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Sustainability Opportunities with Pavements
Sustainability is essentially the capacity to endure. There are numerous definitions provided in the dictionary, including “to give support,” “prolong,” and “withstand” (Merriam Webster 2010). Today the term is applied very broadly to almost every facet of life, although it is increasingly being used in the context of human sustainability on Earth - particularly so, as the causes of global warming and climate change are debated. In 1987, the World Commission on Environment and Development defined sustainable development as “meet[ing] the needs of the present without compromising the ability of future generations to meet their own needs” (WCED 1987), while an ancient Native American proverb captures the concept of sustainability in the following way: “We do not inherit the earth from our ancestors; we borrow it from our children”.
In recent years it has been suggested that in order for human activity to meet present needs without compromising the prospects of future generations, we need to carefully balance economic, environmental, and societal demands. This concept is often referred to as the “three pillars of sustainability” or the “triple-bottom-line” (Figure 1).
For the roadway engineer, consideration of sustainability or sustainable practices often leads to a focus on the structural design, the pavement materials, or the construction operation because these are items that are largely within the roadway engineer’s control. Practices such as recycling, use of industrial by-products (fly ash, slag cement, etc.), resource conservation, CO2 footprint and even embodied energy tend to get a fair amount of attention in this context. There may, however, be significant sustainability opportunities missed by ignoring the operational or use-phase of a pavement’s life.
Recent research suggests that the long- term cumulative benefits derived from a sustainable pavement design can be staggering. Specifying a rigid pavement in order to realize improved vehicle fuel efficiency can have enormous benefits over the life of the pavement. Likewise, specifying and maintaining a smooth pavement can result in similar cumulative benefits. The energy savings realized during the entire operational phase of an urban pavement’s life associated with reduced lighting needs for a high-albedo pavement can also be significant. Moreover, the long-term energy and CO2 savings associated with mitigating the urban heat island effect through the use of high-albedo, cool pavements are quite large.
Therefore, the central question for roadway engineers and administrators is: In the context of sustainable practices, “are we properly accounting for the long term operational sustainability benefits when we make pavement selection and design decisions?”
In order to answer this important question, it is useful to first address the following:
- What is sustainability in the context of highway and roadway pavements?
- What are we currently doing in the sustainability arena?
- What should or could we be doing?
Sustainability in the Context of Pavements
How does a roadway engineer balance the natural environment, societal needs and economic vitality when building pavements? In other words, how do we minimize environmental impacts while improving user experience and maintaining or even reducing costs? Even though much has been written about sustainable practices in the roadway sector over the last decade, significant confusion remains about how to tackle this challenge. There have been significant advancements in the use of recycled mate- rials and industrial by-products in pavements, as well as efforts to improve the efficiency of construction machines and processes. There has also been much focus on ways to reduce the energy footprint of building materials by optimizing mixtures and using less energy intensive cementitious systems.
ACPA’s, “Green Highways: Environmentally and Economically Sustainable Concrete Pavements,” Special Report lists many of these sustainability opportunities with concrete pavements (ACPA 2007). It emphasizes longevity as the primary sustainability opportunity. A long-lasting concrete pavement does not require rehabilitation or reconstruction as often, thereby consuming fewer raw materials in the long run. This longevity impacts our environment in other ways as well. Energy savings are realized because rehabilitation and reconstruction efforts consume energy. Also, congestion is reduced (with accompanying energy savings and a reduction in vehicle pollutants) because there are fewer construction zones impeding traffic flow with long- lasting concrete pavements.
Beyond longevity, the Special Report also lists a number of other features of concrete pavements that further enhance their sustainability:
- The rigidity of concrete pavement means lower vehicle fuel consumption and emissions.
- Less fuel-intensive construction operations for concrete pavement as compared to traditional hot-mix asphalt pavement result in enormous economic and CO2 savings.
- Using industrial by-products (e.g., fly ash and slag cement) in concrete paving mixtures improves pavement longevity, reduces the structure’s CO2 footprint, lowers cost, conserves natural resources and reduces disposal needs.
- Concrete pavement’s renewability and 100% recyclability lead to improved longevity and reduced demand on non- renewable resources.
- The strength and rigidity of concrete means fewer and thinner required layers of subbase materials, and reduces the need for material resources.
- The light-colored and cool surface of concrete pavement leads to improved night-time visibility, reduced lighting requirements, and reduced heat island effects.
- Optimized surface textures can be imparted on new and existing concrete pavements that result in long-lasting, smooth and quiet pavements.
