Investigation of In-Place Asphalt Film Thickness and Performance of Hot-Mix Asphalt Mixtures

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Investigation of In-Place Asphalt Film Thickness andPerformance of Hot-Mix Asphalt Mixtures

Xinjun Li1; R. Christopher Williams2; Mihai O. Marasteanu3; Timothy R. Clyne4; and Eddie Johnson5

Abstract: This research work investigates approaches for computing the average asphalt film thickness in asphalt mixtures and examineswhether the in-place asphalt film thickness has a rational relationship with the performance of hot-mix asphalt mixtures. Two methods forcalculating the aggregate surface areas, the surface area factor and index methods, are discussed in this research and the analytical resultsshow that both the fine aggregate particles and aggregate shapes significantly affect the calculation and should be taken into considerationto obtain greater accuracy for the surface area. A computation method is proposed and significantly improves the current conventionalmethod for the asphalt film thickness calculation by incorporating shape factors and flat surface factors for all sieve sizes. Fieldperformance data from MnROAD and rutting data from laboratory fabricated mixtures, including both coarse and fine gradations, wereanalyzed to investigate the asphalt film thickness and corresponding performance of asphalt mixtures. Both the field data and laboratoryexperimental results show that the asphalt film thickness is a significant factor affecting the rutting performance for asphalt mixtures.However, more research work is needed to investigate the relationship between the asphalt film thickness and the other performanceparameters of asphalt mixtures, such as fatigue cracking, before a film thickness specification can be proposed.

DOI: 10.1061/�ASCE�0899-1561�2009�21:6�262�

CE Database subject headings: Asphalts; Mixtures; Thickness; Cracking.

Introduction

Hot-mix asphalt �HMA� is a mixture of asphalt cement, mineralaggregates and air, and the properties of this material are signifi-cantly influenced by its components. The asphalt mixture designprocess generally requires a balance of various desirable mixtureproperties with an attempt to optimize the selection and propor-tions of these different components. Durability, which reflects amaterial’s ability to provide long-term performance without ab-normal raveling and cracking of the paving surface, is one of theprimary set of distresses addressed in a mix design.

An increasing number of researchers have recently discussedproblems in achieving the minimum voids in the mineral aggre-gate �VMA� requirements specified in the Superpave volumetricmix design. Generally, meeting the minimum VMA requirementscan be difficult to achieve in the mix design process. On the otherhand, not attaining VMA requirements can result in mixes beingrejected that may have acceptable performance, whereas some

1Research Associate, Dept. of Civil, Construction, and EnvironmentalEngineering, Iowa State Univ., Ames, IA 50011. E-mail: [email protected]

2Associate Professor, Dept. of Civil, Construction, and EnvironmentalEngineering, Iowa State Univ., Ames, IA 50011.

3Associate Professor, Dept. of Civil Engineering, Univ. of Minnesota,Minneapolis, MN 55455.

4Research Forensic Engineer, Office of Materials, Minnesota Depart-ment of Transportation, Maplewood, MN 55109-2044.

5Research Project Engineer, Office of Materials, Minnesota Depart-ment of Transportation, Maplewood, MN 55109-2044.

Note. This manuscript was submitted on January 7, 2008; approvedon December 29, 2008; published online on May 15, 2009. Discussionperiod open until November 1, 2009; separate discussions must be sub-mitted for individual papers. This paper is part of the Journal of Mate-rials in Civil Engineering, Vol. 21, No. 6, June 1, 2009. ©ASCE, ISSN
0899-1561/2009/6-262–270/$25.00.
262 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / JUNE 2

J. Mater. Civ. Eng. 200

mixes meeting the VMA requirements may experience poor per-formance. Very little rational data correlating pavement perfor-mance with the currently specified minimum VMA values for mixdesign has been presented in previous research studies regardingthe volumetric properties of asphalt mixtures �Kandhal et al.1998; Coree and Hislop 1999�. Alternatively, some asphalt tech-nologists �Kandhal and Chakraborty 1996; Hinrichsen and Heg-ger 1996; Heitzman 2006� recommend replacing the minimumVMA requirement and adopting a minimum average film thick-ness, because the film thickness approach represents a more di-rect, equitable, and appropriate method of ensuring mixdurability. Similar to VMA, the average film thickness of an as-phalt mixture has never been a physically measured parameter,but a computed number. The method employed by most highwayagencies to calculate the film thickness in asphalt mixtures wasdeveloped half a century ago and now this method should beupdated to attain a more accurate computation. Further, a limitedamount of work has been done to investigate the performance ofthe HMA in the previous studies regarding asphalt film thickness.

The intent of this research is to examine whether the in-placeasphalt film thickness has a rational relationship with the perfor-mance of HMA. The methods to calculate asphalt film thicknesswere reviewed and the assumptions and limitations of these meth-ods were analyzed. A new method was proposed to compute as-phalt film thickness with greater accuracy. Field data fromMnROAD and rutting data from laboratory fabricated mixtures,including both coarse and fine gradations, were analyzed to in-vestigate asphalt film thickness and its effect on HMA perfor-mance.

