Effect of aggregate properties on asphalt mixtures stripping and creep behavior

Construction

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Construction and Building Materials 21 (2007) 1886–1898

and Building

MATERIALS

Effect of aggregate properties on asphalt mixtures strippingand creep behavior

Saad Abo-Qudais, Haider Al-Shweily *

Civil Engineering Department, Jordan University of Science and Technology, Irbid 22110, Jordan

Received 23 July 2004; received in revised form 24 July 2005; accepted 31 July 2005Available online 26 March 2007

Abstract

The purpose of this paper is to look at some aspects of the effects of aggregate chemical and physical properties on the creep andstripping behavior of hot-mix asphalt (HMA). Two types of aggregates evaluated in this study were limestone and basalt. The effectsof the aggregates type were evaluated on three different aggregate gradations and two types of asphalt used in preparing the HMA.The percent of increase in static creep strain of HMA due to conditioning was utilized in this study to assess the stripping.

Test results indicated that unconditioned HMA specimens prepared using basalt aggregate resist creep better than those preparedusing limestone. However, after conditioning, mixes prepared using basalt were less resistant to creep strain than those prepared usinglimestone aggregate. Percent absorbed asphalt was found to be directly related to stripping resistant. Also, mixes prepared using aggre-gate following ASTM upper limit of dense aggregate gradation presented the highest resistance to stripping. The results of the calculatedadhesion work were able to detect the effect of stripping on creep behavior for mixes prepared.� 2005 Published by Elsevier Ltd.

Keywords: Asphalt mixes; Stripping; Performance evaluation; Aggregate gradation

1. Introduction

Asphalt pavement failure is a complicated phenomena.It is a result of cumulative damage in different pavementlayers [11]. The influence of moisture on hot-mix asphalt(HMA) stripping is difficult to characterize due to the pres-ence of many factors affecting this damage. One of the ma-jor problems affecting the performance of hot-mix asphaltis stripping.

Many studies indicated that asphalt binder chemistry,aggregate mineralogy, aggregate surface texture, and theinteraction between asphalt and aggregate significantly af-fect moisture susceptibility. The large numbers of differentaggregate mineralogies and the different types of asphaltbinders used across the world, coupled with varied environ-mental conditions, traffic, and construction practices, have

0950-0618/$ - see front matter � 2005 Published by Elsevier Ltd.

doi:10.1016/j.conbuildmat.2005.07.014

* Corresponding author.E-mail address: [email protected] (H. Al-Shweily).

made testing to predict accurately hot-mix asphalt mois-ture susceptibility a difficult task [12].

Aggregate mineral and chemical composition, exposurehistory (e.g., freshly crushed versus days of exposure toenvironmental weathering after crushing) have significanteffects on stripping. Hydrophilic (water loving) aggregatesshould be avoided unless an antistripping additive is used.Angular aggregates, sometimes, increase the stripping po-tential. This can be explained by the fact that angularaggregates increase the potential of film rupture at theaggregate sharp edges [12].

Using high-viscosity asphalt produces hot-mix asphaltwith higher resistance to stripping. However, low viscosityasphalt is desirable during mixing operations, since low vis-cosity asphalt has more spreading ability which producesbetter aggregate coating during mixing [12].

Variations in temperature, freeze–thaw cycles, andwetting–drying cycles increase the stripping potential. Also,the nature of the water to which the mix is exposed (saltcontent, pH) affects stripping. Traffic imposes cyclic

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S. Abo-Qudais, H. Al-Shweily / Construction and Building Materials 21 (2007) 1886–1898 1887

loading on pavement as well as abrasion of its surface; inaddition, traffic will increase the pore water pressure, lead-ing to an increase of stripping potential [12].

Many studies were performed to evaluate stripping andits effect on pavement performance. Abo-Qudais and Al-Shweily [2] evaluated 24 different HMA types using differ-ent mix parameter combinations for the effect of strippingon static creep deformation. The mix parameters include;two types of asphalt cement (60/70 and 80/100 penetra-tion grade), three types of aggregate gradations, two typesof additives, and two modes of conditioning (conditionedand unconditioned). The findings of this study indicatedthat conditioning of HMA specimens has a significant ef-fect on the increase of creep deformation. This is espe-cially true for open-graded aggregate gradation mixes.Also, aggregate gradation, asphalt type, and the type ofantistripping additive have a considerable effect on creepdeformation. This is especially true for conditioned spec-imens. For both conditioned and unconditioned mixesprepared using mid limits and upper limit of ASTM spec-ification for dense graded aggregate gradations, the creepdeformation of mixes prepared using 80/100 asphalt wasless than that for mixes prepared using 60/70 asphaltand tested at the same temperature. The opposite trendwas noticed for conditioned specimens prepared usingopen-graded aggregate gradation, the creep deformationof mixes prepared using 80/100 asphalt was more thanthat for mixes prepared using 60/70 asphalt. Antistrippingadditives showed a significant effect on reducing strippingand creep behavior. Mixes prepared using calcium stea-rate hydroxide antistripping additive showed less strippingand creep deformation than those containing limestonedust additive.

