Design considerations can help mitigate the toll that changing environmental factors like temperature and precipitation may take on infrastructure.

Temperature can affect pavement performance. Most drivers have driven on roads with blowups, buckling, or rutting of pavements. These pavement distresses may necessitate emergency repairs or require State highway agencies to issue advisories cautioning drivers.

Data from the Federal Highway Administration’s Long-Term Pavement Performance program show that 36 percent of the total damage to flexible pavements and 24 percent of the total damage to rigid pavements studied is caused by environmental factors.

Blowups, buckling, and rutting are just a few ways that weather and climate impact pavement structural and functional performance. Pavements are designed based on typical historic climatic conditions, reflecting local temperature ranges and precipitation levels. However, warming trends, changing precipitation patterns, and changes in the frequency and severity of extreme weather events are all projected for the coming decades. As a result, the assumption that historic climatic conditions are a good proxy for future environmental conditions may no longer hold true, threatening pavement performance, road users’ safety, and investments in transportation infrastructure.

To mitigate this threat, FHWA has created a methodology to assess the vulnerability of various transportation assets to changing temperatures and precipitation patterns, and identified some best practices to designing pavements for these changing conditions.

Climate Impacts on Pavements

Environmental factors that may impact pavement performance include temperatures, precipitation, and sea-level rise. As a result, many pavement distresses are a function of climate parameters.

Pavement blowups are not the only distress caused by environmental conditions: shoulder rotation, thermal cracking, rutting, shoving, and corrugation are all affected by temperature; corrugation, frost heave, deflection, cracking, and reduced load-bearing capacity are all affected by subsurface moisture; and pumping is affected by precipitation. Many of these distresses (for example, rutting and thermal cracking) are also impacted by other factors such as material properties and traffic.

Although engineers often assume stationarity (i.e. that the mean, variance, and other statistical properties of a time series are constant over time) as a matter of practicality, in reality climate-related environmental conditions have never been truly constant. In fact, the Intergovernmental Panel on Climate Change predicts that environmental conditions will change at an increasing rate over the coming decades. While the magnitude and speed of projected change are uncertain, even the most optimistic scenarios project substantial change to temperature, precipitation, and related parameters over the next century.

Some of the future trends most significant to pavement performance include:

• Temperature impacts such as general increase in temperature, higher extreme temperatures, increase in the frequency and duration of extreme temperatures, and fewer freezing days.
• Precipitation impacts such as changes in average annual precipitation, with some regions seeing more precipitation while others see less, generally wetter winters and drier summers, and increased precipitation intensity.
• Sea-level rise including flooding and rising ground water, and saltwater encroachment.

Climate parameters have other impacts on pavement performance such as pavement safety and load carrying limits. For example, many States that have long, cold winters take advantage of the additional strength of pavements under frozen conditions by increasing the allowable axle load limits during wintertime pavement subgrade hardening. Then, during the spring thaw period when the pavement structure and subgrade (as well as the layer interfaces) may be subjected to significant amounts of melt water, which greatly weakens the load capacity of the pavements, the States put vehicle weight limits into place. Earlier spring melt will likely reduce the period of these winter weight premiums.

A case study by FHWA found that the Maine Department of Transportation’s seasonal policies for truckload restrictions may be vulnerable to anticipated warming trends. Over the coming decades, shorter freezing seasons will lead to shorter seasons for the winter weight premium, and smaller premiums as well. This may have a significant economic impact on the trucking industry in Maine, requiring either pavement strengthening measures across the network of trucking routes or replanning the freight networks.

Current Pavement Design Methods

When designing pavements–whether it is the mixture design or the structural design–pavement design and material engineers must consider traffic loads, subgrade properties, and environmental conditions. For example, in asphalt mix designs, Superpave® (SUperior PERforming Asphalt PAVEments) asphalt binder performance grades are based on expected pavement temperature extremes, while the freeze-thaw cycles are important for specifying minimum entrained air content for concrete mix designs. Further, in pavement design key environmental parameters include temperature, moisture, and frost actions. The Mechanistic-Empirical Pavement Design Guide (MEPDG) requires five weather-related parameters for the entire design life: (1 )hourly air temperature, (2) hourly precipitation, (3) hourly wind speed, (4) hourly percentage of sunshine, and (5) hourly relative humidity.

