Application and Validation of Remaining Service Interval Framework for Pavements

CHAPTER 6. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS

SUMMARY

The RSI concept was developed to provide an alternative to the long-standing and confusing RSL terminology. The RSI concept does not provide an alternative to assessing the health of the network or making decisions about how to spend available funds. It simply provides a clear terminology and a logical process that will create a consistent construction event-based terminology and understanding (i.e., types of construction events and the timing of those events within the concept of LCC and/or other prioritization approaches based on streams of future construction events). An added benefit of adopting the RSI terminology is that the methodology provides a readily available way to communicate impacts of alternate budget scenarios. The RSI concept considers the complete M&R activity of the pavement system and does not simply consider the end of life as promulgated by the RSL philosophy.

While threshold values can be used to determine the time until future construction events are needed, in the original formulation of RSI, LCC optimization was recommended to determine when preservation, rehabilitation, and reconstruction are needed. During this project, an LCC algorithm was developed that finds the optimum time for treatment subject to threshold constraints based on meeting minimum LOS. This algorithm was used during the validation effort based on the MDSHA network.

Reconstruction is one cost (with the caveat that the user costs are not included), and the resulting condition is the same (new) no matter the pretreatment condition, unlike preservation and rehabilitation treatment. The threshold for reconstruction can be considered the minimum LOS and structural condition when lower-level treatments are no longer cost effective. At this time, the RSI for any treatment should remain zero, essentially reflecting that treatment is needed. This is different than RSI numerics for preservation and rehabilitation because once either of these values reach zero, non-zero values of RSI should still exist for at least one other treatment category. As the condition of the pavement continues to deteriorate beyond the minimum LOS, the RSI remains zero, signaling that treatment is still needed. As a way to improve communication of this need, the years past due can also be used in conjunction with an RSI of zero. For instance, take two pavement sections that have both exceed the reconstruction thresholds; one section has just exceeded the threshold this year, and the other section exceeded the threshold 2 years ago. Both pavement sections have an RSI of zero for all treatments, but the second pavement section could be said to be past due by 2 years. An agency should not have many pavements with an entire string of zeros representing the RSI, and therefore, each of the cases should be treated individually as needed.

As the RSI concept evolved to represent a more ideal system based on optimizing the treatment selection considering all possible treatments and treatment timings instead of being threshold driven, it is no longer an issue to consider more than one type of construction trigger. The evolution from a change in terminology to a change in approach resulted in all construction triggers (i.e., preservation, rehabilitation, or reconstruction) and minimum LOS being considered in selecting the optimum treatments. The resulting RSI numerics represent the optimum and do not focus on one type of construction trigger.

Summary Observations from Project-Level Analyses

The following observations sumarize the most important findings and conclusions from the RSI concept validation effort performed at the project level presented in chapter 4:

• Regular pavement structural evaluation provides the opportunity to identify the optimum treatment sequence that yields the LLCC for a given pavement section.
  
  
• Use of surface cracking as a structural indicator limits the PMS to potentially reactive treatment options because it is a lagging indicator of level of structural deterioration below the pavement surface and therefore may not lead to selection of the LLCC option.
  
  
• Tensile strains can be a leading indicator to identify pavement structural deterioration before surface cracks appear. Depending on road classification, threshold tensile strains can be used to select optimal treatment and maintain the pavement in a state of good repair.
  
  
• At the network level, tensile strains are a better parameter to evaluate and track pavement structural condition. A simple and robust correlation between strains and deflection parameters can be used along with performance models to evaluate and predict future performance. For example, the same fatigue damage model used in pavement design can be used in pavement management when tensile strains are estimated from SCI, as demonstrated in this study.
  
  
• Treatment sequences evaluated using LCCA should yield pavement in a state of good repair at the end of the analysis period. Longer analysis periods should be used in the LCCA. With a shorter analysis period (such as 6 years), the LCCA could yield preservation treatments (such as resurfacing), which would satisfy the minimum LOS but lead to pavements in poor structural condition at the end of the analysis period.
  