Mixture Volumetric Properties and Durability

A mixture is said to be durable when it offers long-term resistance
to weathering and aging and provides good performance without
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abnormal raveling and cracking of the paving surface �Kumar andGeotz 1977�. The durability of asphalt mixtures cannot be dis-cussed separately by individual components due to the fact that adurable asphalt binder and a durable aggregate do not necessarilyensure a durable mixture. Numerous research efforts have beenperformed to investigate the relationship between asphalt mixturevolumetric properties and material durability.

In one of his first efforts to study the volumetric properties ofasphalt mixtures, McLeod proposed VMA as a volumetric param-eter to ensure that mixture gradation had sufficient space for boththe air voids and a desired minimum asphalt content �McLeod1956�. The research effort by Campen et al. �1957� showed thatthe VMA for asphalt mixtures can be controlled quantitatively bygradation. The authors defined in this research a satisfactory mix-ture as the aggregate in this mixture to contain enough voids topermit the addition of sufficient asphalt to provide comparativelythick films without filling all the voids in the aggregate.

In another study, Campen et al. �1959� investigated the rela-tionships between voids, surface area, film thickness, and stabilityin asphalt mixtures. The asphalt film thickness was calculated inthis study by assuming that all the asphalt exists in uniform films.An average film thickness ranging from 6 to 8 �m was found inthis research to provide the most desirable pavement mixtures.This study also illustrated that although the asphalt required toproduce minimum aggregate voids does increase with surfacearea, the increase is not directly proportional to the surface area.Another research effort by Goode and Lufsey �1965� also con-cluded that an average film thickness of 6 �m is adequate toachieve stable mixtures. This study focused on relating asphalthardening to voids, permeability, and asphalt film thickness. Itwas found that the air voids combined with asphalt film thicknesswere important factors affecting the rate of asphalt hardening,whereas the air permeability or air void content was found to notsignificantly affect the rate of asphalt hardening. Kumar andGoetz �1977� also recognized that a longer period of time will beneeded for the HMA with a thicker film to age harden.

In a more recent study, Hinrichsen and Heggen �1996� pro-posed using average asphalt film thickness in mixture design byquestioning the use of rigid minimum VMA specifications.Kandhal and Chakraborty �1996� examined the rationale behindthe minimum VMA requirements and recommended an averagefilm thickness of 9–10 �m for asphalt mixtures compacted at 8%air voids. In another study, Kandhal et al. �1998� concluded thatthe current minimum VMA requirements are inadequate for en-suring mixture durability and they recommended replacing theminimum VMA requirement with a minimum average film thick-ness of 8 �m.

Nukunya et al. �2001� evaluated VMA film thickness, asphaltcontent, and aggregate surface area on the durability �age-hardening rate�, fracture resistance, and rutting resistance of Su-perpave mixtures. The research results were that neither VMA norfilm thickness, as currently calculated, provide a parameter thatadequately reflects the age hardening rate of binders in asphaltmixtures. In another recent research study, Ruth et al. �2002� rec-ognized the strong dependency of fine aggregate gradation andmineral filler content on the surface area of mixtures.

Anderson and Bentsen �2001� showed that the gradation orVMA had no significant effect on the fatigue cracking resistanceof the asphalt mixtures, whereas more significant differences be-
tween 13 and 15% VMA were apparent with shear test results.
JOURNAL


Asphalt Film Thickness Calculation

There is a general agreement that the asphalt film thickness of anasphalt mixture is a computed number and it can not be obtainedby measuring the actual film thickness for a small amount ofmixture material. From this perspective, asphalt film thickness isa conceptual average value or an index, rather than a physicallymeasured number. Therefore, it has been a priority to develop anapproach to calculate film thickness in asphalt mixtures accu-rately. Different calculation methods were developed as the con-cept of film thickness was originally promoted.

Most asphalt film thickness calculations are based upon theaggregate surface area determination, which assumes that the ef-fective asphalt binder is coating aggregate particles over a flatsurface with equal film thicknesses and that all aggregate particlesare spherical. It should be noted that the assumption of taking afilm around a spherical particle and “laying” it flat overestimatesthe film thickness by ignoring the three dimensional aspect. Theassumption of spherical aggregate particles also limits the appli-cation of this method. A new approach was recently developed byRadovskiy �2003� based upon a theory in statistical geometry ofrandom heterogeneous materials such as particulated composites,liquids, suspensions, colloidal dispersions, etc. �Lu and Torquato1992�. By assuming that asphalt film thickness is a function of airvoids and the volume fraction of effective aggregates, this methodinvolves treating all aggregate particles as spheres being dis-persed in a liquid matrix and allowing them to achieve their de-sired positions according to equilibrium statistics. It is wellknown that the asphalt binder content is related to the target airvoids or compactive effort during a mixture design process. Thismeans that the compactive effort should be taken into accountduring the asphalt film thickness calculation if this thicknessvalue is used for mixture design. However, the effect of compac-tive effort should not be taken into consideration during the filmthickness calculation when the asphalt content is known. In arecent extensive study, Heitzman �2006� developed a method toinclude the aggregate particle shape effect in the calculation andhe also did significant work on the sensitivity analysis with vari-ous factors for different calculation methods. However, limitedanalysis was presented for the aggregate shape effect and theapproach is still based upon the assumption that asphalt forms afilm over a plane surface.

