8
27 Transportation Research Record: Journal of the Transportation Research Board, No. 2295, Transportation Research Board of the National Academies, Washington, D.C., 2012, pp. 27–34. DOI: 10.3141/2295-04 Department of Civil and Environmental Engineering, Worcester Polytechnic Insti- tute, 100 Institute Road, Worcester, MA 01609. Corresponding author: M. Tao, [email protected]. susceptibility of the mixes. To assess the influence of moisture content of WMA mixes on their mechanical properties, it is necessary to pre- pare WMA mixes with different moisture contents and to determine moisture contents of compacted WMA samples with a simple labora- tory procedure. Such a laboratory procedure was developed during the first stage of this research study, which was used to prepare asphalt mixes with different moisture contents for mechanical property tests reported in this paper (7 ). In a real pavement, both mechanisms behind moisture damage (e.g., cohesion and adhesion damage) are most likely present and are not easily separable (8). These same mechanisms must also be simulated in the laboratory conditioning and testing procedures, and therefore the use of a single parameter to evaluate moisture suscep- tibility of asphalt mixtures is questionable. The laboratory testing procedure currently used by most state highway agencies for evalu- ating hot-mix asphalt moisture susceptibility is the comparison of tensile strength before and after moisture conditioning (AASHTO T283). However, the reliability of results based on tensile strength ratio (TSR) for assessing moisture susceptibility has been questioned because of its inherent limitation, and poor agreement has often been reported in the literature between the TSR prediction and field perfor- mance (8, 9). Therefore, a proper composite parameter that accounts for changes in key mixture properties is needed to effectively evaluate the effects of moisture damage in WMA mixtures. In this study, frac- ture mechanics energy parameters that integrate the effects of moisture damage on key mixture properties (i.e., stiffness, tensile strength, and tensile failure strain) were used. Specific details about this research are presented as follows. OBJECTIVE The objective of this study is to evaluate moisture susceptibility of WMA mixes through laboratory mechanical testing. The mechanical properties tests, from which the fracture energy (FE) parameters were derived, were conducted on asphalt mixes that simulate insufficient drying of aggregates during WMA production and asphalt mixes with infiltrated moisture over the service life of WMA mixes in the field. SCOPE As listed in Table 1, four sample sets were prepared and tested for resilient modulus (M r ), creep compliance, and indirect tensile strength (ITS) at 5°C: S1 and S2, prepared with fully dried aggregates, and S3 and S4, prepared with incompletely dried aggregates. In addition, S2 and S4 were subjected to moisture conditioning before mechanical Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing Wenyi Gong, Mingjiang Tao, Rajib B. Mallick, and Tahar El-Korchi Moisture can lead to serious damage and failures in hot-mix asphalt concrete pavements. This is an even greater concern for warm-mix asphalt because the much lower production temperatures may not completely dry the aggregates. In this Maine Department of Transportation study, the use of fracture energy parameters was evaluated to determine the influence of incomplete drying of mixes on their mechanical properties. Fracture energy–based parameters [energy ratio (ER); ratio of energy ratio (RER)] were determined from the following testing of mixes with fully and partially dried aggregates, some of which were subjected to moisture conditioning: resilient modulus, creep compliance, and indirect tensile strength (ITS) at 58C. The results indicate that (a) resilient mod- ulus, creep compliance, and ITS were all affected by the presence of moisture in mixes; (b) the trend and the degree of influence of moisture for different mechanical parameters were different; (c) the moisture conditioning process caused larger decreases in modulus and ITS values than did incomplete drying of aggregates; however, the same moisture conditioning process caused much larger decreases in modulus and ITS in mixes prepared with incompletely dried aggregates than did the counterparts prepared with fully dried aggregates; and (d) frac- ture energy–based parameters (ER and RER) appeared to be more- distinctive moisture effect and damage indicators than are the other parameters. Warm-mix asphalt (WMA) technologies, initiated in Europe in the late 20th century, have gained a lot of interest in the United States in recent years (1–5). These mixes require lower than conventional temperatures for mixing, laydown, and compaction. As a result, a significant amount of savings is achieved in fuel and emissions. Furthermore, longer haul distances can be achieved and, in colder climates, the construction season can be extended. Last, a lower pro- duction temperature also means a lesser extent of short-term aging of the asphalt binder during mixing (6). There is a concern that the use of much lower production tempera- tures for the production of WMA (e.g., 130°C instead of approximately 150°C for conventional hot-mix asphalt) may not be high enough to completely dry aggregates, given the limited drying time during pro- duction. It is quite possible that some residual moisture entrapped within the aggregate–WMA matrix can further increase the moisture

Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing

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Page 1: Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing

27

Transportation Research Record: Journal of the Transportation Research Board, No. 2295, Transportation Research Board of the National Academies, Washington, D.C., 2012, pp. 27–34.DOI: 10.3141/2295-04

Department of Civil and Environmental Engineering, Worcester Polytechnic Insti-tute, 100 Institute Road, Worcester, MA 01609. Corresponding author: M. Tao, [email protected].

susceptibility of the mixes. To assess the influence of moisture content of WMA mixes on their mechanical properties, it is necessary to pre-pare WMA mixes with different moisture contents and to determine moisture contents of compacted WMA samples with a simple labora-tory procedure. Such a laboratory procedure was developed during the first stage of this research study, which was used to prepare asphalt mixes with different moisture contents for mechanical property tests reported in this paper (7).

In a real pavement, both mechanisms behind moisture damage (e.g., cohesion and adhesion damage) are most likely present and are not easily separable (8). These same mechanisms must also be simulated in the laboratory conditioning and testing procedures, and therefore the use of a single parameter to evaluate moisture suscep-tibility of asphalt mixtures is questionable. The laboratory testing procedure currently used by most state highway agencies for evalu-ating hot-mix asphalt moisture susceptibility is the comparison of tensile strength before and after moisture conditioning (AASHTO T283). However, the reliability of results based on tensile strength ratio (TSR) for assessing moisture susceptibility has been questioned because of its inherent limitation, and poor agreement has often been reported in the literature between the TSR prediction and field perfor-mance (8, 9). Therefore, a proper composite parameter that accounts for changes in key mixture properties is needed to effectively evaluate the effects of moisture damage in WMA mixtures. In this study, frac-ture mechanics energy parameters that integrate the effects of moisture damage on key mixture properties (i.e., stiffness, tensile strength, and tensile failure strain) were used. Specific details about this research are presented as follows.

Objective

The objective of this study is to evaluate moisture susceptibility of WMA mixes through laboratory mechanical testing. The mechanical properties tests, from which the fracture energy (FE) parameters were derived, were conducted on asphalt mixes that simulate insufficient drying of aggregates during WMA production and asphalt mixes with infiltrated moisture over the service life of WMA mixes in the field.

ScOpe

As listed in Table 1, four sample sets were prepared and tested for resilient modulus (Mr), creep compliance, and indirect tensile strength (ITS) at 5°C: S1 and S2, prepared with fully dried aggregates, and S3 and S4, prepared with incompletely dried aggregates. In addition, S2 and S4 were subjected to moisture conditioning before mechanical

Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing

Wenyi Gong, Mingjiang Tao, Rajib B. Mallick, and Tahar El-Korchi

Moisture can lead to serious damage and failures in hot-mix asphalt concrete pavements. This is an even greater concern for warm-mix asphalt because the much lower production temperatures may not completely dry the aggregates. In this Maine Department of Transportation study, the use of fracture energy parameters was evaluated to determine the influence of incomplete drying of mixes on their mechanical properties. Fracture energy–based parameters [energy ratio (ER); ratio of energy ratio (RER)] were determined from the following testing of mixes with fully and partially dried aggregates, some of which were subjected to moisture conditioning: resilient modulus, creep compliance, and indirect tensile strength (ITS) at 58C. The results indicate that (a) resilient mod-ulus, creep compliance, and ITS were all affected by the presence of moisture in mixes; (b) the trend and the degree of influence of moisture for different mechanical parameters were different; (c) the moisture conditioning process caused larger decreases in modulus and ITS values than did incomplete drying of aggregates; however, the same moisture conditioning process caused much larger decreases in modulus and ITS in mixes prepared with incompletely dried aggregates than did the counterparts prepared with fully dried aggregates; and (d) frac-ture energy–based parameters (ER and RER) appeared to be more-distinctive moisture effect and damage indicators than are the other parameters.