To make sense of all of these opportunities and to better characterize their relative impacts requires the concept of “cradle-to-grave” analysis.
Carrying an analysis from cradle-to-grave is the central idea of an environmental life cycle assessment (LCA). LCA has its origin in finding ways to cumulatively account for resource use and availability, which stemmed from concerns about the limitations of raw materials and energy resources in the 1960’s. LCA has been through a significant evolution since then, and in the late 1990’s it was standardized by the International Organization for Standardization (ISO), partly out of concerns about the lack of uniform methods for conducting the assessments, as well as the inappropriate use of the methods to make broad marketing claims by manufacturers (EPA 2006). Established in 1947 and headquartered in Geneva, Switzerland, ISO is an international standard-setting body composed of representatives from national standards organizations representing 163 of the world’s nations. These ISO guidelines are provided in the ISO 14040 series of standards.
The purpose of such an LCA approach is to ensure that all the effects, such as loads, raw materials factors, construction-related variables, etc. are accounted for in the analysis, from the moment any component of the subject of the study (pavement in this case) is extracted or processed, all the way to its end of life. It essentially involves a cumulative analysis of a product’s environmental impact throughout all stages of the product’s life cycle, including impacts not usually considered in more traditional analyses.
The use of the term “life cycle” is an acknowledgement that all the major activities in the course of the product’s life- span must be considered, from its manufacture (including the raw material acquisition required to manufacture the product), use, and maintenance, to its final disposal (EPA 2006). Figure 2 illustrates the concept of a LCA, including inputs, outputs and the system boundary.
LCA allows a roadway engineer to focus on any number of variables – including energy, emissions, raw materials, global warming potential, water use, and hazardous materials. Even though most current LCA approaches do not consider variables such as traffic impact during the use phase (given that vehicle movement is not considered part of the road), the “cradle-to-grave” LCA approach clearly captures these kinds of impacts.
For example, to evaluate the CO2 impacts of a concrete roadway, a LCA would include impacts of such typically- considered activities as extraction of aggregates at a quarry, the processing of raw materials at a cement plant, the transport of these materials to the batch plant, the construction activities, the maintenance and upkeep of the concrete pavement, as well as salvaging the pavement at the end of its life (i.e., the impacts associated with materials production, construction, maintenance, and end-of-life). However, it would also include the CO2 impact from the use-stage, or operation-phase of the pavement, including factors such as rolling resistance (vehicle fuel consumption rates) and albedo (as it relates to urban heat island, lighting and global cooling) (Santero et al. 2010).
In order to properly assess sustainable practices in the pavement arena, and capture the long-term operational sustainability benefits, the techniques and tools provided by a proper “cradle-to-grave” LCA are necessary. Only through such a comprehensive assessment will highway administrators and decision-makers be able to properly consider and account for the cumulative impacts of their decisions. In other words, LCA will enable decision-makers to ascertain what environmental sustainability really means in the context of roadway pavements. However, it is important to acknowledge that LCA will not necessarily determine which pavement is most effective or works best – the information developed in a LCA will be used as one component of a more comprehensive decision process that also includes tradeoffs with performance and cost (EPA 2006).
Current U.S. Sustainability Initiatives
Given the increased interest and focus on sustainability and “green” approaches, what are roadway agencies and pavement engineers currently doing to meet the challenge? By far, the items that have received the most attention revolve around the material production and construction phases, while very little emphasis has been placed on the use phase (Arkedani and Sumitsawan 2010). In fact, a recent comprehensive review of existing pavement LCA models suggests that although the models are progressing towards a state of comprehensiveness and quality that should eventually facilitate utilization on a wider scale, the scopes of current models are bounded by the more commonly- analyzed phases of the life cycle (materials production, construction, maintenance, and end-of-life), with little attention paid thus far to important contributors such as the use phase and traffic delay (Santero et al. 2010).
Within the material production and construction phase of the life cycle, much of the emphasis by U.S. highway agencies has been directed toward increasing the use of recycled materials in our pavements (both recycled concrete aggregates (RCA) and recycled asphalt pavement (RAP)), as well as increasing the use of industrial by-products such as fly ash and slag cement in concrete. In addition, there is a significant interest in using permeable pavements to address stormwater quality issues, and using warm-mix asphalt to reduce energy use and emissions during asphalt production.
