New research argues that focusing on the remaining service interval is a more effective management strategy than fixing the worst first or threshold-driven approaches.

The public has high expectations of transportation infrastructure. Namely, pavements and bridges should be in good repair and provide consistent, high-quality service. To meet these goals, the State and local departments of transportation charged with managing these assets work to achieve and sustain a desired state of good repair over the life cycle of the assets at minimum practicable cost.

Through pavement management systems, DOTs employ a strategic and systematic process that focuses on managing each asset over its life cycle. That process typically involves a structured sequence of maintenance, preservation, rehabilitation, and reconstruction actions.

The challenge is making the best use of limited agency resources, while providing an optimum level of service to road users. Accomplishing that goal requires monitoring the condition of the pavement network and forecasting its performance in order to plan effectively for future construction actions. Predicting when to apply treatments to achieve and sustain a desired level of service at the minimum practicable life-cycle cost is critical to managing pavements. In essence, knowing--or being able to estimate--the future condition of pavement sections is the rational basis for making informed decisions regarding pavement infrastructure.

However, multiple ambiguities are associated with the commonly used terminology--“remaining service life”--from how to define it to how different agencies interpret, apply, and exchange data on pavement conditions. For example, does the phrase refer to the time from the present to when the pavement is expected to fail--meaning it needs major, costly work--or from the present to when it reaches an unacceptable level of service requiring some intervention, which could be less extensive and less costly? As a result, agencies have tended to focus on dealing with the worst, more costly problems first, rather than using an approach based on the lowest life-cycle cost for managing their assets.

Seeking to eliminate the ambiguities inherent in the existing terminology, and ultimately reduce the costs of maintaining the Nation’s transportation assets, researchers working with the Federal Highway Administration are exploring an alternative terminology based on the concept of remaining service interval. The key difference between the two concepts is that while remaining service life computes the time until a pavement reaches a predefined terminal condition, remaining service interval computes the time until any treatment is applied to achieve and sustain a desired level of service over the life cycle of the assets at the minimum practicable cost.

Here’s what you need to know about the remaining service interval concept and how it can improve the practice of managing pavements.

What’s Wrong with Remaining Service Life?

Engineers typically define the term “remaining service” as the period over which a pavement section adequately performs its desired function or performs to a desired level of service. The phrase “remaining service life” often refers to the time from the present to when a pavement reaches an unacceptable condition, requiring a construction intervention. Although predicting the time until a treatment should be applied is a critical component at all levels of decisionmaking, the current terminology poses a number of challenges with regard to interpreting and using relevant data properly, as well as exchanging information among agencies on pavement condition and performance.

“Because pavements are repairable systems, use of the word ‘life’ is an improper concept, given that pavements do not ‘die,’” says Gary Elkins, senior associate engineer with Amec Foster Wheeler. “Correctable component failures do not define the system life.”

Another common definition of remaining service life is the time until the next rehabilitation or reconstruction action. But rehabilitation and reconstruction are two different actions in terms of the condition of the pavement at the time of construction, and their associated costs vary dramatically. Attempting to interpret estimates of remaining service life from mixed rehabilitation and reconstruction segments provides little information to decisionmakers. Also, the timing of future rehabilitation or reconstruction will depend on what lower level treatments are applied during the rehabilitation period.

An unintended consequence of using current remaining service life terminology is that it may promote the more costly “worst-first” approaches to correcting pavement deficiencies, where the pavement is allowed to deteriorate to poor condition or its “threshold limit” before taking steps to preserve or rehabilitate it. By expressing pavement condition in terms of remaining service life, decisionmakers and laymen alike expect pavements in the worst condition to be treated first. This is not ideal for managing pavements, as it tends to cost the most and results in an overall pavement condition that is inferior to that achievable through other approaches.

