Evaluation of Rut Resistant Asphalt Mixtures for Intersection

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Evaluation of Rut Resistant Asphalt Mixtures forIntersectionElie Y. Hajj a , George Tannoury b & Peter E. Sebaaly aa Pavements/Materials Program, Dept of Civ. & Env. Engineering MS257 , University ofNevada , Reno, NV, 89557, United States E-mail:b Terracon Consulting Engineers and Scientists , Roseville, Kansas, 67209, United StatesE-mail:Published online: 17 Oct 2011.

To cite this article: Elie Y. Hajj , George Tannoury & Peter E. Sebaaly (2011) Evaluation of Rut Resistant Asphalt Mixturesfor Intersection, Road Materials and Pavement Design, 12:2, 263-292

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Road Materials and Pavement Design. Volume 12 – No. 2/2011, pages 263 to 292

Evaluation of Rut Resistant Asphalt

Mixtures for Intersection

Elie Y. Hajj*— George Tannoury**— Peter E. Sebaaly*

* Pavements/Materials Program, Dept of Civ. & Env. Engineering MS257

University of Nevada

Reno, NV 89557, United States

[email protected]

[email protected]

** Terracon Consulting Engineers and Scientists

Roseville, Kansas 67209, United States

[email protected]

ABSTRACT. The Nevada Department of Transportation (NDOT) sponsored a two-phase

research study to develop specific requirements to be used for HMA mixes at intersection. The

phase I of the study evaluated the impact of different aggregate gradations on the mixtures’

resistance to rutting. The rutting resistance of a HMA mix was found to be highly dependent

upon the aggregate gradation. The phase II consisted of a laboratory evaluation of the mixes

with the most promising gradations identified in phase I for fatigue cracking resistance.

Additionally, a full mechanistic analysis is conducted to effectively evaluate the relative

fatigue and rutting performances of the evaluated HMA mixes under heavy braking 18-wheel

truck. Based on the data generated in this study a new aggregate specification limit was

recommended for asphalt mixtures at intersections. The recommended gradation is intended

to improve the asphalt mixture’s resistance to rutting without jeopardizing its resistance to

fatigue cracking.

KEYWORDS: Intersection, Rutting, Fatigue, 18-Wheel Truck, Mechanistic Design, 3D-Move.

DOI:10.3166/RMPD.12.263-292 © 2011 Lavoisier, Paris

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264 Road Materials and Pavement Design. Volume 12 – No. 2/2011

1. Introduction

During the last decades there have been dramatic changes in traffic volumes,

traffic weights, and tire pressures, resulting in a significant increase in permanent

deformation of the Hot Mix Asphalt (HMA) pavements. Permanent deformation is a

major mode of failure in flexible pavements consisting of both rutting and shoving.

Typically, a rutting failure alone occurs under traffic loads moving at highway

speed, while both rutting and shoving failure may occur under traffic loads at

intersections. In many instances, the same HMA mixtures that have a history of

good performance in rutting did not perform well at intersections. Permanent

deformation of HMA is a common problem at intersections where the HMA mixture

exhibits lower stiffness when subjected to slow moving or stopped vehicle loads.

This problem is more common in hot climates weather where the stiffness of the

HMA is further decreased with the increase of the pavement temperature.

Additionally, the different behaviour of HMA pavement at intersection is related to

the more complex stresses imposed at the pavement surface layer by the braking,

accelerating and turning movements of heavy loaded trucks. Therefore, different

type of HMA mixtures may be required at intersections from the one used on the

main lane.

Permanent deformation is the largest and the most frequently occurring problem

when dealing with HMA at intersections. Rutting in the HMA layers develops

gradually as the number of load application increases, usually appearing as

longitudinal depressions in the wheel paths accompanied by small upheavals to the

sides. There are three general types of HMA pavement rutting, which are described

below, that can result from different mechanism.

Consolidation can occur when there is insufficient compaction during the

construction of the pavement. An HMA mix with insufficient density is prone to

further compaction under traffic, especially in hot weather and at intersections where

the loads are slow moving or static. With consolidation, a depression occurs in the

wheel path with no humps on either side of that depression. Furthermore, surface

wear takes place because of the surface abrasion under chains and studded tires used

in the winter season (Marker, 1981; Sousa and Weissman, 1994).

Instability of the HMA mixture at intersection is a significant performance

problem that leads to plastic flow at the surface of the pavement. Some of the more

common reasons for mixture instability are: excessive amount of asphalt binder and

insufficient air voids, too much rounded aggregate, or too high of the minus 0.075-

mm material. Plastic flow will normally appear as longitudinal ruts in the pavement

near the centre of the applied load with humps of material on either side of the rut

(Marker, 1981; Sousa and Weissman, 1994).

Lastly, structural rutting results from insufficient structural capacity in the

pavement system and can occur when the strength and/or thickness of the pavement

layers are insufficient to support the design traffic on the existing subgrade. The

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Evaluation of Rut Resistant Asphalt Mixtures for Intersection 265

compaction of the supporting layers results in the development of ruts in the

pavement surface. A rut resulting from this type of action will generally be

accompanied by longitudinal and/or alligator cracking ((Marker, 1981, Sousa and

Weissman, 1994; Rosenberg and Buncher, 1999).

The rutting resistance of an HMA mix is highly dependent upon the integrity of

the aggregate structure. Mr. Robert D. Bailey of the Illinois Department of

Transportation (retired 1997) developed a specific procedure termed “Bailey

method” to select aggregate gradation for asphalt mixtures. This method focused on

creating aggregate structure which facilitates interlock in the coarse aggregate,

resulting in a strong “aggregate skeleton”, and thus providing high shear strength

and rutting resistance in the HMA pavement. Additional information on the Bailey

method can be found in Vavrik et al. (2001 and 2002).

Williams (2003) explored the relationships between HMA mixture properties

and rutting susceptibility as measured under wheel-tracking devices. As a result, an

increase in rut depth was observed with the decrease in VMA. On the other hand, a

decrease in rut depth was observed with the increase in the PG high temperature

binder grade. Additionally, an increase in the asphalt binder content and film

thickness resulted in a reduction in the mixtures’ resistance to rutting. Therefore, it

was concluded that a change in the binder content, binder properties, and/or

aggregates gradation can improve the mixtures’ resistance to rutting while it may

greatly jeopardize its resistance to fatigue and thermal cracking.

In 2003, a study performed by the US Army Corps of Engineers (Partl and

Newman, 2003) pointed out the need for selecting an asphalt modifier that can resist

multiple distresses such as rutting, fatigue, thermal cracking, and moisture damage.

Given the good resistance of polymer-modified binders against rutting, it was found

that the type of polymer might have a significant impact on fatigue properties.

Mixtures contained reactive styrene–butadiene cross-linked polymer exhibited the

highest fatigue life.