A variety of programs have emerged in recent years that focus on improving the sustainability of our highway and street pavements. These programs include environmental LCA programs, sustainability rating systems and direct implementation efforts of what is regarded as sustainable practices. Likely because of the complexity and unfamiliarity with performing a complete LCA, most software applications and ratings systems focus exclusively on the pavement construction period (De Larrard 2009). A recently published critical review of research and literature in the LCA arena provides a good summary of the various systems (Santero et al. 2010). The most prominent and commonly referred to programs in the U.S. today include the Green Highways Partnership (GHP), the GreenroadsTM system and, most recently, the Federal Highway Administration’s (FHWA’s) Green Pavement Technologies Program.
Green Highways Partnership – The GHP was launched as a pilot program by the U.S. Environmental Protection Agency (EPA) in 2005 as a diverse, public-private partnership, bringing together ideas and strategies for greening high- ways to help enable a sustainable infrastructure. Green highways are not defined by a list of specific requirements – instead, they are defined as an effort to leave the project area “better than before”. Through partnering and collaboration, the GHP is essentially a vehicle to discuss the many characteristics of a green highway that will in fact differ from project to project, and location to location. Figure 3 highlights some of the technologies that the GHP promotes as sustainable and green. As is evident from this figure, the scope is larger than just the pavement surface, and includes the surrounding ecosystems and environment.
As part of the collaborative GHP effort, a series of Partnership Teams were established around the following three themes: Watershed Based Stormwater Management, Industrial By-product Recycle & Reuse, and Conservation & Ecosystem Protection. The primary focus of the Industrial By-product Recycle & Reuse Team is on promoting environmentally sound and technically acceptable uses of industrial materials and practices that conserve non-renewable resources, reduce impacts to landfills, reduce greenhouse gas (GHG) emissions and save energy (GHP 2008). The GHP also provides exposure to a select number of pilot projects that demonstrate the program goals of increasing the visibility of creative solutions and inspiring others to pursue green choices. However, the primary emphasis of the GHP to date has been their recognition program, where they acknowledge people, projects, and activities that demonstrate excellence in pursuing the Green Highways goals. The award categories revolve around sustainable planning, design, construction, maintenance, and materials recycling.
In summary, the GHP is not intended to be a LCA tool, nor is it meant to be a comprehensive approach to sustain- ability measurement. It is primarily a strategy to encourage “green” practices. In the case of the GHP, these green practices all relate to the materials production and construction phases of the highway life cycle.
GreenroadsTM Rating System – Most notable among the emerging rating systems in the U.S. is the GreenroadsTM rating system. According to their website, (www.green-roads.us) a Greenroad is defined as roadway project that has been designed and constructed to a level of sustain- ability that is substantially higher than current common practice. It is intended to be a sustainability performance metric for roadways that awards points (or credits) for certain sustainable practices. The rating system awards these credits in seven different categories, each with different amounts of available credits, for a total of 129 credits achievable (11 of these are required, leaving 118 voluntary credits possible). Based on the amount of voluntary credits earned, the project can become Greenroads certified at one of four levels (Certified, Silver, Gold or Evergreen).
The developers of GreenroadsTM released version 1.0 of the rating system in January 2010, so as of yet, there is very little experience with the program, but it currently is undergoing a period of peer-review as well as public commenting. It is expected that the rating system will evolve and improve as stakeholders are given the opportunity to review and provide input.
As noted in recent literature, the GreenroadsTM rating system is designed simply to “score” the environmental (and to a degree the societal) impact of a particular road project, but it is not intended to be a comprehensive LCA tool (Santero et al. 2010). The main emphasis of the rating system is on impacts associated with materials production and construction; figure 4 shows the distribution of the voluntary credits.
Of the 129 total credits attainable, only 2 credits are awarded for the completion of a detailed LCA for the entire project. There are credits possible for cool pavements and long-life pavements, but their weights do not appear to be commensurate with their level of importance from a sustainability perspective. The use of cool pavements is worth the same amount of credits (5) as the incorporation of recycled materials in the pavements. Likewise, the use of long-life pavements is worth the same amount of credits (5) as the use of regional materials sources for construction. As mentioned, it is expected that this rating system will be modified once the developers have the benefit of important input from the various stakeholders.
Green Pavement Technologies Program – In January 2010, the FHWA announced a new initiative entitled Every Day Counts (EDC). The goal of the EDC initiative is to reduce highway project delivery times, accelerate the national deployment of innovative technologies, and reduce the Agency’s environmental footprint internally. The EDC initiative is a major priority for the FHWA, and significant resources are being dedicated to ensure that the goals are met under this program. In the area of accelerating the deployment of innovative technologies, Green Pavement Technologies was early identified as one of the focus areas. A technology deployment team was quickly established and a number of green pavement technologies identified, including:
- Recycled Concrete Aggregates (RCA) - reduces energy use when considering mining, processing and transportation, and decreases construction waste.
