1. Chapter 9. Highway Design for Rideability
Chapter objectives covered by CE361: By the end of this chapter the student will be able to:
Determine the loading that vehicles exert on the highway pavement system
Design a section of flexible pavement
Evaluate cost tradeoffs between material attributes and the thickness of pavement layers
Determine the thickness of a Portland Cement Concrete slab (not covered in CE361)
Implement a simple pavement management system (9.5.1 – only)
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2. 9.1 Factors in Pavement Design
By the end of this section, the student will be able to...
Objectives of 9.1 & 9.2
List the components in a pavement system
List the factors that affect pavement design
Describe the impacts that trucks have on pavements
Explain load equivalency factors
Identify truck types by axle configurations
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3. 9.1 Factors in Pavement Design
9.1.1 History – Read
9.1.2 Two kinds of pavements
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4. 9.1.4 Measuring pavement quality
Measured by Rideability (or comfort to the rider) and Visual observation of faults (distresses such as cracking, rutting, and shoving)
PSI (present serviceability index 0 -5) – surrogate for PSR by physical measurement
PSR (present serviceability rating 0 – 5) – subjective rating by expert raters.
Terminal serviceability index (TSI) PSI = 2.0 to 2.5 (unacceptable ride quality)
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5. IRI (International Roughness Index) – measure of roughness inches/mile or meters/km – ride-based which is directly a function of pavement distress – cracking, patching, etc.
Tab 9.3
An IRI over 120 in/mi indicates time for rehabilitation and corresponds to a PSI of about 2.2.
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6. When CBR (California bearing ratio) < 6, Mr ≈ 1500 x CBR
9.1.5 Soil characteristics
Mr: Modulus of resiliency = the elastic properties of soil is used in flexible pavement thickness design
When CBR (California bearing ratio) < 6, Mr ≈ 1500 x CBR
Mr has seasonal changes – see the next page. This chart was omitted in the textbook. Need to determine an annual average Mr to use in pavement design.
This topic is covered in detail in CEEn563, taught by Dr. Guthrie.
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7. Environment Step 1 Step 2 Step 3
Temperature and rainfall affect the level of strength of the subgrade, reflected on the value of resilient modulus. AASHTO developed a chart that helps you to estimate the effective roadbed soil resilient modulus using the serviceability criteria (in terms of “relative damage, uf.”)
Determine the average uf. value and obtain Mr from the chart or the equation of uf. .
The bar on the right is used twice: Once to read uf value for each month’s sample Mr, then to read annual average Mr using the average uf value.
Step 2
Step 3
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8. 9.2 Determining Loads from Truck Traffic
Truck traffic is the major cause of pavement damages. And truck loading is converted to ESALs (equivalent single-axle loads) for pavement thickness design (the AASHTO design method)
The objective is to estimate the ESALs that will cause a degradation of the PSI to 2 or 2.5 from its initial value of 4.5 to 5. The annual ESALs are calculated from the expected mix of daily truck traffic and summed for the year.
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9. 9.2.1 Determining equivalent single-axle load
Fig 9.5
Read and
Fig 9.6
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10. Chapter 9