Warm-mix asphalt (WMA) technologies, initiated in Europe in the late 20th century, have gained a lot of interest in the United States in recent years (1–5). These mixes require lower than conventional temperatures for mixing, laydown, and compaction. As a result, a significant amount of savings is achieved in fuel and emissions. Furthermore, longer haul distances can be achieved and, in colder climates, the construction season can be extended. Last, a lower pro-duction temperature also means a lesser extent of short-term aging of the asphalt binder during mixing (6).

There is a concern that the use of much lower production tempera-tures for the production of WMA (e.g., 130°C instead of approximately 150°C for conventional hot-mix asphalt) may not be high enough to completely dry aggregates, given the limited drying time during pro-duction. It is quite possible that some residual moisture entrapped within the aggregate–WMA matrix can further increase the moisture

Page 2: Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing

28 Transportation Research Record 2295

property tests. The FE-based parameters (discussed later) were calculated from these measured mechanical properties.

MethOdOlOgy

testing Materials

In this study, aggregates with low water absorption (1.8%) were used, with their gradation listed as follows: 12.5 mm, 8%; 9.5 mm minus, 30%; 9.5 mm, 9%; sand, 31%; and washed ledge sand, 22%. A PG 64-28 binder at 6.3 wt %, plus 1.5% Sasobit by weight of the total binder, was used to prepare WMA samples.

Sample preparation

The preparation of WMA specimens followed the laboratory proce-dures developed in the first stage of this research, which involves the following steps: aggregate soaking, aggregate oven drying at 90°C plus binder heating at 125°C for 4 h, aggregate and binder mixing, mixture aging at 90°C for 2 h, and mixture compaction. Incompletely dried aggregates were realized by drying soaked aggregates in a 90°C oven for 4 h. For more details on the sample preparation, see the paper by Mallick et al. (7). The moisture conditioning followed the process specified in AASHTO T283 (10). All the specimens were prepared with a target air void content of 7% ± 1%. A testing temperature of 5°C was chosen because of the following consider-ations: (a) fracture resistance of asphalt mixes is more critical at lower temperatures, and (b) the influence of moisture content on fracture resistance should not be complicated by the freezing effect of entrapped water, which means that the testing temperatures need to stay above zero.

Mechanical testing procedures

Mr Test

Mr is a fundamental property that characterizes the stiffness of pavement materials and can be calculated as follows:

MP

H tr = ( ) +( )cyclic

∆0 27 1. ( )µ

where

Mr = instantaneous or total resilient modulus of asphalt mixes [MPa (psi)];

ΔH = recoverable horizontal deformation [mm (in.)];

µ = Poisson’s ratio, which can be determined either through testing or from suggested values in testing specification (ASTM D4123-95);

t = thickness of specimen [mm (in.)]; Pcyclic = Pmax − Pcontact = applied cyclic load [N (lb)]; Pmax = maximum applied load [N (lb)], which is suggested to

be 10% of peak load obtained from ITS test; and Pcontact = contact load [N (lb)].

Creep Compliance Test

Creep is another important characteristic of viscoelastic materials, which describes the relationship between the time-dependent strain and applied stress (10). Creep compliance, D(t), is calculated as follows:

D tX D t

PCt( ) =

× ××

×∆ tm avg avg

avgcmplGL

, ( )2

where

ΔXtm,t = trimmed value of horizontal deformation (mm), Davg = average diameter of three testing samples (mm), tavg = average thickness of three testing samples (mm), D(t) = creep compliance at time t (kPa−1), GL = gauge length (mm), Ccmpl = 0.6354 × X Y( )−1

− 0.332 = creep compliance coefficient, and

X/Y = ratio of horizontal to vertical deformations.

The creep compliance test provides not only the time-dependent creep characteristics, but also asphalt mixes’ creep compliance parameters (i.e., m and D1), which were used to calculate the FE parameters.

