Ning LEE, Chai-Pei CHOU, Kuan-Yu CHEN
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Benefits in Energy Savings and CO2 Reduction by Using Reclaimed
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Asphalt Pavement
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Submission date: July, 31, 2011
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Word count: 5,584+250*7=7,334
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a
Ning LEE, b Chai-Pei CHOU, cKuan-Yu CHEN
a
PhD Candidate
Department of Civil Engineering, National Taiwan University
No. 1, Sec. 4, Roosevelt Road, Taipei, 106, Taiwan
Fax: +886-2-23639990
Tel: +886-2-23625920 ext. 319
E-mail: [email protected]
b
Professor
Department of Civil Engineering, National Taiwan University
No. 1, Sec. 4, Roosevelt Road, Taipei, 106, Taiwan
Fax: +886-2-23639990
Tel: +886-2-23625920 ext. 302
E-mail: [email protected]
c
Undergraduate
Department of Civil Engineering, National Taiwan University
E-mail: [email protected]
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TRB 2012 Annual Meeting
Paper revised from original submittal.
Ning LEE, Chai-Pei CHOU, Kuan-Yu CHEN
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Abstract
This study uses the Pavement Life-cycle Assessment Tool for Environmental and Economic
Effects (PaLATE) and energy consumption data provided by local hot mix asphalt plants to
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confirm the benefits of energy savings and CO2 reduction derived from using Reclaimed
Asphalt Pavement (RAP). Other effective factors, such as the manufacture of binder, a plant’s
energy saving strategies, and the performance of RAP pavements, are also discussed.
According to the results of the analysis, producing 30% RAP mixture has only 84% of
energy consumption and 80% of CO2 emission of a virgin hot mix asphalt mixture. The
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reduction of energy consumption and CO2 emission is mainly due the reuse of asphalt
concrete. Although there are some studies that claim that mixtures containing RAP do not
perform as well as virgin mixtures, as long as the life of RAP pavement can achieve over
80% of the new mixture’s life, there will be positive benefits on reduced energy consumption
and CO2 reduction from a life cycle approach. Using RAP in pavement mixture is indeed a
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feasible and potential way to make pavement construction greener.
Keywords: RAP (Reclaimed Asphalt Pavement), HMA (Hot Mix Asphalt), Energy
consumption, CO2 Emission
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TRB 2012 Annual Meeting
Paper revised from original submittal.
Ning LEE, Chai-Pei CHOU, Kuan-Yu CHEN
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1. INTRODUCTION
To maintain the serviceability of roads, every year considerable amounts of reclaimed asphalt
pavement (RAP) are milled from existing pavement due to maintenance. After proper
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treatments, such as crushing and sieving, the RAP can be reused in new mixtures, by mixing
with specific amount of virgin binder and aggregates. Since RAP is easy to obtain,
inexpensive, and reusable in new pavement mixtures, it is considered a good recycled
material from both economic and environmental perspectives. In some developed countries,
RAP has been used for more than 25 years (1-4). Many authorities and agents allow RAP to
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be added to new asphalt mixtures. Some of them even advocate using RAP in new asphalt
mixtures because using recycled materials may be more environmentally friendly. Given the
energy saving and carbon reduction trends, RAP has become a viable new choice of
pavement material.
The three advantages of using RAP in new mixtures, (low cost (5,6), environmental
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friendliness (7), and higher resistance to some types of pavement distress (8, 9)), are often
studied by researchers. However, compared with studies on economical efficiency and
performance, studies on the environmental benefits of using RAP are relatively few. There
are not many studies that present the environmental benefits of using RAP quantitatively, so
there is still a lack of evidence that RAP is indeed a good choice for reducing environmental
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impact.
Because CO2 is one of the most significant sources of carbon emission, and energy
consumption is also directly related to the amount of carbon emission, the amount of CO 2
emission and energy consumption are selected in this study as the indices in evaluating the
benefits derived from using RAP in HMA mixture. The aim of this study is to confirm
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whether using RAP in pavement construction is really feasible and effective on saving energy
and reducing CO2 emissions. An overview of the characteristics of asphalt mixtures
containing RAP is first reviewed, and then the Pavement Life-cycle Assessment Tool for
Environmental and Economic Effects (PaLATE), which is a Life Cycle Assessment (LCA)
tool developed for pavement engineering, is used to quantify the pollution of asphalt concrete
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production. Related information of five local HMA plants were investigated for the analysis.
