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Effects of coarse aggregate angularity and asphaltbinder on laboratory-measured permanent deformationproperties of HMABaoshan Huang a , Xingwei Chen b , Xiang Shu a , Eyad Masad c & Enad Mahmoud da Department of Civil and Environmental Engineering , University of Tennessee , Knoxville,TN, USAb Louisiana Transportation Research Center, Louisiana State University , Baton Rouge, LA,USAc Department of Civil Engineering , Texas A&M University , College Station, TX, USAd Zachry Department of Civil Engineering , Texas A&M University, Texas TransportationInstitute , College Station, TX, USAPublished online: 22 Dec 2008.

To cite this article: Baoshan Huang , Xingwei Chen , Xiang Shu , Eyad Masad & Enad Mahmoud (2009) Effects of coarseaggregate angularity and asphalt binder on laboratory-measured permanent deformation properties of HMA, InternationalJournal of Pavement Engineering, 10:1, 19-28, DOI: 10.1080/10298430802068915

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Effects of coarse aggregate angularity and asphalt binder on laboratory-measured permanentdeformation properties of HMA

Baoshan Huanga*, Xingwei Chenb1, Xiang Shua2, Eyad Masadc3 and Enad Mahmoudd4

aDepartment of Civil and Environmental Engineering, University of Tennessee, Knoxville, TN, USA; bLouisiana Transportation Research

Center, Louisiana State University, Baton Rouge, LA, USA; cDepartment of Civil Engineering, Texas A&M University, College Station,

TX, USA; dZachry Department of Civil Engineering, Texas A&M University, Texas Transportation Institute, College Station, TX, USA

(Received 7 December 2007; final version received 10 February 2008 )

Rutting has been identified as one of the primary distresses in asphalt pavements. Rutting in hot-mix asphalt (HMA)

mixtures can be attributed to either the lack of interlocking of aggregate structure or insufficient bonding between aggregate

and asphalt binder, or both. In the present study, efforts have been made to identify the contributions of aggregate structure

and asphalt binder to the rutting characteristics of a dense-graded surface HMA mixture. Coarse gravels at five different

angularity levels (100, 85, 70, 50 and 35% of aggregate with two or more fractured surfaces) were used to produce mixtures

with similar aggregate gradations. Three different asphalt binders (PG 64-22, PG 76-22 and PG 82-22) were used to make

mixtures for laboratory rut evaluations. The aggregate imaging system (AIMS), uncompacted voids in coarse aggregate

(VCA) and tri-axial shear tests were conducted to evaluate the coarse aggregate angularity (CAA). The US Army Corps of

Engineers’ gyratory testing machine (GTM), creep and the asphalt pavement analyser (APA) tests were selected to

characterise the rut resistance of asphalt mixtures.

The results from this study indicated that coarse aggregate AIMS, VCA and tri-axial tests were related to the CAA and

laboratory-measured rutting indices. At temperatures close to the binder’s upper grade limit, aggregate structures played a

critical role in the rut resistance of HMAmixtures; whereas, at temperatures below the binder’s upper grade limit, the stiffness

of the asphalt binder played a more important role in the rut resistance of asphalt mixtures evaluated in this study.

Keywords: CAA; asphalt binder; permanent deformation; asphalt concrete

1. Introduction

Rutting is one of the primary distresses in hot-mix asphalt

(HMA) pavements. In a well-designed HMA mixture,

aggregate should be well-proportioned to develop a stable

skeleton and provide enough resistance against shearing

load. An optimised gradation usually ensures enough

contacts between coarse and fine aggregates. In addition,

the shapes and surface textures of aggregate (both coarse

and fine) intimately influence the shear resistance of

aggregate structure. HMAmixtures containing angular and

rough aggregates have been believed to be more rut-

resistant (Meier and Elnicky 1989, Brown et al. 1992,

Fletcher et al. 2002, Prowell et al. 2005). During the

development of the Superpave mix design procedure, the

Strategic Highway Research Program researchers estab-

lished the minimum coarse aggregate angularity (CAA)

requirements for asphalt mixtures according to the traffic

levels and proximities of the mixtures to the pavement

surface. The results from the National Cooperative

Highway Research Program 9–35 report also indicated

that increased coarse aggregate fractured faces would

increase rutting resistance (Prowell et al. 2005).