Unlike the treatment of traffic conditions, which considers future traffic loads in the design of pavements, these environmental conditions are assumed stationary and are based on historical climate data. Because of this assumption, pavements constructed today may not be realistic for future climate conditions in some cases. As a result, future pavement performance may be vulnerable to distresses ranging from thermal cracking to weakening from frost heave and thaw.

“Including climate change considerations is an important aspect for DelDOT’s [Delaware Department of Transportation] long-term planning, design, construction, and maintenance of the State’s critically important infrastructure,” says Jim Pappas, P.E., deputy director of Transportation Solutions at DelDOT. “To ensure we can be as knowledgeable as possible, the department is investing significant resources into understanding these impacts and working closely with other interested stakeholders across the State, region, and country to increase the knowledge base and adapt and implement best practices as changes occur.”

Designing for Changing Conditions

Because current pavement design and analysis tools such as the MEPDG generally use historical climate data sources, a number of new steps are required in the design process to use these tools with future climate parameters. First, practitioners need to make decisions on which and how many future climate scenarios to use to adequately bracket the range of potential futures. Then, practitioners need to translate temperature and precipitation projections into project-level parameters, such as the pavement temperatures and moisture conditions, using downscaled global climate model results (which are available online, for example, at https://gdo-dcp.ucllnl.org/downscaled_cmip_projections/dcpInterface.html and www.narccap.ucar.edu).

The empirical models used to predict pavement performance (such as ride quality, rutting, fatigue cracking, slab cracking, faulting, and punch-outs) also need to be calibrated to reflect how pavement designs and materials interact in a changing environment. Sometimes the data required and data available are mismatched. For instance, the American Association of State Highway and Transportation Officials’ AASHTOWare Pavement ME Design methodology requires hourly records of five climate inputs: (1) temperature, (2) precipitation, (3) wind speed, (4) cloud cover or percent sunshine, and (5) relative humidity. However, future climate models are not designed to provide hourly records at acceptable levels of data accuracy. Often, only daily results are available.

In addition to structural design, practitioners may need to make improvements to the foundation to ensure the pavements are more resistant to flooded conditions as well as extreme weather events.

Climate Projections in Texas

Although climate model projections are not “plug and play” with current pavement design and analysis tools, practitioners can develop workarounds frequently. For example, FHWA partnered with the Texas Department of Transportation (TxDOT) to study how to incorporate future climate into possible pavement designs for the proposed State Highway 170 (SH–170). Because SH–170 has not yet been fully designed and built, the study of these items provides an opportunity to help influence the eventual design. The study evaluated the potential impacts of projected temperature and precipitation changes on pavement performance. FHWA obtained climate projections from the U.S. Bureau of Reclamation, which provided peer-reviewed, statistically downscaled data of the World Climate Research Programme’s Coupled Model Intercomparison Project (CMIP) Phase 5. Over a 50-year analysis period, the climate projections indicated a steady increase in ambient temperature and, possibly, aridity. These changes in turn impact secondary climate variables such as relative humidity and soil moisture, and consequently could affect the performance of pavement materials and subgrade.

To study the impact of the projected temperatures and precipitation on the pavement, FHWA considered both flexible and rigid pavement designs. The future climatic conditions were used in MEPDG and other performance models to analyze the impact on the pavement performance. For example, the researchers used simplified versions of the AASHTOWare Pavement ME Design analytical models to determine the percent change in design parameters for crack width, crack spacing, and punchouts for continuously reinforced concrete pavement. For these analyses, relative humidity is a critical value needed. However, this information is not readily available from climate models, so FHWA used empirical models relating temperature and humidity at the site to develop estimates of future relative humidity based on temperature projections.

The results of the study showed both beneficial and detrimental effects on the performance of both the flexible and rigid pavement design options. The decreased precipitation increases the subgrade support and reduces the soil shrink-swell cycles, improving the pavement smoothness. However, the increased temperature increases the potential for cracking and rutting, reducing the overall pavement performance.

