Asset Management

Pavement Health Track Remaining Service Life (RSL) Forecasting Models, Technical Information

Overview

A cornerstone of highway pavement management is to ensure that a given network of pavements provides a minimum level of service to the user public at all times. One way of ensuring that pavements perform adequately is to (1) monitor their condition using performance measures/indicators and (2) forecast future pavement performance with reasonable accuracy so as to be able to plan future maintenance and rehabilitation needs cost effectively. Knowing the current condition and estimating the future pavement condition allows managers to answer the following:

• What is the current condition of my pavement network?
• How does the current condition compare to the required level of service/performance?
• Which individual pavement sections are critical to sustained performance?
• What are my best "operations and maintenanc" and "capital improvement” investment strategies to ensure sustained performance over the short to medium time horizon?
• What is my best long-term funding strategy to ensure sustained performance?

The main performance indicator used in answering the questions posed above is pavement remaining service life (RSL).

Service life of a new pavement is defined as the period over which a pavement section adequately performs its desired function or performs to a desired level of service. For an existing pavement, RSL is simply the amount of service life left.

Pavement RSL has been in use, directly or indirectly, for many years, and it provides engineers and managers with an objective assessment of how many more years (under predefined conditions of traffic, environments, terminal level of service, and other factors) a given highway section or network will continue to be in existence.

Approaches to Determining Pavement RSL

Traditional Approach

Traditionally pavement condition has been defined based on only the structural and functional condition of the pavement—the period of time under specified site conditions during which a pavement’s structural or functional condition is expected to remain within stated limits, provided that appropriate routine and preventive maintenance are carried out. Examples of stated design and site conditions include:

• Demands actually placed on the pavement while in use (e.g., traffic applications)
• Environmental conditions (e.g., harshness of the environment [freeze-thaw or wet-dry])
• Planned routine, preventive, and corrective maintenance and other preservation activities
• End condition level of the pavement.

Figure 1 shows an example of how future pavement performance is forecast and used to estimate pavement RSL. Note that RSL could be significantly different if traffic applications and terminal performance are varied. The ability to forecast future pavement condition both at the project and network levels is the bedrock to a successful determination of pavement RSL. For this traditional definition of pavement RSL, pavement condition is characterized using an overall pavement condition index such as pavement serviceability rating (PSR) or smoothness.

Figure 1. Example of pavement RSL
[based on smoothness (International Roughness Index, or IRI) loss only]

PHT Tool Approach

Current practice is to apply a multi-condition approach for determining pavement RSL. Several publications, including the American Association of State Highway and Transportation Officials (AASHTO) Asset Management Guide and National Cooperative Highway Research Program (NCHRP) Report 551: Performance Measures and Targets for Transportation Asset Management, have described performance measures that can be used in characterizing pavement condition. Commonly used pavement performance measures/indicators used to characterize pavement structural and functional condition are presented in Table 1.

Table 1. Multi-Condition Approach for Determining Pavement RSL

Pavement Performance Measure	Estimate of Pavement RSL
Structural Condition Period until pavement reaches renewal terminal condition level	Time until pavement structural condition deteriorates to renewal intervention level (excessive alligator cracking, transverse cracking, rutting, and faulting)
Functional Condition Period until the pavement no longer provides the required level of service	Time until pavement smoothness deteriorates to renewal intervention level (excessive roughness)

With these performance measures, pavement RSL can be determined to satisfy varying objectives [e.g., remaining functional life, remaining structural life, remaining service life (overall), etc].

Using multiple performance measures, RSL is determined first for each individual pavement performance measure as follows:

RSL_PMX = AGE_TD - AGE_NOW   (1)

Where

RSL_PMX = RSL based on performance measure X
AGE_TD = forecasted pavement age at threshold value of performance measure X
AGE_NOW = current pavement age.

The RSL of a newly constructed pavement is equal to the pavement design life, and any pavement section with a condition worse than the threshold has an RSL of 0.

The PHT Tool allows for overall pavement RSL for a given pavement section to be determined using two methods presented below:

Method 1

Overall pavement section RSL is basically the minimum of all the individually computed RSLs for each performance measure considered. It is defined mathematically as follows:

Overall RSL = Min (RSL_PM1, RSL_PM2, RSL_PM3, …… RSL_PMN)    (2)

Method 2

Overall pavement section RSL is basically the weighted average of all the individually computed RSLs for each performance measure considered. It is defined mathematically as follows:

Where

Wi = weight assigned to a given performance measure i
RSL_PMi = RSL computed for given performance measure i.