Emily B. Lorenz, P.E., LEED AP BD+C
The following article is a reprint from Aspire Magazine, Spring 2013 Edition.
Sustainability
Envision,™ a rating system for sustainable infrastructure and developed by the Institute for Sustainable Infrastucture (ISI), was first released for public comment in July 2011. ISI is a non-profit organization founded jointly by the American Council of Engineering Companies (ACEC), the American Public Works Association (APWA), and the American Society of Civil Engineers (ASCE). Shortly after this first public-comment period, the Zofnass Program for Sustainable Infrastructure at Harvard University partnered with ISI to further develop the Envision rating system. Project certification under the Envision rating system began in September 2012.
The intent of the Envision rating system is to standardize evaluation of the sustainability of infrastructure projects. It is applicable to projects in sectors such as energy, water, waste, transportation, landscaping, and information. In the transportation sector, project types that can use Envision include airports, roads, highways, railways, public transit facilities, and bridges.
Infrastructure is critical to a functioning society. It enables humans to have clean drinking water, travel between our homes and work, and ensures a reliable energy supply. However the earth’s resources are not infinite, and thus to maintain sustainable development, we must attempt to reduce negative environmental, economic, and social impacts in infrastructure design. The Intergovernmental Panel on Climate Change defined sustainable development as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs.”
Similar to other green or sustainability rating systems, credits are grouped in categories related to environmental, social, and economic impacts. A total of 60 credits are distributed across five categories, each of which is explored further in the following sections. Within each credit, point levels are set based on meeting different levels of achievement, and points are weighted within Envision based on the importance of the credit related to overall infrastructure sustainability. An assessor assigned to the project will determine the level of achievement that the project team has reached for each individual credit using a predetermined set of evaluation criteria. The level of achievement for the entire project is determined by the number of points achieved in the different credit categories.
Envision levels of achievement include:
- Improved
- Enhanced
- Superior
- Conserving
- Restorative
In the following sections, all credits and their intents are listed. However due to space limitation, only some of the credits to which concrete bridges can contribute are discussed in more detail.
Quality of Life (QL)
Strategies in this category relate to a project’s impact on the community. Broad credit categories include purpose, well being, and community. Table 1 lists the credits in this category and their intents. Two strategies in the Quality of Life category that relate to concrete bridges are explained in more detail in the following sections.
QL2.3 Minimize light pollution
The metric for this credit is that “lighting meets minimum standards for safety but does not spill over into areas beyond site boundaries, nor does it create obstrusive [sic] and disruptive glare.” Concrete bridges can contribute to this credit because light-colored concrete requires fewer lights for the same amount of visibility. This reflectability also reduces energy costs associated with outdoor lighting because more reflective surfaces reduce the amount of fixtures and lighting required. Concrete bridges can reduce outdoor lighting requirements and can contribute to lessening the associated light pollution.
QL2.4 Improve community mobility and access
For this credit, the metric is “extent to which the project improves access and walkability, reductions in commute times, traverse times to existing facilities and transportation. Improved user safety considering all modes, e.g., personal vehicle, commercial vehicle, transit and bike/ pedestrian.” There are synergies between reducing environmental impacts and reducing construction-related user delays. During initial construction, various concrete bridge types can minimize on-site construction activities, thereby lessening the amount of time that drivers are inconvenienced. Likewise, by choosing a concrete bridge that has greater durability and fewer maintenance requirements, user delays during the service life of the bridge can also be reduced. This in turn reduces energy consumption of user vehicles and the resultant emissions to air.
Leadership (LD)
Strategies in this category relate to incentivizing more-credible management and leadership related to a project’s sustainability. Broad credit categories include collaboration, management, and planning. Table 2 lists the credits in this category and their intents. Most of the strategies in the Leadership category relate to the project team, thus aren’t as related to the structural system chosen for a bridge. There are bridges where stakeholder input (LD1.4) has guided the selection of the structural system. However, no strategies in the Leadership category are explained in more detail in this article.
Resource Allocation (RA)
Strategies in this category relate to reducing a project’s embodied energy and use of virgin, non-renewable resources. Broad credit categories include materials, energy, and water. Table 3 lists the credits in this category and their intents. Four strategies in the Resource Allocation category that relate to concrete bridges are explained in more detail in the following sections.
RA1.3 Use recycled materials
To contribute to this credit, a “percentage of project materials that are reused or recycled.” Concrete bridges can contribute to this credit by using industrial wastes such as fly ash, slag cement, and silica fume as part of the cementitious materials—with certain aesthetic (color) and early compressive strength considerations. This strategy reduces the environmental impact of the concrete and also uses by-product materials that may otherwise be disposed of in a landfill.
