Construction and Building Materials 28 (2012) 525–530

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Construction and Building Materials

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Investigation into causes of in-place rutting in asphalt pavement

Tao Xu ⇑, Xiaoming HuangSchool of Transportation, Southeast University, Nanjing 210096, Jiangsu, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 July 2011Received in revised form 13 August 2011Accepted 1 September 2011Available online 6 December 2011

Keywords:Asphalt pavementRuttingAir voidsAggregate gradationAsphalt content

0950-0618/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2011.09.007

⇑ Corresponding author. Address: School of TranspoSipailou 2#, Nanjing 210096, Jiangsu, China. Tel.: +88379 5184.

E-mail address: seuxt@hotmail.com (T. Xu).

The field measurement and laboratory tests were conducted to investigate potential causes of in-placerutting. The results indicate that the major rutting is attributed to the decrease in thickness of middleand lower layer, and the driving lane shows a severer rutting. Inadequate compaction is a major causefor the final rutting depth. Also the aggregate gradation has a major contribution to rutting. The gradationbetween 1.18 and 4.75 mm in sieve size becomes finer for the three layers. The upper and middle layershow an increase in asphalt content, but the lower layer presents a decrease in asphalt content.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Asphalt mixture is a heterogeneous complex composite mate-rial of air, binder, and aggregates used in modern pavement con-struction [1]. More than 95% of the paved roads in China areasphalt pavement. Asphalt pavement is subjected to many dis-tresses during their service life. One of the main distresses is rut-ting. Recently, rutting in asphalt pavements has become one ofthe major distress forms with the increase in traffic volume, tirepressure and axial load. It often happens within the first few yearsafter opening to traffic [2].

Rutting is the accumulation of permanent deformation in thepaving layers. It is caused by a combination of densification andshear deformation, appearing as longitudinal depressions in thewheel paths and small upheavals to the sides [3]. Rutting has a sig-nificant impact on the performance of asphalt pavement, in partic-ularly the ruts trap water to cause hydroplaning [4]. Moreover,significant rutting can lead to major structural failures [5]. Hence,the rutting not only reduces the service life, but it may also affectthe safety of highway users.

For instance, asphalt pavement in China needs frequent mainte-nance to ensure the pavement meeting the structural and functionalrequirements. This induces high maintenance cost. Therefore, it isimportant to make efforts to minimize rutting, and it is necessaryto investigate the causes that lead to rutting in asphalt pavement.

ll rights reserved.

rtation, Southeast University,6 25 8379 1654; fax: +86 25

In the mechanistic-empirical pavement design system, rutting is ac-cepted as one of the primary design criteria for asphalt pavement [6].

It is well known that variations in material characteristics are amajor factor to influence the rutting resistance of asphalt mixtures[7]. Many researchers have conducted a lot of work on it [2,4]. Atthe same time, different test methods were utilized to estimatethe rutting performance of asphalt pavement. In recent years, vis-co-elasticity theory, especially linear visco-elasticity models, havebeen developed to predict the rutting in asphalt pavements [8].However, up to now it is difficult for established theory modelsto predict accurately the depth and propagation of rutting in as-phalt pavement [9], since those models are based on some specificconditions.

Hence, some in situ tests were performed to study field perfor-mance of asphalt mixtures including rutting, bleeding, crack, age-ing, temperature variation, etc. Because of the potential ruttingdistresses, there is a large effort to determine a value or range ofoptimal desired in-place density [10,11].

In China, a great number of asphalt pavements on the express-way have been constructed in the past 20 years. Meanwhile, thetraffic volume has also increased dramatically, in particular trucktraffic. As a result, rutting has increasingly become one of the mainconcerns for highway agencies nationwide. Furthermore, shortlyafter opening to traffic, severe rutting occurs in many newly con-structed asphalt pavements. The complexity of the rutting devel-opment is due to many influential factors, such as binder type,asphalt content, mixture type, load level, temperature, and initialcompacted density. Therefore, rutting resistance of the paving as-phalt mixture is one of the important considerations in standardprocedures for asphalt mix design [12].

