86

Transportation Research Record: Journal of the Transportation Research Board, No. 2306, Transportation Research Board of the National Academies, Washington, D.C., 2012, pp. 86–94.DOI: 10.3141/2306-10

I. L. Al-Qadi, Illinois Center for Transportation and Department of Civil and Envi-ronmental Engineering, and Z. Leng and M. Doyen, MC-250, Department of Civil and Environmental Engineering, University of Illinois at Urbana–Champaign, 205 North Mathews Avenue, Urbana, IL 61801. J. Baek, Department of Civil and Environmental Engineering, Sejong University, 98 Gunja-Dong, Kwangjin-Gu, Seoul, 143–747, South Korea. H. Wang, Department of Civil and Environmental Engineering, Rutgers University, 623 Bowser Road, Piscataway, NJ 08854. S. L. Gillen, Illinois State Toll Highway Authority, 2700 Ogden Avenue, Downers Grove, IL 60515. Current affiliation for Z. Leng: Department of Civil and Structural Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. Corresponding author: Z. Leng, zleng2@illinois.edu.

for an extended construction season. Three WMA techniques have been commonly used to facilitate the mixing of binder and aggre-gates: organic additives, foaming techniques, and chemical addi-tives. Use of organic additives and foaming techniques reduces the effective viscosity of the binder, and chemical additives serve as a surface-active agent to reduce friction at bitumen–aggregate interfaces. Table 1 is a list of some of the most typical WMA prod-ucts used in the United States and shows the degree to which each reduces a mixture’s production temperature (1). However, because there are various WMA preparation techniques, the physical and chemical properties of the mixture can be altered, which can result in different short-term and long-term mechanical behaviors of the mixture. Therefore, many studies evaluated the performance of mixtures produced with various WMA techniques (2–5). Most of these studies reported equivalent performance of WMA with respect to HMA. However, because the WMA technology is relatively new, many questions remain about WMA’s characteristics, such as increased susceptibility of permanent deformation caused by early-age traffic, unknown long-term field performance, higher risk of moisture susceptibility, and unclear traffic opening time.

WMA could provide significant benefits to the industry and in northern states if it could be applied to heat-sensitive mixes, such as polymer or ground tire rubber modified stone mastic asphalt (SMA) mixes designed for high traffic volume or heavy load pavements. Because these SMA mixes must be produced and compacted at high temperatures, placing them during colder temperatures in the fall or early spring is not practical. Use of warm-mix additives with these mixes would allow for a significantly extended construction season. Before SMA mixes with warm-mix additives are used, however, their early-age performance must be determined to be comparable to that of the conventional SMA mixes.

The early-age performance of WMA is a concern mainly because of the curing process. Within a relatively short period after construc-tion, time-dependent hardening, or curing, may occur in WMA as the asphalt binder regains its original viscosity or as a certain amount of entrapped moisture is evaporated from the WMA. Insufficient cur-ing time may cause deterioration of WMA at an early stage, which could consequently affect its long-term performance. Therefore, the early-age performance of WMA must be investigated, especially within the first 24 h of placement to ensure the roadway is not opened to traffic too soon.

Several previous studies reported on the effects of curing time on the strength of WMA. The earliest references to WMA in the United States focused on the way in which curing conditions affected the moisture content and stability of WMA prepared with foaming tech-

Short-Term Performance of Plant-Mixed Warm Stone Mastic AsphaltLaboratory Testing and Field Evaluation

Imad L. Al-Qadi, Zhen Leng, Jongeun Baek, Hao Wang, Matthew Doyen, and Steven L. Gillen

Warm-mix asphalt (WMA) is gaining attention in the pavement indus-try because of its environmental benefits. WMA would be especially useful if it could be applied to heat-sensitive mixes, such as stone mastic asphalt (SMA). However, before these SMA mixes are used with warm mix additives, the performance of the mixes must be examined. A study was conducted to verify the early-age performance of a warm SMA with an Evotherm 3G additive. An Evotherm SMA and a conventional SMA were evaluated and compared with in situ lightweight deflectometer testing and comprehensive laboratory tests, including complex modulus, wheel track, indirect tensile strength, and semicircular bending fracture. Gyratory-compacted specimens made with fresh plant mixtures were tested in the lab at various curing periods; lightweight deflectometer tests were conducted in the field directly on an Illinois expressway over-lay immediately after compaction up to 24 h. The results showed that at the same curing period, the early-age performance of the Evotherm SMA was generally comparable to that of the conventional SMA. The curing process increased the dynamic modulus of both Evotherm SMA and conventional SMA but did not significantly affect other laboratory test results. The onsite modulus of the Evotherm SMA was found to be less than that of the conventional SMA at the same temperature. However, because of the lower compaction temperature, the pavement constructed with the Evotherm SMA could be opened to traffic earlier than could the pavement constructed with the conventional SMA while providing the same modulus.

