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doi:10.1016/j.bu
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Building and Environment 42 (2007) 3621–3628
www.elsevier.com/locate/buildenv
Effect of asphalt film thickness on the moisture sensitivity characteristicsof hot-mix asphalt
Burak Sengoza,�, Emine Agarb
aFaculty of Engineering, Department of Civil Engineering, Dokuz Eylul University, 35160, Izmir, TurkeybFaculty of Civil Engineering, Istanbul Technical University, Istanbul, Turkey
Received 13 October 2006; received in revised form 13 October 2006; accepted 13 October 2006
Abstract
Temperature, air and water are the common factors that profoundly affect the durability of asphalt concrete mixtures. In mild weather
conditions, distresses such as permanent deformation, fatigue cracking can be encountered on the pavements due to traffic loading. But
when a severe climate is in question, these stresses increase in poor materials; under inadequate control; with traffic as well as with water
which are key elements in the degradation of asphalt concrete pavements. Many variables affect the amount of water damage in asphalt
concrete layer. Among them, mixture design properties such as air void level, permeability, asphalt content and asphalt film thickness are
the ones that must be investigated carefully.
This study is aimed to determine the relationship between the various asphalt film thicknesses and the susceptibility characteristics to
water of hot mix asphalt (HMA) so that an optimum asphalt film thickness that minimizes the moisture damage of HMA can be
obtained. For this purpose, the modified Lottman Test (AASHTO T283) is performed on the Superpave Gyratory compacted specimens
that contain 5 different asphalt film thicknesses. A good correlation between the asphalt film thickness and the modified Lottman test
results as well as an optimum asphalt film thickness of 9.5–10.5 mm is obtained.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Asphalt film thickness; Moisture susceptibility; Water damage; Stripping
1. Introduction
Many highway agencies have been experiencing pre-mature failures that diminish the performance and servicelife of the pavements. One of the major causes of prematurepavement failure is the moisture damage of the asphaltconcrete layer. However, the causes of the increase inpavement distress because of moisture susceptibility havenot been conclusively identified. Researchers suggest thatchanges in asphalt binders, decreases in asphalt bindercontent to satisfy rutting associated with increases intraffic, changes in aggregate quality, increased widespreaduse of selected design features and poor quality control areprimarily responsible for increased water sensitivity pro-blems [1,2].
e front matter r 2006 Elsevier Ltd. All rights reserved.
ildenv.2006.10.006
ing author. Tel.: +90 232 412 7072; fax: +90 232 412 7253.
ess: [email protected] (B. Sengoz).
Moisture damage in the asphalt concrete pavement occursdue to the loss of adhesion (stripping) or loss of cohesion (i.e.softening of asphalt that weakens the bond between asphaltand aggregate). The stripping of asphalt from the aggregatesresults in the reduction of strength of asphalt concretemixture [2]. The reduction in strength may contribute to thedevelopment of various forms of pavement deterioration suchas rutting, raveling, cracking [3].Many variables affect the amount of the moisture
damage which occurs in an asphalt concrete mixture. Thecharacteristics of aggregate and asphalt concrete mixtureproperties in terms of permeability, air voids and asphaltfilm thicknesses are probably the most important factors[4]. Researchers have carried out laboratory experimentsrelated to the effect permeability, air voids and aggregategradation on the moisture susceptibility of asphalt concretemixtures [1,4,5], however no experimental study has beenconducted for evaluating the effect of asphalt film thicknesson the water damage of hot-mix asphalt (HMA).
