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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-01-167
Date: April 2005

Structural Factors of Jointed Plain and Concrete Pavements: SPS-2 - Initial Evaluation and Analysis

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CHAPTER 7. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS

The SPS-2 project, Strategic Study of Structural Factors for Rigid Pavement of Jointed Plain Concrete Pavements, is one of the key experiments in the LTPP program. The main objective is to determine the relative influence and long-term effectiveness of the JPC pavement strategic factors that affect its performance. There are some concerns about the ability of the SPS-2 experiment to meet those expectations, given that several SPS-2 sites were not constructed. In addition, at the SPS-2 sites that were constructed, some construction deviations and data collection deficiencies exist.

This study provides the first comprehensive review and evaluation of the SPS-2 experiment. Issues of experimental design, construction quality, data availability and completeness, and early performance trends are addressed in this report. The key findings are summarized in this chapter. This chapter also presents the research team's recommendations on improving the SPS-2 experiment and its data availability, expectations of the SPS-2 experiment, and future data collection and analysis topics.

SPS-2 Experimental Site Status

As of January 2000, 13 SPS-2 sites have been constructed throughout the United States. An additional site in California has been nominated and is currently under construction. The full factorial of the original experimental design is not completely satisfied by the constructed sites, leaving a portion (31 percent) of the desired inference space (subgrade and climate) with no experimental sites.

These 14 sites fill in 11 of the 16 SPS-2 experiment factorial cells and provide good coverage of major areas of the United States. Five sites (31 percent) are missing from the original experiment design, as listed below:

Five additional SPS-2 sites (60 sections total) are needed to complete the design factorial. While it is impossible to determine the exact effects at this time, the lack of data from these missing sites will limit the results obtainable from the SPS-2 experiment, as summarized below:

Some of these deficiencies can be overcome through use of mechanistic data analysis. However, there is no mechanistic analysis method that considers all factors involved, and the missing data cells will always limit in the verification and calibration of any performance models.

In addition to the 12 core sections at each site required by the SPS-2 experiment, SHAs have built a total of 40 supplemental sections with SPS-2 projects. The main value of the supplemental sections will be as a direct comparison to the core sections. For example, one supplemental section in Washington did not have dowels, and this will provide a direct comparison to a similar design with dowel bars. Various other comparisons are possible, including skewed joints, base types, subdrainage, slab thickness, asphalt concrete pavement, jointed reinforced concrete, special dowels, and variable slab thickness. These sections are valuable to the States, and efforts should be made to ensure that their construction and monitoring data are collected and stored.

Data Availability and Completeness

The SPS-2 project is an extremely valuable source of performance data for modern concrete pavements. The performance of these sections will be of great interest to all highway agencies building concrete pavements, and of course to the concrete industry as well. This report will be the major reference document for all future SPS-2 research studies, and will be the major reference document for the 14 SHAs that have constructed SPS-2 project sites.

Data elements that were considered to be essential to the SPS-2 experiment analyses have been assessed. The data availability and completeness for the SPS-2 sites are considered good overall. A high percentage of the SPS-2 data is at level E-greater than 82 percent for all data types, and greater than 99 percent for many.

However, a significant amount of data is still missing, especially traffic, distress and faulting surveys, and key materials testing data. These deficiencies need to be addressed before serious analysis of the SPS-2 experiment can occur.

The SPS-2 data deficiencies are summarized below:

Note that the LTPP program is embarking on a systemwide effort to resolve all missing data. Some missing data have already been obtained. This effort will greatly improve the data availability for future analysis.

Experimental Design versus Actual Construction

Required experimental design factors were compared with the actual constructed values stored in the IMS database. This database includes both the site condition factors and pavement design features. Most SPS-2 sections follow the experiment design for the large majority of the design factors. Most deviations from the experiment design are found in the concrete slab thickness and 14-day flexural strength. A number of sections were more than 12 mm too thick or too thin, or were more than 10 percent too high or too low in flexural strength.

Out of the 13 SPS-2 project locations, only the recently constructed Wisconsin site does not have enough data in the IMS database to be evaluated. These data are currently in the pipeline. Of course, data from the California site under construction also are not in the database at this time.

