INFLUENCE OF WARM MIX ASPHALT ADDITIVE AND …docs.trb.org/prp/14-3900.pdf · 1 INFLUENCE OF WARM...

INFLUENCE OF WARM MIX ASPHALT ADDITIVE AND DOSAGE RATE ON 1

CONSTRUCTION AND PERFORMANCE OF BITUMINOUS PAVEMENTS 2

3

Ashley Buss 4 Graduate Research Assistant (Corresponding Author) 5

Iowa State University 6

Civil Construction and Environmental Engineering Department 7

174 Town Engineering Building, Ames, IA 50011 8

Phone: 563-880-8098 9

[email protected] 10

11

Yu Kuang 12 Graduate Research Assistant, Iowa State University 13




17

R. Christopher Williams 18 Professor, Iowa State University 19


Phone: 515-294-4419 21


23

Jason Bausano 24 Research Engineer, MeadWestvaco 25

5255 Virginia Avenue, North Charleston, SC 29406 26

Phone: 843-740-2292 27


29

Andrew Cascione 30 Graduate Research Assistant, Iowa State University 31



Phone: 520-481-4127 34


36

Scott Schram 37 Bituminous Engineer 38

Materials Office 39

Iowa Department of Transportation 40

Ames, IA 50010 41

Phone: 515-239-1604 42


44

45

Submitted on: August 1, 2013 46

47

TEXT: 5237 48

FIGURES and TABLES: 9 49

TOTAL WORDS: 7487 50

51

TRB 2014 Annual Meeting Paper revised from original submittal.

Buss, Kuang, Williams, Bausano, Cascione, and Schram 2

ABSTRACT 1 Warm mix asphalt (WMA) technology is an effective way to reduce emissions and save energy during asphalt 2

paving by reducing production temperatures of hot mix asphalt (HMA). As development of WMA additives 3

evolve, many owner-agencies do not know the effect WMA dosage rates have on moisture susceptibility, rutting 4

resistance, and mixture compaction at different temperatures. The overall influence of time and temperature on 5

mixture performance is also important. In this research, two versions of a commonly used WMA additive 6

derived from the forest products industry are evaluated for performance during construction and traffic loading. 7

Laboratory specimens with different additive contents (0%, 0.5%, and 1.0%) were compacted at different 8

temperatures (115°C, 130°C, and 145°C), to evaluate shear capability parameters. Statistical analyses of the 9

compaction force index (CFI) indicated the stability of asphalt mixtures at various compaction temperatures. 10

Evaluation of moisture sensitivity and rutting performance was conducted using the indirect tensile strength test 11

and Hamburg wheel tracking test. Improvements were realized at the 0.5% dosage level; therefore, no economic 12

benefit is achieved by increasing the dosage level. Findings from the laboratory were tested in the field to 13

evaluate the effect of curing time and temperature on WMA compared with HMA. A plant-produced mix 14

included HMA and a WMA mixture produced for the same project. The WMA additive used in the plant project 15

is the same additive used in the laboratory study. Loose mix was collected in order to evaluate the influence of 16

curing time and temperature of a WMA mixture. 17

18

Keywords: warm mix asphalt, compaction, anti-strip, rutting 19

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INTRODUCTION 1 WMA technology contributes to the construction of sustainable roadways by reducing plant mixing 2

temperatures 20°C to 55°C (35°F to 100°F) lower than typical HMA (1). This not only reduces the emission of 3

greenhouse gases that include carbon dioxide (CO2) and sulfur dioxide (SO2), but also lowers fuel consumption 4

which saves energy and production costs. During jobsite placement of WMA, the lower temperatures reduce the 5

amount of fumes and odors inherent when asphalt is produced and placed at high temperatures. Producing 6

asphalt at lower temperatures also reduces binder oxidation, which can increase asphalt pavement cracking 7

resistance and service life (1). 8

There are three main types of WMA technologies. These include foaming, organic wax additives, and 9

chemical processes. According to a recent survey (2), Evotherm®, Sasobit® and Double Barrel Green® are 10

widely used WMA additives or processes used in the United States. Evotherm 3G is a chemical based additive. 11