In 1996, Harvey and Tsai evaluated the effects of a broad range of asphalt

contents and air-void contents on fatigue life and stiffness for a typical California

mix. It was found that the fatigue life was significantly affected by the strain level,

air-void content, and asphalt content. A higher laboratory fatigue life (i.e. better

fatigue resistance) was observed for lower air-void content and higher asphalt

content. Additionally, a decrease in air-void content and asphalt content resulted in a

significant increase in initial stiffness.

Historical field measurements in Nevada show that a significant number of HMA

pavements carrying very heavy and channelized traffic in southern Nevada are

experiencing severe permanent deformation near signalized intersections. Realizing

the need for different asphalt mixes at or near intersections, the Nevada Department

of Transportation (NDOT) sponsored a two-phase research study at the University

of Nevada to develop specific requirements for HMA mixes at intersections. Phase I

of the study consisted of a post-mortem evaluation of in-service intersections and a

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laboratory evaluation of mixtures with different aggregate gradations in terms of

their resistance to rutting. Phase II of this study consisted of a laboratory evaluation

of the rut resistant HMA mixes identified in phase I for fatigue cracking resistance.

This phase is essential as an improved resistance to rutting of a mixture may

partially jeopardize its resistance to fatigue cracking. In addition, the phase II of the

study included a re-evaluation of the rutting characteristics of the rut resistant mixes

using laboratory techniques proposed by the new AASHTO mechanistic-empirical

pavement design guide (MEPDG) (NCHRP, 2004). Additionally, a full mechanistic

analysis was conducted to effectively evaluate the relative performance of the

evaluated HMA mixes under moving and braking heavy loads.

This paper presents in details the research effort that was conducted under phase

II of the overall research effort with a brief summary of the findings from phase I

that was completed in 2006. More information on phase I can be found in Hajj

(2005) and Sebaaly et al. (2005).

2. Research objective

The overall objective of this research study was to identify an HMA mix with

good resistance to rutting and shoving at intersections. Phase I of this study

evaluated the impact of aggregate gradation on the mixtures resistance to permanent

deformation. The objective of phase II of this study was to evaluate the fatigue

resistance of the rut resistant mixtures identified in phase I. Additionally, the rutting

resistance of the mixes wasre-evaluated according to the test methods referred to in

the new AASHTO Mechanistic Empirical Pavement Design Guide (MEPDG).

3. Findings of phase I

Two laboratory evaluations were conducted: I. Postmortem evaluation of in-

service intersections and II. Laboratory evaluation of mixes with different aggregate

gradations. The full documentation and findings of Phase I are available in

published reports and papers (Sebaaly et al., 2005; Hajj, 2005; Hajj et al., 2007).

The first part of phase I evaluated and compared the volumetric properties of

field cores from rutted and non-rutted intersections in southern Nevada. Several in-

service HMA intersections were identified, some with severe rutting and some

without any rutting failures. Core samples from between the wheel path and

150 meters away from the intersection were obtained from thirteen different projects

in southern Nevada.

The idea behind the sampling plan was that cores from between the wheel path

and away from intersection provide HMA mixes that have not been damaged due to

the traffic loading while the wheel path cores at the intersection represent the

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damaged mixes. The cores taken were tested for bulk specific gravity and aggregate

gradations. For the projects experiencing severe rutting, the Air Voids (AV) and

voids in the Mineral Aggregate (VMA) were lower at intersection when compared to

the ones at 150meters away from the intersection. The AVs at the intersection were

less than 2.5%, indicating excessive densification of the HMA mixes under heavy

slow/stopped traffic.

Two gradation zones were identified by grouping the extracted aggregate

gradations of the mixes that experienced rutting and mixes that experienced no

rutting. Figure 1 shows that the no rut mixes zones covered denser and coarser

gradations than the rut mixes zones.

Figure 1. Aggregate gradation limits for rut and no rut mixtures from 13 different

projects

Additionally, the field performance of HMA intersections was used in an attempt

to identify a laboratory test that identifies HMA mixes that are prone to permanent

deformation at intersections. To this end, the sampled cores from the various

intersections were tested under the Asphalt Pavement Analyzer (APA) and the

Repeated Shear at Constant Height (RSCH) tests. The analysis of the generated data

led to the conclusion that neither the APA nor the RSCH can be effectively used as a

post-mortem device to explain the performance of HMA mixes at intersections.

In the second part of phase I, five aggregate gradations were used with a PG76-

22 polymer-modified asphalt binder and were evaluated in the laboratory for rutting

resistance.

– No Rut Mixture gradation, designated as NRM,

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– Caltrans gradation used for intersections, designated as CT,

– NDOT Type 2C gradation, designated as T2C,

– NDOT Type 2 gradation, designated as T2,

– Rut Mixture gradation, designated as RM.

The RM and the NRM gradations belong to the rut mixtures and no rut mixtures

zones, respectively, as identified in Figure 1. The NDOT Type 2C gradation is used

by NDOT very successfully to resist rutting under highway traffic load throughout

the entire state of Nevada but its performance has not been as good at intersections

in the southern part of the state. The T2C gradation belongs to the no rut mixtures

zone for sieve sizes larger than 4.75 mm and for the rut mixtures zone for sieve sizes

smaller than 4.75 mm. The NDOT Type 2 gradation is a finer gradation than the

NDOT Type 2C gradation and belongs to the rut mixtures zone. On the other hand,

the CT gradation belongs to the rut mixtures zone for sieve sizes larger than 9.5 mm

and for the no rut mixtures zone for sieve sizes smaller than 9.5 mm. It should be

noted that both NDOT gradations meet the Superpave specification for a nominal

maximum aggregate size of 19 mm. Figure 2 shows all five aggregate gradations

evaluated in the laboratory.

Figure 2. Laboratory produced aggregate gradations – phase I

The laboratory evaluation assessed the resistance of the mixes to permanent

deformation using the triaxial compressive strength test, the Repeated Shear

Constant Height (RSCH) test, the repeated Load Triaxial Test (RLT), and the

Asphalt Pavement Analyzer (APA) test. All laboratory performance test results

yielded a ranking of mixtures from best to worst rutting resistance as follows: NRM,

CT, T2C, T2, and RM.

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The mixes with the aggregate gradation identified from the intersections that

experienced no rutting (NRM) showed the best performance in all the permanent

deformation tests conducted in this phase of the study. However, this gradation was

the coarsest one among all the evaluated five gradations. Therefore, it was

recommended that fatigue resistance of this mix should be evaluated prior to its

implementation at intersections in southern Nevada. On the other hand, the NDOT

Type 2C gradation showed, in the laboratory, a significant improvement in rutting

resistance when compared to the mixes that experienced rutting at intersections in

southern Nevada. However, NDOT started recently specifying the PG76-22 binder

for the southern part of the state after using the AC-30 asphalt binder for the past 15

years. Therefore, it was also recommended to continue monitoring the performance

of the Type 2C mixes with the PG76-22 binder at intersections in southern Nevada.