Image analysis-based aggregate morphology assess-

ment has been the focus of recent research efforts to

successfully link imaging-based indices to results from

manual testing methods as well as to pavement response

and performance (Masad and Button 2000, Tutumluer

et al. 2000, Masad 2003). Shape properties of aggregate

samples measured by image analysis have been

associated to laboratory strength data and field rutting

performances of HMA (Rao et al. 2001, 2002, Masad

et al. 2005, Pan et al. 2005).

Previous researches have studied the effect of coarse

aggregate morphology on the performance of HMA

(Huang and Ebrahimzadeh 1972, Kuo and Freeman 1998,

Chen et al. 2001, Fletcher et al. 2002, Pan et al. 2005,

2006). Shu et al. (2006) investigated the characterisation

methods of CAA of gravel by direct shear test and

resilient modulus test, and related CAA level to the rutting

performance of HMA by asphalt pavement analyser

(APA) test. It appears that coarse aggregate direct shear

ISSN 1029-8436 print/ISSN 1477-268X online

q 2009 Taylor & Francis

DOI: 10.1080/10298430802068915

http://www.informaworld.com

*Corresponding author. Email: bhuang@utk.edu

International Journal of Pavement Engineering

Vol. 10, No. 1, February 2009, 19–28

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test and resilient modulus are not effective in the

evaluation of CAA.

2. Objective

In order to better specify the crushed gravels that are

widely used for HMA mixtures in Tennessee, research has

been conducted to identify the contributions of aggregate

structures and asphalt binder to the rutting characteristics

of a dense-graded surface HMA mixture, and to correlate

coarse aggregate tests (such as the aggregate imaging

system (AIMS), uncompacted voids in coarse aggregate

(VCA) and tri-axial tests) with HMA rutting performance

tests (such as gyratory testing machine (GTM), creep and

APA tests).

Coarse gravels at five different angularity levels and

three different asphalt binders were used to make mixtures

for laboratory rut evaluation.

3. Laboratory experiment

A laboratory experiment was conducted to characterise

the rut resistance of a dense-graded surface HMA

mixture. Coarse gravels at five different angularity levels

(100, 85, 70, 50 and 35% of aggregate with two or more

fractured surfaces) were used to produce mixtures with

similar aggregate gradations. Three different asphalt

binders (PG 64-22, PG 76-22 and PG 82-22) were used

to make mixtures for laboratory rut evaluations.

The aggregate imaging system (AIMS), VCA and Tri-

axial shear test were considered to evaluate the CAA. The

US Army Corps of Engineers’ GTM was employed to

evaluate the aggregate structures. Creep and APA tests

were selected to characterise the overall rut resistance

of asphalt mixtures. A list of the laboratory experiment

plan is presented in Table 1.

3.1 Materials

The coarse aggregates used in this study were gravel with

nominal maximum size of 12.5mm. The fine aggregates

consisted of no. 10 screenings and clean natural sand. The

gradation of the blended aggregates is shown in Figure 1.

Three types of asphalt binder, a conventional PG64-22,

two SBS polymer modified PG 76-22 and PG 82-22 were

used in this study. Their properties meet the Superpave PG

binder requirements.

3.2 Aggregate imaging test

The coarse aggregate characteristics (sieve #4-sieve 5/8 in.)

were evaluated using image analysis techniques through the

AIMS system (Figure 2; Masad 2003). AIMS is a computer

automated system that includes a lighting table where

Table 1. Test factorials.

Material Coarse aggregate test Mixture performance test

Aggregate CAA (%) AIMS VCA Tri-axial test Asphalt binder Asphalt content (%) GTM test Creep test APA test

35 Yes Yes Yes PG 64-22 5.8 Yes Yes YesPG 76-22 5.8 / Yes YesPG 82-22 5.8 / Yes Yes

50 Yes Yes Yes PG 64-22 5.8 Yes Yes YesPG 76-22 5.8 / Yes YesPG 82-22 5.8 / Yes Yes

Gravel 70 Yes Yes Yes PG 64-22 5.8 Yes Yes YesPG76-22 5.8 / Yes YesPG 82-22 5.8 / Yes Yes

85 Yes Yes Yes PG 64-22 5.8 Yes Yes YesPG 76-22 5.8 / Yes YesPG 82-22 5.8 / Yes Yes

100 Yes Yes Yes PG 64-22 5.8 Yes Yes YesPG 76-22 5.8 / Yes YesPG 82-22 5.8 / Yes Yes

Note: CAA is the coarse aggregate angularity; AIMS is the aggregate imaging system; VCA is the uncompacted void content of coarse aggregate test; GTM is the gyratorytesting machine; APA is the asphalt pavement analyser; ‘/’ indicates that testing was not conducted.