4cm SMA-13 Base asphalt binder

6cm AC-20 Base asphalt binder

7cm AC-25 Base asphalt binder

34cm 5% cement-stabilized aggregate base course

20cm 10% lime-flyash soil subbase course

Fig. 1. Schematic diagram of pavement structures in selected expressway.

526 T. Xu, X. Huang / Construction and Building Materials 28 (2012) 525–530

However, few efforts were made to investigate directly the rela-tionship between rutting and property changes of asphalt mixtureon an in-service pavement. The objective of the present study is toinvestigate potential causes of in-place rutting in asphalt pave-ment by identifying the contributions of measured changes inphysical properties of asphalt mixtures. This may provide somesuggestions for asphalt mixture design to improve the rutting dis-tress and prolonging the service life of pavement. To this end, weconducted an in situ investigation by field measurement of ruttingdepth and laboratory tests on core specimens extracted from thesouthern section of the Ningsuxu expressway. It is one of the pri-mary trunk highways in Jiangsu province, China. The pavementstructure in the selected expressway is shown in Fig. 1. It is impor-tant to note that this pavement structure was selected for researchbecause it is one of the most commonly utilized pavement struc-tures for highways in China. After obtaining the core samples, var-ious laboratory tests were conducted on the core samples toidentify the performance change of asphalt mixtures.

Fig. 2. Photo of (a) rutting on the selected pavement and (b) measuring method ofrutting depth.

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1 6 11 16 21 26 31 36

Cross-section numbering

Rut

ting

dept

h (m

m)

Passing lane Driving lane

Fig. 3. Testing results of rutting depth in driving and passing lane.

2. Field measurement

2.1. Measurement plan

After opening to traffic for approximately 7 years, rutting havebecome a very serious issue on the selected expressway (Fig. 2a).Taking into account the severity level of rutting, we selected a typicalpavement segments for more detailed investigations. Therefore, atesting plan was devised to investigate causes of the rutting forma-tion, including field measurements and laboratory tests.

The former was rut depth measurement. We measured the rut-ting depths at the interval of 10 m. Thus the 36 testing results ofrutting depth in the typical pavement cross-sections were ob-tained. The rutting depths were measured using 3 m elevatedstraight edge and displacement transducer (Fig. 2b). The 3 m ele-vated straight edge is used to establish a horizontal reference line.The laboratory tests included each layer thickness, bulk specificdensity, maximum theoretical specific density, air void content,aggregate gradation, and asphalt content, etc.

2.2. Rutting depth

The rutting depths of driving and passing lane in the selectedtypical pavement segment are shown in Fig. 3.

From Fig. 3, severe rutting occurs in the selected asphalt pave-ment section after opening to traffic for approximately 7 years. Theaverage depth is 9.2 mm and 11.1 mm for passing and driving lane,respectively. Moreover, the maximum depth approaches 15 mm.This is because the plastic shear movement of the asphalt mixturesand further compaction under the combined effects of repeatedaxle load and continued hot weather in this area [2].

Additionally, results of field measurement show there are loca-tions where the material had shoved outward adjacent to the ridgyband and shoulder (see Fig. 2a). The lateral plastic flow of the asphaltmixtures is an important reason for the rutting. The use of excessiveasphalt binder in the mixture causes the loss of internal friction

between the aggregate particles. Also, a lack of angularity of theaggregates and an insufficient surface texture can facilitate the plas-tic flow of asphalt mixture due to the decrease in inter-particle fric-tion [13].

On the other hand, the densification in wheel paths is the fur-ther compaction of asphalt mixtures owing to repeated trafficloads. When compaction is inadequate during construction, thechannelized traffic provides a repeated kneading action in thewheel paths and completes the consolidation. Then a substantialamount of rutting may occur [4]. The aforementioned plastic flowcan be minimized by using large size aggregates, angular andrough textured coarse and fine aggregate, stiffer binders, as wellas by providing suitable compaction during construction. However,because the distribution of repeated traffic volume is different inpassing and driving lane, the rutting depth is various in the twolanes. The rutting depths in passing lane focus on the range be-tween 5 and 10 mm, while the more rutting depths in driving lane

T. Xu, X. Huang / Construction and Building Materials 28 (2012) 525–530 527

is in the range between 10 and 15 mm. This suggests the rutting indriving lane is severer than that in passing lane, since the drivinglane is subjected a greater traffic volume.