To address these challenges, FHWA researchers identified potential adaptation strategies: use a stiffer binder in the case of flexible pavement design and use additional steel for rigid pavement design. TxDOT has not started design and construction of the pavements; however, if it incorporates these recommendations into its design it could implement the adaptation measures gradually. The recommended materials are widely available, reasonable in cost, and already in use in some places. Because the adaptation measures do not require complex redesign or development of engineering innovations, TxDOT can avoid significant obstacles (such as a learning curve for a new technology) and the recommendations may prove economically beneficial over a longer term. Adaptation measures also may be appropriate on a larger scale because all roadways in the area may be exposed to the same climate factors.

Monitor Pavement Performance and Trends

Most predicted changes to environmental variables are projected to occur relatively slowly in relation to a typical pavement lifecycle–think changes on the scale of decades–allowing agencies to monitor trends and implement adjustments to their practices when appropriate. Pavement designers can compare current weather and climate values used in design to projected values over the life cycle of the existing or proposed pavement. Where these values differ significantly, the engineers can consider ways to update the climate-related design inputs with data reflecting future climate projections.

In some cases where trends are slow enough or pavement rehabilitation cycles quick enough, a simpler approach is to ensure that the historical data used in analyses are up to date (including data from the past decade). Similarly, another approach is to monitor key pavement performance parameters to see if they are exhibiting trends that suggest the need for revised practices. These indicators can help agencies understand if and when to modify current design and maintenance practices.

In cases where the trends differ, agencies should consider taking the next step of evaluating the vulnerability of their pavement network to future climate impacts by conducting an analysis of the network or specific projects.

Assess Vulnerability Of Pavement

State agencies can evaluate where weaknesses exist in the transportation system to improve pavement performance, optimize funding, and increase the health and longevity of the Nation’s highways. FHWA has developed several tools to help with this type of assessment.

A starting point is FHWA’s Vulnerability Assessment and Adaptation Framework (FHWA-HEP-18-020), a guide to analyzing the impacts of extreme weather and changing environmental conditions on transportation infrastructure. The framework aids agencies by providing a structured process for conducting an assessment and identifying key considerations, questions, examples, and resources for the major tasks involved. The framework is available at www.fhwa.dot.gov/environment/sustainability/resilience/adaptation_framework/index.cfm.

Transportation engineers often are unfamiliar with sources of climate data projections. To provide easy access to download and synthesize the types of data used in the case studies described previously, FHWA developed the CMIP Climate Data Processing Tool. This large Microsoft® Excel®-based tool processes raw climate projection data, downloaded from a U.S. Bureau of Reclamation data portal website. The tool’s outputs are projected temperature and precipitation changes in a local area. The tool provides a relatively quick and easy way for users to determine the potential magnitude of future changes in their area. The tool is available at https://www.fhwa.dot.gov/environment/sustainability/resilience/tools/.

Finally, the Vulnerability Assessment Scoring Tool enables users to design and structure an indicator-based vulnerability assessment. Users must input local asset data and then the tool provides a flexible framework for creating a relative vulnerability score for each asset evaluated, enabling a structured prioritization of potential vulnerabilities. This tool is available at https://www.fhwa.dot.gov/environment/sustainability/resilience/tools/.

More Resilient Pavement Design Strategies

As aging infrastructure is rebuilt or upgraded, opportunities exist for State highway agencies to plan and design infrastructure to meet future environmental conditions. FHWA’s Adaptation Decision-Making Assessment Process (ADAP) is a risk-based approach for planners, designers, or engineers to account for the increasing role of future climate impacts in transportation projects, focused on the project or engineering level. The process aids decisionmakers in determining which project alternative makes the most sense in terms of life-cycle cost, resilience, regulatory, and political settings. The framework lays out specific steps, but some situations may warrant adjustments within the general confines of the framework. The process can be tailored to meet an agency’s specific requirements. In addition to being used for designing more resilient infrastructure projects, planners, designers, and engineers can use the ADAP to assess existing assets’ sensitivity to projected climate trends.

Not all steps of the ADAP are required in all situations. The process is designed to be flexible to minimize the level of effort needed in the analysis where the consequences of asset failure are low or the cost of adapting to future climate trends is relatively small.