ARTICLE IN PRESS
Table 1
Factors which influence moisture damage [3]
1. Aggregate
� Aggregate compositionJ Degree of acidityJ Surface chemistryJ Types of mineralsJ Source of aggregate
� Physical characteristicsJ AngularityJ Surface roughnessJ Surface areaJ GradationJ PorosityJ Permeability
� Dust and clay coatings
� Moisture content
� Resistance to degradation
2. Asphalt
� Chemical composition
� Hardness
� Crude source and refining process
3. Mixture design and construction
� Air voids level and compaction
� Permeability and drainage
� Film thickness
4. Environment
� Temperature
� Freeze-thaw cycles
� Dampness and pavement age
5. Trafik
6. Anti-stripping additives properties
B. Sengoz, E. Agar / Building and Environment 42 (2007) 3621–36283622
The objectives of this study are to conduct the modifiedLottman Test (AASHTO T 283) on the SuperpaveGyratory compacted samples that contain different asphaltfilm thicknesses so as to obtain optimum asphalt filmthickness. In this way, relationship between the asphalt filmthickness and the moisture susceptibility of hot-mix asphalt(HMA) can be determined.
2. Factors affecting moisture susceptibility of asphalt
concrete pavement
Moisture damage in asphalt concrete pavement isaffected by many factors. The type of aggregate, bothcoarse and fine, must be examined carefully in evaluatingthe water damage of the mixture. Some aggregates such asgranite, gravel and other siliceous type materials aresensitive to moisture and are prone to stripping whenincorporated in asphalt concrete. Other aggregates such aslimestone are less susceptible to moisture damage [6]. Insome cases, the majority of the stripping takes place in thecoarse aggregate portion of the mixture. In some cases, thefine aggregate is more moisture sensitive and moststripping occurs in that part of the mixture.
The second factor is the type of source of crude oil andrefining process which is used to manufacture the asphaltcement. Most asphalt cements are relatively inert in regardto moisture damage. The asphalt cements, from one toanother; do not show much difference in the degree ofstripping. In other words, the source of asphalt cement ismuch less dominant than the type of aggregate [7].
The third factor is the asphalt concrete mixture proper-ties. The air void level and the permeability of the mixture,which are influenced by the degree of compaction, asphaltcement and the aggregate gradation, are important sincethey control the level of water saturation and drainage. Athigh air void contents, above 6%, a given mixture cansuffer a considerable degree of moisture damage. Exceptionis made for open graded mixtures where air void levels of15%–25% allow water to drain [8].
The asphalt film thickness has also an influence on themoisture susceptibility characteristics of HMA because itaffects durability of the mixture. Thick films which areassociated with black flexible mixtures are known to bedurable. On the other hand, thin films which are associatedwith brownish, brittle mixtures tend to crack and ravelexcessively thus shortening the service life of the pavement.Mixtures with thick asphalt film are less susceptible towater damage than the mixtures with thin asphalt film sincevery little quantities of water can move through the mixturethat contains thick asphalt film thicknesses [8,9].
Environmental conditions and traffic affect the amountof stripping which happens in a particular mixture. Moremoisture damage typically occurs in areas where there areconsiderable amount of rain and/or snowfall. Both the typeof traffic and the volume are important variables. As thetraffic becomes heavier and as the truck volume increases,the amount of stripping becomes greater [9].
The summary of the factors that affect the amount ofmoisture damage are given in Table 1.
3. Materials
3.1. Aggregate
The Superpave mixtures are produced with limestoneaggregate from Redland Genstar’s Frederick Marylandquarry. Although asphalt mixtures prepared with limestoneaggregate are less susceptible to moisture damage, thereason for the utilization of limestone aggregate is tosimulate the real pavement conditions in Turkey, wheremost of the asphalt pavements are constructed usinglimestone aggregate. In order to find out the properties ofthe aggregate used the sieve analysis, specific gravity, LosAngeles abrasion resistance test, sodium sulfate soundnesstest, fine aggregate angularity test, fractured faces, sandequivalent and flat and elongated particles tests wereconducted on each aggregate group. The results of thesetests conducted on aggregate groups and specificationlimits corresponding to each test method are presented in
ARTICLE IN PRESS
Table 2
Results of experiments conducted on aggregate groups
Test 7# 8# 10# Washed 10# Specification Spec. Limits
Bulk specific gravity 2.701 2.700 2.586 2.663 AASHTO T84/T85 —
Surface saturated SG. 2.717 2.710 2.646 2.687 AASHTO T84/T85 —
Apparent SG 2.736 2.730 2.729 2.729 AASHTO T84/T85 —
Absorption (%) 0.4 0.4 0.9 0.9 AASHTO T84/T85 —
Los angeles abrasion (%) 26 26 — — AASHTO T96 45% (max)
Sodium sulfate soundness 0.1 0.1 1.2 1.2 AASHTO T104 10—20% (max)
Fine aggregate angularity — — 45.6 45.6 AASHTO TP33 40% (min.)