Some potential effective factors, such as the energy consumption and CO2 emission when
manufacturing the binder, energy saving strategies of HMA plants, and the service life of
RAP pavements, are also discussed.
Finally, the environmental benefits of using RAP can be determined by comparing the
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energy consumption and CO2 emission of asphalt mixtures containing RAP with those of
producing virgin hot mix asphalt (HMA) mixtures.
TRB 2012 Annual Meeting
Paper revised from original submittal.
Ning LEE, Chai-Pei CHOU, Kuan-Yu CHEN
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2. CHARACTERISTICS OF ASPHALT CONCRETE CONTAINING RAP
2.1 Process of Producing Hot Asphalt Mixtures Containing RAP
Almost all HMA plants in Taiwan are batch plants. FIGURE 1 shows the material flow
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diagram of producing HMA mixture with RAP in a batch plant. The process of producing
HMA mixture with RAP is similar to that of producing virgin mixtures with entirely virgin
materials. Because the RAP contains aged bitumen, the temperature in the RAP dryer cannot
be as high as what is needed to dry virgin aggregates. The temperature to dry RAP is about
135℃, and virgin aggregates are dried at about 140℃ to 170℃. After drying, the RAP and
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virgin aggregates are first mixed together (dry mix), and then the virgin binder is sprayed into
the mixer and mixed with the RAP and virgin aggregates (wet mix). Finally, the production of
HMA mixture containing RAP is complete.
Raw virgin aggregates
Raw RAP
Crushing
Sieving
Storage
Storage
Virgin aggregates
RAP (ready to use)
Dry Virgin aggregates
Dry RAP
Odor removal
Storage
(with heater)
Directly heating
Indirectly heating
Gas emission
Virgin binder (if used)
Weighing
Sieving
Storage
Weighing
Dry mixed aggregates
Feeding
Adding recycling
agents (optional)
Virgin binder and
recycling agents (if used)
Dry mix
Aggregate mixture
Spray
Wet mix
RAP mixture
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FIGURE 1
Material flow diagram of producing HMA mixture with RAP in a batch plant.
2.2 Performance Studies of RAP Contained Pavement
Because of the importance of pavement serviceability, many studies have evaluated the
performance of asphalt mixtures containing RAP. Those studies survey the physical
characteristics of RAP mixtures, like their resistance to pavement distresses (5, 10-12),
TRB 2012 Annual Meeting
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inputting factors of the Mechanical-Empirical Pavement Design Guide (MEPDG) such as the
materials’ Master Curve (E*) and Resilience Modulus (13), and the Superpave design factors,
like the G*/sinδ. There are other studies that try to find out how to input proper information
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into the MEPDG design software to predict the performance of pavement containing RAP
(14). Generally, the control factors in the experiment design in those RAP-related studies
include RAP content (12, 13), source of RAP (2), grades of virgin binder (2, 14), and type of
additives, such as recycling agents and warm-mix agents, added into the asphalt mixtures
(15).
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The amount of RAP in the asphalt mixture affects the performance of pavement. The
effects on stiffness caused by adding RAP is not significant when the RAP content in the
mixture is low (16), but as the percentage of RAP increases, the mixture becomes stiffer (10,
13, 17). As a result, its resistance to permanent deformations, such as rutting and roughness,
can be improved (11, 18-20). However, the increase of stiffness due to adding RAP into the
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mixture is not limitless (8). Also, overly high RAP content may results in poor resistance to
moisture, stripping, and cracking (2, 8, 10). Although RAP mixture still has its disadvantages,
it seems that all of these problems can be easily solved. Some studies suggest those problems
can be resolved by limiting the maximum RAP content no more than 40% (8) or 45% (11) of
the total weight of the mixture. On the other hand, the poor resistance to moisture and
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stripping can be solved by using additives to improve the RAP mixture’s properties. Adding
proper addictives, such as lime, can solve stripping problems (19). By properly adding
recycling agents or warm-mix agents, the pavement containing RAP has similar or even
better performance compared to pavements containing only virgin materials.
