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Publication Number: FHWA-RD-01-166
Date: November 2003

Structural Factors for Flexible Pavements—Initial Evaluation of The SPS-1 Experiment Final Report

SUMMARY AND CONCLUSIONS

The SPS-1 experiment, entitled Strategic Study of Structural Factors for Flexible Pavements, is one of the key experiments of the LTPP program. The main objective of this experiment is to determine the relative influence and long-term effectiveness of the HMA pavement strategic factors that affect performance. Most of the site factorial cells have companion projects within each cell and it is believed that the construction deviations and discrepancies will not have a detrimental impact on the ability of the experiment to accomplish its original objectives.

This report has presented the results from the first comprehensive review and evaluation of the SPS-1 experiment. Issues of experimental design, construction quality, data availability and completeness, and early performance trends have been addressed.

The unavailable data identified in this report do not necessarily mean that the data were not collected or submitted by the SHAs that built the individual projects. There can be several instances where good data can be delayed before reaching Level E status. The following are some examples of why some data elements could be shown as unavailable when the data actually were collected:

The LTPP program is continuing on a systemwide basis to resolve all unavailable data so that they will be available to future studies. Some data have already been located and forwarded to the IMS during the course of this study. The key findings from this detailed review are summarized in this chapter.

SPS-1 EXPERIMENT STATUS

As of January 2000, 18 SPS-1 projects have been constructed throughout the United States (refer to figure 1). The full factorial of the original experiment design has been completely filled, with the exception of two site cells that do not have companion projects. The missing site factorial cells are the coarse-grained soils in a dry-no-freeze climate and the fine-grained soils in a wet-no-freeze climate (site cells 2 and 8 in table 3). These two missing companion projects are not believed to be critical. The completeness of the experimental factorial is a major benefit in calibrating and validating performance prediction models for new flexible pavements. To completely fill the factorial, the following projects would need to be constructed:

All of the SPS-1 projects have 12 core test sections and some of the projects have additional supplemental test sections that were built by the agency. A total of 216 core test sections and 32 supplemental test sections are available from this experiment. This number of test sections should provide excellent data for future studies.

The primary value of the supplemental sections will be to serve as a direct comparison to the core test sections within that specific SPS-1 project. However, the supplemental sections built at each site can be used in regional or national studies through the application of mechanistic analysis principles.  Therefore, efforts should be made to ensure that their construction and monitoring data are collected and stored in the IMS for future use.

An important issue in the experimental factorial is the imbalance in the number of projects between the different soil classifications. Eleven projects have been built over fine-grained soils while only seven have been built over coarse-grained soils. This imbalance is not believed to be critical, but should be considered when analyzing the data to determine the effects of the subgrade on performance. The other important observation from the experiment is that the ages of the projects are reasonably distributed between the different cells of the site factorial (table 3).

DESIGN VERSUS ACTUAL CONSTRUCTION

Experimental design factors were compared to the actual values measured during construction that were included in the IMS database. This includes both the site condition factors and pavement design features. Most SPS-1 sections follow the experiment design for the large majority of the design factors. Overall, very few construction deviations have been reported for the SPS-1 projects, with the exception of layer thickness.

Most layer thickness measurements deviate more from the experiment design than allowed by the project requirements for each layer. However, none of the thickness data for the thin and thick layers overlap. The following summarizes notable deviations when comparing the designed to the as-constructed values.

The other construction deviations are primarily related to the HMA layers. For example, the stability of the HMA mixture placed along the Arkansas project was less than the value specified in the project documents. In addition, the percentage passing the number 4 sieve for the HMA mixture and the percentage passing the number 200 sieve for the dense-graded aggregate base exceeded the specified values for many of the test sections. These are considered minor deviations and should not be critical to the overall experiment.

DATA AVAILABILITY AND COMPLETENESS

The data availability and completeness for the SPS-1 experiment are good overall (more than 95 percent of all data types collected are at Level E) with the exception of two major data elements—materials testing and traffic data. Furthermore, a significant amount of monitoring data must still be collected and/or checked to fill in the missing gaps of the time-history data for selected projects. 

Four projects (Florida, Montana, Oklahoma, and Wisconsin) do not have the initial transverse profile data and are missing some of the time intervals to establish the performance trends. The Montana and Wisconsin projects are less than 2 years in age while the other two have been in-place for more than 2 years. Transverse profiles were measured shortly after construction for the Montana, Oklahoma, and Wisconsin projects, but these data are not at Level E in the IMS. The reasons data have not achieved Level E status need to be ascertained and the situation rectified before detailed analyses of the SPS-1 experiment can be completed.

The critical data deficiencies for the SPS-1 experiment are summarized below:

It is recommended that a significant effort be put forth to obtain these missing data as soon as possible. The following sections summarize the availability of each data element and its effect on future studies, such as for the 2002 Design Guide (NCHRP Project 1-37A).

Construction Reports/Data

The construction and deviation reports are extremely valuable in reviewing and explaining performance anomalies of the individual test sections.  Construction and deviation reports were available for review for all of the projects with the exception of Michigan, Wisconsin, and Montana. The construction report has been recently submitted for the Wisconsin project and the one for the Montana project is in the process of being prepared.   

Materials Data

The materials data are partially complete for all of the projects with the exception of the relatively new projects.  However, none of the projects were found to have all testing completed. Tables 9 through 13 summarize selected test data by material type for each of the projects and show that extensive test data were unavailable at the time of the data extraction. In fact, none of the projects have any indirect tensile resilient modulus, strength, and creep compliance test data. 

Materials test data are a key data element, especially for use in mechanistic studies (such as NCHRP 1-37A) because the test results are used to determine the material physical properties needed for distress predictions and to help explain performance anomalies.  Without the materials tests, the SPS-1 experiment will be severely limited in its application to future studies.  As stated above, the materials testing is currently under way and materials test data are being submitted to the RCOs on a periodic basis.  Completion of the materials testing program should be a high priority to ensure that the full benefit of the SPS-1 experiment can be realized.