The use of warm-mix asphalt (WMA) in the United States has grown rapidly in recent years because of its significant environmental ben-efits. Compared with traditional hot-mix asphalt, production of WMA consumes less energy, emits less greenhouse gas, and allows

Al-Qadi, Leng, Baek, Wang, Doyen, and Gillen 87

niques (6–8). These investigations discovered a strong correlation between moisture content and strength of the mixtures. Consider-able strength gain was observed for the specimens subjected to short (within 1 day) and intermediate (1 to 7 days) curing periods. The effect of curing for WMA prepared with organic and chemical addi-tives has also been investigated (3). However, no strong evidence was found to support that curing time affects the tensile strength of Sasobit WMA and Evotherm WMA. Hence, depending on the type of warm-mix technique and asphalt mixture, the curing effects of WMA may vary.

REsEaRch ObjEcTivE and scOpE

This study determined the early-age performance of the warm-mix SMA containing an Evotherm 3G additive with both laboratory per-formance testing and on-site stiffness measurement. The following research tasks were conducted:

1. Evaluate the early-age mechanical properties of the Evotherm SMA with respect to a control conventional SMA through labora-tory performance testing. Laboratory tests were conducted on both the Evotherm SMA mixture and the control SMA mixture. These tests included complex modulus, loaded wheel track, indirect ten-sile (IDT) strength, and semicircular bending (SCB) fracture.

2. Evaluate the effect of curing time on various mechanical prop-erties of the Evotherm SMA based on the laboratory test results. For both the Evotherm SMA and the control SMA, each performance test was conducted at curing periods of between 3 h and 7 days after compaction. Because the fracture property is not time critical, the SCB fracture tests were conducted at curing times of between 12 h and 12 weeks after compaction.

3. Compare the in situ stiffness of the field Evotherm SMA pave-ment section and the control SMA pavement section with a light-weight deflectometer (LWD) test. The LWD test was conducted at various periods after compaction to monitor the change in pavement surface modulus as the pavement cooled and continued to cure.

4. Determine the appropriate traffic opening time for the Evo-therm SMA on the basis of the field LWD testing results. For determining the traffic opening time for the Evotherm SMA, it was decided that the Evotherm SMA and control SMA should provide the same in situ stiffness at traffic opening.

TEsTing MaTERiaL and ExpERiMEnTaL pLan

Testing Material

Warm-Mix Additive

A chemical additive, Evotherm 3G, was selected to produce a warm-mix SMA. Evotherm 3G, developed by MeadWestvaco, is a water-free chemical package containing surfactant and an antistripping agent, which improves aggregate coating, workability, and com-paction. Evotherm 3G can lower mix temperatures 60°F to 85°F (33°C to 45°C). In the asphalt plant where the SMA mixtures for this study were produced, the Evotherm 3G additives were stored in tanks connected to the asphalt tank with pipes (Figure 1).

SMA Mixture Design

A 1/2-in. (12.5-mm) SMA, one of the typical SMAs that have been used by contractors in the Chicago, Illinois, area on many large-scale expressway overlay projects, was selected as the control for this study. As Table 2 shows, the Evotherm SMA and the control SMA share a mixture design, although the Evotherm SMA con-tains 0.5% Evotherm 3G by weight of binder. Both mixtures use PG 64-22 asphalt binder with 12% ground tire rubber and 8% fine fraction of fractionated recycled asphalt pavement. The compac-tion temperatures of the control SMA and the Evotherm SMA were 305°F and 260°F (152°C and 127°C), respectively. The volumet-ric properties of the control SMA, which meet the standard Illi-nois Department of Transportation requirements for SMA mixture design, as modified by Illinois Tollway special provisions are as follows:

• Ndes: 80,• Asphalt content: 6.2%,• Bulk specific gravity: 2.440,• Maximum specific gravity: 2.529,• Air void: 3.5%,• Voids in mineral aggregate: 15.7%,• Voids filled with asphalt: 77.7%, and• Tensile strength ratio: 0.94.