In another study, Abo-Qudais [1] studied the effect ofusing different evaluation techniques on the predicted strip-ping of 24 different HMA combinations prepared using dif-ferent mix parameters. Similar mix parameters as those in aprevious study were used. The stripping evaluation tech-niques include percent reduction in both indirect tensilestrength and Marshall stability, percent increase in creepdue to stripping, in addition to stripping visual evaluationusing the Texas boiling test. The findings of this study indi-cated that the estimated stripping is affected significantly bythe method of evaluation. The reduction in indirect tensilestrength and Marshall stability were found to be less sensi-tive to stripping than the percent increase in creep. Also,percent increase in creep was the only one among the meth-ods used that was able to determine the effect of used as-phalt and aggregate gradation on the stripping of HMA.

Guirgus et al. [8] investigated the use of cement-coatedaggregate mixtures to improve the pavement performance.It was found, that the resistance of cement-coated aggre-gate mixtures to the action of water is as high as those trea-ted with hydrated lime, and the retained strength afterimmersion in hot water is almost 100%.

Portland cement has been used as an agent in hot-mixbituminous mixtures to prevent stripping of asphalt cement

from aggregate. It has been reported, that the addition of1% of Portland cement will increase stability by 250–300% over that of untreated hot-mix asphalt. Also, Flex-ural fatigue resistance was increased when cement treatedaggregate mixtures were used [7].

Maupin [10] performed field investigations on the effec-tiveness of antistripping additives in Virginia. The resultsof the study indicated that significant visual stripping wasdetected at many sites, also chemical additives performedno better than hydrated lime. The pavement voids at manysites were too high for good durability. The degree of strip-ping damage in underlying layers could influence perfor-mance at many sites.

In another study, Maupin [9] used Lottman�s test toevaluate the effect of different types of asphalt cementsand antistripping agents on the stripping susceptibility ofHMA. The results indicated that the new test method mea-sured no differences in stripping susceptibility of HMAwith different types of asphalts, while significant differencesin stripping susceptibility were detected when differentadditives were used.

Brown and Bassett [5] evaluated five hot-mix asphaltmixes with different maximum aggregate sizes of crushedlimestone used in preparing the specimens. The asphaltcontent of all mixes was selected to provide air void contentof 4%. Specimens were evaluated using the Marshall, indi-rect tensile strength, creep, and resilient modulus tests. Thecreep test results indicated that the permanent strain of4 in. specimens increased with an increase in the maximumsize of aggregate.

2. Objectives

The main purpose of this paper was to evaluate the ef-fects of aggregate and its properties on stripping andcreep behavior of HMA. To accomplish this, a laboratorystudy was initiated to evaluate the effect of aggregateproperties on stripping and creep behavior susceptibilityof HMA.

3. Laboratory program

3.1. Variables

The effect of aggregate type on HMA stripping andcreep behavior were evaluated at different mix parametersincluding three aggregate gradations, two types of asphalt,and two mode of conditioning, as shown in Fig. 1 andTable 1. In an earlier study by Abo-Qudais [1] the effectof aggregate gradation and asphalt type on HMA preparedusing limestone aggregate was evaluated. In the presentstudy, different parameters were evaluated for their effecton mixes prepared using either limestone or basalt aggre-gate. Half of the specimens were exposed to conditioningwhile the other half were tested without conditioning, threespecimens were tested at each mix parameter and condi-tioning combination.

HMA Specimens(72 specimens)

Basalt *(36 specimens)

Limestone(36 specimens)

Gradation A**(12 specimens)

Gradation C **(12 specimens)

Gradation B(12 specimens)

60/70 Asphalt(6 specimens)

80/100 Asphalt ***(6 specimens)

Unconditioned(3 specimens)

Conditioned(3 specimens)

*Same variables as those for limestone aggregate were considered.** Same variables as those for gradation B aggregate were considered.*** Same variables as those for 60/70 Asphalt were considered.

Fig. 1. Evaluated hot-mix parameters and conditioning mode.