ITS Test

The indirect tensile (σ) stresses are calculated with the following equation:

σπ

= ×× ×2

3P

t D( )

where

P = axial load, t = thickness of specimen, and D = diameter of specimen.

TABLE 1 Testing Matrix to Determine FE Parameters of Various Asphalt Mixes

Sample ID Aggregate ConditionMoisture Conditioning Mr

Creep Compliance ITS Air Void (%)

Moisture Content (%)

S1 Totally dry No √ √ √ 7.2 ± 0.08 0.2 ± 0.00

S2 Totally dry Yes √ √ √ 7.1 ± 0.10 1.8 ± 0.30

S3 Wet + 4-h oven drying No √ √ √ 7.8 ± 0.09 0.2 ± 0.03

S4 Wet + 4-h oven drying Yes √ √ √ 7.6 ± 0.07 1.8 ± 0.50

Note: ID = identification; √ = corresponding experiment performed on sample set.

Page 3: Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing

Gong, Tao, Mallick, and El-Korchi 29

Because of the limitation of the testing apparatus, the horizontal strains cannot be accurately recorded, and an approximate calcula-tion approach was used to obtain the indirect tensile strains, which is expressed in the following equation:

ε εV

H

v= ( )4

where

εV = approximate strain in vertical direction, εH = approximate strain in horizontal direction, and v = Poisson’s ratio.

Fe-based performance indicators

Fracture Energy

FE is defined as the total energy that has been applied to the speci-men during a complete crack propagation process. On the basis of the results obtained from ITS test, FE can be calculated as follows (11, 12):

FE = ( )∫ σ ε εε

i df

05( )

where εf is the failure strain at the fracture point, which is defined as the point at which the difference between the vertical and horizontal deformations reaches the maximum (13). According to the definition by Birgisson et al., fracture point usually occurs earlier than the point at which the tensile strength is achieved (13). However, considering that the fracture point is too close to the yield point to identify its position, the tensile strength point (i.e., the peak loading point) is taken as the fracture point for the sake of calculation convenience in this study.

Dissipated Creep Strain Energy at Fracture or Failure

Dissipated creep strain energy (DCSE) is the portion of the FE that is dissipated during one loading cycle, whereas the recoverable por-

tion is the elastic energy (EE). Figure 1a illustrates the relationship between DCSE, FE, and EE, which is also shown in the follow-ing equation, where DCSEf is the dissipated creep strain energy at fracture.

DCSE FE EEf = − ( )6

Fracture Energy Ratio

The fracture energy ratio (ER) concept, originally developed by Roque and his associates at University of Florida, is defined as the ratio of DCSEf to the minimum DCSE (DCSEmin) (12–17). DCSEmin is the minimum DCSE for having adequate cracking resistance, which is a function of the asphalt mix’s creep compliance characteristics (i.e., m and D1 shown in Figure 1b) and the anticipated tensile stress magni-tude in the asphalt mix layer of a pavement under traffic loading. DCSEf was determined as the hatched area in Figure 1a, on the basis of ITS and Mr test results. The procedure to calculate DCSEmin from the creep compliance of asphalt mixes and the anticipated tensile stress at the bottom of a pavement asphalt layer is illustrated in Figures 1b and 2.

ER is defined as

ERDCSE

DCSE

DCSE= =

×f fa

m Dmin.

( )2 98

1

7i

where D1 and m-values were backcalculated from the indirect creep compliance testing results by nonlinear regression analysis with the aid of Matlab. Parameter a is a function of the asphalt mix’s ITS and the anticipated tensile stress magnitude at the bottom of the asphalt layer in a pavement section: a = 0.0299 • σ−3.1 • (6.36 − ITS) + 2.46 × 10−8, with σ being the tensile stress (in psi) and ITS in the unit of MPa (12).

As illustrated in Figure 1b, the creep compliance parameters (i.e., m, D1, and D0) were obtained by nonlinear curve fitting of the experimental results based on the following equation:

D t D D tm( ) = +0 1 8( )

Fracture Point

DCSEf

Mr

m-value

Log

D(t

)

Log t 1

D1

Str

ess,

σ

Strain, ε

DCSEf=AREA (in shadow) DCSEmin=m2.98D1/a

(a) (b)

EE

AreaOAB=FE=DCSE+EE

O

A

B

FIGURE 1 (a) Relationship between FE, EE, DCSE, and determination of DCSE f from ITS and Mr testing results and (b) creep compliance parameters used to calculate minimum DCSE.