Evotherm does not contain any water but is in the form of a liquid additive which includes chemical agents 12

derived from the forest products industry. Sasobit, an organic wax WMA additive, is a Fisher-Tropsch wax. 13

These are created by the treatment of hot coal with steam in the presence of a catalyst. Double barrel green is an 14

asphalt foaming system that uses water to produce foamed WMA. All of the commonly used WMA products are 15

designed to improve bitumen coating of aggregates, workability, and aggregate-binder adhesion. 16

As use of WMA technology grows, owner/agencies are interested in knowing how the WMA additives 17

affect the compactability of the asphalt mixture during lay down as well as the moisture susceptibility of the 18

asphalt pavement. The addition of WMA additives will improve compaction of the asphalt mixture at lower 19

temperatures but effectiveness as a compaction aid over a range of temperatures or dosage rates has not been 20

widely studied. Compactability of WMA using variable dosage rates of Sasobit and Rediset® were evaluated 21

and compared with a 0.6% dosage rate of Evotherm 3G. Evotherm 3G was found to provide the best 22

compactability (3). Further study of dosage rates for surfactant-based chemical WMA additives is needed. 23

Likewise, little research has been conducted on the dosage rate of WMA additives as it relates to the 24

moisture sensitivity of an asphalt mixture as well. Traditionally, the moisture susceptibility of WMA has been a 25

concern for pavement engineers due to the lower mixing temperatures, addition of water that is necessary for 26

some WMA technologies, or the change in asphalt-water affinity caused by chemicals or waxes. In most 27

chemical based WMA additives, the chemical additives are designed to improve aggregate-binder adhesion to 28

improve the moisture susceptibility of the asphalt pavement. Moisture susceptibility improvement has been 29

experimentally proven (4); however, it is not known whether dosage rate that optimizes the compactability of 30

the asphalt mixture is the same dosage rate that optimizes its resistance to moisture damage. Moreover, little 31

research has been conducted to determine if different dosage rates of a WMA additive have different effects on 32

mixture compactability and performance. Curing time and temperature is also a factor that has shown to 33

influence the performance of WMA (5-7). Recent studies have shown that the stiffness of asphalt mixtures is 34

sensitive to curing temperature and temperature of WMA (5-7). The effect that the curing time and temperature 35

has on the Hamburg Wheel Tracking Device (HWTD) results should be evaluated to determine if the reduced 36

reheating temperature and/or curing time will cause mixes to fail the 14,000 stripping inflection point 37

requirement for mixes with equivalent single axle load (ESAL) designs greater than 3 million (8). 38

39

OBJECTIVES 40 This research addresses three main objectives. The first objective is to evaluate the effectiveness of additives A-41

1 and B-1 as compaction aids by using shear force index parameters obtained from the Superpave Gyratory 42

Compactor (SGC). The second objective is to study how effective these two types of additives perform as liquid 43

anti-strip agents by conducting the following two moisture sensitivity analyses: 44

1. Evaluate the indirect tensile strength of moisture conditioned and unconditioned specimens by 45

following AASHTO T-283 “Resistance of Compacted HMA to Moisture-Induced Damage”. 46

2. Utilize the HWTD to test the mixture’s susceptibility to moisture damage. 47

The third objective is to evaluate B1 additive’s effect on curing time and temperature on a plant-produced WMA 48

and HMA. 49

50

MATERIALS 51 The two WMA technologies used in this study are generically referred to as: WMA-A1 and WMA-B1. The 52

properties of each material are listed in Table 1. The main difference between the materials is their viscosity 53

with WMA-A1 having a higher viscosity than WMA-B1. 54

55



TABLE 1 WMA additive properties 1

WMA-B1 WMA-A1

Physical form Dark amber liquid Dark liquid

Specific gravity at 25°C (77°F) 0.97 0.999

Conductivity at 25°C (77°F) (μS/cm) 2.2 4.3

Dielectric Constant at 25°C (77°F) 2 - 10 2 - 10

Viscosity (Pa · S)

at 27°C (80°F) 0.28 – 0.56 1.05 – 1.90

at 38°C (100°F) 0.15 – 0.30 0.47 – 0.85

at 49°C (120°F) 0.08 – 0.16 0.20 – 0.40

2

TABLE 2 Aggregates and combined gradation for lab produced mix and plant produced mix 3