Table 1Experimental design

Variable Number ofvariables

Cumulativenumber of samples

Aggregate type Limestone aggregate 2 2Basalt aggregate

Gradation Gradation A 3 6Gradation BGradation C

Asphalt type 60/70 Asphalt 2 1280/100 Asphalt

Conditioning Conditioned 2 24Unconditioned

Repetition Specimen 1 3 72Specimen 2Specimen 3

1888 S. Abo-Qudais, H. Al-Shweily / Construction and Building Materials 21 (2007) 1886–1898

3.2. Materials

The materials used in this study were different types ofaggregate and asphalt and are described as follows.

3.2.1. Aggregate

Aggregate from two sources were utilized in asphaltmixtures: crushed limestone and basalt. The limestone

was obtained from Al-Huson quarries in the northern partof Jordan, while basalt was obtained from Al-Safawi in theeastern part of Jordan. Table 2 summarizes the physicalproperties of these aggregates; While Table 3 summarizesthe chemical composition of the two aggregates. For eachaggregate type, three aggregate gradation were evaluated:

� Gradation A. Upper limit of ASTM specifications fordense aggregate gradation. The nominal size of this gra-dation was 12.5 mm.� Gradation B. Mid limits of ASTM specifications for

dense aggregate gradation. The nominal size of this gra-dation was 19.0 mm.� Gradation C. Mid limits of ASTM specifications for

open aggregate gradation. The nominal size of this gra-dation was 19.0 mm.

Fig. 2 shows the aggregate size distribution of the threegradations.

3.2.2. Asphalt

Two types of asphalt cement with different penetrationswere used in this study:

� Asphalt 1. Performance grading (PG) 70–10, 60/70 pen-etration grading, and AC-20 viscosity grading.

Table 2Physical properties of aggregate used

Aggregate ASTM test designation Dry bulk specific gravity Apparent specific gravity Absorption (%)

Limestone Basalt Limestone Basalt Limestone Basalt

Coarse aggregate C127 2.424 2.531 2.573 2.621 3.1 3.1Fine aggregate C128 2.485 2.632 2.590 2.680 4.6 3.6Mineral filler C128 2.552 2.692 2.625 2.710 5.1 5.0

Table 3Chemical composition of aggregates used

Compound Limestone Basalt

SiO2 1.1 1.7Al2O3 0.86 17.9Fe2O3 2.0 7.2FeO – 1.0MgO 0.86 2.8CaO 54.6 6.9Na2O – 4.2K2O – 1.6H2O – 1.2CO 41.8 –

Table 4Properties of asphalt used

Test Methods Result

AC 60/70 AC 80/100

Ductility at 25 �C (cm) ASTM D113 110 118Penetration at 25 �C,

100 g (0.1 mm)ASTM D5 64 92

Softening point (�C) ASTM D36 50 45.5Flash point (�C) ASTM D92 319 312Fire point (�C) ASTM D92 325 318Specific gravity at 25 �C ASTM D70 1.010 1.010


� Asphalt 2. PG 58–22, 80/100 penetration grading, andAC-30 viscosity grading.

The two asphalt types were obtained from a local petro-leum refinery. Table 4 summarizes the physical propertiesof the asphalts.

3.3. Mix design methodology (optimum asphalt content)

To determine the optimum asphalt content by weight oftotal mix, for each aggregate gradation, Marshall mix de-sign procedures (ASTM D1559) were followed. The Mar-

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shall compaction method was used instead of theGyratory compaction method, because HMA specimensprepared using basalt aggregate and compacted by Gyra-tory compactor were found to have high air voids evenwhen a high number of gyrations was used. Three speci-mens of each asphalt content (3.0%, 3.5%, 4.0%, 4.5%and 5.0% for mixes prepared using gradation C, and4.5%, 5.0%, 5.5%, 6.0% and 6.5% for mixes prepared usinggradations A and B) were prepared. A total of 45 speci-mens were tested for stability, flow, air voids, unit weight,and voids in mineral aggregate.

The optimum asphalt content was calculated as theaverage of asphalt content that meets maximum stability,

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ate gradation (A)

te gradation (B)

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n of aggregates used.


maximum unit weight, and 4% air voids. The resultingoptimum asphalt content was checked whether it achievedthe specification limits of the five parameters (stability,flow, air voids, unit weight, and voids in mineral aggregate(VMA)). The resulting optimum asphalt content of mixesprepared using limestone aggregate were 5.6%, 5.3%, and4.2% for mixes prepared using gradation A, B, and C,respectively, while for mixes using basalt aggregate theoptimum asphalt content was 5.7%, 5.4%, and 4.4% formixes prepared using gradation A, B, and C, respectively.