- Warm Mix Asphalt - allows for a reduction in the asphalt mixture production and placement temperatures, and allows for corresponding reductions in energy use and emissions/odors.
- High Reclaimed Asphalt Pavement (RAP) - reduces the demand for non-renewable resources and decreases construction waste.
- Two-lift Concrete Pavements - allows for the use of marginal materials, including RCA, in the lower (bulk) layer of a concrete pavement structure while providing a highly durable surface layer with optimized aggregate properties.
- Concrete Mixtures with a Reduced Clinker Factor - lower energy and CO2 footprint per cubic yard of concrete when compared to conventional concrete mixtures.
Although FHWA ultimately selected only one of these green pavement technologies (warm mix asphalt) under the EDC initiative, all of the technologies considered involve improving the sustainability footprint of either the materials production or the construction phase of a pavement’s life cycle.
Clearly, the state of practice within the U.S. highway sector is focused primarily on strategies to improve the sustain- ability profile of materials production operations and construction operations; little is being done to capture the long-term operational sustainability benefits during the pavement’s life cycle.
What We Should be Focusing on
Every one of the commonly adopted sustainability strategies mentioned in the previous sections are important because significant benefits can be derived by embracing each of these practices. However, it is important to know which strategies are most impactful, so efforts can be focused on those practices. This will aid agencies in making the most informed decisions about the sustainability impacts of the various pavement infrastructure choices available to them. Luckily, LCA is aiding in this process.
A comprehensive LCA study undertaken by the Center for Energy and Processes at the École Nationale Supérieure des Mines de Paris (also known as Mines ParisTech) examined the impact of six different pavement structures in reference to twelve different environmental factors (including GHG’s, energy, ecotoxicity, smog, odor, solid waste, etc). The analysis concluded that the overall impact from the use-phase dwarfs impacts from all other phases of the pavement’s life cycle. In fact, with the sole exception of the solid waste factor, the impact of the use-phase (traffic in this case) was at least ten-times greater than all other phases (Rens 2009). An EAPA/Eurobitume joint report published in 2004 refers to this same study and asserts that from the energy use perspective, the construction and maintenance phase represent only about 2% of the total energy consumption during the entire life cycle of the pavement – the balance being represented by the use- phase (EAPA/Eurobitume 2004).
Figure 5 illustrates how the different life-cycle phases of a typical road contribute to the total footprint across the twelve different environmental factors. In this case, truck traffic and car traffic dominate the impacts. A 2% or 3% improvement in the truck traffic and car traffic portions of the roadway’s overall sustainability footprint (or ecoprofile) would essentially offset the entire construction and maintenance ecoprofile.
This follows a similar pattern to that for buildings. Work completed by the Athena Institute for the International Green Building Challenge demonstrates that over the lifetime of a building, the energy footprint associated with the operation of the building (use-phase) represents between 87% and 97% of the total energy footprint over its lifetime (CAC 2003). Therefore, focusing attention on ways to reduce this portion of the footprint makes sense from a holistic perspective. Because pavements remain in service for decades, lying exposed every hour of every day, and typically support millions of vehicles during that time, it becomes quite clear that use-phase impacts are likely to be the dominant factor when assessing sustainability (Santero et al. 2010, Arkedani and Sumitsawan 2010, Rens 2009, and Taylor and Van Dam 2010).
The most prominent use-phase impact is likely to come from either vehicle fuel consumption rates (related to pavement rigidity and smoothness), or pavement albedo (as it relates to urban heat island, lighting and global cooling).
Vehicle Fuel Consumption – Several comprehensive LCA studies indicate that the impact of traffic can have a drastic influence on a roadway’s overall sustainability footprint (or ecoprofile). Mostly this has to do with the fuel that vehicles consume as they travel the roadway. Therefore, any strategies that can positively impact fuel consumption will be of great importance (Rens 2009). Even small changes in the traffic’s fuel consumption would likely result in hundreds of thousands of gallons (liters) of marginal fuel consumption over the pavement’s service life (Santero et al. 2010). Figure 6 illustrates the marginal fuel consumption associated with just 1.0% improvement in fuel economy for a roadway with a traffic level of 20,000 vehicles per day. The impact this improvement would have over a 30-year design life is 93,500 gallons per mile (220,000 liters per kilometer). This value is quite staggering, considering that there are 459,000 miles (738,000 kilometers) of Interstates, tollways and arterials on the U.S. network alone (FHWA 2008). Assuming that this level of traffic is representative of the average travel on this network, the total volume of fuel saved would be over 43 billion gallons (162 billion liters), or roughly 1.4 billions gallons (5.4 billion liters) per year, for just 1.0% improvement in fuel economy. Can this level of fuel economy savings be realized via pavement type alone? Recent research suggests that it can.
