Expected Service Life and Performance Characteristics of HMA Pavements in LTPP

Transcription

1 Expected Service Life and Performance Characteristics of HMA Pavements in LTPP

2 Expected Service Life and Performance Characteristics of HMA Pavements in LTPP Submitted to: Asphalt Pavement Alliance Prepared by: Harold L. Von Quintus, P.E. Jag Mallela Jane Jiang Applied Research Associates ERES Consultants Division 102 Northwest Drive, Suite C Round Rock, Texas 78664

3 TABLE OF CONTENTS i Page No. LIST OF FIGURES ii LIST OF TABLES iii EXECUTIVE SUMMARY iv 1. INTRODUCTION Background Study Objectives Scope of Work and Report EXPERIMENTAL FACTORIAL AND KEY FACTORS/PROPERTIES EXTRACTED FROM LTPP Factors Included in Analysis Plan Data Source and Data Availability Data Elements Extracted from LTPP Database Number of Test Sections Summary Data Availability DISTRESS USED IN SURVIVABILITY ANALYSIS GLOBAL SURVIVABILITY ANALYSIS EXPECTED SERVICE LIFE Load Related Area Fatigue Cracking Load Related Longitudinal Cracking in Wheel Path Non-Load Related Cracking Longitudinal Cracking Outside Wheel Path Non-Load Related Cracking Transverse Cracking Rutting Smoothness or Roughness Summary PAVEMENT FEATURES AND SITE FACTORS RELATED TO PERFORMANCE OF HMA PAVEMENTS Analysis of Variance Localized Survival Analysis and Comparative Studies SUMMARY OF FINDINGS AND FUTURE RECOMMENDATIONS Findings Future Recommendations REFERENCES APPENDICES A Detailed Experimental Factorial B LTPP Test Sections More Than 20 Years in Age with No to Minor Levels of Distress C LTPP Test Sections with High Levels of Distress (Extensive Levels of Distress) D Statistical Values from ANOVA for Each Performance Indicator

4 LIST OF FIGURES Figure Page No. Figure Caption No. 1 Frequency histogram of pavement age at the time of the distress survey for the LTPP GPS test sections included in the feasibility study Histogram of the number of test sections with fatigue cracking Graphical illustration of the results from a survivability analysis or probability of exceeding a specified area of fatigue cracking Probability of occurrence for fatigue cracking Histogram of the number of test sections with longitudinal cracking in wheel paths Graphical illustration of the results from a survivability analysis or probability of exceeding a specified length of LCWP Probability of occurrence for LCWP Histogram of the number of test sections with longitudinal cracking outside the wheel paths Graphical illustration of the results from a survivability analysis or probability of exceeding a specified length of LCNWP Probability of occurrence for LCNWP Histogram of the number of test sections with transverse cracks Graphical illustration of the results from a survivability analysis or probability of exceeding a specified length of transverse cracking Probability of occurrence for transverse cracking Histogram of the number of test sections with various rut depths Graphical illustration of the results from a survivability analysis or probability of exceeding a specified rut depth Probability of occurrence for rutting Histogram of the number of test sections with various values of roughness Graphical illustration of the results from a survivability analysis or probability of exceeding a specified roughness Probability of occurrence for roughness Graphical illustration of the results from a survivability analysis or probability of exceeding ten percent fatigue cracking for HMA pavement built over different soils Graphical illustration of the results from a survivability analysis or probability of exceeding a rut depth of 12 mm for different HMA thickness Graphical illustration of the results from a survivability analysis or probability of exceeding a moderate level of transverse cracking for different types of HMA pavements ii

5 LIST OF TABLES Table Page No. Title of Table No. 1 List and source of data elements recovered from LTPP database for use in the feasibility study Number of LTPP test sections in the pavement structure categories Number of LTPP test sections in the climate categories Number of LTPP test sections in the foundation categories Distress magnitude used in the performance evaluations Number of LTPP GPS-1 and 2, SPS-5 (control sections), and SPS-9 test sections in each distress category Expected service life based on different levels and magnitudes of distress Average age of the LTPP test sections that exceed moderate and excessive levels of distress Summary of p-values from a one-way ANOVA of the test sections used in the global survivability analysis Summary of p-values from a one-way ANOVA of the SPS-1 test sections Summary of p-values from a one-way factor ANOVA of the SPS-1 test sections, Von Quintus and Simpson, Results from the ANOVA for the LTPP test sections included in the global survival analysis description statistical values grouped by different factors and for selected performance indicators (GPS sections) Results from ANOVA for the LTPP SPS-1 test sections description statistical values grouped by different factors and for selected performance indicators iii

