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SR 385 Green Roadways: Environmentally and Economically Sustainable Concrete Pavement
- 1 Introduction
- 2 Longevity
- 3 Beyond Longevity
- 3.1 Reduced Fuel Consumption and Emissions
- 3.2 Lower Energy Footprint
- 3.3 Reduced Use of Natural Resources
- 3.4 Use of Industrial Byproducts
- 3.5 Pavement Renewal
- 3.6 Quiet Surface Textures
- 3.7 Improved Water Quality
- 3.8 Pavement Recycling
- 3.9 Light-Colored and Cool
- 4 Conclusions
- 5 References
The concepts of “sustainability” and “ sustainable development” are receiving considerable attention as the causes of global warming and climate change are debated. The World Commission on Environment and Development has 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).
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.”
A major focus of this concept involves ensuring that sustainable practices in pavements go hand in hand with economic success. This is indeed true of concrete pavements. Particularly because of its long life, concrete is an economical, costeffective pavement solution that consumes minimal materials, energy, and other resources for construction, maintenance, and rehabilitation activities over its lifetime. Beyond longevity, other features of concrete pavement further enhance its sustainability:
- Because of its rigidity, concrete pavement deflects less under vehicle loading, which results in reduced vehicle fuel consumption.
- The construction of concrete pavements consumes less fuel (particularly diesel) during materials production, transportation, and placement than the construction of asphalt pavements.
- Concrete pavements exhibit a lower energy footprint associated with their production, delivery, and maintenance than asphalt pavements do over a 50 year time period.
- Due to its inherent rigidity, concrete pavement requires less subbase aggregate materials for structural support than asphalt pavements.
- Concrete pavement mixtures incorporate industrial byproducts (i.e., fly ash and slag cement), which lowers the disposal needs, reduces the demand on virgin materials, and conserves natural resources.
Figure 1. An example of a green, sustainable concrete pavement; see ACPA’s QD025P-1, “Nall Avenue: A Green Street,” for details on the sustainability-related benefits of this project (photo courtesy of The City of Overland Park, Kansas).
- Concrete pavement can periodically be renewed through diamond grinding, prolonging its lifespan, enhancing its smoothness, and improving its skid resistance.
- New concrete pavement surface textures have recently been developed to produce quieter pavements over longer periods of time, reducing noise pollution.
- Concrete pavements designed with pervious concrete shoulders can minimize surface water discharge and help replenish groundwater aquifers.
- Concrete pavement is 100% recyclable, suitable for use in new concrete pavement, unstabilized subbase materials or controlled fill.
- Concrete pavements’ lighter color and increased reflectivity improve nighttime visibility, reduce the amount of power needed to illuminate roads at night, help mitigate urban heat island and can help offset global warming.
Although cement is a relatively energy intensive and carbon dioxide (CO2)intensive material to manufacture, cement manufacturing accounts for less than 1.5% of U.S. CO2 emissions; the balance of emissions comes from sources such as electricity production (40%), transportation (33%), residential heating (6%), and various other commercial and industrial processes (DOE 2006).
Furthermore, it is critical to recognize that cement is only one of the materials that make up the final product—concrete. Figure 2 illustrates how 92% of the volume of typical paving concrete is comprised of materials that require very little energy to obtain and have a low CO2 footprint, including sand, gravel, water, air, and industrial byproducts (including supplementary cementitious materials or SCMs).
In addition, the cement industry has taken dramatic steps to reduce any environmental impact of the manufacturing process, reducing the amount of energy to make a ton of product by more than 33% since 1972. The Cement Manufacturing Sustainability (CMS) Program continues this success with a national pledge to reduce CO2 emissions by an additional 10% per ton of product by 2020, from a 1990 baseline.
The overall positive effects of reducing energy use and minimizing the environmental footprint of development through efficient and sustainable use of concrete dramatically outweigh the impact of the cement manufacturing process (Carter 2006). Put another way, concrete is one of the world’s most CO2 efficient and sustainable building materials.
The balance of this report provides more information about the features of concrete pavements that contribute to their legacy as a costefficient, sustainable, and green choice for roadway pavements.
Figure 2. Constituents of a typical concrete paving mixture.
The longevity of concrete pavements is well documented. Countless concrete roadways in North America have lasted 50 years or more, supporting traffic volumes much greater than originally anticipated. Such longlasting concrete pavements are not confined to one region of North America, nor to a specific type of environment or climate. A few notable U.S. examples are provided here:
- Interstate 10 in the San Bernardino Valley in California was originally constructed in 1946 as part of historic Route 66. Portions of this concrete highway are still carrying traffic today at an impressive volume of more than 200,000 vehicles per day. After being renewed three times by surface grinding during its 65 year life, this highway is a true testament to concrete pavement’s sustainability (ACPA 2006).
