperature increased, the modulus of the m i x t ~ ~ r e significantly increased with the addition of the modifiers. The result also shows that at high temperatures, 37°C and 60°C, the modulus of the modified

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TABLE 10. Regression coefficient for permanent deformation curves for I- l [nix

Binder

AC-5

AC-5 + carbon

AC-5 + latex

AC-5 + polymer

AC- 10

AC-10 + carbon

AC- 10 + latex

AC- I0 + polymer

AC-20

AC-20 + carbon

AC-20 + latex

AC-20 + polymer

Coefficient

Intercept Slope

Intercept Slope

lntercept Slope

Intercept Slope

Intercept Slope

Intercept Slope

Intercept Slope

Intercept Slope

Intercept Slope

Intercept Slope

Intercept Slope

lntercept Slope

AC-5 is more or less comparable to the modulus of AC-20 conventional paving mixture.

It?crz.tneritcil stcitic loaclirlg test The test was conducted at five test temperatures, and the

permanent strain versus loading time was plotted on a log- log scale. The best straight line was fitted, where the slope of the line and the intercept were determined by the least square linear regression (Fig. 4). The slope of the line pro- vided the rate at which permanent deformation accumulated with time; the intercept indicated the expected perinanent deformation from applied load for 0.1 s. The resulting slope and the intercept values for the 12 binders are presented in Table 10.

Results from this test were analyzed using a linear regression technique to test the effect of the original asphalt grade and modifier on the mixture. A comparison test was conducted at each testing temperature level using each asphalt grade and modifier as a group.

The test results in Table 1 I show that the modifier has a significant effect on the slope and intercept para1neter.s of the permanent strain curves at all test temperatures where the original asphalt grade has no significant effect. This result seems to contradict the traditional expectation that the grade of base asphalt of the modified binder plays an important role in the permanent deformation of the paving mixture. In fact, however, this result shows that the addition of the modifier causes a significant reduction in the permanent deformation for all paving mixtures, which decreases the effect of the original asphalt grade.

INCREMENTAL LOADING TIME (s)

F I G . 4. Permanent strain curve using the incremental creep test.

The test results in Table 11 also show that the individual combination of each asphalt grade and modifier (binder) has statistically different parameters from the permanent deformation curves at cr = 0.05. In addition, the t-test results, which are presented in Table 12, show an individual corn- parison between each modified binder and the conventional binder. The results indicate that each modifier-asphalt grade combination (binder) is significantly different from the corresponding conventional binder.

In general, it can be concluded that permanent deformation of the modified paving mixture is somewhat less than that of

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TABLE 1 I. Summary of regression analysis test on the effect of the independent variables on permanent deformation characteristic of the paving mix

- 17°C 4°C 21°C 37°C 70°C Independent

variable Coefficient R' P > F R' P > F R' P > F R' P > F R' P > F

Modifier Slope 0.962 0.1725 0.980 0.0003 0.930 0.0899 0.974 0.0003 0.936 0.3745 Intercept 0.959 0.0001 0.97 1 0.0001 0.920 0.0001 0.962 0.0001 0.927 0.0020

Asphalt grade Slope 0.792 0.9927 0.923 0.8219 0.672 0.9973 0.551 0.9490 0.915 0.4878 Intercept 0.794 0.1091 0.922 0.1008 0.672 0.3052 0.55 1 0.6232 0.912 0.0484

Binder Slope 0.987 0.2341 0.991 0.0074 0.955 0.4698 0.988 0.0016 0.970 0.4522 Intercept 0.793 0.0033 0.550 0.5248 0.514 0.0501 0.670 0.0010 0.772 0.4875

TABLE 12. Summary of t-test between conventional mix and modified mix for permanent deformation coefficients

-- - -

Carbon Latex Polymer Temperature

("c) Conventional Slope 1ntercept'"lope Intercept Slope Intercept

- 17 AC-5 S S N N S S AC- 10 S S N N S N AC-20 S S N N S S

2 1 AC-5 S S S S S S AC- I0 S S S S S S AC-20 S S S S S S

37 AC-5 S S S S S S AC- 10 S S S S S S AC-20 S S S S S S

70 AC-5 - - - - - -

AC- 10 - - - - - - AC-20 S S S S S S

NOTES: S indicates that comparison is significant at CY = 0.05; N, comparison is not significant at CY = 0.05; a dash denotes data are missing.

