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Publication Number: FHWA-RD-01-168
Date: July 2006

Rehabilitation of Asphalt Concrete Pavements: Initial Evaluation of The SPS-5 Experiment-Final Report

Chapter 6. Summary And Conclusions

The SPS-5 experiment entitled Rehabilitation of Asphalt Concrete Pavement is a key experiment of the LTPP program. The main objective of SPS-5 is to determine the long-term effectiveness of different rehabilitation techniques on pavement performance and service life. There are some concerns about whether SPS-5 can meet its expectations given that several projects were constructed on coarse-grained, subgrade soils. Although this is considered a significant deviation from the experimental plan, it is not believed to be detrimental to the overall expectations for this experiment as long as it is fully considered in the data analyses.

This report presents results from the first comprehensive review and evaluation of SPS-5. Issues of experiment design, construction quality, data availability and completeness, and early performance trends have been addressed. That unavailable data have been identified here does not necessarily mean that the data were not collected or submitted by the RCO or owner agency that built the individual projects. There can be several reasons that good data can be delayed before reaching Level E status. Following are some reasons that some data elements could be shown as unavailable when the data actually had been collected.

Some initially unavailable data were located and forwarded to the IMS during the course of this study. The key findings or observations from this detailed review are summarized in this chapter.

SPS-5 EXPERIMENTAL SITE STATUS

As of January 2000, 18 SPS-5 projects had been identified throughout North America. The full factorial of the original experiment design had been completely filled except for 2 cells. One project was required in a dry-freeze environment on a pavement in poor condition. This project would serve as a replicate to the Manitoba project (refer to table 8). Two projects were required for the wet-no-freeze climate with a pavement in fair condition. These missing projects were not believed to be critical because the factorial covered the range of environments and pavement conditions previously included in the experiment design.

Each SPS-5 project had 9 core test sections, and some had supplemental test sections built by the individual agency; 162 core test sections and 48 supplemental test sections were available, a total of 210 test sections. This number of test sections should provide excellent data for future studies for calibrating and validating design procedures.

The primary value of the supplemental sections is to serve as a direct comparison to the core test sections within that specific SPS-5 project. However, these supplemental sections also can be used in regional or national studies through the use of mechanistic analysis principles. Thus, efforts should be made to ensure that their construction and monitoring data are collected and entered in the IMS for future use.

An important issue in the experimental factorial is the different soil classifications. Half the projects were built over coarse-grained and half over fine-grained soils. This deviation from the original experiment design is not believed to be critical, but should be considered when analyzing the data to determine the main factor effects on performance.

DESIGN VERSUS ACTUAL CONSTRUCTION

Experimental design factors were compared to the actual values measured during construction. These include both the site condition factors and rehabilitation design features in the IMS database. Most SPS-5 sections followed the experiment design for the large majority of design factors. Overall, very few construction deviations were reported for the SPS-5 projects, with the exception of overlay thickness.

Most overlay thickness measurements deviated from the project’s experiment design requirements. However, none of the thickness data for the thin and thick overlays overlapped. Two projects (Maine and Manitoba) had significant thickness deviations from the planned overlay thickness. In both cases, the SHA was attempting to correct problems in the cross-slope of the pavement.

The other construction deviations were primarily related to the HMA mixtures. The percentage passing the number 4 sieve for the HMA mixture and air voids of the compacted mat exceeded the specified values for many test sections. These deviations were considered minor and should not be critical to the overall experiment. Other minor deviations were noted in the construction of these projects. For example, breakdowns of the hot mix facility and paving equipment caused delays in construction. Most of these types of deviations were considered minor and should not be critical to the overall experiment.

Three projects incorporated a control section that was overlaid during project construction of the other test sections: Colorado, Montana, and New Mexico. In each case, the condition of the existing pavement was believed to be a risk to the traveling public.