- Route 23 through Kanabec County, Minnesota, was originally paved with concrete in 1948. According to a Minnesota Department of Transportation (DOT) pavement condition survey conducted in 2000, the 52 year old concrete pavement still has a present serviceability rating† (PSR) of 4.1 (Very Good).
- More than half of the concrete pavements older than 50 years remaining in Minnesota have a PSR greater than 3.1 (corresponding to ratings of Good or Very Good) (Wathne and Smith 2006).
- U.S. 52 in Charleston, South Carolina, which local resi dents call Rivers Avenue, has provided Charleston residents with uninterrupted service for more than 50 years. A two mile (3 km) section of this east–west thoroughfare was paved with concrete in the early 1950s and has required little in terms of maintenance since.
- Belknap Place, one of the first concrete streets in San Antonio, Texas, was paved with concrete in 1914. It is still performing well today, 92 years after it was constructed (Taubert 2006).
Such longlived concrete pavements have demonstrated economic advantages in terms of lifecycle costs. In addition, they contribute directly to the system’s sustainability in several important ways.
A longlasting concrete pavement does not require preservation/rehabilitation or reconstruction as often, and therefore consumes less raw materials (e.g., cement, aggregates and steel) in the long run. Energy savings are realized as well, because preservation/rehabilitation and reconstruction efforts consume large amounts of energy – not just associated with the efforts themselves, but through raw material processing and, more importantly, via traffic congestion. Work zones cause congestion and congestion wastes vast amounts of energy. Therefore, employing longlasting concrete pavements that allow for fewer construction zones impeding traffic flow will result in less congestion related energy waste, reducing pollution generation and vehicle emissions.
Finally, there are positive safety implications as well. Many of our highway fatalities occur in and are associated with highway construction zones. If we can reduce the number and frequency of these construction zones by employing long lasting concrete pavements, it will save lives directly.
Because of this longevity, concrete pavements have the potential to help society address the challenges of sustain able development in numerous ways. Ultimately, all these environmental and social benefits add up to greater long term economic benefits to the public. Clearly, pavement longevity is a crucial element of roadway sustainability.
There are a whole host of sustainability benefits of concrete pavements beyond longevity. Some of these are very familiar to most roadway engineers while others are not as well known. Figure 3 illustrates the most pronounced sustainability benefits, beyond longevity, of concrete pavement. Of course, pavement longevity impacts most of these benefits in a positive way as well. The most important thing to remember about these sustainability benefits is that roadway engineers and highway administrators DO have the ability and the tools to minimize the sustainability footprint of their roadway pavements – each one of the benefits shown can be achieved through proper selection, design and/or mixture optimization procedures.
Figure 3. Sustainability benefits beyond longevity; all these can be achieved through proper selection, design and/or mixture optimization.
Reduced Fuel Consumption and Emissions
The construction, operation and use of pavements have a significant fuel footprint associated with them. However, a growing body of research suggests that there are signification fuel savings potentials with the use of rigid concrete pavements over any alternative paving materials. These savings can be realized during the production and construction phase of a pavements life cycle as well as (primarily) during its use phase (when it is being trafficked).
Fuel Savings During Use
Profile stability is a term used to describe the ability of a pavement surface to resist deformation and deflection caused by sustained and repeated loading. Unlike flexible pavements, which are viscoelastic and therefore sensitive to both temperature and loading, concrete pavement’s rigid surface does not deform or deflect significantly under heavy vehicle loading. This not only makes concrete pavement less susceptible to the formation of heavy vehicle wheel ruts and the associated increased hydro planing potential, it also positively impacts vehicle fuel consumption.
Recent Life-Cycle Assessment (LCA) research suggests that this feature of rigid concrete pavements is likely the most significant sustainability opportunity available to roadway decision makers. Because pavements remain in service for decades, supporting millions of vehicles during that time, incremental fuel efficiency gains associated with the use of rigid or stiff pavements (such as concrete) can have enormous benefits over the life of the pavement (Santero et al. 2010, Arkedani and Sumitsawan 2010, Rens 2009, Milachowski et al. 2010, and Taylor and Van Dam 2010). Ongoing research at the Massachusetts Institute of Technology (MIT) supports this as well. The potentially lower fuel consumption of vehicles on concrete pavements can lead to significantly lower life-cycle carbon emissions compared to an asphalt pavement (MIT 2010).
Figure 4. Exaggerated depiction of a truck tire deflecting an asphalt pavement (left) but not a concrete pavement (right).
Several studies to date suggest that the resistance (amount of deflection) encountered by vehicle wheels on asphalt pavements is measurably greater than resistance encountered on concrete pavements (Figure 4). Thus, more energy and more fuel are required to move vehicles on flexible pavements (Taylor Consulting 2002). A 2010 ACPA Special Report on Sustainability Opportunities with Pavements (SR306) provides an overview of the numerous studies to date that examine the link between fuel consumption rate and pavement type.