Figure 1. Aggregate gradation.

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aggregates are placed in order to measure their physical

characteristics (form, angularity and texture). It is equipped

with an autofocus microscope and a digital camera, and is

capable of analysing the characteristics of aggregate sizes

retained on sieve #100 (0.15mm sieve) up to aggregates

retained on 1 in. sieve (25.4mm).

AIMS evaluate aggregate characteristics in terms of

form, angularity and texture (Masad 2003). Many ways to

calculate the aggregate shape parameters have been

proposed by various researchers (Masad and Button 2000,

Tutumluer et al. 2000, Rao et al. 2001, 2002, Masad 2003,

Pan et al. 2005). This study focused on aggregate

angularity and it can be calculated as follows using the

gradient method (Masad 2003).

Angularity index ¼XN23

i

jui 2 uiþ3j ð1Þ

where N is the total number of points on the edge of the

particle with the subscript i denoting the ith point on the

edge of the particle.

3.3 Uncompacted voids in coarse aggregate test

The VCA are measured using a calibrated cylindrical

measure. The coarse aggregate is struck off, and its mass is

determined by weighing. VCA can be calculated as the

difference between the volume of the cylindrical measure

and the absolute volume of the coarse aggregate collected

in the measurement. VCA provides an indication of the

aggregate’s angularity, sphericity and surface texture

(AASHTO 2003).

3.4 Coarse aggregate tri-axial shear test

The coarse aggregate tri-axial test for coarse aggregate

was conducted in a tri-axial chamber. The coarse

aggregate was enwrapped with membrane, and moulded

with vacuum in a 100mm diameter and 200mm height

mould. The confining pressures were 69, 138 and 207 kPa.

The shearing rate was selected as 25.4mm/min. The

shearing force and vertical displacements were continu-

ously recorded while the aggregate material was sheared.

The Mohr–Coulomb shear strength of coarse aggre-

gate can be expressed as

s ¼ cþ s tanf ð2Þ

where s ¼ shear stress at failure; c ¼ cohesive strength;

s ¼ normal stress; and f ¼ angle of internal friction.

A higher friction angle value always indicates higher

shear strength, and the aggregate structure will have higher

rut-resistance.

3.5 HMA mixture design

Standard Marshall mix design procedure was employed to

design the asphalt mixture. The mixture satisfied the ‘411-

D’ mix as specified by the Tennessee Department of

Transportation (TDOT 2006). In order to compare the

performance of mixture with different CAA levels under

the same conditions as much as possible, the same asphalt

content of 5.8% was adopted as optimum asphalt content

for each CAA level. Besides, two other asphalt content

levels (4.9 and 4.0%) were also used in GTM test to

evaluate the effect of asphalt content on the aggregate

structure of HMA mixtures.

3.6 GTM test

The GTM has been used for decades as an engineering tool

for the design and characterisation of both bound and

unbound paving materials. The device was developed and

refined in the 1960s as a mechanisation of the original

Texas gyratory compactor, and is intended to serve as a

combination compaction and shear testing machine.

Samples are subjected to compaction effort using a

rotating head that must maintain a minimum angle via two

adjustable rollers positioned on opposite sides of the

cylindrical test specimen (Prowell 2003).

Three 150mm diameter samples of each mixture were

compacted under a 0.84MPa vertical ram pressure in the

GTM. The mixture samples were compacted at 1508C

with 185 revolutions. The following parameters from

GTM test were investigated in this study (Kuo and

Freeman 1998).

Gyratory stability index (GSI) is the ratio of the

maximum gyratory angle to the minimum gyratory

angle. For a rut-resistant mixture, the shear resistance

from the aggregate structure will prevent the tilting trend

from the ram head as the mixture is being compacted.

Figure 2. A picture of the AIMS system.

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Therefore, a lower GSI value always indicates higher

stability (or rut-resistance) for the mixture.

3.7 Creep test

The creep test was used to evaluate the rutting potential of

asphalt mixtures. This test is conducted by applying a

static load to an HMA specimen and measuring the

resulting total and permanent deformation with time.

In this study, the static unconfined creep test was

performed. The test temperature was 408C (close to the

pavement effective temperature), and applied pressure was

69 kPa. The static load was applied for 1 h, and the load

was removed and the sample was allowed to rebound for

another hour. The permanent strain is the total strain minus

the recoverable strain. The total strain is related to the

creep stiffness; whereas, the permanent strain is more

closely related to permanent deformation properties of

asphalt mixtures.

