Issues, Future Directions, And Emerging Technologies Related To Pavement Materials
Issues, future directions, and emerging technologies for enhanced sustainability of aggregates in transportation include:
- Increased shipping of aggregates by truck from long distances increases emissions, energy use, and noise, whereas local quarrying of aggregates has implications for land use, noise, dust, and other factors. As local aggregate sources are exhausted and the development of new sources stymied by community opposition, pressure will be exerted to use aggregates with less desirable characteristics. This could affect the long-term performance of pavements.
- Increasing pressure to use higher volumes of recycled, co-products and waste materials (RCWMs) will likely renew pressure to make complete use of all materials from a construction site (e.g., current practice often is to waste crushed concrete fines, but their use may be highly encouraged in the future). Further, pressure to use non-conventional RCWMs (such as steel slag aggregate or recycled glass, for example) may also increase. If done without sufficient research, the increased use of RCWMs may compromise pavement performance unless it is accommodated in the design stage and utilizes effective construction practices.
- As readily available sources of aggregates of the highest quality become exhausted, the use of "marginal" aggregates will increase. In many cases, these aggregates can be used without negatively affecting pavement life. Yet if such aggregates are used inappropriately, premature pavement failures will likely occur.
Specialty aggregates are at times needed to fulfill a specific need driven by a sustainability goal. For example, highly durable aggregates will be needed on an exposed aggregate surface, or a light-colored aggregate may be specified to increase surface reflectivity to reduce lighting requirements. In addition, other aggregates might be sought to improve the quality of the pavement material such as prewetted, lightweight fine aggregate added to cementitious mixtures to enhance curing.
A number of strategies for reducing impacts from asphalt binders, modifiers, additives, and aggregate have been presented. Some future directions and emerging technologies that should be monitored and implemented, when and where beneficial, are:
- A reduction in material quantities through improvements in mixture design, construction practices, and, in some cases, new materials such as warm-mix asphalt (WMA) or, where traffic, climate and existing condition warrant, inclusion of polymers, rubber, and other modifiers.
- Greater use of RCWMs, including reclaimed asphalt pavement (RAP), recycled asphalt shingles (RAS), and others, to reduce the mining, extraction, manufacture, and transport of non-renewable virgin materials, provided that performance is not compromised. For individual projects, this requires analysis of whether suitable RCWMs are locally available because long transportation distances may reduce the energy and environmental benefits of using RCWMs.
- Greater use of locally available pavement materials provided that those benefits are not offset by reduced performance. For asphalt materials, locally available aggregates are the primary consideration.
- Development of alternatives, namely bio-based alternatives, to nonrenewable feedstocks such as petroleum. The environmental, economic, and societal impacts of producing these alternatives will need to be evaluated to determine their overall feasibility.
Cement and Concrete Materials
There are a number of issues and emerging technologies that have the potential to affect the production and use of concrete materials in the near future. These include:
- The EPA released an amended air toxics rule for portland cement manufacturing that significantly restricts emissions (especially of mercury which comes from both the burning of coal and calcination of the calcium carbonate) by U.S. cement plants by September 2015P P. The impact of this new rule is uncertain, but it is clear that it will result in lowering the environmental impact of cement production. Switching to alternative fuel sources can address some of the issues related to mercury released during coal combustion, but mercury released during calcination of the calcium carbonate will result in increased capital cost for some cement plants as they install mercury capture equipment and the likely closing of others where it is not economically viable.
- If fly ash becomes scarce, the market share of slag cement would be expected to increase. As U.S. slag production is expected to remain relatively constant, the long-term growth in the supply of slag cement is likely to hinge on imports, either of ground or unground material (USGS 2013). The environmental impact of importation will be closely linked to the mode of transportation, with transport by barge/ship having significantly lower impact than by truck.
- One innovation is the high-volume supplementary cementitious material (SCM)/portland limestone cement mixtures that are becoming more common. As state highway agencies accept this technology, it has the potential to significantly lower the greenhouse gas (GHG) emissions associated with paving concrete.
- Photocatalytic cement is another innovation that potentially offers an opportunity to create a highly reflective surface that remains clean while treating air pollution through a photocatalytic reaction involving nanoparticles of titanium dioxide (TiO2). The reactions result in a chemical reduction of nitrous oxides (NOx), which prevents the formation of ozone and associated smog. In addition to this pollution-reducing quality, these cements are often very lightly colored and have very high albedo (reflectance) properties, which can result in a lowering of pavement and near surface temperatures (see Chapter 6 (.pdf) of the Reference Document for more details) while providing an aesthetically pleasing appearance due to their self-cleaning properties. The environmental benefits of photocatalytic cements have been documented in laboratories and on paving projects throughout Europe (Guerrini et al. 2012; Beeldens 2012), where more than 2.4 million yd2 (2 million mP2P) of photocatalytic surfaces have been constructed, with horizontal surface applications like pavements (including both paving block and single-lift concrete pavement) comprising about half of that total. Reductions in NORxR have been reported to be as high as 60 percent, depending upon local environmental conditions and the technique for dispersing the TiOR2R in the concrete (Beeldens 2012). Pavement uses of photocatalytic cements in the U.S. have included paving blocks, porous concrete, and slurry-infiltrated asphalt pavement (Guerrini et al. 2012). One acclaimed project is the reconstruction of Cermak Road in Chicago, where pervious pavers with a photocatalytic surface have been employed (Oberman 2013). An effort to implement this technology featuring its use in the top layer of a two-lift concrete pavement project constructed on Route 141 near St. Louis, Missouri in 2010 was not as successful as hoped, demonstrating the need for continued research on this technology to determine the best avenue for implementation.
- Low carbon and carbon sequestering cementitious systems are emerging including geopolymers and alkali-activated fly ash (Van Dam 2010). Work continues on a number of other cementitious systems that have the potential to actually sequester carbon dioxide as they harden, lowering the carbon footprint of concrete mixtures. However, at the current time, none of these systems is currently viewed as economically viable for large-scale adoption.
U.S. Geological Survey (USGS). 2013. 2011 Minerals Yearbook, Cement. Advance Release. U.S. Department of the Interior, U.S. Geological Survey, Reston, VA. (Web Link (.pdf)).
Guerrini, G. L., A. Beeldens, M. Crispino, G. D'Ambrosio and S. Vismara1. 2012. "Environmental Benefits of Innovative Photocatalytic Cementitious Road Materials." Proceedings, 10PthP International Conference on Concrete Pavements, Quebec City, Quebec, Canada.
Beeldens, A. 2012. "A Double-Layered Photocatalytic Concrete Pavement: A Durable Application with Air-Purifying Properties." Proceedings, 10PthP International Conference on Concrete Pavements, Quebec City, Quebec, Canada.
Oberman, M. 2013. "Smog-Eating Pavement on 'Greenest Street in America.'" Phys.org. Omicron Technology Limited, Douglas, Isle Of Man, United Kingdom. (Web Link).
Van Dam, T. 2010. Tech Brief: Geopolymer Concrete. FHWA-HIF-10-014. Federal Highway Administration, Washington, DC. (Web Link (.pdf)).