Module 4: Pavement Design

Lecture 28 IRC method of design of flexible pavements

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Failure Criteria

Critical locations in pavement

A and B are the critical locations for tensile strains $(\epsilon_t)$. Maximum value of the strain is adopted for design. C is the critical location for the vertical subgrade strain $(\epsilon_z)$ since the maximum value of the $(\epsilon_z)$ occurs mostly at C.
Fatigue Criteria:
Bituminous surfacings of pavements display flexural fatigue cracking if the tensile strain at the bottom of the bituminous layer is beyond certain limit. The relation between the fatigue life of the pavement and the tensile strain in the bottom of the bituminous layer was obtained as

$$N_f = 2.21\times10^{-4}\times\left(\frac{1}{\epsilon_t}\right)^{3.89}\times\left(\frac{1}{E}\right)^{0.854}$$ (1)

in which, $N_f$ is the allowable number of load repetitions to control fatigue cracking and $E$ is the Elastic modulus of bituminous layer. The use of equation 1 would result in fatigue cracking of 20% of the total area.
Rutting Criteria
The allowable number of load repetitions to control permanent deformation can be expressed as

$$N_r = 4.1656\times10^{-8}\times\left(\frac {1}{\epsilon_z}\right)^{4.5337}$$ (2)

$N_r$ is the number of cumulative standard axles to produce rutting of 20 mm.