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hick

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Upper Middle Lower

3. Experimental

3.1. Core specimen preparation

To better understand the causes that leaded to rutting, and identify the contribu-tion of the mixture properties in each asphalt layer to the total rutting depth, somefull-depth core samples (150 mm in diameter) were extracted. These samples wereextracted from wheel paths at the same cross-section in passing and driving lane,ridgy band between wheel paths, and pavement shoulder, respectively. Six typicalcross-sections with various rutting depth were selected to extract full-depth coresamples for laboratory tests. For all the six cross-sections, 120 full-depth core sam-ples were extracted. Then each full-depth core sample was separated into three spec-imens from the two tack coats, designating as upper, middle and lower layer from topto down, respectively. For each layer, several groups of specimens were prepared forfurther testing. Each group consisted of at least three effective specimens.

3.2. Layer thickness test

The layer thickness can reflect intuitively the rutting depth and rutting distribu-tion in each pavement layer. According to previous study [14], the total pavementrutting in this type of typical pavement structure derived solely from the three as-phalt layers on the semi-rigid base course. Therefore, we tested only the thicknessof the three asphalt mixture layers.

3.3. Density test

Asphalt mixture density varies throughout the life of the pavement owing toaxle loads. Hence, it is important to better understand how sensitive asphalt mix-tures are to rutting variation. First the apparent density of full-depth core samplewas measured to find the correlation between apparent density and rutting depth.Then the bulk and theoretical maximum specific gravity of core specimen for eachlayer were tested to estimate the variation of the air voids, identifying the relation-ship between the specific gravity or density and rutting depth. One of the core spec-imens in each sampling locations was reheated to about 135 �C and broken-up fortesting the theoretical maximum specific gravity and extraction experiments. Thespecific gravity tests were conducted in accordance with ASTM D22726 and ASTMD2041, respectively.

3.4. Air void test

For the evaluation of the rutting, it is also necessary to identify the change of theair voids in pavement asphalt layers. Because rutting in asphalt pavement was dueto the further compaction of asphalt mixtures, the air voids must stay high enoughto prevent permeability of water and air, and low enough after a few years to pre-vent plastic flow. Therefore, another analysis focused on the changes in air voids ofthe three layers after opening traffic for about 7 years. Also, the test results forwheel paths were compared to other locations to identify differences that could ex-plain the observed rutting distress.

3.5. Extraction test

To analyze the changes in asphalt content and aggregation gradation of asphaltmixture, extraction tests were conducted. The asphalt mixtures were obtained fromcore specimens of the three layers, respectively. Furthermore, the core specimenswere extracted from various locations at the same cross-section of the pavement.Three specimens in each sampling location were tested to evaluate changes in as-phalt content and aggregation gradation. The asphalt content of the samples wasdetermined according to ASTM D 2172 and the aggregation gradation was deter-mined according to ASTM C117/C136, which has been the most widely appliedfor designing and controlling paving mixtures.

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Passing lane Driving lane Ridgy band Shoulder Design value

Sampling locations

Ave

rage

spe

cim

en t

Fig. 4. The thickness changes in each layer.

4. Results and discussion

4.1. Thickness change in each layer

To identify directly the contribution of thickness changes ineach layer to total rutting depth, the thickness were measuredfor the core specimens that were extracted, for example, at thecross-section with rutting depth of 10 mm. Hereafter, the corespecimens used to test other properties in this study are extracted

from the same rutting depth. The testing results of thickness foreach layer are shown in Fig. 4.

From Fig. 4, the three pavement layers exhibit different changesin thickness after opening to traffic for about 7 years. For the upperlayer, there is a slight decrease in thickness of core specimens ex-tracted from wheel paths as compared with the design value. How-ever, the core specimens extracted from shoulder and ridgy bandpresent an increase in thickness. The reason for this is that thethickness decrease at wheel paths is due to the further densifica-tion and shear deformation under the combined effects of repeatedaxel loads and high temperature. Additionally, SMA-13 with a bet-ter skeleton structure facilitates the upper layer to increase its rut-ting resistance, although the nonmodified asphalt binder is used inthis layer. At the same time, the shear deformation causes the as-phalt mixture to shove at high temperature, leading to the increasein layer thickness at the ridgy band and shoulder.