The most in-depth and statistically rigorous studies into this phenomenon were conducted in several phases over multiple years by the National Research Council Canada (NRC). The studies 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 [IRI < 120 in./mi (1,900 mm/km)].
The findings from these studies (Taylor Consulting 2002; Taylor and Patten 2006) are illustrated in Figure 5 and Table 1. Figure 5 shows that fuel consumption for two common truck types—tractor tanker semi-trailer and tractor van semi-trailer—can be reduced an average of about 1% to 6% when traveling on concrete versus asphalt pavement, depending on truck type and vehicle speed.
Figure 5. Fuel consumption savings on concrete versus asphalt pavements (after Taylor and Patten 2006).
Table 1. Yearly Potential Savings in Cost, CO2, NOx, and SO2 for a Typical Major Arterial Roadway (62 mi [100 km] long)
Using these values, Table 1 identifies the potential yearly savings for a major arterial roadway that is 62 miles (100 km) long and carries 20,000 vehicles per day with 15% trucks. Calculations assumed that the tractor-trailer unit has an average engine fuel consumption of 5.5 miles per gallon (43 liters per 100 km).
In the context of overall transportation sustainability, these fuel savings and pollutant reductions are enormous. In the case of greenhouse gases (CO2) alone, the average savings realized by the 62 mile (100 km) long major arterial roadway in Table 1 over its 30 year design life [165,000 tons (150,000 metric tons)] are more than three times greater than the CO2 emitted during the manufacture of cement used for the construction of the concrete pavement [52,800 tons (48,000 metric tons)]‡.
Put another way, all of the CO2 emitted during the manufacture of cement used to construct a concrete roadway pavement is compensated for during the first nine years of service by virtue of the reduced pavement deflection and improved truck fuel efficiency. The remaining 22 years of service and the proportionate CO2 savings are a further testament to the sustainability of concrete pavements.
The difference in fuel consumption as a function of pavement type should be an important consideration for government agencies when analyzing potential pavement structures for new or reconstructed pavements. Significant greenhouse gas and cost savings can be realized when operating vehicles on concrete pavements versus asphalt pavements.
Fuel Savings During Construction
The construction of roadway pavements requires a lot of energy. The production of paving materials, the transportation of these materials to the site, and their actual placement consume significant amounts of fuel, mostly diesel.
The Federal Highway Administration (FHWA) reports on fuel usage for various elements of construction, including roadway paving, in its Technical Advisory T 5080.3 on Price Adjustment Contract Provisions (FHWA 1980). The Technical Advisory lists the average diesel fuel usage factor for asphalt pavements as approximately 2.90 gallons per ton (12 liters per metric ton) of asphalt and the fuel usage factor for concrete pavements as 0.98 gallons per cubic yard (4.9 liters per cubic meter) of pavement surface course. Using this information, the amount of fuel required to produce and place one lane mile of asphalt and concrete pavement can be calculated and compared.
Figure 6 illustrates this comparison — it shows that the amount of fuel required per lane mile of concrete roadway is less than one-fifth of the fuel required to produce and place a lane mile of asphalt pavement. Though equivalently designed concrete pavement thicknesses are typically between two and three inches thinner than an asphalt pavement designed for the same scenario, this example conservatively assumes that the pavements have the same thickness. NOTE: It is important to note that the FHWA fuel usage factors do not take into account fuel (or energy) use for production of the liquid asphalt binder or portland cement.
Figure 6. Typical amounts of diesel fuel required to construct one mile (kilometer) of concrete and asphalt pavements; assumptions: FHWA average fuel usage factors with a haul distance of 0-10 miles (0-16 km) (FHWA 1980), lane width = 12 ft (3.7 m), thickness of concrete and asphalt pavement = 10 in. (250 mm), and asphalt density = 140 lb/ft3 (2,240 kg/m3).
In order to illustrate the magnitude of the fuel saving potential, the following example is useful. FHWA estimates that roughly 500 million tons (450 million metric tons) of asphalts is placed annually in the U.S. (FHWA 2007). If concrete roadways were constructed in place of just half of this amount of asphalt tonnage, the savings in fuel from production/construction operations alone would amount to nearly 600 million gallons (2.3 billion liters) — every year.
In addition to the obvious economic benefits, there are enormous environmental advantages. These savings in diesel fuel would eliminate the emission of approximately 7 million tons (6 million metric tons) of CO2 into our atmosphere each year. This is the equivalent of taking 1.4 million cars off the road (the average passenger car emits about five tons (4.5 metric tons) of CO2 annually (EPA 1997)).
Lower Energy Footprint
Embodied primary energy is a measure of all energy use associated with the production, delivery, and maintenance of a facility over a predetermined time period. It includes both feedstock energy (the gross combustion heat value of any fossil hydrocarbon that is part of the pavement, but is not used as an energy source; e.g., bitumen) as well as primary energy (fossil fuel required by system processes including upstream energy use).
