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

PROJECT REQUIREMENTS

Each SPS-1 project had to meet certain criteria. There were limitations on the methods and materials used in construction of the sections, as well as requirements for testing and continued monitoring. Each of these criteria is outlined in this chapter.

CONSTRUCTION MATERIAL REQUIREMENTS

Construction requirements were provided in the "Construction Guidelines" section of the Specific Pavement Studies of Structural Factors for Flexible Pavements (SPS-1) Guide.⁽⁴⁾ The overall length of each section was required to be 183 m with 152.4 m for monitoring and 15.25 m on each end for materials sampling. The distance between each of these sections had to be long enough to allow sufficient space for changes in materials and thicknesses during construction. The suggested length for these transitions was 30.5 m.

Subgrade Requirements

The finished subgrade elevations were not to vary from the design by more than 12 mm. This was to be determined using rod and level readings taken on the lane edge, outer wheel path, midlane, inner wheel path, and the inside lane edge at 15-m intervals throughout the project. Surface irregularities were not to exceed 6 mm between two points in any direction in a 3.05-m interval. If a working platform at the top of the subgrade was required, adding lime, portland cement, or other suitable material to the subgrade to alter the index properties of the soil could create it. The strength of the subgrade was not to be unduly increased.

Base Layers

Two types of bases are included in each project—drained and undrained. The drained bases include a permeable asphalt treated base with edge drains. The undrained bases consist of dense graded materials. The undrained bases were used on sections 1–6 and 13–18, and were defined as dense graded aggregate base (DGAB), asphalt treated base (ATB), or a combination of these two materials. The requirements for the DGAB were as follows:

• Minimum 50 percent retained on the No. 4 sieve.
• 38-mm top-size aggregate, unless the State agency normally specified and used less than 38 mm.
• Less than 60 percent passing the No. 30 sieve and less than 10 percent of the total same passing the No. 200 sieve.
• Liquid limit less than 25 and plasticity index less than 4 for the fraction passing the No. 40 sieve.
• If the L.A. Abrasion test was used by the agency, the loss shall not exceed 50 percent at 500 revolutions.
• The compacted lift thickness must not be greater than 152 mm.
• The DGAB was to be compacted to at least 95 percent of maximum density.
• In-place density of the DGAB was to be determined prior to the application of an asphalt prime coat, if used.
• A prime coat of low viscosity asphalt cement was specified for use prior to placement of the permeable asphalt treated base (PATB) layer (on sections that include a PATB layer).
• Final DGAB elevations were not to vary from design by more than 12 mm, as measured using the rod and level measurements taken on the lane edge, outer wheel path, mid lane, inner wheel path, and inside lane edge at a 15-m interval.

The requirements for the ATB layer were as follows:

• The aggregate used in the ATB layer had to meet the same requirements as the aggregate for the DGAB layer.
• Asphalt emulsions were not allowed.
• Experimental modifiers were not to be used in the core test sections, but could be used in supplemental sections.
• No recycled AC was allowed in the ATB.
• If the Hveem mix design procedure was used by the SHA, the ATB mixture had to meet the following requirements:
  
  

  Swell	0.7 mm
  Stabilometer Value	35 min.
  Moisture Vapor Susceptibility	25
  Design Air Voids	3 to 5 percent

  
  
• If the Marshall mix design procedure was used, the ATB had to meet the following requirements:
  
  

  Compaction blows	50
  Flow	2 mm to 5 mm
  Stability	4.4 kN
  Air Void	3 to 5 percent

• A low-viscosity asphalt was to be used as a tack coat on top of the ATB prior to placement of the HMA surface material.
• A track paver was specified for placing the ATB on the PATB layer.
• The maximum compacted lift thickness of the first lift was not to exceed 152 mm, and subsequent lifts were not to exceed 102 mm.
• The minimum compaction requirement was 90 percent of the maximum theoretical specific gravity for the first lift and 92 percent for subsequent lifts. There was no maximum compaction requirement for any of the HMA lifts.
• Final ATB elevations were not to vary from design more than 12 mm, as measured using the rod and level.
• The base layer thickness was not to vary from design by more than 6 mm.