TABLE 1 Typical WMA Products and Temperature Reductions

Production Temperature Decrease

WMA Technique WMA Product30° to 50°F (17° to 28°C)

>50°F (28°C)

Organic additive Sasobit xx x

Foaming technique Double Barrel Green xx xLow energy asphalt xxAspha-Min xx xAdvera WMA xx xWAM Foam xx

Chemical additive Evotherm xx xxRediset WMA xx xREVIX x xx

Note: x = observed; xx = frequently observed.

FIGURE 1 Injection of Evotherm 3G.

88 Transportation Research Record 2306

Laboratory Testing and specimen preparation

As shown in Figure 2, four laboratory performance tests were con-ducted in this study: dynamic modulus test, loaded wheel track test, IDT strength test, and SCB fracture test. Table 3 is a summary of the laboratory testing matrix.

Dynamic Modulus Test

The dynamic modulus test was conducted according to AASHTO TP-62, Determining Dynamic Modulus of Hot-Mix Asphalt Con-

crete Mixtures. For each mix and curing period, three replicates were prepared for testing. Dynamic modulus tests were con-ducted at room temperature [77°F (25°C)] with frequencies of 25, 10, 5, 1, 0.5, and 0.1 Hz. The measured dynamic modulus and phase angle for each test were reported as defined by the following equations:

E * ( )= σε

0

0

1

σ σ ω= ( )0 2sin ( )t

TABLE 2 Comparison of SMAs

Mix Ndes

NMAS [in. (mm)] Binder FRAP (%)

Warm Mix Additive

Compaction Temperature [°F (°C)]

Control SMA 80 0.5 (12.5) PG 64-22 12% GTR 8 na 305 (152)

Evotherm SMA 80 0.5 (12.5) PG 64-22 12% GTR 8 0.5% of binder 260 (127)

Note: NMAS = nominal maximum aggregate size; FRAP = fractionated recycled asphalt pavement; GTR = ground tire rubber; na = not applicable.

(a)

(c)

(b)

(d)

FIGURE 2 Laboratory test setup: (a) dynamic modulus test, (b) loaded wheel track test, (c) IDT strength test, and (d) SCB fracture test.

Al-Qadi, Leng, Baek, Wang, Doyen, and Gillen 89

ε ε ω φ= −( )0 3sin ( )t

where

|E*| = dynamic modulus, σ0 = applied steady state stress amplitude, ε0 = measured strain amplitude, ω = angular frequency (2πf, where f = frequency), and ϕ = phase angle in radians (ωΔt, where Δt = time lag between

stress and strain).

Loaded Wheel Test

A Hamburg-type loaded wheel tester, manufactured by PMW, Inc., was used to assess the rutting performance of mixtures. This test was done in accordance with a Texas Department of Transporta-tion procedure (9), except that it was conducted in a dry condition at 86°F (30°C). The dry condition was selected to better represent the short-term performance immediately after construction. The test was performed by rolling a 158-lb (738-N) steel wheel on the speci-men surface at 50 passes a minute and stopping after 20,000 passes. The rut depth at a specified number of wheel passes or the number of passes until failure was reported. For each mix and curing period, two pairs of replicates were prepared for testing.

IDT Test

The IDT strength test was performed in accordance with AASHTO T-322-07, Standard Method of Test for Determining the Creep Com-pliance and Strength of Hot-Mix Asphalt (HMA) Using the Indirect Tensile Test Device, on a universal testing machine manufactured by Instron, Inc. For each mix and curing time, two replicates were prepared. The testing specimen was loaded until failure at a rate of 0.5 in./min (12.7 mm/min) at room temperature (77°F). The IDT strength was calculated with the following equation:

SP

d bt = 24

π i i( )

where

St = tensile strength, P = maximum applied load, d = diameter of specimen, and b = thickness of specimen.

