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1. Introduction

Because of the viscosity–temperature dependency (Ukwuoma and Ademodi, 1999;Gardiner and Brown, 2000; Bahia et al., 2001; Petersen et al., 1994; Coplantz et al., 1993) of 

asphalt materials, some experts (Sousa et al., 1994; Kandhal et al., 1995; Huner and Brown,

2001; West, 2005; McDaniel et al., 2000) believe that the relevant performance of hot mix

asphalt mixture has a strong relationship with mixing temperatures in the central plant and

compaction temperatures in the field. This has been widely implemented to determine the

mixing and compaction temperature both in laboratory and field construction (Gudimettla

et al., 2003). Some problems, such as segregation, high air void content resulting in inad-

equate mixing and compaction temperature, are considered to be the major contributors to

early or premature distresses of asphalt pavement (Gardiner and Brown, 2000; Krishnan

and Rao, 2001; Epps et al., 2000, 2002; Rauhut et al., 1994).

Great interests have been paid to recycled hot mix asphalt mixture due to environmental

conservation and energy crisis since 1970 (Ngowi, 2001; McDaniel et al., 2000). Recycled

hot mix asphalt mixture is a multiphase system, which consists of sized recycled asphalt

material, new asphalt, and/or recycling agents andnew aggregates. It meets the requirements

of standard material and mix specifications for the type of mix being produced. Considering

mixing temperature, recycled aggregates are preheated at a temperature ranging from 110 to

140 ◦C, which is much lower than the conventional preheated temperatures of fresh aggre-

gates and asphalt binders. This means that the recycled aggregates may be heated by other

components during the mixing process. Therefore, a wide range of ultimate temperature

of blended mixture would be expected, which is a great disadvantage for the temperature

control during transportation and construction, considering energy consumption.

Generally, the optimum mixing and compaction temperatures are determined by the

viscosity–temperature curve of asphalt binders. In this way, the preheated temperatures of various components could be designed to be economical. Under the optimum design, the

ultimate temperature of recycled hot mix asphalt mixture would fully meet the requirements

of optimum mixing and compaction temperatures with the least energy consumption.

In this paper, the temperature characteristics of recycled hot mix asphalt mixtures are

illustrated through the viscosity–temperature curve of asphalt binders containing both fresh

asphalt binders and recycled asphalt binders. The effects of various preheated temperatures

of different components on the ultimate temperature of recycled hot mix asphalt mixtures

were studied, and the temperature storage stability was evaluated over time.

2. Experimental

2.1. Materials and gradation

Recycled asphalt pavement (RAP) was taken from Jingzhu Expressway in Hubei

province, PR China. In order to get a better gradation of recycled hot mix asphalt mix-

ture, RAP was milled and divided into two groups: coarse RAP (RAP1) with 4.26% asphalt

content on average and fine RAP (RAP2) with 7.54% asphalt content on average. Table 1

lists the particle size distribution of RAP1 and RAP2 aggregates. Recycled asphalt was

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Table 1

Sieves distribution of RAP1 and RAP2

Sieves (mm) 16 13.2 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075RAP1 (%) 100 96.1 84.3 46.7 31.7 25.2 20.1 15.2 11.9 9.4

RAP2 (%) 100 100 99.5 93.1 66.8 49.3 36.8 26 20.4 15.7

extracted from RAP with penetration of 26 dmm at 25 ◦C, ductility of 12 cm at 15 ◦C, and

softening point of 67 ◦C.

AH-70 paving asphalt was obtained from Zhonghai Asphalt (Taizhou) Ltd., Co. in

Jiangsu province, PR China, with penetration of 66 dmm at 25 ◦C, ductility of 150 cm

at 15 ◦C, and softening point of 48.3 ◦C.

Fresh limestone aggregate was obtained from Wuhan Duanlingmiao Aggregate Plant in

Hubei province, PR China, with crushed stone value at 19.8%, Los Angeles abrasion loss

value at 18.2%, and flat and elongated particles in coarse aggregate at 3%.Table 2 presents the blended aggregates gradation of AC-16, which was selected as

the test gradation in this paper. The percentages of RAP added were selected as 30 wt%

(16 wt%RAP1 plus 14 wt% RAP2) and 50 wt% (35 wt% RAP1 plus 15 wt% RAP2). The

total asphalt content of recycled hot mix asphalt mixture was selected as 4.3 wt%.