3.4. HMA specimens fabrication

The specimens were prepared according to the AsphaltInstitute Manual (MS-2). The asphalt cement and aggre-gate (limestone or basalt) were mixed at 164 and 155 �Cfor specimens� prepared using asphalt 1 and asphalt 2,respectively. The HMA specimens were compacted usingthe Marshall compactor at about 148 and 143 �C for spec-imen prepared using asphalt 1 and asphalt 2, respectively.

3.5. Moisture conditioning

The moisture conditioning was used to evaluate theeffects of water saturation and accelerated water condi-tioning with a freezing–thawing cycle of compacted bitu-minous mixtures in the laboratory. The hot-mix asphaltspecimens conditioning was performed according toAASHTO T283 by immersing the specimens in waterand exposing them to a vacuum for 10 min to achievesaturation levels between 55% and 80%. Then the speci-mens were exposed to freezing at a temperature of�18 ± 3 �C for 16 h and thawing at 60 �C for 24 h. Theconditioned specimens were left for 24 h before perform-ing the creep test. The effect of conditioning on HMAstripping was evaluated using the static creep test. Theresults of creep tests on conditioned specimens were com-

%Increase in creep ¼ Creep of conditioned specimens� Creep of unconditioned specimen

Creep of unconditioned specimen� 100%. ð2Þ

pared to those on unconditioned (control) specimens (seeTable 5).

3.6. Static creep test

This test is considered to be very important in obtainingdata for estimating potential deformation of vehicle wheelpaths and ranking bituminous mixtures on the basis oftheir resistance to permanent deformation. The creep testwas selected, for the evaluation of stripping in this study.Based on a previous study by Abo-Qudais [1], the creep testproved to have a better capability to predict stripping thanthe tensile strength ratio or the Marshall stability ratio. The

static creep test was conducted, at 30 �C, using the Univer-sal Testing Machine (UTM). The tests were performedaccording to the following procedures: after capping thetwo sides of the specimen, it was placed in the loading ma-chine, Fig. 3, under a conditioning stress (initial loading) of10 kPa for 10 min. Then the conditioning stress was re-moved and a stress of 100 kPa was applied for 1 h, afterwhich the load was removed and the deformation recoverywas monitored for 15 min. Three specimens were evaluatedfor the same: type of aggregate, type of asphalt, type ofaggregate gradations, and mode of conditioning.

The initial height of the specimens was measured beforecapping, while the axial deformation was monitored duringthe creep test using the linear vertical displacement trans-ducers (LVDTs). Accumulated microstrain was calculatedas the ratio between the measured deformation to theinitial specimen height according to the following equation:

e ¼ Dh=h0; ð1Þwhere e is the accumulated microstrain occurred in thespecimen during a certain loading time at a certain temper-ature, h0 is the initial specimen height (the initial distancebetween specimen loading surfaces) (mm), and Dh is the ax-ial deformation (reduction in distance between specimenloading surfaces) (mm · 10�6).

Creep test results for different mixes and conditioningare summarized in Figs. 4–9; while Fig. 10 and Table 6summarize the percent increase in creep after 60 min ofloading. It should be noted that each point on these figurerepresent the average results of three specimens preparedfrom the same mix parameters and tested under the sameconditions.

The stripping effect on creep behavior was evaluatedbased on percent increase in creep due to conditioning. Itwas calculated by dividing the difference in creep value be-tween unconditioned and conditioned specimens on that ofunconditioned specimens

3.7. Adhesion and cohesion works analysis

The attraction force between asphalt and aggregate wasanalyzed for different combinations of asphalt and aggre-gate types. This attraction force across the interphase wasmeasured as the reversible work of adhesion by usingDupre�s relationship [3]

W adh ¼ cs þ cb � csb; ð3Þwhere Wadh is the work of adhesion between asphalt andaggregate, cb is the surface tension of asphalt, cs is the sur-face tension of aggregate, and csb is the interfacial tensionbetween asphalt and aggregate.