6 EXECUTIVE SUMMARY This report documents the results from a study that was requested by the Asphalt Pavement Alliance through the Asphalt Institute to determine the average service life or time to an unacceptable surface condition for new HMA-surfaced pavements. The study is similar to the one sponsored by the Federal Highway Administration (FWHA) to determine the probability of time to a specific level of distress for HMA overlays. As for the FHWA sponsored project, the data used within this study was extracted from the LTPP database version 13.1/NT3.1, which was released in January All applicable sections with the sufficient data in the LTPP database were used in the study. An LTPP section is 500 ft. long by one lane wide. Six distress types were used to determine the average time to various surface conditions or magnitudes of distress. These distress types are: area fatigue cracking, longitudinal cracking in the wheel path area, longitudinal cracking outside the wheel path area, transverse cracking, rut depth, and smoothness as measured by the International Roughness Index (IRI). The time to a specific surface condition, or distress magnitude, was determined using a global survivability or probability of failure analysis. In addition, an analysis of variance and localized survival analysis were used to identify the pavement and site features related to performance of new HMA-surfaced pavements. Based on a review of the LTPP distress data, there are 109 flexible pavement sections, out of 392 sections, that are older than 20 years with moderate or lower levels of distress, while there are more than 50 sections that are older than 20 years with no to minor levels of distress, as of There ware simply too few flexible pavement sections with excessive levels of distress in the LTPP database to conduct any detailed studies, using simple regression techniques to identify those factors related to the higher levels of distress. The following tabulates the percentages of test sections with different levels of distress. Distress Type Fatigue cracking Longitudinal cracking in wheel paths Longitudinal cracking not in wheel paths Transverse cracking Rutting Roughness Levels of Distress None, % Nominal, % Greater Than Nominal, % Low Moderate Excessive NA NA A moderate level of distress was used to determine the time or age of the flexible pavements to an unacceptable surface condition for each distress using global survivability or probability of failure analysis. [The definition of each distress level is included in the report.] iv

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8 Expected Service Life and Performance Characteristics of HMA Pavements in LTPP 1. INTRODUCTION 1.1 Background The service life of a Hot Mix Asphalt (HMA) pavement is an essential and important piece of information to any life cycle cost procedure for estimating equivalent annual or present worth costs for a specific design strategy. Service life is defined as the time in years from construction to the first major structural rehabilitation or to an unacceptable condition of the pavement surface. Values of 10 to 17 years have been used in calculating the life cycle costs of HMA pavements. 1 Many State agencies have based the service life on historical information that may not be based on the more recent advances in mixture design and construction procedures. As a result, industry and agencies have argued over this estimate of service life and over the accuracy of the data used to determine its value. One of the objectives of the Long Term Performance Program (LTPP) was to develop improved design methodologies for HMA pavements. To achieve the objective, hundreds of HMA test sections have been established and are being monitored to collect the necessary data and determine the performance characteristics of HMA pavements. The data from this program can be used to develop and validate improved design methodologies and to establish and confirm the HMA pavement s service life. Industry and some agencies, however, have expressed concern over the accuracy of the LTPP database, because of the discrepancies and problems that have been identified by various researchers using this data. Thus, the Asphalt Pavement Alliance requested that a feasibility study be conducted to ensure that there are sufficient data and that those data are accurate to determine the probability of time to different surface conditions, because of its effect on life cycle costs and design strategy selection. This study was prepared in response to that request from the Asphalt Pavement Alliance to determine the average service life, or time to an unacceptable surface condition, of HMA pavements in the LTPP program. This study is similar to a previous study sponsored by the FHWA to determine the probability of time to a specific level of distress for HMA overlays using the data from LTPP flexible pavement rehabilitation experiments Special Pavement Studies (SPS)-5 and General Pavement Studies (GPS)-6. 2 A description of the LTPP program, experiments, and database can be found in the website 1

9 1.2 Study Objectives Stated simply, there are two objectives of this study, which are listed below. 1. To determine whether the LTPP data can be used to define the time to an unacceptable surface condition for HMA pavements that have not been rehabilitated, and identify the design features and site factors that have an effect on that time. 2. If found to be feasible, conduct an initial analysis of that data as well as develop a final analysis plan to determine the expected service life and other performance characteristics of different types of HMA-surfaced pavements. 1.3 Scope of Work and Report To accomplish the two objectives listed above, seven tasks grouped into two phases were included in the work plan. These seven tasks are listed below: Phase I o Task 1 Develop Experimental Factorial for Analysis Plan. o Task 2 Extract Data from LTPP Database and Organize within Experimental Factorial. o Task 3 Assess Data Availability within Cells of Experimental Factorial. o Task 4 Prepare Feasibility Report and Future Study Plan. Phase II o Task 5 Complete Detailed Analysis of Time to Unacceptable Surface Condition. o Task 6 Determine the Effect of Design Features and Site Factors on the Performance Characteristics of HMA Pavements. o Task 7 Prepare Design Feature and Site Factor Report and Future Detailed Study Plan. This report documents the results from both phases, and is subdivided into six sections, including this introduction. The second section, after this introduction, presents the experimental factorial, identifies the data elements and their availability from the LTPP database, and lists the number of test sections within each part of the factorial. The third section lists the distresses used in the survivability or probability of failure analysis, while the fourth section discusses the analyses completed for this feasibility study. The fourth section also includes examples of the expected service life for the different distresses. The fifth section presents the analysis of variance and detailed studies to identify the site factors and pavement features that affect surface distress. The sixth section of the report provides a summary of the key findings from this study and future recommendations. 2

