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Publication Number: FHWA-RD-01-166
Date: November 2003

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

GENERAL OVERVIEW OF EXPERIMENT

The first step in the evaluation of the SPS-1 experiment is to assess how much of the original experiment was actually constructed and what effect any missing sites will have on the usefulness of the SPS-1 data. The original SPS-1 experiment design, the SPS-1 experimental sites actually constructed, the effects of missing experimental design cells, and information available from the SPS-1 supplemental sites are discussed in this chapter. The January 2000 release of the IMS that contains only Level E data was used for the detailed review.

ORIGINAL SPS-1 EXPERIMENT DESIGN

The SPS-1 experiment examines the effects of climatic factors (wet versus dry and freeze versus no-freeze) and type of subgrade (fine-grained and coarse-grained) on pavement sections incorporating different structural factors. These factors include:

The original SPS-1 experiment factorial is shown in table 1. A total of 24 combinations of structural factors are presented by the factorial. Because 24 sections at one site would produce an undue burden on the SHAs, the projects were developed so that only 12 sections were built at any one site. Therefore, a complete factorial of all factors is made up of two columns of the factorial. The sections in the first half of a complete factorial are numbered 1–12, while the second half are numbered 13–24.

Each section varies from the others in terms of the structural factors mentioned above. The shaded cells in the factorial are those that were not expected to be filled by a section. As shown, the experimental factorial was completely filled with candidate projects that were nominated originally for the SPS-1 experiment. In total, the SPS-1 experiment has 216 core test sections.

Table 1 also illustrates which State projects were nominated initially to fill which design cells. The table shows that some of the cells contain replicate sections. In particular, the projects built in Iowa and Ohio are both in the wet-freeze environmental zone on fine-grained subgrades. The same set of sections was built on both of these projects. In addition, the projects built in Virginia and Michigan are both in the wet-freeze environmental zone on fine-grained subgrades. These two projects contain the offsetting sections from those built on the Iowa and Ohio projects. As of August 1999, the SPS-1 experiment has 18 projects located throughout the United States and Canada. A map of the selected sections is shown in figure 1.

Table 1. Factorial for the SPS-1 experimental design and the sites/projects originally nominated for each cell within the experiment.

Pavement Structure Combinations Factors for Moisture, Temperature, Subgrade Type, and Location
Wet Dry
Freeze No-Freeze Freeze No-Freeze
Drainage Base Type Total Base Thick Surface Thick Fine Coarse Fine Coarse Fine Coarse Fine Coarse
J K L M N O P Q R S T U V W X Y

No

AGG

8"

4"

  VA, MI   WI   LA

 

AR

  NE   MT   OK  

AZ

7"

IA, OH   DE   AL   FL   KS   NV   NM   TX  

12"

4"

IA, OH   DE   AL   FL   KS   NV   NM   TX  

7"

  VA, MI   WI   LA   AR   NE   MT   OK  

AZ

ATB

8"

4"

IA, OH   DE   AL   FL   KS   NV   NM   TX  

7"

  VA, MI   WI   LA   AR   NE   MT   OK   AZ
12"

4"

  VA, MI   WI   LA   AR   NE   MT   OK  

AZ

7" IA, OH   DE   AL   FL   KS   NV   NM   TX  
ATB

 

4" AGG

8"

4"

IA, OH   DE   AL   FL   KS   NV   NM   TX  

7"

  VA, MI   WI   LA   AR   NE   MT   OK  

AZ

12"

4"

  VA, MI   WI   LA   AR   NE   MT   OK  

AZ

7"

IA, OH   DE   AL   FL   KS   NV   NM   TX  

Yes

PATB
AGG

8"

4"

IA, OH

  DE   AL  

FL

 

KS   NV   NM   TX  

7"

  VA, MI   WI   LA   AR   NE   MT   OK  

AZ

12"

4"

  VA, MI   WI   LA   AR   NE   MT   OK  

AZ

7"

IA, OH   DE   AL   FL   KS   NV   NM   TX  

16"

4"

  VA, MI   WI   LA   AR   NE   MT   OK  

AZ

7"

IA, OH   DE   AL   FL   KS   NV   NM   TX  

ATB
PATB

8"

4"

  VA, MI   WI   LA   AR   NE   MT   OK  

AZ

7"

IA, OH   DE   AL   FL   KS   NV   NM   TX  

12"

4"

IA, OH   DE   AL   FL   KS   NV   NM   TX  

7"

  VA, MI   WI   LA   AR   NE   MT   OK   AZ

16"

4"

IA, OH   DE   AL   FL   KS   NV   NM   TX  

7"

  VA, MI   WI   LA   AR   NE   MT   OK   AZ

Shaded cells are not required by the experiment.
1 inch = 25.4 mm

 

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Figure 1. Location of the SPS-1 projects.