Eight projects can be characterized as good to excellent when comparing designed versus constructed values, while the remaining four projects are considered poor to fair.

In addition to the comparison of the designed versus constructed values from the IMS database, construction reports of from all 13 SPS-2 reports sites were reviewed to identify construction deviations and difficulties. Four projects were found to have experienced more than minor construction difficulties or deviations.

The following summarizes significant deviations when comparing the designed versus constructed factorial factors, and more than minor construction deviations:

Early Performance Trends

The SPS-2 sections are relatively young, and a large majority show little distress. As of January 2000, only 43 of 155 sections (28 percent) are showing any noticeable distresses. Ninety-five percent of the SPS-2 sections have less than 1 mm of edge joint faulting. Eighty-seven percent of the SPS-2 sections show zero transverse cracking, and 78 percent of the sections have zero longitudinal cracking.

Based on the initial statistical analyses and comparisons, the following initial performance trends are noted (note that long-term performance may be different from short-term performance):

States' Expectations from the States for the SPS-2 Experiment

Two national workshops were held where input was received from the States on the SPS-2 project. The meetings were held on November 2-3, 1999, in Columbus, OH, and on April 27, 2000, in Newport, RI. The research team made presentations at both conferences about the status of SPS-2 data collection, analysis, and availability, and the near- and long-term LTPP products. Several participating States made presentations on the status and analyses of their SPS-2 projects, as well as the States' expectations of the SPS-2 experiment. There were many discussions of the future directions of the SPS-2 experiment and the analyses of the SPS-2 data at both conferences. Those discussions are summarized below.

In general, the States are satisfied with the SPS-2 experiment and fully expect to get valuable information about different design features from the project. Many States have been conducting or planning their own analyses of their SPS-2 projects. Some analyses have already yielded useful results. The States would like to see a focus on implementation of SPS-2 findings as they evolve over time.

First and foremost, what the States want to know the effect on pavement performance and cost-effectiveness of the SPS-2 experimental factors, such as:

In addition to the structural design features, the States also would like to know what major site condition factors influence performance of concrete pavement, including:

Other specific expectations from the States include:

As to the future analysis of SPS-2 data, the States believe that it is worthwhile to complete the missing data (backcast if necessary) to obtain traffic and materials data. Participants believed that many fundamental studies can be conducted to see how SPS-2 sections are responding to load and environmental stresses. It was also suggested that an integrated analysis plan is needed for future research.

Can SPS-2 Meet Expectations?

The specific experiment expectation of the SPS-2 project 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 evaluation has identified several significant problems that will limit the results that can be obtained from the SPS-2. Specifically, SPS-2 project sites are missing for certain subgrade-climate combinations. Some SPS-2 sites had construction deviations. Significant materials data and traffic data are missing from some sites or sections. The missing traffic data and key materials data must be obtained or forecasted before meaningful global analysis can be performed. The Nevada site has excessive early cracking that will limit its usefulness.

However, these problems do not mean that many important and useful findings and results cannot be obtained from SPS-2 experiments. Some interesting and important early trends have already been identified that will be useful in the design and construction of JPCP, even though the sections are only a maximum of 7.5 years old. As time and traffic loadings accumulate on the SPS-2 sites, additional valuable performance data will be obtained.

Because of FHWA's intense ongoing effort to obtain missing data (construction, materials, traffic, and monitoring), valuable results can be obtained from the SPS-2 sites. It is further believed that even more results can be obtained if a concerted effort is made to perform proper analyses of the data.

