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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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| This report is an archived publication and may contain dated technical, contact, and link information |
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| Publication Number: FHWA-HRT-13-038 Date: November 2013 |
Publication Number: FHWA-HRT-13-038 Date: November 2013 |
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 binary or two-level acceptable/not acceptable or pass/fail rating scale: While this type of forced choice acceptance scale purposefully limits the range of response to identify the acceptance threshold, it also limits analysis of the results.
A five-level Likert scale: This type of scale provides a measure of the range in acceptance criteria to be considered. An example rating scale for the condition of a pavement test section is as follows:
A key feature of this type of rating scale is that it allows greater options in the analysis of the ratings. The neutral center point value is a measure of the gray area between acceptance and non-acceptance. The ratings can be converted to a three-level scale by combing the two acceptable ratings and two unacceptable ratings.
In this example, to improve the repeatability of the results, it is useful to provide the raters with a more refined definition of each category that is related to the attribute of interest. The cardinal ordering of the scale can also have an impact on the scoring. In this example, the largest value is assigned as the worst condition state, although most grading scales use the largest value to represent the best condition state.
Subjective ratings can be influenced by a wide range of factors, some of which may not be desired or can overly influence the ratings. For example, road roughness is commonly expressed using the IRI parameter computed from measured longitudinal road profiles. A method to determine levels of acceptability to riders is to have a panel rate the acceptability of a set of test sections with a range of roughness by driving over them. In this case, the following factors can influence the ratings provided by the panel:
Suspension characteristics of the vehicle used by the rating panel: People riding in vehicles with soft suspensions will feel less vibration than those riding in stiff suspension vehicles.
Vehicle speed: The suspension response of a vehicle is greatly influenced by vehicle speed. Since IRI is standardized to a constant speed, test sections with the same IRI can have different degrees of acceptance depending on how fast the vehicle is travelling.
Distress condition or appearance of the test section: While the purpose of the rating is to judge the level of pavement roughness, it is known (from the AASHO Road Test and other studies) that raters can be influenced by other factors such as the amount of cracking and visual appearance of the test section surface.(4,6)
Climate conditions during rating sessions: High winds can cause movements in the measurement vehicle that are not due to the roughness of the pavement.
Quietness of the test vehicle: The noises heard while driving can be more related to the texture characteristics of the pavement surface than the pavement roughness.
Type of pavement: The roughness producing characteristics of a jointed PCC pavement are different than those of an asphalt concrete (AC) pavement.
Alignment of the pavement test sections: Horizontal curves, cross slope changes, and vertical grades can influence the response of the rating vehicle.
Driver of the rating vehicle: The method the driver uses to maintain speed can influence the response of the rating vehicle.
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:
It can be postulated that when cracks extend completely through the bound layers of a pavement, they allow entry of surface water into the unbound layers. Accumulation of water in the unbound layers can advance the rate of pavement deterioration and cause damage that is expensive to repair. Applying a corrective treatment before the cracks reach this state can help preserve the integrity of the foundation support and extend the time until rehabilitation or reconstruction is needed.
Research has shown that hydroplaning potential of a vehicle is related to the depth of water and vehicle speed. It is possible to establish a threshold water depth for the design vehicle speed of a route. Assessing the potential water holding capacity of a rut requires consideration of more than rut depth. Factors such as shape of the rut, rut location, and pavement cross slope can be determined from the modern generation of high-speed road condition survey devices that measure the pavement's transverse profile. Computer programs can determine the actual water depth potential from these types of field measurements.
The perpetual pavement concept developed for AC pavements is based on the premise that an adequately thick asphalt pavement placed on a stable foundation will resist distresses that form at the bottom of a pavement structure and are costly to correct. By limiting distress formation to the pavement surface layer, the pavement life can be extended in perpetuity by rejuvenating the pavement surface materials in a remove and replace type of maintenance operation. The prime consideration in this type of pavement system is to plan future construction events to limit the depth of the top-down cracks. However, measuring the depth of top-down cracks from project-level field measurements is a challenge.
As pavement roughness increases, the level of dynamic load impacts on the pavement structure from heavy trucks also increases. Since mechanistic principles of pavement performance are primarily based on the magnitude and frequency of applied truck loads, in theory a limit on pavement roughness should exist where the magnitude of the applied dynamic loads accelerate the structural damage to the pavement. The algorithm created to assess the adequacy of pavement smoothness for location of weigh-in-motion scales is based on computation of the increase in the level of dynamic truck wheel loads from the same field data used to compute IRI.
Pavement structural capacity diminishes to the point where stress and strain levels of applied traffic loads accelerate pavement damage. The results of non-destructive pavement deflection testing can be used to assess changes in pavement structural properties related to resulting increases in stress/strain responses in critical pavement structural layers.
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.
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:
Conventional practice is that preservation or preventive maintenance treatments should be applied while a pavement is still in relatively good condition. In theory, pavements still in excellent condition with no defects should not require treatments. The issue becomes determining when treatments are best applied. When possible, the upper limit should be based on specific pavement defects which can be corrected to a good-as-new condition without the need for structural restoration treatments. If aggregated distress-based indices are being used, then use of expert subjective opinions can be used to determine a nominal threshold value when preservation maintenance activities can be used.
One strategy for setting threshold limits for pavement condition to determine when preservation treatments are applicable is by considering when the extent and severity of distress types have reached levels requiring corrective treatments for structural improvements and preservations treatments are no longer adequate. There is a point when the extent of the required treatment represents a structural improvement or crosses the capital improvement cost threshold. Some examples include the following:
The number of locations per mile requiring full-depth repair exceeds 10–30 percent of the surface area depending on the pavement and distress type.
Extent of faulting of jointed PCC pavement requires the combination of grinding, ultra-thin overlays, and dowel bar retro fit to correct.
The severity and extent of alligator or fatigue-related cracking in the wheel paths requires full-depth patches on more than 10 percent of the section length.
When structural and restoration treatments cost less than alternative spot repairs.
When pavement roughness reaches a level that causes a significant increase in vehicle operating costs.
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.