In summary, phase I of the study led to the conclusion that permanent

deformation at intersections in southern Nevada may be resisted by certain

aggregate gradations and mixtures that meet specific permanent deformation testing

requirements. However, the fatigue characteristics of the identified rut resistant

mixtures, i.e., T2C, CT, and NRM, were unknown which led to phase II of the

research.

4. Materials and mixtures characteristics

In phase II of the research study, an experimental program was carried out to

evaluate the rutting and fatigue behaviors of the mixes with the NRM, CT, and

NDOT type 2C gradations (Figure 2). All three mixes were designed with a

polymer-modified PG76-22 asphalt binder and limestone aggregates sampled from

atypical quarry located south of Las Vegas, Nevada.

All mixes were designed using the Hveem Mix Design Method as specified by

NDOT. Additionally, all mixes were treated with 1.5% hydrated lime by dry weight

of aggregate following NDOT’s specifications. NDOT mandates the use of lime in

all mixes to improve the mixtures’ resistance to moisture damage. The mixes were

designed for heavy traffic which corresponds to a minimum Hveem stability of 37.

5. Laboratory evaluation

The laboratory-produced mixes were evaluated in terms of the following

properties: stiffness, resistance to rutting, and resistance to fatigue cracking. The

laboratory evaluation will provide mixtures properties that are used in the following

two analyses: a) comparison of the relative performance of the various mixes, and b)

mechanistic analysis of HMA pavements under moving and braking heavy loaded

trucks.

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5.1. Stiffness of the mixtures

The Dynamic Modulus (|E*|) is the primary material property of HMA mixes

that is used in structural pavement design. Due to the viscoelastic behavior of the

HMA pavement, this property varies with temperature and time of loading. The

dynamic modulus test (AASHTO TP62-07, 2008) was used to develop the dynamic

modulus master curve of the three HMA mixes. For linear viscoelastic materials

such as HMA mixes, the stress-strain relationship under a continuous sinusoidal

loading is defined by its complex dynamic modulus (|E*|). Mathematically, the

“dynamic modulus” is defined as the absolute value of the complex modulus, i.e.,

|E*| = 10/00, where 10 and 00are peak (amplitude) stress and strain, respectively.

The dynamic modulus master curve is represented by the sigmoidal function

described by Equation [1a]. The general form of the shift factors is provided in

Equation [1b].

�[1a]

[1b]

where, tr is the time of loading at the reference temperature, D andGare fitting

parameters; for a given set of data, G represents the minimum value of log(|E*|), D+

G represents the maximum value of log(|E*|), E + J = parameters describing the

shape of the sigmoidal function, a(T)is the shift factor as a function of temperature,

T is the temperature of interest and tis the time of loading at a given temperature of

interest.

According to the new AASHTO MEPDG (NCHRP, 2004), a(T) can be

expressed in terms of asphalt binder viscosity and time of loading at a given

temperature of interest as shown in Equation [2]. The parameter c is one of the

values returned by the numerical optimization. The optimization was performed

using the “Solver” function in Microsoft Excel®. This calculation is performed by a

spreadsheet to compute the sum of the squared errors between the logarithm of the

average measured dynamic moduli at each temperature/frequency combination and

the values predicted by Equation [1a] (AASHTO PP62-09, 2010). The shift factor

depends only on the binder viscosity for the age and temperature of interest (K), and

the Rolling Thin Film Oven (RTFO) aged viscosity at the reference temperature

(KTR)

� [2]

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For the RTFO-agedPG76-22 asphalt binder, the viscosity was measured at

multiple temperatures and the viscosity-temperature (in Rankine) relationship was

developed:

loglog(K) = 4.704 – 2.481 x log( TR ) [3]

Figure 3 shows the dynamic modulus master curves |E*| for all the three

evaluated mixes at the reference temperature of 21ºC. Figure 3 shows that the |E*| of

the NRM mix is higher than the CT and the T2C mixes for a reduced frequency

above 0.0004 Hz. However, the dynamic modulus property of the CT mix is higher

than the T2C mix only at a reduced frequency between 0.1 and 3500 Hz. At a

reduced frequency below 0.0003 Hz the dynamic modulus of the T2C exceeded the

ones of the CT and NRM mixes.

Figure 3. Dynamic modulus master curves at 21°C

Additionally, the storage (E’) and loss (E”) moduli master curves of the various

mixes at 21°C are shown in Figure 4. The data show that E’ of the NRM mix is

higher than the CT and T2C mixes at all frequency above 0.0001 Hz. However, the

E’ property of the CT mix is higher than the T2C mix only at a frequency of 0.1 to

4000 Hz. At a frequency below 0.0001 Hz the E’ of the T2C exceeded the ones of

the CT and NRM mixes. On the other hand, the E” of the NRM mix is higher than

the CT and T2C mixes at a frequency above 0.001 Hz. However, the E” property of

the CT mix is higher than the T2C mix only at a frequency of 0.1 to 450 Hz.

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Figure 4. Storage and loss modulus master curves at 21°C

5.2. Resistance of the mixtures to rutting

The resistance of the HMA mixes to rutting can be evaluated under two types of

testing: empirical and fundamental. In the category of the empirical tests, this study

used the Asphalt Pavement Analyzer (APA) to evaluate the relative performance of

the various mixtures. However, in the category of fundamental tests, this study used

the repeated load triaxial test (RLT) to evaluate the fundamental behavior of the

various mixtures.

5.2.1. Rutting behavior in the asphalt pavement analyzer test

The APA test is standardized under AASHTO TP63-03 (2008), where a loaded

concave steel wheel travels along a pressurized rubber hose that rests upon the HMA

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sample. Four 152-mm diameter cylindrical samples were compacted from each mix

using the Superpave Gyratory Compactor (SGC) to a height of 76 mm and air voids

of 7±0.5%. Samples are secured within form-fitting acrylic blocks during testing.

The APA wheel load is 445-N and the hose pressure is 689kPa. A data acquisition

program records rut depths during testing at 2 points within each sample and their

average is reported.

Figure 5 summarizes the rut depth data from the APA test at 60°C. The data

indicate that all mixes meet and are far below the NDOT APA criterion of 8mm rut

depth after 8,000 cycles at 60°C for highway speed. No significant difference was

observed between the various mixes at 60°Cthus indicating similar resistance to

rutting. These results could be an indication that the APA test is not sensitive

enough to test for any significant difference in the rutting resistance of the mixtures

when tested at a temperature lower than the high performance temperature of the

binder (i.e. PG76-22).

Following the above observation, the various mixes were tested under the APA

at 76°C. Figure 5 shows that all three mixtures exhibited a rut depth at 76°C that is

less than 8mm with the NRM having the lowest (2.0 mm) followed by the CT mix

(2.8 mm) and the NDOT T2C mix (3.6 mm). The various APA rut depths after 8000

cycles were compared using the least square mean (LS mean) statistical technique.

