Guide for Pavement-Type Selection (2011)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

73.1 Overview This chapter presents a process for identifying feasible pavement-type alternatives for a given project by consider- ing life-cycle strategies required to achieve a desired per- formance level throughout a specific period. The process begins with identifying potential alternatives to be con- sidered. From this broad group of alternatives, unfeasible options are eliminated by applying project-specific con- straints. Strategies (including initial pavement structural design and probable M&R activities) over the life cycle of the pavement are identified for each of the remaining alternatives. 3.2 Pavement-Type Selection Committee It is suggested that agencies form a pavement-type selec- tion committee that includes representation from pave- ment design, materials, construction, and maintenance groups. The committee should provide a formal mecha- nism for seeking input from the paving industry. The pur- pose of the committee is to identify a broad range of alternatives for consideration in a systematic and unbiased manner. The following are the key responsibilities of the committee: • Developing and maintaining a list of strategies that should be considered in the pavement-type selection process. • Addressing sustainability and other nonengineering considerations. • Making the pavement-type selection on projects where mul- tiple feasible alternatives are identified, with no clear advan- tage to any of the alternatives. • Performing periodic reviews of the pavement-type selection process and recommending modifications for improvement. 3.3 Development of Potential Alternatives The pavement-type selection process should be compre- hensive and transparent. Engaging a pavement-type selection committee composed of representatives from the agency, industry, and the research community would ensure that a broad range of input was received on existing as well as inno- vative techniques. The selection process would provide a set of alternatives that should be considered. It is expected that these alternatives reflect the findings of national and state research studies, regional experience, type and size of projects, and type of traffic the pavement is expected to carry. Figure 2 presents a suggested process for the selection of potential alternatives. One of the key components of identifying alternatives is determining what does or does not work in the geographic area where the project will be constructed. An agency’s pavement management system is a good source of data for making this determination. Pavement management systems usually contain condition data that can be used to determine the performance of the various pavement designs and materials. Where pave- ments perform better than expected, this input can be fed into the LCCA. Where pavement performance is not as good as expected, an evaluation should be made to determine if the defi- ciencies can be corrected through design or material modifica- tions. This process also may identify regions or specific traffic conditions where there are differences in performance. How- ever, using these data should not preclude the consideration of designs for which the agency has little or no experience. In those cases, performance estimates can be based on the experiences of other agencies, together with the application of analysis tools such as the American Association of State Highway and Trans- portation Officials (AASHTO) Mechanistic-Empirical Pavement Design Guide, Interim Edition: A Manual of Practice (AASHTO 2008). The sustainability of the various alternatives also must be considered. C H A P T E R 3 Identification of Pavement Alternatives and Development of Pavement Life-Cycle Strategies

It is important that the selection process allows for the con- sideration of innovative approaches that may be identified by the pavement industry and other sources. The industry associ- ations generally are familiar with the designs and techniques being used under a variety of conditions and the extent of their success. Therefore, the pavement-type selection commit- tee should request input from industry associations. The perfor- mance of these innovative approaches needs to be quantified for the LCCA and M&R schedules used in the LCCA process. The third major component of the alternative identification process is a program for monitoring and evaluating the results of ongo- ing research programs at both the national and regional levels. 3.4 Identification of Alternatives for a Specific Project Within the broad group of alternatives, certain choices may be inappropriate for a specific project under considera- tion. The following factors should be considered in evaluat- ing alternatives for a specific project: • Functional class. There are three functional classifications: arterial, collector, and local roads. All highways and road- ways are grouped into one of these classes, depending on the character of the traffic (i.e., local or long distance) and the degree of vehicle access permitted. Some pavement-type alternatives may not be appropriate for specific functional classes. • Traffic level/composition. The percentage of commercial traffic and frequency of heavy load applications have a major effect on the alternatives appropriate for a specific proj- ect. Agencies may choose to establish minimum structural requirements to ensure adequate performance and service life for minor facilities where traffic is unknown. For heav- ily trafficked facilities in congested locations, the need to minimize the disruptions and hazards to traffic may dictate the selection of strategies having long initial service lives with little M&R needed, designed at a high level of reliability. • Existing pavement condition and historical condition trends. The condition of the existing pavement and its his- torical performance, as determined through manual or auto- mated distress surveys and smoothness testing, can impact the identification of alternatives for both reconstruction and rehabilitation projects. Overall condition indicator values; distress types, severities, and amounts; and ride quality mea- surements help define the structural and functional needs of 8 Figure 2. Process for determining alternatives for consideration in pavement-type selection.