Table 5Properties of different evaluated HMA mixtures

Gradation used Property Type of aggregate used in preparingHMA specimens

Limestone Basalt

A Optimum asphalt content (%) 5.6 5.7Stability (kgf) 1483 1220Flow (0.25 mm) 16 11.2Bulk specific gravity Mean 2.42 2.49

Maximum 2.45 2.52Minimum 2.40 2.48Standard deviation 0.026 0.023

Air voids (%) Mean 4.8 5.0Maximum 5.0 5.1Minimum 4.7 4.9Standard deviation 0.173 0.100

Voids in mineral aggregates (%) Mean 13.7 14.5Maximum 14.2 14.5Minimum 13.0 14.4Standard deviation 0.610 0.057

B Optimum asphalt content (%) 5.3 5.4Stability (kgf) 1426 1120Flow (0.25 mm) 16.5 10Bulk specific gravity Mean 2.31 2.46


Air voids (%) Mean 4.1 4.8Maximum 4.4 5.2Minimum 3.9 4.6Standard deviation 0.261 0.321


C Optimum asphalt content (%) 4.2 4.4Stability (kgf) 665 525Flow (0.25 mm) 12.8 15.8Bulk specific gravity Mean 2.24 2.25


Air voids (%) Mean 12.8 12.3Maximum 13.4 14.2Minimum 13 10.9Standard deviation 0.721 1.653



Here, cb and cs were determined by measuring the con-tact angle of a series of test liquids, with known surface ten-sion, placed on the asphalt and aggregate, respectively. Thefollowing equation was then applied:

cb ¼ c1

ð1þ cos /Þ2

4U2; ð4Þ

where c1 is the surface tension of test liquid, / is the mea-sured contact angle between aggregate and test liquid, andU is the factor function of molar volume of asphalt andaggregate, can be assumed equal to 1.0 [3].

Similarly, the surface tension of the asphalt (cs) wasdetermined. The interfacial surface tension between asphaltand aggregate (csb) was determined from asphalt andaggregate surface tensions using the following formula:

cbs ¼ cb þ cs � 2UðcbcsÞ1=2. ð5Þ

The attraction force in the asphalt body itself was mea-sured as the reversible work of cohesion, for the cohesionwork in asphalt [3]

W coh ¼ 2cb; ð6Þwhere Wcoh is cohesion work.

Fig. 3. Static creep test setup.

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Fig. 4. Effect of aggregate type and conditioning on creep of hot-mix asphalt specimens prepared using 60/70 asphalt, and mid limits for dense aggregategradation.


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Fig. 5. Effect of aggregate type and conditioning on creep of hot-mix asphalt specimens prepared using 80/100 asphalt, and mid limits for dense aggregategradation.

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Fig. 6. Effect of aggregate type and conditioning on creep of hot-mix asphalt specimens prepared using 60/70 asphalt, and upper limit for dense aggregategradation.


The results gave attraction forces (adhesion and cohe-sion), for different combinations of asphalt and aggregatetypes, which were used in explaining why a certain combi-nation of asphalt and aggregate experienced less strippingresistance and higher creep deformation.

The same method of calculating the work of adhesionbetween asphalt and aggregate was used to calculate theadhesion work between water and asphalt or aggregate.Comparing the work of adhesion between water andasphalt or aggregate to that between asphalt and aggre-gate can be used to determine whether submerging spec-imens in water will cause stripping of asphalt fromaggregate.

4. Results and discussion

The static creep behavior of the prepared specimens wasevaluated using the Universal Testing Machine (UTM).The effects of aggregate type and gradation, type of asphaltand amount of absorbed asphalt on hot-mix asphalt strip-ping and creep behavior are summarized in Table 6 and dis-cussed in the following sections. The effects of conditioning,aggregate chemical composition and adhesion work betweenaggregate and asphalt on the creep behavior of hot-mix as-phalt are also discussed. As mentioned earlier each point inthe results presented in this study represent the average ofthree specimens prepared from the same mix parameters

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Fig. 7. Effect of aggregate type on creep of hot-mix asphalt specimens prepared using 80/100 asphalt, and mid limits for dense aggregate gradation.

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Fig. 8. Effect of aggregate type and conditioning on creep of hot-mix asphalt specimens prepared using 60/70 asphalt, and mid limits for open gradedaggregate gradation.


and tested under the same conditions. Statistical analysis ofcollected data indicated that the variability in results (basedon the calculated standard deviation) between specimensprepared from the same mix parameters and tested underthe same conditions are low and within the expected variabil-ity of these parameters. Also, statistical analysis, using t-testat 95% confidence level, indicated that there is a significantdifference between specimens prepared from different mixparameters or tested under different conditions.

4.1. Effect of aggregate type

The effect of aggregate type on creep strain of differenttypes of hot-mix asphalt specimens is shown in Figs. 4–9.