An embodied primary energy analysis in this context accounts for the energy needed to extract materials from the ground (e.g., aggregates, raw materials for cement production, oil for asphalt, etc.), process these materials, produce the paving mixtures, construct the roadway, maintain it, and rehabilitate it over a predetermined time period. This approach is an effective means to evaluate the energy footprint a facility itself has over its lifetime.
A recent study conducted by the Athena Institute presents embodied primary energy and global warming estimates for the construction and maintenance of equivalent concrete and asphalt pavement structures for several different road types in various geographic regions in Canada (Athena Institute 2006). The study period was 50 years, a period that takes into account original road construction and all maintenance and rehabilitation activities for both pavement alternatives. It is noteworthy to mention that this study did not account for the use-phase of a pavements life cycle (i.e., operational fuel consumption).
For all six pavement structural design comparisons, the concrete pavement alternatives clearly require significantly less energy than their asphalt pavement counterparts from a life-cycle perspective. Results show that concrete pavements require from 1/5 to 1/2 the amount of energy that equivalent asphalt pavement alternatives do. As an example, Figure 7 shows the comparative embodied primary energy for the high-volume roadway pavement structures analyzed (with 0% recycled asphalt pavement (RAP) included in the final pavements). As this figure illustrates, the embodied primary energy associated with concrete pavement alternatives is only about 33% of the embodied primary energy associated with asphalt pavement alternatives.
Also evident from Figure 7 is that the feed-stock energy component is the largest contributor to total energy for the asphalt pavements. However, even when feed-stock energy is excluded from the analysis, concrete pavements still present a significant energy advantage over their asphalt counterparts for all pavement structures analyzed. Even with the inclusion of 20% RAP in the asphalt pave ment alternatives, the energy advantage of the concrete pavement counterparts is still significant, especially at the embodied energy level (Athena Institute 2006).
This study reflects the embodied energy and greenhouse gas emissions related only to production, transportation, and placement of materials for initial construction, maintenance, and rehabilitation. As mentioned earlier, operational considerations, such as truck fuel savings by operating on different pavement types (see Reduced Fuel Consumption and Emissions on page 4) and energy savings due to the different light reflectance properties of the pavement types (see Reduced Lighting Requirement on page 12), are not considered. The results would be even more dramatic in favor of rigid and light colored pavements with the use phase included. In fact, the report does suggest that fuel savings and urban heat island effects should be taken into account in any decisions predicated on life-cycle environmental analyses.
Reduced Use of Natural Resources
Another consideration is the volume of granular base/ subbase materials needed to provide structural support for pavement. Because of concrete’s rigidity and stiffness, the slab itself supplies a major portion of the pavement system’s structural capacity and distributes heavy vehicle loads over a relatively wide area of subgrade. An asphalt pavement is not as rigid and does not spread loads as widely. Therefore, asphalt pavements typically require more layers of base granular material at a greater thickness when compared to an equivalent concrete pavement design (ACPA 2007). This essentially means that a concrete pavement is more resource efficient.
Based on an analysis performed on equivalent pavement designs for asphalt and concrete pavements for an arterial road on a low-strength subgrade, approximately twice as much granular material is needed for an asphalt pavement structure than for a concrete structure (ARA 2003). The environmental effect of this increased demand on granular material may be amplified if suitable aggregate sources are not locally available. Longer haul distances result in aggregate haul trucks consuming more fuel and emitting more CO2.
Another way that concrete pavement is more resource efficient relates directly to its improved structural capacity. Because concrete pavement is not very sensitive to applied loads (i.e., does not rut, shove, etc.), it can accommodate increased truck weights and higher tire pressures. Concrete pavement is used routinely at industrial facilities and port facilities with great success. This benefit of concrete pavement may be useful in the sustainability arena for roadway pavements as well.
Figure 7. Comparison of embodied primary energy for high-volume. roadways (0% RAP)
If allowable truck weights are raised by 50% from 80,000 lbs to 120,000 lbs (36,300 kgs to 54,400 kgs), movement of goods that previously required three trucks would now require just two trucks; this is true because the vast majority of trucks tend to “weigh out” (e.g., reach the allowable weight limit) before they “cube out” (e.g., run out of volume in the truck). This could in turn cut highway truck fuel consumption by nearly one third. Given that we today have approximately 1.2 million fully loaded trucks on U.S. highways, that consume almost 11 billion gallons (42 billion liters) of diesel fuel per year (USDOT 2008), the fuel savings potential via raising truck weights amount to nearly 3.6 billion gallons (14 billion liters) per year and roughly 42 million tons (38 million metric tons) of CO2.