Cylindrical samples, 150mm in diameter and 170mm

in height were compacted by the superpave gyratory

compactor. A nominal 101.6mm diameter specimen was

cored from the centre of the gyratory specimens. The two

ends of the specimen were trimmed to get a 150mm height

specimen. Three samples were made for each mixture. The

air voids of 7% was used for this test.

3.8 APA test

In the present study, the 150-mmdiameter by 75-mmheight

laboratory compacted cylindrical specimens were tested, at

648C, 444.4N load and 0.7MPa hose pressure (as per

AASHTO TP63-03). The air voids for the APA test was

selected to be 7 ^ 1%. Six samples were made for each

mixture. Rut depth versus loading cycle curves and final rut

depth after 8000 cycles (16,000 passes) under dry

conditions were recorded. The wheel speed was approxi-

mately 0.6m/s.

4. Results and discussions

4.1 Aggregate tests

4.1.1 AIMS test

The coarse aggregate were composed of three aggregate

sizes; retained on 12.5, 9.5 and the 4.75mm sieve.

The ratio of weight of these three sizes of aggregate is

11:44.5:44.5. Each size was scanned separately using

AIMS. Table 2 provides the results (average) for

sphericity, texture and angularity.

A high angularity index indicates a higher aggregate

angularity, and a higher texture index means that the

aggregate has more texture. The values of texture index for

all the five aggregates are considered within the low range,

that is, the aggregates are not highly textured. As shown by

Fletcher et al. (2002), crushing of most gravel does not

improve texture which is consistent with the results in

Table 2. The angularity index shows a clear trend on an

increase in angularity with more crushing. The sphericity

indices for the five aggregates are similar. The analysis

will focus on the angularity index as it is the only property

that shows differences among the coarse aggregate

samples.

Figure 3 presents the relationship between angularity

index of coarse aggregate and CAA level. Generally, an

increase in CAA levels resulted in increased angularity

index.

4.1.2 VCA test

Figure 4 presents the VCA with different CAA levels.

Although not significant (VCA ranged from 43.3 to 44%),

the CAA had obvious effects on the VCA. Coarse

aggregate with higher CAA level has higher VCA. This

indicated the coarse aggregate with higher CAA level was

Table 2. AIMS test results of coarse aggregate.

Sample 12.5mm 9.5mm 4.75mm Average

Sphericity indexCAA – 35 0.647 0.694 0.688 0.676CAA – 50 0.699 0.720 0.694 0.704CAA – 70 – 0.718 0.660 0.689CAA – 85 0.672 0.723 0.676 0.690CAA – 100 0.666 0.742 0.651 0.686

Texture indexCAA – 35 72.0 86.1 68.6 75.6CAA – 50 60.8 82.6 72.4 71.9CAA – 70 78.4 71.9 70.6 73.7CAA – 85 82.6 90.5 71.6 81.6CAA – 100 64.7 76.0 72.7 71.1

Angularity indexCAA – 35 2547 2713 2658 2639CAA – 50 2483 2582 2669 2578CAA – 70 2875 2520 3080 2825CAA – 85 2855 2844 2954 2884CAA – 100 3122 2947 3312 3127

Figure 3. CAA level versus angularity index.

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easier to form interlock structure. The VCA of coarse

aggregate with 35 and 50% CAA exhibited lower VCA

than that of other coarse aggregates.

4.1.3 Coarse aggregate tri-axial test

Theoretically, the shear resistance of aggregate structure

should be reflected by the friction angle of compacted

aggregates. Since a previous study indicated a poor

relationship between the direct shear friction angle and

the HMA rutting property, efforts were made during the

present study to employ triaxial shear testing in order to

eliminate the effect of complex boundary conditions

existing in direct shear. Figure 5 presents the triaxial

friction angle of coarse aggregate with different CAA

levels. From the results, it appears that the friction angle

was not sensitive to CAA levels between 50 and 100%.

Coarse aggregate with higher CAA level generally had

higher friction angle value. The friction angle of coarse

aggregate with 35% CAA exhibited much lower friction

angle value than other coarse aggregates. The friction

angles of coarse aggregate with 50 and 70% were

relatively lower than that of coarse aggregate with 85

and 100%.