Since 1989, several important studies have examined the link between vehicle fuel consumption rate and pavements. Zaniewski reported that at every test speed above 19 mi/h (30km/h), trucks used less fuel on concrete pavements as compared to asphalt pavements (Zaniewski 1989). The reported fuel consumption savings were approximately 20%, although these findings were not statistically significant. The hypothesis presented is that because trucks cause more deflection on flexible pavements than on rigid pavements, more of the energy intended for propelling the truck is “absorbed” causing that deflection.
Since then, several more statistically rigorous studies have been undertaken. In 2006, the National Research Council Canada (NRC) released a report describing a comprehensive, multi-phase study on the effects of pavement structure on vehicle fuel consumption. The study concluded that tractor-trailers traveling on concrete pavements have statistically significant lower fuel consumption than those traveling on asphalt pavements throughout the summer to winter temperature range for fully loaded trucks operating on smooth pavements [International Ride Index (IRI) < 120 in./mi (1,900 mm/km)] (Figure 7) (Taylor and Patten 2006). Fuel consumption for two common truck types—tractor tanker semi-trailer and tractor van semi-trailer—were an average of between 1% to 6% lower on concrete versus asphalt pavement, depending on truck type and vehicle speed.
A Swedish study published in 2008 investigated fuel consumption of passenger vehicles on different pavement types at highway speeds 55 mi/h (90 km/h). The study indicated a statistically significant improvement in fuel consumption of 1.1% (at a 95% confidence level) on the concrete pavement tested versus the equivalent asphalt sections (Jonsson 2008). The hypothesis given as to the reason for this difference related primarily to the greater rigidity of concrete that consequently does not deflect as much under traffic loads.
A research report submitted by The University of Texas at Arlington to the Ready Mixed Concrete (RMC) Research & Education Foundation in March 2010 examined the effect of pavement type on fuel consumption for city driving (roughly 30 mi/h [50 km/h]). The study aimed to identify any statistically significant differences that may exist in fuel consumption rates on typical concrete and asphalt city streets (in this case for a passenger van). It was found that the fuel consumption rates were consistently lower on the concrete sections regardless of the test section, driving mode (acceleration vs. constant speed), and surface condition (dry vs. wet). In all cases, the differences were found to be statistically significant (at a 90% confidence level). On average, the fuel consumption rates were between 3.2% and 4.7% lower on the concrete city streets. The study did not include any theoretical assessment of tire-pavement interaction or other mechanical reasons as to why these differences in fuel consumption rates exist (Arkedani and Sumitsawan 2010).
Work recently conducted at the Michigan State University also suggests fuel consumption advantages on concrete pavements. Univariate analyses (with IRI as covariate and pavement type as a fixed factor) were conducted for five different classes of vehicles traveling at three different speeds during winter and summer conditions. The analysis suggests that trucks driven at lower speeds (35 mi/h [56 km/h]) during summer conditions on flexible pavements consume more fuel than when driven on rigid pavements. In this study however, the effect of pavement type was only pronounced during summer conditions and for trucks at lower speeds (Chatti 2010).
It is important to acknowledge that it is not only pavement rigidity that can contribute positively on vehicle fuel consumption rates – pavement smoothness is also a factor. The smoother a pavement is, the less fuel that will be required to propel vehicles along the roadway. Any roughness along the way will translate into vertical motion and consequently heat in vehicle suspension systems, leaving less energy available for forward motion. This concept is very similar to the hypotheses associated with rigid versus flexible pavements. Any energy that is “bled- off” to do such things as deflect the pavement, or excite the suspension system, will not be available to propel the vehicle forward. Hence, more energy is required to propel the vehicle, and fuel economy suffers.