SCB Test

The SCB fracture test was used to conduct fracture characteriza-tion of asphalt mixtures. Cylindrical specimens were sliced into cylinders 2 in. (50 mm) thick and cut in half along the diameter. A 0.6-in. (15-mm) notch was cut into each half of the specimen. The test was performed at a temperature of 10.4°F (−12°C), which is 18°F (10°C) warmer than the low-temperature binder grade. The test was conducted in a constant crack mouth opening displacement (CMOD) rate mode and the load, displacement, and CMOD were recorded. The work of fracture was calculated from the SCB test by using the load–CMOD curve in the following equation:

W Pduf = ∫ ( )5

where Wf is the work of the fracture and u is the CMOD.

Specimen Preparation and Transportation

To eliminate the effect of reheating and to better simulate the curing that occurs in the field, all laboratory testing specimens in this study were compacted in the plant immediately after production. Six gyra-tory compactors were operated simultaneously in the plant to ensure that additional curing time did not occur in the loose mixture while each sample was waiting to be compacted.

The specimens were prepared in an asphalt plant in Aurora, Illinois, and then transported to the Advanced Transportation Research and Engineering Laboratory of the University of Illinois in Rantoul for testing. Because the asphalt plant is 150 mi (241 km) from the labora-tory, to meet the strict performance testing schedule the specimens to be tested 3 h after compaction were flown by chartered airplane. The remaining specimens were transported by trucks.

A detailed rehearsal was performed before work began with the mix-tures that were prepared for testing. The exact amount of time needed for measuring bulk density and for cutting and coring the specimens for each test was recorded. To minimize the effect of density mea-surement and specimen preparation on testing results, these were con-ducted right before the test was started. The procedure was consistently followed throughout the study.

On-site LWd Test

Concurrent with the laboratory performance tests, LWD tests were conducted in the field for monitoring of the early-age on-site stiff-ness of the SMA mixtures. As Figure 3 shows, an LWD is a type of

TABLE 3 Laboratory Testing Matrix

Test Complex Modulus Wheel Track IDT Strength SCB

Material property |E*|, phase angle Rut depth Strength Work of fracture

Temperature, °F (°C) 77 (25) 86 (30) 77 (25) 10 (−12)

Specimen dimension, 3.94 (100) (D) 2.44 (62) (H) 5.91 (150) (D) 2.96 (75) (R) in. (mm) 5.91 (100) (H) 1.97 (50) (H) 1.97 (50) (H)

Condition 25, 10, 5, 1, 0.5, 0.1 Hz Up to 20,000 cycles (dry) Max. 2,248 lbf (10 kN) CMOD control at 0.028 in./mm (0.7 mm/min)

Curing period 3 h, 6 h, 12 h, 1 day, 3 days, and 7 days

3 h, 6 h, 12 h, 1 day, and 3 days

3 h, 6 h, 12 h, 1 day, 3 days, and 7 days

12 h, 1 day, 3 days, 7 days, 3 weeks, 6 weeks, and 12 weeks

Note: D = diameter; H = height; R = radius; max. = maximum.

90 Transportation Research Record 2306

field testing equipment that determines the stiffness of pavement material. The basic principle of the LWD is to use geophones to measure surface deflections induced by dropping weight. The cal-culation of the pavement surface deflection modulus is based on the Boussinesq solution:

Ef a

d0

20

0

16=

−( )µ σ( )

where

E0 = surface deflection modulus (MPa), f = factor for stress distribution (2 is a standard value for flexible

plate), µ = Poisson’s ratio (normally 0.35 for asphalt mixture),σ0 = stress under plate (kPa), a = radius of plate (mm), and d0 = center deflection (µm).

Figure 4a shows the construction site on I-355 in the Chicago area, where the Evotherm SMA and control SMA used in laboratory testing were placed. The underlying structures of both SMA layers contain a 1.9-mm (0.75-in.) leveling binder course, a 30.5-cm (12-in.) cement concrete layer, and a 30.5-cm (12-in.) aggregate subbase on top of the subgrade. For each mixture, a leveled area was selected for the LWD test to eliminate the effect of the surface slope on the testing results. Three test locations were selected, as illustrated in Figure 4b: one in the left wheelpath, one between wheelpaths, and one in the right wheelpath. The LWD test started immediately after the final compac-tion pass. To monitor the surface modulus change over time after

construction, the test was repeated every 30 min until 3 h and then every 3 h until 24 h. Before each test, an infrared temperature gun was used to measure the pavement surface temperature.

