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

Chapter 2. General Overview of Experiment

The first step in evaluating the SPS-5 experiment assessed how much of the experiment actually was constructed and what effect any missing sites might have on the experiment. This chapter discusses the original SPS-5 experiment design, the experimental sites constructed as of the time of this report, the effects of the missing experimental design cells, and the information available from the supplemental sites. The January, 2000, release of the IMS that contained only Level E data was used for the detailed review.

ORIGINAL SPS-5 EXPERIMENT DESIGN

The SPS-5 experiment examines the effects of climatic factors (wet versus dry and freeze versus no-freeze) and pavement condition (fair versus poor) on pavement sections incorporating different rehabilitation structural features. The features include:

• Amount and type of surface preparation before overlay.
• Type of material used for the overlay (recycled versus virgin asphalt mix).
• Variations in overlay thickness (51 or 127 mm).

The original SPS-5 experiment factorial is shown in table 1. A site is defined by the environmental conditions shown within the design. Nine combinations of rehabilitation factors are presented by the factorial. In table 1, intensive surface preparation denotes those test sections in which 51 mm of the surface was milled off and patching was performed as needed to rectify localized failures. Minimum surface preparation indicates that only patching was performed. The experimental plan specified that the recycled mixtures should contain 30 percent RAP and that the RAP incorporated into the mix should be the material that was milled from the intensive surface preparation test sections.⁽²⁾ As part of the experiment design, one section was to have no treatment and serve as the control section for the project.

Two projects were required for each of the nine combinations of rehabilitation factors. Table 1 illustrates which State projects were nominated initially to fill specific design cells of the factorial. As shown, at least two projects were nominated for each cell with the exception of the dry-freeze-poor condition cell of the factorial. Some cells contain triplicate sections. As of August 1999, the SPS-5 experiment had 18 projects located throughout the United States and Canada. A map of the selected sections is shown in figure 1. These projects are well distributed across North America.

Figure 1. Map. Location of the SPS-5 projects.

Each test section has an identifying number that is common for all of the SPS-5 projects, and this test section number indicates the following characteristics:

• 501:Control–no treatment.
• 502: Thin overlay (51 mm)–recycled HMA mix.
• 503:Thick overlay (127 mm)–recycled HMA mix.
• 504: Thick overlay–virgin mix.
• 505: Thin overlay–virgin mix.
• 506: Thin overlay–virgin mix– with milling.
• 507: Thick overlay–virgin mix–with milling.
• 508: Thick overlay–recycled mix–with milling.
• 509: Thin overlay–recycled mix–with milling.

Requirements set for all test sections on each project are as follows:

• Length: 152.4 m–for performance monitoring with time.
• Subgrade: Fine-grained.
• Minimum estimated traffic loading: 85,000 equivalent single axle loads (ESALs) per year.

SUPPLEMENTAL SECTIONS

In addition to the nine core sections located at each project, the States were allowed to add supplemental sections that would be monitored by LTPP. These supplemental sections are usually a variation of the experiment and represent typical rehabilitation practices by the individual agency. Table 2 provides a list of the SPS-5 projects and the number of supplemental sections that were built at each. These supplemental sections represent a much more diverse range of overlay materials and rehabilitation strategies than were considered in the core experiment. The 48 supplemental sections are considered a valuable source of data for future pavement engineering needs.

CURRENT STATUS OF DESIGN FACTORIAL

The status, as of the year 2000, of the SPS-5 design factorial is provided in table 3. All projects have been located in the appropriate cells based on the actual environmental data, which will be discussed in detail in chapter 4. As shown, all of the cells have at least two projects, except for the wet-no-freeze, fair condition cell and the dry-freeze, poor condition cell. The distribution of these projects across North America represents the diverse environmental conditions required for this experiment. In summary, a total of 210 test sections (162 core test sections of the experiment plus 48 supplemental sections) had been built as part of the SPS-5 experiment.

Note: Unmentioned states and provinces had no supplemental sections.

There is one major difference or deviation from the SPS-5 experimental plan. The subgrade soils for all SPS-5 projects were to be fine-grained soils. However, the soils for many of the projects are classified as coarse-grained soils. Table 3 identifies those projects that have fine-and/or coarse-grained soils. Only five of the SPS-5 projects have fine-grained soils: Maryland, Minnesota, Mississippi, Oklahoma, and Texas. The soils supporting the Missouri project were not yet classified. Four projects have soils that vary between fine- and coarse-grained: California, Colorado, Georgia, and Manitoba. The subgrade soils for the remaining eight projects are classified as coarse-grained. Although this is considered a significant deviation from the experimental plan, it is not believed to be detrimental to achieving the overall expectations for this experiment.

Note: The numbers in parentheses indicate supplemental sections, which are followed by the age of the project as of January 2000.

*Missouri is located in the cell for which it was nominated because the data for determining the correct cell assignment were unavailable as of the time of this report.