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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

 

Reformulated Pavement Remaining Service Life Framework

APPENDIX A. PAVEMENT DESIGN AND MANAGEMENT CONCEPTS

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.

PAVEMENT DESIGN BASICS

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.

This graph shows the basic concept of modern pavement  design. The x-axis shows time, and the y-axis shows pavement condition and ranges  from poor at the bottom to good at the top. The graph consists of a single  curve representing the relationship between pavement condition and time. The  curve begins near the top of the y-axis, just below good, and extends out  toward the x-axis in a concave downward manner. The curve terminates toward the  end of the x-axis just above the poor condition on the y-axis. A dashed line  extends horizontally from the y-axis and intersects the termination point of  the curve. Above the line is the phrase "Minimum Acceptable Condition." At the  intersection of the horizontal line and the curve termination point is a  vertical solid line that extends downwards but not all the way to the x-axis.  The horizontal distance from the y-axis to this vertical line is labeled  "Pavement Design Life."

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. This graph shows  the service histories of several trial pavement designs incorporating future  overlays from the 1986 American  Association of State Highway and Transportation Officials (AASHTO) Guide for  Design of Pavement Structures. The y-axis shows Serviceability Index (SI),  and the x-axis shows time or accumulated traffic. Below the x-axis is a legend  defining two different times (TA (life of design trial A) and TD  (design life desired)), which are found on the x-axis. The graph consists of  four different concave downward curves. The first two curves originate from the same point along the top of the y-axis. The first  curve is labeled "Trial A." This curve continues to the point denoted TA  on the x-axis. The y-axis value at this point is labeled "Minimum  Acceptable SI." The second curve is labeled "Trial B." This curve continues  just past the point denoted TD on the x-axis. The y-axis  value at this point is labeled "Minimum Acceptable SI," which is further right  than where the Trial A curve ends. At the point labeled TD on the  x-axis, there is a long dashed vertical line that intersects both the  horizontal Minimum Acceptable SI line and the Trial B curve. At the  intersection of the two lines, which is below the Trial B curve, there is a  black circle outlined in blue. The distance on the x-axis from the origin to  time TD is labeled "Required performance period of design life" and  is placed just below the Minimum Acceptable SI line. Two arrows extend outward  from the label—one reaching to the y-axis and the other reaching to the  vertical line at TD to signify the distance. At the intersection of the Trial A curve and the minimum acceptable SI line is  a vertical dashed line that extends to the y-value where the original blue curve began.  From this point, two additional concave downward curves originate. The first curve is depicted  by a line that is labeled "Trial A1." The second curve is labeled "Trial A2."  The Trial A2 curve extends further across the x-axis and has a smaller slope  than the Trial A1 curve. Both curves end when they reach the Minimum Acceptable  SI line.

Figure 4. Graph. Illustrated service histories of trial pavement designs incorporating future overlays

STAGED CONSTRUCTION AND PERPETUAL PAVEMENTS

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. This graph shows  the relationship between pavement condition and time. The x-axis shows time,  and the y-axis shows pavement condition and ranges from poor at the bottom to good  at the top. The graph consists of a single curve representing the relationship  between pavement condition and time. The curve begins near the top of the  y-axis and extends out toward the x-axis in a concave downward manner. There is  a horizontal dashed line extending from the y-axis just above the poor  condition on the y-axis. The area above the line is labeled "Minimum Acceptable  Condition." When the curve reaches a distance about halfway from the point it  originated, it extends upward to the height on the y-axis where the curve  began. This vertical line is labeled "Application of planned structural  overlay." At the top of the vertical line, the relationship between time and  pavement condition continues in a concave downward manner and extends toward  the x-axis but with less slope than the first curve. The curve terminates near  the end of the x-axis around the middle of the y-axis. There is a vertical dashed  line extending up from the x-axis and intersecting the horizontal dashed line  just prior to the termination of the curve. The curve and this vertical line do  not intersect. The distance from the y-axis to this vertical line is labeled "Desired  Pavement Design Life."

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. This graph shows the  relationship between pavement condition and time. The x-axis shows time, and  the y-axis shows pavement condition and ranges from poor at the bottom to good  at the top. The graph consists of a single curve representing the relationship  between pavement condition and time. The curve begins near the top of the  y-axis and extends out toward the x-axis in a concave downward manner. There is  a horizontal dashed line extending from the y-axis about one-third on the  distance from the top of the y-axis. This line is labeled "Surface Distress  Extends Through Surface Layer." At the point just before the curve reaches this  dashed line, the curve stops and proceeds upward with a vertical line to the  height on the y-axis that the curve began. From the top of the vertical line,  the relationship between time and pavement condition continues in a concave  downward manner again, extending toward the x-axis but with less slope than the  first curve. This repeats two more times in the same manner for a total of four  curves and three vertical lines. The relationship never intersects or falls  below the line labeled "Surface Distress Extends Through Surface Layer." There  are three arrows pointing to these vertical lines labeled "Remove and Replace  Surface Layer." There is a horizontal dashed line extending from the y-axis  just above the poor condition labeled "Minimum Acceptable Condition." There is  a vertical dashed line extending up from the x-axis, intersecting the  horizontal dashed line just prior to the end of the third curve and third  vertical line. The curve and this vertical line do not intersect. The distance  from the y-axis to this vertical line is labeled "50-Year Pavement Design Life."

Figure 6. Graph. Perpetual pavement design concept based on construction of a pavement where distresses occur in the pavement surface layer.

