What can we do to improve pavement sustainability?
Sustainability is context sensitive. Because a pavement must exist and function within larger systems, practices that support sustainability must contribute to more sustainable systems and thus depend on context. As a result, a full accounting of surrounding systems and a pavement's influence on them is necessary in order to define the most appropriate sustainability practices associated with a particular pavement system. This requires an understanding of the pavement life cycle. Approaches to improve the sustainability of pavements can be from any one of these stages.
The pavement life cycle includes the material production, design, construction (which includes new construction as well as preservation, maintenance, and rehabilitation), use, and end-of-life stages.
A wide range of materials (primarily aggregates, asphalt materials, and cementitious materials) can be used for paving applications. The most well known approach to improve the sustainability of the pavement materials is the use of recycled, co-products or waste materials (RCWMs). There are several other approaches available and emerging technologies to monitor that may improve pavement sustainability with regard to aggregate production, asphalt materials, and concrete materials. It is important that each approach is assessed from a life-cycle perspective to determine the role it plays in contributing to the sustainability of a pavement system. This allows decision makers to examine potential economic, environmental, and social impacts that may occur throughout the life cycle, and also to evaluate potential trade-offs.
The design process results in the development of alternative pavement structures (including structural layers and thicknesses), specifications for materials that meet the performance objectives of the individual layers as well as the system as a whole, considerations for subsurface drainage (as appropriate), and governing construction specifications needed for the pavement to perform as intended. It is important that sustainability is considered when defining the project objectives, the first step in the pavement design process.
There are a number of trade-offs that must be considered, as improvements in one area might be detrimental to another, with the ultimate goals of the owner/agency ultimately determining which approach may be most suitable for a particular project. Some examples of sustainable design strategies include: longer life pavements, designs incorporating local materials to reduce transportation costs, accelerated construction materials, and single lane rehabilitation.
Pavement construction practices have changed significantly over the last several decades. New construction techniques and technologies that have significantly improved pavement quality and construction efficiency while decreasing construction environmental impacts. These construction practices, in concert with an appropriate pavement structural designs that use appropriate materials, can provide significant improvements to the overall sustainability of a pavement system. Many software tools are available to support the sustainability of pavement construction operations.
The impacts rely on the influence of pavement on vehicle operations and the interaction between the pavement, the environment, and humans. Several pavement characteristics have been linked to various use stage impacts such as noise, vehicle fuel consumption, safety, stormwater runoff, and potentially the urban and global climate. Often times tradeoffs exist.
Some of the decisions regarding use stage effects that can affect sustainability are made at the network level and can be implemented through effective pavement management systems (PMS), while others can only be implemented at the project level through design and construction decisions. There are trade-offs that may be considered within many of these decisions, including important safety issues. Many of these trade-offs are sensitive to project context, particularly traffic levels and climate. Project context also often has a large influence on the relative importance of environmental impacts of different phases of the pavement life cycle; for example, use-phase impacts on routes with heavy traffic are often much greater than material production and construction phase impacts, while the opposite may be true for low-traffic routes.
Maintenance and Preservation
Pavement preservation is inherently a sustainable activity. It often employs low-cost, low-environmental- impact treatments to prolong the life of the pavement by delaying major rehabilitation activities. This conserves energy and virgin materials while reducing emissions over the life cycle. Well-maintained pavements provide smoother, safer, and quieter riding surfaces over a significant portion of their lives, resulting in higher vehicle fuel efficiencies, reduced crash rates, and lower noise impacts on surrounding communities, which positively contributes to their overall sustainability.
When a pavement reaches its end-of-life, it may: (a) remain in place and be reused as a part of the supporting structure for a new pavement, (b) recycled, or (c) be removed and landfilled. Each of these activities has economic and environmental costs that should be considered.