'!:Intercepts of regression lines are taken at 0.1 s.

the control paving mixture, especially at high temperatures where the pavement is more susceptible to rutting. Carbon black leads the way in reducing permanent deformation of the paving mixtures, followed by paving mixture modified with polymer; paving mixtures modified with latex do not sub- stantially reduce permanent deformation among the modified paving mixtures tested.

Fcrtig~ie life A multiple regression analysis was conducted to deter-

mine fatigue life as a function of strain for the original asphalt grade and each modifier. A regression analysis using the least square method of best straight line fitting was performed to obtain the best-fitted line between induced strain and load applications to failure with logarithmic transfor- mation of the data. The summary of the regression equation coefficients for each binder is presented in Table 13. As can be seen from Table 13, the regression equation fits the data very well with the coefficient of determination (R') ranging between 0.92 and 0.99 and the standard error esti- mate ranging between 0.01 and 0.07. The test results in Table 14 indicated that mixtures made with each modified

binder were significantly different from the corresponding mixtures made with conventional asphalt. A test on the binders showed that each binder has significantly different fatigue properties.

The fatigue parameters, K , and Kz , which were obtained from the fatigue test curves, are listed in Table 15. The values of K , and K, are a direct measure of how the modifier affects the fatigue mechanism of the paving mixture. The flatter the slope of the fatigue curves, the larger the value of K, becomes; that is, if two materials have an equal K , value, then a large value of K, for one material would indicate the potential for longer fatigue life of that material. A low value of K , usually indicates a low fatigue life, assuming the fatigue curves are parallel, that is, the K, values are equal; however, when two fatigue curves intersect, it indicates that the magnitude of induced strain will determine which material will have a longer fatigue life.

The results of the fatigue tests on the mixtures in Table 15 indicate that, compared to conventional paving mixture, mixes with polymers have a significantly increased K , value and a moderately increased K, value. Compared to conventional mixes, mixes with latex show an increased K, value,

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TABLE 13. Regression coefficient of fatigue curves for 1-1 niix

Coefficient of Standard Binder Slope Intercept determination, R' deviation

AC-5 AC-5 + carbon AC-5 + latex AC-5 + polymer

AC- 10 AC- 10 + carbon AC-10 + latex AC-10 + polymer


TABLE 14. Summary of t-test between conventional asphalt mix and modified mix for fatigue coefficients

Carbon Latex Polymer Conventional

asphalt Slope Intercept Slope Intercept Slope Intercept

AC-5 S S S S S S AC- 10 S S S S S S AC-20 S S S S S S

NOTES: S indicates that comparison is significant at cx = 0.05.

which indicates an increase i n the fatigue life of the paving TABLE 15. Fatigue parameters mixture under high stress (large induced strain); this effect was more obvious when latex was added to low grade asphalt Binder K I K2 - such as AC-5 and AC-10.

Table 15 shows that when carbon black was added, there is a marginal decrease in the K , value, coupled with an increase in the K2 value, since the effect of the K , value in the fatigue equation is linear and the effect of the K2 value is a power function. Thus a rate of change in the fatigue life value, N,, is more affected by the K, value than by the K , value. Therefore, it can be concluded that the addition of carbon black gives rise to a sinall increase in fatigue life behaviour when added to hard asphalt (AC-20). However, the addition of carbon black to the soft conventional asphalt, AC-5 and AC-10, produced relatively higher significant fatigue life in the mixtures.

Part 11. Performance predication In order to assess the influence of the modified mixtures

on the pavement performance, a representative pavement section was selected for analysis. The composition of this pavement section is shown in Fig. 5. For the purpose of predicating performance, the VESYS IIIA viscoelastic model was used (Kenis 1977). VESYS model was developed by the Federal Highway Administration under the direction of Fred Moavenzadeh (Moavenzadeh et al. 1973, 1974). In field verification studies (Khosla 1987; Sneddon 1986; Kennedy 1980; Anderson et al. 1978; Sharma et al. 1977), the actual field performance results have been very close to the model predicted performance results. The output of the VESYS IIIA includes a prediction of distress parameters such as rutting, roughness, and cracking over the design period of the pavement. In addition, based on the above

AC-5 6.97 X 2.65 AC-5 + carbon 2.56 X lo-' 3.19 AC-5 + latex 9.41 X 2.74 AC-5 + polymer 1.71 X 10--2.5