Climatic Data

The SPS-1 experimental design called for a project to be located in one of four different climates to ensure a diverse range of climatic factors (refer to table 3). The climatic data are obtained from actual measurements of weather data for the specific sites that are monitored with time and from historical data from nearby weather stations.  All projects were found to be in compliance with the appropriate cell requirements based on the NOAA and historical climatic data.  AWS equipment is installed at every SPS-1 project site. Table 15 listed the number of days of data from the AWS at each project site, and shows that there is much more data in the IMS for the older projects.   

Traffic Data

The SPS-1 experimental design called for continuous WIM monitoring, as permitted by WIM scale operating divisions. Table TRF_MONITOR_BASIC_INFO was examined to identify the SPS-1 records with WIM, AVC, and annual ESAL estimates.

Table 15 summarized the amount of data for the SPS-1 sites and identified those projects with no traffic data at Level E.   In summary, nine (50 percent) of the SPS-1 sites do not have any traffic data in the IMS while only five projects do not have any WIM or AVC equipment installed at the site.  Most of the older projects have the traffic monitoring equipment installed at the site, while the newer projects do not have the monitoring equipment and are missing traffic data.  

All of the projects have an annual estimate of the number of ESALs, but the reliability of this data is unknown for nearly 50 percent of the projects.

Performance Indicator Data

Several types of monitoring data are included in the LTPP IMS. These monitoring data include:  distresses (from both manual and video surveys), longitudinal profiles, transverse profiles, deflection, and friction. Table 16 provided a summary of the number of distress and other performance indicator measurements made at each of the SPS-1 project sites. Table 17 provided a summary of the average number of years between the surveys for each performance indicator, except for friction. 

The data that were unavailable at the time of the data extraction include four projects without transverse profile data and two of those projects were less than 2 years old. The following summarizes the data that should be measured in the near future, if it has not already been collected by the RCOs:

In summary, the amount of performance indicator data is good. The time-series data for each measure of performance will be a significant benefit for future studies regarding the design and performance of new flexible pavements. 

Friction Data

With few exceptions, friction surveys have not been performed on the SPS-1 projects. This testing is not considered essential to the SPS-1 experiment. Thus, the missing friction data will have no impact on future studies on structural behavior and performance.

Summary

Table 27 summarizes the unavailable and limited data for the SPS-1 experiment as of January 2000.

EARLY PERFORMANCE TRENDS

Most of the SPS-1 projects are relatively young and show little or no distress.  As of January 2000, less than 10 percent of the test sections have distress magnitudes that exceed values believed to be necessary to complete meaningful comparisons.  Based on the statistical analyses and comparisons documented in chapter 5, the key experimental factors were found to have an effect on the performance indicators.  Caution should be used in extrapolating these early findings because the long-term performance trends could be significantly different from these early observations.

The specific experimental expectations of the SPS-1 experiment were to determine the primary effects and interactions of the following key design features:

These effects and interactions were to be determined for each of the following subgrade and climatic conditions:

Table 27.  Summary of missing or limited data for the SPS-1 experiment.

Subgrade Soil Type

Pav't. Structure, Cell Numbers

Climate, Moisture-Temperature

Wet-Freeze

Wet-No-Freeze

Dry-Freeze

Dry-No-Freeze

Fine-Grained

1–12

IA(1)-7.0:

Limited AWS climatic data.

Limited WIM/AVC traffic data.

Limited distress surveys.

Subgrade—Missing classification, moisture density (M-D), & permeability data.

Aggr. Base—Missing resilient modulus & permeability data & limited classification & M-D data.

ATB—Limited mix & asphalt data & missing moist. susc. data.

HMA—Missing mix, moist. susc., asphalt & aggregate data.

LA(0)-2.1:

No WIM equipment installed.

Missing recent longitudinal profile.

Subg.—Missing permeability data.

Aggr. Base—Missing M-D, classification, & permeability data & limited resilient modulus data.

ATB—Missing mix, moist. susc., aggregate & asphalt data.

HMA—Missing mix, moist. susc., asphalt & aggregate data.

KS(6)-5.8:

Limited WIM/AVC traffic data.

Missing recent deflection survey & transverse profile.

Aggr. Base—Missing classification, M-D, resilient modulus, & permeability data.

ATB—Missing mix, aggregate, asphalt, & moist. susc. data.

HMA—Missing moist. susc., mix, asphalt & aggregate data.

NM(0)-3.7:

No WIM equipment installed.

Missing recent deflection survey & longitudinal profile.

Aggr. Base—Missing classification, M-D, & resilient modulus data.

   

OH(2)-4.6:

No WIM equipment installed.

Subg.—Missing classification, M-D, and resilient modulus data & limited permeability data.

Aggr. Base—Limited classification & permeability data & missing M-D & resilient modulus data.

ATB— Missing moist. susc. data & limited mix, aggregate & asphalt data.

HMA—Missing moist. susc. data & limited mix, asphalt & aggregate data.

AL(3)-6.4:

No WIM equipment installed.

Missing recent distress survey & transverse profile.

Subg.—Missing permeability data.

Aggr. Base—Missing resilient modulus & permeability data & limited classification & M-D data.

ATB—Missing asphalt, aggregate, & moist. susc. data & limited mix data.

HMA—Missing moist. susc., asphalt & aggregate data & limited mix data.

   
 

13–24

MI(1)-4.0:

Limited AWS climatic data.

Limited WIM/AVC traffic data.

Missing recent deflection survey.

Subg.—Missing permeability data & limited classification, M-D, & resilient modulus data.

Aggr. Base—Missing resilient modulus & permeability data, & limited classification & M-D data.

ATB—Missing mix, aggregate, asphalt, & moist. susc. data.

HMA—Missing mix, moist. susc., asphalt & aggregate data.

No Projects

NB(1)-4.1:

Limited WIM/AVC traffic data.

Subg.—Limited resilient modulus & permeability data.

Aggr. Base—Missing resilient modulus & permeability data & limited classification data.

ATB—Missing moist. susc. data & limited aggregate & asphalt data.