Aggregate % in Mix Source Location Gsb %Abs FAA

1/2 Crushed Eagle City 32 Ames Mine/Martin Marietta 2.581 2.65 47.0

3/4 CL Chip Eagle City 5 Ames Mine/Martin Marietta 2.625 1.92 47.0

1/2 X 4 Quartzite 13 Dell Rapids E. Minnehaha Co/Everi. 2.641 0.14 47.5

3/8 CL Chip Lime Creek 8 Ames Mine/Martin Marietta 2.680 0.44 47.0

Manf. Sand Lime Creek 24 Ames Mine/Martin Marietta 2.659 0.78 45.0

Sand 17 Ames South/Hallett Materials Co. 2.594 1.35 40.0

Hydrated Lime 1 commercially produced

1/2 X 4 Quartzite

1/2 ACC Stone

Manf Sand

Concrete Sand

RAP

9.0%

31%

26%

17%

17%

New Ulm Quartzite Quarry

Greene Limestone- Warnholtz

Greene Limestone- Warnholtz

Greene LS- Cedar Acres Resorts

2RAP09-06 (4.63%)

2.620

2.606

2.705

2.606

2.635

0.72

2.45

1.41

0.76

1.65

45.0

45.0

45.0

38.0

42.4

Job Mix Formula- Combined Gradation

1" 3/4" 1/2" 3/8" #4 #8 #16 #30 #50 #100 #200

Upper Tolerance

100 100 100 91 63 43

18

5.0

100 100 95 84 56 38 25 14 5.8 3.4 3.0

100 100 88 77 49 33

10

1.0

Lower Tolerance

Upper Tolerance

100 100 100 94 72 53

24

1.0

100 100 95 87 65 48 32 20 8.8 5.8 4.7

100 100 100 94 72 53

24

1.0

Lower Tolerance

4

The A-1 and B-1 additives were each blended with a PG 64-22 asphalt binder at 0%, 0.5%, and 1.0% 5

by weight of binder. 6

A 12.5 mm (0.5 in.) nominal maximum aggregate size (NMAS) was developed using Superpave 7

specifications for the 10 million ESAL level. The optimal binder content that met all Superpave volumetric 8

criteria was 5.3%. All samples were compacted in the Superpave Gyratory compactor. The non-shaded portion 9

of Table 2 shows the combined gradation for the laboratory mixture as well as each aggregate type, bulk specific 10

gravity, percent absorption and fine aggregate angularity. The variable factors for the samples are the two types 11

of WMA additive, (A1 and B1) with three different dosage contents (0%, 0.5%, 1%). Therefore, one control 12



and four experimental mixes were developed. The mixes are abbreviated as 0%, A1-0.5%, A1-1%, B1-0.5% and 1

B1-1% for subsequent discussion. 2

The laboratory study was expanded to include a plant produced (field) mixture in order to identify the 3

effects of curing time and temperature on WMA compared with HMA. The field mixture included a control 4

HMA mix (no additive) and a WMA (additive B1) mixture. The field mixture is a 12.5 mm (0.5 in.) NMAS and 5

a 10 million ESAL level design. The shaded portion of Table 2 shows the combined gradation and aggregate 6

properties. The mixture was produced in Floyd County Iowa on a section of US 218 near Charles City. Reduced 7

plant temperatures were achieved on this project. 8

9

TEST METHODOLOGY 10 To evaluate the performance of additives A1 and B1 as a compaction aid, the samples were compacted using a 11

Superpave Gyratory Compactor at three different mixing/compaction temperatures: 160/145°C (320/293°F), 12

145/130°C (293/266°F), and 130/115°C (266/239°F), respectively. The selected design number of gyrations 13

