Initial Service Life Determination in LCCA

Highway agencies often use life cycle cost analysis (LCCA) to select the most cost-effective alternative when planning new construction or reconstruction of a roadway. A key input in an LCCA is the service life, also known as performance period, of the pavement alternatives. Most agencies use different service lives for initial construction and rehabilitation activities. The initial service life represents the average number of years for a newly constructed or reconstructed pavement to reach the agency’s threshold for rehabilitation. The rehabilitation performance period is the length of time for a rehabilitated pavement to reach the agency’s rehabilitation threshold.

The initial performance periods can be significantly different for competing pavement alternatives. The timing of future maintenance and rehabilitation activities have an effect on the life cycle cost of each pavement alternative. Therefore, an important question for agencies conducting LCCA is: what is the actual initial service life for each pavement type? The National Asphalt Pavement Association and the State Asphalt Pavement Associations recently sponsored research at NCAT to answer this question. The objectives for this study included: (1) reviewing the methods that DOTs currently use to set initial service lives for both asphalt and concrete pavements, (2) determining actual service lives based on the age of the pavements at first rehabilitation using historical data, and (3) determining the pavement ride quality at the time of first rehabilitation.

A literature search and a survey of state DOTs were conducted to gather information about pavement service lives and rehabilitation activities considered in LCCA for both asphalt concrete (AC) and portland cement concrete (PCC) pavements. Long-term Pavement Performance (LTPP) program data were analyzed to determine the actual timing of first rehabilitation of AC and PCC pavements and the ride quality based on International Roughness Index (IRI) at the first rehabilitation for pavement sections in the U.S. and Canada.

The initial performance periods used in LCCA by the majority of agencies ranged between 10 and 15 years for AC pavements and between 20 and 25 years for PCC pavements. A common method agencies use to determine pavement performance periods was found to be the utilization of historical data from their state pavement management system (PMS). Other methods reported included using expert opinions, engineering judgement, and the pavement design life.

State DOT practices for determining the actual timing of rehabilitations for both AC and PCC pavements are unique to each agency and often include various pavement condition indices. The individual distresses reported to be part of the indices were typically cracking, IRI, and rutting for flexible pavements, and cracking, IRI, and faulting for rigid pavements. While cracking was reported for both pavement types, cracking is not quantified in the same way across all pavement types and cannot be compared directly. Given the difference in distress types and cracking definitions for each pavement, condition indices and associated thresholds are not comparable between unlike pavement types. Therefore, actual practices and criteria for determining timing of rehabilitations may not be based on equal levels of performance.

However, IRI is a common performance measure used in most decision-making processes for determining the actual timing of rehabilitation. Some agencies have threshold values associated with IRI, but they vary widely from state to state. Therefore, a nationwide consensus among DOTs does not exist for IRI values that indicate the need for pavement rehabilitation.

Data from the LTPP General Pavement Study (GPS) experiments and in the Specific Pavement Study (SPS) experiments were examined to determine the timing of the first rehabilitation events. This database includes AC and PCC pavements. Initial service lives for all sections were determined based on the dates of the first rehabilitation activity and the original construction reported in the LTPP database.

The first rehabilitation activity used by DOTs differs among pavement types, with diamond grinding being the most common for PCC pavements, and mill and inlay as the most common for AC pavements. From the analysis of the LTPP data, the average asphalt pavement age at time of first rehabilitation was found to be approximately 18 years (Table 1). This is much longer than the 10 to 15 years for the initial performance periods currently used by most DOTs for LCCAs. For concrete pavements, the LTPP data shows that the age at first rehabilitation is about 24 years (Table 1). This is consistent with the survey responses of what DOTs are using for initial performance period in LCCAs.

Table 1. Summary of Middle 90% of Pavement Ages at Time of First Rehabilitation
Table 1. Summary of Middle 90% of Pavement Ages at Time of First Rehabilitation

The mean roughness index (MRI) values (the average of the left and right wheelpath IRI measurements) measured just prior to the first rehabilitation were also examined from the LTPP database. The MRI values for the pavement sections were compared to the FHWA categories for ride quality (very good, good, fair, poor, and very poor) associated with IRI measurements (Table 2). In general, AC pavements were smoother than PCC pavements at the time of rehabilitation. AC pavements were most often rehabilitated while in good or fair ride quality, while PCC pavements were rehabilitated in fair or poor ride quality. For both AC and PCC pavements, more than 85% of the pavement sections were rehabilitated before reaching the threshold of 170 in/mile for the very poor category. Given this high percentage, an MRI of 170 in/mi is too high to be used as a first rehabilitation trigger.

Table 2. Ride Quality (IRI) Prior to Rehabilitation
Table 2. Ride Quality (IRI) Prior to Rehabilitation
*Sum is not 100% due to rounding
**FHWA Categories for Ride Quality from 1999 Status of the Nation’s Highways, Bridges, and Transit: Conditions and Performance Report. U.S. Department of Transportation, 2000

As shown in Table 3, the 95% confidence intervals of both pavement types overlap between 119 in/mi and 121 in/mi, which corresponds with the early FHWA threshold of 120 in/mi dividing fair and poor ride quality. Thus, an MRI value of 120 in/mi should be considered a functional performance threshold to determine initial performance periods of AC and PCC pavements for use in LCCA.

Table 3. Summary of MRI Values Just Prior to the First Rehabilitation by Pavement Type
Table 3. Summary of MRI Values Just Prior to the First Rehabilitation by Pavement Type

This study recommends using MRI, the performance measure common to all pavement types, for determining initial service lives for LCCA to ensure consistent levels of performance are being compared among unlike pavement types. A functional performance threshold of 120 in/mi is recommended since this value represents the separation of fair and poor ride quality. The study also found that most highway agencies currently use initial performance periods for asphalt pavements that are much lower than the actual age at time of first rehabilitation based on national LTPP data. Given the ongoing advancements in materials, pavement design, and construction, the pavement initial service lives used in LCCA should be re-examined periodically to ensure they are representative of actual performance.