2.2. Methods

2.2.1. Viscosity–temperature curve test 

Brookfield Viscometer (Model DV-II+, Brookfield Engineering Inc., USA) was

employed to measure the viscosities of asphalt binders which consist of fresh asphalt binders

and recycled asphalt binders according to ASTM D4402. The amount of recycled asphaltbinder added was selected as 0, 25, 50, 75, and 100 wt%. These fresh asphalt binders

and recycled asphalt binders were blended together at 140 ◦C using different components

percentages listed above, respectively. Viscosity of each asphalt binder was tested at a tem-

perature ranging from 125 to 185 ◦C, and the viscosity–temperature plots of asphalt binders

were drawn on the base of the testing data.

2.2.2. Mixing and temperature storage stability test 

In this study, the percentages of RAP added were 30 and 50 wt%. RAP and fresh

aggregate, whose accurate temperatures were measured by handheld thermocouple digital

thermometers, were preheated at temperatures with a range of 110–140 ◦C and a range of 

180–220 ◦C, respectively. Fresh asphalt was heated at 160 ◦C and the mixer was controlled

at 170 ◦C. Filler was dried, not preheated.

Eight groups of mixing process are shown in Table 3 from group A to group H. The

integrations of fresh aggregate and RAP at different temperatures are under careful consid-

Table 2

AC-16 aggregate blended gradation

Sieves (mm) 19 16 13.2 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075

Pass (%) 100 98.1 89.7 70.6 51.2 33.4 23.5 17.2 12.8 9.4 6.6

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Table 3

Results of mixing and temperature stability test

Preheated temperature

of RAP (◦C)

Preheated temperature of 

fresh aggregate (◦C)

Percentage of 

RAP added (%)

Actual mixing

temperature (◦C)

Optimum mixing

temperature (◦C)

Final temperatur

after 1 h storage

A 120 200 30 166.5 175.9± 2.5 161.8

B 130 200 30 167.8 164.6

C 130 210 30 175 172.1

D 110 220 30 179.4 178.2

E 120 190 50 160.4 181.1± 2.5 156.8

F 130 200 50 169.2 165.9

G 130 210 50 174.3 171.1

H 140 220 50 182.3 180.6

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614 S.P. Wu et al. / Resources, Conservation and Recycling 51 (2007) 610–620

eration. First, RAP and fresh aggregates were blended for 90 s. Fresh asphalt of required

amount was then added and blended for another 90 s. Finally, filler (if necessary) was added

and blended for 90 s. The total mixing time was 270 s. After mixing, mixtures were imme-diately stored in an attemperator with a handheld thermocouple digital thermometer. The

changes of mixtures’ temperatures were monitored and recorded over time.

3. Results and discussions

3.1. Viscosity–temperature dependency of asphalt binders

Fig. 1 presents the viscosity–temperature dependency of asphalt binders with different

percentages of fresh asphalt binders and recycled asphalt binders. The typical relationship

between viscosity and temperature is plotted on a semi-log scale with viscosity on the logscale. However, in this figure, viscosity is plotted on a linear scale (Gudimettla et al., 2003).

The reason is that if viscosity is plotted on the log scale the relationship would appear linear

and the relative effect of temperatures on viscosity could not be distinguished. Fig. 1 shows

that as the temperature increased from 125 to 185 ◦C, the viscosity of all these binders

decreased. Although the magnitude of viscosity was different at a particular temperature

for all these binders, the general trend of the curves was all the same.

In addition, Fig.1 indicates that the inclusion of recycled binders in fresh binders resulted

in a modification of viscosity at the same temperature. For example, the viscosity of fresh

asphalt binder is 0.42 Pa s at 135 ◦C, and the binders containing 25 and 50 wt% recycled

Fig. 1. Viscosity–temperature curve of asphalt binders containing recycled and fresh asphalt binders.

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S.P. Wu et al. / Resources, Conservation and Recycling 51 (2007) 610–620 615

Fig. 2. Effect of percentage of recycled asphalt on optimum mixing and compaction temperature.

binder have modified the viscosity to 0.92 and 1.12 Pa s, as shown in Fig. 1, respectively.