However, some studies that conducted field tests or analyzed historical data on RAP
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pavements have different opinions. One of those studies indicates that the service life of test
sections with RAP are shorter than that of the sections without RAP, because cracking and
potholes occur more easily on RAP pavements (21). Another study compared over 10-years
of data obtained from the Long-Term Pavement Performance database of several pavement
sections in Texas (9). It built performance models to predict the service life of overlays with
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and without RAP relatively, and then compared the life cycle costs of each scenario. That
study implied that there is no reason to support using RAP in pavement construction, since
regardless of whether thin or thick overlays are used, the service life of overlays containing
20% RAP are shorter than those of entirely virgin overlays. The service life of RAP overlays
is 30% to 60% shorter than the overlays using virgin materials. Therefore, using RAP may
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cost more from a life-cycle approach.
According to the literature review, it is obvious that the conclusions of studies based on
lab tests and field test data are not consistent. Because of the uncertainty of RAP pavements’
TRB 2012 Annual Meeting
Paper revised from original submittal.
Ning LEE, Chai-Pei CHOU, Kuan-Yu CHEN
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performance, many road authorizes allow the use of RAP in pavement construction, but still
set limitations of the RAP content. The maximum amount of RAP content is usually set
between 20% and 40% (1, 9, 10). The variability in performance should be considered when
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evaluating the environmental benefits of using RAP to get a more actual result.
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3. ENVIRONMENTAL BENEFITS OF USING RAP
3.1 Environmental Impact Assessment of Pavement
In addition to physical characteristics and performance, researchers also study the
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environmental benefits of using RAP. The three main environmental advantages of using
RAP are 1) savings over using virgin or natural materials, 2) reducing the accumulation of
milled asphalt concrete, and 3) decreasing the environmental impact due to disposal in
landfills. Most studies use the LCA concept to compare energy consumption or the amount of
pollution caused by mixing RAP and virgin mixtures (7, 13, 22, 23). Of the three advantages,
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the last two are more difficult to quantify, and the environmental impact of landfills often
need to be observed for a long period of time (24). Therefore, the environmental benefit in
savings from using virgin materials is the most commonly used index. By calculating the
savings in energy and pollution reduction, the environmental benefits of using each recycled
material can be evaluated quantitatively.
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The life cycle of pavements can be divided into four phases: 1) materials manufacture, 2)
construction, 3) operation and maintenance, and 4) demolition or reconstruction (25). The
scope of phases and items to inventory vary in different studies. Some studies make the
assumption that the recycled materials discussed can entirely replace virgin materials, namely
that pavements that contain recycled materials have the same performance as the virgin
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pavement. Consequently, they do not consider the operation and maintenance phase and the
demolition or reconstruction phase, since the environmental impact during the two phases
will be the same in all scenarios (22, 24, 26). However, this assumption is not necessarily true
when evaluating RAP pavements since the conclusions of related studies are not consistent.
There are several methods or tools that can quantify the environmental impact of
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manufacture and other human activities. For example, Chiu (2008) used the Eco Indicator 99
database to evaluate the environmental impact of using conventional HMA, the mixture
containing recycled glass, and the mixture containing 40% RAP. Chiu’s study found that
using RAP has the least impact on the environment (7). That study considered the difference
in performance for each material by setting different intervals for each according to previous
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studies. However, the Eco Indicator 99 database is not built for pavement engineering but for
general industries. The most important component of HMA, asphalt binder, is not included in
its database, so the data on a similar ingredient was selected as a replacement in that study.
TRB 2012 Annual Meeting
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Ning LEE, Chai-Pei CHOU, Kuan-Yu CHEN
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Energy consumption and other pollutions, such as odor (1), SO2, NOx and CO2
emissions (25) are also commonly used inventory targets when assessing pavement
constructions. A previous study inventoried the cradle-to-gate CO2 emission of a HMA batch
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plant in Taiwan to evaluate the respective carbon footprints of producing virgin HMA and
RAP asphalt mixtures (22). In that study, the transportation emission of the ingredients was
also calculated according to the average values of historical data. The result showed that
producing per unit weight of 30% RAP mixture emits only 85% of CO2 emission when
producing virgin mixture. However, the scope of inventory in that study did not cover the
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odor removal equipment, which is required by the Taiwanese government, so the amount of
CO2 emission when producing RAP mixtures was underestimated. Besides, that study
assumed that using RAP does not save virgin binder, and 90% of CO2 reduction was from the
reduction of natural aggregate usage.