DATA AVAILABILITY AND COMPLETENESS

The data availability and completeness for the SPS-5 experiment were good overall, with the exception of two data elements. These two elements were materials testing and traffic data. Furthermore, some monitoring data still needed to be collected and/or checked to fill in the gaps of the time-history performance data for selected projects. Three projects (Minnesota, New Mexico, and Oklahoma) did not have sufficient time-history data for transverse profile to establish performance trends. The transverse profile should be measured at each of these sites. The reasons that data had not achieved Level E status need to be ascertained and the situation rectified before detailed analyses of the experiment can be completed. Most other data elements that had been collected at each site were at level E.

The SPS-5 data deficiencies are summarized below.

It is recommended that a significant effort be put forth to obtain these missing data. The following sections summarize the availability of each data element and its effect on following studies, such as for the 2002 Design Guide (NCHRP 1-37A). (10)

Construction Reports/Data

The construction and deviation reports were extremely valuable in reviewing and explaining performance anomalies of individual test sections. Construction and deviation reports were available for all of projects except Missouri. The Missouri project was recently constructed, but the construction report was unavailable at the time of the detailed review.

Materials Data

The materials data were partially complete for all of the projects, with the exception of the new projects that had no test data at Level E. The laboratory material testing was divided into preconstruction and postconstruction tests. Preconstruction tests were to be performed on each existing pavement layer and the subgrade, while postconstruction tests were confined to the HMA overlay mixture.

Tables 10 through 12 summarized the availability of selected test data by material type for each project. Extensive test data were unavailable at the time of the data extraction–especially for the HMA materials. In general, the younger the project, the less testing had been completed. None of the resilient modulus, indirect tensile strength, and creep compliance tests had been completed on the HMA mixtures for any project.

Unavailable materials test data to determine the physical properties of the pavement and soils will be a significant limitation in the SPS-5 project’s use in mechanistic studies (such as NCHRP 1-37A). Completion of the materials testing program should be a high priority to ensure achieving the full benefit of the SPS-5 experiment. The RCOs recognized the importance of this data element and were pursuing these data. The materials testing program was still underway, and materials test data were being submitted to the RCOs periodically.

Climatic Data

No climatic data were missing. The SPS-5 experiment design called for a project to be located in one of four different climates (refer to tables 1 and 3). The climatic data are estimated from historical data from five nearby weather stations (virtual weather stations). The IMS contains monthly and average annual summary statistics for all 18 projects.

Traffic Data

The SPS-5 experiment design calls for continuous AVC monitoring, with WIM data collected at least 2 days of the year, as permitted by WIM scale operating divisions. Continuous AVC monitoring was defined as more than 300 AVC monitoring days in a given year. Table TRF_MONITOR_BASIC_INFO was examined to identify the SPS-5 records with WIM, AVC, and annual ESAL estimates.

Table 15 summarizes the amount of data for the SPS-5 sites and identifies those projects that had no traffic data at Level E. In summary, 14 (about 75 percent) of the SPS-5 sites had no traffic monitoring equipment at the site. Most of the older projects did have some traffic data, while most of the newer projects were missing the traffic data. All projects had an annual estimate of the number of ESALs; however, the reliability of these data was unknown for 15 of the 18 projects.

Performance Indicator Data

Several types of monitoring data are included in the LTPP IMS, including distresses (from both manual and photographic surveys), longitudinal profiles, transverse profiles, and deflection. Performance data are collected for both preconstruction and postconstruction time frames. All projects have preconstruction information on the surface condition of the pavement. Table 16 summarizes the number of postconstruction distress and other performance indicator measurements made at each project site; table 17 summarizes the average number of years between the surveys for each performance indicator.

Performance indicator monitoring data were available for all projects. However, the time interval between data collections was beginning to exceed the recommended frequency for a few of the projects, as noted below.

The RCOs had taken steps to collect some missing data or submit data that had been collected but not forwarded to the IMS. In summary, the amount of performance indicator data was good. The time-series data for each measure of performance will be a significant benefit for future studies on the design and performance of rehabilitated flexible pavements.