Of course, this is only true if all roadway pavements consisted of concrete. As it is, most U.S. roadway pavements are not surfaced with concrete so the impact of increasing truck weights on the performance of our pavements and bridges would need to be further investigated. However, when new highway corridors are contemplated, or when existing corridors are being reconstructed, the enormous fuel and natural resource savings possible through the use of rigid and strong concrete pavements should not be ignored.
Use of Industrial Byproducts
In most concrete used for roadways in North America, some of the portland cement is replaced or supplemented with one or more industrial byproducts. These byproducts are often referred to as supplementary cementitious materials (SCMs). The three most commonly used SCMs are fly ash (byproduct of coal burning), slag cement or ground granulated blast furnace slag (byproduct of iron production), and silica fume (byproduct of silicon or ferrosilicon alloy manufacturing).
Using SCMs in concrete pavement has several environmental benefits. First, recovering industrial byproducts avoids the use of virgin materials needed for cement manufacturing. Additionally, beneficial utilization reduces the amount disposed in landfills. More importantly, however, are the greenhouse gas and energy reductions achievable by using SCMs to replace a portion of portland cement. CO2 and energy savings are related to the percentage of SCM used in the concrete mixture design. Many state highway agencies allow up to 25% of portland cement to be replaced with fly ash and 50% to be replaced with slag cement; some states even allow higher SCM replacement levels.
While portland cement is an essential ingredient in concrete, its production requires significant energy use and generates greenhouse gases. The cement industry in North America has made significant strides in reducing energy and emissions associated with cement manufacturing. Beyond these production technology improvements, the environmental impact of portland cement can be further reduced through partial replacement of cement with SCMs.
One study (Marceau and VanGeem 2005) showed that, for a typical Maryland DOT concrete pavement mixture, a replacement of 50% of the portland cement with slag cement resulted in a 35% reduction in embodied primary energy and a 45% reduction in embodied greenhouse gas per cubic yard (cubic meter) of concrete. This calculation includes all the energy utilized and emissions generated in mining, manufacturing and transporting concrete’s constituent materials, as well as the manufacturing processes involved with producing concrete.
The EPA also recognizes the importance of SCMs in disposal, energy, and greenhouse gas reduction. Their Comprehensive Procurement Guidelines require that slag cement, fly ash, and silica fume be included in all construction project specifications utilizing federal funding (greater than $10,000), unless a valid technical or market reason for not using them can be documented.
Besides these environmental benefits, SCMs can enhance concrete properties when used in appropriate quantities. For example, they can improve workability of the mixture, decrease concrete permeability, improve durability, and enhance strength. FHWA’s Fly Ash Facts for Highway Engineers gives an excellent overview of some of the characteristics, applications and benefits of fly ash in concrete pavements (FHWA 2003). Cost savings may also result in markets where SCMs are less expensive than portland cement, or where mixture optimization can provide engineering properties (e.g., strength or durability) that would be more expensive to achieve without SCMs.
Details on several U.S. and Canadian DOTs’ allowances for the use of SCMs can be found in A Synthesis of Data on the Use of Supplementary Cementing Materials in Concrete Pavement Applications Exposed to Freeze/Thaw and Deicing Chemicals (Cement Association of Canada 2005), as well as in ACPA’s Agency Practices Explorer, accessible in the online Application Library at www.apps.acpa.org.
At the end of its useful life, a concrete pavement surface can be renewed via concrete pavement restoration (CPR) activities, such as full/partial depth repairs, dowel-bar retro-fitting, and grinding. Diamond grinding is a particularly useful technique used to restore pavements and improve ride quality, noise, and surface texture — in essence, renewing the pavement surface.
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 hard-wood floor with a drum sander. Based on a study of 76 test sections nationwide (including pavements in freezethaw 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 typically is possible to diamond grind a candidate concrete pavement up to three times without compromising its fatigue life. 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. In addition, this enhanced smoothness and longevity is accomplished without extracting or processing additional raw materials such as aggregates or binders.
An excellent example of this is a section of I10 (San Bernardino Freeway) just east of Los Angeles. It was originally constructed in 1946 as part of historic Route 66. In 1965, it was ground to correct the considerable amounts of joint spalling and faulting that had developed during its more than 20 years of service. This first ever continuous grinding project in North America provided 19 years of additional service. In 1984, this pavement got a third lease on life when Caltrans decided to restore the pavement again using diamond grinding. In 1997, the 51year old pavement was ground for a third time. After 65 years of service, the concrete pavement is currently carrying more than 200,000 vehicles each day, a true testament to concrete pavement’s sustainability (ACPA 2006).
Quiet Surface Textures
Noise pollution is a growing concern in North America. The surface texture of a roadway pavement controls many important factors, including a portion of traffic noise. Noise generated at the tire–pavement interface often becomes dominant at highway speeds (TCPSC 2005).
