Sustainable Asphalt Pavements: 

Technologies, Knowledge Gaps and Opportunities 

Timothy D. Miller, Research Assistant 

Department of Civil and Environmental Engineering, University of Wisconsin–Madison 

tdmiller2@wisc.edu 

Hussain U. Bahia, Ph.D., Professor 

Department of Civil and Environmental Engineering, University of Wisconsin–Madison 

bahia@engr.wisc.edu 

Prepared for the Modified Asphalt Research Center (MARC), 

Part of the Asphalt Research Consortium (ARC) 

February 20, 2009 

The purpose of this investigation is to increase 

awareness about gaps in technology regarding asphalt 

pavement sustainability. Presented here are initial 

sustainability challenges and available methods, models 

and tools that may be used to estimate pavement 

sustainability. Also presented are the energy, emissions 

and environmental impact of current and promising 

asphalt technologies as well as thoughts on next steps for 

research required in these areas. 

Heat used in the production of hot mix asphalt (HMA) is 

one of the main targets in reducing its energy and environmental impact. Low-temperature mixes represent substantial energy savings and an associated mitigation of 

emissions. These mixes include cold mix asphalt (CMA), 

half-warm mix asphalt (HWMA) and warm mix asphalt 

(WMA). All show promise at reducing energy consumption, emissions and natural resource use, though more information is needed to draw definitive conclusions 

regarding use of these technologies from a pavement performance perspective. Recycled asphalt pavement (RAP) 

shows promise at reducing natural resource consumption 

and construction energy requirements. While reducing heat 

and increasing use of recycled asphalt are obvious techniques to make asphalt pavement more sustainable, the 

quantification of their benefits and impact on service life 

are unknown. 

 One opportunity for asphalt researchers is to continue 

developing and refining estimation tools so the industry can 

assess where it currently stands in key sustainability indicators and determine how much it can improve using lowenergy and more recycled materials. Such estimation tools 

could allow the industry and road agencies to recognize the 

impacts of different sustainable construction techniques, 

materials and methods, as well as the potential cost and 

resource savings. 

 The Leadership in Energy and Environmental Design 

(LEED) program certifies environmentally friendly, lowimpact materials and design methods for buildings. While a 

program like LEED does not currently exist for pavements, 

there have been discussions and efforts taken toward creating one. The creation of such a program may be attractive 

for the industry, users, and state and federal agencies. 

INITIAL SUSTAINABILITY CHALLENGES 

The initial challenges facing sustainability analyses of 

asphalt pavements are defining what a sustainable asphalt 

pavement is, collecting data that accurately reflect industry 

practices, and setting system boundaries for the sustainability analysis. 
2

Challenge 1: Defining Sustainable Asphalt 

Pavements 

The push toward low-energy, low-emissions and environmentally friendly construction methods is taking root in the 

asphalt industry, but it comes with the challenge of defining 

what “green” or “sustainable” actually mean in practice. 

How might a green or sustainable asphalt pavement be 

defined? One definition, drawn from the United Nations’ 

Brundtland Commission report, considers a sustainable 

pavement to be a safe, efficient and environmentally 

friendly pavement that meets today’s transportation needs 

without jeopardizing the ability to meet such needs in the 

future (1). Another definition suggests an environmentally 

friendly road should minimize ground disturbance; be welldrained and appropriately surfaced to control erosion and 

loss of material; employ effective erosion control measures; 

and be regularly maintained while continuing to meet user 

needs (2). 

For the purposes of this paper, a sustainable pavement 

may be defined as a pavement that minimizes environmental impacts through the reduction of energy consumption, natural resources and associated emissions while 

meeting all performance conditions and standards. 

Challenge 2: Collecting Data 

A second challenge lies in collecting data that accurately 

reflect industry practice. Industry surveys often provide 

information about contractor practices but then fail to capture proprietary methods and techniques that lead to competitive advantage within the industry. While common 

energy values and conversions are applied, variability exists even in well-established values. Developing surveys 

that are easily accessible to private contractors and road 

authorities would supplement the data currently available. 

Data related to energy consumption, resource consumption 

and plant efficiency would be necessary for accurate analyses of asphalt pavement sustainability. 