Friction Data

With few exceptions, friction surveys had not been performed on the SPS-5 projects. This testing, however, was not required at the time nor is it essential to the SPS-5 experiment. That friction data were missing should have no impact on future studies on structural behavior and performance of HMA overlays.

Summary

Table 28 summarizes the unavailable and limited data for the SPS-5 experiment, as of January 2000, in terms of the revised experimental factorial. Table 29 summarizes the limitations and action items to correct these deficiencies in each SPS-5 project. Every effort should be made to obtain these data elements.

EARLY PERFORMANCE TRENDS

Most SPS-5 projects were still relatively young. As of January 2000, less than 45 percent of the test sections had distress magnitudes that exceeded values believed necessary to complete meaningful comparisons. Based on preliminary statistical analyses and comparisons, age, surface preparation, and pre-existing surface condition were found to have an effect on performance indicators. The long-term performance trends could be significantly different from these early observations as more and more data are collected for these test sections.

The specific experimental expectations of the SPS-5 experiment were 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:

The following conclusions were drawn from the preliminary performance analyses conducted for this report.

This evaluation has shown that several problems will limit results that can be obtained by the SPS-5 experiment, two of minor and two of major importance. Of minor importance are (2) the misinterpretation of the different distress types with time by the distress surveyors and (2) the measurement error of low levels of distress, which results in difficulties in interpreting performance trends and in determining the effects between the experiment factors.

Table 28. Summary of unavailable and limited data for the SPS-5 experiment.
Pavement
Condition
Before Overlay
Subgrade Soil TypeClimate, Mois Pavement ture-Temperature by State, Section, and Age1,2,3
Wet-FreezeWet-No-FreezeDry-Freeze1Dry-No-Freeze1
FairFine grainedGA(8)-6.2:
No WIM/AVC equipment
installed=Subgrade–Limited
classification, resilient modulus
and M-D data.
Existing HMA–Limited mix,
asphalt and aggregate data.
HMA overlay–Missing asphalt
and aggregate data, and limited
mix data.
Preconstruction transverse
profile unavailable.
No projectsMN(3)-8.9:
Missing recent transverse
profile.
Subgrade–Missing resilient
modulus data.
Exist. HMA–Missing asphalt
and aggregate data, and limited
mix data.
HMA overlay–Missing mix,
asphalt, and aggregate data.
Preconstruction transverse
profile unavailable.
OK(1)-2.1:
No WIM/AVC equipment
installed.
Subgrade–Missing
classification, resilient modulus,
and M-D data.
Existing HMA–Limited mix
data
HMA overlay–Missing
aggregate and asphalt data,
and limited mix data.
Preconstruction transverse
profile unavailable.
CO(2)-7.9:
Existing HMA–Limited mix,
asphalt and aggregate data.
HMA overlay–Limited mix,
asphalt, and aggregate data.
TX(0)-7.8:
HMA overlay–Missing
aggregate data, and limited
asphalt and mix data.,
Preconstruction distress survey
Coarse grainedNJ(2)-7.0:
Subgrade–Missing M-D data
and limited classification and
resilient modulus data.
Exist. HMA–Missing asphalt
and aggregate data, and limited
mix data.
HMA overlay–Limited asphalt
data.
No projects AB(0)-8.9:
Subgrade–Limited resilient
modulus data.
Preconstruction transverse
profile unavailable.
NM(0)-2.9:
No WIM/AVC equipment
installed.
Missing recent transverse
profile.
Subgrade–Missing
classification, resilient modulus,
and M-D data.
Existing HMA–Limited mix
data.
HMA overlay–Missing mix,
asphalt, and aggregate data.
Preconstruction transverse
profile unavailable.
MT(2)-8.0:
No WIM equipment installed.
Subgrade–Limited resilient
modulus data.
PoorFine grainedMD(1)-4.1:
Subgrade–Missing
classification, resilient modulus,
and M-D data.
MO(0)-0.0:
The Missouri project was
constructed recently–no data
are available as of January
2000.
MS(1)-8.9:
Limited distress surveys and
transverse profiles.
Existing HMA–Missing
aggregate and asphalt data,
and limited mix data.
HMA overlay–Missing asphalt
and aggregate data, and limited
mix data.
MB(0)-10.0:
No WIM equipment installed.
Subgrade–Missing resilient
modulus data & limited
classification data.
Existing HMA–Missing mix,
asphalt, and aggregate data.
HMA overlay–Missing asphalt
and aggregate data, and limited
mix data. Preconstruction transverse and longitudinal profiles unavailable.
No projects
 Coarse grainedME(1)-4.1:
No WIM/AVC equipment installed. Subgrade–Missing resilient modulus data. Existing HMA–Missing asphalt data. HMA overlay–Limited mix data.
FL(6)-4.3:No WIM/AVC equipment installed. Subgrade–Limited classification, resilient modulus, and M-D data. Existing HMA–Missing asphalt and aggregate data, and limited mix data. Preconstruction transverse profile unavailable. No projectsAZ(2)-9.2:Subgrade–Missing resilient modulus data. Preconstruction transverse profile unavailable.
AL(2)-7.7:No WIM/AVC equipment installed. Limited transverse profiles. Existing HMA–Missing mix, asphalt, and aggregate data. HMA overlay–Missing mix, asphalt, and aggregate data. Preconstruction transverse profile unavailable. CA(13)-9.2:Subgrade–Missing resilient modulus data. Existing HMA–Missing mix, asphalt, and aggregate data. HMA overlay–Missing asphalt, mix, and aggregate data.
Notes:
  1. The values in parentheses are the numbers of supplemental test sections for each project. The other value provided for each project is the age of that project in years, as of January 2000.
  2. The moisture susceptibility tests for the existing HMA surface and HMA overlay are missing for all projects, with the exception of the Florida, Maryland, and Maine projects.
  3. The indirect tensile resilient modulus, strength, and creep compliance test for the existing HMA surface and HMA overlay are missing for all projects.

