value and obtain Mr from the chart or the equation of uf. 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. Materials of construction (AC surface), a1= Resilient modulus, MrStructural number of the AC surface, a10.44Įnvironment Temperature and rainfall affect the level of strength of the subgrade, reflected on the value of resilient modulus. Materials of construction (Base course), a2Base!Structural number of the base course, a2Use CBR, R-value, or Mr to find a2 values Materials of construction (Subbase), a3 Charts are available to convert the properties of pavement construction materials to structural numbers: a3, a2, and a1Structural number of the subbase, a3Use CBR, R-value, or Mr to find a3 values 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) = 1000 + 555 x (R-value) for R During the structural design, only Mr values are used. 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.Higher SN means stronger pavement, thus the impact of traffic on pavement deteriorations is less. So, you must check if the final SN3 is similar to the assumed SN. Traffic In the AASHTO flexible pavement design, traffic is considered in terms of ESAL for the terminal PSI (Table 20.13 for pt = 2.5.) You must assume the structural number of the pavement. Pavement performanceStructural 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 5.0 Road closed5 Just constructed4.2 Initial PSI (pi)Terminal PSI (pt) 2.5 to 3.0 for major highways 2.0 for lower class highways 1.5 for very special casesPSI What we discuss in class todayDesign considerations Use of tables and figures related to the factors consideredStructural design procedureĭesign considerations for theAASHTO Flexible Pavement Design Pavement performance Traffic Roadbed soils (subgrade material) Materials of construction Environment Drainage ReliabilityThe following factors are considered in the pavement thickness design. 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 20.18 and Figures 20.15 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