Challenge 3: Setting System Boundaries 

The third challenge lies in defining sustainable pavement 

system boundaries to establish what will and will not be 

included in the analysis. The availability of data influences 

the system boundaries that necessarily confine a pavement 

analysis. For purposes of this paper, the system boundaries 

will encompass five critical processes (as shown in Figure 

1): 

 Extraction of raw materials 

 Manufacturing or production of paving products 

 Construction or placement of materials 

 Maintenance 

 Removal, recycling or disposal 

EXTRACTION

Natural

aggregates

Bitumen

production

Excavation

Crushing/

screening

Crude oil

extraction

Oil refining

Bitumen

storage

MANUFACTURING

Hot mix

asphalt

production

Drying and

heating

Mixing

Cold mix

asphalt

production

Mixing

Emulsifier

production

Emulsifier

production

Mixing

Transport

CONSTRUCTION

Tack coat

application

Transport

Asphalt

paving

Rolling/

compaction

Transport

MAINTENANCE

Seal coat

application Resurfacing

RECYCLING

(PETROLEUM PRODUCTS)

Transport

Production of recycled

secondary aggregates

Waste

collection

Plant

recycling

RAP

In-situ

recycling

Figure 1. Five main processes comprise the 

system boundaries for the asphalt pavement 

life cycle analysis. This analysis excludes petroleum products. 

Questions raised regarding pavement system boundaries include: 

 Is earthmoving included? 

 Is transportation of water or solvents on liquefied 

asphalts considered? 

 Is manufacturing of the production plant hardware 

included? 

 Is energy used to maintain the temperature of hot 

asphalt during storage and application considered? 

 How is the production plant fueled and what is its 

operating efficiency? 

 How are trucks modeled when carting raw materials? 

 What is the pavement service life? 

Setting system boundaries determines the resolution of 

the analysis. Figure 1 depicts the unit processes in asphalt 

pavement construction for the first three processes of raw 

material extraction, manufacturing of construction products 

and construction (3). As the amount of data collected increases, system boundaries may be expanded beyond the 

critical processes mentioned here. Defining system boundaries is critical in determining which factors are included in 

the analysis. 
3

METHODS, MODELS AND TOOLS 

Relevant methods, models and tools may be identified for 

estimating pavement sustainability indicators. Typical life 

cycle analysis (LCA) models capture lifetime costs but may 

neglect other important sustainability indicators such as 

energy consumption and emissions. A redefined LCA 

model with coherent system boundaries based on available 

data would include the following characteristics (3): 

 Process parameters—data on transport distances, 

fuel efficiency and energy consumption in transport, 

materials production and construction 

 Pavement processes—data on pavement dimensions, mix design and service life 

 Unit inventory—inventory for unit operations of 

transport, materials production and construction 

 Project inventory—aggregated data for unit processes of production, transport and construction 

 Results—impact categories, including global warming potential, eco-toxicity, human toxicity and other 

sustainability indicators 

Incorporating elements of this redefined LCA model 

may be useful in quantifying non-cost factors and benefits. 

One methodology, the eco-efficiency methodology developed by BASF, is an alternative LCA tool that suggests a 

framework for conducting sustainability analyses. The goal 

of this method is to quantify the sustainability of products 

and processes while ensuring short project times and low 

costs (4). The method output is the ecological fingerprint, 

which includes: 

 Energy consumption 

 Emissions 

 Health effect potential 

 Risk potential 

 Resource consumption 

 Land use 

Values calculated from the ecological fingerprint are multiplied by weighting factors and normalized to obtain a 

graphical depiction that portrays each alternative as it relates to each of the six outputs (5). As shown in the example of the fingerprint (Figure 2), each of the six axes is 

independent, so an alternative that performs favorably in 

terms of risk potential might perform poorly in terms of 

energy consumption. Using the ecological fingerprint, 

optimizing the efficiency of the system is possible by identifying critical points where improvements can be made. 

Several estimation tools have been developed to evaluate energy consumption per unit of material produced, 

hauled or constructed. Other models estimate emissions, 

benefits of using recycled materials and user costs (6). In 

addition to determining energy consumption, emissions and 

raw materials consumption, these tools can evaluate other 

environmental impacts and costs. Another promising tool 

was developed by the Waste Recycling Action Program 

(WRAP) (7). The WRAP tool synthesizes the best components of 26 sustainability and assessment models to promote the benefits of recycled secondary aggregates and 

carbon dioxide. Other analytical models based on spreadsheets are also available for obtaining estimates. Clearly the 

Hot chip seal with

ground tire rubber

Cold chip seal modified

with SBR polymer

Cold chip seal modified

with SBS polymer

1.0 is the worst position

for each output.