 

Table 29. Deficiencies and action items for each SPS-5 project.
SPS-5 ProjectDeficiency - IssueSuggested Action
AL, FL, GA, ME, MO, NM, OKNo traffic measuring equipment installed at the site.Install traffic monitoring equipment at the sites.
AB, LA, FL, GA, ME, MO, MB, NM, OKNo traffic data has Level E status.Process the traffic data that has been collected or address the reasons data do not have a Level E status.
All projectsInsufficient materials test data available for the essential material properties.Complete the test program. Use back-calculation of elastic layer modulus until laboratory test data become available.
All projectsLayer thickness deviates from the planned thickness more than allowed by project requirements.None–Adjust or normalize the performance to account for the thickness difference between the test sections.
All projectsIn-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.
AZ, AB, AL, FL, GA, MN, MB, NM OKPreconstruction transverse profile not at Level E.Process data or determine reasons data are not at Level E.
TXPreconstruction distress data not at Level E.Process data or determine reasons data are not at Level E.
MBPreconstruction longitudinal profile not at Level E.Process data or determine reasons data are not at Level E.
AL, MO, NM, OKData from IMS table TST_L05B not at Level E.Process data or determine reasons data are not at Level E.
MN, MOData from IMS table SPS5_LAYER not at Level E.Process data or determine reasons data are not at Level E.
Ms, MT, TXLimited manual distress data.Take immediate action to collect these data or process the data collected.
AL, MN, NMLimited transverse profile data.Take immediate action to collect these data or process the data collected.
AL, NMLimited longitudinal profile data.Take immediate action to collect these data or process the data collected.

The two problems that could result in major limitations to the value of SPS-5 are the materials test data and traffic data that are unavailable in the IMS. 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.