(Ndes) was 96 and the maximum number of gyrations (Nmax) was 152. Three replicate samples for each 14

mixing/compaction temperature combination were compacted to Nmax. 15

The Superpave Gyratory Compactor used in this research was equipped with several advanced 16

functions that include measurement of the force and shear capability applied to the specimen. From this data, 17

resistive effort curves can be constructed to analyze the stability of the asphalt mixtures at the three different 18

mixing/compaction temperature combinations. The resistive effort represents the work done by the SGC per 19

unit volume per gyration, assuming the material is perfectly viscous or plastic (9). From the resistive effort 20

curve, the compaction force index (CFI) and the traffic force index (TFI) are developed to estimate the shear 21

force effect from compaction and traffic on asphalt pavements. 22

The resistive effort curve is separated at 92% of the asphalt mixture maximum theoretical specific 23

gravity (Gmm) into a construction effect zone and a traffic effect zone. The CFI refers to the construction side 24

and relates to the area under the resistive effort curve below 92% Gmm. For the traffic effect zone, the TFI is 25

measured by the area between 92% and 98% Gmm under the resistive effort curve. In essence, low resistive effort 26

is desirable for a contractor to easily compact an asphalt pavement, saving compaction time/effort and reducing 27

cost. Therefore, asphalt mixtures with lower CFI values are desired for improved constructability. Inversely, 28

higher TFI values are desired for asphalt mixtures as they indicate a greater resistance to stress from traffic 29

loading and a reduction in pavement rutting, ultimately extending the pavement’s service life (10). 30

31

Moisture Sensitivity 32

In order to evaluate the contribution of the WMA additives as an anti-strip and to determine which type has the 33

ability to mitigate moisture sensitivity at the optimum dosage rate, Indirect Tensile Strength (IDT) testing and 34

HWTD were conducted. For the laboratory produced mixes, six replicate samples for each test were compacted 35

to 7%±0.5 air voids using the Superpave Gyratory Compactor. Dimensions for AASHTO T-283 cylindrical 36

samples are 100 mm (4 in.) diameter and 63.2 ±2.5 mm (2.5±.1in.) in height. Mixing and compaction 37

temperatures were 155°C (311°F) and 145°C (293°F), respectively. Samples for each test were randomly 38

assigned into two subsets of three samples. Moisture conditioning of the samples was conducted according to 39

AASTHO T-283. 40

One of two subsets was randomly selected to be tested under the dry condition. The dry samples were 41

conditioned to a temperature of 25 ± 0.5°C (77±1°F) for two hours prior to testing. The moisture-conditioned 42

specimens underwent vacuum saturation. The degree of saturation was between 70 and 80 percent for the tested 43

specimens and they were each wrapped with a plastic film and then placed in a plastic bag which contained 10 ± 44

0.5 ml of water and sealed. Afterwards, the sealed samples were stored in a freezer at a temperature of -18± 3°C 45

(0 ± 5°F). After a minimum of 16 hours, all of samples were removed from the freezer, unwrapped, and 46

submerged in a water bath at 60 ± 1°C (140±2°F) with 25mm (1in.) of water above their surface for 24 ±1 47

hours. For the last step, before testing is same as control group samples as all of conditioned samples were 48

placed in a 25 ± 0.5°C (77±1°F) water bath for two hours prior to testing. 49

The laboratory evaluation of optimum dosage rate and compactability will ultimately be tested in the field 50

where moisture susceptibility requirements are important. Therefore, the influence of curing time and 51

temperature was studied for a plant-produced/laboratory-compacted mixture. The curing of WMA field samples 52

is currently performed at the reduced compaction temperatures. This portion of the study focuses on how HMA 53

and WMA performance results in the HWTD change due to different curing times and temperatures. The intent 54

is to determine how long curing should take place and at which temperature in order to have comparable test 55



results in the HWTD between HMA and WMA as well as determine which temperature and time combination 1

best simulates the field core HWTD test results. The curing durations chosen were 2 and 4 hours with curing 2

temperatures of 120°C (248°F), 135°C (275°F), and 150°C (302°F). A curing time of greater than four hours is 3

not generally practical in industry. The cured samples were tested in the HWTD. Only WMA samples were 4

cured at reduced temperatures. Table 3 shows the Hamburg pairs that were tested. Each “X” represents a sample 5

that was paired and tested in the HWTD. Cores taken from the field two years after construction were also 6

tested. 7

8

TABLE 3 Plan of study for investigating curing time and temperature in the HWTD 9