10 2. EXPERIMENTAL FACTORIAL AND KEY FACTORS/PROPERTIES EXTRACTED FROM LTPP 2.1 Factors Included in the Analysis Plan The experimental factorial and analysis plan used to evaluate the performance characteristics and expected service life of HMA pavements that have yet to be rehabilitated is similar to the LTPP SPS-1 factorial or experiment. 3 The study factorial consists of five primary tiers, which are listed below and were selected to be somewhat consistent with the SPS-1 experiment. 1. Pavement or base type 2. HMA layer thickness 3. Soil type 4. Climate 5. Drainage layers Appendix A includes the experimental factorial, and shows the magnitudes of the key factors in the primary cells to determine which pavement features and site factors have a significant effect on the expected service life of flexible pavements in the LTPP program. The secondary factors considered in the analysis plan are listed below. Traffic is a secondary factor related to the HMA thickness. The annual number of 18- kip equivalent single axle loads (ESALs) was extracted, because it is readily available for most of the test sections in the LTPP database. HMA mixture properties considered as secondary factors included: o Gradation (those sieves used in Witczak s dynamic modulus regression equation). 4 o o Asphalt type and/or grade with respect to climate. Volumetric properties, such as air voids and asphalt content by weight. Effective asphalt content by volume is a required input in Witczak s dynamic modulus regression equation, but not available in the LTPP database. The following factors were included in the analysis plan as related to base type (unbound crushed aggregate, unbound uncrushed gravel, asphalt-treated base or black-base mixtures, cement-treated base or semi-rigid pavements). o Unbound aggregate base thickness. o Unbound aggregate repeated load resilient modulus at a specific stress state. Soil classification and structural response properties included: o Plasticity index. o Frost Susceptibility as defined by the Corp of Engineers frost classification chart. o Soil repeated load resilient modulus at a specific stress state. The factors normally used to discriminate between climates were used as secondary factors in the analysis plan: o Annual precipitation or rainfall. o Temperature and/or freezing index. 3

11 Deflection basin classification categories as developed by Von Quintus for FHWA in conducting back-calculation of layer modulus. This information is available in the computed parameter database. 5,6 Obviously the number of factors or parameters that can be considered in the analysis plan is dependent on the total number of test sections with sufficient data and the number of test sections that fall within each of the primary tiers of the factorial. Appendix A shows the number of LTPP test sections that fall into each primary cell of the experimental factorial. The remainder of this section of the report identifies the data elements, their availability, and the number of test sections that can be used in the study. Each test section in the LTPP database is 500 ft. long by one lane wide. 2.2 Data Source and Availability Those LTPP experiments that are applicable to this study include the GPS-1 and 2 and SPS-1 and 9 experiments, as a minimum. However, the GPS-6B and SPS-5 experiments can also be used when the condition of the existing pavement prior to overlay and the time of construction for the original HMA pavement are known. More importantly, for a test section to be included or eligible for use in this study, it must have been monitored at least twice before the section was overlaid. The reason for requiring at least two distress surveys is the large variation in the distress data --- two observations are needed to ensure that the measurements are reasonable. As noted above, HMA sections in the GPS-1 and 2 and SPS-1 and 9 experiments are candidates and were identified for use in this study. Additionally, most of the control sections of the SPS-5 experiment are also eligible because these test sections were not overlaid when they entered the LTPP program. Overall, there are 643 sections eligible for this study, including the SPS-1 test sections. However, the SPS-1 test sections were initially excluded for use in the feasibility study (Phase I) to determine the expected service life to an unacceptable surface condition. The reasons for excluding these test sections is that the structural features were varied within a SPS-1 project, such that a constant design life is not appropriate. It was assumed that the GPS-1 and 2, SPS-9, and the control sections of the SPS-5 projects, in all probability, have a design life of about 20 years. The assumption of a similar design life is believed to be reasonable based on the survey of current state practices, as reported by Darter, et al.7,8 The SPS-1 projects, however, will be included in the analysis to determine the performance characteristics of HMA pavements and to identify the design features and site factors that have a significant effect on the performance characteristics. 2.3 Data Elements Extracted from LTPP Database LTPP database version 13.1/NT3.1 released in January 2002 was used for this study. Table 1 lists the LTPP data tables and fields corresponding to the key factors that are needed for this study. Only Level E data was made available for use from LTPP. Level E represents the highest quality data that has passed all of the quality checks included in the LTPP program. The data availability summarized in table 1 is based on the 643 test sections that are applicable to the feasibility study. 4