The LS mean results show that the three mixes were significantly different at

76°C,with the NRM having the best rutting resistance followed by the CT and the

NDOT T2C.

Figure 5. APA rut depths at 60 and 76°C (numbers above bars represent mean

values for 4 replicates and whiskers represent mean +/- one standard deviation)

5.2.2. Rutting behaviour in the repeated load triaxial test

HMA samples of 100 mm diameter by 152 mm height specimens were

compacted in the SGC to 7r0.5%air voids. All mixes were evaluated at 30, 50 and

70ºC. Compressed air was used to apply a constant confining pressure of 207kPa.

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Adeviator vertical stress of 307kPa was cycled by applying a haversine load pulse of

0.1 second duration followed by a 0.6 second rest period (i.e., unload). The 0.1

second load duration was selected in order to simulate the loading of a truck moving

at highway speed. This complete cycle was repeated 12,000 times. Axial

deformations were continuously measured over the middle 100mm of the sample.

The RLT test results were used to develop the rutting performance models for

the three evaluated mixes. The performance model suggested in the MEPDG

(Equation [4]) to assess rutting in the HMA layer relates the ratio of axial permanent

strain (0p)over the resilient axial strain (0r), to the number of loading cycles (N) and

temperature (T). The a, b, and c in Equation [4] are experimentally determined

regression coefficients. It is clearly shown from the generalized rutting model that

the lower the 0p/0r the higher the rutting resistance of the mix.

[4]

The data from the RLT test were plotted in the form of the ratio of permanent

strain over the resilient strain (0p/0r) versus loading cycles (N) as shown in Figure 6.

Each curve represents the average response of three replicate samples tested at each

of the specified temperatures (i.e. 30, 50 and 70ºC). In general, the lower the RLT

curve (i.e. lower intercept and slope of the RLT curve) the higher the resistance of

the mixture to permanent deformation. The data in Figure 6 show that the resistance

of the mixtures to permanent deformation increase as the temperature of the test

decreases. Additionally, the data show that 0p/0r of the T2C mix increased at higher

rate than the 0p/0r of the NRM and CT mixes at all three temperatures. Consequently,

the T2C mix will exhibit a faster build-up of the permanent strain under multiple

loading indicating a lower resistance to rutting than the NRM and CT mixes.

The RLT data at the three testing temperatures were used to develop the

generalized rutting model for each mixture as presented in Equation [4]. Table 1

summarizes the regression coefficients for the generalized rutting models of each

mix.

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Figure 6. RLT test results at 30, 50, and 70°C

Table 1. Rutting regression parameters of the various mixtures

Mix a* b* c* R2

NRM 3.0047×10-4 0.35443 1.9155 0.986

CT 3.9653×10-5 0.34572 2.5212 0.992

T2C 7.1050×10-6 0.41377 2.7634 0.999

* , T in qC

The ratio of the permanent axial strain over the resilient axial strain (0p/0r) after

12,000 loading cycles were compared for any statistically significant differences

using the LS mean technique. Figure 7 summarizes the 0p/0r values along with the

significance difference after 12,000 loading cycles. The statistical analysis shows

that, at the 30°C, the NRM mix exhibits the highest 0p/0r, followed by the NDOT

T2C mix, and the CT mix. At 50°C, the NDOT T2C mix exhibits the highest 0p/0r,

followed by the NRM mix and the CT mix. Consequently the CT mix will have a

better resistance to rutting than the other two mixes at 30°C and 50°C. On the other

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hand, at 70qC the NRM mix exhibited the lowest 0p/0r, followed by the CT mix, and

the NDOT T2C mix. Therefore, the NRM mix will have a better rutting resistance

with the NDOT T2C mix having the least resistance to rutting.

Figure 7. Statistical comparison of Hp/Hr from RLT test results after 12,000

cycles(SH and SL denotes, respectively, “significantly higher” and “significantly

lower” 0p/0r of the NRM or CT mix than the 0p/0r of the NDOT T2C mix)

5.3. Resistance of the mixtures to fatigue cracking

The resistance of the three HMA mixes to fatigue cracking was evaluated at

4.4°C, 21°C and 43°C using the flexural beam fatigue test (AASHTO T321-07, 2008)

under the strain controlled mode of testing. Fatigue life (Nf) or failure was defined as

the number of cycles corresponding to a 50% reduction in the initial stiffness. The

initial flexural stiffness of the mix was measured at the 50th load cycle. The model

shown in Equation [5] was used to characterize the fatigue behavior of the HMA

mixes at each of the tested temperatures:

[5]

Where 0t is the applied tensile strain, and k1 and k2 are experimentally determined

coefficients. The beam fatigue samples were compacted using the kneading

compactor to 7±0.5 percent air voids. It is clearly shown from the beam fatigue

relationship that the higher the k1 and k2 values, the more fatigue resistant the mix is.

Figure 8 shows the fatigue relationships of the three mixes at each of the testing

temperatures. The data show a better fatigue resistance for the CT and NDOT T2C

mixes at 4.4°C when compared to the NRM mix. On the other hand, a better fatigue

resistance for the CT mix at 21°C was observed when compared to the other two

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mixes which exhibited similar fatigue characteristics. Additionally, a better fatigue

resistance was observed for the CT mix at 43°C when compared to the other two

mixes with a slightly better resistance for the NDOT T2C mix over the NRM mix.

Figure 9 summarizes the fatigue data of the various mixes in terms of the number

of cycles to failure and ratios under tensile strains of 500 microns at a temperature of

4.4°C, and 800 microns at a temperature of 21°C, representing medium and high

levels of tensile strains at the bottom of the HMA layer of a flexible pavement. The

selection of the strain levels was based on the typical stains encountered in

pavements and the availability of data for the statistical comparison of the Nf values

of the various mixes. No statistical analyses were conducted at 43°C because two

out of the three mixes were not actually tested at 800 microns. The statistical

analyses show that at 4.4°C, the CT mix has significantly higher number of cycles to

failure than the NRM and NDOT T2C mixes. Additionally the NDOT T2C mix has

significantly higher fatigue life than the NRM mix. At 21°C, the CT mix has

significantly higher number of cycles to failure than the NRM and the NDOT T2C

mixes.

The recommended MEPDG generalized model that correlates the number of

cycles to failure (Nf) to the tensile strain (0t) and mixture’s stiffness (E) was

developed for each of the mixes evaluated in this study (Equation [6]) by fitting a

multiple linear regression for the test data at the various temperatures. The k1, k2 and

k3 are experimentally determined coefficients (Table 2). The short-term aged

dynamic moduli of the various mixes at 10 Hz and at the various temperatures

(4.4°C, 21°C, and 43qC) were determined using the dynamic modulus master curves

and their corresponding shift factors at the specified temperature. The developed

mix specific generalized models will be used in the mechanistic analysis described

later to evaluate the impact of mixture type on the fatigue performance of an HMA

pavement.