For the middle and lower layer, a larger permanent deformationoccurs in all core specimens regardless of sampling locations. Thedecrease in thickness at driving and passing lane is greater dueto more axle loads than that at other locations. Another importantreason for this is that the nonmodified asphalt binder was used inthe two layers. Under heavy axel loads and continued hot weather,the two layers are easier to deform permanently. This also agreeswith the results obtained from a theoretic analysis using the elasticlayer system. The calculating results indicate that the middle issubjected a greatest compression and shear deformation [14].Meanwhile, the thickness at pavement ridgy band and shouldershows a smaller reduction because of less axle loads. Moreover,the asphalt mixture shoving to two sides of wheel paths at hightemperature also offsets the decrease in thickness of the middleand lower layer.

4.2. Test results of apparent density

Based on the testing results, a regression analysis is performedon the apparent density and rutting depth in order to show the var-iation trends of the rutting depth because of changes in density le-vel. Fig. 5 presents the relationship between apparent density ofthe studied full-depth core samples and rutting depth measuredat passing and driving lane, respectively.

As shown in Fig. 5, a linear trend exists between the permanentdeformation and apparent density measured from full-depth coresamples. It is observed that the higher the apparent density inthe wheel paths, the larger the rutting depth. Additionally, thechange trend at passing lane is slower than that at driving lane ow-ing to more traffic volume on the latter. Another important reasonis the smaller compaction density during construction. It leads tothe further densification in asphalt mixtures after opening to

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2.470 2.480 2.490 2.500 2.510 2.520 2.530 2.540

2.480 2.490 2.500 2.510 2.520 2.530 2.540 2.550

Apparent density (g/cm3)

Rut

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h (m

m)

(b)

Fig. 5. Relationship between apparent density and rutting depth at (a) passing and(b) driving lane.

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Sampling locations

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void

s (%

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Upper Middle Lower

Fig. 6. Variations of air voids of core specimens for each layer.

528 T. Xu, X. Huang / Construction and Building Materials 28 (2012) 525–530

traffic. Subsequently, rutting depth increases in asphalt pavement.The higher density also increases the likelihood of shoving andbleeding [11]. The variations in layer thicknesses measured fromthe core samples extracted at wheel paths also confirm theabove-mentioned observation.

Additionally, because nonmodified asphalt binder is used in theselected pavement segment, the binder acts as a lubricant betweenthe aggregates and reduces point-to-point contact pressures in hotweather [4]. This also causes the plastic movement in asphalt mix-tures, resulting in rutting.

4.3. Variation of air voids

The air void content is an indicator of the permanent deforma-tion behavior of asphalt pavement. To evaluate the contribution offurther compaction to rutting depth, the bulk specific gravity andair voids of core specimens extracted from various sampling loca-tions were tested. The testing results of air voids are provided inFig. 6.

From Fig. 6, the air voids of the core specimens in each layer ex-tracted from wheel paths are lower than that from ridgy band andshoulder. Furthermore, the air voids at driving lane is less than thatat passing lane. This is because the wheel paths experienced addi-tional compaction under repeated traffic loads, and in particularlyat the driving lane.

As shown in Fig. 6, it is noted that some air voids in the threelayers are below 3%. The excessive decrease in in-place air voidsis closely related with plastic deformation of the pavement, whichincreases the likelihood of shoving and rutting. It is well knownthat field air voids in the range 3–6% are desirable for pavementperformance [15,16]. Therefore, the plastic deformation is ex-pected to increase significantly at so low air voids level, resultingin rutting. Previous research efforts have shown that the in-place

air voids should never fall below approximately 3% during the ser-vice life of the asphalt pavement [11]. When the air voids drops be-low 3%, the binder acts as a lubricant between the aggregates andreduces point-to-point contact pressures [4]. This causes perma-nent deformation changes either in volume or in shear, whichmostly occurs in hotter weather or because of heavy axle loads[17]. Additionally, it is interesting to note that core specimen ex-tracted from shoulder also exhibits a decrease in air voids,although there are few traffic loads on it. Maybe this is becausesome impurities on the pavement permeate into the mixtures,and they are absorbed into internal structures of the mixtures,allowing the measured air voids to decrease.