11. 9.2.4 Determining lifetime ESALs for pavement design
This is like calculating a future value of a series of annual payments with interest rate, g.
AnnualESAL
N years: design horizon
Future total ESAL
We will walk through Examples 9.1 and 9.2.
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12. 9.3 Flexible Pavement Design
9.3.1 The AASHTO Road Test
Read to know the history of the AASHTO flexible pavement thickness design formula,
Done in Ottawa, Ill
9.3.2 Materials for asphalt pavements
Asphalt concrete (surface layer):
Must resist deformation from loads
Be skid resistant (even when wet)
Be impervious to most weather and deicing chemicals
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13. 9.3.3 Pavement layers and the structural number
Surface layer of hot mix asphalt (typically 3 to 4 inches)
“black top” = bitumen + aggregates
Base course (typically 6 to 12 inches)
Unbound, untreated course aggregate; sometimes treated
Subbase course (depends on the subgrade)
Right below base course; heavy control of the fines
Subgrade – soil under pavement
Either local material or replaced
Structural Number (SN)
Reflects strength of the total pavement thickness
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14. Structural number of a pavement
SN reflects the total pavement thickness, including the capability of the soil or subgrade to provide sufficient resiliency during the repeated loadings to maintain the desired serviceability (PSI).
a1, a2, a3 = coefficients of relative strength per inch
d1, d2, d3 = thickness in inches, d1 being surface layer
mi = modifiers for more than normal amounts of moisture, mi = 1.0 unless otherwise specified (A table contains m values was omitted in 2nd edition. In this class, m = 1.0 is used.)
hma = hot mix asphalt
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15. Definition of drainage quality and finding recommended mi values
Time required to drain the base/subbase layer to 50% saturation of free water.
Step 1
If “Fair” and 30% exposure, then mi is 0.80.
Step 2
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16. 9.3.4 Fitting the design variables together
The properties (i.e. layer coefficients in the equation above) of each layer plays a major role in determining the thickness of each layer.
Once the required structural number of a pavement section is determined, the layers are computed by using the AASHTO layered analysis method.
It is assumed that the structural capacity of the pavement is the sum of the structural capacity of each of its layers.
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17. Simplify this as f(W18) = f(ZRSo) + f(SN)
We will keep the ESAL value (W18) constant and try to prove whether ZR must be negative or not. Note that So and SN are always positive. Standard deviation is always positive because it is a physical difference from the mean value, and SN is also positive because it implies pavement thickness.
ESAL is an estimated value. It may actually more or less. In the design formula, however, the ESAL value is set to a constant. Then, to make sure the pavement survive, you have to have a thicker one than the thickness that the estimated ESAL requires. To make that happen in the design formula, we need to subtract a value from the RHS. Hence, the reliability factor must be negative (see Table 9.7). The only way to make ZRSo a subtraction is to have a negative value of ZR because So is always positive. In return f(SN) must be bigger to make up the difference, resulting in larger SN needed. S02 accounts for the chance variation in the traffic forecast and the chance variation in actual pavement performance for a given design period traffic, W18.
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18. Nomograph for the AASHTO flexible pavement design method
Pivot pt.
Nomograph for the AASHTO flexible pavement design method
Finding SN for base course, subbase, & subgrade (see p.9.22).
Data for point D. Mr of subgrade assumed to be 5000 psi
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19. 9.3.5 Performing the layered analysis
Once the overall structural number for the pavement has been found, the layered analysis to find the thickness of each pavement layer can begin.
ai and mi values for the layers must be determined given material strength (Mr) and drainage characteristics data
Must meet minimum thickness standards
Tab 9.8
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20. Structural layer coefficients (ai)
Fig 9.14
Asphalt Concrete, a1
Fig 9.15
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Granular base layer for untreated, a2

21. Structural layer coefficients (cont)
Omitted
Fig 9.15
Cement-treated base, a2
Subbase, a3
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22. Determining mi values (mi = 1 unless otherwise specified)
Time required to drain the base/subbase layer to 50% saturation.
Step 1
If “Fair” and 30% exposure, then mi is 0.80.
Step 2
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23. Problem 9.13 as an example
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24. Example 9.4: Pavement Design Alternatives
We will walk through this example, and you must read it before you come to class.
Example 9.4 shows you how we can minimize the cost of pavement construction, while meeting SN requirements, by modifying layer thicknesses.
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25. 9.5 Pavement Management System
PMS = A pavement management system (PMS) is a set of tools or methods that assist decision makers in finding optimum strategies for providing and maintaining pavements in a serviceable condition over a given period of time.
Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA 91) required all States to have a PMS that covered all Federal-Aid highways; but this requirement was rescinded in 1995.
1990 AASHTO Guidelines for PMS
“A Pavement Management System is designed to provide objective information and useful data for analysis so that highway managers can make consistent, cost-effective, and defensible decisions related to the preservation of a pavement network.”
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26. Performance Analysis
Without routine maintenance, but with reconstruction every 5 years.
With routine maintenance
Fig. 9.27
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27. Three components of a PMS
database which contains, as a minimum, the data required for PMS analysis:
analysis methods to generate products useful for decision making: and,
feedback process which uses ongoing field observations to improve the reliability of PMS analysis.
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28. Data collection components
Inventory: physical pavement features including the number of lanes, length, width, surface type, functional classification, and shoulder information
History: project dates and types of construction, reconstruction, rehabilitation, and preventive maintenance, and their costs
Condition survey: roughness or ride, pavement distress, rutting, and surface friction
Traffic: volume, vehicle type, and load data
Data base: compilation of all data files in the PMS
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29. Analysis Components
Condition analysis: ride (PSI, IRI), distress (see Figures 9.29 and 9.30), rutting, and surface friction
Performance analysis: pavement performance analysis and an estimate of remaining service life
Investment analysis: an estimate of network and project level investment strategies (single- and multi-year period analyses and life-cycle cost evaluation (trade-offs analysis, e.g. rehabilitation or maintenance)
Engineering analysis: evaluation of design, construction, rehabilitation, materials, mix design, and maintenance
Feedback analysis: evaluation and updating of procedures and calibration of relationships using PMS performance data and current engineering criteria
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30. Pictures from the UP North SLC Intermodal Transfer Yard
Cracks
Joint failures & corner cracks
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Alligator cracks
Potholes

31. Condition Analysis Outputs
The outputs from this module can include:
ranking of all pavement segments according to types of distress and condition scores as a function of traffic or road classification;
(2) identification of MR&R strategies, which define a set of criteria (e.g., combinations of different distress levels and traffic) for assigning a particular action to each pavement segment; and
(3) estimates of funding needs for the selected treatments. The outputs are indicative of current needs based on current conditions.
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