Estimation of layer thicknesses

• The thickness of the pavement is adjusted in such a way that the stress/ strain developed is less than the  allowable values obtained from past performance information.
• The two major modes of structural failure of pavement are fatigue and rutting.
  • Fatigue: Traffic applies repetitive load to the pavement surface, and the cracks start from bottom the bound layer/ slab and propagate upwards. When the extent of surface cracks reaches a predefined level, the pavement is said to have failed due to flexural fatigue.

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

        Fig. 4  Propagation of fatigue cracking

  • The horizontal tensile stress/ strain at the bottom of bound layer (bituminous surfacing, cemented base or concrete slab, as the case may be) is used as the governing parameter for fatigue failure.
  • Conventionally, for design of concrete pavement stress is used as parameter, and for design of bituminous pavement strain is used as parameter.

  • Rutting: Rutting is the accumulation of permanent deformation. This is the manifestation of gradual densification of pavement layers, and shear displacement of the subgrade.

  Fig 5. Development of Rutting

   

  • The vertical strains on the pavement layers, mainly the vertical strain on the subgrade is assumed to be governing factor for rutting failure.
  • The rutting is generally not considered for concrete pavement design.

• The fatigue/ rutting equations are developed from field or laboratory studies, where fatigue / rutting lives are obtained with respect to respective stress/ strain for fatigue/ rutting. For a given design life, thus, allowable fatigue and rutting stress/ strains can be estimated using the fatigue/ rutting equations.
• The various other types of pavement failures could be shrinkage, thermal fatigue, top down cracking (for bituminous pavement) etc.