4.4. Aggregate gradation changes

The influence of aggregate gradation of asphalt mixtures on therutting was evaluated by a series of extraction tests. The changes inaggregate gradation for the three layers are presented in Tables 1–3, respectively. The limit values are according to Technical Specifi-cation for Construction of Highway Asphalt Pavement (JTG F40-2004) in China.

As given in Tables 1–3, the aggregate gradations of asphalt mix-tures show similar change trends, regardless of sampling locations.Meanwhile, the gradations become significantly finer and ap-proach or exceed the upper limits, because the rutting in asphaltpavement is also influenced by the properties of aggregates [4].

Table 1 shows that the variations in aggregate gradations ofupper layer. It is observed that the gradations of various samplinglocations vary significantly. Furthermore, the passing ratios insome sieve sizes are beyond the upper limit, indicating the aggre-gate gradations of upper layer become considerably finer. The rea-sons for these are that the asphalt mixture of SMA13 type has abetter skeleton structure between the coarse aggregates bypoint-to-point contact. This contact is prone to be crushed underaxle loads. Particularly, when the nonmodified asphalt becomesvery soft like a lubricant in the mixtures at high temperature, theupper layer is undergone directly heavy axle loads. This more eas-ily causes the aggregate gradation to become finer.

For the middle layer, the changes in aggregate gradations are gi-ven in Table 2. AC20 belongs to a dense gradation. The gradationchange is not as obvious as that of SMA13. The passing ratios be-tween 1.18 and 4.75 mm in sieve sizes approach the upper limits.This indicates that some aggregate are also crushed under axleloads, causing the aggregate gradation much finer. The finer grada-tions directly result in the densification of asphalt mixtures. This isan important reason for rutting formation, in particular at earlystage after opening to traffic.

Table 3 provides the extraction results of the mixtures from thelower layer. It is observed that the passing ratios between the sievesizes of 1.18 and 9.5 mm approach or exceed the upper limits. The

Table 1Passing ratio (%) of various sampling locations for upper layer.

Sampling locations Sieve size (mm)

16 13.2 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075

Passing lane 100 93.8 75.9 39.2 29.3 24.3 20.3 17.4 14.5 11.8Ridgy band 100 93.6 75.3 38.9 27.8 22.7 18.3 15.3 13.8 11.8Driving lane 100 94.2 75.7 38.4 27.7 24.7 19.8 16.3 14.0 11.4Shoulder 100 93.5 75.2 38.7 27.7 22.7 18.6 15.1 13.2 10.1

Lower limit 100 90.0 50.0 20.0 15.0 14.0 12.0 10.0 9.0 8.0Upper limit 100 100 75.0 34.0 26.0 24.0 20.0 16.0 15.0 12.0

Table 2Passing ratio (%) of various sampling locations for middle layer.

Sampling locations Sieve size (mm)

26.5 19 16 13.2 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075

Passing lane 100 97.3 86.9 74.5 63.6 51.1 42.6 32.2 20.7 13.2 10.2 6.2Ridgy band 100 96.8 85.4 75.2 63.9 52.6 43.7 30.7 18.8 15.0 11.7 6.7Driving lane 100 97.5 87.3 75.9 65.5 52.4 41.2 32.0 20.0 14.1 11.5 6.3Shoulder 100 97.3 87.2 75.3 67.4 55.2 42.5 33.6 20.0 13.4 11.1 6.0

Lower limit 100 90.0 78.0 62.0 50.0 26.0 16.0 12.0 8.0 5.0 4.0 3.0Upper limit 100 100 92.0 80.0 72.0 56.0 44.0 33.0 24.0 17.0 13.0 7.0

Table 3Passing ratio (%) of various sampling locations for lower layer.