Generally, asphalt binder is a combination of asphaltenes and maltenes (resins and oils).

Asphaltenes are more viscous than either resins or oils. It plays a major role in determining

asphalt viscosity Airey (2003). Oxidation of aged asphalt binder during construction andservice causes the oils to convert to resins and the resins to convert to asphaltenes, resulting

in age hardening and a higher viscosity binder than fresh binder (Kandhal et al., 1995).

Fig. 2 shows the variation of optimum mixing and compaction temperatures of recycled

hot mix asphalt mixtures with recycled asphalt binder concentrations. It is well known that

the viscosity range of 0.17± 0.02 Pa s is usually used as guidance for mixing temperatures

and 0.28± 0.03 Pa s for compaction temperatures during mix design (Gudimettla et al.,

2003). Experimental data shown in Fig. 2 indicate that the relationship between relevant

temperatures and the percentages of recycled asphalt binder added followed the following

function:

ln T b = W 0.5 ln T r + (1−W 

0.5) ln T f  (1)

where T b is optimum relevant temperature of blended binder; T r and T f  denote optimum

relevant temperatures of recycled binder and fresh binder, respectively; W  is the weight

percentage of the recycled binder.

For optimum mixing temperature in this paper (at the viscosity of 0.17 Pa s):

ln T bm = W 0.5 ln 187.5+ (1−W 

0.5)ln155 R2= 0.9997 (2)

where T bm is optimum mixing temperature of blended binder.

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groups of curves after peak, it can be seen that the temperature characteristics of group D and

group H show higher temperature stability over time than other groups, and the temperature

characteristics in group E show the lowest temperature stability obtained from the highestdecreasing slope.

Table 3 gives the results of mixing and temperature storage stability test. The actual

mixing and the actual compaction temperatures were observed to vary with the change of 

preheated temperatures of fresh aggregate and RAP, as shown in Table 3. Comparing the

actual mixing temperatures with optimum mixing temperatures, the result proves that the

actual mixing temperatures of groups C, D and H are all in the range of optimum mixing

temperatures. In addition, all of the final storage temperatures are beyond the range of 

optimum compaction temperatures except group E. It means that adjusting actual mixing

temperature to the optimum one is much more important than adjusting actual compaction

temperature to the optimum compaction temperature. Thus, the actual mixing temperature

is one of the most important controlling factors in recycled hot mix asphalt mixing process.

Data obtained in experiments, presented in Table 3, also indicated that the sensitivity of 

preheated temperature of RAP to the relevant temperatures is significantly lower than the

sensitivity of preheated temperature of fresh aggregate with the same percentage of RAP

added. For example, comparing group A with group B, the effect of increasing preheated

temperature of RAP by 10 ◦C results in 1.3 ◦C increasing of actual mixing temperature;

while the effect of increasing preheated temperature of fresh aggregate by 10 ◦C results in

7.2 ◦C increasing of actual mixing temperature, comparing group B with group C.

The sensitivity of preheated temperatures of fresh aggregate to the relevant temperatures,

when 50 wt% RAP was added, was significantly lower than the sensitivity of preheated

temperature of fresh aggregate with 30 wt% RAP added, as shown in Table 3. For example,

the effect of increasing preheated temperature of fresh aggregate by 10 ◦C results in 7.2 ◦C

increasing of actual mixing temperature when 30 wt% RAP added. The effect of increasingpreheated temperature of fresh aggregate by 10 ◦C results in 5.1 ◦C increasing of actual

mixing temperature with 50 wt% RAP added, comparing group F with group G.

3.3. Methodology of mixing temperature control

Experimental data presented in Fig. 3 and Table 3 indicates that the actual mixing tem-

perature of recycled hot mix asphalt mixture has a strong relationship with preheated

temperature of RAP and fresh aggregate. As discussed in Section 3.2, the sensitivity of 

preheated temperature of RAP to the actual mixing temperature is significantly lower than

the sensitivity of preheated temperature of fresh aggregate with the same percentage of RAP

added. The sensitivity of preheated temperature of fresh aggregate to the actual mixing tem-perature with high percentage of RAP added is significantly lower than the sensitivity of 

one with small percentage of RAP added.