RA1.4 Use regional materials
The metric for this credit is that “percentage of project materials by type and weight or volume sourced within the required distance.” For concrete, the distance requirement is 100 miles. Using local materials reduces the environmental impact (energy and emissions) related to transporting heavy building materials. Most concrete plants (ready-mixed and precast) are close to project sites, and likewise the cement, aggregates, and reinforcing steel used to make the concrete, and the raw materials to manufacture cement, are usually obtained or extracted from local sources.
RA1.5 Divert waste from landfills
For this credit, the metric is “percentage of total waste diverted from disposal.” Precast concrete girders can be reused when bridges are expanded, and concrete can be recycled as road base, fill, or aggregate in new concrete at the end of its useful life. Concrete pieces from demolished structures can be reused to protect shorelines. Most concrete from demolition in urban areas is recycled and not placed in landfills. Also important is that concrete generates a small amount of waste with a low toxicity.
RA1.7 Provide for deconstruction and recycling
To contribute to this credit, the project must use a “percentage of components that can be easily separated for disassembly or deconstruction.” Precast concrete bridge girders can be reused for pedestrian crossings or other applications. To reuse components effectively, engineers need to be able to determine the residual service life of the components. Precast concrete construction provides the opportunity to disassemble the bridge should its use or function change, and the components can be reused in a different application. These characteristics of precast concrete make it sustainable in two ways: by diverting solid waste from landfills and by reducing the depletion of natural resources and production of air and water pollution caused by new construction.
Other ways that the concept of reuse is facilitated with concrete components are:
- Concrete pieces from demolished structures can be reused to protect shorelines and create fisheries.
- Wood forms can generally be used 25 to 30 times without major maintenance while fiberglass and steel forms have significantly longer service lives.
Natural World (NW)
Strategies in this category relate to a project’s impact on biodiversity. Broad credit categories include purpose, well being, and community. Table 4 lists the credits in this category and their intents. Most of the strategies in the Natural World category relate to the where the project is located, thus aren’t as related to the structural system chosen for a bridge. The use of longer spans, segmental construction, or top down construction can be used to minimize the impact at ground level, however, no strategies in the Natural World category are explained in more detail in this article.
Climate and Risk (CR)
Strategies in this category relate to minimizing emissions and ensuring a project is resilient. Broad credit categories include emissions and resilience. Table 5 lists the credits in this category and their intents. Four strategies in the Climate and Risk category that relate to concrete bridges are explained in more detail in the following sections.
Resilience
Credits CR2.1, CR2.3, and CR2.4 relate to the ability of a structure to withstand, and continue to function to some degree, after a natural or man-made disaster. The metric for each of these credits is:
- CR2.1 Assess climate threat: prepare a plan that is a “summary of steps taken to prepare for climate variation and natural hazards.”
- CR2.3 Prepar e for long-term adaptability: “the degree to which the project has been designed for long-term resilience and adaptation.”
- CR2.4 Prepare for short-term hazards: “steps taken to improve protection measures beyond existing regulations.”
Concrete bridges can contribute to these three credits because concrete structures are resistant to tornados, hurricanes, wind, floods, and earthquakes. Concrete can be economically designed to resist tornadoes, hurricanes, and wind.
In general, concrete is not damaged by water; concrete that does not dry out continues to gain strength in the presence of moisture. Concrete submerged in water only absorbs very small amounts of water even over long periods of time, and typically this water does not damage the concrete.
Concrete structures can be designed to be resistant to earthquakes. Appropriately designed concrete systems have a proven capacity to withstand major earthquakes.
CR2.5 Manage heat islands effects
The metric for this credit is “[maximize] surfaces with a high solar reflectance index (SRI) to reduce localized heat accumulation and manage microclimates.” Concrete without added pigment can meet the high SRI value (29) required in this credit. Concrete bridges provide reflective surfaces that minimize the urban heat island effect and contribute to this credit. Urban heat islands are primarily attributed to horizontal surfaces, such as roads, decks, and walkways, which absorb solar radiation. Two methods of mitigating heat islands are providing shade and increasing albedo. Using materials with higher albedos (solar reflectance values), such as concrete, will reduce the heat island effect, save energy, and improve air quality.
Application
Project teams use the assessment tools provided by the Envision system to evaluate the community, environmental, and economic benefits of projects. Currently two tools are available, with two new tools projected for release after 2012. The available tools include:
Stage 1—Self-assessment checklist: this tool can be used for educational purposes or to track project progress related to sustainability.
Stage 2—Third-party, objective rating verification: in this scenario, the project team’s assessment is validated by an independent, third-party verifier. This allows for public recognition of the project. Using this tool, projects can earn points in 60 potential credits within the five credit categories.