[6]

Table 2. Fatigue generalized models of the various HMA mixes

Mix K1 K2 K3 R2

NRM 0.62908 4.2395 2.1470 0.916

CT 0.26784 4.3172 2.0598 0.860

T2C 0.03493 4.4143 2.0374 0.926

* , Ht in mm/mm and E in MPa

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Figure 8. Fatigue characteristics of the various mixes

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Figure 9. Laboratory measured fatigue life of the various mixes and ratio of fatigue

life with respect to T2C mix

6. Mechanistic analysis

A better laboratory performance of an HMA mix in rutting or fatigue may not

necessarily translate into a better performance in the field as the performance life of

an asphalt pavement is highly dependent on both the stiffness and the characteristics

of the HMA mix (e.g. rutting and fatigue resistance) and their interactions. For

example, in a mechanistic pavement analysis, an HMA layer with a higher stiffness

will produce under field loading a lower tensile strain at the bottom of the HMA

layer. But on the other hand, the same mix will show a lower laboratory fatigue life

(lower resistance to fatigue cracking) under strain controlled mode of testing when

compared to mixes with lower stiffness. Therefore, depending on the magnitude of

the reduction in the calculated strain at the bottom of the HMA layer, the HMA layer

with the higher stiffness may result in a longer fatigue life in the field even though it

exhibited a lower laboratory fatigue life. Therefore, a full mechanistic analysis needs

to be conducted to effectively evaluate the performance of an HMA pavement.

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A mechanistic-empirical design method represents the most advanced method

for designing HMA pavements. The mechanistic portion is based on the mechanics

of materials that relates an input, such as a wheel load, to an output or pavement

response, such as stress or strain. The empirical portion is when the response values

are used to predict distresses based on laboratory relationships calibrated with field

performance data. Consequently, predicting the rutting and fatigue life of HMA

pavements necessitates the development of permanent deformation and fatigue

models in the laboratory followed by a mechanistic analysis to determine the

pavement responses in the HMA layer, which in turn are used as input to the

permanent deformation and fatigue models.

The permanent deformation and fatigue models for the three mixes (i.e. NRM,

CT, and T2C) developed in this study are used in the mechanistic analysis. It should

be mentioned that the developed performance models are statistical relationships

based on the laboratory analysis of the asphalt mixes and therefore field

shift/adjustment factors are required to provide reasonable estimates of the actual

performance in the field. The field shift factors are outside the scope of this research

and the laboratory-based performance models were only used for a relative

comparison.

Pavement responses were determined under the loading of an 18-wheel truck at

normal highway speed (without braking) and at intersection (with braking). In this

study the computer code 3D-Moving Load Analysis (3D-MOVE) developed by

Siddharthan et al. (1998) was used to estimate the pavement dynamic responses.

6.1. Description of the computer code 3D-MOVE load analysis

The computer code 3D-MOVE incorporates the approach of the formulation of a

continuum-based “finite-layer” model to evaluate the response of a layered medium

subjected to a moving surface load (Siddharthan et al., 1998). The pavement system

is characterized through a combination of viscoelastic and elastic horizontal layers

with each layer characterized using a set of uniform properties that rest on a rigid

impermeable layer. The 3D-MOVE model can handle multiple moving loads, non-

uniform tire-pavement normal contact stress distribution, and non-uniform interface

shear stresses caused by braking and turning forces.

Since the 3D-MOVE program can only simulate moving loads at a constant

speed, the response of the asphalt pavement to the decelerated truck during the

braking period was estimated at two different speeds: 64 and 3.2 km per hour, before

it starts braking and as it approaches the intersection, respectively. The braking

forces as a result of deceleration were incorporated in the analysis to simulate the

braking effect.

The mechanistic-empirical analysis is used in conjunction with the dynamic

modulus, fatigue characteristics data, and rutting characteristic data that were

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measured in the laboratory on all three mixes to assess the fatigue and rutting

performance of HMA pavements. The analysis considered a three layers pavement

consisting of an HMA layer on top of a crushed aggregate base on top of the

subgrade. A total of six pavement structures were analyzed in this study (Table 3).

Table 3. Analyzed pavement structures

Thickness [mm]

Structure No.Pavement layers

1 2 3 4 5 6

NRM 100 200 -- -- -- --

CT -- -- 100 200 -- --

T2C -- -- -- -- 100 200

Crushed aggregate base

(CAB)200 200 200 200 200 200

Subgrade Infinite

The first step of the above mechanistic analysis is to estimate the load

distributions on various tires of the 18-wheel tractor-semitrailer during normal

highway traffic and at intersections. Braking at intersections causes the vehicle to

decelerate and the loads to transfer to the front of the vehicle. The resulting axle load

can be higher or lower than the initial static load, depending on the axle location.

Figure 10 shows the major forces acting on an 18-wheel tractor-semitrailer during

braking on a downward sloping pavement. The axles include: the tractor steering

axle, the tractor tandem axle (i.e., driving axle), and the semitrailer tandem axle (i.e.,

trailer axle). Since an intersection located on a downward sloped pavement

represents the worst case scenario, the tire loads distributions were determined for

an 18-wheel truck braking on a 6% downward sloping pavement (Hajj, 2005; Hajj et

al., 2006).

In order to calculate the normal load on each axle, the tractor and the semitrailer

are considered as free bodies separately and combined. The vertical, horizontal, and

moment equilibrium equations for the tractor, semitrailer unit, and tractor-

semitrailer combination were derived as a function of truck loads and geometry. By

solving the system of equilibrium equations, the normal loads on the various axles

during braking on a downward slope can be expressed as a function of the linear

deceleration of the truck along the longitudinal axis, the angle of the slope with the

horizontal, the tractor and semitrailer total weights and the truck geometry. The full

formulation and solutions are presented by Hajj (2005). Table 4 summarizes the load

distributions on the various axles of the 18-wheel tractor-trailer combination under

the normal highway traffic (without braking) and at intersections (with braking).

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Figure 10. Forces acting on a tractor-semitrailer during braking on a downward

slope

Table 4. Axle load distributions of a fully loaded 18-wheel tractor-trailer

Speed – Braking

Action AxleVertical load per tire

[kN]

Horizontal load per tire

[kN]

Steering 26.7 0

Front driving

Rear driving

18.9

18.9

0

064km/h –

no braking

Front trailer

Rear trailer

18.9

18.9

0

0

Steering 37.7 9.6

Front driving

Rear driving

22.8

14.9

13.2

13.23.2km/h –

braking

Front trailer

Rear trailer

19.5

12.7

11.4

11.4

The stress distribution at the tire-pavement interface is significantly affected by

tire type, inflation pressure, and load. Previous studies showed that the stress

distribution at the tire-pavement interface is not uniform, and influences the HMA

pavement resistance to rutting and fatigue failures (Sebaaly, 1992; De Beer and

Fisher, 1997; Myers et al., 1999; Weissman, 1999). This study used the non-uniform

stress distributions under the Goodyear G159A 295/75R22.5 tire measured by the

Kistler device, for an inflation pressure of 827kPa. The Kistler MODULAS Quartz

Sensor Array device measures the vertical stress as the tire moves over a measuring

pad. The loading of the MODULAS sensors was accomplished by driving a truck

with a single trailer at a preset speed. Vertical stress distributions are available for a

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wide range of tire loads and vehicle speeds. Braking forces at each tire were

included as interface shear stresses with their distribution estimated by multiplying

the vertical stress distribution by the calculated coefficient of friction between the

tire and the pavement surface. The coefficient of friction at each tire was determined

by dividing the longitudinal load at the tire by the associated vertical load.