In this context, roughness refers to longer wavelength textures, typically termed mega texture (2-20 in. [50-500 mm] wavelengths). Smaller wavelength textures (microtexture and macrotexture) are primarily important for skid resistance (FHWA 2005). A number of studies published since 1990 suggest that there are significant fuel consumption efficiency gains associated with pavement smoothness gains (Santero et al. 2010). A FHWA report published in 2000 suggests that a reduction in IRI from 150 in./mi (2.4 m/ km) to 75 in./mi (1.2 m/km) (i.e., improvement in smoothness) on asphalt pavement results in an accompanying 4.5% improvement in truck fuel economy (FHWA 2000). The tests were conducted on the FHWA’s WesTrack facility in Nevada. Work currently finishing up under the National Cooperative Highways Research Program’s (NCHRP’s) project 1-45 is focusing specifically on how pavement conditions (including smoothness) affect vehicle operating costs. Results of this work are expected to reinforce findings from earlier studies.
These benefits of smooth pavements from a sustainability perspective are relevant not only for new pavements (i.e., specifying smooth pavements), but also when deciding on maintenance strategies and schedules. Diamond grinding and other concrete pavement restoration activities are particularly useful techniques used to restore pavements and improve ride quality, noise, and surface texture. Diamond grinding involves removing a thin layer of a concrete pavement’s surface using closely spaced diamond saw blades; the process is analogous to refinishing a hardwood floor with a drum sander. Figure 8 shows concrete pavement diamond grinding in progress on an interstate pavement in Colorado. Note how the pavement albedo (light reflectivity) in this case is improved as well.
Transportation agencies typically combine diamond grind- ing with at least one other restoration procedure when sig- nificant structural distresses are present. Diamond grinding enhances ride or smoothness, while the other procedures address structural problems.
Based on a California Department of Transportation study of 76 test sections nationwide (including pavements in freeze-thaw zones), the average longevity of a diamond- ground project is about 14 years. In California, this value was determined to be closer to 17 years (Stubstad et al. 2005). This is important because it is possible to diamond grind candidate concrete pavements up to three times before major reconstruction is needed. This could extend the service life of a concrete pavement to twice its normal design life by adding only these two or three diamond grinding activities and continuing normal routine maintenance. The fuel economy advantages associated with the improved and prolonged smoothness are clearly quite significant. In addition, this enhanced smoothness and longevity is accomplished without extracting or processing additional raw materials such as aggregates or binders.
In the context of overall transportation sustainability, the potential fuel savings and GHG reductions associated with selecting and maintaining rigid and smooth pavements can prove to dwarf the sustainability benefits from all other phases of the pavement life-cycle.
As an illustration of the enormity of the possible CO2 savings using rigid pavement alone, ACPA’s, “Green Highways: Environmentally and Economically Sustainable Concrete Pavements,” Special Report presents an example where fuel economy has been positively impacted via the use of rigid pavements versus flexible pavements, using the data from the NRC study. The cumulative CO2 savings realized from a 3.8% improvement in truck fuel economy associated with rigid versus flexible pavement along a 62 mile (100 km) long major arterial highway with 20,000 vehicles per day, 15% trucks and a 30-year design life, is more than three times greater than the CO2 emitted during the manufacture of cement used for the construction of the concrete pavement (ACPA 2007).
Put another way, for this particular example, all of the CO2 emitted during the manufacture of cement used to construct a concrete highway pavement is compensated for during the first nine years of service by virtue of the reduced pavement deflection and improved truck fuel efficiency. The saving realized during the remaining 22 years would essentially be a credit when compared to the CO2 footprint associated with a flexible pavement facility.
With additional improvements in fuel economy as a result of incrementally improved smoothness during the pavements service life through judicious concrete pavement restoration activities, the possible fuel savings and resulting CO2 reductions are even greater.
Pavement Albedo – The other critical operational-phase impact that should be considered in a sustainability assessment relates to concrete pavement’s capacity to reflect light. This characteristic of pavement, generally referred to as albedo or solar reflectance, is a function of both type and age of the material. Albedo is a measure of how much solar radiation is reflected from a surface. It is expressed as the ratio of incoming light to reflected light, where a perfect reflector is 1 and a perfect absorber is 0. Typical albedo values reported for concrete and asphalt pavements range from 0.25 to 0.45 and 0.05 to 0.20, respectively. The higher albedo that concrete pavement can provide is advantageous for multiple reasons. High albedo pavements can:
- Significantly reduce the amount of energy needed for artificial roadway illumination during night-time,
- Reduce the amount of energy needed to cool urban environments associated with the urban heat island effect, and
- Mitigate the greenhouse effect and contribute to global cooling by reducing the amount of solar radiation absorbed by the earth’s surface.
Because pavements remain in service for decades, lying exposed every hour of every day, the cumulative impact of these factors over the pavement service life are enormous.