ACPA co-sponsored research at Purdue University’s Herrick Laboratory developed a new approach to diamond grinding concrete pavements. This new texture, called the Next Generation Concrete Surface (NGCS) represents the first major modification of a concrete texture in almost thirty years. The NGCS surface, shown in Figure 8, provides a more consistent texture with minimal positive or upward texture, resulting in less tirepavement noise. To date, the NGCS surface has been constructed at thirteen locations in nine states.
Because most of the noisy concrete pavements already exist, the development of a diamond grinding solution as a corrective measure for existing roadways was given a high priority by the concrete pavement community. Through partnerships, the implementation of the newly developed NGCS texture has been greatly accelerated with two test sections constructed within approximately one year of its inception. The oldest NGCS test section is now four years old and an NGCS texture was recently constructed into an exposed aggregate texture.
Research by the National Concrete Pavement Technology Center examined conventional textures in order to define construction variability (in terms of depth, width, spacing, etc.) and to develop an understanding of the relationship between three-dimensional (3D) surface texture and noise. The 3D texture was measured using a lightweight, remote controlled, line-laser measurement device that provides real-time graphic displays of both micro- and macrotexture (Rasmussen et al. 2008 and 2010).
Figure 8. An example of the Next Generation Concrete Surface (NGCS) as constructed in a section of Interstate 94 alongside the MNRoad test facility (photo courtesy of Diamond Surface, Inc.).
Results from research conducted to date indicate that for ordinary concrete pavements, longitudinal textures— including tining, grooving, and grinding—are particularly favorable in terms of low noise generation (Ardani and Outcalt 2005).
FHWA, in its Technical Advisory on Surface Texture for Asphalt and Concrete Pavements, includes longitudinal tining, grooving, and grinding as recommended practices to provide the desired texture over the performance life of the pavement and to minimize objectionable levels of tire– pavement noise (FHWA 2005).
Ongoing research is also looking at other innovative textures that may provide desired environmentally sensitive surface characteristics. Although experience with exposed aggregate textures is very favorable in Europe, in terms of both noise mitigation and cost effectiveness, there is very little experience and only a few installations with exposed aggregate textures in North America.
Through all of the public and private sector research, public highway agencies and the paving industry now have even better guidelines for constructing concrete pavements that generate minimal noise at the tire–pavement interface, while still meeting or exceeding dry- and wet-weather friction requirements.
Another consideration when evaluating any pavement’s performance in terms of noise is acoustic longevity, or how tire–pavement noise changes over time as the pavement surface wears.
A pavement’s noise profile must be considered not only immediately after construction but over the life of the pavement. Concrete pavements’ inherently rigid structure reduces the potential for erosion of the desirable surface texture features under traffic loading. Unlike asphalt pavements, which are viscoelastic and therefore sensitive to both temperature and sustained or repetitive loading, concrete pavements much more readily retain their initial surface textures and the associated noise characteristics for the life of the pavement. As detailed in previous sections, this characteristic of concrete pavements also positively impacts both truck fuel economy and safety.
Improved Water Quality
Stormwater quality can be improved through the use of pervious concrete pavements. Pervious concrete pave ments are comprised of specially graded coarse aggregates, cementitious materials, admixtures, water, and little or no fines. Mixing these products in a carefully controlled process creates a paste that forms a thick coating around aggregate particles and creates a pavement with interconnected voids on the order of 12 to 35%. This results in a pavement that is highly permeable, with drainage rates in the range of 2.5 to 18 gallons per minute per square foot (100 to 730 liters per minute per square meter) (Brown 2003).
Pervious concrete has the potential to provide an environmentally sensitive product for specific applications. Currently, the most common uses of pervious concrete are parking lots (Figure 9), low traffic pavements, and pedestrian walkways. However, interest in its use for highway pavements and shoulders is increasing.
Pervious concrete pavements reduce stormwater runoff and help recharge groundwater aquifers. They also reduce the amount of pollutants, such as car oil, anti-freeze, and other automobile fluids, contained in non-runoff stormwater. By allowing some rainfall to percolate into the ground, pervious concrete promotes natural filtration and “treatment” of rainwater via soil chemistry and microbial activity (Brown 2003).
Figure 9. Pervious concrete pavement parking area in Normal, Illinois.
Experience with pervious concrete pavements has been promising, particularly in warmer southern climates. Research is also continuing to address the freeze-thaw durability of pervious concrete to establish parameters for its durability and application as shoulders in colder climates. One research project at the National Concrete Pavement Technology Center at Iowa State University in Ames, Iowa, suggests that, with a properly proportioned mixture, freeze-thaw performance can be excellent (Kevern et al. 2005; Schaefer et al. 2006).
Clogging of pores within the concrete matrix has also been noted as a challenge of pervious paving materials. Even with significant deposition of fines within the pore structure of pervious concrete, however, the drainage rate is still sufficient to maintain flow during the majority of significant rainfall events. Routine sweeping or vacuuming of the pavement surface will remove much of the deposited material and restore the porosity of clogged pervious concrete to nearly new conditions.
