Green highways are no longer just a concept, but are increasingly becoming a reality as agencies turn to concrete pavements to meet the three-pronged requirements of sustainable development.
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,” according to a report to the United Nations General Assembly in August 1987.
More precisely, in terms of highway and roadway construction, sustainable development involves being good stewards of the environment, balancing the needs of business and providing societal benefits. Within the broad framework of sustainable development, green highways are environmentally responsible and sustainable in all aspects, including design, construction and maintenance. The green highways initiative was begun in 2005 as a pilot program by the U.S. Environmental Protection Agency (EPA). The program has stayed true to its original mission of coordinating transportation needs and environmental requirements.
A friend of energy
Concrete pavements offer some inherent features and benefits that are well-suited for sustainability goals and objectives. The following sections describe those attributes in greater detail.
Longevity: The durability and longevity of concrete pavements are both well-known and well-documented. There are many examples of concrete pavements that have far exceeded their original design lives.
Among those are the very first concrete pavement placed in Bellefontaine, Ohio, in 1891. The pavement sections on Court Street there are still carrying traffic more than 117 years later. Belknap Place in San Antonio, Texas, was paved with concrete in 1914 and is still carrying traffic today.
I-10 in California’s San Bernardino valley was originally constructed as part of historic Rte. 66. Portions of this concrete pavement highway are still carrying traffic today, and with more than 200,000 vehicles per day, the traffic volume is substantially higher than originally intended. The pavement has been restored by diamond grinding three times since originally placed, but otherwise has required few repairs and little maintenance after more than six decades of service.
Heading due north to Minnesota, more than half of the concrete pavements older than 50 years still have a present serviceability rating of 3.1 (considered good to very good) or greater. Other examples abound, each of which underscores the durability and long-term value of concrete pavements.
Further contributing to pavement longevity is concrete pavement restoration, including full- or partial-depth repairs, dowel-bar retrofitting and diamond grinding. A concrete pavement surface can be renewed by diamond grinding, which improves ride quality, noise and surface texture. Studies by the California Department of Transportation suggest the average time between additional rehabilitation needed for a diamond-ground pavement is approximately 17 years.
The longevity of concrete pavements not only provides significant economic advantages in terms of life-cycle costs, but also contributes directly to the system’s sustainability in several important ways. A long-lasting concrete pavement does not require rehabilitation or reconstruction as often and therefore consumes less raw materials in the long run. This longevity benefits our environment in other ways as well. Energy savings are realized, since rehabilitation and reconstruction efforts consume energy. Also, congestion is reduced (with accompanying energy savings and reduction in vehicle pollutants) by employing long-lasting concrete pavements because of fewer construction zones impeding traffic flow.
Because of this longevity, concrete pavements have the potential to help society address the challenges of sustainable development in numerous ways. Ultimately, all these environmental and social benefits add up to greater long-term economic benefits to the public. In a sense, longevity is a crucial element of sustainability.
Reduced vehicle fuel consumption and emissions: One key to reduced fuel consumption and emissions is the profile stability of a pavement, the ability of the surface to resist deformation and deflection caused by repeated or sustained loadings, such as heavy truck traffic.
Concrete pavements, being rigid substrates, do not deform under heavy vehicle loadings and, therefore, deflect less. In sharp contrast, asphalt pavement is viscoelastic and more sensitive to both temperature variations and applied wheel loads. This not only makes asphalt susceptible to rutting and shoving, as well as increased hydroplaning potential, but it also increases fuel consumption.
The reason is that fuel consumption is partly a function of the degree of pavement deflection in response to the load applied as the wheels move along the surface. Several studies suggest the resistance (amount of deflection) encountered by heavy-vehicle wheels on asphalt pavements is measurably greater than the resistance on concrete pavements. Therefore, it takes more energy and fuel to move heavy vehicles on flexible pavements.
The most in-depth studies on this phenomenon were conducted by the National Research Council of Canada (NRC). The final study included collaboration between National Resources Canada (NRCan) and the Cement Association of Canada, with input from various departments of transportation.