These Figures indicate the relationship between accumu-lated strain (microstrain) and time for both conditionedand unconditioned HMA specimens prepared using differ-ent types of aggregate. Unconditioned specimens indicatedthat hot-mix asphalt prepared using limestone aggregateexperienced higher creep value than those prepared usingbasalt aggregate. The same trend was noticed regardlessof the types of asphalt and the aggregate gradations usedin preparing mixes. After 60 min of loading, the creepmicrostrains, of HMA prepared using 80/100 asphalt andmid limits of dense aggregate gradation, were 7926 and3846 for mixes prepared using limestone and basalt aggre-gate, respectively. These results can be explained by the factthat the basalt aggregate is rougher than the limestone

Fig. 10. Percent increase in creep strain due to conditioning after 60 min of static loading for different types of hot-mix asphalt.

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Fig. 9. Effect of aggregate type and conditioning on creep of hot-mix asphalt specimens prepared using 80/100 asphalt, and mid limits for open gradedaggregate gradation.

Table 6Bulk specific gravities, percent increase in creep due to conditioning, and absorbed asphalt for different types of hot-mix asphalt

Aggregate Gradations Asphalt csb cse cse � csb Increase in creep (%) Absorbed asphalt (%)

Limestone A 1 2.455 2.557 0.102 17.4 0.10732 2.455 2.564 0.109 32.5 0.114978

B 1 2.447 2.548 0.101 25.2 0.106222 2.447 2.543 0.096 53.7 0.100764

C 1 2.432 2.460 0.028 39.0 0.0286062 2.432 2.451 0.019 100.9 0.01934

Basalt A 1 2.569 2.593 0.024 83.3 0.0244662 2.569 2.591 0.022 139.8 0.02241

B 1 2.556 2.567 0.011 196.3 0.0111582 2.556 2.571 0.015 295.0 0.015239

C 1 2.541 2.552 0.01 252.9 0.0111582 2.541 2.549 0.008 357.9 0.008105



aggregate, so the mechanical interlock between asphalt andbasalt aggregate will be higher leading to higher resistanceto creep. For the unloading portion of the creep test (defor-mation recovery), the recovered deformations of mixes pre-pared using limestone aggregate and mix prepared usingbasalt aggregate was closed to each other.

In contrast to unconditioned hot-mix asphalt specimens,conditioned HMA specimens prepared using basalt aggre-gate showed higher creep values than those prepared usinglimestone aggregate. A similar trend was noticed regardlessof the type of asphalt and aggregate gradations used in pre-paring the HMA. After 60 min of loading, the creep micro-strains for HMA prepared using 80/100 asphalt and midlimits of dense aggregate gradation, were 12,490 and14,601 for mixes prepared using limestone and basaltaggregate, respectively. The above mentioned results canbe explained by the fact that the limestone aggregate hasbetter resistance to stripping than basalt aggregate. Lime-stone rock is hydrophobic (water hating) aggregate whilebasalt rocks tend to be hydrophilic (water loving) aggre-gate. This agree with the results of other studies that usedMarshall stability to evaluate stripping in HMA. Thesestudies, indicated that conditioned hot-mix asphalt speci-mens prepared using limestone aggregate have higher sta-bility value than those prepared from basalt aggregate[4,6,11].

The conditioning of hot-mix asphalt specimens preparedusing open graded aggregate gradation with basalt aggre-gate caused complete failure of the specimens after shortperiods of loading, as shown in Figs. 8 and 9.

4.2. Effect of absorbed asphalt

According to the mechanical theory, the composition ofthe rock is important only to the extent that it effects sur-face texture. It is thought that the rougher the surface tex-ture, the better the adhesion. Porous aggregate usually

Fig. 11. Percent increase in creep and percent

shows better adhesion to asphalt due to better mechanicalinterlock. However, in some cases it is difficult to removewater from the pores during the drying process; this maybe detrimental.

In this section the stripping of hot-mix asphalt is relatedto the amount of asphalt absorbed in aggregate permeablevoids. The results, shown in Fig. 11 and Table 6, indicate astrong direct relationship between resistance to stripping(creep strain resistance) and the amount of absorbed as-phalt. This result can be explained by the fact that as ab-sorbed asphalt increases, the mechanical bonding betweenasphalt and aggregate will increase as the amount of ab-sorbed asphalt increases.

Fig. 11 indicates that the amount of absorbed asphalt iscapable of explaining not only the effect of aggregate type,but also the effect of aggregate gradation and type of as-phalt used in preparing the HMA. It should be noted thatthere are other factors that explain why HMA preparedwith limestone aggregate has better resistance to strippingbetween asphalt and aggregate. These factors includechemical reactions, surface energy relationships, and polar-ity of the aggregate.