Mixes 120°C (248°F) 135°C (275°F) 150°C (302°F)

2 Hours 4 Hours 2 Hours 4 Hours 2 Hours 4 Hours

Field Mix HMA --* -- -- -- XXXXXX XXXX

Field Mix WMA XXXXXX XX XXXXXX XXXX XXXXXX XX

*-- indicates no samples were tested for this category 10

11

Indirect Tensile Strength 12 The IDT test was conducted according to AASHTO T-283 to evaluate the mixture’s resistance to stripping. 13

Moisture damage in asphalt can be influenced by the presence of moisture in the asphalt mixture and will result 14

in a loss of strength through the weakening of the bond between the asphalt cement and the aggregate (11). The 15

loss of strength due to moisture in the asphalt mixture can be reflected from the tensile strength ratio (TSR). The 16

TSR is a numerical index that expresses an HMA pavement’s resistance to moisture damage as the ratio of 17

retained strength after freeze-thaw conditioning to that of its original strength. 18

For testing, the samples were placed between steel loading strips in a hydraulic universal testing 19

machine (UTM) within an environmental chamber set at 25°C (77°F). A load was applied to the specimen at a 20

constant rate of 50 mm/min (2 in/min). The maximum compressive load was recorded from which the tensile 21

strength can be calculated. The average tensile strength of the moisture conditioned subset group was divided by 22

the dry subset group to calculate the TSR. 23

24

Hamburg Wheel Track Test 25 The HWTD is one of several wheel tracking tests used in the United States. It was developed in the 1970s by 26

Esso A.G of Hamburg, Germany, (12). The HWTD is used to test an asphalt mixture’s susceptibility to moisture 27

damage and its resistance to permanent deformation. 28

AASHTO T-324 followed for specimen preparation and test setup. Mixing and compaction 29

temperatures were 155°C (311°F) and 145°C (293°F), respectively. Two cylindrical specimens 150 mm (6 in.) 30

in diameter and 61±1 mm (2.4±0.04 in.) in height were butted into molds and placed under water at 50°C 31

(122°F). Two solid steel wheels with 0.73 MPa (145 psi) contact stress were loaded on the samples and repeated 32

20,000 times at 1.1km/h wheel passes for about 6.5 hours or until failure. The test ends automatically when 50 33

mm (1.6 in.) rut depth occurs or the preset number of 20,000 wheel cycles is reached (11). 34

An important indication moisture damage measured by the HWTD is called the stripping inflection 35

point (SIP). The SIP is the number of wheel passes at the intersection of the creep slope and the stripping slope. 36

After the number of wheel passes at that point, the moisture damage tends to dominate performance. The 37

Colorado Department of Transportation (CDOT) points out that any inflection point below 10,000 wheel passes 38

is an indication of moisture susceptibility (13). The HWTD rutting result is defined as the rut depth at 20,000 39

wheel passes. Currently, there is no AASHTO specification to limit the maximum rut depth for the HWTD 40

testing in the U.S.; however, the Texas Department of Transportation (TxDOT) uses 12.5 mm (0.5 in.) after 41

10,000 passes for mixes with a PG 64-22 and the Colorado Department of Transportation (CDOT) suggested 42

that a rut depth of 10 mm (0.4 in.) after 20,000 passes as the criterion (14). 43

44

TEST RESULTS AND DISCUSSION 45 Test results were evaluated based upon how the additive types, A1 and B1, contribute to both the stability of the 46

asphalt mixtures and the results for IDT and HWTD, while taking into consideration the various additive dosage 47

levels as well as the mixing and compaction temperature combinations. Statistical analysis was conducted using 48