Defining Remaining Service Interval

Beginning in 2008, FHWA spearheaded research to develop the concept of remaining service interval under contract DTFH61-08-D-00033-T-09001, “Definition and Determination of Remaining Service and Structural Life.” The results of that research are reported in Reformulated Pavement Remaining Service Life Framework (FHWA-HRT-13-038) and Pavement Remaining Service Interval Implementation Guidelines (FHWA-HRT-13-050).

Remaining service interval does not simply consider the end of life as promulgated by the remaining service life philosophy, but instead takes into account the complete spectrum of maintenance and rehabilitation activity applied to the pavement system.

The concept of remaining service interval is based on the idea that a pavement’s maintenance and rehabilitation requirements cannot be defined by a single value representing the end of its life. Instead, pavements should be described based on intervals used to communicate the amount of time before a treatment is required to provide an acceptable or above-acceptable level of service at the lowest practicable life-cycle cost. Implicit in this change in terminology is the idea that describing a pavement using service intervals more closely reflects how pavements are maintained. That is, not all pavements are allowed to reach terminal serviceability. Also implicit in this change in the terminology is that a given pavement can be described using a string of numbers that represents an optimal treatment sequence and timing.

An Example of the Remaining Service Interval Concept

Applying the Remaining Service Interval at the Project Level

The remaining service interval considers life-cycle costs in proposing a structured sequence of actions to maintain, preserve, repair, rehabilitate, and replace pavements to provide needed functions safely and reliably over the life cycle of the asset at minimum practicable cost. Further, remaining service interval has the ability to unify the outcome of different management approaches for determining needs by focusing on when and what treatments are needed, as well as the service interruption created. This approach to managing pavements also can enhance communication, because the remaining service interval provides details of the sequenced actions needed to manage the assets, as opposed to assigning a single ambiguous term to the pavements.

Implications for Asset Management

The two most recent transportation funding bills, the Moving Ahead for Progress in the 21st Century Act (MAP-21), passed in July 2012, and the subsequent Fixing America’s Surface Transportation (FAST) Act, passed in December 2015, helped set the stage for the remaining service interval concept by requiring performance management and a State asset management plan for the National Highway System to improve or preserve the condition of the assets and the performance of the system, in accordance with section 1106(a) of MAP-21, codified as 23 U.S.C. 119. MAP-21 requires State DOTs to develop processes to manage their pavements and bridges for their whole life. Now, the remaining service interval concept is poised to help DOTs manage their assets based on the optimum timing to place a treatment, rather than waiting for a given threshold.

MAP-21 defined asset management under 23 U.S.C. 101 as a “strategic and systematic process of operating, maintaining, and improving physical assets, with a focus on both engineering and economic analysis based upon quality information, to identify a structured sequence of maintenance, preservation, repair, rehabilitation, and replacement actions that will achieve and sustain a desired state of good repair over the life cycle of the assets at minimum practicable cost.”

Asset management offers a coordinated approach over entire life cycles. In following an asset management approach, agencies use data, economic analysis, performance measures, and performance-based goals to make decisions. Specifically, those decisions consider the most effective combination of maintenance and rehabilitation actions while minimizing reconstruction.

The remaining service interval concept is framed upon the same principle: identifying an optimized sequence of construction actions through engineering and economic analyses that minimizes the life-cycle cost while providing acceptable or above-acceptable level of service to the users. The concept quantifies future needs and liabilities, and it can help agencies as they move away from dealing with the worst first and toward an approach based on the lowest life-cycle cost for managing their pavements. Further, it can help asset managers communicate this information to agency leaders and stakeholders. As DOTs develop performance- and risk-based asset management plans, set targets, and measure their performance, as required by MAP-21 and the FAST Act, using the remaining service interval can contribute to the conscientious application of the same principles and processes.

According to Jim Mack, director of market development at building materials company CEMEX, the remaining service interval concept “provides insight on the performance requirements [that is, the time extension until the next activity] that a pavement treatment must meet at the project level so that there is never too much of the network needing repair at any given time. It allows agencies to develop and compare different ‘programs of projects’ at the project level--and see their impact at the network level--so that better programming decisions can be made.”