Sections 7–12 and 19–24 incorporate the drained bases. Each of these sections includes a PATB layer with edge drains to permit water to drain out of the pavement structure. The requirements for the PATB layer were as follows:

• An asphalt emulsion was not allowed as a binder for the PATB layer.

• The gradation for the PATB was to have no more than 2 percent passing the No. 200 sieve. The following gradation was recommended:

   

  38 mm	100 percent
  25 mm	95 to 100 percent
  13 mm	25 to 60 percent
  No. 4	0 to 10 percent
  No. 8	0 to 5 percent
  No. 200	0 to 2 percent

• More than 90 percent of the aggregate was to have at least one crushed face.
• No recycled AC was permitted in the PATB.
• A static steel wheel roller was specified for compacting the PATB layer.
• No portion of the PATB was to be day-lighted.
• Other than the paver and the roller, no other equipment was allowed to travel or park on the PATB.
• Transverse interceptor drains were to be placed in the transition zone between the drained and undrained base structure sections on the down slope end of the PATB layers. They were to be placed at least 30 m past the end of the test section.

Drainage Materials

Filter fabrics were to be used on sections that include PATB layers. These were specified to prevent clogging of the PATB layer. The filter fabrics used were to meet the American Association of State Highway and Transportation Officials-American Building Contractors-American Road and Transportation Builders Association Task Force 25 recommendations, which include the following requirements:

• Nonwoven or woven geotextile materials had to conform to Class B drainage applications. However, fabric used where the PATB was constructed as the first layer and for the transverse interceptor drains had to meet Class A requirements.
• For sections where the PATB layer was placed on the subgrade, the filter fabrics were to be placed directly on the subgrade and extend around the outside edge drain trench, across the travel lanes, and around the inside edge drain.
• For sections where the PATB layer was placed on the DGAB, the filter fabrics were to extend around each edge drain and wrap around the outer edge of the PATB layer, but did not need to extend the full-width of the lane.
• Filter fabrics were to be installed in accordance with the manufacturer’s specifications.
• Exposure of the geotextiles to the elements between laydown and cover could not exceed 14 days.
• Any fabric that was damaged had to be repaired with a patch that extended 914 mm beyond the perimeter of the damage, unless the fabric was replaced.
• The fabric had to be overlapped a minimum of 610 mm at all longitudinal and transverse geotextile joints.

• Inside and outside edge drains had to be constructed for crowned pavements.
• Edge drains could be no closer than 914 mm to the edge of the travel lanes.
• The edge drains had to run continuously throughout the sections incorporating the PATB layers.
• The PATB was recommended for backfill around the edge drains; however, other open graded material could be used, if approved.
• Collector pipes had to be at least 76-mm-diameter slotted plastic pipes.
• Outlet pipes had to be a minimum 76-mm-diameter unslotted rigid plastic pipe.

Drainage pipes were to be sized for the expected flows determined as part of design. Discharge outlet pipes were to be located at maximum intervals of 76.2 m. Outlets were to be at least 152 mm above the expected 10-year flow elevation of the collector ditches to prevent backflow.

It should be noted that the construction requirements did not include video inspection of the edge drains after construction of the project or site was completed.