Each of the 12 sections was required to be 152.4 m in length. In addition, each project was required to have a minimum traffic loading of 100,000 equivalent single axle loads (ESALs) per year.

STATE SUPPLEMENTAL SECTIONS

In addition to the core sections located at each project, the States were allowed to add supplemental sections that are monitored by LTPP. These supplemental sections are usually a variation of the experiment and represent typical construction practices by the State. Thus, the main value of the supplemental sections will be as a direct comparison to the core sections. Table 2 provides a list of the SPS-1 projects and a general description of the supplemental sections that were built at each one.

A total of 32 supplemental sections have been built as part of the SPS-1 experiment, but only 25 sections are listed in table 2. The other seven supplemental sections have been built, but descriptions of the structures of these sections are not in the LTPP database at this time. The supplemental sections not included in the database are from Arizona (two sections are included and two are excluded), Arkansas (one section), Iowa (one section), Ohio (two sections), and Oklahoma (one section). These additional sections should add significant value to the experiment and can be used to estimate the expected variability in distress with time under the same conditions.

CURRENT STATUS OF DESIGN FACTORIAL

The current status of the SPS-1 site design factorial is provided in table 3. All projects have been located in the appropriate cells based on the actual test data and construction details, which will be discussed in greater detail in chapter 4. As shown, all cells have two projects, with the exception of the coarse-grained subgrades in a dry-no-freeze climate. However, it is important to note the following observations regarding the final experiment design:

  1. The cell for the coarse-grained subgrades in a dry-no-freeze climate has only one project, while the cell for the fine-grained soils in a wet-freeze climate has four projects. This replication will be useful; however, if not analyzed properly, it could cause a bias in computing mean national trends.
  2. Eleven SPS-1 projects have been built over fine-grained soils, while only seven have been built over coarse-grained soils. This imbalance is not critical, but it should be considered when evaluating and analyzing the data for determining the effects of the subgrade on performance.
  3. The Alabama and Louisiana projects are located in the same cell (fine-grained soils in a wet-no-freeze climate) but are not paired as a complete factorial. Both of these projects contain pavement structure cells 1 to 12 (refer to table 1). Thus, this cell also contains only one of the companion projects, similar to the coarse-grained soils in a dry-no-freeze climate (the Arizona project).

Table 2. Supplemental sections constructed on SPS-1 projects.