the pavement. Such needs are better addressed by some alternatives than others, thus helping narrow the list of fea- sible alternatives. • Detailed evaluation of existing pavement properties. Cor- ing, nondestructive testing, and other on-site evaluations (drainage, friction) provide information on the causes, extent, and variability of pavement distress and the struc- tural and functional capacities of the existing pavement. • Roadway peripheral features. Peripheral features such as guardrails, curbs and gutters, traffic control devices, over- head clearances, and on-grade structures may play impor- tant roles in the selection of alternatives. Such features may have special bearing on rehabilitation work where grade changes are limited. For example, in some cases, recycling or reconstruction may be more desirable than an overlay. In practice, the broadest range of alternatives (including the various forms of recycling) should be considered on each proj- ect. However, certain alternatives may not be appropriate for certain classes of roads or under certain traffic conditions. In addition, there may be project features that limit the number of feasible alternatives. Decisions regarding the identifica- tion of alternatives at the project level should be documented in the pavement-selection document. This step is presented in Figure 3. 3.5 Development of Pavement Life-Cycle Strategies For each alternative identified and designed, a life-cycle strategy is developed to sustain the desired functional and structural performance level of the pavement over the chosen analysis period. The life-cycle strategy includes the construc- tion of the original pavement structure, as well as rehabilita- tion, preventive maintenance, and corrective maintenance activities. Figure 4 presents a flow chart of this step. Each alternative pavement should be designed for the same conditions of traffic level, reliability, life, and terminal per- formance thresholds that trigger rehabilitation. However, the required M&R activities, their timing, and associated costs will be different for design alternatives over the life of the pave- ment. In the long term, these differences result in different cost 9 Figure 3. Identification of pavement alternatives for a project. Next step: see Figure 4 Select alternatives from the agency pool Conduct engineering review and analysis Project-specific data Is alternative feasible for project ? Project-specific feasible alternatives Develop project-specific criteria for eliminating unfeasible alternatives Eliminate alternative YES NO Pavement-type selection committee identifies a pool of alternatives (see Figure 2)

streams for alternatives developed with similar design and performance criteria. Establishing the life-cycle model requires knowledge of the expected service lives of the original or structurally rehabili- tated structure and the sequence of expected timing and extent of future M&R treatments, as illustrated in Figure 5. Viable strategies are selected for structural rehabilitation based on the predicted terminal pavement condition in the future. A viable strategy should be feasible from an engineering and eco- nomic standpoint, address existing causes of deterioration, and mitigate the recurrence of the same distress over the analysis period. The selection of life-cycle strategies requires the collection and meticulous evaluation of a large amount of data. Such data pertain to the current project and similar past projects (e.g., in terms of pavement-structure design, costs, performance, traf- fic characteristics, climate, materials, or subgrade). Historical project data may consist of direct source data or could be man- ifest as practical-experience information held by the agency, its engineering consultants, paving contractors, or various other project stakeholders. Data should be updated and expanded as necessary to accommodate the continuous evolution of struc- tural design, materials, specifications, and construction prac- tices. As noted earlier, the suggested approach is to use data housed in an agency’s pavement management system database. If sufficient data are unavailable to incorporate new technolo- gies, the past data can be considered along with research find- ings, specific evaluation and testing results, experience in other states, and modeling tools such as the Mechanistic–Empirical Pavement Design Guide (MEPDG) (AASHTO 2008). 10 Pavement life-cycle models for alternatives Figure 4. Development of a life-cycle model for pavement alternatives. Figure 5. Example pavement life-cycle model. Initial Construction Maintenance Maintenance Maintenance t1 t2 t3 t40 Time, years Maintenance Rehabilitation 1 Rehabilitation 2

