Long-Term Pavement Performance (LTPP) Data Analysis Support: National Pooled Fund Study Tpf-5(013)

Chapter 9. Cost Consideration

An additional objective of the study involved evaluating the costs associated with performance in the various environments. Cost differences associated with maintaining pavements in deep frost or multiple freeze-thaw climates relative to costs in other areas (i.e., no-freeze) were of particular interest. The following excerpt was taken from the problem statement for this study:

LCCA was used to evaluate pavement costs in the various climatic settings because it produces comparable results (i.e., equivalent uniform annual costs). Comparisons were made using both deterministic and probabilistic LCCA methods. The deterministic method does not account for variation inherent in the inputs, and therefore, it does not provide information on the distribution of the resulting annualized costs. Because only mean input values are used in the deterministic approach, the results are in terms of mean values with no indication of variability. Probabilistic analysis does incorporate the variation of the inputs and provides distribution statistics of the resulting annualized costs. Realcost version 2.2 was used to conduct the probabilistic analysis. Distributions of construction cost and treatment timings were modeled as a triangle distribution, which required maximum, minimum, and most likely values as inputs. The maximum and minimum unit costs received from the PFS were used to develop the maximum and minimum distribution inputs for construction cost. The average unit costs were used as the most likely value. For the treatment timing distributions, the upper and lower 95-percent confidence intervals were used to determine the maximum and minimum inputs, respectively, while the mean prediction was used as the most likely value.

Annualized and present worth costs for maintaining pavements over a 30-year period were used in the following comparisons. Preventive maintenance activities were assumed to be consistent for all regions, and they were not included in the analysis. It should be noted that some northern SHAs have implemented the routine application of chip seals to pavement sections to mitigate freeze-thaw deterioration. Chip seals are not represented in the performance models; therefore, the contribution to the reduction in deterioration cannot be incorporated in the LCCA, and they were not included in the cost analysis. User costs were assumed to be constant, and they were not included in the analysis. The models developed in this study were used to predict pavement performance for typical roadway sections in the following five region climates:

• Deep-freeze wet (low FTC).
• Deep-freeze dry (low FTC).
• Moderate-freeze wet (high FTC).
• Moderate-freeze dry (high FTC).
• No-freeze wet region.

The performance trends for both new and overlay flexible pavements were determined for fatigue cracking, transverse cracking, rut depth, and ride. Similarly, the performance trends for rigid pavement designs were determined for longitudinal cracking, transverse cracking, faulting, and ride.

Performance trends were evaluated only for pavement aged less than 30 years, so that the inference space of the data used to develop the models was not exceeded. For the rigid pavement designs, the performance trends indicated that no specific damage category reached an action level before 30 years. Because any performance prediction beyond 30 years would greatly exceed the inference range, it was decided not to continue with the economic analysis of the rigid pavements.

Standard flexible pavement sections were developed based on the standard 1993 AASHTO Guide for Design of Pavement Structures design procedures⁽¹⁾ using the input variables contained in the questionnaire. One cost comparison was performed using this standard section for all environmental zones; therefore, the initial and rehabilitation costs were constant for all regions. Cost differences were the result of treatment timing differences because of performance differences between the regions. The resulting roadway sections were similar to the average pavement sections provided by the PFS.

To account for local adaptations used to mitigate damage associated with freezing and thawing climates, an additional cost evaluation was performed in which the initial costs of the deep- and moderate-freeze regions included additional frost-freeze material (i.e., unbound base) to obtain a pavement structure with a total depth of 915 mm (36 inches). The 915 mm (36 inches) depth was used because it represents a typical frost-free depth for many SHAs where 1 to 1.2 m (3 to 4 ft) of frost is experienced. The standard section derived from the 1993 AASHTO design guide was used for the no-freeze region. In this evaluation, variations in cost resulted from differences in treatment timing as well as in initial cost (rehabilitation treatment costs were constant).

For the flexible pavement design, the action timing for resurfacing was based on the following distress levels:

• Fatigue cracking—35 deduct points or a fatigue cracking index of 65.
• Transverse cracking—50 deduct points or a transverse cracking index of 50.
• Rut depth—12 mm (0.5 inch).
• Ride—IRI of 2.7 m/km (171.2 inch/mi).

The fatigue cracking deduct value of 35 represents about 10 to 15 percent medium severity fatigue cracking in the wheelpaths. This lower level of fatigue cracking was picked so that a 50-mm (2-inch) overlay would provide reasonable service, and a more intensive overlay design process would not need to be incorporated.

The transverse crack deduct level of 50 represents fairly extensive transverse cracking with medium severity transverse cracking occurring at about 9-m (30-ft) spacing. This is about the level where many SHAs decide to take action to resurface the pavement.^(10,27) The common treatment at this level is to place a 50-mm (2-inch) overlay. The rut depth of 12 mm (0.5 inch) is a common level to initiate some type of treatment.

The ride value of 2.7 m/km (171.2 inch/mi) represents a roughness level at which many SHAs will generally place a 50-mm (2-inch) overlay. FHWA has also established this as the maximum acceptable roughness level for primary highway facilities in the Federal aid system.⁽²⁸⁾

The timing at which the various distress categories reached the level to trigger improvement was then determined for each of the five environmental zones. Table 29 summarizes the service life at which treatment was initiated along with the distress type driving the event. The timeline includes multiple treatment events for each of the climates. Only fatigue and transverse cracking were included in the table because they were achieved before the rutting and ride levels by significant margins.