Most pervious concrete pavements constructed to date have been designed with hydrological requirements as the sole design criteria. Building on previous advances in thickness design methodologies for jointed plain concrete pavements and hydrological design methodologies for stormwater management best practices, the ACPA developed a user-friendly structural and hydrological design software for sustainable pervious concrete pavements in 2010. This software, called PerviousPave, is available at www.acpa.org/bookstore.
Concrete pavement is 100% recyclable. At the ultimate end of its fatigue life, concrete pavement can be crushed and reused in many ways (e.g., subbase material for a new concrete pavement). A 2005 study conducted by the Construction Materials Recycling Association (CMRA) revealed that about 130 million to 140 million tons (118 million to 127 million metric tons) of concrete were crushed and recycled in 2004. In fact, by weight, concrete is the most recycled material in the United States (CMRA 2011).
Some state DOTs allow up to 100% of coarse aggregate in concrete mixtures to be recycled concrete aggregate. By using recycled concrete aggregates in new pavements, agencies can virtually eliminate the need for mining and transporting virgin aggregates. In addition, recycling the existing concrete pavement onsite (Figure 10) eliminates the need for transporting the old concrete to an offsite crusher and back to the concrete batch plant. The strategy of using recycled aggregate in new concrete pavements has proven successful in many applications across the U.S., resulting in excellent long-term concrete performance. One notable example is I–35 near Guthrie, OK, where the Oklahoma DOT reconstructed a 6 mile (10kilometer) section in 1988. Today, almost 23 years later, there is no evidence of any difference in performance between the southbound section paved with recycled aggregate, and the adjacent northbound section paved with virgin limestone aggregate.
Figure 10. Paradigm in-place concrete recycling equipment in operation in Oklahoma (photo courtesy of Duit Construction).
Using recycled concrete pavement, particularly in applications that expose it to the atmosphere (e.g., embankment fill, gravel roads, roof ballast, and railroad ballast) has additional sustainability benefits resulting from a process called carbon sequestering.
Approximately 60% of the CO2 emitted during the manufacture of portland cement results from a process known as calcination, a chemical reaction among the raw materials in the cement kiln. When cement is subsequently used to produce concrete, another chemical reaction, called hydration, occurs that allows the cement to form compounds that bind the aggregates together. Later, when hardened concrete is exposed to air, the calcination reaction reverses in a process called carbonation.
In essence, carbonation recaptures CO2. Carbonation occurs naturally in all concrete, albeit at very slow rates due primarily to the low permeability of concrete. However, exposing a large surface area of concrete to the atmosphere, for example through crushing, dramatically accelerates the carbonation process. Eventually, such expo sure may allow for the recapture of all the CO2 originally evolved from the cement raw materials during calcination (RMRC 2005).
Light-Colored and Cool
Concrete surfaces readily reflect light. This characteristic of concrete, generally referred to as albedo, is advantageous for several reasons. It can significantly improve both pedestrian and vehicular safety by enhancing nighttime visibility on and along concrete roadways. It reduces the amount of energy needed for artificial roadway illumination during the night. It also reduces the amount of energy needed to cool urban environments associated with the urban heat island effect. Light colored and cool concrete pavements can also mitigate the greenhouse effect and contribute to global cooling by reducing the amount of solar radiation absorbed by the earth’s paved surfaces. These effects are further discussed in the following sections.
Reduced Lighting Requirement
Lighting fixtures are important elements of most urban roadway facilities. Enhanced nighttime visibility is intuitively related to improved traffic safety. Figure 11, which shows highway Castello Branco in Sao Paulo state, Brazil, at night, clearly illustrates how visibility is improved in the lanes paved with concrete (ABCP 2005). In addition, because of the more reflective nature of concrete pavements, a specified luminance level can be achieved with lower-wattage and/or fewer light fixtures than would otherwise be needed. Ultimately, this translates to lower costs and lower energy consumption over time.
Several highway agencies in the U.S. acknowledge this opportunity, and classify different pavement types by their ability to reflect visible light. The Minnesota DOT uses the general guidelines and classification scheme published in the AASHTO standards. Table 2 lists the minimum average maintained illuminance by facility classification and pavement type, as published in the 2003 Minnesota 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 U.S. 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.
Some suggest this difference presents a potential cost savings of over 30% (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.
Figure 11. Brazil roadway illustrating concrete (left) vs. asphalt albedo (photo courtesy of Brazilian Portland Cement Association).
Table 2. Mn/DOT Illumination Demands by Facility Classification/Pavement Type (Mn/DOT 2003)
Heat Island Mitigation
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, as illustrated in Figure 12 (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.
Figure 12. EPA illustration of an urban heat-island profile (EPA 2004).