EXPECTATIONS FROM THE OWNER AGENCIES

At one national workshop, input was received from the States and Provinces on the SPS-5 experiment (April 27, 2000, in Newport, Rhode Island).(5) Several agencies made presentations on the status of their individual SPS-5 project and on their expectations for the experiment. Panel discussions on the future direction and analysis of the SPS-5 data are summarized in this section.

In general, the owner agencies seem to be satisfied with the experiment and believed that it would produce valuable information on different design factors and features. Many agencies had been conducting or were planning their own analyses on their individual SPS-5 projects. Some of these analyses were yielding useful results; however, the agencies wanted a focus on implementation.

First and foremost, agencies wanted a research-quality database from SPS-5. Second, the agencies wanted to be able to determine the impacts of the rehabilitation design features on overlay performance and the effectiveness of the SPS-5 experiment design factors, such as:

In addition to the structural design features, the agencies also wanted to know what major site condition factors influence the performance of HMA overlays over flexible pavements, including:

Other expectations from the agencies included:

As to the future analysis plan for SPS-5, the agencies believed that it was worthwhile first to fill in the missing data–specifically, obtain traffic and materials test data. Some presenters at the SPS conference requested that fundamental studies be conducted to determine how the SPS-5 sections were responding to load and environmental stresses and loads. It also was suggested that an integrated analysis plan be developed for future research studies.

CAN THE SPS-5 EXPERIMENT MEET EXPECTATIONS?

This data review and evaluation of early performance trends showed that several significant data issues will limit the results that can be obtained from SPS-5. The missing traffic data and key material test data must be obtained before meaningful global analysis can be performed. A few SPS-5 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 findings cannot be obtained from the SPS-5 experiment even if no more traffic and materials data become available. Some important early trends were already identified that will be useful for the design and construction of HMA overlays of flexible pavements, even though all projects were less than 10 years old. Continual monitoring of the projects and test sections will provide valuable performance data that can be used in future studies to answer the questions asked by the owner agencies. Thus, it is concluded from this comprehensive study of the SPS-5 experiment that the expectations from the owner agencies and HMA industry can be met.

RECOMMENDATIONS FOR FUTURE ACTIVITIES

As stated in chapter 1, the key objective of the SPS-5 experiment is to determine the relative influence of different rehabilitation techniques on flexible pavement performance. It is believed that the experiment will be able to achieve this key objective with time. At the time of this report, the oldest SPS-5 project was just over 10 years (most were 5 to 7 years old), so many test sections had a moderate amount of surface distress, but only a few had been taken out service. The real benefit from this experiment will occur in the years ahead, as a greater percentage of test sections exhibit higher levels of distress–magnifying the effect of the experimental and other rehabilitation factors on performance.

The SPS-5 assessment report focused on the quality and completeness of SPS-5 construction and monitoring data, and on the adequacy of the experiment to achieve the original expectations and objectives. Some data were unavailable, but this will not significantly limit the value of the results. Detailed analysis of the effect of different design factors on performance was outside the scope of work for this study. Thus, future studies using SPS-5 experimental data should be planned and prioritized so they can be initiated as the SPS-5 projects exhibit higher levels of distress.

These future studies should be planned in two stages that focus on local and national expectations from SPS-5. The first stage would be a detailed assessment of each structural cell in the experiment to support local interests. The second would examine selected data elements to evaluate the effect of different structural features across the whole experiment. Both are discussed below.

Initial Stage–Analysis of Local Expectations or Individual Factorial Cells

A detailed evaluation of the projects within each cell should be completed as soon as some test sections begin to exhibit higher levels of at least one distress type. The purpose of these 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 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-5 experiment can contribute to the following specific analyses outlined in the strategic plan.

A description of some future studies that could be pursued at the national level using all of the SPS-5 experimental data are summarized in tables 30 through 41. 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-5 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-5 experiment; objectives in the second are related to other experimental factors.

