1

The Effect of Water Conditioning for Extended Periods on the Properties of Asphalt 1

Binders 2

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Submission date: August 1, 2012 5

Word count 3077 plus 4 Tables and 11 Figures 6

Total number of words: 6827 7

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Ana S. Figueroa I (Corresponding author) 10

PhD Candidate 11

Department of Civil Engineering 12

Javeriana University–Colombia 13

Email: figuerosofia@gmail.com 14

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Raul Velasquez 17

Research Associate 18

Department of Civil and Environmental Engineering 19

University of Wisconsin – Madison 20

Email: ravelasquez@wisc.edu 21

22

Fredy Alberto Reyes 23

Professor 24

Department of Civil Engineering 25

Javeriana University–Colombia 26

fredy.reyes@javeriana.edu.co 27

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Hussain Bahia 30

Professor 31

Department of Civil and Environmental Engineering 32

University of Wisconsin – Madison 33

Email: hubahia@facstaff.wisc.edu 34

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Paper submitted for presentation and publication at the 38

Transportation Research Board 39

92nd

Annual Meeting 40

January 13-17, 2013 41

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2

1 THE EFFECT OF WATER CONDITIONING FOR EXTENDED PERIODS ON 2

THE PROPERTIES OF ASPHALT BINDERS 3

4

ABSTRACT 5

6

The selection of materials that are less susceptible to moisture damage (i.e., stripping) is 7

critical to guarantee the performance of asphalt pavements. Stripping in the asphalt mix can 8

accelerate damage and consequently reduce pavement life. Most of the current research on 9

moisture damage is related to the effect of water on the mastic, the aggregate, and the 10

adhesive bond between the aggregate and the binder. However, limited research has been 11

conducted to determine the effect of extended water exposure on the properties of asphalt 12

binders. This paper investigates the influence of extensive water exposure on the stripping 13

potential of asphalt binders by measuring rheological properties, bond strength and the 14

wettability of a Colombian binder before and after immersion in water for three, six, and 15

nine months. In this study, thin films of asphalt (i.e., height= 2 mm) were immersed in 16

water for three, six, and nine months and comparisons were made between experimental 17

results of unconditioned and conditioned binders. Master curves for complex modulus and 18

phase angle were obtained using frequency sweep test in the Dynamic Shear Rheometer 19

(DSR). The bond strength between the binder and aggregates was measured with the 20

recently developed Binder Bond Strength (BBS) test. The wettability potential of the 21

conditioned and unconditioned binder was estimated using the Sessile Drop method. 22

Further, Dynamic Modulus testing of mixes prepared with the unconditioned and 23

conditioned binder was conducted. Experimental results indicate that there are significant 24

changes in the properties of the binder after nine months of water conditioning. Further, 25

dynamic modulus of the mixes prepared with the binder conditioned for nine months is 26

significantly higher than the modulus of the unconditioned mix. 27

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Key words: Stripping, Cohesion, Adhesion, Binder Bond Strength (BBS), Sessile Drop 29

Method, Contact Angle, Wettability, Moisture Damage, Master Curves for Complex 30

Modulus and Phase Angle, Dynamic Modulus. 31

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INTRODUCTION 34

35

Moisture damage is one of the major failure modes of asphalt mixtures and occurs when 36

asphalt is removed from the aggregate surface due to the presence of water. This process 37

known as "stripping" has been widely studied in terms of the influence of water on the 38

adhesive bond between binder and aggregate, the importance of aggregate mineralogy, and 39

on the development of experimental devices to evaluate the impact of water in asphalt 40

mixtures. 41

42

This paper studies the changes in the mechanical properties of asphalt binder after water 43

conditioning for extended periods of time (i.e., 3, 6, and 9 months). The asphalt binder 44

(i.e., Pen 80-100) and aggregate used in this research correspond to materials typically used 45

in Colombia for the construction of asphalt pavements. 46

47

3

BACKGROUND 1

2

The environmental conditions can significantly change the physical and mechanical 3

properties of asphalt mixtures. This climatic effect adds another level of complexity in the 4

design and analysis of asphalt pavements subjected to dynamic loading from traffic. 5