3.5.1 Service Lives of Initial Pavement and Future Rehabilitation Treatments The service life of the initial pavement structure before the first major rehabilitation and the service life of future rehabil- itation treatments are estimated for each proposed alternative. The service life until rehabilitation depends on factors such as material durability, climate condition, construction quality, and traffic loading and may be shorter or longer than the design life of the pavement (Darter and Hall 1990). Pavement service life can be estimated using various tech- niques, ranging from expert modeling using the opinions of experienced engineers to detailed performance-prediction modeling using historical pavement performance data to con- struct survival curves. The following guidelines are suggested when considering a technique for developing service-life estimates: • Historical pavement performance data documented in the agency’s pavement management system should be the first and foremost source. • Expert opinions are prone to biases and should be consid- ered only when reliable historical performance data are unavailable or are greatly limited, such as if the pavement or rehabilitation types being considered are substantially different, due to changes in traffic or use of new materials or technologies. • Experience-based estimates, in conjunction with data trends from other sources, can be used in the absence of reliable and adequate historical performance data. The following items should be taken into consideration when developing service-life estimates: • The reliability and accuracy of estimates greatly depend on the data quality and the number of data points available. • It is important to assess how closely the pavement manage- ment sections used in the analysis represent the pavement alternative under consideration. This requires grouping of pavement sections with similar characteristics as a “family” of pavements. A factorial table can be used to construct such pavement families. The grouping factors include, but are not limited to, similarities in structural design, traffic loading, functional class, climate, geographic region, pavement type, design features, design type, subgrade, and materials. For simplicity and clarity, the number of factors should be min- imized as much as possible while remaining comprehensive enough to cover the entire range of distinct characteristics. • If the design, materials, construction specifications, funding, or operating conditions of the pavement alternative are dif- ferent from those available from pavement management files, the pavement-type selection committee should discuss appropriate adjustments. The commonly used approaches in pavement management–based estimations are performance trends and survival analyses. Performance-Trend Analysis In performance-trend analysis, historical pavement con- dition data representing a given strategy are compiled and regressed over time. A threshold pavement condition level is then established that indicates the need to restore deteriorat- ing functional and/or structural capacity. On the regression curve, the time at which the pavement reaches the threshold condition level is determined. The estimated time reflects the average age for a rehabilitation need or the estimated service life of the pavement. Performance-trend analysis is essentially a four-step process: 1. Existing pavement sections with similar structural fea- tures, traffic loadings, subgrade type, and functional classes to those of the proposed alternatives are identified. The historical condition data of these sections are extracted from the pavement management system or other sources. Careful attention is given to the “representative” aspect of the selected pavement sections, and the data are screened for any factors that might have an unusual effect on pave- ment condition (e.g., design or construction issues and traffic loading). Sections that are not representative may warrant removal from the dataset. 2. Time-series performance plots are created using the con- dition data for each family of pavements. Best-fit linear or nonlinear models relating pavement condition to age are developed. To the extent possible, the time-series plots include separate trends for structural and functional indi- cators. Some data filtering usually is needed in eliminating the time-series data that reflect any significant improve- ment in pavement condition due to M&R activities. 3. Acceptable condition thresholds that serve as triggers for any major intervention should be identified. The threshold trigger values depend on factors such as the roadway type (rural or urban), functional class, size of highway network, agency’s resources, policies, and programmatic constraints. 4. Service-life estimates are made for each family of pavements based on their condition trends and threshold values. These estimates can be developed using both deterministic and probabilistic analyses. Figure 6 illustrates the estimation of functional service life for a family of pavements using smoothness data. In this exam- ple, a threshold International Roughness Index (IRI) value of 150 in./mi was used. The model trend line intersects the thresh- old IRI where the estimated service life is about 18 years. There- fore, in this example, the estimated mean for functional life 11

(ride quality) is about 18 years, and the estimated standard deviation is about 2.5 years. Survival Analysis The “survival” of a pavement section is defined as the non- occurrence of failure—or, in other words, the nonoccurrence of major rehabilitation. This technique uses historical con- struction and rehabilitation data for a family of pavements to construct a survival curve that plots the percent survival as a function of time (or traffic loadings). Using a value of 50 per- cent pavement sections surviving (or, conversely, 50 percent sections failed), an estimate of the median life (and standard deviation) for a pavement with similar features and loading conditions can be developed. Figure 7 shows an example sur- vival curve developed for a family of pavements. The estimated median service life and the standard deviation are 17.5 and 1.5 years, respectively. The preferred method of survival analysis is to develop pave- ment survival curves within each district or region and to con- firm those survival relationships periodically over time. Trying to segregate the data into too many groups, however, can result in too few roadway segments for a survival analysis to be repre- sentative of the agency’s operational policies and procedures. Another drawback to this approach is the inability to account for the benefits or improvements in pavement design, materi- als, and specifications. This drawback can be overcome by using survival analysis in conjunction with mechanistic-empirical methods to estimate pavement lives. Such a distress-dependent approach can be used to determine the effect of using materials and design features that are not represented adequately in the agency’s pavement management system database. 3.5.2 Timing and Extent of M&R Treatments The timing and extent of future rehabilitation treatments for functional improvement (e.g., thin overlays and diamond grinding), as well as future maintenance treatments (e.g., rou- tine maintenance, preventive maintenance, and minor repairs), should be estimated based on pavement management system and/or history records. The focus of maintenance costs should be on the timing and extent of preventive and major forms of maintenance. 12 IRI = 51.13e0.0521AGE R² = 0.52 N=242 0 50 100 150 200 250 300 0 5 10 15 20 25 30 R o u gh ne ss (IR I), in ./m i Age, years Threshold IRI = 150 in./mi Est. std. dev. ≈ 2.5 years Est. Service life ≈ 20.5 years Figure 6. An example of functional life estimation for a family of pavements (N  number of observations). Figure 7. An example of pavement-family survival analysis. 0% 20% 40% 60% 80% 100% 0 5 10 15 20 25 Se ct io n s Su rv iv in g Age, years Est. std dev. ≈ 1.5 years Est. median life ≈ 17.5 years 50% sections surviving