Recommendations for the SPS-2 Experiment

Missing SPS-2 Sites. To complete all the SPS-2 factorial cells, construction of the following sites is recommended:

Missing SPS-2 Data. Significant effort is recommended to obtain the following missing data:

Expectations from SPS-2.The overall objective is for the SPS-2 project's results to provide SHAs with documented findings to help them improve their management, design, construction, and materials procedures related to JPCP. The following specific information is expected to be gained from the SPS-2 project:

Specific design, subgrade, climate, and traffic effects

Data for use in calibration of mechanistic-empirical distress models

Data for use in empirical performance modeling (for pavement management)

Data for use in a variety of mechanistic modeling (backcalculation, structural analysis)

Data for use in a variety of cost/benefit analyses

Future Data Ccollection.It is recommended that the following areas receive special emphasis in SPS-2 data collection:

Recommended Future Analyses for SPS-2 Experiment

As stated previously, a very small percentage of the SPS-2 test sections currently have significant levels of distress, and only a few have been taken out service. The real benefit from this experiment will occur over the next 15 years, as more and more test sections exhibit higher levels of distress, magnifying the effect of the experimental and other structural factors on performance.

This report focuses on the quality and completeness of the SPS-2 construction and monitoring data and on the adequacy of the experiment to achieve the original expectations and objectives. 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-2 experimental data should be planned and prioritized so they can be initiated as the SPS-2 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 experimental cell in the project (companion SPS-2 sites that define a full factorial experiment) to ensure data adequacy, assess construction deficiencies, and support local interests and expectations. The second stage evaluates the effect of different structural features across the entire national experiment. Both analysis stages are briefly discussed in the following sections. After the sections are 15 to 20 years of age, a third-stage analysis will ultimately be needed to fully reap the benefits of the SPS-2 experiment.

Initial Stage-Analysis of Individual Factorial Cells

Each major cell in the SPS-2 experiment consists of at least two companion projects. One of these companion projects contains experimental sections 1 through 12, and the other contains sections 13 through 24. These companion SPS-2 sites constitute a full factorial of design factors and make it possible to evaluate the main effects and interactions of each experimental factor for those site conditions. A detailed evaluation of the companion projects within each major cell should be completed as soon as possible to ensure that all of the data exist and are acceptable. The purposes of the case studies in the first stage are listed below:

This first-stage analysis is considered absolutely essential prior to initiation of the second-stage analyses.

Second Stage-Analysis of Experimental Findings

The second-stage analyses should not be pursued until the first-stage analysis has been completed. It is expected that the analyses performed at this stage will be coordinated with the Strategic Plan for LTPP Data Analysis. The SPS-2 experiment can contribute to the following specific analyses outlined in the strategic plan:

In summary, the following future analysis objectives are recommended for the SPS-2 experiment. These analysis topics are discussed in more detail in figures 25 through 31.

  1. Perform site-by-site analyses of SPS-2 projects to resolve data problems and gain understanding of performance of individual test sections (figure 25).
  2. Determine the effect of the SPS-2 experimental factors on the performance of the jointed plain concrete pavements (figure 26).
  3. Determine the optimum JPCP design features for specific site conditions and traffic loading (figure 27).
  4. Determine the effect of concrete slab thickness variations on LTPP and initial ride quality (figure 28).
  5. Calibrate and validate relationships (transfer functions) between pavement structural response and individual distress types (figure 29).
  6. Conduct mechanistic analyses of SPS-2 sites (particularly Ohio and North Carolina) to gain knowledge of critical stresses and deflections to explain their performance in terms of joint faulting and slab transverse and longitudinal cracking (figure 30).
  7. Conduct cost-benefit analyses of SPS-2 data to determine the cost-effectiveness of various design features (figure 31).

The full results from the SPS-2 experiment will require 20 years of monitoring for the majority of sections. Additional studies beyond these proposed will be required.

Objective no. 1
Perform site-by-site analyses of SPS-2 projects to gain understanding of performance of individual test sections. (Initial stage, expected timeframe 2001 to 2002)

TOPIC AREA
Pavement design.

PROBABILITY OF SUCCESS
High.

LTPP STRATEGIC PLAN
7A, 7B, and 7C.
(Study of the Experimental Factors)

SUPPLEMENTAL XPERIMENTS
None.

END PRODUCT

  • Identification of test sections that perform well and poorly at each SPS-2 site, including supplementals.
  • Determination of the effect of any construction difficulties and material noncompliance issues on pavement performance and response.

POTENTIAL PRODUCT USE
Design of new or reconstructed cost effective and reliable jointed plain concrete pavements.