Page 4: Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing

30 Transportation Research Record 2295

The anticipated tensile stress (σ) at the bottom of an asphalt layer in a typical Maine pavement section, which is schematically shown in Figure 3, was determined with the aid of WinJulea (a linear elastic analysis software package for pavement structures) (18). The essential input parameters for carrying out WinJulea calculations are also included in Figure 3. The computed σ values correspond-ing to different asphalt sample sets are listed in Table 2, along with the respective Mr values, are listed according to size and mass percentage as follows: 12.5 mm, 8%; 9.5 mm minus, 30%; 9.5 mm, 9%; sand, 31%; and washed ledge sand, 22%.

Ratio of Energy Ratio

A new parameter, ratio of energy ratio (RER), is proposed as an indi-cator of the effects of moisture damage on the fracture resistance of asphalt mixtures and defined as follows:

RERER

ERconditioned

control

= ( )9

where ERconditioned and ERcontrol are the energy ratios of asphalt mix samples with and without moisture conditioning, respectively.

ReSultS and diScuSSiOn

volumetric data

The air voids and moisture contents of tested sample sets are sum-marized in Table 1. The sample set that was prepared with the incompletely dried aggregates (S3) did not retain much moisture compared with the control set (S1). After the moisture conditioning process, the sample sets (S2 and S4) absorbed roughly 10 times the amount of water that was absorbed by the samples without moisture conditioning (S1 and S3).

Resilient Modulus

Figure 2 summarizes the Mr results of the four sample sets (S1 to S4). Although the testing samples within either of the following two groups (S1 and S3 in Group 1; S2 and S4 in Group 2) had very close moisture contents (approximately 0.2% in Group 1 and 1.8% in Group 2), Figure 2 indicates that there are appreciable differences in resilient moduli and that the incomplete drying of the aggregates did cause some weakening of the asphalt mixes. Such weakening is probably attributable to the negative influence of the residual moisture on asphalt binders’ coating of the aggregate, as demon-strated by the comparison of two photos shown in Figure 4. More aggregates remain uncoated in the testing samples (Figure 4b) pre-pared with the incompletely dried aggregates. Figure 2 also shows that the moisture conditioning process (after the sample compaction)

S1 S2

1600

(±115)

(±101)(±111)

(±94)

(±68)

(±74)

(±121)

(±122)

1400

1200

1000

Res

ilien

t M

od

ulu

s (k

si)

Instantaneous Mr

Total Mr

800

600

400

200

0S3 S4

FIGURE 2 Mr results of the four asphalt mix sample sets (the numbers above the bars are the standard deviation).

FIGURE 3 Typical pavement section in Maine and input parameters for WinJulea calculations.

R=4in.

WMA

Base Layer

Subbase

Subgrade Layer

External Tire Load=5,000 lb

3”6”

8”

EWMA=245.7 ksi ν=0.30

ν=0.35

ν=0.35

ν=0.40

E=14.5 ksi

E=14.5 ksi

E=10.9 ksi

TABLE 2 Resilient Moduli and Calculated Tensile Stresses at Bottom of Asphalt Layer Corresponding to Asphalt Mixes

Sample Set Mr (ksi)

Calculated Tensile Stress (σ) (psi)

S1 1,131.4 324.5

S2 886.3 289.0

S3 978.0 303.2

S4 562.5 225.3

Page 5: Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing

Gong, Tao, Mallick, and El-Korchi 31

has greater influence than the incomplete drying process (before the sample compaction) for reducing the asphalt mixes’ stiffness (i.e., S3 having a larger Mr value than S2). Moreover, Figure 2 indicates that the testing sample S4 prepared from incompletely dried aggre-gates is more prone to moisture damage compared with its counter-part (S2) prepared from fully dried aggregates, as evidenced from a larger decrease in stiffness.