12 Table 1 List and Source of Data Elements Recovered from LTPP Database for Use in the Feasibility Study 5

13 Table 1 List and Source of Data Elements Recovered from LTPP Database for Use in the Feasibility Study, continued 6

14 The question that was asked and answered for the feasibility study: Is there a sufficient number of sites or test sections to determine the time to an unacceptable surface condition and to identify those design features and site factors that have a significant effect on that service life? The next part of this section looks at the number of test sections within the cells of the analysis plan or factorial by age and condition. 2.4 Number of Test Sections This part of the report presents the experimental factorial and the number of test sections that can be used to determine the expected service life of HMA pavements. Appendix A includes the detailed experimental factorial and shows the number of LTPP test sections within each cell of that factorial. The following summarizes the number of test sections within each primary tier of the factorial. Pavement Structure: Pavement/Base Type and HMA Thickness Table 2 lists the number of appropriate LTPP test sections as classified or grouped by pavement structure. As shown, there are probably too few of the semi-rigid pavements to complete any performance comparisons beyond pavement structure cells included in the factorial. The LTPP test sections are weighted more towards the conventional-flexible type pavement structures. The definitions for the pavement type categories used in this study (table 2) follow the definitions used by LTPP. These definitions generally follow those also used in NCHRP Project 1-37A. (11) Table 2 Number of LTPP Test Sections in the Pavement Structure Categories Climate: Temperature and Moisture Table 3 lists the number of appropriate LTPP test sections as classified or grouped by climate. As shown, there are many fewer LTPP test sections that were built in climates with an average annual rainfall less than 35 inches and with a mean annual air temperature greater than 70 F. However, there should be a sufficient number of sites within each climate cell to determine the effect of climate on performance. 7

15 Table 3 Number of LTPP Test Sections in the Climate Categories Foundation: Soil Type and Drainage Condition Table 4 lists the number of appropriate LTPP test sections, as classified or grouped by foundation. As shown, there are many fewer LTPP HMA test sections that were built over finegrained silty soils. In addition, most of the HMA test sections were built without any drainage layer or feature. Obviously, that statement is not correct for the SPS-1 experiment, where drainage was a key parameter in the design of the experiment. Thus, both factors were included in the detailed analyses (discussed in Section 5 of this report). Table 4 Number of LTPP Test Sections in the Foundation Categories 2.5 Summary Data Availability In summary, there is a sufficient number of test sections to determine the expected service life based on survivability analyses of conventional, full-depth, and deep strength HMA pavements (refer to table 2). There are too few semi-rigid pavements to be included in the detailed analyses. All other factors included in the primary tiers of the factorial can be used. Most of the data elements in table 1 are available for use in the analysis of variance (ANOVA) and more detailed studies. Those data elements that are unavailable for a sufficient number of test sections include indirect tensile strength and creep compliance. These data elements were excluded as secondary factors in the analysis plan. The data elements that are only available for a limited number of sites include repeated load resilient modulus for unbound paving materials and soils, HMA gradation and asphalt content by weight. These data elements were extracted but can only be used in a limited analysis of selected cells in the factorial. 8

16 3. DISTRESSES USED IN SURVIVABILITY ANALYSIS Six distress types or performance indicators were used to evaluate the performance trends or characteristics of new HMA pavements included in the LTPP database. They include fatigue cracking, longitudinal cracking in the wheel path (LCWP), longitudinal cracking outside the wheel path (LCNWP), transverse cracking, rutting, and smoothness (as measured by the International Roughness Index [IRI]). The definition and measurement for each distress is described in the LTPP Distress Identification Manual. 9 The extent of these distresses was grouped into different categories for relative comparisons and for determining the expected service life of each category. The different levels of distress used in the study are defined in table 5. These distress levels or ranges in magnitudes were selected to be consistent with the typical ranges that have been used in design and/or may trigger some type of rehabilitation activity. These levels of distress are also similar to those used by Rauhut, Von Quintus, and Eltahan for evaluating the performance trends of HMA overlays. 2 Table 5 Distress Magnitude Used in the Performance Evaluations of LTPP Sections Note: LTPP sections are 500 ft. long by one lane wide. Table 6 identifies the number of LTPP test sections within each distress and magnitude category identified in table 1. As shown, there are a sufficient number of test sections within each category of distress, with the exception of the excessive category. There are too few test sections that have an excessive level or magnitude of distress for any detailed analyses. This in itself is a significant finding that there are so few test sections with excessive levels of distress. 9