The related equation obtained from the least error fitting was shown as follows:

lnT AM =

P 

2−

1

122P 

lnT RAP + (1− P 

2)lnT FA (4)

where T AM is the actual mixing temperature of recycled asphalt mixture; T RAP the pre-

heated temperature of RAP; T FA the preheated temperature of fresh aggregate; P denotes

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618 S.P. Wu et al. / Resources, Conservation and Recycling 51 (2007) 610–620

Fig. 4. Actual mixing temperature simulated when 30% RAP added.

for percentage of recycled asphalt pavement (RAP) added. Eq. (4) meets the requirement

that the simulated temperatures should vary as the actual mixing temperatures mentionedin Table 3 with the modeled error percent of ±3 ◦C when P equals to 30 and 50 wt%.

Figs. 4 and 5, respectively, present the actual mixing temperatures simulated when P

equals to 30 and 50 wt%. The optimum preheated temperature range is determined by the

optimum mixing temperature range, and the latter is determined by viscosity–temperature

curve. It can be seen from Figs. 4 and 5 that the combinations of preheated temperature of 

RAP and fresh aggregate which meet the optimum mixing temperature range mentioned

above are shown as either a lower RAP temperature plus a higher fresh aggregate temper-

ature group or a higher RAP temperature plus a lower fresh aggregate temperature group.

This means the preheated temperature of fresh aggregate should increase as the preheated

temperature of RAP decreases. Hence, a relatively low preheated temperature of fresh

aggregate could meet the requirements mentioned before as the preheated temperature of RAP becomes higher. All of the mixtures could meet the optimum mixing temperature

range determined by viscosity–temperature curve when preheated temperatures of fresh

aggregate are beyond 209 and 233 ◦C, as mentioned in Figs. 4 and 5, respectively.

It can be concluded from Figs. 4 and 5 that the fresh aggregate should be preheated to an

extremely high temperature to ensure the final mixing temperature fitting into the range of 

optimum mixing temperature even if the preheated temperature of RAP is low. Generally,

when the temperature is higher, the mix workability would be better, since the viscosity of 

the binder decreases as the temperature increases (Gudimettla et al., 2003).

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S.P. Wu et al. / Resources, Conservation and Recycling 51 (2007) 610–620 619

Fig. 5. Actual mixing temperature simulated when 50% RAP added.

There are many problems associated with high mixing temperatures. It is known that the

temperature differentials would appear in combination of the RAP and the fresh aggregate.

The bigger the temperature gradients emerged, the more temperature differentials occurred.

Temperature differentials were considered as one of the important contributors to segrega-tion even early distresses of asphalt mixture (Gardiner and Brown, 2000). Another problem

is that the binders are ageing rapidly when they are combined with fresh aggregate of high

preheated temperatures. Excessive heating may result in oxidation of binders, especially

fresh binders (Gudimettla et al., 2003). The higher the temperature of fresh aggregate pre-

heated is, the faster the binders are ageing. Research carried out by McDaniel et al. (2000)

indicated that the properties of RAP binders showed no significant difference between

preheat temperature of 110 and 150 ◦C within 2 h.

Based on the above discussions, it is suggested to use a high preheated temperature of 

RAP in combination with a relatively low temperature of fresh aggregate. For example, if 

RAP is preheated at a high temperature of 140 ◦C, the fresh aggregate could be preheated at

a relatively low temperature of 206 ◦C to meet the optimum mixing temperature of 30 wt%

RAP mixture, as shown in Fig. 4.

4. Conclusions

The rheological properties of asphalt binders which contain recycled and fresh asphalt

binders have advantages with the optimization control of recycled hot mix asphalt mix-

ture. The optimum mixing and compaction temperatures can be determined by the

viscosity–temperature curve, which are increased by the recycled binder concentration.

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The optimization control of actual mixing temperature can be implemented by adjusting

the temperatures of in-material components to meet the requirements of optimum tempera-

ture range determined by the viscosity–temperature curve. It is much more economical forrecycled hot mix asphalt mixture to use a higher preheated temperature of RAP.

Acknowledgement

The authors are grateful to Hubei Province Department of Transportation for its financial

support for this paper (the 2005 Transportation Science and Technology Key Project, Ref.

No. 2005361).

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