  
• The asset value of each pavement section at the start and end of the analysis period is required for a comprehensive LCCA at the network level. The need for asset value can be substituted by an approach in which, at the end of the analysis period, the chosen optimum treatment sequence yields tensile strains that are similar to other, costlier alternatives.

Summary Observations from Network-Level Analyses

The following observations sumarize the most important findings and conclusions from the RSI concept validation effort performed at the network level presented in chapter 5:

• As part of the network-level RSI validation effort, optimal strategies (based on LLCC) were developed for a pavement network. It was found that the RSL was not related to the time until each pavement should be scheduled for work (from the optimization). In other words, the RSL may provide an indication of the current network condition, but it bears no relationship to the future maintenance needs of the pavement.
  
  
• The analysis in chapter 5 compared two objective functions, each of which was directly linked to specific objectives. In any decision analysis framework, agency goals are mapped to the decisionmaking process through the construct of the objective function. Consequently, the objective function and underlying assumptions used in the analysis have a significant impact on the long-term condition and projected yearly costs of the pavement network. Therefore, many key decisions, such as the chosen analysis period, should be reflective of the objective function used in the analysis.
  
  
• The analysis period that an agency uses to analyze lifecycle costs should be directly linked to the objective function defined by an agency. The objective function that was used for the RSI validation (defined in chapter 5) was to minimize lifecycle maintenance costs for the pavement network while also ensuring that no pavement fell below a specified threshold condition. In this case, it was found that the time horizon for the LCCA needed to be approximately 20 years longer than the desired analysis period.

CONCLUSIONS

The results from the validation efforts presented in this report support the conclusion that the RSI represents a valid approach to determining and communicating future M&R needs of a pavement instead of defining pavement life using a single number. The results in chapter 5 showed that the remaining life is essentially not related to the time until the next pavement treatment in an optimal strategy. In addition, developing optimal strategies for pavement management at the project level (chapter 4) and network level (chapter 5) represents enhanced approaches to planning pavement M&R needs.

Based on the validation results from chapters 4 and 5, it can be concluded that optimal pavement management decisions should not be predicated on condition-based threshold values for treatments. Instead, optimal pavement management strategies may include the application of treatments well before a threshold condition is reached. Therefore, an important step toward the implementation of the RSI is the development of a procedure to determine optimal strategies for pavement M&R scheduling.

RECOMMENDATIONS

As a result of the validation and application of the RSI concept efforts at the project, network, and strategic levels, the following recommendations are provided:

• Improvements to HPMS 2010+ data for data consistency and completeness for strategic level inputs are needed.
  
  
• Evaluation of simplified MEPDG models used in the PHT analysis tool is required to determine the cause of erroneous predictions and should consider both models and input data.⁽²⁴⁾
  
  
• Asset valuation is a critical input to LCCA, as demonstrated in chapter 4.The asset value cannot be obtained from the last treatment applied to the section. A comprehensive method that accounts for the pavement system as a whole should be developed.
  
  
• Use of TSDDs in structural evaluations should be explored.They can be used to overcome limitations of using FWD testing at the network level.
  
  
• Chapter 4 showed the merit of using tensile strain as a robust pavement structural condition indicator and for identifying optimum treatment sequences.It was shown that a pavement is optimally maintained when the tensile strain is within certain range.Further research is needed in identifying the optimum tensile strain range that typically changes with pavement classification.
  
  
• As part of implementing the RSI at agency levels, agencies should reevaluate their approach to treatment selection and strategy optimization to ensure that the objective function used in the analysis adequately captures agency goals.
  
  
• Additional research regarding modern network-level optimization techniques should be conducted in an effort to move agencies away from threshold-driven decisionmaking. The continuous growth of computational resources has brought optimization techniques that used to be too computationally intensive into the realm of implementability.
  
  
• Agencies should enhance their performance prediction models to demonstrate the relationship between the effectiveness of an M&R treatment and the condition of the pavement just prior to treatment. These models would provide critical information required for making more optimal decisions at the network level.