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Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
REPORT |
This report is an archived publication and may contain dated technical, contact, and link information |
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Publication Number: FHWA-HRT-13-038 Date: November 2013 |
Publication Number: FHWA-HRT-13-038 Date: November 2013 |
It is helpful to review pavement engineering design basics in order to develop a common terminology to apply to pavement life issues. In this appendix, the basic concepts of pavement design and management are reviewed to establish a framework for pavement remaining life definitions presented in this report.
Figure 3 shows the basic concept of modern pavement design. This illustration starts with the construction of a new pavement structure. The pavement design life is the predicted time it will take for the structure to reach a minimum acceptable condition value. It is important to note that all current pavement design methods base the minimum acceptable condition on functional service criteria provided by a pavement structure and not an extreme condition that represents severe structural defects or prevents vehicle passage.
Figure 3. Graph. Basic concept of modern pavement design.
The definitions of pavement life cycle become more complicated when future pavement maintenance, restoration, rehabilitation, resurfacing, and reconstruction events are introduced into the design process.
Figure 4 presents pavement design concepts from the 1986 AASHTO Guide for Design of Pavement Structures when the initial design life of an alternative pavement trial requires consideration of overlays to meet the required performance period or design life. (11) The Serviceability Index (SI) is used to measure pavement conditions. The design period or life is shown as time or accumulated traffic. In this example, the required performance period or design life exceeds the predicted life of the trial A pavement design. An overlay is required for trial A at the time its condition equals the minimum SI. The trial A1 overlay example does not satisfy the required performance period, while the trial A2 overlay is expected to exceed the required performance period. This is contrasted to the trial B pavement design that exceeds the required performance period without need for an overlay. In this case, the life of trial A is required to be extended with an overlay to achieve the desired performance period because the initial design life of trial A is less than required.
Figure 4. Graph. Illustrated service histories of trial pavement designs incorporating future overlays
The concept of staged pavement construction was introduced to reduce initial pavement construction costs with the planned application of a structural overlay early in the life of a pavement to extend its life to a desired performance period. While the thickness of the initial pavement structure is initially constructed less than required, the added thickness of the overlay is applied early enough to meet the initial pavement design life requirements. This concept is illustrated in figure 5.
Figure 5. Graph. Staged pavement construction design concept.
A perpetual pavement is defined as an asphalt pavement designed and built to last longer than 50 years without requiring major structural rehabilitation or reconstruction and needing only periodic surface renewal in response to distresses confined to the top of the pavement.(12) The basic premise is that an adequately thick asphalt pavement placed on a stable foundation will resist distresses that form at the bottom of a pavement structure and are costly to correct. By limiting distress formation to the pavement surface layer, the pavement life can be extended in perpetuity by rejuvenating the pavement surface materials and remove and replace the type of maintenance operation, as illustrated in figure 6.
Figure 6. Graph. Perpetual pavement design concept based on construction of a pavement where distresses occur in the pavement surface layer.
Pavement design methods based on the development of multiple pavement distresses, such as the MEPDG, use multiple threshold values for each distress considered.(7) Pavement design life is defined as the shortest time it takes for one of the distresses to reach a terminal threshold condition. Figure 7 illustrates a hypothetical distress-based pavement design approach consisting of pavement roughness and two distress types. In this example, the pavement design life is defined by the time that distress 2 reaches the maximum tolerable threshold. Time to distress initiation is also illustrated in this figure. The time to distress initiation or first display of a distress feature such as cracking can be an important event in the pavement life for maintenance and repair planning activities and used to define maintenance-free time periods.
Figure 7. Graph. Multiple disstress-based pavement design where one of the distresses reaches a maximum threshold limit.
Selecting appropriate corrective treatments to restore the serviceability of a pavement depends on its condition at the time of application. As a pavement deteriorates, there comes a point when less costly rehabilitation treatments should not be applied, and the pavement must be reconstructed. Maintenance, repair, and preservation treatments have a tendency to be more cost effective if applied early in the life of a pavement before distresses cause damage to a pavement structure, which requires more costly rehabilitation treatments.
Figure 8 illustrates the general concept of application of corrective pavement treatments as a function of pavement condition. Unlike the other time-history pavement condition concept figures, the shape of the curve in this figure follows distress progression incorporated into some popular PMSs. This illustration contains three treatment zones as a function of pavement condition. The maintenance, repair, and restoration zone occurs early in the pavement life when distresses have not progressed to a state where a rehabilitation option, such as a structural overlay, is required. The rehabilitation zone occurs after maintenance, repair, and restoration treatments alone are effective and extends to the point where pavement reconstruction is required. The wait and reconstruct option represents allowing a pavement structure to deteriorate beyond what is considered functionally acceptable.
LCCA is an engineering economic tool that is useful in comparing the relative merit of competing project implementation alternatives. By considering all of the costs (agency and user) incurred during the service life of an asset, this analytical process helps transportation officials select the lowest cost option or, more commonly, make tradeoff decisions. Additionally, LCCA introduces a structured methodology that accounts for the effects of agency activities on transportation users and provides a means to balance those effects with the construction, rehabilitation, and preservation needs of the system itself.(13) LCCA is the preferred approach used to determine pavement type choice and timing of preservation, rehabilitation, and reconstruction treatments.
Agency costs to construct and maintain pavement structures are, in practice, still the primary consideration in the pavement management process. Figure 9 illustrates the conceptual tradeoffs between pavement resistive capacity, construction costs, and maintenance, repair, and restoration costs that can be generated using LCCA. In this illustration, costs are expressed in total LCCs. Construction costs increase to provide a pavement structure with greater resistance to applied structural and environmental loads. For example, it costs more to build a thicker pavement structure to resist structural wheel loads and with better materials to resist environmental degradation. As the resistive capacity of the initial pavement structure increases, maintenance, repair, and restoration costs decrease. For a single pavement structure, a conceptual optimum pavement design exists at the minimum total pavement LCC. Under-designed pavements typically have a higher maintenance, rehabilitation, and repair cost ratio to construction cost than over-designed pavement structures.
Figure 9. Graph. Conceptual tradeoffs among pavement resistive capacity, construction costs, and maintenance, repair, and restoration costs.
Figure 10 illustrates the economic concept of application of corrective pavement treatments. This concept is based on pavement deterioration rates accelerating as the pavement accumulates more damage. Because of the accelerated deterioration rate, the cost of deferring a treatment also increases at an accelerated rate.(14)
Figure 10. Graph. Concept of increasing repair cost as a function of pavement deterioration.
The following list summarizes pavement design, rehabilitation, and reconstruction concepts:
Pavement design is based on providing a pavement that satisfies a functional service requirement relative to the role of the pavement structure in the transportation system.
Application of maintenance, repair, preservation, and rehabilitation treatments can be used to extend the service provided by a pavement past the initial design life. Engineers implicitly or explicitly consider these actions during initial design stages.
Some pavement design concepts are based on construction of a pavement structure so that renewal of the upper surface layer can provide a pavement structure that provides adequate serviceability as long as the renewal treatments are provided as needed.
The initial design life of distress specific pavement design methods are based on one of the distresses reaching a terminal limiting threshold.
Pavement condition thresholds are used to indicate when pavement rehabilitation treatments or reconstruction are predicted to be needed.
A theoretical optimum mix of pavement construction, maintenance, and rehabilitation alternatives exists for each pavement structure.