HMA—Missing moist. susc. data & limited mix, asphalt & aggregate data.

TX(8)-2.3:

Limited AWS climatic data.

Subg.—Missing M-D & permeability data & limited classification data.

Aggr. Base—Missing M-D, classification, & permeability data.

ATB—Missing mix, moist. susc., aggregate & asphalt data.

HMA—Missing mix, moist. susc., asphalt & aggregate data.

   

VA(1):

Limited WIM/AVC traffic data.

Missing recent deflection survey.

Subg.—Missing resilient modulus data.

Aggr. Base—Missing M-D, resilient modulus & permeability data.

ATB—Limited asphalt data.

HMA—Limited mix, moist. susc., asphalt & aggregate data.

   

OK(2)-2.1:

No WIM equipment installed.

Subg.—Missing M-D & permeability data & limited classification data.

Aggr. Base—Missing M-D, classification, resilient modulus, & permeability data.

Coarse-Grained

1–12

DE(2)-3.2:

No WIM equipment installed.

Subg.—Missing classification, M-D, and permeability data.

Aggr. Base—Missing classification, M-D, & resilient modulus data & limited permeability data.

ATB—Missing moist. susc. & asphalt data & limited mix & aggregate data.

HMA—Missing moist. susc. data & limited mix data.

FL(1)-3.7:

Limited WIM/AVC traffic data.

Missing transverse profile data.

Missing recent deflection survey & longitudinal profile.

Subg.—Missing gradation & permeability data & limited resilient modulus data.

Aggr. Base—Missing resilient modulus & permeability data & limited classification & M-D data.

NV(0)-4.0:

No AWS equipment installed.

Limited WIM/AVC traffic data.

Subg.—Limited permeability data.

Aggr. Base—Missing M-D & resilient modulus data & limited classification data.

ATB—Missing moist. susc., asphalt, & aggregate data & limited mix data.

HMA—Missing moist. susc., asphalt & aggregate data & limited mix data.

No Projects

 

13–24

WI(0)-1.8:

No construction report.

No AWS equipment installed.

No WIM equipment installed.

Missing recent deflection survey & transverse profile.

Subg.—Missing classification, M-D, resilient modulus, & permeability data.

Aggr. Base—Missing classification, M-D, resilient modulus & permeability data.

ATB—Missing mix, moist. susc., asphalt, & aggregate data.

HMA—Missing mix, moist. susc., asphalt, & aggregate data.

AR(1)-5.7:

Limited WIM/AVC traffic data.

Missing recent distress survey & longitudinal & transverse profiles.

Subg.—Missing resilient modulus & permeability data.

Aggr. Base—Missing M-D, resilient modulus, classification, & permeability data.

ATB—Missing mix, asphalt, aggregate, & moist. susc. data.

HMA—Missing moist. susc., asphalt & aggregate data & limited mix data.

MT(0)-0.8:

No construction report.

No WIM equipment installed.

Subg.—Missing classification, M-D, resilient modulus, & permeability data.

Aggr. Base—Missing classification, M-D, resilient modulus, & permeability data.

ATB—Missing mix, moist. susc., asphalt & aggregate data.

HMA—Missing mix, moist. susc., asphalt & aggregate data.

AZ(5)-6.0:

Subg.—Limited resilient modulus and missing permeability data.

Aggr. Base—Missing permeability & limited resilient modulus data.

HMA—Missing moist. susc. data & limited asphalt data.

Note:    All projects are missing indirect tensile resilient modulus, indirect tensile strength, and indirect tensile creep compliance tests. The values in parentheses are the number of supplemental test sections for each project; the other value provided for each project is the age of that project in years.

This evaluation has shown that several problems will clearly limit the results that can be obtained from the SPS-1 experiments. The misinterpretation of the different distress types over time by the distress surveyors and the potential measurement error of low levels of distress increase the difficulty of interpreting performance trends and of determining the effects between the experimental factors. The misinterpretation of distress types leads to wide variations in distress quantities over time for a given section (e.g., longitudinal and transverse cracking are sometimes called block cracking). The measurement error also contributes to wide variations in distress quantities over time for a given section.  Both of these problems make data analysis very difficult. This problem may lead to the need to provide "smooth" curves for specific distress types for each section for analysis purposes.

Other major limitations are related to the value of the SPS-1 experiment, including the missing materials test data and traffic data.  Without these data, the experimental objectives can be accomplished only in an empirical sense in terms of the general performance of different sections, but the development and calibration of mechanistic procedures will not be possible. Table 28 summarizes the limitations and action items to correct these deficiencies related to each of the SPS-1 projects.

The following list highlights some early performance trends from the SPS-1 experiment:

Table 28. Deficiencies and action items for each SPS-1 project.

SPS-1 Project

Deficiency

Suggested Action

Alabama, Delaware, Ohio, Louisiana, Montana,(1) Oklahoma, New Mexico, Texas, and Wisconsin

No traffic data (WIM and/or AVC).

Collect and/or process traffic data.

Alabama, Delaware, Louisiana, Oklahoma and New Mexico

No traffic measuring equipment installed at site.

Install traffic WIM and AVC measuring equipment.

Michigan, Montana(2)

No construction reports available.

Ensure that the construction reports become available for future studies—even if the contractor, SHA personnel, and other personnel on site during construction have to be interviewed from memory.

Nevada(3) and Wisconsin

No AWS data.

Collect and/or process climatic data.

Nebraska, Wisconsin, Michigan and Montana

Construction data—rod & level measurements.

Process data that was collected during construction.

Alabama, Arkansas, Delaware, Iowa, Kansas, Louisiana, Michigan, Montana, Nevada, Oklahoma, Ohio,(4) Wisconsin,(4) and Texas

Insufficient test data available for the essential material properties.

Complete the test program.  Use backcalculated elastic layer modulus until laboratory test data become available.

All Projects

Layer thickness deviates from the planned thickness more than allowed by the project requirements.

None—Adjust or normalize the performance to account for the thickness difference between test sections.

All Projects

In-place air voids deviate from the recommended values for the HMA layers.

