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

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

Chapter 2. Evaluation of The SPS-2 Experiment

The first step in the evaluation of the SPS-2 experiment is to assess how much of the original experiment was actually constructed and what effect missing sites will have on obtaining expected findings. The original SPS-2 experiment design, the SPS-2 experimental sites actually constructed, the effects of blank experimental design cells, and potential information available from the SPS-2 supplemental sites are discussed in this chapter.

Original SPS-2 Experiment Design

The complete experiment design factorial, the site classification category, and individual section identification numbers are given in table 1. This design factorial requires a total of 16 SPS-2 sites and 192 core sections be evenly distributed within the factorial matrix of climate and subgrade to satisfy the requirement of the SPS-2 experiment design.

The experiment design for SPS-2 included eight main factors. Three are project site factors, and the remaining five are related to the pavement structure.

Project Site Factors

Three factors describe site conditions at the test site:

1. Climatic—Temperature - Freeze and no-freeze
2. Climatic—Precipitation - Wet and dry
3. Subgrade - Fine-grained and coarse-grained

Traffic loading was incorporated as a covariant. Each of the SPS-2 sites required relatively high rate of traffic of at least 200,000 rigid equivalent single-axle loads (ESALs) per year.

The experiment design factorial at the project site level is provided in table 2. The design factorial calls for a total of 16 SPS-2 sites to be constructed throughout the United States.

Pavement Structure Design Factors

In addition to the three site factors, five experiment design factors were allocated to the pavement structure. Two factors were allocated to the surface layer: one for thickness and one for strength. Two other structural factors were allocated to the base/sub base: one for strength/stability and the other for drainage. The last factor was assigned to the width of the pavement slab. In summary, the SPS-2 experiment incorporates the following pavement structural factors:

1. Slab thickness - 203 and 279 millimeters (mm)
2. Concrete flexural strength - 3.8 and 6.2 megapascals (MPa) at 14-days
3. Base type and drainage - Dense-graded aggregate base (DGAB)
   - Lean concrete base (LCB)
   - Permeable asphalt-treated base (PATB) with edge drains
4. Slab width - 3.66 and 4.27 meters (m)

Notes: Test section numbers included in cells from 01 to 24 represent a full factorial experiment.

AGG = Dense-graded untreated aggregate base.

LCB = Lean concrete base.

PATB = Permeable asphalt-treated base.

Note: Two related sites (e.g., J and K, T and U) make up a full factorial of design features.

The total full factorial designs for combination of the study factors result in 24 different experimental pavement structures (2 thickness x 2 strength x 3 base type x 2 lane width = 24). The experiment design was implemented using a one-half fractional factorial approach that permitted construction of 12 test sections at one site and construction of the 12 complementary sections at another site within the same climatic region and with a similar subgrade type. Both of these complementary sites must be available to evaluate the main effects and all interactions of the design factors within a given climate and subgrade type.

Each SPS-2 test section length is 152 m long. As a result, an SPS-2 project may be constructed over a length greater than 3.2 kilometers (km). Other key design features common to all SPS-2 sections include the following:

• Joint spacing-4.6 m uniform spacing.
• Joint load transfer-Doweled perpendicular transverse joints, with 32-mm dowel bars for the 203-mm-thick pavements and 38-mm dowel bars for 279-mm-thick pavements. Dowels are to be epoxy coated, 457 mm long, and spaced at 305 mm.
• Longitudinal joints-Between lanes the joints should be saw cut, preferably using up to an 8-mm-wide blade, to a depth of D/3 (where D equals slab thickness). The sealant reservoir may be formed later using a second saw cut to provide for an 8-mm-wide by 25-mm-deep cut.
• Joint sealing-All joints shall be sealed before opening to traffic. Joint sealing shall be accomplished using only silicone sealant.
• Shoulder-Either AC or portland cement concrete (PCC) shoulders. PCC shoulders shall not be tied to the mainline pavement. If the concrete shoulder is placed monolithically with the traffic lanes, then the shoulder joint shall be sawed full depth.