creep compliance test

The results of creep compliance tests on the four asphalt mix sample sets are presented in Figures 5 to 7. Figure 5 shows that both the incomplete drying process and the moisture conditioning resulted in increases in creep compliance, and consequently sample set S4 had the highest creep compliance. However, there is no clear trend in the influence of moisture on m and D1 values among these sample sets, except that sample set S4 had by far the largest D1 value.

itS test

Figure 8 shows that ITSs decreased under the influence of mois-ture, either resulting from the incomplete drying of aggregates or the moisture conditioning process. Therefore, sample set S4 had the lowest ITS. The results of ITS tests are further discussed in the following section by comparing them with ER values.

Fracture energy Ratio

ER is the ratio of DCSEf and DCSEmin, and the values for the four sample sets are presented in Figure 9. It is recommended that an ER value larger than 1 be an indication of an asphalt mix with good frac-ture resistance, whereas an ER value less than 1 implies an asphalt mixture with poor fracture resistance (12, 19). All the sample sets have an ER value smaller than 1 (as shown in Figure 9), which means that these WMA mixtures might not have adequate fracture

(a) (b)

FIGURE 4 Photographs of compacted asphalt mix sample prepared with (a) fully dried aggregates and (b) incompletely dried aggregates.

0

0.00001

0.00002

0.00003

0.00004

0.00005

0.00006

0.00007

0 20 40 60 80 100 120

Cre

ep C

om

plia

nce

(1/

psi

)

Time (second)

S1

S2

S3

S4

FIGURE 5 Results of creep compliance tests.

Page 6: Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing

32 Transportation Research Record 2295

0.0E+00

5.0E-07

1.0E-06

1.5E-06

2.0E-06

2.5E-06

3.0E-06

S1 S2 S3 S4

D1 Value

D1

FIGURE 6 Backcalculated D1 values from indirect creep compliance testing results.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

m-V

alu

e

S1 S2 S3 S4

FIGURE 7 Backcalculated m-values from indirect creep compliance testing results.

0

50

100

150

200

250

300

Ind

irec

t T

ensi

le S

tren

gth

(p

si)

S1 S2 S3 S4

(±5)

(±11)

(±4)

(±5)

FIGURE 8 Results of ITS tests.

Page 7: Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing

Gong, Tao, Mallick, and El-Korchi 33

resistance. However, the excessively low ER values for these asphalt mixes might in part be because the regression equation for approxi-mating a-value was originally developed for pavement and traffic conditions in Florida, whose applicability for pavements in Maine remains to be confirmed. Moreover, sample set S4 prepared with the incompletely dried aggregates experienced a much larger decline in ER value compared with its counterpart (S2) after being subjected to moisture conditioning.

Ratio of energy Ratio

Figure 10 indicates that the TSR values between the two sample groups (Group 3: S1 and S2; and Group 4: S3 and S4) are nearly indistinguishable and that the RER values between these two groups are significantly different. Note that the TSR values of these two groups are smaller than 0.8, which indicates that both sample groups are prone to moisture-related damage. Given that the difference in other mechanical properties (e.g., Mr, ITS, and creep compliance) of these two groups is appreciable, it seems that RER is a more distinctive indicator of moisture susceptibility.

A summary of mechanical properties of the asphalt mixes as well as their dependence on moisture is given in Table 3, which indicates the following:

1. Moisture, no matter whether it comes from the incompletely dried aggregates or the infiltration subsequently during the mois-ture conditioning process, has an appreciable influence on the over-all behavior of the asphalt mixes, as evidenced by the variation of such mechanical properties as Mr, creep compliance, and ITS values.

2. The tendency and degree of moisture susceptibility on the mechanical parameters (e.g., Mr, m-value, D1, ITS, TSR, and RER) are different.

3. By comparing the Mr and ITS values among all the sample sets listed in Table 3, one can notice that the moisture conditioning process has caused larger decreases in those mechanical properties than the incomplete drying of aggregates. However, the moisture conditioning process caused more weakening in the sample sets pre-pared from incompletely dried aggregates (S3 and S4) than in their counterpart (S1 and S2).