Fractured faces (%) 100 100 100 100 PTM 621 —
Sand equivalent — . 89 89 AASHTO T176 40% (min.)
Flat and elongated particles 7.5 9.7 — — ASTM D4791 10% (max.)
Table 3
Stockpile proportions and final 12.5mm gradation
Sieve no (mm) Passing %
19.0 100
12.5 97
9.5 87
4.75 58
2.36 35
1.18 21
0.600 13
0.300 9
0.150 8
0.075 6.1
B. Sengoz, E. Agar / Building and Environment 42 (2007) 3621–3628 3623
Table 2. As seen from Table 2, all results obtained fromexperiments are within Superpave Specification limits.
After determining the properties of aggregate groups,mixture ratios were chosen based on the Superpaveconsensus aggregate criteria related to control points andrestricted zones for 12.5mm nominal sized aggregate. Fourstockpile proportion and final gradation is presented inTable 3.
3.2. Asphalt cement
The asphalt cement used in each of the Superpavemixtures is an unmodified PG 64-22 obtained from thePaulsboro, New Jersey Terminal of the Citgo AsphaltRefining Company. An extensive testing program isperformed to characterize the rheological properties ofthe asphalt cement using both conventional and Superpavetests. Table 4 summarizes the results of the tests conductedon PG 64-22 asphalt cement. The tests carried out (given inTable 4) complying with the Superpave requirements andSpecifications. As seen from Table 4, all results obtainedfrom the experiments are within the Superpave Specifica-tion limits.
4. Experimental
After the properties of the aggregate and the asphaltcement were determined, the surface area of the aggregate
was calculated by multiplying the surface area factors givenin MS-2. [10], by the gradation values presented in Table 3.The surface area factors are presented in Table 5. Thesurface area factors used to calculate asphalt film thickness(given in Table 5) are also adopted in Superpave DesignMethod. For the aggregate gradation used, the surface areawas calculated to be 5.1827m2/kg. This value was used inthe calculation of asphalt content corresponding to theasphalt film thicknesses chosen in the study.In this study asphalt paving mixtures were prepared at
each of the following five effective asphalt film thicknesses:4.9, 5.8, 7.7, 9.6 and 11.4 mm. The utilization of thesethicknesses are based on the Shell Bitumen Handbookwhich states that the average asphalt film thickness inHMA ranges from 5 to 15 m [11]. The required asphaltcontents corresponding to 4.9, 5.8, 7.7, 9.6 and 11.4 mmasphalt film thicknesses were calculated as 3.08%, 3.58%,4.56%, 5.5% and 6.45% taking the percentage of asphalt(unmodified PG 64-22) absorption for limestone aggregateinto consideration.The Modified Lottman Test (AASHTO T283) was then
performed on the asphalt specimens prepared by using thecalculated asphalt contents mentioned above.The aim of the modified Lottman Test is to evaluate
susceptibility characteristics of the mixture to waterdamage. This test is performed by compacting specimensto an air void level of 7%71%. Three specimens areselected as a control and tested without moisture con-ditioning; and three more are selected to be conditioned bysaturating with water (55%–80% saturation level) followedby a freeze cycle (�18 1C for 16 h) and subsequently havinga warm-water soaking cycle (60 1C water bath for 24 h).The specimens are then tested for indirect tensilestrength (ITS) by loading the specimens at a constant rate(50mm./min vertical deformation at 25 1C) and the forcerequired to break the specimen is measured. The indirecttensile strength (ITS) of the conditioned specimens iscompared to the control specimens in order to determinethe tensile strength ratio (TSR).In this study, specimens were sorted into two subsets
(both control and conditioned) of three specimens eachso that average air voids of two subsets are equal.