17 Table 6 Number of LTPP GPS-1 and 2, SPS-5 (Control Sections), and SPS-9 Test Sections in Each Distress Category 10

18 4. GLOBAL SURVIVABILITY ANALYSIS EXPECTED SERVICE LIFE This section of the report presents the global survivability analysis or probability of failure performed on each distress type. Figure 1 is a histogram of pavement age for those LTPP-GPS test sections included in the analysis. The median age of these test sections is 17 years, and there are 109 test sections that are older than 20 years. The older test sections provide a valuable source of information to support the fact that flexible pavements can and do perform satisfactorily in excess of 20 years. 4.1 Load Related Area Fatigue Cracking Fatigue cracking is defined as a series of interconnected cracks (characteristically with a chicken wire or alligator pattern) and is measured in square meters at each severity level. 9 Fatigue cracking usually develops as multiple, short longitudinal cracks in the wheel path that eventually become connected laterally. Thus, increases in fatigue cracking over time (or with cumulative traffic) can be accompanied by decreases in longitudinal cracking in the wheel paths. Fatigue cracking is dependent on a number of factors; two of the more important ones are total HMA layer thickness and truck traffic. For this study, it was assumed that all of the GPS-1 and 2 test sections were designed for twenty years and that the pavement layer thicknesses were determined based on the truck traffic expected over that 20-year time period. 11

19 Table 6 summarized and listed the number of test sections within each category of fatigue cracking. As tabulated, there are a sufficient number of test sections up to the moderate distress category. There are too few test sections in the excessive category for any detailed analysis in the factorial. Figure 2 includes a histogram of the number of LTPP-GPS test sections with different amounts of fatigue cracking. Out of 280 GPS-1 and 2 test sections, 177 have no fatigue cracking. Figure 3 graphically presents the results of the survivability analysis or probability of exceeding a specific value of fatigue cracking. As shown, the time difference between 10 and 20 percent fatigue cracking levels is short, and consistent with previous experience. It is also important to note that there are only 29 test sections that have excessive levels of fatigue cracking. This number is less than 10 percent of the applicable LTPP test sections, excluding the SPS-1 projects. Figure 4 graphically illustrates the occurrence of different fatigue cracking levels for different probability levels. As shown the average (50 percent probability of occurrence) service life is about 25 years for the moderate level of fatigue cracking. The moderate level of cracking is that amount of cracking that typically will trigger some type of rehabilitation activity. This average expected service life exceeds the values that have been typically used in life cycle cost analysis, even though most of the HMA mixtures were designed and placed based on older procedures that have been replaced by Superpave. Figure 2. Histogram of the number of test sections with fatigue cracking. 12

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21 4.2 Load Related Longitudinal Cracking in Wheel Path Longitudinal cracking in the wheel path (LCWP) is defined as cracks predominantly parallel to the pavement centerline located in the wheel paths and is measured in meters at each severity level. 9 LCWP can be the initial stages of fatigue-area cracking or cracks that initiate at the surface of the pavement but stay longitudinal with continued traffic loading. Table 6 summarized and listed the number of test sections within each category of LCWP. As tabulated, there are a sufficient number of test sections up to the moderate distress category. There are too few test sections in the excessive category for any detailed analysis in the factorial. Figure 5 includes a histogram of the number of LTPP-GPS test sections with various lengths of LCWP, while figure 6 graphically presents the results of the survivability analysis or probability of exceeding a specific value of LCWP. As shown, there is a relatively long time period between a low to moderate level of cracking (10 and 75 m), which is consistent with the author s previous experience. Figure 5. Histogram of the number of test sections with longitudinal cracking in wheel paths. Figure 7 graphically illustrates the occurrence of different LCWP levels for different probability levels. As shown the average (50 percent probability of occurrence) service life is about 28 years for the moderate level of LCWP. The moderate level of cracking is the amount of cracking that typically will trigger some type of rehabilitation activity. This average expected service life significantly exceeds the values that have been typically used in life cycle cost analysis. It is also important to note that there are only 8 test sections that have excessive levels of LCWP. 14

22 This number is less than 5 percent of the applicable LTPP test sections, excluding the SPS-1 test sections. 15

23 4.3 Non-Load Related Cracking Longitudinal Cracking Outside Wheel Path Longitudinal cracking not in the wheel path (LCNWP) is described as cracks that are predominantly parallel to the pavement center line but not in the wheel paths. 9 This category of cracking (LCNWP) includes longitudinal construction joints, also referred to as paving lane cracks. It needs to be noted that there can be three cracks not in the wheel paths: one near the outside edge of the lane, one between the wheel path, and one near the inside edge of the lane. However, two parallel cracks in either of these three locations are considered together and are not measured individually and the lengths added, so the maximum amount of LCNWP is 457 m. Table 6 summarized and listed the number of test sections within each category of LCNWP. As tabulated, there are a sufficient number of test sections up to the moderate distress category. There are too few test sections in the excessive category for any detailed analysis in the factorial. Figure 8 includes a histogram of the number of LTPP-GPS test sections with various lengths of LCNWP, while figure 9 graphically presents the results of the survivability analysis or probability of exceeding a specified length of LCNWP. As shown, the increase in the amount of LCNWP is fairly constant over time, which is consistent with the author s previous experience. 16

