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. To reduce the amount of stress on soil sub-grade, provide thick layers of base course, Sub-base course and surface course.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 The sub-grade should be strong enough to take the stresses and also it is important to keep the stresses coming from top layers should be within the limit of sub-grade capacity. This layer is termed as foundation for the pavement system.

Subgrade is the bottom most layer which is nothing but natural soil layer compacted up to required depth generally about 150 to 300 mm to receive the loads coming from top layers. A good quality base course and Sub-base courses provided can also serves frost protection layer.

The function of frost protection layer is to prevent damage of pavement from frost heaves, which are formed by freezing of groundwater. It is generally provided between Subbase and sub-grade soil. Frost Protection Layerįrost protection layer is provided for the pavements in colder regions where temperatures are very low.

Flexible pavement design aashto profile