TEsTing REsuLTs and discussiOn

Results of Laboratory performance Test

Air Void Contents of Prepared Specimens

The air void contents of the compacted gyratory specimens were measured before each test. Table 4 is a summary of the measured air void contents of the specimens prepared for various tests. The data show that the air void contents are generally within the range of 6.0% ±0.5%, with only one exception: the control SMA for the wheel track test.

Complex Modulus Test

Figure 5 shows the dynamic modulus test results at 25 Hz and 0.1 Hz. At both frequencies, the modulus of both mixtures increases with the curing time. However, at the same curing time, no significant difference was found between the Evotherm SMA and the control SMA, as supported by the overlapped error bars at all curing times. The error bars represent the mean dynamic modulus plus and minus one standard deviation. If the error bars at the same curing time overlap, the two measurements are considered to be not statisti-cally different. Thus the data presented in Figure 5 indicate that the

(a) (b)

Deflect Load

0 0

30

60

90

120

10 20 30

Loading time (ms)

Def

lect

ion

(m

icro

n)

0

6

3

9

12

15

Lo

ad (

kN)

40 50 60

FIGURE 3 LWD test: (a) equipment and (b) typical data.

(a)

One lane

Left wheelpath

Between wheelpaths

10ft

10ft

Right wheelpath

(b)

FIGURE 4 Construction site at I-355 driving lane: (a) compaction and (b) LWD test locations.

Al-Qadi, Leng, Baek, Wang, Doyen, and Gillen 91

dynamic moduli of the Evotherm SMA and control SMA are com-parable at the same curing stage. The dynamic moduli of both mixes show a trend to increase during the curing process.

Loaded Wheel Track Test

Figure 6 shows the wheel track test results at 5,000 and 20,000 load-ing cycles. Because the tests for the specimens with 3-h curing were terminated before 20,000 loading cycles to start the tests for the specimens with 6-h curing, no data were collected at 20,000 loading cycles for the specimens with 3-h curing time.

Figure 6 shows no clear trend between the rut depth and the cur-ing time for the mixtures. This lack of a trend indicates that curing time does not significantly affect the early-age rutting performance of either Evotherm SMA or control SMA. A comparison of the rut depth of both mixtures at the same curing time showed that the rut depths of the Evotherm SMA and the control SMA are statisti-

cally the same in most cases except at the 72-h curing time, when the Evotherm SMA resulted in a slightly greater rut depth than the control SMA.

IDT Strength Test

The early-age tensile strengths of the two mixtures were obtained through the IDT test. Figure 7 shows the tensile strengths of the Evotherm SMA and the control SMA at various curing stages. The Evotherm SMA provides statistically higher tensile strength only at 12 h, whereas the control SMA provides equal or higher tensile strength at all other curing times. In general, the tensile strength of the control SMA is comparable to, or higher than, that of the Evo-therm SMA. The tensile strength of the Evotherm SMA increases during the first 12 h and then stays relatively constant. The tensile strength of the control SMA increases at between 24 and 72 h and then remains relatively constant.

TABLE 4 Air Void Contents of Prepared Specimens

Dynamic Modulus Test Loaded Wheel Track Test IDT Strength Test SCB Fracture Test

Mix Average (%) COV (%) Average (%) COV (%) Average (%) COV (%) Average (%) COV (%)

Control SMA 6.1 2 6.6 2 6.5 4 6.5 5

Evotherm SMA 6.1 2 6.0 2 5.8 2 5.9 2

Note: COV = coefficient of variance.

3 0

200

400

600

Dyn

amic

Mo

du

lus

(ksi

)

800

1000

1200

Evotherm SMA

6 12

Curing Time (hr) 24 72 168

(a)

(b)

3 0

50

100

Dyn

amic

Mo

du

lus

(ksi

)

150

200

250 Evotherm SMA

6 12

Curing Time (hr) 24 72 168

FIGURE 5 Dynamic modulus test results at (a) 25 Hz and (b) 0.1 Hz.

(a)

(b)

3

Evotherm SMA

6 12

Curing Time (hr) 24 72

0.00

0.04

0.08

Ru

t D

epth

(in

.) 0.12

0.16

6 0.00

0.04

0.08

Ru

t D

epth

(in

.)

0.12

0.16

0.20 Evotherm SMA

12

Curing Time (hr) 24 72

FIGURE 6 Loaded wheel track test results after (a) 5,000 cycles and (b) 20,000 cycles.