None—Adjust or normalize the performance to account for the difference in air voids between the same test sections.

Florida, Montana,(5) Oklahoma, and Wisconsin

No transverse profile data.

Take immediate action to collect and/or process these data.

Arkansas and Louisiana

Transverse profile measured only once at these sites.

Take immediate action to collect these data.

Louisiana, New Mexico, and Oklahoma

Longitudinal profile measured only once at these sites.

Take immediate action to collect these data.

Arkansas, Florida, Iowa, and Wisconsin

Only one manual survey performed at these sites.

Take immediate action to collect these data.

Notes: 

  1. WIM equipment was installed at the Montana site.  The traffic data are on hold pending the new traffic processing software.
  2. The construction report for the Montana project is in draft form and awaiting additional construction information.
  3. AWS equipment was installed at the Nevada project, but is not at Level E in the IMS.
  4. Materials testing has been completed for the Ohio and Wisconsin projects, but that data are not at Level E.
  5. Transverse profiles were measured at the same time that the distress surveys were performed on the Montana project, but these data are not at Level E in the IMS.

EXPECTATIONS FROM THE STATES

Two national workshops were held where input was received from the States on the SPS-1 experiment.  The meetings were held on November 2–3, 1999, in Columbus, OH, and on April 27, 2000, in Newport, RI.  Several State agencies made presentations on the status of their SPS-1 project and their expectations from the SPS-1 experiment.  Panel discussions on the future direction and analysis of the SPS-1 data were held at both conferences. Those discussions are summarized in this section.

In general, the States seem to be satisfied with the experiment and believe that it will produce valuable information on the different design factors and features.  Many States have been conducting or are planning their own analyses on their SPS-1 project.  Some of these analyses have already yielded useful results; however, the States would like to see a focus on implementation.

First and foremost, the States want a research-quality database from the SPS-1 experiment.  Secondly, the States want to be able to determine the effects of the design features on pavement performance and the effectiveness of the SPS-1 experiment design factors, such as:

In addition to the structural design features, the States also want to know what major site condition factors influence performance of new flexible pavements, including:

Other expectations from the States include:

As to the future analysis plan for SPS-1, the States believe that it is worthwhile first to fill in the missing data—specifically obtain traffic and material test data.  Some presenters at the SPS conference requested that fundamental studies be conducted to determine how the SPS-1 sections are responding to load and environmental stresses and loads. It also was suggested that an integrated analysis plan be developed for future research.

CAN THE SPS-1 EXPERIMENT MEET EXPECTATIONS?     

The specific experiment expectation of the SPS-1 was to determine the main effects and interactions of the following key design features.

These main effects and interactions were to be determined for each of the following subgrade and climatic conditions:

This data review and evaluation of early performance trends has shown that several significant data issues will limit the results that can be obtained from the SPS-1 experiment.  The missing traffic data and key materials data must be obtained before meaningful global analysis can be performed.  A few of the SPS-1 sites had significant construction deviations.  However, these construction deviations will not have a detrimental effect on the value of the experiment if the materials test data become available. 

This does not mean that many important and useful results cannot be obtained from the SPS-1 experiment.  Some important early trends have already been identified that will be useful for the design and construction of HMA pavements, even though all projects are less than 7 years old. Thus, it is concluded from this comprehensive study of the SPS-1 experiment that the expectations from the State agencies and HMA industry can be met.

RECOMMENDATIONS FOR FUTURE ACTIVITIES

As stated in chapter 1, the key objective of the SPS-1 experiment is to determine the relative influence of different structural factors on flexible pavement performance.  It is believed that the experiment will be able to achieve this key objective with time.  Since the oldest SPS-1 project is just over 7 years (most are 3 to 5 years old), only a small percentage of the SPS-1 test sections have significant levels of distress and only a few have been taken out service.  The real benefit from this experiment will occur within the next 5 years, as a greater percentage of test sections exhibit higher levels of distress—magnifying the effect of the experimental and other structural factors on performance.

This SPS-1 assessment report focused on the quality and completeness of the SPS-1 construction and monitoring data, and on the adequacy of the experiment to achieve the original experimental expectations and objectives.  Some data are unavailable, but not enough to significantly limit the value of the results from this experiment. Detailed analysis of the effect of different design factors on performance was outside the scope of work for this study. Thus, future studies using the SPS-1 experimental data should be planned and prioritized so they can be initiated as the SPS-1 projects exhibit higher levels of distress.

These future studies should be planned for in two stages that focus on local and national expectations from the experiment. The first stage is to conduct a detailed assessment or case study on each structural cell in the experiment to support local interests and expectations. The second stage takes selected data elements to evaluate the effect of different structural features across the whole experiment.  Both stages are discussed briefly in the following sections.

Initial Stage—Analysis of Local Expectations or Individual Factorial Cells

A detailed evaluation of the companion projects within each major cell should be completed as soon as some of the test sections begin to exhibit higher levels of at least one distress type. The purpose of the case studies is to:

Second Stage—Analysis of National Expectations or Experimental Findings

The second-stage analyses should not be pursued until the first-stage analysis is complete. It is expected that the analyses performed as a part of the second stage can be coordinated with the Strategic Plan for LTPP Data Analysis. The SPS-1 experiment can contribute to the following specific analyses outlined in the strategic plan:

A description of some of the future studies that can be pursued at the national level using all of the SPS-1 experimental data are summarized in tables 29 through 42. The future research studies were prepared based on the discussions with and presentations from SHA personnel at the various SPS conferences that were held in 1999 and 2000. These future analysis objectives are believed to be achievable from data collected within the SPS-1 experiment and have been subdivided into two categories. The first category includes the analysis objectives that are related to the main factors of the SPS-1 experiment and the second category objectives are related to other experimental factors. 

The following second-stage analysis objectives are recommended for the SPS-1 experiment, which are presented in more detail in tables 29 through 42:

Future Analysis Objectives Related to Main Experimental Factors (table number)

29.    Perform test section-by-test section analyses of the projects included in the SPS-1 experiment to gain an understanding of the performance of the individual test sections and how the performance and response of each test section compare to the other test sections within that project and for the companion project. This objective is the initial analysis of the individual factorial cells.