4.3. Effect of aggregate chemical composition

The chemistry of the aggregate surface affects the degreeof the water sensitivity of the asphalt aggregate bond. Thechemical compositions of the two types of aggregates usedare summarized in Table 3. This table indicates that lime-stone is composed mainly of CaCO3, while basalt iscomposed principally of Al2O3. Compared to basalt aggre-gate, limestone aggregate contains less SiO2. Silica usuallycauses a reduction in the bond between asphalt and aggre-gate. Limestone is considered to bear a positive charge,while basalt bears a mixed charge. Usually stronger bondsare associated with more electro-positive charge. Thismakes the basalt aggregate fall in the hydrophilic (water

absorbed asphalt of different HMA types.


loving) category, while the limestone aggregate falls in thehydrophobic (water hating) category.

4.4. Aggregate shape

Both aggregates evaluated in this study were manufac-tured, elongated, angular aggregate. No different aggregateshapes, for the same type of aggregate, were evaluated inthis study, so the aggregate shape was not a factor thatcan be evaluated at this stage on the study.

4.5. Aggregate gradation

HMA prepared using gradation C (open-graded grada-tion) was found to be less resistant to stripping. This iscaused by a large amount of air voids and the large diam-eter of pores in the mix. HMA prepared using gradation Ashowed the highest resistance to stripping although it hasmore air voids than mixes prepared using B aggregate gra-dation as shown in Table 5. The higher air voids in mixesprepared using B aggregate gradation can be referred tousing less amount of fine compared to mixes preparedusing A aggregate gradation.

For unconditioned HMA specimens, HMA preparedusing gradation C experienced the highest creep strain,while mixes prepared using A aggregate gradation experi-enced the least creep strain. This can be explained by thefact that HMA prepared using C aggregate gradation con-tain less fines; this will reduce adhesion by fines causing lessresistance to stripping.

4.6. Adhesion work

The results of the adhesion work calculations for differ-ent combinations of asphalt and aggregate types are sum-marized in Table 7. This table indicates that the adhesionbetween aggregate and asphalt in HMA prepared usinglimestone aggregate is higher than that of mixes preparedusing basalt aggregate. This means that the HMA preparedusing limestone aggregate have higher resistance to strip-ping since the bond strength between asphalt and limestoneaggregate, as reflected by the adhesion work, is strongerthan that between asphalt and basalt aggregate. The adhe-sions between aggregate and 60/70 asphalt were 4.88 and0.24 dyne cm for mixes prepared using limestone and basaltaggregate, respectively.

As the effect of types of asphalt used in preparing theHMA were considered, it was found that mixes prepared

Table 7Adhesion work between aggregate and asphalt or water

Asphalt 60/70

Surface tension (dyne cm) 35.2Adhesion work with 60/70 Asphalt (dyne cm) –Adhesion work with 80/100 asphalt (dyne cm) –Adhesion work with water (dyne cm) –

using 80/100 asphalt have higher adhesion than that of60/70 asphalt. This means that mixes prepared using 80/100 asphalt have better stripping resistance and thereforeless increase in creep due to conditioning compared to thatof HMA specimens prepared using 60/70 asphalt. Thisagreed with the percent increase in creep obtained formixes prepared using A and B aggregate gradation, but itdisagreed with those for mixes using C gradation. Mixesusing C gradation showed higher creep when asphalt 80/100 was used in preparing the mix.

As shown in Table 7, the adhesion between water andaggregate was higher than that between asphalt andaggregate, regardless of the types of asphalt and aggregateused in preparing the mix. For example the adhesionwork between limestone aggregate and 60/70 asphaltwas 4.88 dyne cm, while it was 11.63 dyne cm betweenthe same aggregate and water. This means that the bondstrength between water and aggregate is higher than thatbetween asphalt and aggregate, i.e., water has the ten-dency to replace asphalt coating the aggregate, and socause stripping.

5. Conclusions

This study evaluated the effect of aggregate type on thehot-mix asphalt stripping and static creep behavior. On thebasis of the results, the following conclusions can bedrawn:

1. The HMA stripping resistance was found to be signifi-cantly effected by the type of aggregate used in preparingthe mix. Unconditioned HMA asphalt prepared usinglimestone showed better stripping resistance than thatprepared using basalt aggregate. This trend was reversedas the HMA was exposed to conditioning.