Adaptation strategies depend on pavement type and the project climate parameters. Core adaptation strategies for pavements to compensate for potential increases in pavement distress as a result of higher temperatures or changing precipitation patterns may include:

• Adjusting the pavement binder and mix design specifications to better match expected future environmental conditions. For example, by moving to a stiffer asphalt grade suited to higher temperatures.
• Adjusting the pavement structural design. For example, for concrete pavements, robust designs that limit moisture damage and shrinkage are a good alternative. Stabilized subbases and base materials may be a good alternative to unbound bases especially in areas where the ground water table may rise or precipitation is increasing.

Often, because of the long timeframes of these future trends and the uncertainty in the exact magnitude of future changes, phased adaptation options provide a useful way to address potential changes without incurring unnecessary upfront costs. For instance, one strategy for asphalt pavements is to design a thick asphalt layer with multiple layers of varying levels of stiffness (also known as a perpetual pavement). The surface layer, which is most severely impacted by temperature, can easily be rehabilitated when needed through micromilling and replaced with a new surface layer to accommodate changes resulting from climate impacts over time.

The Iowa Department of Transportation uses perpetual pavements as an overall strategy to reduce life-cycle cost and increase system sustainability. In situ testing on the agency’s most recent project on Iowa Highway 100 in Linn County has shown minimum strain levels and can be expected to yield nearly infinite fatigue life of the lower layers. The agency should be able to respond to surface distresses caused by environmental factors during future rehabilitation efforts. The project recently won the Perpetual Pavement Award from the Asphalt Pavement Alliance and the pavement is performing well.

In many cases, adaptation options will increase project costs by a relatively small amount. For example, the Texas and Maine case studies discussed in this article found project cost increases ranging from 2 to 13 percent over the next three decades. Although a 2 percent project cost increase may seem minimal with regard to the total project funds, for network level strategies, implementation of these strategies cost increases may be a significant financial concern.

Closing the Gaps

Current approaches to improving pavement’s resilience to future climate and weather conditions have many gaps including developments to improve pavement foundation designs to withstand flooding and other changes.

Pavement design models and national temperature-based design maps need to be updated with information that is available from existing climate models. For instance, the engineering tools used to aid in the selection of asphalt binder grade, such as the Long-Term Pavement Performance Bind (LTPPBind) 3.1 software, use nationwide mapping of design temperatures based on historical weather data. FHWA recently improved the data in this tool by incorporating the National Aeronautics and Space Administration’s Modern-Era Retrospective Analysis for Research and Applications (MERRA) data for a more complete historical climate dataset. FHWA presented the data findings and advantages to AASHTO and AASHTO plans to incorporate the MERRA data into the AASHTOWare PavementME design software tool. Further incorporating future climate parameters into the data and design methodologies would provide practitioners with the flexibility to work with a range of possibilities using both historical and forecasted data.

Nonetheless, the approaches available today can help agencies monitor trends in climate-related pavement performance, assess vulnerability to present and future changes in environmental conditions, and design more resilient pavements. Together, these strategies can help to reduce long-term costs by ensuring the resilience and durability of the Nation’s pavements.

“Preparing for the future climate and incorporating designs or treatments now that are more resilient is an important way to preserve the state of good repair of the transportation system,” says Carol Lee Roalkvam, environmental policy branch manager with the Washington State Department of Transportation’s Environmental Services Office. “Agencies can take steps now to be better prepared for future climate conditions.”

Heather Dylla, Ph.D., is a sustainable pavement engineer on FHWA’s Pavement Materials Design and Performance Team. Dylla holds a doctorate and a master’s degree in engineering science from Louisiana State University and B.S. in civil engineering from Bradley University.

Rob Hyman is an environmental protection specialist on FHWA’s Sustainable Transportation and Resilience Team. Hyman holds master’s degrees in civil and environmental engineering and also in technology and policy from the Massachusetts Institute of Technology and a bachelor’s degree in earth and planetary sciences from Harvard University.

For more information, contact Heather Dylla at heather.dylla@dot.gov, or Rob Hyman at robert.hyman@dot.gov.