ARTICLE IN PRESS
Table 4
Results of the experiments conducted on PG 64-22 asphalt
Condition Test Specification Results Specification limits
Unaged asphalt
Specific gravity (25 1C) AASHTO T228 1.021 —
Viscosity, 135 1C ASTM D4402 0.420Pa.s —
Viscosity 165 1C ASTM D4402 0.114Pa.s —
Dynamic shear rheometer (G*/sin d), 10 rad/sec., 64 1C AASHTO TP5 1.260 kPa 1.00 kPa (min)
(Indicator to resistance to permanent deformation)
RTFO aged residue
Mass change AASHTO T240 0.14% —
Dynamic shear rheometer (G*/sin d), 10 rad/s., 64 1C AASHTO TP5 2.516kPa 2.200 kPa (min)
(Indicator to resistance to permanent deformation)
PAV aged residue
Dynamic shear rheometer (G*/sin d), 10 rad/s., 25 1C AASHTO TP5 4154kPa 5000 kPa (max)
(Indicator to fatigue cracking)
Bending beam Rheometer 60 s, �12 1C AASHTO TP1 209MPa 300Mpa (max)
(Indicator to low temperature cracking)
m value 60 s., �12 1C AASHTO TP1 0.342 0.300 (min.)
Table 5
Surface area factors
Sieve no (mm) Surface area factor
19.0 0.41
12.5 0.41
9.5 0.41
4.75 0.41
2.36 0.82
1.18 1.64
0.600 2.87
0.300 6.14
0.150 12.29
0.075 32.77
B. Sengoz, E. Agar / Building and Environment 42 (2007) 3621–36283624
The specimens were prepared to give target air void contentlevel of 7%. A Superpave Gyratory Compactor was usedfor this purpose. The total number of specimens testedwere 5 (asphalt film thickness)*2 (control and conditionedspecimen)*3 (replicates) ¼ 30. The flow chart of theexperimental study and design parameters are presentedin Fig. 1 and Table 6 respectively.
5. Results and discussion
The ITS test results of control and conditioned speci-mens which are a part of the modified Lottman test aregiven in Tables 7 and 8 respectively.
In order to see the effect of the asphalt film thickness onthe moisture susceptibility characteristics of the samplesand to determine the optimum asphalt film thickness,asphalt film thicknesses were plotted against the values ofITS for both control and conditioned specimens.
The concept of power regression analysis was used as atool to fit the observed data to the curve, which quantifiesthe relationship between the independent and the depen-
dent variables. The independent variable is the asphalt filmthickness whereas the dependent variables whose valueswere given in Tables 7 and 8 were the ITS test results forthe control and the conditioned specimens. Fig. 2 showsthe relationship between the asphalt film thickness and theITS test results of specimens.Regression analysis leads to power functions in the data
as modeled by the following equations:
ITScontrol ¼ 8097:4 h�1:0026 R2 ¼ 0:99 (1)
ITScond ¼ 5453:5 h�0:8694 R2 ¼ 0:98 (2)
where ITScontrol is the Indirect tensile strength of controlspecimens, kpa, ITScond the Indirect tensile strength ofconditioned specimens, kpa, h the Asphalt film thickness,mm, R2 the Determination of coefficient.A fairly good relationship was obtained between the
asphalt film thickness and the ITS of control andconditioned specimens both of which were compacted with7% air voids. Although the database is small, the very highvalue of R2 (determination of coefficient) indicates that theinvestigated functions exactly represent the relationshipbetween the asphalt film thickness and the ITS of speci-mens. It should be noted the mentioned functions are onlyvalid between the asphalt thicknesses of 5–12 mm.It can be seen in Fig. 2 that the slope of the curve