30.    Determine the effect of the main SPS-1 experimental factors on the performance of flexible pavements.

31.    Quantify the benefits of a good drainage system for flexible pavements.

32.    Determine the effect of layer thickness variations on long-term pavement performance and initial ride quality.

33.    Determine and quantify the effect of base material selection and type on the performance of flexible pavements.

34.    Estimate the effect of seasonal conditions or changes on the modulus of unbound pavement materials and subgrade soils.

35.    Quantify the effect of soil type on pavement performance measures (specifically ride quality) and minimum pavement thickness over the foundation.

Future Analysis Objectives Related to Other Experimental Factors (table number)

36.    Determine the effect of base condition (such as moisture content, compaction, and degree of saturation) on the performance of flexible pavements.

37.    Determine the effect of HMA compaction and material properties (gradation and resilient modulus) on pavement performance.

38.    Quantify the remaining life of cracked or damaged HMA layers.

39.    Quantify the applicability of the subgrade protection criteria—limiting the subgrade vertical compressive strain and deflection.

40.    Confirm the hypothesis of surface initiated fatigue cracks and identify those HMA mixture properties and pavement conditions most conducive to the occurrence of fatigue cracks initiating at the surface of the pavement.

41.    Compare and quantify any differences between backcalculated modulus values using "MODCOMP" and laboratory measured resilient modulus.

42.    Conduct mechanistic analyses of the SPS-1 project sites and test sections to gain knowledge of critical stresses, strains, and deflections to explain their performance in terms of fatigue cracking, permanent deformation within each layer, and ride quality.

Table 29. Identification of future research studies from the SPS-1 experiment—
initial analysis of the individual factorial cells and companion projects.

OBJECTIVE NO. 1

Perform test section-by-test section of the SPS-1 projects to gain an understanding of the performance of the individual test sections as compared to the performance and behavior or response of the other test sections within that project and those of the companion project.  (Expected timeframe—2001 to 2002).

TOPIC AREA

Pavement design and performance prediction.

Impact of specific design features on performance.

PROBABILITY OF SUCCESS

High

LTPP STRATEGIC PLAN

7.A, 7.B, 7.C

SUPPLEMENTAL EXPERIMENTS

END PRODUCT

Impact of specific design features and level of significance on pavement performance and the occurrence of pavement distress.

  • Identify the test sections that perform well and poorly at each of the SPS-1 project sites.
  • Prepare case study reports that identify and define the effect of any construction difficulty or anomaly and material noncompliance with the project specifications on pavement performance and response.
  • Compare the companion projects within a specific cell of the factorial and determine any bias in performance differences that may be caused by construction anomalies and/or material noncompliance.

POTENTIAL PRODUCT USE

Future analysis projects.

GENERAL TASKS

  • Resolve construction and monitoring data anomalies and experimental cell differences for those projects that changed cell locations from the original experiment design, as they relate to the specific cell in the experiment.
  • Conduct comparative analyses of the individual test sections at each site, including the supplemental test sections, to identify differences in pavement performance and response.
  • Determine the effect of any construction difficulties and problems and material noncompliance issues with the SPS-1 project specifications, if any, on pavement performance and response.
  • Develop findings regarding comparisons made between the companion projects and test sections and prepare a case study report that will be useful for the SHAs involved and also will be useful for the national studies.

Table 30. Identification of future research studies from the SPS-1 experiment—
overall effect of the main experimental factors on performance.

OBJECTIVE NO. 2

Determine the effect of the main SPS-1 experimental factors on the performance of the flexible pavements. 

(Expected timeframe—2002 to 2003).

TOPIC AREA

Pavement design and performance prediction.

Impact of specific design features on performance.

PROBABILITY OF SUCCESS

High

LTPP STRATEGIC PLAN

7.A, 7.B, 7.C

SUPPLEMENTAL EXPERIMENTS

END PRODUCT

Impact of specific design features and level of significance on pavement performance and the occurrence of pavement distress.

  • Determine the effect of drainage on performance and identify the site conditions (type of subgrade soil, climate, traffic) where permeable bases will and will not contribute to improved performance.
  • Identify the flexible pavement design features and properties that are compatible with permeable base drainage and contribute to improved performance.
  • Identify the site conditions where different base types will contribute to improved pavement performance.
  • Determine the significance of seasonal changes on the response of the pavement and materials related to performance and incremental deterioration.
  • Confirm and quantify the effect of the subgrade on pavement performance and minimum pavement thickness above the subgrade.

POTENTIAL PRODUCT USE

  • Design new or reconstructed cost-effective and reliable flexible pavements.
  • Calibration and validation of new pavement design procedures/methods and distress prediction models.

GENERAL TASKS

  • Review results and findings from each SPS-1 test section and companion project.
  • Conduct statistical analysis to determine significant factors and interactions on performance.
  • Conduct mechanistic-empirical analyses for cracking, rutting, and IRI.
  • Based on the statistical and mechanistic analyses, determine the effect of different experimental factors or design features on pavement performance and response.
  • Prepare practical presentations of the results, including software, decision trees, etc., for use by practicing engineers, that aid them in determining the end products above.

Note:  The future research topics or objectives that follow for the individual main or primary factors of the experiment are included as individual project objective statements.

Table 31. Identification of future research studies from the SPS-1 experiment—benefits of drainage.

OBJECTIVE NO. 3

Quantify the benefits of a good drainage system for flexible pavements.  (Expected timeframe—2004 to 2005).

TOPIC AREA

Design and performance predictions.

PROBABILITY OF SUCCESS

Moderate to high(1)

LTPP STRATEGIC PLAN

7.A, 7.B, 7.C

SUPPLEMENTAL EXPERIMENTS

GPS test sections with drainage features

END PRODUCT

Impact of a positive drainage system on pavement performance and the occurrence of pavement distress.

A decision tree to identify those site conditions requiring drainage for enhancing the performance of flexible pavements and for selecting the drainage design features.