Better results are ordered

relatively less than 1.0.

Energy

consumption

Emissions

Health effect

potential

Risk

potential

Resource

consumption

Land use

1.0

0.5

0

Figure 2. One modeling approach is BASF’s 

eco-efficiency methodology, which generates 

an ecological fingerprint. Each axis compares 

a different ecological output for the alternatives. (Image adapted from BASF) 

methods and tools for conducting life cycle assessments are 

not only applicable to the road industry, but are improving. 

At this point, no single model or tool seems to be the 

desired “black box” that would allow the user to input 

given design values for any type of road and obtain a depiction of expected impacts and costs. Building on existing 

conventional models for life cycle assessments by incorporating WRAP tools into an eco-efficiency model could be a 

promising path forward. 

A more comprehensive model would incorporate the 

six critical areas identified in the BASF eco-efficiency 

model while accounting for lifetime costs and pavement 

performance. Tools developed by WRAP and Land Transport New Zealand could be rendered to reflect the weighted 

importance of each factor, and provide DOTs and industry 

with a coherent estimation tool. 

ENERGY, EMISSIONS AND 

ENVIRONMENTAL IMPACT 

Envisioning a sustainable road may be easier than actually 

constructing it, but pathways to practically developing 

sustainable pavements lie in energy savings and natural 

resource conservation. Therefore, it is necessary to investigate the construction techniques, estimation methods and 

alternative technologies as they relate to these critical sustainability issues.

Several existing technologies have been introduced, all 

of which rely on the possibility of pavements being manufactured and constructed at lower temperatures. Construct-
4

ing pavements using any one of these methods presents 

opportunities for energy savings and emissions reductions. 

But do these technologies actually save energy and reduce 

emissions? How much energy is conserved and how much 

are emissions reduced? Would these new types of pavements last as long? 

Energy 

Energy savings may be realized by reducing process temperatures for HMAs while developing warm, half-warm 

and cold mix techniques and pavement preservation strategies (8). Only recently has the cost of energy become a 

driving change in the road-building industry, and until now, 

little has been done to monitor energy use (9). Using lowtemperature asphalt techniques can vastly reduce energy 

requirements, emissions and environmental impact. Given 

the five major processes within the system boundaries, each 

process may be considered in terms of energy consumption. 

Real data need to be collected and organized so that tools to 

calculate energy savings can be developed. 

Extracting raw materials 

The first process under consideration is the extraction of 

raw materials. Data are relatively constant across studies 

regarding fuel usage for the heavy machinery used to extract the materials. Relative to the other processes in the 

LCA, material extraction imposes minimal energy requirements. A push for recycling old aggregates rather than 

extracting virgin materials has emphasized the downstream 

process of recycling. RAP is being widely implemented in 

pavement projects. A need to quantify energy required for 

milling RAP and processing is necessary. 

Manufacturing asphalt materials 

Several studies confirm that the most energy-intensive 

process is the production of asphalt pavement materials, 

especially mixing and drying aggregates and producing 

HMA (1, 3, 10). Estimates suggest that nearly 50 percent of 

total production energy is required for mixing and drying 

aggregates; 40 percent of total production energy is required to produce bitumen (10). 

Changes in aggregate storage and drying processes can 

substantially reduce energy consumption in asphalt pavement production. Producing low-temperature asphalts can 

vastly reduce the amount of energy required for these processes. Producing WMAs can reduce manufacture energy by 

as much as 15 percent, while HWMAs may lead to energy 

reductions on the order of 50 percent (11). Other studies 

estimate fuel savings of 11 percent to 35 percent for WMAs 

and up to 50 percent for low-energy asphalts such as CMAs 

or HWMAs (12). Energy conservation may be further increased with the use of foamed bitumen because aggregates 

need not be heated and can be mixed with the foam while 

cold or damp (13). 

Figure 3. Asphalt production is the most energy-intensive process in the pavement life 

cycle. As a consequence, changes in production methods have the potential for the 

greatest energy savings. (Image courtesy of 

Rock Road Companies) 

Another issue related to material manufacturing processes is plant efficiency. Some studies suggest targeting 

operating inefficiencies as a means of reducing energy 

consumption. Plant energy usage is controlled by several 

factors, including: 

 Ambient temperature 

 Aggregate moisture content 

 Plant combustion efficiency 

Regulating these variables may ensure that fuel use is optimized. Monitoring plant efficiencies can be achieved 

through careful placement of monitoring devices and technologies and regular maintenance of critical production 

equipment. A comprehensive list of operational measures 

and energy-saving guidelines is also available (9). 