The studies concluded tractor-trailers traveling on concrete pavements have statistically significant lower fuel consumption than those traveling on asphalt pavements throughout the summer to wider temperature range for fully loaded trucks operating on smooth pavements.
The findings from studies by Taylor Consulting (2002) and Taylor and Patten (2006) show that fuel consumption for truck types (a tanker and a van, both tractor trucks with semitrailers) can be reduced by 1% to 6% when traveling on concrete versus asphalt. Table 1 shows the yearly potential savings in dollars, carbon dioxide (CO2), nitrogen oxides (NOx) and sulfur dioxides (SOx), all of which are significant. In this model, the assumptions include a tractor-trailer traveling 100,000 miles per year; an average engine fuel consumption of 5.5 miles per gallon; and a diesel-fuel cost of $3.99 per gallon.
Lower construction fuel demand: Constructing highway pavements requires a large amount of energy, most of which comes from fossil fuels, which of course we are dependent on oil imports to produce. Of that fuel, most of it is diesel, which has experienced sharp price increases recently.
The Federal Highway Administration (FHWA) reports on fuel usage for various aspects of construction, including highway paving, in its “Technical Advisory T 5080.3 on Price Adjustment Contract Provisions” (FHWA 1980).
The document shows the fuel usage factor for asphalt pavements is 2.90 gal per ton and 0.98 gal per cu yd for concrete pavements. Converting construction fuel usage factors to fuel required per mile of roadway constructed, the figures are as follows: 10,718 gal of fuel for a 10-in. asphalt pavement, compared with 1,916 gal of fuel for a 10-in. concrete pavement.
This means if concrete pavements were constructed in place of half of the roughly 500 million tons of asphalt pavements constructed each year, the savings in diesel fuel from construction alone would be over 0.5 billion gal.
Use of industrial by-products: In most concrete used for highways in North America, some of the portland cement is replaced or supplemented with one or more industrial by-products. Known as supplementary cementitious materials (SCMs), the two most common include fly ash (from coal burning) and slag cement (from iron production).
Using SCMs in concrete pavement has several environmental benefits. First, recovering industrial by-products reduces the use of virgin materials needed in cement manufacturing. It also reduces or, more precisely, diverts materials away from landfills.
Equally important, using SCMs also saves energy and reduces emissions associated with cement production. 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 allow even higher SCM replacement levels.
Besides these environmental benefits, SCMs generally enhance concrete properties when used in appropriate quantities. For example, they improve workability of the mixture, decrease concrete permeability, improve durability, increase strength and, most important, enhance longevity. Cost savings also may result in markets where SCMs are less expensive than portland cement, or where mixture optimization can provide engineering properties (for example strength, durability and longevity) that would be more expensive to achieve without SCMs.
Recyclability and reusability: Concrete is the most recycled material in North America, according to the Construction Materials Recycling Association (CMRA). In fact, CMRA reports that in 2004 somewhere in the range of 130 million to 140 million tons of concrete were crushed and recycled. At the ultimate end of its service life, concrete pavement can be crushed and reused in many ways, including as granular fill, sub-base material or base course for new pavement.
Recycled concrete pavement also can be used as an aggregate for new concrete pavement. Some state departments of transportation allow up to 100% of coarse aggregate in concrete mixtures to be recycled concrete aggregate. This leads to reduced demand for nonrenewable natural resources.
Using recycled concrete aggregates from pavement, particularly in applications that expose it to the atmosphere (such as embankment fill, gravel roads, roof ballast and railroad ballast) has additional benefits toward mitigating global warming, notably from a process called carbon sequestration. The Recycling Materials Resource Center reported in 2005 that such exposure might allow for the recapture of all the CO2 originally evolved from the cement raw materials associated with calcination during cement manufacture. Carbon sequestration, according to the U.S. Department of Energy, is “the provision of long-term storage of carbon in the terrestrial biosphere, underground or the oceans so that the buildup of carbon dioxide (the principal greenhouse gas) concentration in the atmosphere will reduce or slow.”