92 Transportation Research Record 2306

SCB Test

The fracture resistances of the two mixtures were characterized through the SCB test. Figure 8 shows the SCB test results at various curing stages. The test results do not show any significant effect of curing time on the work of fracture for both mixtures. Although the average work of fracture energy for the control SMA appears to decrease with curing time, the error bar data show this trend is not statistically significant. At all curing times, the error bars of the Evotherm SMA and the control SMA overlap, which means the fracture resistance of the Evotherm SMA is comparable to that of the control SMA at the same curing time.

On-Site LWD Test Results

To further evaluate and compare the on-site stiffness of the Evo-therm SMA and the control SMA, LWD tests were conducted on the field test sections built with these two mixtures.

Measured Surface Temperature and Surface Modulus

Because an asphalt mixture is a viscoelastic material, its modulus is sensitively dependent on the temperature. Therefore, it is impor-tant to record the temperature for each LWD test. The temperature of the asphalt mixture always decreases during the first couple of hours after compaction, and its cooling rate is dependent on various

factors, such as compaction temperature, pavement layer thick-ness, existing surface, and environmental conditions (e.g., ambient temperature and wind speed).

Figure 9a shows the measured surface temperatures up to 3 h after compaction for both mixtures. After the final compaction, the pavement surface temperature of the Evotherm SMA was approxi-mately 120°F (49°C), and the surface temperature of the control SMA was 20°F (11°C) higher. The cooling rates for the two mixtures within 3 h after compaction are very similar.

After the asphalt mixture was placed and compacted in the field, its modulus changed over time, which was captured in the LWD data. The change of the surface modulus over time caused by the change in mixture temperature and possibly the mixture’s curing. Figure 9b plots the surface moduli measured with LWD against the corresponding temperatures for both mixtures. As pavement sur-face temperature decreases, the surface modulus of both mixtures increases. However, at the same temperature, the control SMA resulted in a higher surface modulus than that of the Evotherm SMA.

Traffic Opening Time for Evotherm SMA

One of the challenges of WMA construction is to determine its appropriate traffic opening time. The LWD testing provides a way to

3 0

20

40

60

Ten

sile

Str

eng

th (

psi

)

80

100 Evotherm SMA

6 12

Curing Time (hr)

24 72 168

FIGURE 7 IDT test results.

1 0

1

2

3

Wo

rk o

f F

ract

ure

(J)

4

5

6

7 Evotherm SMA

3 7

Curing Time (day) 49 84

FIGURE 8 SCB test results.

(a)

(b)

Evotherm SMA Control SMA

1.0 0.0 0

50

100

150

200

Time After Compaction (hr)

Su

rfac

e T

emp

erat

ure

(°F

)

2.0 3.0

Evotherm SMA

50 70 90 110 130 150 170 190 0

100

200

300

400

500

600

700

Surface Temperature (°F)

Su

rfac

e M

od

ulu

s (k

si)

Control SMA

FIGURE 9 Field testing results: (a) pavement surface temperature versus time and (b) surface modulus versus surface temperature.

Al-Qadi, Leng, Baek, Wang, Doyen, and Gillen 93

determine the opening time for WMA based on the surface modulus measurement. In the criterion used here, the warm mix SMA and the control SMA should provide the same modulus when they are opened to traffic.

The following regression equations between the surface modu-lus E and the surface temperature T for the control SMA and the Evotherm SMA can be obtained from the data in Figure 9b:

E Tcontrol control= − +8 2489 1195 6 7. . ( )

E TEvotherm Evotherm= − +7 3626 1019 2 8. . ( )

Equation 7 (R2 = .9494) is used to determine the surface modulus of the control SMA at a certain traffic opening temperatures. This surface modulus can then be plugged into Equation 8 (R2 = .7854) for determining the traffic opening temperature for Evotherm SMA. Because the control SMA and Evotherm SMA were constructed at the same thickness and on the same underlying structure, the same LWD surface modulus will lead to the same modulus for the two mixtures.

According to Figure 9a, the relationship between the surface tem-perature T and the time after construction t for the control SMA and the Evotherm SMA can be described with the following two equations, respectively:

T t tcontrol control3

control2= − + −0 0209 1 0388 1. . 33 649 137 4 9. . ( )tcontrol +

T t tEvotherm Evotherm3

Evotherm= − +0 0319 1 2371. . 22Evotherm− +12 199 115 44

10. .