4. For two groups (S1 and S2 in Group 3 versus S3 and S4 in Group 4), TSR values are indistinguishably close (0.66 versus 0.65), despite that their other mechanical properties are quite different (e.g., Mr, ITS, ER, and RER). However, considerably different values are observed for FE-based parameters (ER and RER). This indicates that ER and RER are probably good indicators to evaluate moisture susceptibility of asphalt mixes.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

En

erg

y R

atio

(E

R)

S2 S3 S4S1

FIGURE 9 Calculated values of ER.

FIGURE 10 Comparison of TSR (dark) and RER (light) values between Asphalt Mix Group 3 (S1 and S2) and Group 4 (S3 and S4).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

TS

R a

nd

RE

R

TSR

RER

With fully dried aggregates With aggregate dried for 4 hours in a 90°C oven

Page 8: Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing

34 Transportation Research Record 2295

cOncluSiOnS and RecOMMendatiOnS

From this study, the following conclusions and recommendations can be made:

1. FE-based parameters (ER and RER) seem to be sensitive to moisture susceptibility of asphalt mixes, whereas TSR seems to be insensitive to moisture susceptibility of the asphalt mixes under investigation.

2. The effect of incomplete drying of aggregates on the mechanical properties of asphalt mixes is evident.

3. The effect of moisture after compaction is more pronounced for asphalt mixes that contained incompletely dried aggregates.

A field-based study should be carried out to establish threshold values of the fracture-based parameters such that the tests and properties can be used on a regular basis during mix design.

acknOwledgMentS

The authors gratefully acknowledge the help of the Maine Department of Transportation and especially Dale Peabody, Rick Bradbury, and Wade McClay for help in procuring materials. This study would not have been possible without the help of Don Pellegrino of the Pavement Research Laboratory of Worcester Polytechnic Institute and the following graduate students of the Department of Civil and Environmental Engineering: Karen O’Sullivan, Rudy Pinkham, and Ryan Worsman.

ReFeRenceS

1. Brown, D. Recycling Gets Hot. Public Works Magazine. http://www. pwmag.com/industry-news-print.asp?sectionID=772&articleID=456553. Accessed Nov. 2007.

2. Mallick, R. B., J. E. Bradley, and R. L. Bradbury. Evaluation of Heated Reclaimed Asphalt Pavement Material and Wax-Modified Asphalt for Use in Recycled Hot-Mix Asphalt. In Transportation Research Record: Journal of the Transportation Research Board, No. 1998, Transporta-tion Research Board of the National Academies, Washington, D.C., 2007, pp. 112–122.

3. Mallick, R. B., P. S. Kandhal, and R. L. Bradbury. Using Warm-Mix Asphalt Technology to Incorporate High Percentage of Reclaimed Asphalt Pavement Material in Asphalt Mixtures. In Transportation Research Record: Journal of the Transportation Research Board, No. 2051, Trans-portation Research Board of the National Academies, Washington, D.C., 2008, pp. 71–79.

4. Prowell, B. D., G. C. Hurley, and E. Crews. Field Performance of Warm-Mix Asphalt at National Center for Asphalt Technology Test Track. In Transportation Research Record: Journal of the Transportation Research Board, No. 1998, Transportation Research Board of the National Acad-emies, Washington, D.C., 2007, pp. 96–102.

5. Tao, M., and R. B. Mallick. Effects of Warm-Mix Asphalt Additives on Workability and Mechanical Properties of Reclaimed Asphalt Pavement Material. In Transportation Research Record: Journal of the Transporta­tion Research Board, No. 2126, Transportation Research Board of the National Academies, Washington, D.C., 2009, pp. 151–160.

6. Federal Highway Administration. Warm Mix Asphalt. http://www.fhwa.dot.gov/pavement/asphalt/wma.cfm. Accessed Nov. 2007.

7. Mallick, R. B., M. Tao, B.-L. Chen, K. O’Sullivan, and P. Cacciaator. Practical Method to Understand the Effect of Aggregate Drying on the Moisture Content of Hot-Mix Asphalt. In Transportation Research Record: Journal of the Transportation Research Board, No. 2208, Transporta tion Research Board of the National Academies, Washington, D.C., 2011, pp. 90–96.