Paving urban roadways with concrete is an effective strategy to help mitigate urban heat island effects. With their higher albedo, concrete pavements reflect significantly more sunlight than darker colored asphalt pavements, resulting in cooler pavement surfaces. Research conducted at the Lawrence Berkeley National Laboratory suggests that, when exposed to sunlight, lighter-colored concrete pavements typically have surface temperatures approximately 21°F (12°C) lower than darker-colored asphalt pavements (Pomerantz et al. 2000). Both the LEED and GreenRoadsTM rating systems recognize this as well, and award points directly for use of high albedo pavement surfaces.
The temperature difference can be verified directly as well, as shown in Figure 13. This thermal image of the ramp to State Road 202 in Mesa, Arizona, taken in August 2007, shows that the reduction in heat retention and emittance can be significant for brighter and more reflective concrete pavements. The thermal image illustrates the temperature difference between concrete pavement (foreground) and asphalt pavement (background) in the same ambient conditions. The picture was taken at approximately 5:00 pm during partly cloudy conditions, and the temperature difference between the two pavement surfaces was approximately 20°F (11°C).
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.
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 U.S. 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).
Another benefit of lighter-colored and cool pavements in the urban heat island context relates to the reduced potential for smog formation. Although the exact effect of pavement type on heat retention and resulting air quality issues, such as smog formation, is complicated and not very well understood, it is clear that the process of smog formation is very sensitive to temperature.
Figure 13. Thermal image of a pavement in Mesa, Arizona (note the temperature difference between the concrete pavement (foreground) and the asphalt pavement (background)).
Computer simulations for Los Angeles show that resurfacing about two-thirds of the pavements and rooftops with more reflective surfaces and planting three trees per house can lower temperatures by as much as 5°F (3°C) and can therefore significantly reduce the potential for smog formation (Akbari 2005).
A 1997 report suggests that increasing the albedo of 775 (1,250km) of pavement in Los Angeles by 0.25 (i.e., converting the surface from darker pavement to lighter concrete) would reduce smog-related medical and lost-work expenses by $76 million per year (Pomerantz et al. 1997).
In a way similar to how concrete pavements help mitigate heat island effects, highalbedo concrete pavements also contribute directly to global cooling. By reflecting more of the sun’s energy away from the pavement surface and back out into space, the amount of solar radiation absorbed by the earth’s surface is reduced – 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. If implemented 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.
The staggering magnitude of this offset potential clearly suggests that use of high albedo pavements can and should be an important strategy to consider when contemplating ways to mitigate climate change via our built environment. Despite the enthusiasm Energy Secretary and Nobel Laureate (1997 Physics) Steven Chu continues to express about the cool roof and cool pavement strategies, these tools and the opportunities they afford are not yet widely considered in the highway community or among public sector administrators.
Concrete pavement has long been considered the environmentally and economically sustainable pavement choice simply due to its longevity. This hallmark of concrete pavement ensures that the desirable performance characteristics of the pavement remain essentially intact for several decades. In addition, long-lasting concrete pavements do not require rehabilitation or reconstruction as often and, therefore, consume fewer raw materials and energy over time, and generate fewer pollutants/emissions along the way as well. Of course, the reduced potential for congestion associated with long-lasting concrete pavement is the most significant benefit because congestion wastes enormous amounts of energy (fuel), in-turn generating enormous amounts of emissions. Ultimately, all of these environmental benefits add up to greater long-term economic and social benefits to the public.
However, there are many other features of concrete pavements that also support the case for concrete pavements’ sustainability. As mentioned, these features all contribute to making concrete pavements an environmentally sensitive (and sensible) pavement choice:
- The rigidity of concrete pavement means lower vehicle fuel consumption and emissions.
- Less fuel-intensive construction operations for concrete pavement result in significant economic and CO2 savings.
- Concrete pavement’s overall lower energy footprint means tremendous savings in energy over the life of the pavement facility.
- The strength and rigidity of concrete means fewer and thinner required layers of subbase materials.
- Using industrial byproducts in concrete improves pavement longevity, saves money, reduces the need for disposal in landfills, and greatly reduces both energy use and generation of greenhouse gases.
- Concrete pavement’s renewability and 100% recyclability lead to improved longevity and reduced demand on nonrenewable resources.
- New surface textures that significantly reduce tire-pavement noise have been developed for concrete pavements.
- Pervious concrete pavements capture and filter pavement runoff and help improve storm-water quality.
- The light-colored and cool surface of concrete pavement leads to improved visibility, reduced lighting requirement, reduced heat island effects and significant opportunities for global cooling.
Most importantly, a roadway engineer or highway administrator can in fact elect to incorporate each and every one of these sustainability features into their pavements through proper selection, design and mixture optimization processes. Of course, each of these sustainability benefits typically also result in greater long-term cost-effectiveness to the public. As such, concrete pavements are the clear choice for environmentally sensitive and economically sustainable roadways—truly green roadways, in more ways than one.
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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.