Reformulated Pavement Remaining Service Life Framework

CHAPTER 2. REFOCUS OF PAVEMENT REMAINING LIFE VOCABULARY

REFORMULATING RSL TERMINOLOGY

A primary objective of this report is to provide a definition of pavement remaining life that will promote consistency in the use of the terminology. Many RSL definitions are currently used in the pavement community to describe different events in the construction history of a pavement. Construction-related history best describes the use of RSL models in all levels of the pavement management decision process because the primary purpose of predicting remaining pavement service life (or life within the context of modern pavement management) is to plan for future field construction event(s), whether it be the application of maintenance, preservation, rehabilitation, reconstruction, or other treatments to correct some attribute of the pavement structure. Regardless of the name given to different treatments, these are all construction events that cost the highway agency to provide and impact facility users. Ideally, the definition of RSL should be independent of business decisions.

The major source of uncertainty in the current RSL definition is the use of the term "life" to represent multiple points in the construction history. In the pavement design context, "life" is used to represent the time until the hypothetical pavement structure reaches an unacceptable condition since the pavement designer must make assumptions on the pavement properties. In the pavement management context, after construction of the pavement structure, the as-constructed properties become more important in pavement life expectations than the assumed inputs into the original design process. A pavement structure can be thought of as a system whose components include subgrade treatments, subsurface drainage features, base layers, shoulders, bound structural load bearing layers, and surface layers. As a repairable system, the life of the system is not defined by correctable component failures.

The proposed solution to the problem is to remove the word "life" from the lexicon since it is the basis for confusion. Instead of using RSL or structural life, adopting terminology of time remaining until a defined construction treatment is required to replace the generic ill-defined RSL term. This terminology has the ability to unify the outcome of different approaches to determine needs by focusing on when and what treatments are needed and the service interruption created. Thus, RSL is replaced by RSI.

Moreover, adopting a definition related to construction treatments creates new terminology for treatments related to other factors besides pavement condition. For example, if a construction cycle is defined in terms of time until the next construction event requires lane closures, then capacity improvements, shoulder widening, utility construction, and realignment construction activities can be included in the construction event. In turn, this broadens the application of the definition in the future. In some situations, capacity issues can have more of an effect on the service provided by a pavement structure than the condition of the pavement surface. This shifts the emphasis on the life remaining in a pavement structure to the time remaining until the next planned construction lane closure is required or future type of defined construction treatment is needed.

FUNDAMENTAL PAVEMENT PLANNING ELEMENTS

The fundamental elements required to unify RSL terminology include the following:

1. Development of a high order controlled vocabulary used to define pavement construction events. The objective of this vocabulary is to uniquely define what type of predicted future construction event is need.

2. A common basis for when the future construction event is needed.

3. How future needs are determined to differentiate between different levels of business decisions.

4. The location and extent of the needed treatment.

The logic of this structure is based on separating the definitions of what future construction event is needed from how the need is determined. Although project-level prediction methods require more intensive engineering data than network-level models, they are used for the same purpose in determining similar types of future treatment needs.

Another need for a fundamental RSL model is to incorporate the service interruption concept. Drawing from the concept of a repairable system used outside of the pavement industry, incorporating service interruption into the model and quantifying the effect on user experience is critical. Thus, some form of user experience (i.e., analogous to functional condition percepts) should be a part of a fundamental RSL definition. In addition, there is the need to model performance from an engineering standpoint, and this primarily involves integrating a structural component into the model. Each of these perspectives must be accounted for as part of fundamental pavement planning activities.

CONSTRUCTION EVENT TERMINOLOGY

The proposed replacement for RSL (i.e.,remaining structural life) is RSI (i.e., time until a construction treatment is required). This terminology requires three attributes: time when a treatment is needed, type of construction treatment, and reason for the construction treatment.

The time, or the year, when a treatment is needed is specified as this establishes the funding needs by budget planning years. This is meant to replace prediction models based on traffic applications. Traffic application rates used in the modeling process need to be converted to a time basis. Converting traffic application rates to a time basis is a complex process based on consideration of the design lane (which receives the most truck loadings), multilane facilities, damaged lanes (which are the lanes in worst condition), and other local factors that influence pavement damage from vehicle and environmental effects.

Consistent terminology is needed to describe the construction treatments, promote database integration, and increase the level of aggregation at local, district, State, and national levels. The following examples highlight construction event definitions based on the expanded paradigm of common pavement improvements included in many modern PMSs:

• Crack sealing: Application of sealants in surface cracks.

• Joint sealing: Application of sealants in preformed joints.

• Surface treatment: Application of a layer of material of intended uniform thickness less than 0.5 inches (12 mm).

• Thin overlays: Application of a material layer of intended uniform thickness greater than 0.5 inches (12 mm) and less than 2 inches (50 mm) and which does not increase the thickness of the bound material layers by more than 25 percent.

• Thick overlays: Application of a material layer of intended uniform thickness greater than 2 inches (50 mm) or increases in thickness of bound pavement layers by more than 25 percent.

• Concrete pavement restoration: Application of full-/partial-depth joint repairs, slab replacement, dowel bar retrofit, or other restoration treatments.

• Grinding: Removal of portions of the surface layer of a pavement without placement of a new material layer.

• Grooving: Cutting of grooves in the surface of a pavement without application of a new material layer.

• Milling: Removal of bound portions of a pavement that is associated with placement of a new material surface layer.

• Undersealing: Injection of cementitious material underneath bound pavement layers.

• Reconstruction: Removal and replacement of all bound layers of an existing pavement.

• Addition of lanes: Construction of additional lanes to the facility designed to permit greater traffic capacity.

• Addition of tied shoulders: Removal and construction of portland cement concrete (PCC) shoulders tied to adjacent PCC pavement structures.

• Shoulder widening: Extending the width of the existing shoulder with use of similar materials.

Note that the definitions only attempt to describe what type of construction treatment is being applied to the pavement.

An indication of the reason(s) why a future construction event is predicted is needed to complete the definition since pavement improvements are based on different needs. The following examples highlight controlled terminology that can be used to explain the basis of predicted time to a threshold event:

• Roughness exceeds y International Roughness Index (IRI). The y value is the generally accepted limiting value for pavement roughness in terms of IRI.

• Cracking exceeds the limit requiring major rehabilitation.

• Cracking exceeds the limit requiring reconstruction.

• Rut depth correction requires major rehabilitation.

• Rut depth correction requires reconstruction.

• Skid resistance reaches the safety limit.

• PCI reaches threshold x. For systems based on a PCI-based index, x represents the various thresholds between rehabilitation and reconstruction.

• Present serviceability rating (PSR) reaches x. For systems still using the PSR/Present Serviceability Index (PSI) concept, x is the serviceability value considered by the agency appropriate for the route classification.

Typically, an agency will develop some sort of decision matrix to use as part of its pavement management. This matrix will associate types of pavement deficiencies requiring construction actions with types of construction best suited to correct them. For example, if the roughness exceeds the IRI threshold, then a typical approach would be placing an overlay to correct the roughness.