The following second-stage analysis objectives are recommended for the SPS-5 experiment; they are presented in more detail in tables 30 through 41.

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

(30) Perform test-section-by-test section analyses of the projects included in the SPS-5 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 between the projects. This objective is the initial analysis of the individual factorial cells.
(31)Determine the effect of the main SPS-5 experimental factors on the performance of flexible pavements.
(32)Determine the effect of layer thickness variations on LTPP and initial ride quality.
(33)Estimate the effect of seasonal conditions or changes on pavement response and the response of individual materials and on the subgrade soils.
(34)Quantify the effect of the existing pavement condition and distress extent on the performance measures (specifically ride quality) and minimum overlay thickness over the existing pavement.
(35)Quantify the effect of milling the existing HMA surface before overlay placement.
(36)Determine the effect of HMA mixture characteristics and the use of RAP on the performance of HMA overlays.

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

(37) Determine the effect of HMA compaction and material properties (gradation and resilient modulus) on pavement performance and whether there are consistent differences between HMA mixture with and without RAP.
(38)Quantify the remaining life of cracked or damaged HMA overlays.
(39)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 starting at the surface of the pavement.
(40)Conduct mechanistic analyses of the SPS-5 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.
(41)Quantification of the subgrade protection criteria for limiting the vertical compressive strains in the subgrade and overall deflection for overlay thickness design.

Data-Collection Efforts

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

Table 30. Identification of future research studies from the SPS-5 experiment–Initial analysis of the individual factorial cells and companion projects.
OBJECTIVE NO. 1
Perform test-section-by-test-section analysis of the SPS-5 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.
TOPIC AREA
Pavement design
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-5 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 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-5 project specifications, if any, on pavement performance and response.
  • Develop findings regarding comparisons made between the 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 31. Identification of future research studies from the SPS-5 experiment–Overall effect of the main experimental factors on performance.
OBJECTIVE NO. 2
Determine the effect of the main SPS-5 experimental factors on the performance of the flexible pavements.
TOPIC AREA
Pavement design
PROBABILITY OF SUCCESS
High
LTPP STRATEGIC PLAN
7.A, 7.B, 7.C
SUPPLEMENTAL EXPERIMENTS
END PRODUCT
Improvement in pavement materials characterization as related to performance and construction, and impact of specific design features and level of significance on pavement performance and the occurrence of pavement distress.
  • Minimum overlay thickness needed for different existing surface conditions, different surface treatments, and performance characteristics–minimum IRI levels.
  • The effect of different surface repair techniques (milling versus non-milling the existing surface before overlay placement) on overlay performance.
  • The effect of different HMA mixtures (with and without RAP) on overlay performance and identification of differences in mixture placement properties (for example, expected initial IRI values).
  • Seasonal factors that describe changes in the response of the pavement and materials related to performance and incremental deterioration.
Potential Product Use
  • Design cost-effective and reliable overlays and other rehabilitation techniques of flexible pavements.
  • Calibration and validation of HMA overlay design procedures/methods and distress prediction models for HMA overlays.
GENERAL TASKS
  • Review results and findings from each SPS-5 test section and 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 to 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 32. Identification of future research studies from the SPS-5 experiment–Effect of overlay thickness variations on performance.
OBJECTIVE NO. 3
Determine the effect of thickness variations on long-term HMA overlay performance and initial ride quality.
TOPIC AREA
Design
PROBABILITY OF SUCCESS
Moderate to high*
LTPP STRATEGIC PLAN
2.D, 7.B
SUPPLEMENTAL EXPERIMENTS
SPS-1 experiment
END PRODUCT
Impact of HMA overlay thickness and the variation of that thickness on overlay 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-5 project related to the initial stage.
  • Establish the thickness variability along each test section.
  • Complete a regression study of the variation in overlay 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 asbuilt pavement.
  • Develop reduction in service life based on these increased variances in HMA thickness.