SPS-1 Supplemental Sections
STATE_CODE SHRP_ID LAYER_NO Layer Description (from material codes)
1 0161 1 Silty Clay
2 152 mm Crushed Stone
3 152 mm Bituminous Bound Base
4 102 mm HMAC
1 0162 1 Silty Clay
2 249 mm Bituminous Bound Base
3 114 mm HMAC
1 0163 1 Sily Clay
2 152 mm Crushed Stone
3 Woven Geotextile
4 102 mm PATB
5 150 mm Bituminous Bound Base
6 119 mm HMAC
4 0161 1 Gravel
2 94 mm Crushed Stone
3 145 mm HMAC
4 0162 1 Gravel
2 229 mm HMAC
10 0159 1 Silty Sand
2 1219 mm Silty Sand
3 193 mm Crushed Stone
4 168 mm Bituminous Bound Base
5 142 mm HMAC
7 25mm Porous Friction Course
10 0160 1 Silty Sand
2 991 mm Silty Sand
3 140 mm Cement-Aggregate Mixture
4 142 Bituminous Bound Base
5 175 mm MHAC
7 25 mm Porous Friction Course
12 0161 1 Poorly Graded Sand
2 259 mm Limerock, Caliche
3 102 mm HMAC
20 0159 1 Sandy Silt
2 152 mm Treated Subbase
3 297 mm HMAC
20 0160 1 Sandy Silt
2 152 mm Treated Subbase
3 178 mm Crushed Stone
4 165 mm HMAC
20 0161 1 Sandy Silt
2 152 mm Treated Subbase
3 279 mm Gravel
4 140 mm HMAC
20 162 1 Sandy Silt
2 152 mm Treated Subbase
3 251 mm ATB
4 38 mm HMAC
20 0163 1 Sandy Silt
2 152 mm Treated Subbase
3 203 mm Gravel
4 64 mm ATB
5 38.1 mm Plant Mix (Cutback Asphalt) Material, Cold Laid
20 0164 1 Sandy Silt
2 152 mm Treated Subbase
3 305 mm HMAC
39 0160 1 Silty Clay
2 102 mm Crushed Stone
3 277 mm ATB
4 104 mm HMAC
40 0160 1 Sandy Clay
2 Lime Treated Subgrade Soil
3 137 mm Crushed Stone
4 PATB
5 HMAC
48 0160 1 Silty Sand
2 305 mm Lime Treated Subgrade Soil
3 269 mm Limerock Asphalt
4 145 mm HMAC
48 0161 1 Silty Sand
2 305 mm Lime Treated Subgrade Soil
3 211 mm Limerock Asphalt
4 109 mm HMAC
48 0162 1 Silty Sand
2 305 mm Lime Treated Subgrade Soil
3 203 mm Crushed Limestone
4 121.9 mm HMAC
48 0163 1 Silty Sand
2 305 mm Lime Treated Subgrade Soil
3 251 mm Crushed Limestone
4 117 mm HMAC
48 0164 1 Silty Sand
2 305 mm Lime Treated Subgrade Soil
3 208 mm Crushed Concrete
4 127 mm HMAC
48 0165 1 Silty Sand
2 305 mm Lime Treated Subgrade Soil
3 185 mm Crushed Concrete
4 112 mm HMAC
48 0166 1 Silty Sand
2 305 mm Lime Treated Subgrade Soil
3 343 mm Limerock, Caliche
4 142 HMAC
48 0167 1 Silty Sand
2 305 mm Lime Treated Subgrade Soil
3 8 mm Reinforced Geogrid
4 244 mm Limerock, Caliche
5 122 HMAC
51 0159 1 Silty Clay
2 152 mm Soil Cement
3 188 mm Crushed Stone
4 102 mm PATB
5 140 mm Bituminous Bound Base
6 86 mm HMAC

 

Table 3. Final factorial for the SPS-1 experiment design.

Subgrade Soil Type

Pavement Structure
(Refer to Table 1)

Climate, Moisture—Temperature

Wet-Freeze

Wet-No-Freeze

Dry-Freeze

Dry-No-Freeze

Fine-Grained

Cells 1–12

Site Cell 1.A
IA(1)—7.0
OH(2)—4.6

Site Cell 2.A

Site Cell 3.A
KS(6)—5.8

Site Cell 4.A
NM(0)—3.7

Cells 13–24

Site Cell 1.B
MI(1)—4.0
VA(1)—3.7

Site Cell 2.B

Site Cell 3.B
NB(1)—4.1

Site Cell 4.B
OK(2)—2.1
TX(8)—2.3

Coarse-Grained

Cells 1–12

Site Cell 5.A
DE(2)—3.2

Site Cell 6.A
FL(1)—3.7

Site Cell 7.A
NV(0)—4.0

Site Cell 8.A

Cells 13–24

Site Cell 5.B
WI(0)—1.8

 

Site Cell 6.B
AR(1)—5.7

Site Cell 7.B
MT(0)—0.8

Site Cell 8.B
AZ(5)—6.0

Note: The values in parentheses are the number of supplemental sections for each project. The other value provided for each project is the age of that project in years, as of January 2000.

In summary, a total of 248 SPS-1 sections have been built across the United States and Canada. At least one project is located within each site factorial cell. It is expected that those cells with missing companion projects can be compensated for through the use of mechanistic-empirical studies and proper analyses of the data. Thus, the completeness of the SPS-1 experiment is considered to be in overall good condition to meet the experimental objectives and expectations that were noted in chapter 1.

 


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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.
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