Reflectivity and coolness: Concrete surfaces readily reflect light. This characteristic of concrete, generally referred to as albedo, is advantageous for several reasons. First, it can significantly improve both pedestrian and vehicular safety by enhancing nighttime visibility on and along concrete roadways. Concrete pavements also reduce the amount of energy needed for artificial roadway illumination during the night. There are also other factors related to the reflectivity of concrete pavements, as well as their relative coolness compared to asphalt pavements, including mitigation of the urban heat island effect and smog reduction.
During daytime hours, concrete’s high reflectivity means that more of the sun’s incoming radiation is reflected back into the atmosphere, lowering the amount of heat absorbed by the pavement and its surroundings (this heat retention is often referred to as urban heat island). This in turn reduces the cooling requirement during the summer heat and can substantially lower the associated energy demand. This in turn not only has cost implications, but it also leads to higher emissions from power plants. According to recent work by Lawrence Berkeley National Laboratory, paving urban roadways with concrete is one useful strategy to help mitigate urban heat island effects.
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. 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; for example 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 (such as aggregates, raw materials for cement production and oil for asphalt), process these materials, produce the paving mixtures, construct the roadway, maintain it and rehabilitate it over a predetermined time. This approach is an effective means to evaluate the energy footprint a facility makes during 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.
The study period was 50 years, which takes into account original road construction and all maintenance and rehabilitation activities for both pavement alternatives. In the study, concrete pavement alternatives clearly require significantly less energy than their asphalt pavement counterparts do from a life-cycle perspective. Results show that asphalt pavements require two to five times more energy than equivalent concrete pavement alternatives.
Improved water quality: Storm-water quality can be improved through the innovative use of pervious concrete pavements. Pervious concrete pavements comprise specially graded coarse aggregates, cementitious materials, admixtures, water and little or no fines.
Pervious concrete pavements reduce storm-water runoff and help recharge groundwater aquifers. They also reduce the amount of pollutants, such as car oil, antifreeze and other automobile fluids, contained in nonrunoff storm water. By allowing some rainfall to percolate into the ground, pervious concrete promotes natural filtration and “treatment” of rainwater via soil chemistry and microbial activity.
Quiet surface textures: Noise pollution is a growing concern in North America. The surface texture of a highway pavement controls many important factors, including tire-pavement noise.
One of the advantages of concrete pavements is that virtually any texture can be created during or after finishing operations, including textures that minimize tire-pavement noise while maintaining wet- and dry-weather friction for the life of the pavement.
Research by the concrete paving industry is in the process of identifying optimal textures for low noise generation at the tire-pavement interface in a variety of circumstances. In North America, a significant amount of noise issues are related to existing transversely tined concrete pavements. Consequently, some of the ongoing research is focused on identifying diamond grinding techniques for retexturing and renewing pavement to produce the quietest surface textures. The FHWA, in its recent “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 minimize objectionable levels of tire-pavement noise (FHWA 2005).
Results from research recently conducted at both Purdue University’s Institute for Safe, Quiet, and Durable Highways and the National Concrete Pavement Technology Center indicate that for ordinary concrete pavements, longitudinal textures—including tining, grooving and grinding—are particularly favorable in terms of low noise generation. Some of the quietest concrete pavements measured in North America are longitudinally ground pavements, often called “whisper ground.”
Whisper grinding refers to the practice of grinding concrete pavement specifically for improving its noise profile instead of simply grinding for smoothness and ride quality. Blade width, blade spacing and grinding depth are selected with noise mitigation in mind.
Furthermore, the concrete pavement industry is currently conducting field trials of its Next Generation Concrete Surface (NGCS), which is showing great promise, both in terms of its relative tire-pavement noise features and its durability.
While these examples underscore the sustainability benefits of concrete pavements, there also is more to the story. The industry continues to pursue initiatives that make concrete pavements an even better value to agencies and road users. With sustainability and cost considerations in mind, concrete pavements are increasingly becoming the right choice for a growing number of agencies.