POTENTIAL PRODUCT USE

Design engineers for designing new pavements.

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial stage of the analysis.
  • Subdivide data or test sections into those test sections with and without drainage features.
  • Classify each site as to requiring drainage or not requiring drainage.
  • Complete a regression analysis to determine any differences in performance and establish the relative differences between the two conditions.
  • Complete an analysis of variance and other analyses to relate the site conditions to the effect of the drainage feature on improving performance.

(1)  The probability of success will increase if field investigative studies are initiated to quantify the workability of the drainage system.  Without the confirmation on the drain ability of the drainage system, major assumptions will have to be made regarding the quantification of the benefits from drainage.


Table 32. Identification of future research studies from the SPS-1 experiment—  
effect of thickness variations on performance.

OBJECTIVE NO. 4

Determine the effect of thickness variations on long-term pavement performance and initial ride quality.  (Expected timeframe—2003 to 2004).

TOPIC AREA

Design

PROBABILITY OF SUCCESS

Moderate to high(1)

LTPP STRATEGIC PLAN

2.D, 7.B

SUPPLEMENTAL EXPERIMENTS

SPS-5 experiment

END PRODUCT

Impact of layer thickness and the variation of thickness on pavement performance and the occurrence of pavement distress.

A relationship or tabulation between increased thickness variances or standard deviations (coefficient of variations) and reduced ride quality or reduced pavement service life.

POTENTIAL PRODUCT USE

Development of pay reduction factors based on thickness deviations.

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial stage.
  • Establish the thickness variability along each test section.
  • Complete a regression study of the variation in thickness (HMA) and the different performance measures and determine if threshold limits of variances in HMA thickness affect selected distresses.
  • Accumulate and/or determine the initial IRI measured at each test section.
  • Complete a regression study of the variation in thickness (HMA) and the initial IRI and determine if threshold limits of variances in HMA thickness increase the initial roughness (reduced ride quality) of the as-built pavement.
  • Develop reduction in service life based on these increased variances in HMA thickness.

(1)   The initial IRI values (longitudinal profile measured within 6 months of construction, assuming reasonable performance of the test sections) are needed to obtain the full benefit of the research study. The initial IRI values will need to be predicted from the time series data for some of the test sections or SPS-1 projects.


Table 33. Identification of future research studies from the SPS-1 experiment—
effect of base material type on pavement performance.

OBJECTIVE NO. 5

Determine the effect of base material selection and type on the performance of flexible pavements.  (Expected timeframe—2003 to 2004).

TOPIC AREA

Design

PROBABILITY OF SUCCESS

High

LTPP STRATEGIC PLAN

2.A, 2.D, 7.A, 7.B, 7.C

SUPPLEMENTAL EXPERIMENTS

GPS-1 and GPS-2 experiments

END PRODUCT

Impact of the base material type and condition on pavement performance and the occurrence of pavement distress.

A decision tree and/or specifications for selecting base material types (material specifications) and when to stabilize the base layer.

POTENTIAL PRODUCT USE

To assist design engineers in selecting different base materials for specific site conditions and traffic levels and to determine the conditions when base stabilization will provide the most benefit.

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial analysis stage.
  • Conduct a regression study or analyses of the SPS-1 data to determine the effect of basethickness and type on performance.
  • Evaluate the effect of material noncompliance on performance and make adjustments for those test sections with noncomplying materials.
  • Determine the increase in performance or service life for the stabilized bases and the effect of base thickness on pavement life.

Table 34. Identification of future research studies from the SPS-1 experiment—effect of seasonal changes on pavement response and material responses related to performance.

OBJECTIVE NO. 6

Effect of seasonal conditions or changes on the response of the pavement structure and material response or modulus of unbound pavement materials and subgrade soils as related to pavement performance.  (Expected timeframe—2004 to 2005).  

TOPIC AREA

Materials characterization and pavement management.

PROBABILITY OF SUCCESS

High

LTPP STRATEGIC PLAN

2.A, 3.C, 3.E

SUPPLEMENTAL EXPERIMENTS

GPS-1 and GPS-2

END PRODUCT

Improvement of environmental effects and considerations in pavement design, material selection (or specifications), and performance predictions.

A table summarizing the seasonal modulus ratio and a map showing locations or areas with significant seasonal effects for different pavement types.

POTENTIAL PRODUCT USE

Allow designers and pavement management engineers to identify typical times of low modulus values.

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial analysis stage.
  • Categorize the pavement structure with different soil types and base/subbase types and thicknesses in different climatic areas.
  • Identify and select those projects and test sections with sufficient time series deflection data (three or four measurements during different seasons of the year).
  • Calculate the modulus ratio for each season or measurement date from a "standard" modulus value or time of year.
  • Conduct a regression analysis of the seasonal modulus ratios to determine their correspondence with surface cracking (or permeability), type of pavement structure, layer thickness, subgrade soil type, and various climatic parameters (such as rainfall).

Table 35. Identification of future research studies from the SPS-1 experiment—
effect of soil type and stiffness on pavement performance.

OBJECTIVE NO. 7

Quantify the effect of soil type on pavement performance measures (specifically ride quality) and minimum pavement thickness over the foundation.  (Expected timeframe—2003 to 2004).

TOPIC AREA

Materials characterization and pavement design.

PROBABILITY OF SUCCESS

High

LTPP STRATEGIC PLAN

2.A, 3.A, 7.B, 7.C

SUPPLEMENTAL EXPERIMENTS

GPS-1 and GPS-2

END PRODUCT

Improvement in materials characterization of soils for design and development/confirmation of design criteria to protect the subgrade soils.

Minimum pavement thickness design criteria for selected soil types to protect and maintain ride quality.

POTENTIAL PRODUCT USE

Minimum pavement design standards over specific soil types and identification of minimum pavement thickness to maintain selected ride quality.

