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Chapter 9. Highway Design for Rideability

Chapter objectives covered by CE361: By the end of this chapter the student will be able to:. Chapter 9. Highway Design for Rideability. Determine the loading that vehicles exert on the highway pavement system Design a section of flexible pavement

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Chapter 9. Highway Design for Rideability

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  1. Chapter objectives covered by CE361: By the end of this chapter the student will be able to: Chapter 9. Highway Design for Rideability 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 (section 9.4 not covered in CE361, take CEEn 563) Implement a simple pavement management system (9.5.1 – 9.5.3 only) Chapter 9

  2. 9.1 Factors in Pavement Design • 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 By the end of this section, the student will be able to... Chapter 9

  3. 9.1 Factors in Pavement Design 9.1.1 History – Read 9.1.2 Mechanistic-Empirical Pavement Design (not covered in CE361) 9.1.2 Two kinds of pavements Chapter 9

  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) Chapter 9

  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. Demonstration of the IRI: http://www.youtube.com/watch?list=PLCF257346F5C759BB&v=KYkvXPVCoQ4 Chapter 9

  6. 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 the annual average Mr to use in pavement design. This topic is covered in detail in CEEn563, taught by Dr. Guthrie. Chapter 9

  7. Environment Step 1 • 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 Chapter 9

  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. Chapter 9

  9. 9.2.1 Determining equivalent single-axle load Chapter 9

  10. Read 9.2.2 The Fourth-Power Formula for Trucks Chapter 9

  11. Chapter 9

  12. 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. Chapter 9

  13. 9.3 Flexible Pavement Design 9.3.1 The AASHTO Road Test Read the history of the AASHTO flexible pavement thickness design formula. Done in Ottawa, Ill. 1950 - 1962 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 Chapter 9

  14. 9.3.3 Pavement layers and the structural number Chapter 9

  15. 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 (The table contains m values was omitted in the 2nd edition. In this class, m = 1.0 is used.) hma = hot mix asphalt SN1 HMA SN2 Base SN3 Subbase Subgrade Chapter 9

  16. 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 Chapter 9

  17. 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 (above subgrade), layer thicknesses 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. SN1 HMA SN2 Base SN3 Subbase Subgrade Chapter 9

  18. (Eq. 9-7) 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 (see Table 9.7 They are all negative). 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 (variance) accounts for the chance variation in the traffic forecast and the chance variation in actual pavement performance for a given design period traffic, W18. Chapter 9

  19. Nomograph for the AASHTO flexible pavement design method Pivot pt. SN1 = 2.8 HMA SN2 = 3.8 Base SN3 = 5.0 Subbase Finding SN for base course, subbase, & subgrade (see p.9.19). Data for point D. Mr of subgrade assumed to be 5000 psi Subgrade Chapter 9

  20. 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 Chapter 9

  21. Structural layer coefficients (ai) Untreated granular base course, Mr = 25,000 lbs/in2  a2 = 0.12 HMA Mr = 380,000 lbs/in2  a1 = 0.41 Asphalt Concrete, a1 Chapter 9 Granular base layer for untreated, a2

  22. Structural layer coefficients (cont) Untreated granular subbase Mr = 13,000 lbs/in2  a3 = 0.098 Subbase, a3 Chapter 9

  23. Layer design steps (1) Step 1 & 2: Get required structural numbers, SN1, SN2, and SN3 Step 3: Use SN1 to determine the thickness of the HMA layer Chapter 9

  24. Layer design steps (2) Step 4: Use SN2 to determine the thickness of the base course layer, using the actual SN1* provided by the thickness chosen in Step 3. Step 5: Use SN3to determine the thickness of the base layer, using the actual SN2* provided by the thickness chosen in Step 4. Chapter 9

  25. 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. See the schedule page and click the link to Example 9.4 Step by Step file. Chapter 9

  26. 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.” Chapter 9

  27. Performance Analysis Without routine maintenance, but with reconstruction every 5 years. With routine maintenance Chapter 9

  28. 9.5.1 Road Inventory Data • 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. Chapter 9

  29. 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 Chapter 9

  30. 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 vs. 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 Chapter 9

  31. Condition Analysis Outputs • The outputs from this module can include: • rankingof 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. Chapter 9

  32. 9.3.2 The Life of a Pothole When to fix it? Chapter 9

  33. 9.5.3 Forms of Pavement Distress(Pictures from the UP North SLC Intermodal Transfer Yard) – See Figures 9.29 & 9.30) Cracks Joint failures & corner cracks Chapter 9 Alligator cracks Potholes

  34. 9.5.3 Forms of Pavement Distress(AC Distress vs. PCCP Distress) • Corner breaks (b) Durability cracking • (c) Longitudinal cracking (d) Faulting of transverse joints • (e) Scaling (f) Blowups • Alligator cracking (b) Longitudinal cracking • (c) Reflection cracking (d) Pothole • (e) Rutting (f) Water bleeding and pumping Chapter 9

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