Pavement Illumination – Lighting fixtures are important elements of most urban highway facilities. Enhanced night-time visibility is intuitively related to improved traffic safety. In addition, because of the relatively high albedo of concrete pavements, a specified luminance level can be achieved with lower-wattage and/or fewer lighting fixtures than would otherwise be needed. Ultimately, this translates to lower energy consumption and cost over time.
Several highway agencies in the U.S. classify different pavement types by their ability to reflect visible light. The Minnesota Department of Transportation (Mn/DOT) uses the general guidelines and classification scheme published in the AASHTO standards. Table 1 (see next page) lists the minimum average maintained illuminance by facility classification and pavement type, as published in the 2003 Mn/DOT roadway lighting design manual (Mn/DOT 2003). As the table shows, the illumination demands are roughly 40% to 50% lower for concrete pavements than for asphalt pavements. The US Green Building Council’s Leadership in Energy and Environmental Design (LEED) Rating System acknowledges this as well, as points are awarded for reducing lighting power density, which can be accomplished by using high albedo pavement surfaces.
Recent LCA studies suggest an even more pronounced difference. One study reported a difference in electricity demand for lighting of 0.7 MWh (2.6GJ) over a 50 year life-cycle. This amount accounted for more than 10% of the entire energy footprint of the pavement (Santero et al. 2010). Another report comparing the environmental impacts of concrete pavements to asphalt pavements suggested cost savings of as much as 31% in initial energy and maintenance costs for lighting concrete pavements versus lighting asphalt pavements (Gajda and VanGeem 1997). This is particularly important for municipalities, where the utility costs associated with street illumination is often the 3rd largest budgetary expense item, behind schools and employee salaries. Clearly, there are opportunities to reduce the roadway energy footprint by using high-albedo pavements.
Urban Heat Island – Light colored concrete pavement presents another unique opportunity to reduce the sustainability footprint through cool pavements. Urban heat islands are characterized by urban surface and air temperatures that are higher than the surrounding suburban and rural areas. In many cities in North America, that temperature difference can be as high as 22ºF (12ºC). Heat islands are formed as natural land cover and vegetation is replaced with buildings, pavements and other infrastructure (EPA 2004). One of the major negative consequences of heat islands, apart from public health concerns related to peak temperatures and smog formation, is energy consumption related to increased cooling demand. This not only consumes energy and costs money, it also leads to higher emissions from power plants.
Table 1. Mn/DOT illumination demands by facility classification/pavement type (Mn/DOT 2003)
|Roadway and Walkway Classification||R1||R2 & R3||R4||Uniformity avg/min|
|Foot candles||Lux||Foot candles||Lux||Foot candles||Lux|
|Free ways||0.6 - 0.8||6 - 9||0.6 - 0.8||6 - 9||0.6 - 0.8||6 - 9||3:1 to 4:1|
|Pedestrian Way and Bicycle Lane||1.4||15||2.0||22||1.8||19||3:1|
|Rest Area||Enter/Exit Gores and Interior Roadways||--||--||0.6 - 0.8||6 - 9||--||--||3:1 to 4:1|
|Parking Areas||--||--||1.0||11||--||--||3:1 to 4:1|
Paving urban roadways with concrete can be one effective strategy to mitigate urban heat island effects. With their higher albedo, concrete pavements reflect significantly more sunlight than darker colored asphalt pavements, resulting in cooler pavements. As reported by the U.S. EPA, pavements with higher solar reflectance are cooler in the sun (EPA 2004). Research published by the Lawrence Berkeley National Laboratory (LBNL) suggests that, when exposed to sunlight, lighter-colored pavements typically have surface temperatures approximately 21°F (12°C) lower than darker-colored pavements (Pomerantz et al. 2000). Again, the LEED rating system recognizes this, and awards points directly for use of high albedo pavement surfaces.
Several jurisdictions in the U.S. have embraced cool pavements as part of their strategy to address the urban heat island effect. The City of Miami, Florida, enacted an ordinance in 2009 that requires new construction to use cool roofs, shade from trees, and cool pavements or pervious pavements for at least 50% of the site hardscape (Miami 2009). Cool pavements are paving materials with a solar reflectance of at least 0.30. The City of Chicago, Illinois, has created a sustainable streets pilot program. One of the focus areas of the program is urban heat island. To address this challenge, the City has proposed maximizing landscape opportunities and ensuring that streetscape surface area in roadways, sidewalks and plazas have a minimum solar reflective index of 0.29 (Attarian 2010). Much of their high-albedo cool concrete mixtures used slag cement to enhance the pavement reflectance.
