Chapter: Civil : Highway Planning and Alignment : Design Of Flexible And Rigid Pavements
Design criteria as per IRC
Design criteria as per IRC
The flexible pavements has been modeled as a three
layer structure and stresses and strains at critical locations have been
computed using the linear elastic model. To give proper consideration to the
aspects of performance, the following three types of pavement distress
resulting from repeated (cyclic) application of traffic loads are considered:
vertical compressive strain at the top of the sub-grade which
can cause sub-grade deformation resulting in permanent deformation at the
pavement surface.
horizontal tensile strain or stress at the bottom of the
bituminous layer which can cause fracture of the bituminous layer.
pavement deformation
within the bituminous
layer.
While the permanent deformation within the
bituminous layer can be controlled by meeting the mix design requirements,
thickness of granular and bituminous layers are selected using the analytical
design approach so that strains at the critical points are within the allowable
limits. For calculating tensile strains at the bottom of the bituminous layer,
the stiffness of dense bituminous macadam (DBM) layer with 60/70 bitumen has
been used in the analysis.
Failure Criteria
A and B are the critical locations for tensile strains ( t
). Maximum value of the strain is adopted for design. C is the critical
location for the vertical subgrade strain ( z ) since the maximum
value of the ( 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
in which, Nf is the allowable number of load
repetitions to control fatigue cracking and E is the Elastic modulus of
bituminous layer. The use of equation 28.1 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
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