Based upon the above-mentioned analysis, the asphalt filmthickness can be calculated from the following equation:

FT =Vbe

SAWS=

Pbe

SAPSGb�1�

where FT=asphalt film thickness in mixture �m�; Vbe=volume ofeffective asphalt binder �m3�; SA=aggregate surface area�m2 /kg�; WS=weight of aggregate �kg�; Pbe=effective bindercontent by weight of mixture �%�; Gb=specific gravity of theasphalt binder; PS=percent of aggregate by mix weight; and

PS =1 − Pbe

1 + Pba�2�

where Pba=percent of absorbed asphalt binder by aggregateweight.

Substituting Eq. �2� into Eq. �1�, the following equation isderived:

FT =Pbe�1 + Pba� �3�

SA�1 − Pbe�Gb

OF MATERIALS IN CIVIL ENGINEERING © ASCE / JUNE 2009 / 263

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It should be noted that this method was originally developedbased upon the assumption that asphalt forms a film over theplane surface �two dimensional�, as shown in Fig. 1.

The assumption of a plane surface can lead to errors in thecalculation. For a spherical particle of a diameter D coated by aspherical asphalt shell, as shown in Fig. 2, the total volume of theaggregate and the asphalt binder is shown in

Vtotal = 16��D + 2FT�3 = Vagg + Vshell = 1

6�D3 + Vshell �4�

where Vtotal= the total volume of the spherical aggregate and thecoating film; Vagg= the volume of the spherical aggregate; Vshell

= the volume of the asphalt film coating the spherical aggregate;and D=the diameter of the spherical aggregate.

From Eq. �4�, the exact film thickness FT1 for the aggregateparticle with diameter D coated by a spherical asphalt shell isshown in the following equation �Radovskiy 2003�:

FT1 =1

2D��1 +

Vshell

Vagg�1/3

− 1� �5�

On the other hand, the film thickness calculated from an assumedflat surface is shown in the following equation as FT2:

FT2 =Vshell

�D2 =1

6D�Vshell

Vagg� �6�

The error from the plane surface assumption can then be cal-culated using Error= �FT2−FT1� /FT1. This error can be calculatedfor each sieve size aggregate by assuming a fixed actual asphaltfilm thickness. An analysis for different aggregate sizes with threeactual film thicknesses �5, 10, and 15 �m� that are close to thevalues recommended by other researchers was performed and theresults are shown Table 1. These data are plotted in Fig. 3.

It can be seen from Fig. 3 that the flat surface assumptionoverestimates the film thickness and the error caused by this as-sumption will be significant for aggregates smaller than 0.3 mmin diameter �No. 50 sieve�.

Film thickness

Fig. 1. Illustration of conventional film thickness definition

D D+2*FT

Fig. 2. Spherical aggregate and the coating binder film



From Eq. �3�, the critical component in calculating the asphaltfilm thickness for this method is the determination of the aggre-gate surface area. The following section will introduce two meth-ods for calculating the aggregate surface area.

Aggregate Surface Area Factors Method

The specific surface area of aggregate can be determined from asieve analysis based upon the total percentage passing a stipulatedgroup of sieves:

SA =1

100� �

i=0

N

�PPi � CPi� =1

100� �

i=0

N

�PRi � CRi� �7�

where SA=specific surface area of the aggregate �m2 /kg�; PPi

=percent of aggregate by weight passing the ith sieve; CPi

=surface area factor �m2 /kg�; PRi=percent of aggregate byweight retained in the ith sieve; CRi=specific surface of the par-ticles with diameter in the range �Di ,Di−1�, �m2 /kg�, as shown inFig. 4; i=sieve index �i=0,1 , . . . ,N�; and N=set of sieves for theanalysis �N=7 for the standard set of ASTM sieves for HMA�.

The specific surface of the particles with diameter in the range�Di ,Di−1� can be computed using.

Table 1. Error by Assumption of Plane Surface

D �mm�

Actual filmthickness=5 �m



Calculatedvalue ��

Error�%�


Error�%�


Error�%�

0.03 6.85 37.04 18.15 81.48 35.00 133.33

0.075 5.70 13.93 12.90 29.04 21.80 45.33

0.15 5.34 6.81 11.39 13.93 18.20 21.33

0.3 5.17 3.37 10.68 6.81 16.55 10.33

0.6 5.08 1.68 10.34 3.37 15.76 5.08

1.18 5.04 0.85 10.17 1.70 15.38 2.56

2.36 5.02 0.42 10.08 0.85 15.19 1.28

4.75 5.01 0.21 10.04 0.42 15.09 0.63

9.5 5.01 0.11 10.02 0.21 15.05 0.32

12.5 5.00 0.08 10.02 0.16 15.04 0.24

19 5.00 0.05 10.01 0.11 15.02 0.16

25 5.00 0.04 10.01 0.08 15.02 0.12

0

20

40

60

80

100

120

140

Aggregate size (mm)

Error(%)

5 μ film thickness

10 μ film thickness

15 μ film thickness

0.03 0.3 1.18 2.36 4.75 9.5 12.5 19.5

Fig. 3. Errors by plane surface assumption

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CRi =1

2� Si

mi+

Si−1

mi−1� �8�

where Si=surface of each spherical particle retained on the ithsieve �m2� and

Si = 4�ri2 = �Di

2 �9�

where mi=mass of each spherical particle retained on the ith sieve�kg� and

mi = Vi�i =4

3�ri

3�i =�

6Di

3�i �10�

where Vi=volume of each spherical particle retained on the ithsieve �m3� and �i=effective density of aggregate �kg /m3�.