4.2 Mixture tests

4.2.1 GTM test

Figure 6 presents the GSI of HMA with different CAA

levels. From the results, it was obvious that CAA had

significant effects on the GSI. The GSI value generally

decreased with the increase in the CAA level, which

meant the stability of HMA mixtures would also be

increased with the increase in the angularity of coarse

aggregate. Usually, GSI values less than 1.2 are

considered to be rut resistant for HMA mixtures

(Mohammad et al. 1999). It was noted that all mixtures

(with different CAA) with 5.8% asphalt content had GSI

values higher than 1.2. This indicated that these mixtures

(although satisfying the current specification with other

Figure 4. VCA at different CAA levels.

Figure 5. Friction angle of coarse aggregate with different CAAlevels.

Figure 6. GSI of HMA mixtures from GTM.

Figure 7. Static unconfined creep test. (a) total strain vs. CAAand (b) permanent strain vs. CAA.

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criteria) might be ‘too wet’ if designed with gyratory

compactors.

4.2.2 Creep test

Figure 7 presents the results of the HMA creep test with

different CAA levels and different asphalt binders. From the

results, it is observed that both CAA and asphalt binder

property had significant effects on total strain and permanent

strain. For HMA with PG 64-22 binder, the strain generally

decreased with the increase in CAA level. HMA with 100%

crushed coarse aggregate only exhibited about half the strain

of HMA with 35% CAA coarse aggregate, which indicated

that increasing the CAA level of coarse aggregate would

result in the high resistance of HMA mixtures to rutting.

When a stiffer asphalt binder (PG76-22)was used, the results

showed a similar trend: the strain generally decreased with

the increase in the angularity of the coarse aggregate, while

the strain exhibited was lower than that of HMAwith PG 64-

22.When a much stiffer asphalt binder (PG 82-22) was used,

the total strain generally decreased with the increase in CAA

level. However, the permanent strain of HMA mixture with

different CAA levels did not show significant difference. All

the permanent strains were within the range of 150–300

micro strain.

4.2.3 APA rut test

Figure 8 presents the APA rut test results of HMA with

different CAA levels and different asphalt binders. From

Figure 8. APA rut depths of HMA mixtures.

Figure 9. Correlation between coarse aggregate AIMS angularity index and HMA rutting performance tests. (a) rut depth vs. angularityindex and (b) permanent strain vs. angularity index.

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the results, both CAA and asphalt binder property had

significant effects on rutting. For HMA with PG 64-22

binder, the rut depth generally decreased with the increase

in CAA level. HMA with 100% crushed coarse aggregate

only exhibited about half the rut of HMA with 35% CAA

coarse aggregate, which indicated that increasing the CAA

level of coarse aggregate would result in the high resistance

of HMA mixtures to rutting. When a stiffer asphalt binder

(PG 76-22) was used, the results showed a similar trend.

The rut depth generally decreased with the increase in the

angularity of the coarse aggregate. The rut depth exhibited

only about half the rut of HMA with PG 64-22. However,

when a much stiffer asphalt binder (PG 82-22) was used,

HMA mixture with different CAA levels did not show

significant difference in rut depth. All the rut depths were

within the range of 2–3mm.

4.3 Effects of aggregate test and GTM on ruttingperformance tests

Figure 9 presents the correlation between the coarse

aggregate AIMS angularity index and HMA mixture

properties from APA and creep tests. In Figure 9(a) and

(b), the slopes of the regression lines are negative with

three different asphalt binders (PG 64-22, PG 76-22 and

PG 82-22), which indicates that increase in angularity

index results in decrease in APA rut depth and creep

permanent strain. The absolute value of the slope

decreases as the asphalt binder becomes stiffer which

means HMA with stiffer asphalt binder is less sensitive to

the variation of angularity levels.

Figure 10 presents the correlation between the coarse

aggregate tri-axial friction angle and HMA mixture

properties from APA and creep tests. The slopes of the

trend lines are also negative with three different asphalt

binders (PG 64-22, PG 76-22 and PG 82-22), which

indicates that an increase in friction angle results in the

decrease in APA rut depth and creep permanent strain.

The absolute value of the slope decreases as the asphalt

binder becomes stiffer which means HMA with stiffer

asphalt binder is less sensitive to the variety of friction

angle.

Figure 11 presents the correlation between VCA and

APA rut depth and permanent strain from the creep test.

Figure 10. Correlation between coarse aggregate friction angle and HMA performance tests. (a) rut depth vs. friction angle and (b)permanent strain vs. friction angle.