AC- 10 1.58 X 2.85 AC-10 + carbon 8.31 X 3.34 AC-10 + latex 7.85 X lo-' 3.04 AC-I0 + polymer 6.46 X 3.10

AC-20 1.04 X 1 0 - 9 . 2 7 AC-20 + carbon 1.12 X 3.58 AC-20 + latex 1.10X lo-' 3.23 AC-20 + polymer 3.45 X 3.21

distress parameters, the model predicted, in terms of time intervals, the pavement performance in terms of present serviceability index (PSI). The mean serviceability index at the time of zero was assigned a value of 4.5 and the terminal serviceability index was assigned a value of 2.5.

The overall performance (PSI) of pavements consisting of 12 binders is presented in Table 16. The results indicate that the addition of carbon black or polymer improved the overall performance of the pavement. The addition of carbon black or polymer to low viscosity graded asphalt, such as AC-5, creates a pavement that performs better than the conventional, highly viscous, AC-20. In addition, Table 17 gives the predicted pavement life based on a terminal PSI of 2.5. The results indicate that the service life of the pavement sections is influenced greatly by the type of the binder. The

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FIG. 5. Layer thickness of a representative pavement section.

TABLE 16. Predicted pavement present serviceability index

Present serviceability index

Binder 1 year 5 years 10 years 15 years 20 years




TABLE 17. Predicted service life of a pavement'"

A major finding was that at a low temperature it appears that the mechanical properties of all paving mixtures are affected primarily by the original grade of the asphalt used in making the binder, while the addition of the modifier was not statistically significant in adversely affecting low temperature stiffness. Thus, low temperature cracking s h o ~ ~ l d not be adversely affected by the addition of modifiers.

Binder Time (year)


AC- 10 AC-10 + carbon AC-I0 + latex AC- 10 + polymer


However, as the temperature is increased, the stiffening effect of the modifier on the binder's properties increased, resulting in a significant increase in the modulus value of the mixtures compared with conventional paving mixtures. As a result of the above phenomenon, the study demonstrates that the use of modifiers can improve the temperature susceptibility of both the binders, as indicated by the increase in the Pen-Vis Number (PVN), and the paving mixtures, as indicated by the resilient modulus values obtained from laboratory tests.

The results of the laboratory experiment and the predicted :*Service failure level i s PSI < 2.5 (PSI, present serviceability index). pavement performance show that the three modifiers used in

this research can improve one or more mechanical properties of paving mixtures in a variety of ways. As a result, there is a reduction of one or more pavement distress factors, the overall pavement performance is improved, and pavement life expectancy is increased.

addition of carbon black or polymer to the asphalt binder markedly extended the pavement life while the addition of latex has a moderate effect.

Conclusion The general conclusions were that the mechanical properties

of the paving mixture vary and depend mainly upon the type of the modifier added and the original grade of the asphalt cement. Temperature is by far the most significant of the parameters studied, affecting the resilient modulus of the paving mixtures.

Anderson, D.I., Peterson, D.E., McBride, J.C., and Shephard, L.W. 1978. Field verification and implementation of the VESYS IIM structural subsyslem in Utah. Report No. FHWA- RD-78-510, Federal Highway Administration, Washington, D.C.

Anderson, K.O., Hussain, S., and Jardine, K.G. 1989. Evaluation of low temperature and permanent deformation characteristic

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A L I ET AL. 965

of some engineered asphalt. Canadian Technical Asphalt Moavenzadeh, F., Soussou, J.E., Findakly, H., and Bardemeyer, Association Proceedings, Halifax, N.S., p. 292. B. 1974. Synthesis for rational design of flexible pavement.

The Asphalt Institute. 1984. Design methods for asphalt concrete. Part 111. Operating instruction and program documentation. The Asphalt Institute Manual Series No. 2 (MS-2), Lexington, Ky. School of Engineering, Massachusetts Institute of Technology,

Beagle. C.W. 1967. One-half inch bituminous concrete surface. Cambridge, Mass. Highway Research Record No. 173, Highway Research Board, Moavenzadeh, F.N., Soussou, J .E. , and Findakly, H. 1973. Washington, D.C.. p. 35. Synthesis for rational design of flexible pavements. Part I.