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial analysis stage.
  • Categorize the pavement structure with different soil types and base/subbase type and thickness in different climatic areas.
  • Identify and select those projects and test sections with sufficient time series deflection data (three or four measurements during different seasons of the year).
  • Conduct statistical analyses to determine the significant properties or soil types to determine their correspondence with surface cracking, ride quality as measured by IRI, type of pavement structure, layer thickness, and various climatic parameters (such as rainfall).
  • Based on statistical and mechanistic analyses, identify the minimum layer thickness to maintain a selected ride quality level and minimize the occurrence of pavement distress.

Table 36. Identification of future research studies from the SPS-1 experiment—effect of base condition on pavement performance.

OBJECTIVE NO. 8

Determine the effect of base condition (such as moisture content, compaction, and degree of saturation) on the performance of flexible pavements.  (Expected timeframe—2003 to 2004).

TOPIC AREA

Design and construction.

PROBABILITY OF SUCCESS

Moderate to high(1)

LTPP STRATEGIC PLAN

2.A, 2.B, 2.D, 2.E, 3.A, 7.A, 7.B, 7.C

SUPPLEMENTAL EXPERIMENTS

GPS-1 and GPS-2 experiments

END PRODUCT

Improvement of materials characterization, and impact of unbound aggregate base layer specifications and condition after construction on pavement performance and the occurrence of pavement distress.

A tabulation of base condition (density, moisture content, gradation, and other physical properties) to design values—modulus and moisture sensitivity.

POTENTIAL PRODUCT USE

Development of performance related specifications and design/construction criteria or properties as related to performance, and assistance in developing pay reduction factors.

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial analysis stage.
  • Evaluate the as-built moisture contents, density, gradations, and quality of base material and categorize the different test sections with significant differences.
  • Evaluate the structural response (deflections) and backcalculated layer modulus of the base layer.
  • Correlate the physical properties and response properties to the condition of the base.
  • Determine the effect, if any, on the performance and individual distresses of the pavement, including the decrease in ride quality with time and traffic.  
  • Establish threshold limits or other criteria that can be used in design and construction—effect of construction variability of the base properties and performance.

(1) The probability of success will increase as the materials test data are completed and become available for all pavement layers and for the subgrade soils.

Table 37. Identification of future research studies from the SPS-1 experiment—
effect of HMA properties on pavement performance.

OBJECTIVE NO. 9

Determine the effect of HMA compaction and material properties (gradation and resilient modulus) on pavement performance.  (Expected timeframe—2003 to 2004).

TOPIC AREA

Design and construction.

PROBABILITY OF SUCCESS

High

LTPP STRATEGIC PLAN

2.A, 2.D, 2.E, 3.C, 3.E, 7.B, 7.C

SUPPLEMENTAL EXPERIMENTS

SPS-5, GPS-1, GPS-2 and SPS-9 experiments

END PRODUCT

Improvement of HMA mixture characterization and impact of HMA properties after construction and specifications on pavement performance and the occurrence of pavement distress.

A set of material or mixture properties that can be used in mixture design and material selection, and in structural design for layer thickness determination.

POTENTIAL PRODUCT USE

Assist in the development of performance related specifications, the development of pay reduction factors, and development of material specifications to be used in construction (layer acceptance) and in design for determining layer thicknesses.

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial analysis stage.
  • Determine the physical properties at construction for each HMA layer of each test section.
  • Compare the backcalculated layer modulus with the laboratory measured resilient modulus, define any differences, and determine those factors or variables that have an effect on those differences.
  • Establish if any performance differences in ride quality and pavement distresses (cracking and rut depths) can be attributed to selected or combined material/mixture properties.
  • Establish threshold properties and/or criteria that result in an increased level of distresses or a reduction in ride quality.
  • Establish whether some of the material related distresses (raveling or bleeding) are related to these values.
  • Develop criteria for mixture design and construction acceptance criteria.

Table 38. Identification of future research studies from the SPS-1 experiment—
quantification of remaining life of cracked or damaged HMA layers.

OBJECTIVE NO. 10

Quantification of the remaining life of cracked or damaged asphalt concrete layers.  (Expected timeframe—2004 to 2005).

TOPIC AREA

Pavement management and overlay design.

PROBABILITY OF SUCCESS

High

LTPP STRATEGIC PLAN

4.B, 5.B, 5.C, 6.B

SUPPLEMENTAL EXPERIMENTS

SPS-5, SPS-9, GPS-1, GPS-2, GPS6A & B experiments

END PRODUCT

Improvement in HMA layer characterization and guidance for maintenance and rehabilitation strategy selection and HMA overlay performance prediction.

A reduced modulus scale that is representative of a cracked HMA layer. This scale would be deflection and distress based so that the results from distress surveys can be used to estimate the remaining life of an HMA surface.

POTENTIAL PRODUCT USE

Pavement management studies to determine the expected time for maintenance and/or rehabilitation, and overlay designs and rehabilitation studies.  

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial analysis stage.
  • Backcalculate the modulus of test sections with different types, extents, and severity levels of cracking.
  • Estimate the HMA modulus to the uncracked condition taking into account aging and temperature effects on the HMA modulus.
  • Relate these modulus values to the laboratory test results and compute a modulus damage ratio.
  • Complete a regression analysis of all ratios to define in mathematical terms the equivalent modulus ratio based on the initial or uncracked value.   

Note:  One of the components that will be needed to improve the accuracy of the results is to have comparable time measurements of deflection data and distress surveys. In addition, the resilient modulus of the HMA mixtures will be needed to improve universal application of the results.


Table 39. Identification of future research studies from the SPS-1 experiment—identify those
properties and conditions most conducive to the development of surface initiated fatigue cracks.

OBJECTIVE NO. 11

Confirm the hypothesis of surface initiated fatigue cracks and identify those properties or conditions most conducive to the development of surface initiated fatigue cracks.

TOPIC AREA

Design

PROBABILITY OF SUCCESS

Moderate to high(1)

LTPP STRATEGIC PLAN

2.A, 5.C, 7.B

SUPPLEMENTAL EXPERIMENTS

GPS-2 experiment

END PRODUCT

Improvement in HMA mixture characterization for distress prediction, and development of new pavement response model and performance/distress prediction models applicable to pavement design.