In terms of overall impact of the heat island effect, one study estimates that the increase in temperature due to heat island effects accounts for between 5% and 10% of peak urban electric demand for air conditioning use (Akbari 2005). In fact, estimates in the United States suggest that about 20% of the national air conditioning demand can be avoided through large-scale implementation of heat-island mitigation measures, including cool pavements, cool roofs and planting greenscapes. In the U.S., this amounts to a savings of roughly 40 billion kWh/year (144 million GJ/ year), equivalent to more than $4B USD per year in savings from cooling costs alone (Akbari et al. 2001). Given the massive cumulative benefits associated with using cool pavements to mitigate the heat island effect, a proper assessment of the sustainability profile of a pavement must consider these impacts.
Global Cooling – In the same way they help mitigate heat island effects, high-albedo concrete pavements also contribute directly to global cooling. By reflecting more of the sun’s energy away from the surface and back out into space, they reduce the amount of solar radiation absorbed by the earth’s surface – in effect a change in the global energy balance is made.
A study presented at the 5th Annual California Climate Change Conference in Sacramento, California, details how changing the albedo of pavement surfaces can offset CO2 (Akbari et al. 2008). Because both increasing earth’s reflectivity (albedo) and decreasing earth’s atmospheric CO2 concentration result in global cooling, one can be expressed in terms of the other.
The study indicates that from a global warming perspective, an increase in albedo for pavement of 0.15 (say from 0.10 to 0.25) is equivalent to eliminating 70 lb (38 kg) of CO2 per square yard (meter) of pavement surface. When considered in the 100 largest cities on earth, the equivalent CO2 offset for increasing the albedo for all paved surfaces by 0.15 is over 20 gigatons. This clearly suggests that use of high albedo pavements has a significant sustainability impact and can even be a useful too in mitigating climate change. In fact, a proposal to the United Nations (UN) is being considered with this in mind. Nonetheless, global cooling potential of pavements is not considered in any current LCA studies.
Bringing Things Into Focus
More than ever, sustainability considerations must be included in the decision-making process for roadway administrators and engineers. To date, those people shaping the direction of sustainability consideration for pavement analysis and selection have focused solely on the material acquisition, production and construction phase of the pavement life cycle. Even though these factors are all important, and major benefits can be derived by embracing sustainable practices with each of these, there are significant sustainability opportunities that are missed by ignoring the benefits presented by considering the operational or use-phase of a pavement’s life – the things that contribute in a positive way to sustainability every hour of every day of the pavement’s lifetime.
This challenge is not much different than the challenge that has surrounded cost analysis. Historically, decisions about pavements were based solely on first cost. Over time, as it became evident that this approach was not economically sustainable, agencies started exploring the concept of life cycle cost analysis (LCCA) in order to properly account for the long-term costs. This has resulted in better, more efficient use of available resources. Today, the majority of public agencies in the U.S. use some form of LCCA in their decision-making about pavement selection, enabling them to make better economic decisions on pavements.
As roadway agencies continue to develop sustainable practices, they will develop comprehensive LCA approaches as a means to make better decisions. Tools that properly account for all the impacts, including those taking place during the long time-horizon associated with a pavements life cycle, will become more common.
The development of new tools and guidelines will provide agencies with much needed direction. Work is on-going through the sustainability track of the comprehensive and cooperative U.S. Concrete Pavement (CP) Road Map. A project sponsored by Environment Canada under the Asia Pacific Partnership (APP) program is endeavoring to better capture the life cycle through LCA model development for pavements, and the Sustainability Hub at Massachusetts Institute of Technology has recently begun to address the sustainability and environmental implications of concrete.
Ultimately, long-term operational benefits will be properly accounted for when making decisions about sustainable practices in the roadway sector. From what we know today, rigid, smooth and light-reflective pavement surfaces will be a major focus of sustainable roadway practices moving forward.
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The American Concrete Pavement Association (ACPA) is the premier national association representing concrete pavement contractors, cement companies, equipment and materials manufacturers and suppliers. We are organized to address common needs, solve other problems, and accomplish goals related to research, promotion, and advancing best practices for design and construction of concrete pavements. The opinions, findings, and conclusions expressed in this special report are based on the facts, tests, and authorities stated herein. This publication is intended for the use of professional personnel competent to evaluate the applicability and limitations of the reported findings and who will accept responsibility for the application of the material contained herein. The ACPA and its partners disclaim any liability for the application of the information contained in this document and cannot be held responsible for the accuracy of any of the sources other than work developed by the association.