6

The harmful effects of water in the asphalt pavement structure and its manifestation as 7

fatigue and rutting were first recognized in the 1930's and have been studied in depth in 8

recent years. Generally, the presence of water in the asphalt mixture causes a degradation of 9

its mechanical properties. Moisture damage occurs when the presence of water in the 10

liquid, solid, or gas state reduces pavement functionality by affecting the integrity of the 11

aggregate, the mastic, and the adhesive bond between constituents (1). Moisture damage 12

can also be defined as the progressive deterioration of the material due to the following 13

mechanisms (2, 3): 14

15

a) Loss of adhesion in the asphalt-aggregate interface. 16

b) Loss of cohesive strength of the binder or mastic. 17

c) Degradation of the aggregate. 18

19

The current method to evaluate moisture damage in asphalt mixtures is based on indirect 20

measurements of the tensile strength of the mix before and after moisture conditioning (4). 21

The outcome from this test method is the Tensile Strength Ratio (TSR). A ratio closer to 1 22

indicates that the mix is not susceptible to water. The reliability of the TSR method and its 23

relation to field observations has been questioned by many researchers (5, 6). A field study 24

conducted in Wisconsin (5) showed that the TSR is poorly correlated to moisture damage 25

observed in the field and that it lacks reproducibility. 26

27

Caro et al. (1) and Caro and Alvarez (7) have used the framework of thermodynamics for 28

characterization of asphalt materials to moisture damage. Their research introduced 29

moisture susceptibility indicators which are function of the surface energy of the asphalt 30

binder, aggregate, and water. 31

32

Significant effort has been also placed in the development of simple and reliable testing 33

methods for the characterization of asphalt-aggregate systems to moisture damage. An 34

example of such methods is the Binder Bond Strength (BBS) test (8). This method applies a 35

pull-off tensile load to a thin film of binder which is bonded to an aggregate plate. To 36

address moisture susceptibility, thin film samples are tested before and after water 37

conditioning. This method has been proved successful in determining the effect of 38

modification, rock type, conditioning time, and conditioning media on the moisture 39

susceptibility of asphalt-aggregate systems (8). 40

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Table 1 list the main factors affecting moisture susceptibility of asphalt mixtures. 42

Furthermore, Figure 1 summarizes the stripping mechanisms proposed in the literature. 43

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TABLE 1 Factors affecting moisture damage in asphalt mixtures. 1

MIXTURE DESIGN

- Asphalt and aggregates chemistry

- Aspahlt amount

- Voids

- Additives

PRODUCTION

- Percentage of covering and quality of material pass sive 200

- Mix temparature

- Aggregates Moisture

- Aggregates with Clay

CONSTRUCTION

- Execess of compacting in field

- Hight permeability

- Segregation

- Changes in the mixture in field respect mixture design

TEMPERATURE

- Wet areas

- Freezing and defrost cycles

- Separation of water vapor

ANOTHER ASPECTS

- Surface draniage

- Rehabilitation strategies (marginal materials use)

- Heavy loads (vehicles)

2

3

4 FIGURE 1 Stripping mechanisms. 5

6

5

MATERIALS AND TEST METHODS 1

The materials selected for this study are commonly used in construction of flexible 2

pavements in Colombia. The asphalt binder investigated is a Pen 80-100. The properties of 3

this binder are presented in Table 2. The binder was conditioned in water for 3, 6, and 9 4

months. Thin films of asphalt (i.e., h= 2 mm) were submerged in water as indicated in 5

Figure 2. Note that the quality of the water and the temperature (i.e., 20°C) was carefully 6

controlled during the conditioning time. 7

8

TABLE 2 Asphalt properties. 9

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Property Unit Aging Condition

Original RTFOT PAV

Penetration 25°C 0.1 mm 83.22 50.61 54

Softening Point º C 50.55 54.6 54.6

Penetration Index - 0.26 -0.1 0.06

Ductility cm 145 123 123

Flaming Point º C 318 320.3 320.3

Ignition Point º C 356.9 369.4 369.4

Specific Gravity - 1.007 1.007 1.007

Brookfield Viscosity 135 ºC Pa.s 0.36 0.5 -

Mass lost % N/A 0.00% -

Ic -Colloidal Inestability Index - 0.39 - -

Is- Colloidal Clasification - 5.86 - -

11

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13 FIGURE 2 Asphalt binders submerged in water. 14

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The aggregate used was extracted from Coello river in Tolima-Colombia. These aggregates 16

are obtained after crushing siliceous rocks. The characteristics of the aggregate are showed 17

in Table 3. 18

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TABLE 3 Aggregates characteristics. 1