JMP statistical software (15). 49



Figure 1 displays the average values of the compaction force index (CFI) shown with 95% confidence 1

intervals. The error bars in Figure 1 and Figure 2 show little difference between the additive dosage levels for 2

both CFI and TFI. ANOVA testing confirmed there are no statistically significant differences in additive type 3

and no interaction effects for dosage rate and temperature among additive types A1/B1 at an α=0.05 level. 4

However, for A1 and B1, there are the same significant differences in temperature when Tukey honestly 5

significant difference (HSD) multiple comparison testing was performed. The compaction temperature of 145°C 6

(293°F) is not significantly different with 130°C (266°F) and 115°C (239°F), but 115°C (239°F) is significantly 7

different with 130°C (266°F). The statistical difference between 115°C (239°F) and 130°C (266°F) indicates 8

the compaction temperature of 115°C (239°F) may be too low for this mixture and better compaction is 9

achieved at 130°C (266°F). The CFI at 115°C (239°F) also has the highest average for each mixture tested. The 10

largest average reduction in the CFI values occurred for B1-1% followed by A1-0.5% but variability in the CFI 11

parameter does not allow for statistical conclusions at an α-level of 0.05. An ANOVA analysis of CFI data 12

confirmed that there are no statistically significant differences in the variable dosage levels and no interaction 13

effects between additive type and compaction temperature exist. The ANOVA analysis suggests some statistical 14

differences between compaction temperatures but no overall trend applies to all dosage rates and temperatures. 15

These differences can be observed by the overlapping of confidence intervals in Figure 1. 16

17

18 FIGURE 1 Effects of different additives and dosage levels on the CFI. 19

20

Figure 2 shows the TFI values with error bars that represent 95% confidence interval. The average TFI 21

for the control shows the largest sensitivity to temperature, on average. The ANOVA analysis showed no 22

significant differences in additive type or dosage level and no significant interactions between factors. 23

A comparison between A1-0.5% and B1-0.5%, showed no statistically significant differences in 24

additive type and no interactions between additive and temperature. The highest mean TFI was measured at 25

115°C (239°F) compaction temperature and is statistically different from the mean TFI at 130°C (266°F). On 26

average, the control samples had a higher TFI at the lower temperatures of 130°C (266°F) and 115°C (239°F). 27

Comparing between A1-1% and B1-1% TFI, there are no statistically significant differences in the 28

factors of additive type, temperature and the interaction between additive and temperature. 29

30

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

CF

I

145 °C

130 °C

115 °C

Control B1- 0.5% B1-1% A1-0.5% A1-1%



1 FIGURE 2 Effects of different additives and dosage levels on the TFI. 2

3

The TSR and indirect strength values with 95% confidence intervals are shown in Figure 3. The TSR 4

ratio was calculated by using the conditioned mix strength with an additive as the numerator and the 5

unconditioned mix strength without additive as the denominator. The denominator of the TSR ratio is always the 6

dry strength of the 0% additive content mix. By keeping a consistent denominator, the data does not add a 7

confounding factor. This method accurately reflects the difference in TSR values. 8

All TSR values meet the required 80% minimum. The ANOVA analysis shows no statistical difference 9

between additive type and content; however, on average the 0.5% dosage rate has the highest average TSR 10

values. 11

12

13 FIGURE 3 Iowa DOT TSR values for the control and treatment conditions. 14

15

The mixes were evaluated using the HWTD test using laboratory compacted specimens which contain 16

two types of additives (A1, B1) and three content level (0%, 0.5% and 1%) organized as a full factorial design. 17

Three replicates were prepared at each combination of factor levels, which required a total of 36 specimens. 18

