Based on measurements of roughness, surface distress, skid resistance and deflection, pavements can be assigned a score that reflects their overall condition. This score, sometimes called a pavement condition rating, quantifies a pavement’s overall performance and can be used to help manage pavement networks. By carefully choosing the rating scale (called the condition index), pavement condition scores can be used to (Deighton, 1997):
- Trigger treatment. For instance, once a pavement’s condition rating reaches a certain level, it can be scheduled for maintenance or rehabilitation.
- Determine the extent and cost of repair. A pavement condition score is a numerical representation of a pavement’s overall condition and can thus be used to estimate the extent of repair work and the likely cost.
- Determine a network condition index. By combining pavement condition scores for an entire road network, a single score can be obtained that gives a general idea of the network condition as a whole.
- Allow equal comparison of different pavements. Since a pavement condition score accounts for all types of pavement performance measures it can be used to compare two or more pavements with different problems on an equal footing.
A pavement condition index is simply the scale, or series of numbers, used to describe a pavement condition. Typical pavement condition indices may be based on a scale of 0 to 5 or perhaps 0 to 100. The proper pavement condition index depends upon the objectives of whatever system is used to manage a particular pavement network (called a Pavement Management System or PMS). This section presents two pavement condition index methods.
Present Serviceability Index (PSI)
The present serviceability index (PSI) is based on the original AASHO Road Test PSR. Basically, the PSR was a ride quality rating that required a panel of observers to actually ride in an automobile over the pavement in question. Since this type of rating is not practical for large-scale pavement networks, a transition to a non-panel based system was needed.
PSI ranges from 5 (excellent) to 0 (essentially impassable), and is still used today throughout the country. It is often a good choice for a smaller, less sophisticated pavement rating system.
To transition from a PSR serviceability measure (panel developed) to a PSI serviceability measure (no panel required), a panel of raters during 1958 to 1960 rated various roads in the states of Illinois, Minnesota, and Indiana for PSR. This information was then correlated to various pavement measurements (such as slope variance (profile), cracking, etc.) to develop PSI equations. Further, the raters were asked to provide an opinion as to whether a specific pavement assessed for PSR was “acceptable” or “unacceptable” as a primary highway (see Figure 1). Thus, although PSI is based on the same 5-point rating system as PSR it goes beyond a simple assessment of ride quality. About one-half of the panel of raters found a PSR of 3.0 acceptable and a PSR of 2.5 unacceptable. Such information was useful in selecting a “terminal” (or failure) serviceability (PSI) design input for empirical structural design equations. It is interesting to note that the original AASHO Road Test rater opinions are based on car ride dynamics; it is unclear whether such levels are acceptable for trucks.
Pavement performance can then be defined as “The serviceability trend of a … (pavement segment) with increasing number of axle applications” (Highway Research Board, 1972). Figure 1 further demonstrates this concept.
Other Pavement Condition Rating Systems
One common method for evaluating pavements is to establish a pavement condition rating system that associates deduct (penalty) points with specific distress type, severity, and extent combinations. These points can then be summed and subtracted from some upper limit or maximum value (100 in Washington State’s case) to give an overall rating of a pavement’s structural condition. The equations that describe how to convert from severity and extent of a certain distress type to an index number, or score, vary from state to state and can be rather complex.
In order to manage a pavement network (be it for a town, city, county or state), there must be some means of comparing one pavement to the next. Thus, pavement management systems usually implement some type of pavement condition index. These usually take into account the types of pavement evaluation presented in this Module. Condition indices can be subjective or objective and can vary in complexity, however they should be relevant, reliable, affordable and appropriate (Deighton, 1998).
- Pavement Condition, vol. 3. Videotape from the dTV (Deighton Television) Library video series on pavement management system topics. Deighton Associated, Ltd. Bowmanville, Ontario.↵
- National Cooperative Highway Research Program Synthesis of Highway Practice 14: Skid Resistance. Highway Research Board, National Academy of Sciences, Washington, D.C.↵
- The Factors Underlying a Rational Design of Pavements. Proceedings. Highway Research Board, 1948.↵