becomes steeper as the film thickness falls below a value ofabout 9.5–10.5 mm. This indicates that the asphalt pavingmixture becomes more susceptible to water damage with adecrease in the asphalt film thickness below about9.5–10.5 mm. Therefore, it can be concluded that, the9.5–10.5 mm asphalt film thickness can be accepted as anoptimum asphalt film thickness that minimizes themoisture damage of HMA.The ITS of the conditioned specimens was compared to
the control specimens in order to determine the tensile
ARTICLE IN PRESS
Preparation of asphalt concrete mixtures
using five different asphalt film thickness
Loose mix curing at 60 °C for 16 hours
Short term aging of loose mixture at 135
°C for 2 hours
Compaction
Application of partial vacuum in order to
determine level of saturation between
Indirect Tensile
Strength Test on
control specimens
(S1)
Freezing procedure of
compacted specimen at
-18°C for 16 hours
Thawing procedure of
compacted specimen at
60 °C for 24 hours
Indirect Tensile Strength
Test on conditioned
specimens
(S2)
Tensile Strength Ratio (TSR)=S2/S1
Fig. 1. The flow chart of the modified Lottman Test (AASHTO T283).
Table 6
Design parameters
Type of asphalt Unmodified PG 64-22
Type of aggregate Lime stone aggregate
Asphalt film thickness (mm) 4.9, 5.8, 7.7, 9.6, 11.4
Specimen conditions Control and conditioned
Target air void level (%) 7
Tests performed Indirect tensile strength at 25 1C
Replicates 3
B. Sengoz, E. Agar / Building and Environment 42 (2007) 3621–3628 3625
strength ratio (TSR) which is calculated with the followingequation:
TSR ¼S2
S1(3)
where S1 is the Average indirect tensile strength of controlspecimen, S2 the Average indirect tensile strength ofconditioned specimen.
The results are presented in Table 9.TSR values were then drawn corresponding to each
asphalt film thickness that is given in Fig. 3.Power regression gave an acceptable model for the
relationship between the asphalt film thickness and TSR
values presented below:
TSR ¼ 0:6735 h0:1332 R2 ¼ 0:92 (4)
The very high value of R2 indicates that the abovefunction represents the relationship between the asphaltfilm thickness and TSR values of specimens.Table 9 and Fig. 3 show that as the asphalt film thickness
increases, the TSR values increase as well. This indicatesthat the resistance of asphalt mixtures to the detrimentaleffect of water decrease with increase in asphalt filmthickness.
6. Conclusions and recommendations
Moisture damage in asphalt mixtures is a complexmechanism which is not well understood and has manyinteracting factors. The effect of the asphalt film thicknesson the moisture damage in HMA is one of the factors andhas not been investigated. Therefore the main objective ofthe study is to evaluate moisture susceptibility character-istics of HMA in terms of asphalt film thickness and thefollowing conclusions can be drawn:
�
The relationship between the asphalt film thickness andthe ITS of control and conditioned specimens as well as
ARTICLE IN PRESS
Table 7
Indirect tensile strength tests results of the control specimens
Specimen no Asphalt film
thickness (mm)
Air void (%) Average air void (%) Indirect tensile
strength (kpa)
Average indirect
tensile strength (kpa)
(S1)
1A 4.9 6.8 6.9 1655.05 1677.412
1B 7.0 1683.05
1C 7.0 1694.14
2A 5.8 6.9 7.1 1369.07 1339.603
2B 7.2 1262.30
2C 7.3 1387.44
3A 7.7 6.5 6.7 1106.94 1079.846
3B 7.0 1057.06
3C 6.7 1075.54
4A 9.6 6.6 6.8 858.074 820.588
4B 7.0 864.637
4C 6.7 739.052
5A 11.4 6.5 6.9 702.258 711.207
5B 7.0 719.24
5C 7.1 712.124
Table 8
Indirect tensile strength tests results of the conditioned specimens
Specimen Asphalt film
thickness (mm)