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Property Coarse Fine

LA Abrasion 28.59% N/A

Methylene Blue N/A 4.83%

Flat and Elongated Particles 2.54% N/A

Percentage of Fractured Faces 87.68% N/A

Sand Equivalency - 58.00%

Specific Gravity 2.60 2.38

Specific Gravity in SSD Condition 2.63 2.42

Absorption 1.25% 1.97%

3

Asphalt mixtures were prepared following Colombian standards (i.e., Marshall Method) 4

using the unconditioned and water conditioned binders and the aggregate previously 5

described. The asphalt mixture samples were used for dynamic modulus testing. 6

7

The following test methods were used for the characterization of the materials: 8

a) Asphalt binders: complex modulus and phase angle using the Dynamic Shear 9

Rheometer (DSR), viscosity, penetration grade testing, and contact angle with 10

water. 11

b) Asphalt-Aggregate: Binder Bond Strength (BBS) test before and after water 12

conditioning and contact angle between binder and aggregate. 13

c) Mixture: dynamic modulus test. 14

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Note that at least three replicates for each procedure were tested to ensure repeatability of 16

results. 17

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RESULTS AND DISCUSSIONS 19

Figure 3 shows a summary of the penetration grading testing results for the original and 20

submerged binders. It can be seen that the penetration of the binder decreases as a function 21

of conditioning time. The penetration of the binder submerged for nine months is 22

approximately 48% lower than the original one. Based on the softening point and 23

penetration values, the binder becomes stiffer and more brittle as function of water 24

conditioning time. Also, Figure 3 shows that the submerged binders have Penetration 25

Indices (PI) closer to -1 in comparison to the original binder. This may indicate that the 26

conditioned binders are more susceptible to temperature changes. It is worth noting that the 27

density of the binder does not change significantly after extended periods of water 28

exposure. Figure 3 indicates that the viscosity of the binder at 135°C almost double after 9 29

months of water conditioning. 30

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FIGURE 3 Characterization of unconditioned and conditioned asphalts after 3, 6, and

9 months.

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The viscosity profile of the asphalt binders tested is shown in Figure 4. It can be seen that 1

significant changes in viscosity are observed after 9 months of conditioning. The viscosity 2

at any temperature of the binder conditioning for nine months is higher than the viscosity of 3

the original binder. It is hypothesized that the composition and chemical nature of the 4

binder changes when expose to water for extended periods of time following similar 5

mechanisms as observed for oxidative aging. 6

7

10

100

1000

10000

100000

70 80 90 100 110 120 130 140 150 160 170 180 190

Vis

cosi

ty (c

P)

Temperature (°C)

BRO O KF IEL D VIS COS ITY O RIG INAL AS PH ALT VS AS P H ALT S U BM ERG ED 1 ,3 , 6 AN D M O NTHS

ORIGINAL ASPHALT

S6 MONTHS

S3 MONTHS

S9 MONTHS

8 9

FIGURE 4 Viscosity of original vs. submerged asphalt during 3, 6 and 9 months. 10

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Figure 5 shows the master curves for complex modulus and phase angle for the original and 12

submerged binders. It can be seen that the │G*│ of the binder after 9 months of 13

conditioning is significantly higher than the modulus of the original binder. No significant 14

changes on the complex modulus are observed after water conditioning for 3 and 6 months. 15

Also, it can be observed that the phase angle curves are very similar for all the binders. The 16

high temperature Performance Grade (PG) was estimated for the four binders. The original, 17

3, and 6 months conditioned binders were graded as PG 58. There was a jump in one grade 18

for the asphalt conditioned for 9 months (i.e., PG 64). These results confirm previous 19

observations from the penetration and viscosity testing. The binder after 9 months of water 20

exposure is probably more resistant to rutting but more susceptible to fatigue cracking than 21

the original one. It is important to note that both, the viscous component,│G*│/sin δ, and 22

the elastic component, │G*│sin δ, of the complex modulus of the asphalt increases as 23

function of water conditioning time. 24

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20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0temperature (°C)

10.00

100.0

1000

10000

1.000E5

1.000E6

1.000E7

|G*|

(Pa

)

0

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.0

de

lta (d

eg

re

es

)

0

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.0

de

lta

(d

eg

re

es

)