Although the function of RAP in the mixture is to replace virgin aggregates, since the
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RAP already contains some aged binder, it in fact saves virgin binder. For example, in Texas
experience, if the asphalt content of new mixture is 5%, and the RAP content is 20%, it saves
approximately 1% of virgin binder. In other words, the mixture only needs 4% of virgin
binder (9). According to the investigated HMA plants, adding 30% of RAP into the mixture
saves approximately 33% of virgin binder, which is consistent with the Texas experience.
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When the asphalt content of new mixture is 5%, it only needs 3.5% of virgin binder of the
total weight. Since producing the asphalt binder has the largest environmental impact (7),
savings of virgin binder should be considered when evaluating the environmental benefits of
using RAP.
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3.2 Features of PaLATE
This study uses PaLATE to calculate the energy consumption and CO2 emission of HMA
mixtures. PaLATE is a free life cycle assessment tool used for pavement construction (28). It
was developed by the Consortium on Green Design and Manufacturing and the Recycled
Materials Resource Center. It includes 18 Excel worksheets, which can be divided into three
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categories: Input, Output, and Data. The users first input the analysis information, such as the
types and amounts of materials, construction activities, and equipment, into the Input
worksheets, and then get results from the Output worksheets. Besides the environmental
impact, such as energy consumption and emissions, PaLATE also can be used to do basic
financial analyses. The Data worksheets are actually a database built based on literature
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reviews which include the environmental impact and cost information for several types of
materials and equipment commonly used in pavement construction.
The most important feature of PaLATE is having the information of recycled materials
TRB 2012 Annual Meeting
Paper revised from original submittal.
Ning LEE, Chai-Pei CHOU, Kuan-Yu CHEN
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and on-site recycling processes, which is not provided by other LCA tools (27). Also, users
can adjust all detailed information such as characteristics, emissions, and cost of the materials,
equipments, and activities in the Data worksheets according to the actual conditions (28).
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When using the tool, the users do not need to input information for the entire construction.
They only need to input information relating to the scope of the analysis.
As to the phases of a pavement’s life cycle, PaLATE considers the initial construction
phase and maintenance phase of the pavement. However, it does not provide information
about the performance of pavements. The users must estimate the total amount of
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construction of all initial construction and maintenance during the analysis period. It does not
include the environmental impact caused by traffic (28).
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4. ENERGY CONSUMPTION and CO2 EMISSION of ASPHALT MIXTURES
The information needed to calculate energy consumption and CO2 emissions includes the
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amount of materials (virgin aggregates, virgin binder, and RAP) used and the transportation
distance of each material. The from-cradle-to-gate processes of HMA are as follows:
transport raw aggregates from excavation to the manufacturer; manufacture virgin aggregates;
transport the aggregates to the HMA plants, produce virgin binder; transport the asphalt
binder from the refinery to the HMA plant, apply the materials to hot mix process; and finally,
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transport the ready mixture to the site. Since the distances and vehicles used to transport vary
from case to case, only the manufacture of materials and the hot mix process are considered
here.
The higher the RAP content in the mixture, the more virgin materials can be reserved.
FHWA provided the following equation to estimate the amount of virgin binder needed when
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manufacturing HMA with RAP:
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Pnb  Pb 
100  r Psb
100
(1)
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Where,
Pnb is the asphalt content of the RAP mixture, as percentage (%) in total weight,
r is the percentage of virgin aggregate, as percentage (%) in weight of total aggregates,
Pb is the asphalt content, the amount of binder needed in virgin mixture, and
Psb is the asphalt content in the RAP (plus the weight of recycling agents if used).
According to the experience of some Taiwanese HMA plants, when the asphalt content
(Pb) is 5% and the RAP content is 30%, the amount of virgin binder (Pnb) should be 3.5%.