Constructing pavements 

Pavement construction is a vital process in the analysis 

because it is the process resulting in direct user benefit and 

impact. The energy consumption of this process is minimal—about 1 percent of the total energy use of the cumulative traffic over the pavement lifetime (1). Modest 

reductions in vehicle energy consumption could easily 

offset the energy consumption in pavement construction 

processes. 

Foamed asphalt is also considered beneficial because it 

may serve as a means of achieving base stabilization without significant energy use (13). Using low-temperature 

asphalts can also improve construction under suboptimal 

weather conditions, thereby prolonging the paving season 

(12). Compaction efforts may also be improved with improved workability. It is believed that using bitumen emulsions may be less expensive by roughly 15 percent 

compared to traditional HMA methods (14). 

Maintaining pavements 

Pavement maintenance is critical to preserving the integrity 

of surface layers while prolonging the service life. Maintenance is often considered the least energy-intensive process 

because it requires minimal improvements to the pavement 

structure and focuses on preserving the surface course. One 
5

low-energy maintenance method worth investigating is the 

use of CMA for patching and emulsion-based slurry seals. 

Advantages of these methods include a reduction in production energy and storage life. Using emulsion-based 

slurry seals with asphalts modified using recycled rubber is 

already widely used in parts of the United States. These 

maintenance methods deserve more research to quantify the 

savings in energy and conservation of new resources. 

Removing and recycling pavements 

The final process is pavement removal, recycling and disposal. Nearly 80 percent of all pavements are recycled, 

making asphalt pavement North America’s most recycled 

construction product (15). One New Zealand study attempts 

to identify the primary reasons for the failure of the New 

Zealand road industry to adopt minimization strategies (6). 

The report concludes that waste minimization strategies, 

including recycling, are not widely employed because of a 

lack of experience on the part of the industry and little 

confidence in the use and performance of recycling technologies. 

Figure 4. Asphalt is the most recycled construction product in the United States. The 

energy, emissions and environmental impact 

of recycling must be quantified. (Image 

courtesy of Asphalt Pavement Alliance) 

In-situ recycling is widely regarded as the most energy-efficient recycling process. One study concludes that 

cold in-situ recycling with foamed bitumen consumes less 

energy compared with asphalt overlays or reconstruction 

(16). The data in the study suggest that cold in-situ recycling consumes 15 percent to 35 percent less energy than 

overlay projects, and 60 percent to 70 percent less energy 

than reconstruction projects. Cold in-situ recycling also 

allows for faster construction cycles than does reconstruction. 

Full depth recycling (FDR) using foamed bitumen is 

also gaining popularity worldwide. The advantages of using 

FDR include: 

 Lower life cycle costs 

 Faster construction 

 Reuse of aggregates (17) 

Roads being considered for FDR often have multiple 

overlays, so the recycled material consists of cracked asphalt pavement. Recycling also offers countries lacking 

reliable pavement management systems the best alternative 

for structural rehabilitation (16). 

Emissions 

In addition to energy consumption, greenhouse gas (GHG) 

emissions are a critical issue for developing sustainable 

pavements. One Australian study concludes that ozone 

depletion is not a problem with emissions from bituminous 

binders (18). Other results from this study indicate heating 

of cutback and HMA releases GHG emissions because of 

the large amount of fossil fuels required and the evaporation of kerosene from cutback chip seals. Hydrocarbon 

evaporation may contribute to smog in urban areas, but is 

not a significant problem in rural areas. Contributions to air 

pollution from the volatilization of bituminous binder are 

small compared with emissions from energy production, 

transport and industrial processes. 

Utilizing low-temperature asphalt technologies is 

widely regarded as a means to reduce emissions. As with 

energy consumption, the main process responsible for GHG 

emissions is the manufacture of pavement materials. An 

FHWA study of European practices concludes that expected reductions from using WMAs are: 

 30 percent to 40 percent for carbon dioxide and sulfur dioxide 

 50 percent for volatile organic compounds (VOCs) 

 10 percent to 30 percent for carbon monoxide 

 60 percent to 70 percent for nitrous oxides 

 20 percent to 25 percent for dust 

Technologies that result in greater temperature reductions are expected to have greater emissions reductions 

(12). One estimate of a low-energy asphalt process results 

in a reduction of carbon dioxide emissions of about 9 kilograms per ton of aggregate. In the United States, this would 

translate into a reduction of about 5 million tons of carbon 

dioxide annually (11). 

