Pavement Life Cycle
All pavement will deteriorate over time. The pavement life cycle includes how pavements are built, how their condition changes over time, and how this process can be affected by different forms of maintenance, rehabilitation and reconstruction.
Typically, pavement deteriorates at an ever-increasing rate: at first very few distresses are present and the pavement stays in relatively good condition, but as it ages more distresses develop with each distress making it easier for subsequent distresses to develop. For instance, once a substantial crack occurs it is then easier for water to (1) infiltrate the HMA layer and (2) penetrate and weaken the subgrade.
Maintenance and rehabilitation are the two principal treatments used to extend pavement life. These treatments will (1) immediately increase the pavement condition and (2) affect the future rate of deterioration. In general, maintenance can slow the rate of deterioration by correcting small pavement defects before they can worsen and contribute to further defects. Beyond a certain point, however, defects become too large for correction by mere maintenance. At this point, rehabilitation can be used to effect a wholesale correction of a large number of relatively severe defects, which provides a step increase in pavement condition. Figure 1 illustrates this concept.
Life Cycle Cost Analysis (LCCA)
All new construction, reconstruction, rehabilitation and maintenance projects should employ some level of economic evaluation to determine the most cost effective method and timing. For instance, a rehabilitation that involves placing an SMA wearing course may cost as much as 20 percent more than a typical dense-graded HMA wearing course. However, the SMA wearing course may provide up to 40 percent additional pavement life based on its rut-resistant nature.
Life cycle cost analysis is typically done over a fixed period – often 20, 40 or even 50 years. The costs normally considered are:
- Initial construction costs.
- Maintenance costs. Maintenance costs are those costs associated with maintaining the pavement surface at some acceptable level.
- Rehabilitation costs. These costs represent the costs associated with each rehabilitation alternative (typically they are overlay costs).
- Salvage value. The salvage value is the relative value of the pavement at the end of the analysis period. The value of any generated RAP when replacing a deteriorated pavement should be included in the salvage value.
- User costs. User costs are those costs that are accrued by the user of the facility during the construction, maintenance and/or rehabilitation and everyday use of a roadway section. These costs include vehicle operating costs (tires, gas, oil, etc.) and user delay costs (costs associated with slow downs due to construction and maintenance activities and denial-of-use) (Paterson, 1986). User delay costs are the most difficult and most controversial life cycle cost to accurately calculate because they involve assigning a dollar value to individuals’ delay time and determining future traffic conditions.
These costs are usually summarized over time by discounting all costs that occur at different times using the present worth method to account for the time value of money. The comparative costs may be shown as either total present worth or an annualized cost. LCCA, the availability of funds, project specific and environmental conditions or constraints, project constructability and the ability of each alternative to serve the anticipated traffic volumes should all be used in the decision process for selecting the most appropriate alternative.
For more information concerning life cycle cost analysis, refer to FHWA report FHWA-SA-98-079, Life-Cycle Cost Analysis in Pavement Design (written by Walls and Smith).
Pavement Management Systems
A pavement management system is a set of tools or methods that assist decision makers in determining cost effective strategies for maintaining, upgrading and operating a network of pavements. Pavement management systems (PMS) can be used to determine the most appropriate time to rehabilitate pavement, what the most cost-effective method is, and how many dollars it will take to maintain a roadway system at a desirable condition level (WSDOT, 1994). There are numerous different PMS from which to choose, each one with its own level of complexity. For a small town or rural county a simple system based on visual inspection and maintained in a Microsoft Excel or Access database may be more than sufficient. For a state road network a more complex PMS is usually appropriate.
- International Roughness Index: Relationship to Other Measures of Roughness and Riding Quality. Transportation Research Record 1084. Transportation Research Board, National Research Council, Washington, D.C. pp. 49-59.↵
- Washington State Department of Transportation (WSDOT). (December 1994). A Guide for Local Agency Pavement Managers. The Northwest Technology Transfer Center, Washington State Department of Transportation. Olympia, WA.↵