TRB 2012 Annual Meeting
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The asphalt content in the RAP (Psb) is 4.7%, using the equation (1). This value is similar to
the Texas experience mentioned above (9). Assuming the asphalt content in RAP is 4.7% of
the total weight, the amount of virgin binder needed under different RAP contents can be
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calculated using the equation (1).
The default densities of virgin aggregate, asphalt binder, and RAP are 1.25 ton/yard3,
0.84 ton/yard3, and 1.85 ton/yard3 respectively in the PaLATE database. The volume of 1 ton
of virgin HMA mixture is about 0.819 yard3. Because the densities of mixtures vary with the
material compositions, and a specific volume of mixtures will be needed for a specific
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construction, this study sets 0.819 yard3 as the unit volume, and then calculates the material
composition of HMA with different RAP contents (as shown in TABLE 1).
TABLE 1
Material Composition with Different RAP Content Levels
RAP
Total
Materials
Density
Content
Weight
RAP
Virgin Binder
Virgin Aggregate
3
(ton/yard )
(%)
(ton) Volume (yd3) Weight(ton) Volume (yd3) Weight(ton) Volume (yd3) Weight (ton)
0
1.22
1
0
0
0.059
0.05
0.76
0.95
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10
1.26
1.0319
0.0558
0.1032
0.0553
0.0464
0.7058
0.8823
20
1.31
1.0729
0.1160
0.2146
0.0511
0.0429
0.6523
0.8154
30
1.36
1.1138
0.1806
0.3341
0.0464
0.039
0.5925
0.7407
40
1.41
1.1548
0.4619
0.2496
0.0346
0.0412
0.6582
0.5267
FIGURE 2 shows the energy savings and CO2 reduction of HMA production with
different RAP contents compared to the virgin mixture. The higher the RAP content, the
higher environmental benefits can be obtained. The 30% RAP mixture is the most commonly
used RAP percentage in Taiwan. When the RAP mixture contains 30% of RAP, producing
RAP mixtures saves approximately 16% of energy and 20% of CO2 emission compared to
producing the virgin mixture.
TRB 2012 Annual Meeting
Paper revised from original submittal.
Ning LEE, Chai-Pei CHOU, Kuan-Yu CHEN
Environmental Benefits (%)
Energy Saving
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CO2 Reduction
40.00
35.00
30.00
25.00
20.00
15.00
10.00
5.00
0.00
0%
10%
20%
RAP Content
30%
40%
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FIGURE 2 The environmental benefits on energy saving and CO2
reduction of different RAP content levels.
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5. DISCUSSIONS of OTHER EFFECTIVE FACTORS
5.1 Considering the Manufacture of Binder or Not
Asphalt binder is a byproduct from petroleum refineries. Some studies claim it unnecessary to
consider the environmental impact of byproducts. However, considering the manufacture of
asphalt binder or not may have a significant effect when evaluating the environmental
benefits of using RAP.
TABLE 2 shows the energy consumption and CO2 emission when producing a 30%
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RAP mixture. The table also shows how the energy consumption of binder manufacture takes
more than 70% of the total value, and its CO2 emission takes even more than 80% of the total
value. It is obvious that the manufacture of asphalt binder leads to the greatest environmental
impact. When the manufacture of asphalt binder is not considered in the evaluation, both of
the benefits on energy saving and CO2 reduction decreased considerably. In other words, if
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the binder manufacture is not considered, the advantages of using RAP cannot be shown to be
meaningful. Although the asphalt binder is a byproduct, it is still necessary to consider the
manufacture of binder when evaluating the environmental impact of asphalt mixtures.
Because petroleum reserves are limited, and the demand on asphalt binder will continue to
exist, the asphalt usage of each construction project should be reduced to save the natural
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resource.