In table 29, the fatigue cracking criteria was reached before the other criteria for all environmental settings; therefore, treatment timing was based on fatigue cracking accumulation for all regions. The timing of the first treatment (using fatigue as the driving factor) was then used as a basis for determining the frequency of subsequent treatments, which were obtained from performance curves for overlay pavements.

Based on this information, a standard deterministic LCCA was performed for both the primary highway design and the interstate highway design. Figures 69 through 71 provide the cross sections for primary and interstate standard sections. These initial pavement sections were developed using the design criteria in the PFS questionnaire and the 1993 AASHTO Guide for Design of Pavement Structures.⁽¹⁾ The sections did not include the added surfacing observed in the PFS response nor the literature review. To account for the added surfacing that is used in the frost areas, a second set of roadway sections were set up that included the extra surfacing or layer of frost-free material used by many States to mitigate frost effects. A second cost analysis was performed that considered the local adaptation of additional frost-free material and used the same geometric cross section with the exception of the increased depth of base material for the deep- and moderate-freeze regions. The additional surfacing depth was based on an assumed frost depth of 1 to 1.2 m (3 to 4 ft), which would usually be met with a requirement of around 915 mm (36 inch) of total surfacing depth. The base course under the mainline of the primary section was increased to 762 mm (30 inch), while the interstate section was increased to 660 mm (26 inch) of base course for the deep- and moderate-freeze regions to provide a minimum frost-free surfacing depth of 762 mm (36 inches). These mitigated pavement sections come much closer to matching the pavement sections from the PFSs in response to the questionnaire.

Figure 69. Diagram. Primary highway cross section.

Figure 70. Diagram. Interstate highway, left section.

Figure 71. Diagram. Interstate highway, right section.

As can be seen, a typical two lane section 1.6 km (1 mi) long with shoulders was used for the primary highway section. The interstate highway system consisted of a typical fourlane divided highway 1.6 km (1 mi) in length.

Table 30 provides the treatment events used in the deterministic LCCA over the 30-year analysis period. The timing of the first overlay was based on the new construction performance curves. Subsequent events were obtained from overlay performance curves. All events were estimated from the mean predicted performance values. The values in this table are all with respect to the total pavement age. For example, the second overlay on the deep-freeze wet region takes place 26 years after initial construction and 12 years after the first overlay.

Treatment timing inputs for the probabilistic analysis can be found in table 31. The values in this table are not relative to the total pavement age. Rather, the format of Realcost was such that the performance life of each treatment (relative to the application of that treatment) was required.

Table 32 summarizes the unit cost information provided by the participating SHAs that was used to determine initial and treatment costs. For the deterministic LCCA evaluation, the average unit price for each material was used. The maximum, minimum, and mean values were used to determine the distribution of costs in the probabilistic analysis. A discount rate of 4 percent was used for all analysis. Salvage value was also included in the analysis for remaining life at the end of the analysis period.

The results from the deterministic analysis for both the standard and mitigated sections can be found in tables 33 and 34, respectively. As can be seen, differences do exist between the regions. Using the standard section for all regions resulted in the deep-freeze regions having approximately the same costs as the no-freeze regions. The moderatefreeze regions were slightly lower in costs than the other regions.

The cost differences in this analysis were due solely to changes in treatment timing because of variations in performance. The no-freeze region accumulated fatigue cracking relatively rapidly in comparison with the other regions. The addition of frost-free material to pavements in the Northern States could be contributing to this improved performance period compared to the Southern States. The frost-free material adds structural capacity to the pavement section. Increases in strength relate to smaller stresses and strains leading to slower accumulation of damage, and hence improved performance in the deep- and moderate-freeze regions.

Considering this, the mitigated sections provide more accurate cost comparisons because they include the additional costs associated with placement of a deeper unbound base course, which has contributed to extended performance. Mitigated pavements in the deep-freeze climates have the highest costs followed by the moderate-freeze regions. The no-freeze region exhibits the lowest annual cost.

While there are differences in the deterministic analysis, there is no means to compare these differences to the distribution of data. The probabilistic analysis is a vital component of LCCA because it provides distribution statistics that can be used to determine if cost differences are significant. Figures 72 and 73 provide the distribution of total present worth costs for the primary and interstate standard sections. One standard deviation was used to compute the distributions provided in the figures.

Figure 72. Distribution chart. Annualized costs for standard primary pavement sections.

Figure 73. Distribution chart. Annualized costs for standard interstate pavement sections.

All of the mean cost differences between the regions fall within one standard deviation; therefore, it cannot be concluded that significant differences exist among the different climates based on the standard roadway sections.

However, as noted above, comparisons based on the mitigated sections are more representative of the standard practice of northern SHAs. The results using these mitigated primary and interstate sections can be found in figures 74 and 75, respectively.

Figure 74. Distribution chart. Annualized costs for mitigated primary pavement sections.

Figure 75. Distribution chart. Annualized costs for mitigated interstate pavement sections.

For the mitigated pavement sections on both primary and interstate highways, the annualized cost for the no-freeze region was lower than any other region. These differences fell outside the one standard deviation range.

The mitigated pavement sections provide a more representative comparison of costs between the regions. The costs to maintain pavements in the no-freeze region are lower than the other regions. These differences do fall outside one standard deviation of the data. Using one standard deviation does not provide direct confidence intervals, but it does allow the distribution of the data to be evaluated because it relates to observed cost differences.

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