Further, he adds, “Every pavement network will have pavements at different ages and [in] different conditions, and the right fix depends on both the condition of the pavement and how the different potential treatments will impact the network. As such, some pavements are going to have to be preserved and maintained in order to stay in their current condition, while others will need a more indepth rehabilitation.

“By selectively matching individual pavement section conditions with the performance requirements dictated by the [remaining service interval], and using ... the mix of... treatments available to repair the pavements, agencies can use their limited financial resources to spread the amount of work out among the different categories of maintenance, preservation, rehabilitation, and construction treatments in order to determine the best long-term strategy to maintain the system in a state of good repair.”

Validation at the Project Level

To validate the concept, the FHWA researchers used remaining service interval to assess real-world problems at the project, network, and strategic levels. FHWA project DTFH61-13-C-00016, “Application and Validation of Remaining Service Interval Framework to Pavements,” focused on this validation effort.

The FHWA researchers’ approach to validating the remaining service interval concept at the project level consisted of developing optimal treatment strategies for a given pavement section over a defined timeframe using mechanistic-empirical models. That is, the researchers developed a basic algorithm to facilitate the project-level validation that combined elements from both mechanistic (calculated pavement responses like stress, strain, and deflection and use of those responses to compute incremental damage over time) and empirical (relating cumulative damage to observed pavement distress) principles.

Analyses at the project level require more detailed data that include modeling of pavement design and performance specific to the project’s location. For this effort, the researchers used data from the Long-Term Pavement Performance (LTPP) program. To predict performance, they input project-specific data and treatment scenarios into CalME, a software program for mechanistic-empirical pavement analysis developed by the California Department of Transportation. If the performance prediction resulted in a level of service deemed acceptable or above acceptable over the analysis period, the researchers used the performance prediction and structural condition as inputs for the life-cycle cost analysis. If the level of service needs were not met, the researchers revised the treatment scenario. This approach helped the researchers identify the optimum treatment sequence and quantify potential monetary losses associated with delaying that treatment.

The validation of remaining service interval at the project level showed that regular evaluations of pavement structures provides the opportunity to identify the optimum treatment sequence that yields the lowest life-cycle cost for a given pavement section.

Validation at the Network Level

The overall goal of validating the remaining service interval at the network level was to further develop the concept using real-world data from a State’s pavement management system. The researchers selected the Maryland State Highway Administration as one of the agencies to participate in the validation process.

First, the FHWA researchers gathered the agency’s models and data, and then they recreated the optimization approach for treatment selection used in the software that Maryland employs for pavement management. Next, the researchers compared the outputs from implementation of the remaining service interval concept to the outputs that Maryland supplied. These comparisons helped ensure that the researchers had properly replicated the State’s process and that all models and costs implemented in the validation code matched the outputs supplied by the State agency. Once the researchers had validated the concept using Maryland’s approach, they based the remaining service interval optimization on minimizing total life-cycle cost over the time horizon.

The researchers then compared the results of the remaining service interval implementation to the results from Maryland’s analysis in terms of annual costs, work type, and condition metrics. This comparison showed that the remaining service interval methodology did, in fact, lead to a consistent prediction of treatment needs, performance, and costs over the analysis period.

To demonstrate the differences, the researchers obtained data from the State DOT on the pavements’ remaining service life and compared that to the timing until first treatment from the remaining service interval optimization. Remember: Remaining service life computes the time until the pavement reaches a predefined terminal condition, while the remaining service interval computes the time until any treatment is applied to achieve and sustain a desired level of service at the minimum practicable life-cycle cost. The researchers found that this analysis demonstrated that practically no relationship exists between the information on remaining service life obtained from Maryland and the time until the first pavement treatment as determined by the remaining service interval optimization for treatment selection based on lowest life-cycle cost.