Two vehicle speeds were considered: 64 km per hour (away from intersection)

and 3.2 km per hour (at intersection). Rutting and fatigue analyses were conducted

at two different pavement temperatures of 64°C and 21°C, respectively. The

laboratory developed dynamic modulus master curves of the three mixes for the

analyses temperatures (i.e. 64°C and 21°C) are used to characterize the HMA mixes.

The internal damping for the HMA layer is estimated as a function of the loading

frequency and is included in the analysis by writing the dynamic modulus (|E*|) in

its complex form as follows (Rosset, 1980):

[7]

In which ]AC is the internal damping of the HMA, and E’ and E” are the real and

imaginary modulus components, respectively. The poisson’s ratio (X) for the HMA

layer is assumed to be constant of value 0.4 for the temperature range used in this

analysis.

The base course and subgrade layers are treated in this study as linear elastic

materials with anelastic modulus of 240MPa and 69MPa, respectively. The internal

damping of the unbound layers was assumed to be 5%. The poisson’s ratio was

assumed to be 0.4 for both layers.

Pavement responses such as stresses, strains, and displacements were computed

as a function of time using the 3D-MOVE computer code. Responses at the middle

of the HMA layer were considered for rutting analysis; whereas the responses at the

bottom of HMA layer were considered for fatigue analysis.

6.2. Rutting analysis

The latest development in predicting rutting within the HMA layer calls for the

determination of plastic vertical compressive strain within the HMA layer. The total

rutting within the HMA layer is then calculated by multiplying the permanent strain

times the thickness of the HMA layer.

[8]

Where, RDHMA is the rutting generated in the HMA layer,Hp is the permanent

strain within the HMA layer, and hAC is the thickness of the HMA layer.

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The developed laboratory rutting models for the three mixtures (Table 1) along

with the calculated dynamic resilient axial strain from the mechanistic analysis were

used to predict the permanent strain (Hp) within the HMA layer of the various

pavement structures. It should be noted that the predicted permanent strains were

multiplied by the depth correction function (k1) recommended by the new AASHTO

MEPDG. The depth correction function (k1) is an empirical attempt based on

engineering judgment and very limited field data to adjust the computed plastic

strains for the influence of lateral confining pressure at different depths. Equation

[9] shows k1 as a function of hAC (mm) and the depth (depth, mm) to the

computational point. A k1 value of 3.49 and 0.633 was computed at the mid depths

of the 100 and 200 mm HMA layers, respectively.

[9a]

[9b]

[9c]

The rutting resistance of the various mixes was evaluated with the calculated

permanent axial strain (Hp) in the HMA layer for a fixed number of load repetitions.

Table 5 summarizes the maximum permanent strains and rut depths within the 100-

mm and 200-mm HMA layers under 12,000 loading cycles at normal highway speed

and at intersection. The highest permanent strain and rut depth were found to be

under the steering axle which makes it the most damaging axle in the 18-wheel truck

to the pavement at normal highway speed and at intersection.

The data in Table 5 show that the permanent strains at intersection for 12,000

loading cycles are higher than the one at normal highway speed by a ratio of about

2.0 for both the NDOT T2C and CT mixes and a ratio of 2.5 for the NRM mix.

Additionally, the NRM mix exhibited significantly lower rut depth at normal

highway speed when compared to the NDOT T2C and CT mixes. However, at

intersection all three mixes exhibited similar rut depths after 12,000 loading cycles

with a slightly lower rut depth for the NRM mix. Therefore, it can be concluded that

all mixes exhibited similar rutting resistance under a braking 18-wheel truck with a

slight better resistance for the NRM mix.

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Table 5. Maximum permanent strains and rut depths under 18-wheel truck at 64qCand after 12,000 loading repetitions

Maximum permanent axial strains and rut depths under 12,000 loading repetitions

100-mm HMA layer 200-mm HMA layer

MixesNormal

highway

speed

(64km/h,

no braking)

Intersect-

ion

(3.2km/h,

braking)

Ratio of

Hp-max at

intersection

over Hp-max at

normal

highway

speed

Normal

highway

speed

(64km/h,

no

braking)

Intersect-

ion

(3.2km/h,

braking)

Ratio of

Hp-max at

intersection

over Hp-max at

normal

highway

speed

NRM9.3%

(9.4mm)

22.8%

(23.1mm)2.4

1.6%

(3.3mm)

4.0%

(8.1mm)2.5

CT13.0%

(13.2mm)

25.3%

(25.4mm)1.9

2.1%

(4.3mm)

4.3%

(8.6mm)2.0

NDOT

T2C

14.4%*

(14.7mm)

25.7%

(25.4mm)1.8

2.3%

(4.6mm)

4.4%

(8.9mm)1.9

Ratio of

NRM

over

NDOT

T2C

0.65 0.89

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0.70 0.91

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Ratio of

CT over

NDOT

T2C

0.90 0.98

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0.91 0.98

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Ratio of

CT over

NRM1.40 1.11

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1.31 1.08

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* Rut depth in “mm” is shown in parenthesis.

6.3. Fatigue cracking analysis

The bottom-up fatigue cracking of pavements initiates from the bottom of the

HMA layer and propagates to the pavement surface. The number of load repetitions

is related to the tensile strain at the bottom of the HMA layer and the modulus of the

HMA mix.

The number of load repetitions to fatigue failure of each pavement structure is

estimated using the generalized fatigue model of each mix (Table 2) by applying the

calculated maximum tensile strain at the bottom of the HMA layer (i.e. 3D-MOVE)

and the corresponding stiffness of the mix at 21qC. In order to determine the

mixtures’ stiffness to be used in the fatigue model (Equation [6]), the rate of loading

under normal highway speed and at intersection for each pavement structure is

determined using Barksdale chart (1997). Following Barksdale chart, the loading

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time for both, 100 mm and 200 mm HMA layer, at an operating speed of 3.2km/h

was considered to be 1Hz. At an operating speed of 64km/h the loading time for

both 100 and 200 mm HMA layer was considered to be 10Hz. Following these

steps, the mixtures’ stiffness for the fatigue models were selected from the dynamic

modulus at a loading frequency of 10Hz and 1Hz for the normal highway speed and

intersection, respectively.