RT 80-100 S9 -0001oRT 80-100 S9 -0001o, Temperature sweep step

RT-80-100BARRANCA-O-0001o-2, Temperature sweep step

RT 80-100 S6-0002o-2, Temperature sweep step

RT 80-100 S3-0001o-2, Temperature sweep step

1 FIGURE 5 Complex modulus│G*│ and phase angle (δ) variation for the original and 2

submerged asphalt. 3

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Dynamic modulus testing was conducted at three temperatures (i.e., 5, 25, and 40°C) and 5

three frequencies (i.e., 1, 4, and 16 Hz). Table 4 shows the summary of the dynamic 6

modulus testing of the asphalt mixtures. Similarly to the observations for the complex 7

modulus of the binder, the stiffness of the mixture increases as function of conditioning 8

time (Figure 6). These results indicate that changes in the asphalt phase due to extended 9

water exposure can have a significant impact on the performance of the mixture. Figure 6 10

shows that the effect of exposing the binder to water for extended periods is similar to the 11

effect observed for oxidative aging. It can be seen that changes in dynamic modulus are 12

more pronounce at higher temperatures (i.e., 40°C). 13

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TABLE 4 Dynamic modulus results for mixes prepared with unconditioned and 1

conditioned binder. 2

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Temperature Frequency Dynamic Modulus

(MPa)

(°C) (Hz) 0M 3M 6M 9M

5

1 7640 10568 10771 10614

4 10779 13245 13779 13724

16 14398 16409 16046 17768

25

1 1571 2138 2238 2303

4 2666 3460 3587 4048

16 4427 5293 5694 6215

40

1 505 683 747 779

4 683 913 1042 1150

16 1010 1325 1454 1686

4

5

6 7

FIGURE 6 Dynamic modulus of mixtures prepared with unconditioned and water 8

conditioned binder after 3, 6, and 9 months. 9

The wettability of the original and submerged binders was measured by means of the 10

Sessile Drop method using two conditions: (a) asphalt binder on rock substrate and (b) 11

distilled water on asphalt binder substrate. Figure 7 shows an example of the contact angle 12

measured between the asphalt binder and the rock. 13

14

11

Contact surface

rock

Drop of asphaltContact angle

1 2

FIGURE 7 Contact angle between binder and rock. 3

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Figure 8 shows a summary of the contact angle measurements between the binders and the 5

rock. It can be seen that the wettability of the binder on the rock significantly changes after 6

water conditioning. The contact angles for the submerged binders are higher than the 7

original one. The thermodynamic interaction between the rock and the binder changes as 8

function of submerged time. Figure 9 shows the contact angle between distilled water and 9

the asphalt binders. It can be seen that the wettability of the water on the binder only 10

changes significantly after nine months of conditioning. 11

12

13

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FIGURE 8 Contact angle between binders (original and submerged) and rock. 16

9385 88

128.27

0

20

40

60

80

100

120

140

CA O H2O CA 3M H2O CA 6M H2O CA 9M H2O

AN

GLE

(°)

ASPHALT WETTABILITY

17 18

FIGURE 9 Contact angle between water and original and submerged binders. 19

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The adhesive and cohesive bond strength of the asphalt-aggregate systems was measured 1

using the Binder Bond Strength (BBS) test (Figure 10). The cohesive strength of the 2

original and submerged binders was measured using glass plates as substrates. The 3

adhesive bond was measured before and after subjecting the asphalt-aggregate system to 4

water conditioning as specified in (8). 5

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FIGURE 10 Binder Bond Strength (BBS) test to measure cohesion (glass plates as 9

substrates) and adhesion (aggregate as substrates). 10

Figure 11 shows a summary of the bond strength results using the BBS. No significant 11

changes in the cohesive strength are observed as function of conditioning time. The 12

cohesive bond obtained from testing thin films of the original and submerged binders with 13

glass plates is very similar. Also, the bond strength between the binder and the rock under 14

dry condition does not change as function of conditioning time. An interesting observation 15

is that the susceptibility to moisture as measured with the BBS by the ratio of the wet to dry 16

bond strength for the asphalt conditioned for 9 months is minimal compared to the other 17

binders. 18

19 FIGURE 11 Bond strength (cohesive and adhesive) for the original and submerged 20

binders. 21

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CONCLUSIONS AND RECOMMENDATIONS 1

2

This paper investigated the changes in asphalt binder properties after extended periods of 3

water exposure. In general, it was observed that after nine months of water conditioning the 4

rheological properties of the binder changes significantly. The asphalt binder becomes 5

stiffer as function of conditioning time as it is observed for the case of oxidative aging. 6