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TABLE 2
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Energy Consumption and CO2 Emission of HMA Manufacture
Energy consumption
CO2 emission
Binder production is considered
MJ
Virgin mixture
%
kg-eCO2
%
aggregates manufacture
146.5134
10.83
10.3762
15.25
asphalt binder manufacture
979.1759
72.41
55.6052
81.75
Plant process
226.6097
16.76
2.0413
3.00
1352.2990
100.00
68.0227
100.00
aggregates manufacture
114.2226
10.05
8.0894
14.95
asphalt binder manufacture
770.0638
67.72
43.7302
80.84
Plant process
252.7683
22.23
2.2770
4.21
1137.0547
100.00
54.0966
100.00
Total
30% RAP mixture
Total
Energy savings and CO2 reduction (%)
15.92
20.47
Binder production is not considered
Virgin mixture
aggregates manufacture
asphalt binder manufacture
Plant process
Total
146.5134
39.27
10.3762
83.56
-
0.00
-
0.00
226.6097
60.73
2.0413
16.44
373.1231
100.00
12.4175
100.00
114.2226
31.12
8.0894
78.03
-
0.00
-
0.00
252.7683
68.88
2.277
21.97
366.9909
100
10.3664
100.00
30% RAP mixture
aggregates manufacture
asphalt binder manufacture
Plant process
Total
Energy savings and CO2 reduction (%)
1.64
16.52
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5.2 Energy Saving Strategies in Hot Mix Asphalt Plants
As mentioned previously, the manufacturing processes of RAP mixtures and virgin mixtures
differ slightly. In Taiwan, all RAP mixture plants must set equipment to remove odor from the
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emission gas. This odor-removal equipment in Taiwanese plants is commonly a heating tower
which removes the odor by burning the odor gas with high-temperature flames,
approximately from 700 to 800℃. The PaLATE does not account for the odor removal tower
or other similar equipment. Therefore, it underestimates the environmental impact of
producing RAP mixtures.
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According to interviews with staff at the HMA plants, the fuel consumption of the
odor-removal tower is about 2.5 liters of heavy oil per production of one ton of 30% RAP
mixture. The consumption of 2.5 liter of heavy oil can be converted to 101.365MJ energy and
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7.775kg CO2 emission (29). For each 0.819 yard3 of 30% RAP mixture, the odor-removal
tower requires 112.9MJ energy and generates 8.6598 kg of CO2 emission.
Because saving energy also means savings cost of operations, all five HMA plants
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investigated in this study have implemented some energy saving strategies to reduce the
operating cost. Two of the most important strategies are to reuse the heat provided by the
emissions gas, and to let two pieces of equipment share a heater. The emission gas that passes
through the odor-removal tower is still about 300 to 400℃ or even higher. Therefore, some
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HMA plants reuse the heat of the emission gas to keep the asphalt storage unit warm. Reusing
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heat not only saves energy but also reduces the impact on surrounding wildlife. The other
plants surveyed dry the virgin aggregates and burn the odor at the same time. In this manner,
they do not need two separate flame heaters and the total energy consumption can be limited.
TABLE 3 shows that the environmental benefit of using RAP decreases significantly
when the odor removal is accounted. However, the benefit on energy savings by using RAP
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can be raised with the increasing energy efficiency. When a plant reduces total energy
consumption by 10%, the energy consumption of producing the 30% RAP mixture can be up
to 17% less than that of producing the virgin mixture.
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Removal
TABLE 3
Benefits on Energy Savings and CO2 Reduction with and without Odor
Energy consumption
(MJ)
Energy savings
(%)
CO2 emission
(kg-eCO2)
CO2 reduction
(%)
Without odor removal
Virgin mixture
1352.2990
68.0227
30% RAP mixture
1137.0547
15.92
54.0966
20.47
0%
1249.9547
7.57
62.7564
7.74
5%
1187.4570
12.19
10%
1124.9592
16.81
15%
1062.4615
21.43
20%
999.9638
26.05
25%
937.4660
30.68
With odor removal
30% RAP mixture
energy saving
efficiency
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5.3 Length of Service Life
Pavement’s service life determines the number of maintenance in the analysis period. More
maintenance leads to more material usage. According to the analysis results in this study,
producing RAP mixtures indeed consumes less energy and generates less CO2 emissions than
TRB 2012 Annual Meeting
Paper revised from original submittal.
Ning LEE, Chai-Pei CHOU, Kuan-Yu CHEN
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producing virgin mixtures. Nevertheless, the performance of pavement containing RAP and
that of virgin pavement are different. It is necessary to consider the service life to further
confirm the environmental benefits from the life cycle approach.