The maximum tensile strains at the bottom of the HMA layers were relatively

low and varied between 79 and 238 PH�in the 100-mm HMA layer and between 51

and 127 PH�in the 200-mm HMA layer. The lowest number of cycles to fatigue

failure was under the steering axle which makes it the most damaging axle in the 18-

wheel truck to the pavement at normal highway speed and at intersection.

Table 6. Fatigue lives ratios

100 mm HMA layer 200 mm HMA layer

Fatigue lives ratio Intersection

(3.2km/h,

braking)

Normal

highway speed

(64km/h, no

braking)

Intersection

(3.2km/h,

braking)

Normal highway

speed (64km/h,

no braking)

Nf-NRM/ Nf-NDOT-T2C 1.7 1.4 1.4 1.1

Nf-CT/ Nf-NDOT-T2C 3.2 1.8 3.0 1.4

The critical numbers of load repetitions to fatigue failure in the 100-mm and

200-mm HMA layers under normal highway speed (i.e. 64 km/h no braking) and at

intersection (i.e. 3.2 km/h with braking) were calculated using the appropriate

fatigue model for each of the evaluated mixtures. Since the fatigue models have not

been calibrated against actual field performance data, the ratios of the fatigue lives

were used to evaluate the relative performance of the evaluated mixtures. Table 6

summarizes the ratios of the fatigue lives defined as the ratio of the fatigue life of a

given mix over the fatigue life of the Nevada T2C mix.

Under normal highway speed and in the 100-mm HMA layer, the numbers of

load repetitions to fatigue failure of the CT and NRM mixes were 1.8 and 1.4 times

the number of load repetitions to fatigue failure of the NDOT T2C mix, respectively.

On the other hand, in the 200-mm HMA layer, the numbers of load repetitions to

fatigue failure of the CT and NRM mixes were 1.4 and 1.1 times the number of load

repetitions to fatigue failure of the NDOT T2C mix, respectively.

At intersection and in the 100-mm HMA layer, the numbers of load repetitions to

fatigue failure of the CT and NRM mixes were 3.2 and 1.7 times the number of load

repetitions to fatigue failure of the NDOT T2C mix, respectively. On the other hand,

in the 200-mm HMA layer, the numbers of load repetitions to fatigue failure of the

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CT and NRM mixes were 3.0 and 1.4 times higher than the number of load

repetitions to fatigue failure of the NDOT T2C mix, respectively.

7. Overall summary of findings

The Nevada Department of Transportation (NDOT) is currently specifying the

same type of gradation (i.e. Type 2C) for most of its HMA mixes regardless whether

the mix will be placed on highway or intersection. Although, the NDOT Type 2C

gradation showed excellent rutting resistance under normal highway traffic loading,

its resistance to rutting and shoving at intersections in the hot environment of

southern Nevada was not promising.

The rutting and fatigue resistance of the NRM, CT, and NDOT T2C mixes were

relatively compared and ranked based on the laboratory evaluations and the

mechanistic-empirical analysis. The mix that ranked first was given a score of 1

while the mix that ranked last was given a score of 3. Then, the sums of all scores

were then calculated for each mix and are summarized in Table 7. The mix with the

lowest sum of scores is ranked first overall while the mix with the highest sum of

scores is ranked last.

Based on the data generated from the laboratory experiment and the mechanistic

analyses, the following conclusion can be made.

– When the laboratory rutting resistances of the mixes were compared, the NRM

mix showed higher rutting resistance than the CT mix and the NDOT T2C mix in

the RLT test at 70qC and in the APA test at 76qC. Additionally, the CT mix showed

higher rutting resistance than the NDOT T2C mix in the RLT and in the APA tests.

– When the laboratory fatigue resistances of the mixes were compared, the CT

mix showed higher fatigue resistance than the NRM mix and the NDOT T2C mix.

Also, the NDOT T2C mix showed higher fatigue resistance than the NRM mix.

– The dynamic modulus of the NRM mix is higher than the dynamic modulus of

the CT mix and the NDOT T2C mix at 21°C. The dynamic modulus of the CT mix

was higher than the dynamic modulus of the NDOT T2C mix only at a frequency

higher than 0.1 Hz.

– When combining the laboratory fatigue and rutting characteristics of the mixes

with their mechanical properties (|E*|) into the mechanistic-empirical analysis, the

following observations were obtained:

- Rutting analysis: Under normal highway speed and at intersection, the NRM

mix exhibited a rut depth slightly lower than the CT and the NDOT T2C mixes

which showed similar rut depths regardless of the HMA layer thickness.

- Fatigue analysis: The fatigue life of the CT mix is better than the fatigue life

of the NDOT T2C mix and the NRM mix under both normal highway speed and at

intersection. Additionally, the fatigue life of the NRM mix is better than the fatigue

life of the NDOT T2C mix. However, the polymer-modified PG76-22 mix has

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shown excellent fatigue performance under Nevada’s conditions, and therefore, the

relatively lower fatigue resistance of the T2C mix should not be of a great concern.

Table 7. Relative ranking of the various mixes in terms of rutting and fatigue

resistance

Individual scoresType of

EvaluationProperty NDOT

T2CCT NRM

At 30°C 1 1 1

At 50°C 2 1 1

Rutting resistance in the

Repeated Load Triaxial

(RLT) testAt 70°C 3 2 1

At 60°C 1 1 1Rutting resistance in the

Asphalt Pavement

Analyzer (APA) test At 76°C 3 2 1

At 4.4°C 2 1 3

At 21°C 2 1 2

Laboratory

evaluation

Fatigue cracking

resistance

At 43°C 2 1 3

Normal

highway speed2 2 1Rutting resistance for

12,000 loading

repetitions Intersection 2 2 1

Normal

highway speed3 1 2

Mechanistic

- empirical

analysis

Fatigue resistance

Intersection 3 1 2

Total sum of scores 26 16 19

Overall ranking 3 1 2

In summary, it can be concluded that the CT mix offers advantages in the fatigue

resistance when compared to the other mixes, whereas the NRM mix offers

advantages in rutting resistance when compared to the other mixes. Based on the

overall ranking of the various mixtures, the CT mix ranked the first followed closely

by the NRM mix. The NDOT T2C was ranked last with a significantly higher

overall sum of scores. However, it should be noted that the mechanistic-empirical

analysis showed in general the resistance to rutting of the NDOT T2C mix to be

similar to that of the CT mix, differing from the laboratory evaluation test results.

This is due to the interaction between the rutting characteristics of the mixes with

their stiffness properties (|E*|) in the mechanistic-empirical analysis. On the other

hand, the NDOT T2C mix showed an acceptable resistance to fatigue cracking.