0.0

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

3500.0

4000.0

4500.0

5000.0

TF

I 145 °C

130 °C

115 °C

Control B1- 0.5% B1-1% A1-0.5% A1-1%

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0

200

400

600

800

1000

1200

1400

Iow

a D

OT

TS

R

Ind

irec

t T

ensi

le S

tren

gth

, k

Pa

IDT Ratio

IDT Strength



Mixing and compaction temperatures were 155°C (311°F) and 145°C (293°F), respectively. The laboratory mix 1

samples were cured for 2 hours at the compaction temperature. 2

According to the literature review, it is not inevitable that HWTD results will show all three 3

characteristic variables: creep slope, stripping slope and SIP. For the result of the HWTD test, no stripping 4

deformation occurred. Therefore, only the creep slope and the maximum rut depth at 20,000 passes were used to 5

analyze the data. 6

Based on the data comparison, adding either additives, A1 or B1, can reduce the rutting depth when 7

mixed and compacted at the same temperature. The mix types with the WMA additive (A1 or B1) present better 8

rutting resistance with a reduced creep slope as compared to the HMA samples. The A1-0.5% and A1-1.0% 9

performed almost same as the B1-0.5% and B1-1%, respectively. 10

With respect to the creep slope, the ANOVA analysis indicates statistical differences in additive type 11

and dosage rate. The B1 additive has the lowest mean creep slope and is statistically different than A1. 12

Moreover, the 0%-control specimens show the highest mean creep slope and is significantly different from other 13

mixes containing WMA additives at all dosage rates (0.5%, 1.0%), as shown in Figure 4. 14

Figure 4 shows the rut depth at 20,000 passes. By examining the rut depths, a comparison of dosage 15

level demonstrates the impact the additives have on rutting performance in the asphalt mixes. The mixes with 16

the WMA additive exhibited statistically lower amounts of rutting than the mix with no WMA additive. There 17

were no differences in rutting performance between the 0.5% and 1.0% dosage rate or between the A1 and B1 18

additives. Therefore, adding at least 0.5% of the A1 or B1 additives will improve an asphalt mixture’s resistance 19

to rutting. 20

21

22 FIGURE 4 Average rut depth at 20,000 passes and creep slope for control and treatment samples. 23

24

The curing study was performed on plant produced mixes using the HWTD to investigate the impact of 25

time and temperature on the SIP results. The curing times were either 2 or 4 hours and the temperatures were 26

120°C (248°F), 135°C (275°F) and 150°C (302°F). All of the mixes included in this study used the same WMA 27

additive. The HWTD sample dimensions were 150mm (6 in.) diameter and 60.3 mm (2.374 in.) in height. 28

Cores were sawn to the test sample height. 29

The HWTD test results for the cured-lab compacted samples were compared against the cores taken 30

from the roadway. Figure 5 shows the comparisons for all samples including WMA and HMA. The dash lines 31

represent only 2 hours of curing. The WMA and HMA cores performed well with no evidence of stripping. The 32

HMA mixes are denoted in the graph as red or orange lines. The WMA is shown in blue or green lines. The 33

WMA samples with 2 hours of curing at 120°C (248°F) and 135°C (275°F) were the poorest performing mixes. 34

0.0E+00

2.0E-05

4.0E-05

6.0E-05

8.0E-05

1.0E-04

1.2E-04

1.4E-04

1.6E-04

1.8E-04

0

1

2

3

4

5

6

0% A1-0.5% A1-1% B1-0.5% B1-1%

Cre

ep S

lop

e (m

m/p

ass

)

Ru

t D

epth

at

20

00

0 P

ass

es (

mm

)

Rutting Depth

Creep Slope


Buss, Kuang, Williams, Bausano, Cascione, and Schram

10

HMA and WMA samples cured for 2 hours at 150°C (302°F) displayed similar test results. Increased 1

conditioning time of 4 hours also increased performance in the HWTD. The HMA and WMA both showed 2

similar rutting depths when cured at 4 hours at 150°C (302°F) and this was similar with the rutting depths of the 3

tested cores. The data for the WMA samples cured for four hours at 150°C (302°F) showed some noise in the 4

data but there was not significant rutting or signs of stripping. The SIP values are shown in Figure 6. The HMA 5

samples cured for 2 hours at 150°C (302°F) showed similar values to the WMA samples cured for 4 hours at 6