Substituting Eqs. �9� and �10� Eq. �8�, the specific surface ofthe particles can be determined by �Radovskiy 2003�

CRi = 3� 1

�iDi+

1

�i−1Di−1� �11�

The percentage of aggregate by weight retained on the ith.sieve �PRi� can be obtained from the percentages of aggregate byweight passing the ith and i−1th sieves, as shown in

PRi = PPi−1 − PPi. �12�

Substituting Eq. �12� into Eq. �1�, the specific surface area ofthe aggregate is determined as follows �Radovskiy 2003�:

SA = 0.01��PP0 − PP1�CR1 + �PP1 − PP2�CR2 + ¯

+ �PPN − PPN+1�CRN+1

= 0.01�PP0CR1 + �CR2 − CR1�PP1 + ¯ + �CRN+1 − CRN�PPN

�13�

From Eq. �13�, the relationship between the surface area fac-tors and specific surface of the particles is determined as shown in

CP0 = CR1

CPi = CRi+1 − CRi �14�

Substituting Eq. �11� into Eq. �14�, the following equation isdeveloped �Radovskiy 2003�:

CP0 = 3� 1

�0D0+

1

�1D1�

CPi = 3� 1

�i+1Di+1−

1

�i−1Di−1� �15�

From Eq. �15�, it can be seen that the surface area factor is notonly related to the aggregate particle size, but also the effectivedensity. Therefore, effective density should be an input in thecalculation for the aggregate surface area factor. As an example,Table 2 shows the surface area factors computed using Eq. �15�with effective density of aggregates from Radovskiy �2003�. Thesurface area factors specified in MS-4 �Asphalt Institute 1995�

(i-1)th Sieve

ith Sieve

Fig. 4. Schematic of aggregates retained in two continuous sieves

and introduced by Radovskiy �2003� are also presented in Table 2

JOURNAL


for comparison. It should be noted that the aggregate MS-4 sur-face area factors listed in Table 2 were taken from Radovskiy�2003� and are only available for aggregate sizes ranging from0.075 to 9.5 mm. The results show that the calculated and MS-4surface area factors are very close. It is also shown that the sur-face area factor significantly increases with the decrease in theaggregate size. This illustrates that for two groups of aggregateparticles with one unit weight and the same effective gravity, theaggregate group with a smaller particle size has a much highersurface area than the group with a greater particle size.

It should be noted that the surface area factors calculated usingEq. �15� are based upon the assumption that the aggregate isspherical and it does not take the aggregate shape into consider-ation. This limits the application of this method to some extent. Italso can be noticed from Table 2 that the aggregate particlessmaller than 0.075 mm �No. 200 sieve� have very high surfacearea factor and this fraction of aggregate in a mixture may have asignificant surface area even if just a small percentage. However,the current methods for film thickness calculation use surface areafactors for material smaller than the 0.075 mm �No. 200� sieve asthat for material retained on the 0.075 mm sieve. This indicatesthat the conventional surface factor of the minus 0.075 mm frac-tion is smaller than the specific surface area of this aggregatefraction, which also illustrates that the conventional surface factormethod can greatly underestimate the surface area and thereforeoverestimate the film thickness. It should be noted there is anissue about the function of the fine aggregate in the mixture.Kandhal et al. �1998� pointed out that the fine aggregate particlesmay have a much thicker coating than do coarse aggregate par-ticles, and some of very fine particles may simply be embedded inthe asphalt cement-filler mortar system.

Index Method for Aggregate Surface Area

The index method, on which Heitzman �2006� did significantwork, can take aggregate shape factors into consideration by em-ploying aggregate volume factors and surface area factors. Thismethod takes the gradations for each of the individual aggregate

Table 2. Calculated and Specified Surface Area Factors

Sieveindex, i

Sievesize,

Di �mm�

Effective densityof aggregate

retained,�1,000 kg /m3�

Calculatedsurface

area factor,�m2 /kg�

MS-4specified surface

area factor,�m2 /kg�

0 25 2.34 0.03

1 19 2.34 0.05 N/A

2 12.5 2.34 0.07

3 9.50 2.34 0.17 0.41

4 4.75 2.34 0.41 0.41

5 2.36 2.34 0.82 0.82

6 1.18 2.34 1.59 1.64

7 0.600 2.34 3.01 2.87

8 0.300 2.44 6.06 6.14

9 0.150 2.44 12.30 12.29

10 0.075 2.44 32.79 32.77

11 0.030 2.44 65.57

12 0.015 2.44 122.95 N/A

13 0.0075 2.44 327.87

0.003 2.44 N/A

sources into consideration and determines the retained weight of


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particles from each source on each sieve based on a 1,000 g totalaggregate batch weight. The volume retained on each sieve for a1,000 g batch can be computed using

Vin =Pci�PPi�n−1� − PPin�

Gsbi�16�

where Vin= total volume of aggregate of the ith source retained onthe nth sieve; Pci=percent of the ith source by aggregate weight;PPin=percent of aggregate of the ith source by weight passing thenth sieve; and Gsbi=bulk specific gravity of the aggregate fromthe ith source.