( )t

Then, since the traffic opening temperatures for the two mixtures are already known, the traffic opening times for the control SMA and the Evotherm SMA can be determined with Equations 9 (R2 = .9781) and 10 (R2 = .9781), respectively. These times provide the Evotherm SMA and the control SMA with an equal modulus when they are opened to traffic.

In construction practice, the traffic opening time of the HMA is usually determined by the pavement surface temperature. Table 5 shows the calculated traffic opening times for the Evotherm SMA and control SMA with the traffic opening temperatures for the con-trol SMA of 140°F, 120°F, and 100°F (60°C, 49°C, and 38°C). In Table 5, Column 2 is calculated from Column 1 with Equation 7; Column 3 is calculated from Column 2 with Equation 8; and Col-umns 4 and 5 are calculated from Columns 1 and 3 with Equations 9 and 10. Although the Evotherm SMA had a slightly smaller in situ stiffness than the control SMA at the same temperature, the Evotherm SMA can still be opened to traffic earlier than could the control SMA because of its lower temperature after compaction.

However, the presented traffic opening times for the SMA mix-tures apply only to the environmental conditions in this study. For

the traffic opening times of the same mixtures paved in other envi-ronmental conditions to be determined, the temperature and cooling time relations could be obtained with existing cooling model pro-grams, such as the MultiCool software developed at the University of Minnesota (10). Then the procedure presented in this study could be followed to determine the traffic opening times for the two mixtures in any specific environmental condition.

suMMaRy

For the early-age performance of warm-mix SMA with an Evo-therm additive to be characterized, both laboratory testing and on-site LWD testing were conducted. The following points summarize the findings of this study:

• Laboratory testing results showed that the effect of curing time was generally insignificant on the early-age rutting resistance, tensile strength, and fracture resistance of the Evotherm SMA. The early-age dynamic modulus of Evotherm SMA increased with curing time. However, because a similar trend was observed in the control SMA, this curing effect was not considered to be caused by the Evotherm additive.• The laboratory testing results indicated that the early-age

dynamic modulus and fracture resistance of the Evotherm SMA were comparable to those of the control SMA at the same curing stage. Before 72 h, the Evotherm SMA and the control SMA exhib-ited similar rutting resistance and tensile strength. After 72 h, the Evotherm SMA showed slightly lower rutting resistance and tensile strength.• Field LWD testing indicated that the on-site modulus of the

Evotherm SMA was lower than that of the control SMA at the same temperature. However, because of the lower compaction temper-ature, the Evotherm SMA could be opened to traffic earlier than could the control SMA.

acknOWLEdgMEnTs

This paper was based on the results of the Illinois Tollway project, Short-Term Performance of Modified Stone Matrix Asphalt (SMA) Mixes Produced with Warm-Mix Additives. This project was con-ducted in cooperation with the Illinois Center for Transportation and the Illinois Tollway. The authors appreciate the assistance of research engineers Jeffrey Kern and James Meister and gradu-ate students Ibrahim Abuawad, Sarfraz Ahmed, Qazi Aurangzeb, Seonghwan Cho, Khaled Hasiba, Jaime Hernandez-Urrea, Brian Hill, Hassan Ozer, Alejandro Salinas, Pengcheng Shangguan, and Songsu Son of the Advanced Transportation Research and Engineering Laboratory of the University of Illinois at Urbana–Champaign.

TABLE 5 Traffic Opening Time for Evotherm SMA

Assumed Traffic Opening Temperature for Control [°F (°C)]

Surface Modulus [ksi (MPa)]

Traffic Opening Temperature for Evotherm [°F (°C)]

Traffic Opening Time (h)

Control Evotherm

140 (60) 40.8 (281.3) 132.9 (56.1) 0 0

120 (49) 205.7 (1,418.3) 110.5 (43.6) 1.4 0.5

100 (38) 370.7 (2,555.9) 88.1 (31.2) 3.7 3.2

94 Transportation Research Record 2306

REFEREncEs

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The contents of this study reflect the views of the authors, who are responsible for the facts and the accuracy of the data. The contents do not necessarily reflect the official views or policies of the Illinois Tollway.

The Strength and Deformation Characteristics of Pavement Sections Committee peer-reviewed this paper.