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Publication Number:  FHWA-HRT-13-038    Date:  November 2013
Publication Number: FHWA-HRT-13-038
Date: November 2013


Reformulated Pavement Remaining Service Life Framework

Chapter 5


Threshold limits are used to indicate when a construction trigger reaches a condition when a corrective or preventative construction treatment is needed. This chapter presents various methods and procedures that can be used to establish threshold limits.

In this discussion, maintenance and preservation maintenance treatments do not include routine and catastrophic maintenance. The activities included in this category are planned projects that include pavements longer than 4 mi (6.4 km) that are designed to extend the time until rehabilitation and reconstruction treatments are required. Since the distinction between maintenance, preservation maintenance, and rehabilitation treatments is often dependent on funding sources, the definitions used by the funding sources for allowable treatments should be incorporated into the development of threshold limits.

Rehabilitation treatments include extensive restoration treatments and structural overlays. The general definition of structural overlays is the addition of a new material layer whose thickness is greater than 15–25 percent of the existing bound pavement layers.

A common definition of pavement reconstruction is the removal and replacement of all existing bound pavement layers. This definition includes situations where removal and replacement of the unbound base and subbase layers are also required in addition to the bound surface layers.


Subjective threshold limits are based on ratings determined by panels of judges that include laymen facility users, pavement experts, or a combination. Generally, a formal rating scale is created and used by the judges, and statistical methods are then used to interpret the ratings and establish limits.

Subjective ratings can be used to define an absolute acceptable limit or degrees of acceptability for a measured condition attribute. The following acceptance scales can be used to capture subjective panel ratings:

A field experiment should be developed to attempt to control or measure the statistical significance of cofactors that can influence subjective ratings.


Engineering considerations used to establish threshold limits are based on pavement performance mechanistic concepts or pavement-vehicle interaction factors. Examples of engineering considerations include the following:


Empirical methods are based on observations of events. The historical context of empirical methods is based on the scientific process where findings are based on experimental observations. Within the current pavement engineering nomenclature, the empirical part of the Mechanistic-Empirical Pavement Design Guide (MEPDG) was calibrating the mechanistic prediction models to field performance observations.(7) Thus, establishing threshold limits using an empirical approach is based on observations of past events, which may or may not be based on mechanistic engineering principles.

A critical aspect of empirical models is that they are most applicable to the inference space of the observations from which they were developed. Technology advancements or other changes that are outside of the inference space of the original observations can limit the applicability of existing empirical models to future events.

Examples of empirical approaches used to set threshold limits include the following:

The advantage of this type of analysis is that it does not require a thorough understanding of the mechanism being modeled. For example, one can postulate that the accident rate on a section of roadway is related to the level of friction offered by the pavement surface. One may also recognize that the accident rate on this same section of roadway is related to the speed of the vehicles traveling on that roadway. With an empirical approach, it is not necessary to fully understand all of the mechanisms associated with the accident rate, which may include psychological factors associated with the drivers. Rather, the correlation of the friction with the accident rate can assist in identifying an unacceptable level of friction on the roadway.


Construction limit thresholds can be developed from an economic analysis of construction time-series costs over a long-term period. This analysis depends on knowing or estimating how long alternative construction treatments will last based on the predicted condition of the pavement at the time of the treatment and the cost of the construction treatment. By running Monte Carlo simulations with alternative construction treatments performed at different times, a minimum worth cost can be found as a function of pavement condition. The length of the analysis time period has to be long enough so that the pavement deterioration condition factors eventually force a needed construction treatment.

Figure 2 illustrates the expected outcome of an economic analysis on the most cost effective repair strategies as a function of pavement condition for an individual pavement. Plateaus in repair costs will exist since the same construction treatment will correct a range of pavement distresses and severities. A common construction treatment can be specified for a length of roadway based on the worst condition in that segment. The upper plateau represents the cost to reconstruct the pavement after it has reached a condition state where maintenance and rehabilitation (M&R) treatments do not last long enough to be cost effective. Likewise, the rehabilitation plateau represents a common set of treatments that do not require complete removal and replacement of all bound pavement layers. The significant concept illustrated in this figure is that breakpoints should exist in the pavement condition-repair cost relationship that is expected to be unique between different pavements types, load magnitudes, environmental conditions, agency repair policies, and construction costs.

Figure 2.  Graph. Conceptual relationship between agency repair costs as a function of  pavement condition. This  graph shows the conceptual relationship between agency repair costs as a  function of pavement condition. The y-axis shows repair costs, and the x-axis shows  pavement condition with a scale from left to right ranging from good to bad. The  line starts about one-third of the way across the x-axis. It begins as a slightly  curved parabolic upward trend labeled "Preservation Maintenance." The line then  plateaus to the right, about 25 percent up the y-axis to about the midway point  of the x-axis. The line then continues upward in a slightly curved parabolic  trend again until it reaches another plateau that is about 50 percent up the y-axis and about 75 percent across the x-axis. This plateau is labeled  "Rehabilitation." The line then continues upward in a slightly curved parabolic  trend again until it reaches another plateau near the top of the y-axis and  near the end of the x-axis. This plateau is labeled "Reconstruction."

Figure 2. Graph. Conceptual relationship between agency repair costs as a function of pavement condition.

Critical factors in pavement construction time-series economic analysis include the deterioration rate of the pavement, what type of repair treatments are considered, the effect that pavement condition has on the resulting performance for each repair treatment, and costs included in the analysis. To avoid manipulation of the results from pavement life-cycle cost analysis (LCCA) due to the assumptions of the person performing the analysis, SHAs need to create a set of rules to be used in this type of analysis. To the extent possible, the rules should be based on observations from pavements under agency jurisdiction. However, repeated findings from research studies on this topic indicate that not enough field data exists to create these models with certainty. As a result, agencies may need to use engineering judgment based on available data to create the rules. Establishment of a preliminary set of rules provides a basis to evaluate and update the rule set based on experience. Part of the rules should be standard estimates of error and error distributions forms for use in stochastic/risk-based analysis.


The development of successful threshold limits may best be accomplished through a combination of the approaches previously described. Pavements are complex structures designed to serve a vast array of vehicles operated by users with differing requirements. Development of threshold limits on various measures of pavement condition to indicate the need for a corrective construction treatment must also take into consideration management agency policies, agency managed infrastructure asset-to-budget ratio, multiple funding sources, and user expectations relative to the role of the route in the local transportation system.

The recommended approach to setting threshold limits on need for corrective construction intervention is by use of a combined engineering economic approach. The objective should be to determine condition states where maintenance or preservation treatments, rehabilitation treatments, and reconstructive treatments are most cost effective.

Considerations to determine appropriate pavement state thresholds for application of maintenance or preservation treatments include the following:

One rule of thumb is that an overlay thicker than 2 inches (50 mm) is considered a structural improvement regardless of the depth of material milled from the pavement surface. Another common threshold rule used is that when the treatment cost rises to a certain level set by the agency, it is considered a capital improvement project and requires formal engineering plans and specifications. Like definitions of preservation maintenance, the definition of pavement rehabilitation is also dependent on available funding agency sources. The upper pavement condition threshold limit for rehabilitation is based on when preservation maintenance is no longer effective. Lower limit rehabilitation pavement condition thresholds are based on when reconstruction becomes the most cost effective treatment.

There are also situations when structural defects may require reconstruction to correct even though all other functional aspects of the pavement are acceptable.


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