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25 Table 6 summarized and listed the number of test sections within each category of transverse cracks. As tabulated, there are a sufficient number of test sections up to the moderate distress category, but there are too few test sections in the excessive category for any detailed analysis in the factorial. 18

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29 Figure 16 graphically illustrates the occurrence of different rutting levels for different probabilities. As shown, the average (50 percent probability of occurrence) service life is about 22 years for the moderate level of rut depths. The moderate level of rutting is that amount of cracking that typically will trigger some type of rehabilitation activity. This average expected service life exceeds the values that have been typically used in life cycle cost analysis. It is also important to note that there are only 12 test sections that have excessive levels of rutting. This number is about 5 percent of the applicable LTPP test sections, excluding the SPS-1 test sections. 22

30 Figure 19 graphically illustrates the occurrence of different roughness levels for different probability levels. As shown the average (50 percent probability of occurrence) service life is about 22 years for the moderate level of roughness. The moderate level of roughness is the amount of roughness that typically will trigger some type of rehabilitation activity. This average expected service life exceeds the values that have been typically used in life cycle cost analysis. It is also important to note that there are only 44 test sections that have excessive levels of roughness. This number is slightly more than 10 percent of the applicable LTPP test sections, excluding the SPS-1 test sections. 23

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32 4.7 Summary Table 7 provides a listing of the expected service life to different extents of surface distress. As tabulated, the expected service life to a moderate distress level exceeds 20 years for all distresses. This expected service life is significantly greater than the values used in many life cycle cost analyses that have been performed and reported by various State highway agencies. This finding suggests one of the following: State highway agencies may be using lower levels of distress as trigger values to initiate rehabilitation activities. The values used in previous life cycle cost studies are simply too low. The LTPP test sections are not representative of typical HMA pavements built across the U.S. State highway agencies use a combination of distress types (i.e.; pavement condition index) and functional factors (i.e.; longitudinal surface profile) as a condition to initiate rehabilitation activities. Table 7 Expected Service Life Based on Different Levels and Magnitudes of Distress It is the opinion of the authors that the trigger values used in this study are reasonable and consistent with State highway agency practice, based on recent information accumulated and summarized for NCHRP 1-37A regarding typical failure definitions. 11 It was also found in NCHRP 1-37A that the value of IRI is a reasonable accumulation of all surface distress. Individual distress measurements were used to predict the IRI values measured on both the GPS and SPS test sections. 12 It is more likely that the values used in previous life cycle cost studies are simply too low, especially considering the recent improvements made and adopted for HMA mixture design and construction specifications. Table 8 provides a listing of the average age of the LTPP GPS-1 and 2, SPS-9, and SPS-5 control sections that were used in this study when moderate and excessive levels of distress were observed on the test sections. In other words, only those sections with moderate and extensive levels of distress were used in computing the average age. As tabulated, the average 25

33 age for each group is significantly less than those ages listed in table 7. The reason for this difference is that there are a large number of sections with minimum extents of surface distress that are much older than 20 years. This suggests that there are design features and/or construction parameters that have a significant effect on pavement performance --- it is not just age and traffic that are the key factors in determining the expected service life. The LTPP test sections with minor to no distress are listed in Appendix B, while Appendix C provides a listing of the LTPP test sections with excessive levels of distress. Table 8 Average Age of the LTPP Test Sections that have Moderate and Excessive It is also important to note that for LCWP, the average age to excessive cracking was found to be less than that for the moderate level. Thus, it is inappropriate to only consider the number of sections within each magnitude level alone. The probability to failure considering all of the appropriate LTPP test sections is the correct technique to be used. As tabulated in table 8, only eight test sections have excessive levels of LCWP, while 18 have moderate levels. These sections have relatively thick HMA layers and this cracking, in the author s opinion, is believed to have initiated at the surface. These test sections were included in a group of sites that were identified under NCHRP 1-37A as having surface initiated longitudinal cracks. 11 In summary, it is the perception within the pavement community that flexible pavements can not perform satisfactorily for 20 years, much less past 20 years. The LTPP database confirms that if flexible pavements are properly designed and constructed, they can perform for more than 20 years with no to little surface distress. The next section of this report discusses the detailed analyses that were performed to identify those pavement features related to various distress indicators. 26