The volume of an average particle from each source on eachsieve can be obtained by

Vpin =4

3��1

2�Dn−1 + Dn

2��3

VFin �17�

where Vpin=volume of an average particle from ith source on thenth sieve; Dn=nominal opening of the nth sieve in millimeters;and VFin=volume factor for the particles from ith source on thenth sieve.

The surface area of the average particle from each source onthe different sieves can be determined using

SApin = 4��1

2�Dn−1 + Dn

2��2

SAFin �18�

where SApin=surface area of the average particle from ith sourceon the nth sieve and SAFin=surface area factor for the particlesfrom ith source on the nth sieve.

Once the surface area of the average particle is determined, thetotal surface area for all aggregates from each source and retainedon the same sieve can be calculated by multiplying the number ofthe particles from the same source and retained on the same sievefrom the 1,000 g batch following �Heitzman 2006�

SAin =Vin

VpinSApin =

12VinSAFin

�Dn−1 + Dn�VFin

=12Pci�PPi�n−1� − PPin�SAFin

Gsbi�Dn−1 + Dn�VFin�19�

where SAin=total surface area of the particles from the ith sourceon the nth sieve.

After the aggregate surface area of the mixture is determined,the film thickness can be calculated by �Heitzman 2006�:

FT =Vbe

SAWS=

Pbe

SAPSGb=

Pbe

��n=0N �i=1

n SAin�PSGb

. �20�

An extensive analysis was performed to investigate the influ-ence of different aggregate shape and elongation aspect ratios onsurface areas. It should be noted that this analysis is based on thesieve screen openings to account for the ability of aggregates withdifferent shapes to pass through a square opening. A total of sevendifferent aggregate shapes, which are shown in Table 3 and fiveaspect ratios from 1:1 to 5:1 were analyzed and the surface areafactor �SAF�, volume factor �VF�, and the surface factor ratio,which is the ratio of SAF to VF, were calculated for each aggre-gate shape and aspect ratio. The results are shown in Table 4.

Eq. �19� shows that a higher surface factor ratio leads to ahigher surface area. It can be found from the results in Table 4that both the aggregate shape and elongation aspect ratio can leadto significantly different surface factor ratios. It can be seen that
aggregate particles with a triangle pyramid shape have the highest


surface factor value of 2.45, and illustrates that under a constantweight the surface area from this shape will be 2.45 times that ofaggregate with a shape of sphere, cube or cylinder when all elon-gation aspect ratios for these shapes are 1:1. It can also be seenthat the surface area decreases with the increase of aspect ratio forall shapes analyzed in this research. Overall, the assumption of aspherical shape for all aggregate particles will underestimate thesurface area from the aggregate shape analysis, whereas the as-sumption of spherical aggregates will overestimate the surfacearea from the aspect ratio analysis. The effect of aggregate shapeand aspect ratio on the surface area needs further study to obtaina more realistic approximation.

It also should be noted the film thickness calculated from Eq.�20� is based upon the assumption of a flat surface. The error bythis assumption is analyzed in the previous section and this errorcan be minimized by integrating a flat surface error factor orcoefficient shown in Table 1 with the surface area computation foreach aggregate sieve size. In other words, both the flat surface

Table 3. Aggregate Particle Shapes

Sphere Cube Cylinder Cone

Elongated sphere Square pyramid Triangle pyramid Triangle prism

Table 4. Aggregate Shape Factor

Shape Parameter

Aspect ratio

1:1 2:1 3:1 4:1 5:1

Sphere SAF 1 2 3 4 5

VF 1.0 2.5 4.0 5.5 7.0

SF ratio 1.00 0.80 0.75 0.73 0.71

Cube SAF 1.9 3.2 4.5 5.7 7.0

VF 1.9 3.8 5.7 7.6 9.5

SF ratio 1.00 0.83 0.78 0.75 0.73

Cylinder SAF 1.5 2.5 3.5 4.5 5.5

VF 1.5 3.0 4.5 6.0 7.5

SF ratio 1.00 0.83 0.78 0.75 0.73

Cone SAF 0.8 1.3 1.8 2.3 2.8

VF 0.5 1.0 1.5 2.0 2.5

SF ratio 1.62 1.28 1.18 1.13 1.10

Trianglepyramid

SAF 0.6 0.9 1.3 1.7 2.1

VF 0.2 0.5 0.7 0.9 1.1

SF ratio 2.45 2.07 1.95 1.89 1.86

Triangleprism

SAF 1.2 2.2 3.1 4.1 5.1

VF 0.8 1.7 2.5 3.3 4.1

SF ratio 1.49 1.32 1.27 1.24 1.22

Squarepyramid

SAF 1.0 1.7 2.3 2.9 3.5

VF 0.6 1.3 1.9 2.5 3.2

SF ratio 1.62 1.34 1.18 1.13 1.10

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error factor that is used to minimize the error from the flat surfaceassumption and the actual aggregate shape factor can be inte-grated into Eq. �20�. By taking the actual aggregate shape intoconsideration and minimizing the error by the assumption of a flatsurface, the method shown in Eq. �20� to calculate asphalt filmthickness is a more reasonable approach and this will help toobtain a more accurate computational result.
Asphalt Mixture Performance and Film Thickness

Very limited asphalt mixture performance data were presented inprevious studies regarding asphalt film thickness. Field data fromMnROAD and rutting data from laboratory fabricated mixturesincluding both coarse and fine gradations, were analyzed to in-vestigate the asphalt film thickness and performance of HMA.

MnROAD Field Data

The 10-year HMA mainline sections at MnROAD �Cells 1-4,14-23� were constructed in 1993. Field samples were taken duringconstruction to obtain typical quality control/quality assurancedata. This data are used to calculate the asphalt film thickness forthese cells using the index method. The air voids for most fieldsamples are 3–4%. It should be noted that flat surface factors byassuming a film thickness of 10 �m were employed for the as-phalt surface calculation to minimize calculation errors. Table 5lists the statistics on asphalt film thickness for these cells. Table 5

Table 5. Asphalt Film Thickness ��m� for MnROAD 10-Year Mainline

Cell 1 2 3 4 14 1

Mean 7.26 8.26 7.68 7.12 7.96 7

Median 7.19 8.46 7.70 7.04 8.05 7

Standard deviation 0.37 0.44 0.52 0.42 0.56 0

Minimum 6.86 7.42 6.78 6.52 6.60 6

Maximum 7.85 8.84 8.73 7.88 8.93 9

Count 23 12 12 16 20 20

MS-4 7.46 8.49 7.88 7.32 8.17 8

Table 6. MnROAD Field Performance

CellIRI

�m/km�Rutting�mm�

Transversecracking

�linear m�

1 1.40 5.84 110.1

2 2.02 8.13 150.0

3 1.64 5.59 115.8

4 2.79 8.64 240.0

14 2.16 7.11 277.8

20 1.13 14.48 87.6

21 1.02 10.16 138.6

22 1.60 5.59 125.7

23 1.61 13.46 110.7

15 2.26 6.10 432.6

16 2.75 4.57 400.5

17 2.48 5.08 307.2

18 2.76 5.59 198.0

19 3.02 8.13 350.7aM-35=35 blow Marshall mix design; M-50=50 blow Marshall mix de
Superpave gyratory compactor.
JOURNAL


also lists in the last row the mean value of asphalt film thicknesscalculated using the MS-4 method. It can be seen that the asphaltfilm thickness obtained from MS-4 is always higher than thatcalculated from the index method.

The field performance data for these cells was obtained in2002 and is listed in Table 6. It can be seen that two differentasphalt binders, 120 /150 �PG 58-28� and AC-20 �PG 64-22�, andfour different types of mixture designs were used in these cellsincluding Marshall �35, 50, and 75 blows� and Superpave gyra-tory �100 design gyrations� ones. The differences in performanceare not clear when separating the pavement cells by mix designtypes. The field performance data is separated by the asphaltbinder and are plotted in Figs. 5–8, which shows the averagerutting versus asphalt film thickness, transverse cracking versusasphalt film thickness, international roughness index versus as-phalt film thickness and top-down cracking versus asphalt filmthickness, respectively. However, it should be noted that for mostfield performance plots the correlation coefficient is relatively lowand this is most likely due to the limited data collected in thisresearch. Therefore, the field performance data shown in thispaper is more to illustrate the trend between the asphalt filmthickness and field performance, rather than to give a quantitativeexplanation.

As expected, the field performance data shows that the averagerut depth increases as the asphalt film thickness increases. The rutdepth for the cells with the softer binder PG 58-28 �120 /150�increases more dramatically and it also has a greater amount of

16 17 18 19 20 21 22 23

7.44 7.97 8.70 8.90 8.90 8.35 7.83 8.52

7.50 7.86 8.64 8.97 9.05 8.38 7.80 8.42

0.31 0.51 0.54 0.56 0.57 0.34 0.31 0.64

6.85 7.07 7.78 7.72 7.98 7.44 7.41 7.29

7.94 8.73 9.97 9.93 9.80 9.03 8.51 9.77

16 16 16 16 16 16 16 20

7.64 8.18 8.94 9.14 9.14 8.58 8.05 8.75

op-downcrackinglinear m�

Filmthickness�microns�

Bindertype

Designmethoda

241.5 7.26 120 /150 M-75

256.8 8.26 120 /150 M-35

270.3 7.68 120 /150 M-50

264.0 7.12 120 /150 S-100

37.8 8.17 120 /150 M-75

5.1 9.14 120 /150 M-35

1.8 8.58 120 /150 M-50

12.9 8.05 120 /150 M-75

10.5 8.75 120 /150 M-50

102.3 8.17 AC-20 M-75

6.3 7.64 AC-20 S-100

159.0 8.18 AC-20 M-75

290.4 8.94 AC-20 M-50

267.9 9.14 AC-20 M-35

-75=75 blow Marshall mix design; and S-100=100 gyrations with a

Cells

5

.95

.99

.62

.68

.07

.17

T

�

sign; M


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rutting than the PG 64-22 �AC-20� binder. The field data alsoshows that the increase of asphalt film thickness will decrease theoccurrence of transverse cracking, though a relatively small cor-relation coefficient of less than 0.33 was found for the cells withthe PG 64-22 asphalt binder. This illustrates that a higher asphaltfilm thickness in a mixture may be more durable and resistant tothermal cracking. Cells with the stiffer PG 64-22 �AC-20� binder