Collins, J.H., and Mikols, W.J. 1985. Block copolymer modifi- School of Engineering, Massachusetts Institute of Technology, cation of asphalt intended for surface dressing applications. Cambridge, Mass. Proceedings of the Association of Asphalt Paving Technologists, Omer, M. 1984. Prediction of performance of flexible pavements Vol. 54, p. I . using VESYS IIIA structural subsystem. Ph.D. dissertation,

Gregg, L.E., and Alcoke, W.H. 1954. Investigation of rubber North Carolina State University, Raleigh, N.C. additives in asphalt paving mixtures. Proceedings of the Rostler, F.S., White, R.M., and Dannenbege, E.M. 1977. Carbon Association of Asphalt Paving Technologists, Vol. 23, p. 28. black as a reinforcing agent for asphalt. Proceedings of the

Jew, P., and Woodhams, R.J. 1986. Polyethylene-modified Association of Asphalt Paving Technologists, Vol. 4, p. 376. bitumens for paving applications. Proceedings of the Association Schmidt, R.J. 1972. A practical method for measuring the resilient of Asphalt Paving Technologists, Vol. 55, p. 541. modulus of asphalt treated mixes. Highway Research Record

Kenis, W.J. 1977. Predictive design procedure - a design method No. 404, Highway Research Board, Washington, D.C. for flexible pavements using the VESYS structural subsystem. Sharma, M.G., Kenis, W.J., Larson, J.D., and Gramling, W.L. Proceedings, Fourth International Conference on Structural 1977. Evaluation of flexible pavement design methodology Design of Asphalt Pavements , University of Michigan, based upon field observation at PSU test track. Proceedings, Ann Arbor, Mich., Vol. I, p. l O l . Fourth International Conference on Structural Design of Asphalt

Kennedy, T.W., Roberts, F.L., and Jordahl, P.R. 1980. Required Pavements, University of Michigan, Ann Arbor, Mich., Vol. I, properties for zero maintenance flexible pavements. Proceedings p. 158. of the Association of Asphalt Paving Technologists, Vol. 49, Sneddon, R.V. 1986. Evaluation and verification of the VESYS-3-A p. 550. structural design system for two test sites in Nebraska.

Khosla, N.P. 1987. A field verification of VESYS IIIA structural Transportation Research Record No. 1095, Transportation subsystem. Proceedings of the Sixth International Conference Research Board, Washington, D.C. p. 18. on Structural Design of Pavements, University of Michigan, Terrel, R.L., and Rimsritong, S. 1980. Evaluation of wood lignin Ann Arbor, Mich., Vol. I , p. 486. as a substitute or extender for asphalt. Report FHWAIRD-801

King, G.N., Muncy, H., and Prudhomme, J. 1986. Polymer mod- 125, Federal Highway Administration, Washington, D.C. ification: Binder's effect on mix properties. Proceedings of Terrel, R.L., and Walter, J.L. 1986. Modified asphalt pavement the Association of Asphalt Paving Technologists, Vol. 55, material - the European experience. Proceedings of the p. 519. Association of Asphalt Paving Technologists, Vol. 55, p. 384.

Little, D.N., Button, J.W., White, R.M., Ensely, E.K., Kim, Y., and Thompson, D.C. 1964. Rubber modifier. In Bituminous material. Ahmed, S.J. 1986. Investigation of asphalt additives. Report Edited by Arnold J. Holberg. Interscience, New York. Vol. I, No. FHWAIRD 871001, Federal Highway Administration, Chap. 9, p. 375. Washington, D.C. Yao, Z., and Monismith, C. 1986. Behaviour of asphalt mixtures

Martin, K.G. 1962. Preliminary microviscometer studies of with carbon black reinforcement. Proceeding of the Association carbon black bitumen dispersions. Proceedings of the Australian of Asphalt Paving Technologists, Vol. 55, p. 564. Road Research Board, Vol. I, Part 11. Zanzotto, L., Foley, D., Rodier, C.E., and Watson, R.D. 1987.

Marvillet, J., and Verschave, A. 1979. Liants bitume-polymer Modified asphalt - are you really coming? Canadian Technical pour enduits superficiels. Proceedings, Canadian Technical Asphalt Association Proceedings, Toronto, Ont., p. 92. Asphalt Association, Vol. 24.

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A mechanistic evaluation of modified asphalt paving mixtures