“Adding the remaining service interval concept to our pavement management system could be a great help to our districts as they plan their projects,” says Geoff Hall, chief of the Pavement and Geotechnical Division of the Maryland State Highway Administration. “Our current system using remaining service life is great at communicating current conditions, but it is not effective at communicating how soon a ‘do-nothing’ section--one with large remaining service life--should be preserved. We can let them know what projects should be fixed in the next few years, but we have no way to communicate when the ‘do-nothing’ sections should be fixed, what projects may be just beyond the horizon, or what would be the best alternates if they could not get to the initial list of recommended projects.”

Hall continues, “Remaining service interval will enable us to tell the districts--for every single pavement section--the optimal time range to preserve pavements. For example, although a section with a remaining service life equal to10 is in worse condition than a section with a remaining service life equal to 25, it may be more optimal to preserve [the latter] compared to rehabilitating the former in terms of providing the lowest life-cycle costs. Remaining service interval will tell us that.”

The validation work with the Maryland State Highway Administration demonstrated the feasibility of implementing the remaining service interval approach with relatively few changes to an agency’s existing models and data. In this validation effort, only the optimization method used in the treatment selection was modified, given the state of the models provided by the DOT. Although the change in optimization method is a significant change from the perspective of network-level decisionmaking processes, the researchers did not attempt any changes to data collection, the performance models, or the criteria for selecting treatments.

Takeaways from the Study

As affirmed through this research, the concept of remaining service interval can enhance the decisionmaking process, as well as improve how maintenance and rehabilitation needs related to pavements are communicated to stakeholders at all levels. In addition, remaining service interval is directly in line with MAP-21 and the FAST Act and can help DOTs as they move away from fixing the worst pavements first to an approach based on the lowest life-cycle cost. By implementing this concept, DOTs can optimize the timing of treatments, ultimately leading to lower costs and comparable conditions for road users from year to year.

The key takeaway from the study is this: Optimal decisions about pavement management should not be predicated on condition-based threshold values for treatments. Instead, to minimize the life-cycle costs, DOTs should consider applying treatments well before pavements reach threshold conditions of deterioration. Therefore, an important step toward implementing the remaining service interval concept is the development of a procedure to determine optimal strategies for scheduling pavement maintenance and rehabilitation.

As part of implementing the remaining service interval at the agency level, the researchers recommend that DOTs reevaluate their approach to treatment selection and strategy optimization to ensure that the objective function used in the analysis adequately captures agency goals. To help DOTs move away from threshold-driven decisionmaking, future research could focus on techniques for optimization at the network level. The continuous growth in computational resources has brought optimization techniques that used to be too computationally intensive into the realm of possibility.

Beth Visintine is a senior engineer for Environment & Infrastructure at Amec Foster Wheeler. She holds B.S., M.S., and Ph.D. degrees in civil engineering from North Carolina State University and is a registered professional engineer in North Carolina.

Gonzalo R. Rada is a principal engineer for Environment & Infrastructure at Amec Foster Wheeler. He holds B.S., M.S., and Ph.D. degrees in civil engineering from the University of Maryland, CollegePark, and is a registered professional engineer in five States.

James M. Bryce is a senior consultant for Environment & Infrastructure at Amec Foster Wheeler. He holds a B.S. in civil engineering from the University of Missouri, Columbia, and an M.S. and a Ph.D. in civil engineering from Virginia Tech.

Senthil Thyagarajan is a highway research engineer for Engineering and Software Consultants, Inc. at FHWA’s Turner-Fairbank Highway Research Center in McLean, VA. He holds a Ph.D. in civil engineering from the Washington State University in Pullman, WA.

Nadarajah Sivaneswaran is a senior research civil engineer in FHWA’s Office of Infrastructure Research & Development. He holds an M.S. and a Ph.D. in civil engineering from the University of Washington, Seattle, and is a registered professional engineer in Washington.

For more information, see Pavement Remaining Service Interval (FHWA-HRT-13-039) and Application and Validation of Remaining Service Interval Framework for Pavements (FHWA-HRT-16-053) or contact Nadarajah Sivaneswaran at 202–493–3147 or nadarajah.sivaneswaran@dot.gov.