34 5. PAVEMENT FEATURES AND SITE FACTORS RELATED TO PERFORMANCE OF HMA PAVEMENTS Two types of analyses were performed to identify those site and pavement design features related to key distress indicators: an analysis of variance (ANOVA) and a localized survival analysis using the data and test sections noted in Section 4. This section of the report discusses and reviews the findings from these more detailed studies. 5.1 Analysis of Variance An ANOVA was performed to identify those design features and site factors that have a significant effect on the performance of HMA pavements in the LTPP database. Von Quintus and Simpson completed a similar analysis on the SPS-1 data to determine whether the SPS-1 experimental objectives could be met over time.3 This same type of analysis was completed on the entire database used for the global survivability analysis and on the SPS-1 database using more recent performance data (January 2003). ANOVA for LTPP Sections Used in the Global Survivability Analysis An ANOVA was completed for each of the four major distress types (including IRI) to determine if the main design features or site factors had a significant effect on those distresses and IRI. The majority of test sections included in this category were from the GPS-1 and GPS-2 experiments. Climate was previously found to not be important or have no effect on the key performance measures for both new flexible and rehabilitated flexible pavements and was not reevaluated within this study. 2,11 The major factors included in the ANOVA are listed below. Subgrade type: Fine-grained versus coarse-grained soils. HMA thickness: < 4 inch; 4 to 8 inch; > 8 inch thick. Base type: Unbound aggregate, ATB, and CTB layers. Drainage condition: Permeable versus dense layers. LTPP database version 15, released in January 2003, was used in this analysis. Results from this one-way ANOVA are summarized in table 9. Table 12, included in Appendix D, summarizes the differences in the descriptive summary statistics for the above key performance measures, including mean, variance, standard deviation, number of observations, coefficient of variance, and the p-value from the one-way ANOVA. Most of the results from the ANOVA are consistent with previous experience, with the exception of the effect of HMA thickness on fatigue cracking. The ANOVA suggests that HMA thickness has no effect on fatigue cracking. However, it is well known that as HMA thickness increases, the amount of fatigue cracking decreases. One reason for this result is that most or all of the GPS-1 and 2 sections were probably designed for twenty years. Any difference in traffic and materials between the different sections should be already taken into account by varying layer thickness. Thus, the ANOVA would find no significant effect of HMA thickness on the occurrence of fatigue cracking. It is important to note, that the ANOVA for the SPS-1 test sections, where HMA thickness does vary for the same traffic and climate, does suggest that HMA thickness affects fatigue cracking (refer to table 10). 27

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37 Base Type Found to have an effect on all performance measures, with the exception of transverse cracking for the ANOVA completed on the LTPP 2000 SPS-1 data. The ANOVA completed on the LTPP 2003 data found that base type in the SPS-1 experiment has an effect on all performance measures. The test sections with ATB layers have less rutting, much less fatigue cracking, lower IRI values, and much lower lengths of transverse cracks. Nominal HMA Thickness Found to have an effect on fatigue cracking and IRI, but no effect on rutting and transverse cracking using the 2000 ANOVA. The ANOVA completed on the LTPP 2003 SPS-1 data found that the nominal HMA thickness now has an effect on all performance measures, with the exception of rutting. Subgrade Soil Type Found to have an effect on fatigue cracking and IRI for both the 2000 and 2003 SPS-1 data releases. The ANOVA completed on the 2000 data set found that subgrade soil type had an effect on transverse cracking, but no effect on rutting. Conversely, he ANOVA completed on the 2003 data set found that subgrade soil type affects rutting, but not transverse cracking. Comparison of ANOVAs GPS and SPS-1 Test Sections Results from the one-way ANOVA were compared between the GPS and SPS-1 test sections. This part of section 5 lists and explains the differences that were found (comparison of tables 9 and 10 and tables 12 and 13 in Appendix D). The following summarizes the effect of the key factors of the experiment on the individual performance measures and how these vary between the GPS and SPS-1 test sections. For these comparisons, it should be understood that most of the GPS test sections were designed for about 20 years, whereas, the SPS-1 test sections contained varying structural features and were designed to exhibit early distress measures. Rut Depth Base type, subgrade soil type, drainage condition, and age all showed a statistical effect on the measured rut depths within the SPS-1 experiment. Only HMA thickness was found to have no effect on rutting. On the average, the SPS-1 test sections that were found to have lower rut depths include those with: coarse-grained soils, ATB layers (as defined by LTPP), and permeable base layers. Conversely, only HMA thickness and base type were found to have an effect on the measured rut depths within the GPS sections. Lower rut depths were measured on sections with ATB and CTB layers, and thinner HMA layers. This suggests that most of the rutting has occurred in the HMA layers the thinner the layer the less the rut depth. It should be remembered, however, that the HMA mixtures placed at most of the GPS-1 and 2 and SPS-1 test sections were not designed in accordance with the current Superpave mixture design method and none of the surface mixtures include SMA mixtures. The semi-rigid pavements consistently have the lower rut depths, as expected. However, the total number of semi-rigid pavements is less than 55 within the LTPP program. There are too 30