HMA Layers

The HMA surface had to meet the following requirements, as a minimum:

• If a Marshall mix design method was used, then the mix had to meet the following requirements:

  Compaction blows	75
  Stability (Minimum)	8 kN
  Flow	2 mm to 4 mm

• However, if a Hveem mix design method was used, then the mix had to meet the following requirements:

  Stability (Minimum)	37
  Swell (Maximum)	0.7 mm
  Air Voids	3 to 5 percent

• No recycled materials were permitted in the HMA mixtures placed on the sections.
• The aggregate were to have a minimum 60 percent retained on the No. 4 sieve with two fractured faces, and a minimum sand equivalency of 45.
• The asphalt grade and characteristics were to be selected based on normal agency practice.
• The use of modifiers or experimental additives was discouraged in the main sections; >however, these materials could be used in supplemental sections.
• Lift thicknesses could not exceed 102 mm.
• Longitudinal joints were to be staggered between successive lifts to avoid vertical joints.
• If a distinct surface course mix was used, then the same thickness was to be used on all sections on the project.
• The compacted thickness of any single layer had to be at least 51 mm.
• All transverse construction joints were to be placed outside the sections.
• The thickness of the AC layers (surface and binder) had to be within 6 mm of the thickness specified by the experiment design.
• The riding surface of the pavement had to be smooth. As a target, the as-constructed surface was to have a prorated profile index of less than 158 mm per 1,000 m, as measured by a California type profilograph.

The shoulders placed on these projects had to be a minimum of 1.2-m wide. If possible, the shoulders were to be paved full-width with the surface course to eliminate longitudinal joints. If this was not possible, then the shoulders were to be paved such that the longitudinal joint was to be at least 305 mm outside the travel lane.

Surface friction courses were allowed on the sections if these layers were required by the participating agency. The friction courses were required to be no thicker than 19 mm and were not to be considered as part of the AC thickness required for any specific test section in the experiment.

MATERIALS SAMPLING AND TESTING

Sampling and testing were required on each of the materials being placed. The materials characterization is necessary to evaluate differences between the sections and between projects within the experiment. The parameters measured are those used in most design procedures and those used to assess important performance characteristics of these materials.

A general sampling and testing plan was created for use as a guideline.⁽⁴⁾ >This guideline was then used to develop the sampling and testing plan specific to each project. Because each State was allowed to add supplemental test sections, the number of tests may vary from project to project (test numbers increasing with increase in test sections). These plans were created prior to the construction of each individual project and provided the location of each sample to be taken, where the sample should be sent, and the tests that were to be performed on each sample.

Samples taken from the project include:

• Bulk samples from the upper 305 mm of the subgrade.
• Thin-walled tube samples of the subgrade to 1.2 m from the top of the subgrade.
• Jar samples of the subgrade.
• Bulk samples of the DGAB.
• Jar samples of the DGAB.
• Bulk samples of the PATB.
• Bulk samples of the ATB.
• Bulk samples of the asphalt mixes used in the surface and binder courses.
• Bulk samples of the asphalt cement used in all mixes.
• Cores of the ATB, asphalt binder (if present), and asphalt surface.

In addition to each of these samples, bulk samples were to be taken of the asphalt cement, aggregates, and uncompacted AC mixes to be stored long term. A series of auger probes were to be performed in the shoulder of each test section to a depth of 6 m. This allows for the determination of the depth to a rigid layer. Finally, as part of the field activities during the construction of the project, nuclear density and moisture testing was conducted on top of the bulk sampling areas for the subgrade and on the top of each layer in each test section.

The testing of these samples was divided between FHWA and the SHAs. FHWA conducted the resilient modulus tests, creep compliance tests, and other associated tests. The associated tests are those for which the results are required prior to running the resilient modulus tests. For instance, the protocol for determining the resilient modulus on unbound materials is dependent upon the material classification. Table 4 lists the tests that were to be performed and the minimum number required.

MONITORING REQUIREMENTS

The monitoring of these projects includes several different types of data—distress surveys, deflection measurements, transverse profile measurements, friction measurements, and longitudinal profile measurements. Each of these measurements has different frequency requirements, as noted in the following paragraphs, but these frequencies have been revised over time. The detailed review on the data completeness and availability was based on the cited reference. There can be numerous reasons why a regional coordination office (RCO) was unable to satisfy the monitoring frequency requirements that were in place when a project was built. Some of these reasons are as follows:

• Egress restrictions imposed by the contractor until that project was accepted by or turned back over to the agency.
• Weather conditions, especially on projects built in the northern states and completed during the fall months.
• Equipment breakdowns or maintenance requirements.
• Scheduling difficulties.