R2 = 0.574

R2 = 0.6028

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

7.00 7.50 8.00 8.50 9.00 9.50

Film Thickness, microns

AverageRutting,mm

PG 58-28 (120/150)

PG 64-22 (AC-20)

Fig. 5. MnROAD rutting versus film thickness

R2 = 0.1313

R2 = 0.3259

0

50

100

150

200

250

300

350

400

450

500

7.00 7.50 8.00 8.50 9.00 9.50


TransverseCracking,linearm

eter

PG 58-28 (120/150)

PG 64-22 (AC-20)

Fig. 6. MnROAD transverse cracking versus film thickness

R2 = 0.3252

R2 = 0.2852

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

7.00 7.50 8.00 8.50 9.00 9.50


AverageIRI,m/km

PG 58-28 (120/150)

PG 64-22 (AC-20)

Fig. 7. MnROAD IRI versus film thickness



exhibits much more thermal cracking than cells with a softerbinder. International roughness index �IRI� describes the ridequality of a pavement—the higher the IRI, the rougher the pave-ment surface. For the stiffer binder, the IRI increases with in-creasing film thickness, and the opposite was found for the softerbinder. The ride is much rougher on the stiffer binder sectionsthan on the softer binder sections. Field performance data showsthat top-down cracking is very different for different binders. Forthe stiffer binder, an increase in film thickness led to a significantincrease in top-down cracking, whereas the opposite was foundfor the softer binder. This is consistent with the expectation thatthe stiffer binder leads to increased cracking. However, it alsoshould be noted that the field IRI and top-down cracking datashown in this research is not enough to make a persuasive state-ment and thus more field performance data is needed to do so.

In addition, it should be noted that the film thickness is not theonly factor that affects pavement performance. Consequently, fu-ture research work is necessary to examine the effect by otherfactors such as the structure of the pavement on the field perfor-mance of these cells.

Laboratory Mixture Performance Study

Four gradations including both coarse and fine gradations werestudied in this work, as shown in Fig. 9. One PG 58-28 unmodi-fied asphalt binder was used for the US-31 and US-27 mixturesand a PG 64-22 unmodified binder was used for the I-75 Auburnand I-75 Saginaw mixtures in this study. Three levels of air voids,the design value of 4%, and 8 and 12%, which represent a designvalue, a typical as-constructed value and a high air void �out ofconstruction specification�, respectively, were chosen to study theeffect of air voids on performance. Three levels of asphalt con-tent, the design value, the design value plus 0.5% and the designvalue minus 0.5%, were selected to quantify the effect of bindercontent �and film thickness� on the performance. The mixturecombination for asphalt content and air voids for each gradationis shown in Table 7. All mixtures were gyratory compacted withdimensions of 150 mm in diameter and 75 mm in height using theSuperpave design procedure outlined in SP-2 �Asphalt Institute1996�. Three replicates were tested for each mixture at 58 and64°C for rutting resistance using an asphalt pavement analyzer�APA� based upon the high temperature binder grade. The testing

R2 = 0.5847

R2 = 0.9343

0

50

100

150

200

250

300

350

7.00 7.50 8.00 8.50 9.00 9.50


Top-DownCracking,linearm

eter

PG 58-28 (120/150)PG 64-22 (AC-20)

Fig. 8. MnROAD top-down cracking versus film thickness

load was 445 N with a hose pressure of 689 kPa. The rutting data

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at 8,000 test wheel load cycles was recorded and analyzed.The surface area for each gradation was calculated using the

Index Method with a flat surface factor for each sieve size. Thesurface areas for the three coarse gradations, US-31, US-27 andI-75 �Auburn� are 4.5, 4.9, and 5.0 m2 /kg, whereas the fine gra-dation I-75 �Saginaw� has a much greater surface area of6.8 m2 /kg. This is consistent with the previous analysis that finegradations have higher surface areas. The computed asphalt filmthickness and the average experimental data for the rutting foreach mixture is shown in Table 8. The coefficient of variation�CoV� for the rutting was calculated and is shown as the lastcolumn in Table 8. It is shown that most CoV values for ruttingwere less than 25%. The rutting data was grouped by air voidsand these data were plotted versus asphalt film thickness, whichare shown in Fig. 10.