* 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-5 projects.

Table 33. Identification of future research studies from the SPS-5 experiment–Effect of existing pavement surface condition on overlay performance.
OBJECTIVE NO. 4
Determine the effect of the existing pavement surface condition on selection of repair techniques and on the performance of HMA overlays.
TOPIC AREA
Design/construction
PROBABILITY OF SUCCESS
High
LTPP STRATEGIC PLAN
2.A, 2.B, 2.D, 2.E, 3.A, 7.A, 7.B, 7.C
SUPPLEMENTAL EXPERIMENTS
GPS*-6B
END PRODUCT
Improvement in identifying pavement surface condition/distress as related to overlay performance and providing guidance for maintenance and rehabilitation strategy selection and performance predictions.

A tabulation or decision tree of existing pavement surface condition for selecting repair techniques and minimum HMA overlay thickness required for different performance criteria.

POTENTIAL PRODUCT USE
Overlay design/construction criteria, as related to performance.
GENERAL TASKS
  • Review specific findings from each SPS-5 project related to the initial analysis stage.
  • Evaluate the existing surface condition on construction properties–thickness variations, initial IRI values, performance characteristics–and categorize the different test sections with significant differences.
  • Correlate the physical properties and response properties to the condition of the existing surface and repair techniques.
  • 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 existing surface condition and performance.

* GPS–General Pavement Studies

Table 34. Identification of future research studies from the SPS-5 experiment–Effect of surface repair technique on overlay construction and performance.
OBJECTIVE NO. 5
Determine the effect of repair techniques on HMA overlay construction and on the performance of HMA overlays.
TOPIC AREA
Design/construction
PROBABILITY OF SUCCESS
High
LTPP STRATEGIC PLAN
2.A, 2.B, 2.D, 2.E, 3.A, 7.A, 7.B, 7.C
SUPPLEMENTAL EXPERIMENTS
GPS-6B
END PRODUCT
Guidance for maintenance and rehabilitation strategy selection and performance predictions.

A tabulation for selecting repair techniques for different surface conditions as related to different performance criteria.