Distress Surveys

A distress survey was to be performed on the sections within 6 months of construction. A manual distress survey is to be performed on the sections biennially, with the exception of the "weak" sections. These weak sections are numbers 1, 9, 13, and 21, and they are surveyed annually. ⁽⁵⁾ >If necessary, the surveys may be postponed up to 1 year. In addition to the manual surveys, video distress surveys are performed.

Deflection Surveys

Deflection measurements are to be collected using a falling weight deflectometer (FWD) from 1 to 3 months after the project has been constructed.⁽⁶⁾ Long-term monitoring of these projects is to be completed biennially, except for the "weak" sections. This testing can also be postponed up to 1 year if necessary. Sections 1, 9, 13, and 21 were to be tested every 6 months, but this testing can be postponed up to 6 months.

Table 4. Required testing for the SPS-1 experiment.

Transverse Profiles

Transverse profile measurements are to be taken at the same frequency, and at the same time, as the distress surveys.⁽⁵⁾ As part of a manual distress survey, the surveyor takes transverse profile measurements using a FACE Dipstick ^®. The PASCO units take automated transverse profile surveys in addition to the automated distress surveys.

Longitudinal Profiles

Longitudinal profile measurements are to be taken on the sections within 3 months after construction.⁽⁷⁾ >These measurements can be postponed up to 3 additional months. The "weak" sections (sections 1, 9, 13, and 21) are monitored every 6 months but monitoring can be postponed up to 6 additional months. The other sections are to be monitored biennially. These tests can be postponed up to 1 year, if necessary.

Friction Surveys

Friction measurements were to be taken from 3 to 12 months after the sections were constructed. Long-term monitoring is to be conducted biennially. However, as of January 1, 1999, friction measurements are no longer required on any test section.⁽⁸⁾ These measurements are considered optional and the data are being stored in the national IMS database, if collected.

Traffic Data

Traffic data are to be collected on each of the projects as well. The current requirement states that weigh-in-motion (WIM) data are to be collected continuously on SPS-1 sections. Continuous data collection is defined as the "use of a device that is intended to operate throughout the year and to which the SHA commits the resources necessary to both monitor the quality of the data being produced and to fix problems quickly upon determination that the equipment is not functioning correctly."⁽⁹⁾ WIM devices are to be calibrated biannually. This level of data collection is considered necessary to provide accurate traffic loading measurements.

Climatic Data

Each SPS-1 project was to include the installation of an automated weather station (AWS).⁽¹⁰⁾ The site is to be located close enough to the project to provide weather data that is representative of the weather on the project. The equipment installed at these locations includes a rain gauge. A desiccant, humidity indicator, and conduit putty are used to measure humidity. A wind monitor is included in the installation to measure wind speeds. Equipment also is included to determine the cloud cover and temperature at the site. All of this data is collected and stored by a datalogger. The data is downloaded from the datalogger at least every 6 months.

In addition to the AWS used to collect weather data, data are obtained from four to five National Oceanic and Atmospheric Association (NOAA) weather stations surrounding the project. The data are then averaged using a weighting procedure. This procedure gives weights based on the distance of the weather station from the project. The closer the weather station is to the project, the larger the weight used in the averaging. The data collected from NOAA includes information about the temperature, rainfall, wind, and solar radiation.

Each of the SPS-1 projects was supposed to meet these minimum requirements. Any deviation from these requirements could affect the results obtained from the analysis of the data. The next chapter examines how each of the SPS-1 projects have deviated from the requirements and how these deviations can be expected to affect the results that can be achieved from this experiment.