MULTIPLE DISTRESS-BASED PAVEMENT DESIGN

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 distress-based pavement design where one of the distresses  reaches a maximum threshold limit. This graph shows the hypothetical  distress-based pavement design approach consisting of pavement roughness and  two distress types. The left y-axis shows pavement roughness and ranges from smooth on the bottom to rough  on the top. The right y-axis on the right shows distress amount. The x-axis shows  time with an arrow pointing to the right. There are two times labeled on the  x-axis: T subscript D1 (time to initiation of distress type 1) and T subscript  D2 (time to initiation of distress type 2). The graph consists of three concave  curves that move in the upward direction. The first curve is labeled "Roughness"  and originates from the left y-axis just above the origin. The curve extends  until it reaches a vertical line that extends upward from the x-axis and is located  about 75 percent across the x-axis. The distance from the left y-axis and the vertical line is labeled  "Pavement Design Life." The second curve is labeled "Distress 1" and originates  from point T subscript D1 on the x-axis, which is about 50 percent across the  axis. The curve terminates when it reaches the vertical line. The third curve  is labeled "Distress 2" and originates from point T subscript D2 on the x-axis,  which is about 65 percent across the axis. The curve terminates when it reaches  the vertical line. The distress 2 curve has the steepest slope, followed by the  roughness curve and finally the distress 1 curve. There are three dashed  horizontal lines that intersect the vertical line. From top to bottom, they are  labeled "Maximum Limit Roughness," "Maximum Limit Distress 1," and "Maximum  Limit Distress 2." The roughness curve terminates between the maximum limit distress  1 and maximum limit distress 2 lines. The distress 1 curve terminates below the  maximum limit distress 2 line, and the distress 2 curve terminates at the maximum  limit distress 2 line.

Figure 7. Graph. Multiple disstress-based pavement design where one of the distresses reaches a maximum threshold limit.

REHABILITATION AND RECONSTRUCTION DESIGN

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.

This  graph shows the relationship between pavement condition and time. The y-axis shows  pavement condition and ranges from poor on the bottom to good on the top. The  x-axis shows time. The relationship between the pavement condition and time begins  near the good label on the y-axis and follows a polynomial trend starting as  concave up until it changes to concave down and then changing again to concave  up. The y-axis is divided into three segments by three horizontal lines. The  first line is about one-third from the top of the y-axis below the good label. The  area above this line is labeled "Maintenance, Repair, Restoration." The next  line is located about halfway down the y-axis. The area above this line is labeled  "Rehabilitation." Finally, the last line is located about 75 percent down the  y-axis by the poor label. The area above this line is labeled "Reconstruct."  Just past the halfway mark on the x-axis, the pavement condition is in the  rehabilitation zone and if an overlay is placed on the pavement, which would  cause an increase in the pavement condition. This increase is depicted with a  line extending upward from the original pavement condition line into the  maintenance, repair, and restoration zone. The pavement condition relationship  with time then continues in a concave down manner toward the rehabilitation  zone again. The top of the curve is labeled "Overlay." About three-fourths of the  way across the x-axis, the pavement condition is in the reconstruct zone. If  the pavement is reconstructed, this would cause an increase in the pavement  condition. This increase is depicted by a line extending upward from the  original pavement condition line into the maintenance, repair, and restoration  zone. The pavement condition relationship with time then continues in a concave  down manner towards the rehabilitation zone. Near the intersection of the  original pavement condition curve and the vertical red line is the label "Wait  & Reconstruct."

Figure 8. Graph. Three treatment zones as a function of pavement condition.

PAVEMENT ECONOMICS

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. This graph shows  the relationship between life-cycle cost and pavement structural or environment  resistive capacity. The y-axis shows life-cycle cost, and the x-axis shows pavement  structural/environment resistance capacity. The graph consists of three curves.  The first curve is labeled "Total Cost" and is located toward the top of the y-axis.  The curve begins slightly away from the y-axis in a concave down manner until it  reaches a location about 40 percent across the x-axis. It then continues  upward. Above the vertex, there is a vertical line labeled "Optimum" that does  not intersect the curve. An arrow pointing to the left of the optimum line  indicates under design, and an arrow pointing to the right of the line  indicates over design. There are two additional curves below this initial  curve. One is labeled "Construction Cost," and the other is labeled "Maintenance,  Repair, and Restoration (MR&R)." Both curves are concave up. They intersect  below the vertex of the total cost curve. The curves begin and end at the same  locations on the x-axis as the total cost curve. The MR&R curve starts  above the construction cost curve and continues downward, while the  construction cost curve begins below the MR&R curve and continues upward.

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.  This  graph illustrates how pavement deterioration can affect repair costs. The x-axis shows years and ranges from 0 to 16 in increments of 4 years. The y-axis  is divided into five evenly spaced segments that represent pavement condition  and are labeled, from top to bottom, "very good," "good," "fair," "poor," and "very  poor." There is a single curve that originates from the y-axis in the middle of  the very good zone. The curve is in the shape of a concave downward parabola.  When the curve reaches time equal to 12 years, it is in the fair zone. There is  an arrow extending horizontally from the y-axis, just above the origin of the  curve, across to the point time equals 12 years. This arrow is labeled "75  percent time." The distance from this arrow to the curve is labeled "40 percent  quality drop." At this point, there is an icon of a hand pointing. To the right  of the hand, there is a text box that reads, "Each $1.00 of renovation cost  here…." Below the curve at time equals 12 years, there is another label, "40 percent quality drop," and two black arrows extend outward from the label,  one beginning from the curve and the other to a second arrow that extends  horizontally to the right towards the curve and is labeled "17 percent time."  At this point, just before time equals 15 years, there is another hand icon  with a text box that reads, "Will cost $4 to $5 if delayed to here."

Figure 10. Graph. Concept of increasing repair cost as a function of pavement deterioration.

SUMMARY

The following list summarizes pavement design, rehabilitation, and reconstruction concepts: