When it comes to high-performing pavements that meet the ever-growing needs of the traveling public, asset owners know they can’t continue building roads the way they always have if they want to deliver projects that provide long-term quality of life benefits for all users.
For this reason, forward-thinking states and local agencies are taking it upon themselves to push the limits of pavement mix design to create roads that are not only less expensive to build, but will also last longer. Select DOTs and agencies across the country are implementing innovative design practices to help build sustainable pavements that not only keep their costs down, but also increase pavement performance.
What is a Sustainable Pavement?
There are more than 4 million miles of public roads in the United States. In 2017, over 3 trillion vehicle miles traveled (VMT) were logged over those roadways, consuming more than 169 billion gallons of fuel in the process. In 2016 alone, state and local governments spent $175 billion, or 6 percent of direct general spending, on highways and roads. Highways and roads were the sixth-largest source of direct general spending at the state and local level in 2016 and have been since 1996.
As we know, pavement design and construction are an integral part of this roadway network as they provide a smooth and durable all-weather traveling surface that benefits a range of vehicles and users. Given their key role and widespread use, there is a unique opportunity to improve the sustainability of pavement structures with the potential to deliver tremendous environmental, social, performance and economic benefits.
According to the Federal Highway Administration (FHWA) a sustainable pavement therefore is one that takes in to account the entire lifecycle of the pavement which includes materials, design, construction, use, maintenance and preservation, and end of life. During each phase of life for the pavement, the industry has an opportunity to improve the sustainability of that structure.
Taking these life cycle phases of the pavement into consideration, all pavement types can be designed to be more sustainable by considering costs, environmental impacts and social needs together.
States Putting Sustainable Pavements to Work
All stakeholders in the pavement community—from owner/agencies to designers, and from material and additive suppliers to contractors and consultants—are embracing the need to adopt more sustainable practices in all aspects of their work.
Because of this push for more sustainable pavements, many states and agencies are putting these practices to work proving that pavements can be both cost-effective and long-lasting when designed properly.
New Jersey, faced with a deteriorating transportation infrastructure, decreasing transportation funding and increasing traffic conditions began to implement a performance-based asphalt mixture design system for their “special asphalt mixtures” in 2006. Each of these performance-based mixtures is required to undergo performance testing during mix design, test strip and project construction phase to ensure the produced mixture achieves the desired performance for the specific pavement structure.
Three asphalt mixture performance test methods were utilized to test the New Jersey performance-based specification:
· Asphalt Pavement Analyzer, AASHTO T 340: Determining Rutting Susceptibility of Hot-Mix Asphalt Using the Asphalt Pavement Analyzer;
· Flexural Beam Fatigue, AASHTO T 321: Determining the Fatigue Life of Compacted Hot-Mix Asphalt Subjected to Repeated Flexural Bending; and
· Overlay Tester, Tex-248-F: Test Method for the Overlay Test.
Field performance data since then indicates that all mixtures performed exceptionally well, and in some cases performed better than conventional NJDOT asphalt mixtures. As the performance-based mixtures have become more widely accepted and the methodology of design and production becomes more efficient, NJDOT plans to implement some form of performance-based specifications for all asphalt mixtures.
In 2016, the City of Chicago started to require the use of more warm mix asphalt (WMA) in their pavement design and construction projects between November 1 and May 1 each year. The use of WMA production methods use temperatures 30 to 120°F lower than traditional hot mix asphalt (HMA). Because less energy is needed to heat the asphalt mix, less fuel is needed to produce WMA. Fuel consumption during WMA manufacturing is typically reduced by 20 percent.
The standard specification from the City of Chicago indicated that contractors shall perform a verification of their existing approved mix design with the warm mix additive incorporated at the manufacturers recommended dosage.
The city says verification testing, at optimum AC, shall be comprised of one Gmb and one Gmm sample. The voids shall be within 0.5% of the design value. A Hamburg test (AASHTO T 324) will be required and shall be in accordance with the “HMA Mix Design Requirements (D-1)” Special Provision.
In addition to lowering costs using WMA, the Illinois Tollway has also been serving as a testbed for engineers and industry leaders throughout its current 15-year, $14 billion Move Illinois Capital Program. Illinois Tollway staff and leadership have embraced the philosophy that innovations can lead to cost savings and environmental stewardship while providing equal or improved pavement performance.
With no federal or state funding, and resources independent from the Illinois DOT, the Illinois Tollway has leveraged its latitude for innovation by quickly taking advantage of new products and innovations aimed at testing and implementing product performance.
On multiple preservation and system-wide improvement projects in 2018, the Illinois Tollway provided contractors with a variety of choices for their asphalt mixture ingredients. Contractors had a wide range of tools to choose from including binder types, binder modifiers, rejuvenators, warm-mix asphalt technologies, increased asphalt binder replacement and a wider range of permissible aggregates.
Rejuvenators are the next item to be evaluated by the Illinois Tollway. The Tollway will be allowing the use of rejuvenators on the shoulders as long as they meet current performance standards for Hamburg and DCT, as well as a recovered asphalt binder requirement. Asphalt binders can be extracted and recovered from the mixture designs and evaluated for asphalt binder properties. This provides critical information about the actual performance grade (PG) of the asphalt mix design, which becomes even more important with the increased use of recycled materials.
Not just big players like the state of Illinois and the NJDOT are working towards innovation in their pavement design. The city of Janesville, WI, which owns and maintains around 330 miles of paved streets, is also putting sustainable practices to work. Starting at the beginning of 2020, the city has chosen to increase their RAP content as well as allow the use of additives in their mix designs.