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8. Recommendations

Based on the data generated in this study, the NDOT T2C gradation with the

PG76-22 binder is expected to provide acceptable resistance to rutting with a

marginal fatigue cracking resistance. On the other hand, a significant improvement

in mixtures’ resistance to rutting and fatigue cracking is expected with the CT and

NRM gradations.

a)

b)

Figure 11. (a) Comparison of the NRM and NDOT T2C gradations; (b) Proposed

specification limits for intersection mixes

Consequently, NDOT should consider modifying the NDOT T2C gradation to

closely resemble to the NRM gradation. The NRM gradation was recommended

instead of the CT gradation since it involves fewer modifications to the currently

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used NDOT T2C gradation. Figure 11a compares the NDOT T2C and NRM

gradations along with the upper and lower limits specifications of the NDOT T2C

gradation. The data in Figure 11a show that the NRM gradation violates the NDOT

T2C limits only on the 2-mm sieve. Therefore, it is recommended that, for use on

high traffic intersections, NDOT lowers the limit on the percent passing of the T2C

gradation on the 4.75-mm sieve by 3-5 percentage points which will make the T2C

closer to the NRM gradation. This requires lowering the T2C specification limits

between 4.75-mm and 0.425-mm as shown in Figure 11b to force the gradation to

cross the maximum density line. Based on the results of this study, moving the T2C

gradation closer to the NRM gradation (i.e. lowering its percent passing the 4.75-

mm sieve) should significantly improves its resistance to rutting and fatigue

cracking.

Acknowledgement

The authors gratefully acknowledge the support of the Nevada Department of

Transportation.

9. Bibliography

AASHTO PP62-09, “Developing dynamic modulus master curves for hot mix asphalt

(HMA)”, Standard specifications for transportation materials and methods of sampling

and testing, Provisional Standards, 26th edition, 2010.

AASHTO T321-07, “Determining the fatigue life of compacted hot mix asphalt (HMA)

subjected to repeated flexural bending”, Standard specifications for transportation

materials and methods of sampling and testing, 26th edition, 2008.

AASHTO TP62-07, “Determining dynamic modulus of hot mix asphalt (HMA)”, Standard

specifications for transportation materials and methods of sampling and testing, 26th

edition, 2008.

AASHTO TP63-03, “Determining rutting susceptibility of hot mix asphalt (HMA) using the

asphalt pavement analyzer (APA)”, Standard specifications for transportation materials

and methods of sampling and testing, 26th edition, 2008.

Barksdale R., “Laboratory determination of resilient modulus for flexible pavement design:

final report”, NCHRP Web Doc 14, 1997, National Co-operative Highway Research

Program, Washington DC.

De Beer M., and Fisher C., “Contact stresses of pneumatic tires measured with the vehicle-

road surface pressure transducer array (VRSPTA) system for the University of California

at Berkeley (UCB) and the Nevada Automotive Test Center (NATC)”, CSIR Report CR-

97/053,1997, Council for Sc. and Ind. Res., Pretoria, South Africa.

Hajj E.Y., Hot mix asphalt mixtures for Nevada’s intersections, Ph.D. Dissertation, 2005,

Department of Civil & Environmental Engineering, University of Nevada, Reno, U.S.

Dow

nloa

ded

by [

Nov

a So

uthe

aste

rn U

nive

rsity

] at

06:

25 0

8 O

ctob

er 2

014


Hajj E.Y., Sebaaly P.E., and Siddharthan R.V., “Response of asphalt pavement mixture under

a slow moving truck”, Asphalt Concrete: Simulation, Modeling, and Experimental

Characterization, ASCEGeotechnical Special Publication, No. 146, 2006, p. 134-146.

Hajj E.Y., Siddharthan R.V., Sebaaly P.E., and Weitzel D.C., “Hot mix asphalt mixtures for

Nevada’s intersections”, Transportation Research Records, Vol. 2001, 2007, p. 73-83.

Harvey J., and Tsai B.W., “Effects of Asphalt Content and Air-Void Content on Mix Fatigue

and Stiffness”, Transportation Research Record, Vol. 1543, 1996, p. 38-45.

Marker V., “Problems with asphalt pavements”, Paper presented at the Asphalt State-of-the-

Art Conference, Woodbridge, NJ, May 11, 1981.

Myers L., Roque R., Ruth B., and Drakos C., “Measurement of contact stresses for different

truck tire types to evaluate their influence on near-surface cracking and rutting”,

Transportation Research Records, Vol.1655, 1999, p. 175-184.

NCHRP, “Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement

Structures”, National Cooperative Highway Research Program, Transportation Research

Board, National Research Council, 2004.

Partl M.N., and Newman J.K., “Flexural beam fatigue properties of airfield asphalt mixtures

containing styrene–butadiene based polymer modifiers”, Proceedings of the Sixth

International Rilem Symposium, 2003, Zurich, Switzerland, p. 357-363.

Rosenberg C., and Buncher M., “High performance hot mix asphalt intersection strategy”,

Executive Offices and Research Center, Lexington, Kentucky, 1999.

Rosset J.M., “Dynamic and damping coefficients of foundations”, Proceedings of the ASCE

National Convention on Dynamic Response of Pile Foundations: Analytical Aspects,

1980, New York, p. 1-30.

Sebaaly P.E., “Dynamic forces on pavements: summary of tire testing data”, Report: FHWA-

DTFH 61-90-C-00084, 1992, FHWA, U.S. Department of Transportation.

Sebaaly P.E., Hajj E.Y., Bazi G., Perez J., “Pavement design and materials research: 2003-

2005 activities”, Final Report, 2005, Materials Division, Carson City, NV.

Siddharthan R.V., Yao J., and Sebaaly P.E., “Pavement strain from moving dynamic 3D load

distribution”, Journal of Transportation Engineering, Vol. 124, No. 6, 1998, p. 557-566.

Sousa J.B., and Weissman S.L., “Modeling permanent deformation of asphalt aggregate

mixes”, Journal of the Association of Asphalt Paving Technologist, Vol. 63, 1994, p. 224-

257.

Vavrik W.R., Pine W.J., Huber G.A., Carpenter, S. H., and Bailey, R., “The bailey method of

gradation evaluation: the influence of aggregate gradation and packing characteristics on

voids in the mineral aggregate”, Journal of the Association of Asphalt Paving

Technologists, Vol. 70, 2001, p. 132-175.

Vavrik W.R., Pine W.J., and Carpenter S.H., “Aggregate blending for asphalt mix design:

Bailey method”, Transportation Research Records, Vol. 1789, 2002, p. 146-153.

Dow

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by [

Nov

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aste

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rsity

] at

06:

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Weissman S.L., “Influence of tire-pavement contact stress distribution on development of

distress mechanisms in pavements”, Transportation Research Records, Vol. 1655, 1999,

p. 161-167.

Williams S.G., “The effects of HMA mixture characteristics on rutting susceptibility”,

Transportation Research Board, 2003.

Received: 7 July 2009

Accepted: 29 September 2010

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Documents

Evaluation of Rut Resistant Asphalt Mixtures for Intersection