8. TRB Committee on Bituminous–Aggregate Combinations to Meet Surface Requirements. Moisture Sensitivity of Asphalt Pavements: A National Seminar, San Diego, Calif. Transportation Research Board of the National Academies, Washington, D.C., 2003.

9. Solaimanian, M., R. F. Bonaquist, and V. Tandon. NCHRP Report 589: Improved Conditioning and Testing Procedures for HMA Moisture Sus­ceptibility. Transportation Research Board of the National Academies, Washington, D.C., 2007.

10. Mallick, R. B., and T. El-Korchi. Pavement Engineering: Principles and Practice. Taylor & Francis, Boca Raton, Fla., 2009.

11. Lakes, R. S. Viscoelastic Materials. Cambridge University Press, Cambridge, United Kingdom, 2009.

12. Birgisson, B., R. Roque, and G. C. Page. Performance-Based Fracture Criterion for Evaluation of Moisture Susceptibility in Hot-Mix Asphalt. In Transportation Research Record: Journal of the Transportation Research Board, No. 1891, Transportation Research Board of the National Academies, Washington, D.C., 2004, pp. 55–61.

13. Birgisson, B., R. Roque, M. Tia, and E. Masad. Development and Evalu­ation of Test Methods to Evaluate Water Damage and Effectiveness of Antistripping Agents. Final Report BC354, RPWO#11. Florida Depart-ment of Transportation, Tallahassee, 2005.

14. Roque, R., C. Koh, Y. Chen, X. Sun, and G. Lopp. Introduction of Fracture Resistance to the Design and Evaluation of Open Graded Fiction Courses in Florida. Final Report BD-545 #53. Florida Department of Transporta-tion, Tallahassee, 2009.

15. Zhang, Z. Identification of Suitable Crack Growth Law for Asphalt Mixtures Using the Superpave® Indirect Tensile Test (IDT). PhD dis-sertation. University of Florida, Gainesville, 2000.

16. Roque, R., B. Birgisson, M. Tia, B. Kim, and Z. Cui. Guidelines for Use of Modifiers in Superpave Mixtures: Executive Summary and Evaluation of SBS Modifier. Final Report BC354, RPWO#1, Vol. 1. Florida Depart-ment of Transportation, Tallahassee, 2004.

17. Roque, R., B. Birgisson, Z. Zhang, B. Sangpetngam, and T. Grant. Implementation of SHRP Indirect Tension Tester to Mitigate Crack­ing in Asphalt Pavements and Overlays. Final Report BA-546. Florida Department of Transportation, Tallahassee, 2002.

18. WinJulea. Layered Elastic Analysis Software. Available through the Pavement Transportation Computer Assisted Structural Engineering (PCASE), website: www.pcase.com, 2003.

19. Jailiardo, A. Development of Specification Criteria to Mitigate Top­Down Cracking. MS thesis. University of Florida, Gainesville, 2003.

The Characteristics of Asphalt–Aggregate Combinations to Meet Surface Requirements Committee peer-reviewed this paper.

TABLE 3 Summary of Mechanical Properties of Asphalt Mixes

Sample ID Mr (ksi) m-Value D1 Value ITS (psi) TSR ER Ratio RER

S1 1,131.4 0.57 8.4 E–07 276.6 0.66 0.18 0.56S2 886.3 (M↓) 0.74 (L↑) 5.2 E–07 (M↑) 181.2 (M↓) 0.10 (M↓)

S3 978.0 (S↓) 0.62 (S↑) 1.1 E–06 (S↑) 219.7 (S↓) 0.65 0.12 (S↓) 0.17S4 562.5 (L↓) 0.69 (M↑) 2.6 E–06 (L↑) 141.8 (L↓) 0.02 (L↓)

Notes: S↓ and S↑ = a small reduction (↓) or increase (↑) compared with the values of the control set (S1); M↓ and M↑ = a medium reduction (↓) or increase (↑) compared with the values of the control set (S1); and L↓ and L↑ = a large reduction (↓) or increase (↑). These changes are compared to the corresponding values of the control set (S1).