2. Aggregate gradations was found to have very strongeffect on stripping resistance. HMA prepared usingaggregate gradation followed upper limit of ASTMspecification for dense gradation showed the highestresistance to stripping, followed by HMA preparedusing aggregate gradation followed mid limits of ASTMspecification for dense gradation. HMA prepared usingaggregate gradation followed mid limits of ASTM spec-ification for open graded aggregate gradation showedthe least stripping resistance.

3. The Absorbed asphalt found to have the capability ofreflecting the effect of aggregate type and gradation,and type of asphalt on the HMA stripping resistance.

Asphalt 80/100 Water Limestone Basalt

22.4 72.8 66.3 41.2– 6.76 4.88 0.24– 14.44 11.63 2.84– – 62.03 53.24


The percent of absorbed asphalt was found to have astrong reverse relationship with HMA strippingresistance.

4. Adhesion work was found to have the capability ofreflecting the effect of aggregate type and gradation,and type of asphalt on stripping resistance. However itwas not able to detect the effect of the type of asphaltused in preparing the HMA on stripping resistance.

6. Recommendations

6.1. Recommendations for practical applications

Based on the results of this study, the following pointscan be recommended for practical use of these results:

1. Adhesion work between asphalt and aggregate shouldbe used to select the best combination of asphalt andaggregate for the purpose of preparing HMA.

2. When both limestone and basalt aggregate are available,it is recommended to use limestone in preparing HMA.This is especially true if the mix, in the field will beexposed to long periods of wetting and/or freezing/thawing cycles.

3. In arid areas, it is recommend to use basalt in preparingHMA, especially for roads expected to be exposed toheavy axle loads.

4. Aggregate with higher absorption, and having all thesame other properties, is recommended to be used inHMA to improve stripping resistance.

6.2. Recommendations for further studies

Based on the findings of this research the following rec-ommendation are suggested:

1. The effect of aggregate type on hot-mix asphalt stability,flow, fatigue, dynamic creep and other properties needsto be evaluated.

2. The effect of other types of asphalt, aggregate, andaggregate gradation should be evaluated.

References

[1] Abo-Qudais SA. The effects of environmental damage evaluationtechniques on the prediction of environmental damage in asphaltmixtures. Building and Environment Journal, UK [submitted].

[2] Abo-Qudais SA, Al-Shweily H. Effect of antistripping additives onenvironmental damage of bituminous mixtures. Building and Envi-ronment Journal, UK [accepted].

[3] Aklons JJ, Macknight WJ. Introduction to polymer viscoelastic-ity. New York, NY: Wiley; 1972. p. 249.

[4] Al-Kofahy NA. Development of systematic laboratory testing pro-cedure to predict stripping of asphalt concrete mixtures, Ph.D thesis,Civil Engineering, Jordan University of Science and Technology,Irbid, Jordan; 2003. p. 254.

[5] Brown ER, Bassett CE. Effect of maximum aggregate size on ruttingpotential and other properties of asphalt aggregate mixtures, Trans-portation Research Record, 1259, TRB, National Research Council,Washington (DC); 1990. p. 107–119.

[6] Braik O. Stripping of asphalt mixtures and the effectiveness ofantistripping additives, Master thesis, Civil Engineering Department,Jordan University of Science and Technology, Irbid, Jordan; 1987. p.86.

[7] Eriksen K. Microscopical analysis of asphalt–aggregate mixturesrelated to pavement performance, Report No. 245, Danish RoadInstitute, Denmark; 1993. p. 17.

[8] Guirgus HR, Daoud OEK, Hamdani SK. Asphalt concrete mixturemade with cement-coated aggregate, Transportation ResearchRecord Report No. 843, Transportation Research Board, Washing-ton, DC; 1982. p. 80–85.

[9] Maupin Jr GW. Implementation of stripping test for asphalticconcrete, Virginia Highway and Transportation Council, Charlottes-ville, Transportation Research Record Report No. 712, Transporta-tion Research Board, Washington, DC; 1979. p. 8–12.

[10] Maupin Jr GW. Follow-up field investigation of the effective-ness of anti-stripping additives in virginia, Project Report No.9398-010-940, Virginia Transportation Research Council, VA;1997. p. 22.

[11] Obaidat AF. Evaluation of bituminous mixtures stripping usingconventional and image processing techniques, Master thesis, CivilEngineering Department, Jordan University of Science and Technol-ogy, Irbid, Jordan; 1999. p. 115.

[12] Roberts FL, Kandhal PS, Brown ER, Lee Dah-Yinn, Kenedy TW.Hot mix asphalt materials, mixture design, and construc-tion. MD: NAPA Education Foundation; 1991. p. 490.

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Effect of aggregate properties on asphalt mixtures stripping and creep behavior