Presentation on theme: "Know the factors considered in the AASHTO design method"— Presentation transcript:
1 Lec 29, Ch.20, pp.990-1007: AASTHO flexible pavement design method (objectives) Know the factors considered in the AASHTO design methodBecome familiar with use of Tables 20.13a and b through and Figures through 20.20Know how the structural numbers are used in thickness determinationBecome thoroughly familiar with the structural design process of the AASHTO flexible pavement design method
2 What we discuss in class today… Design considerationsUse of tables and figures related to the factors consideredStructural design procedure
3 Design considerations for the AASHTO Flexible Pavement Design The following factors are considered in the pavement thickness design.Pavement performanceTrafficRoadbed soils (subgrade material)Materials of constructionEnvironmentDrainageReliability
4 Pavement performance Structural Cracking, faulting, raveling, etc. Functional Riding comfort (measured in terms of roughness of pavement.)Serviceability Performance: Measured by PSI Present Serviceability Index with scale 0 to 5.5 “Just constructed”4.2 Initial PSI (pi)PSITerminal PSI (pt)2.5 to 3.0 for major highways2.0 for lower class highways1.5 for very special cases0 “Road closed”
5 TrafficIn the AASHTO flexible pavement design, traffic is considered in terms of ESAL for the terminal PSI (Table for pt = 2.5.)You must assume the structural number of the pavement. So, you must check if the final SN3 is similar to the assumed SN. Higher SN means stronger pavement, thus the impact of traffic on pavement deteriorations is less.
6 Roadbed soils (Subgrade material) CBR (California Bearing Ratio), R-value (Resistance), and Mr (Resilient modulus) are used to describe the property of the subgrade material.During the structural design, only Mr values are used. The following conversion formulas are used if either CBR or R-values are given.Mr (lb/in2) = 1500 x CBR for fine-grain soils with soaked CBR of 10 or less.Mr (lb/in2) = x (R-value) for R <= 20
7 Materials of construction (Subbase), a3 Charts are available to convert the properties of pavement construction materials to structural numbers: a3, a2, and a1Use CBR, R-value, or Mr to find a3 valuesStructural number of the subbase, a3
8 Materials of construction (Base course), a2 Use CBR, R-value, or Mr to find a2 valuesStructural number of the base course, a2
9 Materials of construction (AC surface), a1 0.44Structural number of the AC surface, a1= Resilient modulus, Mr
10 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 2Step 3
11 DrainageThe effect of drainage on the performance of flexible pavements is considered with respect to the effect water has on the strength of the base material and roadbed soil.This effect is expressed by the drainage coefficient, mi. This value is dependent on the drainage quality and the percent of time pavement structure is exposed to moisture levels approaching saturation.
12 Definition of drainage quality and finding recommended mi values Time required to drain the base/subbase layer to 50% saturation.Step 1If “Fair” and 30% exposure, then mi is 0.80.Step 2
13 Reliability The reliability factor (FR) is computed using: The Reliability design level (R%), which determine assurance levels that the pavement section designed using the procedure will survive for its design period (it is a z-score from the standard normal distributionthe standard deviation (So) that accounts for the chance variation in the traffic forecast and the chance variation in actual pavement performance for a given design period traffic, W18.Why do we have a negative sign here? Are ZR values negative? Why not ZRSo! Well the clue is in Eq and the bell curve shown below.One-sided Z-score is used here.SD, SoFlexible pavementsRigid pavementsSurviveFail
14 Structural design Where, The object of the design using the AASHTO method is to determine a flexible pavement SN adequate to carry the projected design ESAL.The method discussed in the text (Example 20-8) applies to ESALs greater than 50,000 for the performance period. The design for ESALs less than this is usually considered under low-volume roads.Where,Simplify this as f(W18) = f(ZRSo) + f(SN)We will keep the ESAL value 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 means pavement thickness.
15 Solving the riddle of the negative values of ZR f(W18) = f(ZRSo) + f(SN)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. The only way to make ZRSo smaller is to have a negative value of ZR because So is always positive.
16 How to use Fig. 20.20 to get structural numbers based on Eq. 20.13 For subbase, Mr=13,500For base course, Mr=31,000SN1= 2.6This line is for the subgrade in Example Mr=1500*6=9000SN3= 4.4SN2= 3.8This line has nothing to do with Example 20-8.
17 Once SN value is set, thickness design begins… Proceed in this directionUse Fig for a3, Fig for a2, Fig for a3, and Tab and m2 and m3. Find the depth that results in a SN value close to the SN value obtained from Fig
18 Example 20-8 Given: ESAL = 2 x 106 One week for water to be drained Saturation level moisture exposure = 30% of the timeAC’s Mr at 68Fo = 450,000 lb/in2CBR of base course =100, Mr = 31,000 lb/in2CBR of subbase =22, Mr = 13,500 lb/in2CBR of subgrade = 6, Mr = 1500CBR= 6*1500 = 9000 lb/in2Rural interstateParameter values:Reliability level (R ) = 99% from TabStandard Deviation (So) = 0.49, Table 20.16, p.973Initial serviceability, pi = 4.5Terminal serviceability, pt = 2.5Drainage mi values = 0.8 for “Fair” category in Tab and “Greater than 25%” category in Tab