GENERAL TASKS

  • Conduct evaluation of permeable base and edge drains and outlets to determine their proper construction performance, and maintenance.
  • 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, problems, and material noncompliance issues with the SPS-2 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 State highway agencies involved and also will be useful for the national studies.

Figure 25. Recommended future analyses for SPS-2-Site-by-site analyses of SPS-2 projects to gain understanding of performance of individual test sections (initial stage).


Objective no.2
Determine the effect of the SPS-2 experimental factors on the performance of the jointed plain concrete pavements. (Expected timeframe 2003 to 2006)

TOPIC AREA
Pavement design.

PROBABILITY OF SUCCESS
High (assuming that subdrainage was evaluated in Objective No. 1).

LTPP STRATEGIC PLAN
7A, 7B, and 7C.

SUPPLEMENTAL EXPERIMENTS
None.

END PRODUCT

  • Effect of a permeable base drainage system on the performance of the jointed plain concrete pavements.
  • Effect of different base types on the performance of the jointed plain concrete pavements.
  • Effect of widened lane on the performance of the jointed plain concrete pavements.
  • Identification of site conditions where thicker concrete slab will and will not contribute to improved performance.
  • Effect of thicker slabs on the performance of the jointed plain concrete pavements.
  • Effect of higher strength concrete on the performance of the jointed plain concrete pavements.
  • Identification of site conditions where these design features will contribute to improved performance of the jointed plain concrete pavements.

POTENTIAL PRODUCT USE
Design of new or reconstructed cost effective and reliable jointed plain concrete pavements.

GENERAL TASKS

  • Review results and findings from each SPS-2 site.
  • Conduct statistical analysis to determine significant factors and interactions on performance.
  • Conduct mechanistic-empirical analyses for cracking, joint faulting, and IRI.
  • Based on statistical and mechanistic analyses, determine the effect of different experimental factors or design features and interaction 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.

Figure 26. Recommended future analyses for SPS-2 experiment-study of the effect of the experimental factors on rigid pavement performance.


Objective no. 3
Determine the optimum design features for specific site conditions and traffic loading for JPCP. (Expected timeframe 2003 to 2005)

TOPIC AREA
Pavement design.

PROBABILITY OF SUCCESS
High.

LTPP STRATEGIC PLAN
7A, 7B, and 7C.
(Study of the Experimental Factors)

SUPPLEMENTAL EXPERIMENTS
GPS-3 and SPS-8.

END PRODUCT
A guideline, catalog, or a design tool for selecting optimum combinations of design features for specific site conditions and traffic level.

POTENTIAL PRODUCT USE
Design new cost effective and reliable jointed plain concrete pavements.

GENERAL TASKS

  • Review results from each SPS-2 site.
  • Conduct statistical analyses to determine significant factors and interactions.
  • Conduct mechanistic-empirical analyses for transverse cracking, joint faulting, and IRI for JPCP.
  • Obtain representative construction cost data for all needed features of JPCP over selected regions that include an SPS-2 experiment.
  • Based on statistical and mechanistic analyses, identify the optimum combination of pavement design features to be used for various site conditions to provide cost effective and reliable JPCP.
  • Prepare practical presentations of the results, including software, guidelines, catalogs, and other tools that aids practicing engineers in determining the end products above.

Figure 27. Recommended future analyses for SPS-2 experiment-determination of the optimum pavement design features.


Objective no. 4
Determine the effect of concrete slab thickness variations on long-term pavement performance and initial ride quality. (Expected timeframe 2005 to 2007)

TOPIC AREA
Pavement design and construction.

PROBABILITY OF SUCCESS
High.

LTPP STRATEGIC PLAN
2C and 2F.

SUPPLEMENTAL EXPERIMENTS
GPS-3 and SPS-8.

END PRODUCT
A relationship between increased thickness variations and reduced pavement service life or reduced initial ride quality.

POTENTIAL PRODUCT USE
Develop pay reduction factors based on concrete slab thickness variation.