At each SPS-2 site, test sections are numbered sequentially either from 01 to 12 or from 13 to 24, depending on climatic and soil type combinations. The numbering within each site depends on the levels of each design factor. Figure 1 graphically presents the section structure and numbering schematic for SPS-2 sections.

Figure 1. Test section details for a full factorial SPS-2 experiment located at two sites (01 to 12 and 13 to 24).

Current Status of Design Factorial

As of August 1999, 13 SPS-2 sites had been constructed throughout the United States An additional site in California had been nominated and may be constructed in the near future. The distribution of the SPS-2 sites is shown on a map in figure 2. The current status of the site design factorial is provided in table 3,showing which State is filling which design cell. As shown in the table, there are five study sites missing from the experiment factorial, indicating a loss of 5 of the 16 (or 31 percent) of the design cells.

Figure 2. Geographic distribution of the constructed SPS-2 projects.

Notes:
1 - Kansas site was planned to fill the dry-freeze zone cell. However, the actual precipitation is quite a bit higher than the specified annual precipitation for this study.
2 - North Dakota site is slightly wetter than the specified annual precipitation.
3 - California site is currently under construction.
? - Represents a missing SPS-2 project.

A list of all the SPS-2 sections, including both core and supplemental sections, is given in table 4. As shown, many SHAs have constructed additional sections at the SPS-2sites. These additional sections are not included in the original SPS-2 experiment factorial and are referred to as State supplemental sections. A total of155 core sections and 40 supplemental sections have been constructed to date. Seven SPS-2 sections are designated as seasonal monitoring program (SMP) sections, where additional sets of climatic and monitoring data are measured.

Potential Effects of Missing Experimental Sites

The current status of the SPS-2 experiment design, showing constructed sections in relation to the original experiment design, is given in table 5. The following five sites (columns) are missing from the original experiment design:

• Two sites in the wet no-freeze climate (southeast U.S.)with a coarse-grained subgrade. Each project fills half of the design factorial.
• One site in the dry freeze climate (northwest U.S.)with a fine-grained subgrade.
• Two sites in the dry no-freeze climate (southwest U.S.), one with a fine-grained subgrade and the other with a coarse-grained subgrade. Each site fills half of the design factorial.

Notes: 1 - California site is under construction.

Five additional projects (60 sections total) are needed to complete the design factorial. These missing projects will definitely limit the results obtainable from the SPS-2 experiment, although it is impossible to determine the exact effects at this time. These limitations are summarized as follows:

• There will be no performance data, and thus, no performance findings from the missing sites.
• The missing section at the Nevada site will reduce the findings for that site and corresponding cell, although there appears to be replication at the Washington site.
• SPS-2 sites exist in wet-freeze climates, making a full inference space of performance data available. All main effects and interactions should be ascertainable in this climate.
• SPS-2 sites are deficient in wet and dry no-freeze climates. There will be difficulties in determining the main effects and interactions in these climates.

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

State Supplemental Sections

In addition to the 12 core sections required by the SPS-2 experiment, SHAs can include additional experimental sections, referred to as State supplemental sections. Although there were provisions in the experimental design to study un doweled JPCP and jointed reinforced concrete pavements (JRCP), no such controlled sections have been constructed other than some uncontrolled supplemental sections. Table 6 lists the design variables selected by SHAs for supplemental sections.

The main value of the supplemental sections will be as direct comparison to the core sections. For example, one supplemental section in Washington did not have dowels;this will provide a direct comparison to a similar design with dowel bars. This comparison is also possible in Arizona and North Dakota. Various other comparisons are also possible, including skewed joints, base types,sub drainage, slab thickness, AC pavement, jointed reinforced concrete, special dowels, and variable slab thickness.

Note: The Arkansas SPS-2 project site does not contain any supplemental sections.