Air void (%) Average air void
(%)
Level of
saturation (%)
Indirect tensile
strength (kpa)
Average indirect
tensile strength
(kpa) (S2)
1A 4.9 7.0 6.9 60.7 1418.56 1389.938
1B 7.0 72.2 1464.010
1C 6.8 60.2 1287.240
2A 5.8 7.3 7.1 61.6 1139.920 1131.126
2B 6.9 61.5 1187.500
2C 7.2 58.9 1065.950
3A 7.7 6.7 6.7 67.1 985.661 976.509
3B 6.6 65.2 1068.315
3C 6.8 59.3 875.551
4A 9.6 6.7 6.8 65.5 734.936 747.192
4B 6.9 64.1 796.691
4C 6.8 64.0 709.948
5A 11.4 6.9 6.8 66.4 656.290 655.094
5B 6.3 69.2 653.898
5C 7.2 73 653.459
B. Sengoz, E. Agar / Building and Environment 42 (2007) 3621–36283626
tensile strength ratio values are quantified by regressionanalysis. The very high determination of coefficientindicates that the functions exactly represent therelationship between the asphalt film thickness andmoisture sensitivity characteristics of the samples.
� Optimum range of asphalt film thickness obtained fromthe figures plotted between the asphalt film thicknessand the modified Lottman test (AASTHTO T283)results is between 9.5 and 10.5 mm.
� The results of the TSR lead to the conclusion that as theasphalt film thickness increases, the TSR values increaseas well. This indicates that the detrimental effect ofwater decrease with increase in asphalt film thickness.
� The modified Lottman Test (AASHTO 283) includes theshort term aging of hot mix asphalt namely the aging ofasphalt concrete during mixing, transporting and
compacting. Since the effect of long term aging andwater damage occurs simultaneously during the servicelife of the pavement, long term aging laboratory testsrepresenting the aging of asphalt binder during theservice life of the road should be adopted in AASHTOT283 method.
� The asphalt film thickness calculation is based on thesurface area factors and the percentage of absorbedasphalt. Further research concerning the measurementof the asphalt film thickness using recently developedimage processing techniques may be helpful.
� In conclusion, since one type of aggregate andperformance graded asphalt cement were utilized, theauthors recommend to conduct more experiments usingdifferent aggregate and asphalt cement combinations forthis purpose.
ARTICLE IN PRESS
ITScontrol = 8097.4h-1.0026
R2 = 0.99
ITScond. = 5453.5h-0.8694
R2 = 0.98
500
700
900
1100
1300
1500
1700
1900
4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12
Asphalt Film Thickness (micronmeter)
Avera
ge I
nd
irect
Ten
sile S
tren
gh
t (k
pa)
Indirect Tensile Str. of conditioned Specimens
Indirect Tensile Str. of control Specimens
Regression line between asphalt film thickness and ITS of control specimens
Regression line between asphalt film thickness and ITS of conditioned specimens
Fig. 2. Relationship between the asphalt film thickness and the indirect strength test results of the control and conditioned specimens.
Table 9
Tensile strenght ratios (TSR) correponding to the asphalt film thicknesses
Asphalt film thickness (mm) TSR (S2/S1)
4.9 0.83
5.8 0.84
7.7 0.90
9.6 0.91
11.4 0.92
TSR = 0.6735h0.1332
R2 = 0.92
0.82
0.84
0.86
0.88
0.90
0.92
0.94
4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12
Asphalt Film Thickness (micronmeter)
TS
R V
alu
e
TSR valuesof compacted specimens
Regressionline between asphaltfilm thickness and TSRvalues of specimens
Fig. 3. Relationship between the asphalt film thickness and the indirect tensile strength ratio (TSR).
B. Sengoz, E. Agar / Building and Environment 42 (2007) 3621–3628 3627
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