POTENTIAL PRODUCT USE
Overlay design/construction criteria, as related to performance.
GENERAL TASKS
  • Review specific findings from each SPS-5 project related to the initial analysis stage.
  • Evaluate the existing surface condition on construction properties–thickness variations, initial IRI values, performance characteristics–and categorize the different test sections with significant differences as related to those test sections with and without milling.
  • Correlate the HMA construction and response properties to the condition repair techniques.
  • Determine the effect of different repair techniques, 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 repair technique and existing surface condition on overlay performance.
Table 35. Identification of future research studies from the SPS-5 experiment–Effect of HMA mixture properties with and without RAP on overlay performance.
OBJECTIVE NO. 6
Determine the effect of HMA material properties or mixtures with and without RAP on overlay performance.
TOPIC AREA
Design/construction
PROBABILITY OF SUCCESS
High
LTPP STRATEGIC PLAN
2.A, 2.D, 2.E,. 3.C, 3.E, 7.B, 7.C
SUPPLEMENTAL EXPERIMENTS
GPS-1, GPS-2, and GPS-6
END PRODUCT
Improvement in HMA mixture characterization for overlay and new pavement design 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 and to develop material specifications to be used in construction (layer acceptance) and in design for determining overlay thickness.
GENERAL TASKS
  • Review specific findings from each SPS-5 project related to the initial analysis stage.
  • Determine the physical properties at construction for each HMA mixture of each test section.
  • Compare the back-calculated 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 mixture type or a combination of material/mixture properties related to mixtures with and without RAP.
  • 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 36. Identification of future research studies from the SPS-5 experiment–Effect of seasonal changes on pavement response and material responses related to overlay performance.
OBJECTIVE NO. 7
Effect of seasonal conditions or changes on the response of the pavement structure and HMA overlay response or modulus and on the other pavement layers and subgrade soils as related to pavement performance.
TOPIC AREA
Materials and pavement management
PROBABILITY OF SUCCESS
High
LTPP STRATEGIC PLAN
2.A, 3.C, 3.E
SUPPLEMENTAL EXPERIMENTS
GPS-6A and B
END PRODUCT
Improvement of environmental effects and considerations in overlay design, mixture 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 and overlays.
POTENTIAL PRODUCT USE
Allow designers and pavement management engineers to identify typical times of year when the pavement and overlay responses change significantly.
GENERAL TASKS
  • Review specific findings from each SPS-5 project related to the initial analysis stage.
  • Categorize the pavement structure with different soil types and pavement types 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 37. Identification of future research studies from the SPS-5 experiment–Effect of HMA overlay properties on pavement performance.
OBJECTIVE NO. 8
Determine the effect of HMA compaction and material properties (gradation and resilient modulus) on overlay performance.
TOPIC AREA
Materials 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-1, GPS-1, GPS-2, GPS-6
END PRODUCT
Improvement of HMA mixture characterization and impact of HMA overlay properties and specifications on performance and the occurrence of 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 the development of material specifications to be used in construction (layer acceptance) and in design for determining overlay thickness.
GENERAL TASKS
  • Review specific findings from each SPS-5 project related to the initial analysis stage.
  • Determine the physical properties at construction for the HMA overlay of each test section.
  • Establish if any performance differences in ride quality and pavement distresses (cracking and rut depths) can be attributed to one or a combination of 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-5 experiment–Quantification of remaining life of cracked or damaged HMA layers.
OBJECTIVE NO. 9
Quantification of the remaining life of cracked or damaged asphalt concrete layers.
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-1, SPS-9, GPS-1, GPS-2, GPS-6A, GPS-6B
END PRODUCT
Improvement of HMA layer characterization and guidance for maintenance and rehabilitation strategy selection and HMA overlay performance predictions.

A reduced modulus scale that is representative of a cracked HMA layer. This scale would be based on deflection and distress 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-5 project related to the initial analysis stage.
  • Back-calculate 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 component needed to improve the accuracy of the results is comparable time measurements of deflection data and distress surveys. In addition, the resilient modulus of the HMA mixtures will be needed to improve the universal application of the results.

Table 39. Identification of future research studies from the SPS-5 experiment–Identification of those properties and conditions most conducive to the development of surface-initiated fatigue cracks.
OBJECTIVE NO. 10
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*
LTPP STRATEGIC PLAN
2.A, 5.C, 7.B
SUPPLEMENTAL EXPERIMENTS
SPS-1 and GPS-2
END PRODUCT
Improvement in HMA mixture characterization for distress prediction, and development of new pavement response model and performance/distress prediction models applicable to overlay 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-5 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) in which surface initiated fatigue are most likely to develop.

* 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-5 experiment–Mechanistic analysis of the SPS-5 sites.
OBJECTIVE NO. 11
Conduct mechanistic analyses of the SPS-5 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.
TOPIC AREA
Pavement design and construction
PROBABILITY OF SUCCESS
Moderate to high
LTPP STRATEGIC PLAN
2.D and 7.B
SUPPLEMENTAL EXPERIMENTS
END PRODUCT
Evaluation and/or development of new pavement response and performance prediction models applicable to overlay 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-5 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.
Table 41. Identification of future research studies from the SPS-5 experiment–Applicability of the subgrade protection criteria for use in overlay 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.
TOPIC AREA
Design
PROBABILITY OF SUCCESS
Moderate to high*
LTPP STRATEGIC PLAN
5.A, 5.B, 5.C, 7.C
SUPPLEMENTAL EXPERIMENTS
GPS-6A and 6B
END PRODUCT
Improvement in subgrade soil characterization for design, and development/confirmation of design criteria to protect the subgrade soil and foundation layers for different rehabilitation strategies.

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-5 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.

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

 


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