Sampling locations Sieve size (mm)

31.5 26.5 19 16 13.2 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075

Passing lane 100 100 88.1 78.4 71.5 64.9 52.9 43.8 33.0 21.6 13.7 10.1 5.6Ridgy band 100 98.8 88.6 77.7 71.5 63.6 53.7 43.8 32.1 21.4 15.1 10.8 5.2Driving lane 100 98.9 88.8 79.7 73.4 65.5 54.1 45.2 32.3 20.9 15.1 10.7 5.9Shoulder 100 100 90.1 79.2 73.3 64.3 53.4 45.3 31.9 21.8 15.9 11.0 5.8

Lower limit 100 90.0 75.0 65.0 57.0 45.0 24.0 16.0 12.0 8.0 5.0 4.0 3.0Upper limit 100 100 90.0 83.0 76.0 65.0 52.0 42.0 33.0 24.0 17.0 13.0 7.0

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T. Xu, X. Huang / Construction and Building Materials 28 (2012) 525–530 529

minor change of the gradation in coarse part may cause rutting se-vere problem in asphalt pavement [16]. Further, the passing ratiosat the sieve size of 2.36 and 4.75 mm are beyond the upper limits.This suggests that the aggregate gradations in lower layer becomefiner. All these directly lead to rutting in asphalt pavement.

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Asp

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Fig. 7. Comparison of asphalt content of each sampling location in selected layer.

4.5. Asphalt content change

The comparison of asphalt content of each sampling location inthe selected layer is illustrated in Fig. 7.

Fig. 7 shows a comparison of asphalt content at various sam-pling locations in each pavement layer. The asphalt contents ofeach location in the selected layer are slightly different. For theupper and middle layer, the asphalt contents at wheel paths andridgy band show greater values as compared with that at shoulder.However, for the lower layer, there is a decrease in the asphalt con-tent at wheel paths as compared with that at shoulder. This is be-cause when the asphalt binder expands at high temperature, andthe air voids in asphalt mixture cannot accommodate it, the as-phalt migrated from down to top under the combined effects of re-peated traffic loads and hot weather [16]. This migration canhappen by the diffusion of asphalt cement into the air voids whenit is subjected to temperature exceeding its softening point andthen by the movement of asphalt due to the pressure gradientdeveloped in asphalt cement. The development of this pressuregradient is due to the reduction of air voids space spatially andtemporally in asphalt mixture from its initial virgin state [18]. Soit results in an accumulation of asphalt cement in upper and mid-dle layer, allowing the measured asphalt content to increase.

Additionally, in hot weather, the asphalt binder behaves likeviscous liquids and tends to migrate easily due to the natural de-crease in its viscosity [19]. Once the hot asphalt binder starts flow-ing, it does not return to their original position [4]. This is whysome asphalt pavements show bleeding under repeated trafficloads and wheel path rut occurs at high temperature.

5. Conclusion

Based on the results of field measurement and laboratory tests,major conclusions can be drawn.

530 T. Xu, X. Huang / Construction and Building Materials 28 (2012) 525–530

(1) The major rutting is attributed to the decrease in thicknessof middle and lower layer on the selected typical pavementstructure, and the driving lane presents a severer rutting. Butthe upper layer shows better rutting resistance. It is recom-mended that the modified asphalt binder should be used atleast in middle layer to increase rutting resistance of asphaltpavement.

(2) Inadequate compaction during construction is responsiblefor a major portion of the final rutting depth measured.There is a significant decrease in air voids for the three layersdue to the combined effects of repeated axle loads and hightemperature. This directly causes the further densification ofasphalt mixtures, leading to rutting on the pavement.

(3) The aggregate gradation has a major contribution to rutting.The gradation between 1.18 and 4.75 mm in sieve sizebecomes finer for the three layers. Selecting an aggregatewith high strength is another important consideration forrutting resistance of asphalt pavement.

(4) The upper and middle layer show an increase in asphalt con-tent due to the combined effects of repeated traffic loadingand hot weather. But there is a decrease in asphalt contentin the lower layer owing to migrating of asphalt binder fromdown to top.

Acknowledgment

This study is funded by the Department of Transportation,Jiangsu Province, China (No. 08Y038).

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