 Another study finds that transporting emulsions results 

in greater fuel usage and related GHG emissions, but that 

this is insignificant compared to production and heating 

emissions (19). Furthermore, the study indicates that replacing cutbacks with emulsions would reduce the amount 

of carbon dioxide produced by almost two-thirds while 

reducing the production of photochemical, smog-generating 

VOC emissions. 

Environmental Impact 

Overall environmental impact can be considered in lieu of 

energy consumption and GHG emissions. Considerations 

such as water quality degradation, eco-toxicity and occupational health are important environmental factors. 

In conducting an eco-efficiency life cycle assessment 

of chip seal alternatives, Wall compared the impacts of 

three chip seals (5). The analysis found that the cold chip 

seal alternative was advantageous in terms of risk potential, 

resource and energy consumption, health effect potential 

and land use compared to hot chip seal alternatives. These 
6

conclusions were based on the fact that the asphalt is applied at lower temperatures and uses fewer resources, energy and land because less asphalt is used in the chip seals. 

Additionally, the aggregate does not need to be precoated 

with asphalt. It was also found that not all recycling leads 

to reduced impacts: Although a ground tire rubber alternative diverted tires from landfills, this advantage was negated by higher emissions due to higher asphalt content and 

precoating of the aggregate as well as disadvantages in 

other environmental categories. 

Emulsions are regarded by some as a safer alternative 

to HMA and cutback bitumen. In an Austroads study, researchers found that the benefits of using emulsions include 

worker health and safety compared to HMA or cutback 

because of lower handling temperatures and nonflammability (18). The study investigated evaporation of 

cutters, bitumen fume emissions, binder heating fuel use, 

binder transport use and general transport fuel use related 

to the road network. Researchers concluded that emulsions 

have a lower impact on the environment in terms of pollution and toxicity relative to cutbacks, especially in urban 

areas. 

It was noted, however, that great care must be taken to 

avoid spilling emulsions into waterways. A New Zealand 

study concurs that emulsions are preferred as the environmentally beneficial method of sealing roads compared to 

cutback bitumen (19). A French study also considered 

emulsions the safest alternative due in part to reduced hydrocarbon volatility and heating temperatures (1). 

Another study considered emulsions in terms of ecotoxicity (20). Findings from the study suggest that emulsions may be considered “slightly harmful” to the aquatic 

environment, while cutbacks may be considered “harmful.” 

The study also found that the emulsifying agent was the 

only component of the emulsion that contributed to ecotoxicity. The report concluded that the major environmental 

danger of emulsions results from spillage and runoff, so 

great care must be taken to avoid overspray and tanker 

accidents. Slaughter’s study confirmed that the water solubility of emulsions posed a greater risk to water contamination than cutbacks, and that cutbacks had not been reported 

to cause water pollution (19). 

NEXT STEPS FOR RESEARCH 

As the asphalt pavement industry continues to make strides 

in sustainable development, growing emphasis must be 

placed on energy consumption, emissions and environmental impact. A green road should be designed not only 

for long service life but for minimal energy consumption 

and environmental impact. 

The authors, working for the University of Wisconsin 

Modified Asphalt Research Center (www.uwmarc.org), are 

mining published literature to raise awareness about opportunities in sustainable asphalt pavement development. 

Some of the promising ideas to move forward are: 

Figure 5. MARC recommends next steps for 

achieving tomorrow’s green asphalts. 

 Refine and/or develop estimation tools to evaluate 

where the industry currently stands in key sustainability 

indicators. This base line could allow contractors and road 

agencies to recognize the potential cost and resource savings in using sustainable methods. 

 Develop an initial certification program for asphalt 

road construction projects similar to LEED building certification. A point system could encourage industry to get 

involved and seriously consider alternative materials and 

methods for production and construction. 

 Advance cold asphalt application specifications and 

test methods. While hot binder technologies have seen 

significant advances due to the Strategic Highway Research 

Program, emulsions and cold mixture technology have 

lagged behind. The science of asphalt emulsification should 

be the focus of significant research and development efforts. 

 Quantify the effect of temperature on emissions and 

developing models that take into account asphalt chemistry, 

temperature, pressure and other climatic factors. 

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