38 few sections to draw any major conclusion from this data. The age of the pavement was found to have no effect rutting for the older GPS sections. On the average, the rut depths measured on the older GPS sections are basically stable within the time frame of the LTPP program. In other words, the slope within the steady state portion of the permanent deformation versus traffic or time relationship is flat. This suggests that moderate to severe rutting will not occur 5 to 10 years after construction, if it does not occur in the early life of flexible pavements, assuming that the traffic loads do not suddenly increase for some reason. Conversely, the rut depth increases with age for the SPS-1 sections. A reason for this difference is that the SPS-1 sections include the primary part (initial densification) of the permanent deformation versus time/traffic relationship, whereas, the GPS sections do not include this data from the early part of a pavement s service life. The larger rut depths were measured on the GPS test sections with very thick HMA layers (exceeding 8 inches), whereas subgrade soil was found to have no effect on rutting of these same sections. A reason for this observation is that the sections included in the GPS experiments have adequate structural thickness, such that rutting in the subgrade is non-existent. This is not the case for the SPS-1 sections, where relatively thin test sections were built such that more rutting has probably occurred in the subgrade soil, in comparison to the GPS sections. It should be noted that there are only 95 GPS test sections that have equaled or exceeded a moderate level of rutting (less than 20 percent), and only 12 have exceeded a severe level of rutting. Fatigue Cracking Nominal HMA thickness, base type, subgrade soil type, and age all showed a statistical effect on the measured fatigue cracking within the SPS-1 experiment, as expected. Only drainage condition was found to have no effect on fatigue cracking. On the average, the SPS-1 test sections that were found to have less fatigue cracking include those with thicker HMA layers, coarse-grained soils, and ATB layers, again as expected. Conversely, only age of the section and subgrade soil type were found to have an effect on the measured fatigue cracking within the GPS sections. All other factors were found to have no effect on fatigue cracking, including HMA thickness. Less fatigue cracking was measured on sections with fine-grained soils, which is in direct contrast to the findings from the SPS-1 experiment. This suggests that the strength of the coarse-grained soils may have been overestimated or the strength of the fine-grained soils underestimated for design, in comparison to the SPS-1 sections where constant layer thickness was used independent of subgrade type. The other finding that does not coincide with practical experience is fatigue cracking not being statistically affected by HMA thickness. A reason for this finding is based on one of the assumptions referred to in Section 2 it is assumed that all of the GPS sections were designed to approximately the same time period of 20 years. If higher levels of traffic are adequately explained by increases in HMA thickness through the structural design procedure, then HMA thickness would not be statistically related to the amount of fatigue cracking measured on the GPS sections, assuming the same design period. This is believed to be a reasonable explanation for this finding based on previous reviews of and experience with state agency design procedures. 1,11 31

39 Transverse Cracking Nominal HMA thickness, base type, and age all showed a statistical effect on the measured transverse cracking within the SPS-1 experiment. Drainage condition and subgrade soil were found to have no effect on transverse cracking. On the average, the SPS-1 test sections that were found to have less transverse cracking include those with: thicker HMA layers and ATB layers. For the GPS test sections, only subgrade soil type was found to have no statistical effect on transverse cracking. On the average, the GPS sections that were found to have less transverse cracking include those with: permeable layers, ATB layers, and thicker HMA layers. The semirigid pavements were found to have significant levels of transverse cracks. In actuality, these transverse cracks are probably reflection cracks from the underlying CTB layer. Roughness or Smoothness in Terms of IRI Value All primary experimental factors showed a statistical effect on the measured IRI values within the SPS-1 experiment. On the average, the SPS-1 test sections that were found to be smoother over time or have lower IRI values include those with: thicker HMA layers and ATB layers, permeable layers, and coarse-grained soils. For the GPS test sections, only subgrade soil type and drainage condition were found to have no statistical effect on the measured IRI values. On the average, the GPS sections that were found to be smoother over time or have lower IRI values include those with: thicker HMA layers and ATB layers. 5.2 Localized Survival Analysis and Comparative Studies A localized survival analysis was completed on the age of the test sections when the HMA pavement reached a moderate level of distress. Section 4 of this report provided the average service life from the global survival analysis for each major distress type of the experimental factorial. Similar survival analysis were completed for each major tier of the experimental factorial (refer to Appendix A) that was found to be statistically important from the ANOVA discussed above. Von Quintus and Simpson completed a similar example in evaluating the preliminary results from the GPS-6 and SPS-1 experiments. 2,3 This localized survival analysis was completed for the primary factors of the experimental plan and some of the secondary factors to identify which design features and site factors have a significant effect on the expected service life. Figures are examples of the localized survivability analysis completed for fatigue cracking, transverse cracking, and rutting. Figure 20 graphically shows that the type of subgrade soils only have a minor effect on the expected service life. The fine-grained clay soils have a slightly lower average service life than the coarse-grained soils. This finding or observation is consistent with previous experience and results from the SPS-1 experiment, but is in direct contrast to the results from the ANOVA on the GPS test sections discussed in Section 4. One reason for this discrepancy between the two 32