Based on limited binder and mixture experimental results the following conclusions can be 7

drawn: 8

9

The penetration and softening point of the binder changes as function of submerged 10

time. The binder submerged for 9 months is stiffer and more brittle than the 11

unconditioned binder. 12

The viscosity profile of the binder submerged for 9 months is significantly different 13

than the one for the original binder. Generally, binders submerged in water for 14

extended periods of time are more viscous than unconditioned ones. 15

There was a jump in one Performance Grade (PG) for the asphalt conditioned for 9 16

months (i.e., PG 64) with respect to the unconditioned binder (i.e., PG 58). 17

The complex modulus of the asphalt binders conditioned in water are in generally 18

higher than the modulus of the original binder. 19

The dynamic modulus of the mix prepared with the binder conditioned for 9 months 20

is significantly higher than the modulus of the original mix. This effect is more 21

important at high testing temperatures (i.e., 40°C). 22

The wettability of the asphalt binder on the rock measured with Sessile Drop 23

method increases as function of submerged time. Moisture susceptibility of the 24

asphalt-aggregate system estimated with the Binder Bond Strength (BBS) test is 25

significantly reduced for the asphalt binder submerged for 9 months. 26

The cohesive strength of the original and submerged binders is very similar. 27

It appears that exposing the binder to water for extended periods of time can change 28

its composition and therefore its rheological properties. These changes can affect 29

mixture performance as indicated by the dynamic modulus testing. 30

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Future work is recommended in terms of expanding the materials tested to cover asphalt 32

binders with different composition (e.g., polymer modification) and aggregates from other 33

sources. Also, mixture testing for fatigue and rutting is needed to validate the importance of 34

extended water exposure of the binder in the performance of the mix. 35

ACKNOWLEDGEMENTS 36

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The authors would like to thank the research team at the Modified Asphalt Research Center 38

(MARC) at the University of Wisconsin-Madison, CONCRESCOL and Prof. Silvia Caro 39

from Los Andes University for their contributions and help to this research. 40

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REFERENCES 42

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1. Caro, S., Masad, E., Bhasin, A., Moisture Susceptibility of Asphalt Mixtures, Part 1: 44

Mechanisms. International Journal of Pavement Engineering, 9(2), 81–98, 2008. 45

14

2. Kigundu, B., and Roberts, F.L., Stripping in HMA Mixtures: State-of-the-Art and 1

Critical Review of Test Methods, NCAT Report 88-02, National Asphalt for 2

Asphalt Technology, Auburn University, Alabama, 1988. 3

3. Rice, J., Relationship of Aggregate Characteristics to the Effect of Water on 4

Bituminous Pavement Mixtures. Symposium on Effect of Water on Bituminous 5

Paving Mixtures, pp 17-34, 1958. 6

4. AASHTO T283, Standard Method of Test for Resistance of Compacted Asphalt 7

Mixtures to Moisture-Induced Damage. American Association of State Highway 8

and Transportation Officials, Washington, D.C., 2003. 9

5. Bahia, H. and Ahmad, S., Evaluation and Correlation of Lab and Field Tensile 10

Strength Ratio (TSR) Procedures and Values in Assessing the Stripping Potential of 11

Asphalt Mixes. WisDOT Highway Research Study SPR#0092-45-91, 1999. 12

6. Kanitpong, K. and Bahia, H., Evaluation of HMA Moisture Damage in Wisconsin 13

as it Relates to Pavement Performance, International Journal of Pavement 14

Engineering, 9(1), 9-17, 2008. 15

7. Caro, S., and Alvarez. A., Evaluación de la Susceptibilidad al Daño por Humedad 16

de Mezclas Asfálticas Empleando Propiedades Termodinámicas, Rev. Fac. Ing. 17

Univ. Antioquia No. 58 pp. 95-104, 2011. 18

8. Moraes, R., Velasquez, R., and Bahia, H., Measuring Effect of Moisture on Asphalt-19

Aggregate Bond with the Bitumen Bond Strength Test, Transportation Research 20

Record No. 2209, Vol. 3, pp. 70-81, 2011. 21

9. Moisture Sensitivity of Asphalt Pavements National Seminar, Transportation 22

Research Board (TRB), San Diego, California, 2003. 23

10. Moraes, R., Validation of the Bitumen Bond Strength Test Using Surface Energy 24

Measurements, Master of Science Thesis, Civil & Environmental Engineering 25

Department, University of Wisconsin-Madison, 2011. 26

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