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Similarly, in Figure 11 (a) and (b), the slopes of the trend

lines are negative with three different asphalt binders (PG

64-22, PG 76-22 and PG 82-22) too, which indicates that

increase in VCA results in decrease in APA rut depth and

creep permanent strain. The absolute value of the slope

decreases as the asphalt binder becomes stiffer which

means HMA with stiffer asphalt binder is less sensitive to

the variety of VCA.

These results indicate that the coarse aggregate tests

(such as AIMS, tri-axial and VCA) and HMA rutting

performance tests (such as GTM, creep and APA) are

intimately related.

Figure 12 presents the correlation between GSI and

APA rut depth and creep permanent strain. The slopes of

the regression lines of GSI versus rut depth, total strain and

permanent strain are positive with three different asphalt

binders (PG 64-22, PG 76-22 and PG 82-22), which

indicates that increase in GSI results in increase in APA rut

depth, creep total strain and creep permanent strain. The

value of the slope decreases as the asphalt binder becomes

stiffer which means HMAwith stiffer asphalt binder is less

sensitive to the variety GSI.

4.4 Effectiveness of aggregate properties to ruttingindices

From the above analyses, it appears that all coarse

aggregate tests were to various degrees related to

laboratory-measured rutting characteristics of asphalt

mixtures. In order to evaluate the effectiveness of each

coarse aggregate property on characterising rutting,

normalisations were made to each aggregate property

versus APA rut depth and creep permanent strain. Since

mixtures with PG 64-22 had the least influence of asphalt

binder, only these mixtures were selected for analyses.

Figure 13(a) gives an example of the normalisation for

creep permanent strain versusGSI. Themaximumvalues of

both GSI and permanent strain were assigned as 1, and the

minimum as 0. The slope of the normalised permanent

strain against the normalised GSI provided a relative

effectiveness of utilising the GSI to characterise the

permanent strain from static creep.

Consequently, the normalised ‘effectiveness slope’ of

each aggregate properties versus both APA rut

depth and creep permanent strain are illustrated in

Figure 13(b). It appears that the CAA was most ‘effective’

Figure 11. Correlation between VCA and HMA performance test. (a) VCA vs. rut depth and (b) VCA vs. permanent strain.

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in characterising creep permanent strain; whereas the

angularity index as measured from the AIMS was most

‘effective’ in characterising the APA rut depth. It should be

noted that although GSI is obtained from the GTM

(a mixture testing), here we compare it along with other

aggregate properties in that the GSI mainly reflects

aggregate structures given fixed asphalt binder content.

5. Summary and conclusions

A study has been conducted to evaluate the contributions of

aggregate structure and asphalt binder to the laboratory

rutting characteristics of a dense-graded HMA mixture.

Based on the laboratory experiments and analyses, the

following can be summarised and concluded:

(1) The AIMS, VCA and tri-axial tests can be

used to characterise the angularities of coarse

aggregates.

(2) Creep and APA tests generally provided consistent

ranking in evaluating the rutting performance of

HMA mixtures.

(3) Aggregate structure and binder stiffness had

significant effects on the rutting performance of

HMA.

Figure 12. Correlation between GSI with APA and creep test. (a) rut depth vs. GSI and (b) permanent strain vs. GSI.

Figure 13. Correlation between GSI with APA and creep test(normalised). (a) normalisation example and (b) effectiveness ofaggregate properties to rutting indices.

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(4) CAA had significant effect on the laboratory

rutting performance of HMA mixtures when a soft

binder was used.

(5) Use of relatively hard asphalt binder could also

lead to high rut-resistance HMA mixture and may

‘compensate’ for the relatively low aggregate

angularity.

(6) The traditional CAA had the strongest correlation

with the laboratory static creep permanent strain;

(7) The angularity index as measured by the AIMS

had the strongest correlation with the APA rut

depth;

(8) The relationships between coarse aggregate

properties and HMA rut resistance can be used

to develop specifications for the selection of

aggregates that would improve the mixture

performance.

Acknowledgements

The authors would like to thank the Tennessee Department of

Transportation (TDOT) for providing financial support for this

study. The authors would also like to thank Mrs Laura

Vukosavljevic, Mr James Bass and many others who have

provided great help during this experimental study.

Notes

1. Email: xchen31@lsu.edu2. Email: xshu@utk.edu3. Email: emasad@civil.tamu.edu4. Email: e-mahmoud@ttimail.tamu.edu

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