GENERAL TASKS

  • Review results from each SPS-2 site.
  • Establish the variation in the concrete slab thickness for each of the SPS-2 test sections.
  • Conduct statistical analyses to determine the effect of the slab thickness variation on pavement performance and response.
  • Develop reductions in service life based on these increased variations in concrete slab thickness.

Figure 28. Recommended future analyses for SPS-2 experiment-quantify the relationships between as-designed and as-built concrete slab thickness and strength.



Objective no. 5
Calibrate and validate relationships (transfer functions) between pavement response and individual distress types. (Expected timeframe 2005 to 2007)

TOPIC AREA
Pavement design.

PROBABILITY OF SUCCESS
High.

LTPP STRATEGIC PLAN
7A, 7B, and 7C.

SUPPLEMENTAL EXPERIMENTS

GPS-3 and SPS-8.

END PRODUCT
A calibrated and/or validated relationship between pavement structural responses (stress) and individual distresses.

POTENTIAL PRODUCT USE
Design of new cost effective and reliable jointed concrete pavements (would contribute to upgrading of 2002 Design Guide).

GENERAL TASKS

  • Establish a comprehensive input database that includes design, construction, materials testing, traffic, climatic, and monitoring data, for the response model.
  • Perform mechanistic analysis to determine the critical response stress and cumulative fatigue damage for the traffic loading applied until the time of the distress measurement (utilize the relationships in the 2002 Design Guide as well as others).
  • Establish the relationships between the cumulative fatigue damage and the measured distress.
  • Perform model assessment and develop calibration coefficients.

Figure 29. Recommended future analyses for SPS-2 experiment-calibration and validation of the pavement transfer functions.


Objective no. 6
Conduct mechanistic analyses of SPS-2 sites (particularly Ohio and North Carolina) to gain knowledge of critical stresses and deflections to explain their performance in terms of joint faulting and slab transverse and longitudinal cracking. (Expected time frame 2005 to 2007)

TOPIC AREA
Pavement design and construction.

PROBABILITY OF SUCCESS
Moderate to high.

LTPP STRATEGIC PLAN
2D and 7B.

SUPPLEMENTAL EXPERIMENTS
None.

END PRODUCT
In-depth, field-verified knowledge as to the effects of critical measured structural responses and curling 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 JPCP a more cost effective and reliable pavement (upgrade 2002 Design Guide).

GENERAL TASKS

  • Establish a comprehensive input database that includes design, construction, materials testing, traffic, climatic, monitoring data, and structural monitoring data (deflections, strains, stresses, others).
  • Analyze slab curling at all sites using longitudinal profile data or other slab curling measurements available (Note: if insufficient data are available, measure curling at several sites in different climates).
  • Perform mechanistic analysis to determine the critical response stress and cumulative fatigue damage for the traffic loading and slab curling.
  • Analyze results and develop findings and recommendations as to impacts of loading and curling on JPCP performance (cracking and faulting).

Figure 30. Recommended future analyses for SPS-2 experiment-mechanistic analyses of JPCP.


Objective no. 7
Conduct cost/benefit analyses of SPS-2 sites to gain knowledge of the cost-effectiveness of design features in different site conditions. (Expected timeframe 2005 to 2007)

TOPIC AREA
Pavement design and construction.

PROBABILITY OF SUCCESS
High.

LTPP STRATEGIC PLAN
7B and 7C.

SUPPLEMENTAL EXPERIMENTS
None

END PRODUCT
In-depth, field-verified knowledge as to the cost-effectiveness of key design features including slab thickness, widened slab, base type, concrete strength, and a permeable base layer.

POTENTIAL PRODUCT USE
Knowledge gained from this experiment will be directly useful to pavement designers in improving the cost-effectiveness of their designs.

GENERAL TASKS

  • Establish a comprehensive input database that includes design, construction, materials testing, traffic, climatic, and monitoring data.
  • Establish typical costs of various design features from the State highway agencies in the States where SPS-2 sites are located.
  • Analyze results and develop findings and recommendations as to the cost-effectiveness of each design feature in each of the main climatic zones covered by the SPS-2 experiment.

Figure 31. Recommended future analyses for SPS-2 experiment-cost/benefit analyses of JPCP.

 

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