Fig. 10 shows that as the average asphalt film thickness in-creases the amount of APA rutting increases. This is consistentwith the field performance data from MnROAD analyzed in theprevious section. The air voids content was also found to signifi-cantly influence the rutting depth for all mixtures. The higher airvoids exhibit higher rutting depth with the other variables thesame. It was found that the correlation for the 12% air voids datais much lower than that for the other two air voids data, whichillustrates that the average rutting depth at high air voids is alsosignificantly related to other variables such as air void content. Atarget film thickness value can be obtained from Fig. 10 with atarget air voids and rutting depth, but should be done consideringfatigue cracking performance as well. Unfortunately, no labora-tory fatigue cracking data exists for these mixtures.

Summary and Conclusions

An increasing number of researchers have recently discussedproblems in achieving the minimum voids in the mineral aggre-

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4Sieve size (^0.45) (mm)

Passingpercent(%)

US-31

US-27

I75 Auburn

I75 Saginaw

Fig. 9. Gradations for the laboratory fabricated mixtures

Table 7. Mixture Combination for Asphalt Content and Air Voids

Asphalt content

Air voids

4% 8% 12%

Optimum−0.5% X X

Optimum X X X

Optimum+0.5% X X

JOURNAL


gate �VMA� requirements specified in the Superpave volumetricmix design and recommended not using the minimum VMA re-quirement and adopting a minimum average film thickness be-cause the film thickness approach represents a more direct,equitable, and appropriate method of ensuring mix durability.This research work investigates the approaches computing the

Table 8. Asphalt Film Thickness and Rutting Data

ProjectAir voids

�%�

Asphalt filmthickness

��m�

Rutting

Mean�mm�

CoV�%�

US-31 4.2 13.02 4.85 11.9

4.1 13.64 7.02 28.5

US-27 3.9 5.22 3.19 8.0

3.8 5.75 3.88 11.5

I-75 Auburn 4.0 10.82 4.88 16.2

4.1 11.35 5.38 19.0

I-75 Saginaw 4.2 7.55 4.16 24.7

4.2 7.94 4.89 20.1

US-31 8.3 12.41 7.22 14.7

8.3 13.64 8.96 22.8

8.2 13.02 7.76 15.1

US-27 7.9 4.70 4.44 8.7

7.7 5.75 5.63 9.2

8.0 5.22 4.27 18.9

I-75 Auburn 8.1 10.29 5.24 6.7

7.9 10.82 7.86 14.8

7.9 11.35 7.63 12.2

I-75 Saginaw 8.1 7.17 5.54 19.2

8.0 7.55 6.5 16.4

8.1 7.94 8.21 14.0

US-31 11.8 13.02 12.61 2.4

11.9 12.41 11.81 11.8

US-27 11.7 4.70 6.39 14.8

11.9 5.22 6.74 16.5

I-75 Auburn 11.6 10.82 9.69 13.2

11.8 10.29 6.1 16.4

I-75 Saginaw 11.8 7.17 11.19 2.4

11.7 7.55 11.37 2.2

R2 = 0.3306 R2 = 0.6155

R2 = 0.7228

0

2

4

6

8

10

12

14

2 4 6 8 10 12 14 16

Film Thickness (micro)

Rutting(mm)

4% AV8% AV12% AV

Fig. 10. APA rutting versus asphalt film thickness for laboratorymixtures


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average asphalt film thickness in asphalt mixtures and examineswhether the in-place asphalt film thickness has a rational relation-ship with the performance of hot-mix asphalt mixtures.

The average asphalt film thickness is not a measured physicalproperty, but a parameter calculated from a material’s volumetriccharacteristics and gradation. The assumption and simplificationduring the computation procedure limits the application of theconventional approach in determining the asphalt film thickness.Two methods for calculating the aggregate surface areas, the sur-face area factor method and Index Method, were discussed in thisresearch and the analytical results show that both the fine aggre-gate particles and aggregate shapes significantly affect the calcu-lation and should be taken into consideration to obtain a greateraccuracy for the surface area. The errors caused by the flat surfaceassumption were also analyzed for different sieve sizes and wasfound to have a significant effect on small aggregate sizes. Acomputation method was proposed to significantly improve thecurrent conventional method for the asphalt film thickness calcu-lation by incorporating shape factors, elongation ratios, and flatsurface factors for all sieve sizes.

Field performance data from MnROAD and rutting data fromlaboratory fabricated mixtures, including both coarse and fine gra-dations, were analyzed to investigate the asphalt film thicknessand performance of HMA. Both the field data and laboratoryexperimental results show that the asphalt film thickness is a sig-nificant factor affecting the rutting performance for asphalt mix-tures. However, more research work is needed to investigate therelationship between the asphalt film thickness and the other per-formance parameters of asphalt mixtures, such as fatigue crackingbefore a film thickness specification can be proposed.

Acknowledgments

The support provided by Minnesota Department of Transportationand Local Road Research Board for part of this research is grate-fully acknowledged.

References

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Campen, W. H., Smith, J. R., Erickson, L. G., and Mertz, L. R. �1959�.“The relationships between voids, surface area, film thickness andstability in bituminous paving mixtures.” Asph. Paving Technol., 28,149–178.

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Documents

Investigation of In-Place Asphalt Film Thickness and Performance of Hot-Mix Asphalt Mixtures