Additives for use must be listed and approved on the AASHTO National Transportation Product Evaluation Program (NTPEP) or the Illinois Tollway Asphalt Modifier Approved Products List. Any use of additives must be listed on the submitted mix design.
The city will also test new mix designs for performance. On the first day of production using these mixes, the city says they will obtain materials for mix design verification.
Verification testing may include any of the following:
1. Asphalt Content
7. PG Grade of the Recovered Binder.
The city says they also encourage the contractor to obtain and test material on the first day of production as well.
The concept of sustainable pavements and green construction practices is not going away. As states continue to design pavements that are more innovative, they will also seek designs that will extend the pavement life, making their roads even more environmentally friendly. The asphalt industry should be embracing these changes and continually investigating technologies at each phase of the pavement lifecycle that can help reach these sustainability goals. While some states and agencies have made strides to better there pavement design and construction to reflect these goals, there is still much work to be done.
The Six Phases of a Pavement Life Cycle
The FHWA says sustainability is very much a system characteristic, and pavements represent but one small part of the transportation infrastructure system. As a result, any improvements to the sustainability characteristics of pavement system can help improve the sustainability goals of the entire network. There are six main phases that can be impacted in a pavement life cycle:
1. Pavement Materials Phase:
a. Includes all processes involved in pavement materials acquisition (e.g., mining, crude oil extraction) and processing (e.g., refining, manufacturing, mixing).
b. Plant processes (e.g., production of asphalt concrete (AC) by mixing aggregate, asphalt cement and additives; production of concrete by mixing aggregate, cementitious materials and additives) used in the materials production phase are typically included.
c. Materials production affects such sustainability factors as air/water quality, ecosystem health, human health and safety, depletion of non-renewable resources and life-cycle costs.
2. Pavement Design Phase:
a. Refers to the process of identifying the structural and functional requirements of a pavement for given site conditions (subgrade, climate, existing pavement structure, traffic loadings) and then determining the pavement structural composition and accompanying materials.
b. Included in this phase are the design processes for not only new pavement design, but also those processes associated with pavement rehabilitation (e.g., structural overlays, bonded/unbonded concrete overlays, crack-and-seat, rubblization).
c. Structural design affects such sustainability factors as performance life, durability, life-cycle costs, construction (e.g., constructability, sequencing, schedule) and materials use.
3. Pavement Construction Phase:
a. Refers to all processes and equipment associated with the construction of pavement systems.
b. Generally, construction activities are associated with initial construction as well as subsequent maintenance and rehabilitation efforts.
c. Construction activities affect such sustainability factors as air and water quality, human health and safety, durability and work zone traffic delay, as well as project costs and time.
4. Pavement Use Phase:
a. Refers to interactions with vehicle operations and the environment.
b. Several key pavement factors (e.g., roughness, viscoelastic energy dissipation, deflection, macrotexture) can have large effects on most sustainability metrics, including fuel economy, vehicle operating costs and associated greenhouse gas emissions and energy use.
c. Environmental interactions (e.g., stormwater disposition, heat capacity/conductivity, and reflectivity) can also impact other sustainability factors such as human health and safety, the urban heat island effect and radiative forcing on a global scale.
5. Pavement Maintenance and Preservation Phase:
a. Refers to actions that help slow the rate of deterioration of a pavement by identifying and addressing specific pavement deficiencies that contribute to overall deterioration.
b. Sealing, patching, seal coats, chip seals, thin overlays, in-place recycling of pavement surfaces, diamond grinding, load transfer restoration and concrete pavement repairs are typically classified as maintenance and preservation activities.
c. Maintenance and preservation impacts sustainability factors such as performance life, durability, life-cycle costs, construction (e.g., constructability, sequencing, schedule) and materials use.
6. Pavement End-of-Life Phase:
a. Refers to the final disposition and subsequent reuse, processing, or recycling of any portion of a pavement system that has reached the end of its useful life.
b. Full-depth reclamation, recycled materials including reclaimed asphalt pavement (RAP) and recycled concrete aggregate (RCA) and landfilling are all typically considered during the end-of-life phase.
c. End-of-life considerations impact sustainability factors such as waste generation and disposition, air and water quality and materials use.
NCHRP Research Report 916 & the Construction Phase
Released in early 2020, the National Cooperative Highway Research Program (NCHRP) Research Report 916: Sustainable Highway Construction Guidebook provides clear and practical information on what constitutes sustainability in the context of highway construction and how to evaluate any proposed construction practice for its sustainability potential.
From procurement to project delivery, highway construction can be scheduled and fulfilled in a sustainable way and many of their best practices come from the FHWA’s Towards Sustainable Pavement Systems: A Reference Document referenced above.
According to this research paper, pavements, their construction, their condition during use and their ultimate disposal can affect energy consumption; greenhouse gas emissions; habitat loss, fragmentation, and change; water quality; the local hydrologic cycle; air quality; mobility; access; freight; community; depletion of non-renewable resources; and economic development.
Their suggestions for sustainable construction practices for asphalt pavements include:
· Density: Improve density to increase pavement life, and/or streamline methods for measuring density in the field.
· Longitudinal Joints: Improve longitudinal joint compaction to reduce the risk of early pavement failure on longitudinal joints.
· Segregation Elimination: Eliminate the separation of coarse and fine aggregate in the paving process as this can lead to early pavement failure.
· Density Differential Elimination: Eliminate isolated cool spots in the paved mat that may be inadequately compacted and lead to early failure.
· Proper Tack Coat Application: Methods to ensure the proper amount of tack coat is applied on existing surfaces (and on surfaces between pavement layers) to ensure proper bonding.