120°C (248°F). Samples conditioned for the two hour curing time at 120°C (248°F) and 135°C (275°F) showed 7

low stripping inflection points and would not pass the 14,000 SIP requirement for this 10 million ESAL mix but 8

longer curing times or higher temperatures would increase the SIP values so that the required minimum SIP 9

would be met. 10

11

12

FIGURE 5 Hamburg results comparing curing temperature and time. 13

14

15 FIGURE 6 SIP comparing HMA/WMA, curing time and temperature. 16

17

-16.0

-14.0

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

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11

CONCLUSIONS 1 In this research, laboratory specimens were produced using two versions of a commonly used WMA 2

technology, generically referred to as WMA-A1 and WMA-B1, with three different dosage rates. The first 3

objective was to use the compaction shear capability test parameter to evaluate the performance of the WMA 4

additives as a compaction aid. The second objective was to determine the use of this additive as an anti-stripping 5

agent. The indirect tensile strength and HWTD tests were executed to evaluate the moisture susceptibility of 6

mixes that utilized both types of WMA additives. The final objective was to determine the impact laboratory 7

curing time and temperature have on a plant-produced/laboratory-cured WMA mix compared with an HMA 8

control mix. Based on the laboratory experiment and statistical analysis, the following conclusions are derived: 9

10

1. The compaction force index (CFI) and the traffic force index (TFI) will not be affected by the additive 11

type (A1, B1) and additive content (0%, 0.5%, 1%). Demonstrating that the shear capability is not 12

sensitive to the effect of the WMA additives. 13

2. The mixtures have better shear capability at the temperature mixing/compaction combination of 14

145°C/130°C (293°F/266°F) than at the combination of 130°C /115°C (266°F/239°F). 15

3. The TSR values were not statistically different but the average TSR values with the dosage level of 16

0.5% were slightly higher. No additional benefit is gained by increasing the additive amount. All 17

samples meet the 80% minimum criteria. 18

4. Laboratory produced samples tested in the HWTD showed no signs of moisture damage. Adding the 19

WMA additive, A1 or B1, can statistically reduce the rut depth and act as an anti-stripping agent. The 20

mix types with the WMA additive (A1 or B1) present better rutting resistance with a reduced creep 21

slope as compared to the HMA samples when mixed and compacted at the same temperature. B1 and 22

A1 produced statistically similar results at both dosage levels. 23

5. Curing time and temperature greatly influences the stripping inflection point in the Hamburg. The 24

lower WMA temperature with curing times below 2 hours, did not perform as well as the samples 25

cured and compacted at HMA temperature or for longer curing durations. WMA and HMA cores taken 26

two years after placement performed well in HWTD tests. 27

6. Based on the data from this research and the literature review (6), a curing time of 2 hours at 135°C 28

(275°F) seems to adequately represent the conditioning of WMA but broader studies which include 29

more mixes and various performance tests should be performed to verify this recommendation. 30

31

The WMA dosage level and additive type did not influence the results of the compaction shear capability. 32

Various compaction temperatures were used when measuring the CFI and TFI parameters and this showed little 33

sensitivity to changes in the binder at the various temperatures but indicated that 115°C (239°F) may be too low 34

for the laboratory mixture. Manufacturer recommendations should be used when choosing mixing and 35

compaction temperatures. The moisture susceptibility tests showed that the WMA additives, A1 and B1, 36

demonstrated moisture resistance ability when compacted at the same temperature as the control samples. The 37

recommended optimal dosage is 0.5% for the mixtures and this is comparable with the manufactures’ 38

recommendation. At the 0.5% dosage rate, the reduced compaction temperature of 130°C (266°F) was achieved 39

and additional resistance to moisture damage was found. Adding dosage levels above 0.5% did not indicate any 40

significant benefits. The curing study data indicates shorter curing times and/or reduced temperatures will 41

influence the SIP in the HWTD. Further research studies in this area will provide a better understanding of how 42

curing time and temperature influence other test parameters. A better understanding of the impact reduced 43

mixing and compaction temperatures have on mix performance can be achieved when using several WMA 44

additives, mix designs and testing procedures at a wide range of temperatures to identify the differences between 45

HMA and WMA. 46

47

ACKNOWLEDGEMENTS 48 The authors would like to thank the Iowa Highway research board for funding the curing study and the Iowa 49

Department of Transportation for their expertise and assistance throughout this study. 50

51



12

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