Mixture design criteria to minimize the occurrence of surface initiated fatigue cracks.

Identification and listing of those factors and/or properties that increase the probability of surface initiated fatigue cracks.

POTENTIAL PRODUCT USE

Identifying the mixture design properties and pavement conditions for which surface initiated fatigue cracks are likely to develop, and determining the criteria to be used in design.

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial analysis stage.
  • Identify and prioritize the test sections that are susceptible to fatigue cracks initiating at the surface.
  • Verify that those sites have fatigue cracks that initiated at the surface of the HMA layer (through distress surveys and coring studies).
  • Conduct statistical studies to identify the properties of the HMA layer and pavement that are conducive for fatigue cracks to initiate at the surface of the pavement.
  • Establish pavement response criteria (for example, deflection criteria) that can be used to design pavements to minimize the occurrence of surface initiated fatigue cracks.
  • Determine the mixture properties and environmental/pavement conditions (soil conditions, base type and thickness, traffic levels and climate) for which surface initiated fatigue cracks are most likely to develop.

(1)   The probability of success will increase greatly if cores are performed as a part of special interim studies and all forensic studies to confirm the location of where the fatigue cracks initiated.


Table 40. Identification of future research studies from the SPS-1 experiment—
applicability of the subgrade protection criteria for use in design of flexible pavements.

OBJECTIVE NO. 12

Quantify the applicability of the subgrade protection criteria—limiting subgrade vertical compressive stains and deflections for use in design of flexible pavements.  (Expected timeframe—2004 to 2005).

TOPIC AREA

Design

PROBABILITY OF SUCCESS

Moderate to high(1)

LTPP STRATEGIC PLAN

5.A, 5.B, 5.C, 7.C

SUPPLEMENTAL EXPERIMENTS

GPS-1 and GPS-2 experiments

END PRODUCT

Improvement in subgrade soil characterization for design, and development/confirmation of design criteria to protect the subgrade soil and foundation layers.

Limiting subgrade vertical strain and deflection criteria if found to be appropriate.

POTENTIAL PRODUCT USE

Identifying the conditions for which subgrade protection is required and would control the design, and determining the criteria to be used in design.

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial analysis stage.
  • Identify and prioritize the test sections that are susceptible to distortions in the subgrade.
  • Verify that those sites have subgrade distortion (either through distress surveys, transverse profiles, or trenches).
  • Determine the limiting subgrade vertical strains and the conditions (soil conditions, traffic levels, and pavement structure) for which the subgrade protection is required.

(1)   The probability of success will increase greatly if trenches are performed as a part of all forensic studies to confirm any subgrade distortion.


Table 41. Identification of future research studies from the SPS-1 experiment—confirm the
C-values or differences between laboratory measured resilient modulus and
backcalculated elastic layer modulus.

OBJECTIVE NO. 13

Compare and quantify any differences between backcalculated modulus values using "MODCOMP" and laboratory measured resilient modulus.  (Expected timeframe—2003 to 2004).

TOPIC AREA

Materials and pavement design.

PROBABILITY OF SUCCESS

High

LTPP STRATEGIC PLAN

2.B

SUPPLEMENTAL EXPERIMENTS

GPS-1 and GPS-2

END PRODUCT

Improvement in pavement and subgrade soils material characterization for pavement design and evaluation.

A table or graph showing the differences (ratios between laboratory and backcalculated modulus values).

Confirmation of the C-value and the factors that affect its magnitude.

POTENTIAL PRODUCT USE

To define those conditions in which the laboratory and field derived values are different for use in design and rehabilitation studies.

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial analysis stage.
  • Determine or extract the backcalculated modulus values for each layer of the pavement and subgrade and confirm the reasonableness of those values.  Remove those sites for which compensating errors have occurred in the solutions.
  • Extract the resilient modulus test data for all pavement layers.
  • Determine the resilient modulus for each layer and subgrade from laboratory measurements for the conditions during each deflection survey.
  • Compare the backcalculated values to the laboratory-measured values at similar stress states and temperatures.

Table 42. Identification of future research studies from the SPS-1 experiment—
mechanistic analysis of the SPS-1 sites.

OBJECTIVE NO. 14

Conduct mechanistic analyses of the SPS-1 project sites to gain knowledge of critical stresses, strains, and deflections to explain their performance in terms of fatigue cracking, permanent deformation within each layer, and ride quality. (Expected timeframe—2003 to 2005).

TOPIC AREA

Pavement design and construction.

PROBABILITY OF SUCCESS

Moderate to high

LTPP STRATEGIC PLAN

2.D, 7.B

SUPPLEMENTAL EXPERIMENTS

END PRODUCT

Evaluation and/or development of new pavement response and performance prediction models applicable to pavement design and performance predictions.

In-depth field verified knowledge as to the effects of critical measured structural responses that will be useful in pavement design, evaluation, and rehabilitation.

POTENTIAL PRODUCT USE

Knowledge gained from this experiment will be useful to researchers and others for improving design procedures to make HMA pavements a more cost-effective and reliable pavement, whose performance can be predicted with structural response models.

GENERAL TASKS

  • Review specific findings from each SPS-1 project related to the initial analysis stage.
  • Establish a comprehensive input database that includes design, construction, materials test results, traffic, climate, monitoring data, and structural monitoring data (deflections).
  • Analyze the cracking and rutting that have occurred at all sites using the longitudinal and transverse profile data and distress data that have been measured with time.
  • Perform mechanistic analyses to determine the critical response stress, strain and/or deflection and cumulative fatigue damage and permanent deformation for the traffic loadings and site-specific conditions.
  • Analyze the results and develop findings and recommendations as to the impacts of loading and material properties on the performance of flexible pavements.

Data Collection Efforts

It is recommended that the following data collection efforts be emphasized in the future in support of the second-stage analyses:

It is recommended that the following specific data analyses be conducted on SPS-1 data:

 


The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
FHWA
United States Department of Transportation - Federal Highway Administration