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It is not easy to predict how long the service life of pavement will be. The latest
AASHTO pavement design guide, MEPDG, developed pavement distress models based on
historical data and mechanical analyses. However, the design software associated with
MEPDG does not have the function to predict the distress of pavements containing RAP.
Since mixtures containing RAP are often stiffer than virgin mixtures, the RAP mixtures are
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likely to have “longer” service lives than those of virgin mixtures by MEPDG predictions. In
other words, there is no reliable performance prediction tool which can be used to predict the
life of RAP pavements so far. To solve the problem, this study considers the lengths of
service life of RAP mixtures using a relative concept.
FIGURE 3 illustrates the energy consumption ratio of the RAP pavements and the virgin
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pavement according to the ratio of their service lives; and FIGURE 4 illustrates the CO2
emission ratio according to the ratio of the service life of RAP pavements and virgin
pavement. When the energy consumption ratio or the CO2 emission ratio exceeds 1, it means
that the RAP mixture is actually more energy-consuming or emitting more CO2 than the
virgin mixture does. The higher the RAP content, the lower the service life ratio needed to
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achieve positive energy savings and CO2 reduction benefits. For example, if the 40% RAP
pavement’s service life is longer than 80% of the virgin pavement’s, using 40% RAP mixture
can be more environmental-friendly than using virgin mixture. On the other hand, the 10%
RAP pavement’s service life must be longer than 90% of the virgin pavement’s to be more
environmentally-friendly than the virgin mixture.
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Lower-content (≤ 20%) RAP pavements can easily achieve performance similar to the
performance of the virgin pavement. As to the higher-content (>20%) RAP pavements,
properly adding recycling agents or other additives will help to improve the performance (30).
Therefore, using RAP in HMA is a feasible way to save energy and reduce CO2 emission in
pavement constructions, although it is still necessary to limit the maximum RAP content in
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the mixtures.
TRB 2012 Annual Meeting
Paper revised from original submittal.
Ning LEE, Chai-Pei CHOU, Kuan-Yu CHEN
Energy consumption ratio
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0% RAP
30% RAP
10% RAP
40% RAP
20% RAP
5.00
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
20%
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2
3
FIGURE 3
30%
40%
50%
60%
70%
80%
90% 100% 110%
Ratio of the service lives of RAP pavements and virgin pavement
120%
The energy consumption ratio of the RAP mixtures and the virgin mixture.
Energy consumption ratio
0% RAP
30% RAP
10% RAP
40% RAP
20% RAP
5.00
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
20%
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FIGURE 4
30%
40%
50%
60%
70%
80%
90% 100% 110%
Ratio of the service lives of RAP pavements and virgin pavement
120%
The CO2 emission ratio of the RAP mixtures and the virgin mixture.
6. CONCLUSIONS
To further confirm whether using RAP is a feasible and effective way to save energy and
reduce CO2 emissions, this study used the PaLATE to calculate the energy consumption and
the CO2 emissions of RAP mixtures and the virgin HMA mixture, and then evaluated the
TRB 2012 Annual Meeting
Paper revised from original submittal.
Ning LEE, Chai-Pei CHOU, Kuan-Yu CHEN
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environmental benefits of using RAP.
According to the analysis results in this study, producing the 30% RAP mixture only
requires 84% of energy and emits 80% of CO2 of producing the virgin mixture dose. The
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higher the RAP content, the higher environmental benefits can be obtained. Although
producing RAP mixtures may require extra equipment, like the odor-removal tower, and
cause extra energy consumption, implementing energy-saving strategies in the HMA plants
can significantly raise the energy saving benefits of using RAP. As to the performance of
RAP pavements, if the RAP pavement’s service life is longer than 80% to 90% of the virgin
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pavement’s, using RAP mixture can be more environmentally-friendly than using the virgin
mixture. Previous studies show that it is not difficult to make low-content RAP mixtures
achieve performance similar to the virgin mixture. Even the performance of high-content
RAP mixtures can be improved by properly adding recycling agents or other additives. Thus,
using RAP is indeed a good method to save energy and reduce CO 2 emissions in pavement
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construction.
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Acknowledgements
The authors would like to thank the National Science Council of Taiwan for their
financial support of this research under Grant NSC 98-2221-E-002-115-MY2, and express
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appreciation to those who contributed their knowledge and support of this project.
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