UNIT – V

DESIGN OF PAVEMENTS
Design Of Pavements: Types of pavements; Functions and requirements of different components of pavements;
Design Factors

Flexible Pavements: Design factors – Flexible Pavement Design Methods – CBR method – IRC method – Burmister
method – Mechanistic method – IRC Method for Low volume Flexible pavements.

Rigid Pavements: Design Considerations – wheel load stresses – Temperature stresses –Frictional stresses –
Combination of stresses – Design of slabs – Design of Joints – IRC method – Rigid pavements for low volume
roads – Continuously Reinforced Cement Concrete Pavements – Roller Compacted Concrete Pavements.

IRC FLEXIBLE PAVEMENTS:

PAVEMENTS: -
Definition: - These guidelines will apply to design of flexible pavements for
Expressway, National Highways, State Highways, Major District Roads, and
other categories of roads. Flexible pavements are considered to include the
pavements which have bituminous surfacing and granular base and sub-base
courses conforming to IRC/ MOST standards. These guidelines apply to new
pavements.

Design criteria:-
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:

1. Vertical compressive strain at the top of the sub-grade which can cause
sub-grade deformation resulting in permanent deformation at the pavement
surface.
2. Horizontal tensile strain or stress at the bottom of the bituminous layer
which can cause fracture of the bituminous layer.
3. 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 strain at the critical points is 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.

A and B are the critical locations for tensile strains (Ԑ t). Maximum value of the strain is
adopted for design. C is the critical location for the vertical subgrade strain (Ԑ z) since the
maximum value of the (Ԑ z) occurs mostly at C.

Fatigue Criteria:
Bituminous surfacing 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. 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

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

Design procedure:-
Based on the performance of existing designs and using analytical approach,
simple design charts and a catalogue of pavement designs are added in the
code. The pavement designs are given for subgrade CBR values ranging from
2% to 10% and design traffic ranging from 1 msa to 150 msa for an average
annual pavement temperature of 35 C. The later thicknesses obtained from the
analysis have been slightly modified to adapt the designs to stage
construction. Using the following simple input parameters, appropriate
designs could be chosen for the given traffic and soil strength:

 Design traffic in terms of cumulative number of standard axles; and

 CBR value of subgrade.

Design traffic:-
The method considers traffic in terms of the cumulative number of
standard axles (8160 kg) to be carried by the pavement during the design
life. This requires the following information:

1. Initial traffic in terms of CVPD

2. Traffic growth rate during the design life

3. Design life in number of years

4. Vehicle damage factor (VDF)

5. Distribution of commercial traffic over the carriage way.

Initial traffic:-
Initial traffic is determined in terms of commercial vehicles per day (CVPD).
For the structural design of the pavement only commercial vehicles are
considered assuming laden weight of three tones or more and their axle
loading will be considered. Estimate of the initial daily average traffic flow
for any road should normally be based on 7-day 24-hour classified traffic
counts (ADT). In case of new roads, traffic estimates can be made on the
basis of potential land use and traffic on existing routes in the area.

Traffic growth rate:-

Traffic growth rates can be estimated (i) by studying the past trends of
traffic growth, and (ii) by establishing econometric models. If adequate
data is not available, it is recommended that an average annual growth rate
of 7.5 percent may be adopted.

Design life:-
For the purpose of the pavement design, the design life is defined in terms
of the cumulative number of standard axles that can be carried before
strengthening of the pavement is necessary. It is recommended that
pavements for arterial roads like NH, SH should be designed for a life of
 15 years, EH and urban roads for 20 years and other categories of roads for
10 to 15 years.

Vehicle Damage Factor:-

The vehicle damage factor (VDF) is a multiplier for converting the number
of commercial vehicles of different axle loads and axle configurations to
the number of standard axle-load repetitions. It is defined as equivalent
number of standard axles per commercial vehicle. The VDF varies with
the axle configuration, axle loading, terrain, type of road, and from region
to region. The axle load equivalency factors are used to convert different
axle load repetitions into equivalent standard axle load repetitions. For
these equivalency factors refer IRC: 37 2001. The exact VDF values are
arrived after extensive field surveys.

Vehicle distribution
A realistic assessment of distribution of commercial traffic by direction
and by lane is necessary as it directly affects the total equivalent standard
axle load application used in the design. Until reliable data is available, the
following distribution may be assumed.

 Single lane roads: Traffic tends to be more channelized on single roads

than two lane roads and to allow for this concentration of wheel load
repetitions, the design should be based on total number of commercial
vehicles in both directions.

 Two-lane single carriageway roads: The design should be based on 75

% of the commercial vehicles in both directions.

 Four-lane single carriageway roads: The design should be based on 40

% of the total number of commercial vehicles in both directions.

 Dual carriageway roads: For the design of dual two-lane carriageway

roads should be based on 75 % of the number of commercial vehicles
in each direction. For dual three-lane carriageway and dual four-lane
carriageway the distribution factor will be 60 % and 45 % respectively.

Pavement thickness design charts:-

For the design of pavements to carry traffic in the range of 1 to 10 msa, use
chart 1 and for traffic in the range 10 to 150 msa, use chart 2 of IRC:37 2001.
The design curves relate pavement thickness to the cumulative number of
standard axles to be carried over the design life for different sub-grade CBR
values ranging from 2 % to 10%. The design charts will give the total
thickness of the pavement for the above inputs. The total thickness consists of
granular sub-base, granular base and bituminous surfacing. The individual
layers are designed based on the recommendations given below and the
subsequent tables.
 Pavement composition:-
Sub-base:-
Sub-base materials comprise natural sand, gravel, Laterite, brick metal,
crushed stone or combinations thereof meeting the prescribed grading and
physical requirements. The sub-base material should have a minimum
CBR of 20 % and 30 % for traffic upto 2 msa and traffic exceeding 2 msa
respectively. Sub-base usually consist of granular or WBM and the
thickness should not be less than 150 mm for design traffic less than 10
msa and 200 mm for design traffic of 1:0 msa and above.

Base:-
The recommended designs are for unbounded granular bases which
comprise conventional water bound macadam (WBM) or wet mix macadam
(WMM) or equivalent confirming to MOST specifications. The materials
should be of good quality with minimum thickness of 225 mm for traffic up to
2 msa a 150 mm for traffic exceeding 2 msa.

Bituminous surfacing:-
The surfacing consists of a wearing course or a binder course plus wearing
course. The most commonly used wearing courses are surface dressing,
open graded premix carpet, mix seal surfacing, semi-dense bituminous
concrete and bituminous concrete. For binder course, MOST specifies, it is
desirable to use bituminous macadam (BM) for traffic upto o 5 msa and
dense bituminous macadam (DBM) for traffic more than 5 msa.

IRC Rigid PAVEMENTS:

PAVEMENTS: -
Modulus of sub-grade reaction:-

Westergaard considered the rigid pavement slab as a thin elastic plate resting on soil sub-
grade, which is assumed as a dense liquid. The upward reaction is assumed to be
proportional to the deflection. Base on this assumption, Westergaard defined a modulus

of sub-grade reaction K in kg/cm3 given by K = where ∆ is the displacement level

taken as 0.125 cm and p is the pressure sustained by the rigid plate of 75 cm diameter at a
deflection of 0.125 cm.

Relative stiffness of slab to sub-grade

A certain degree of resistance to slab deflection is offered by the sub-grade. The
sub-grade deformation is same as the slab deflection. Hence the slab deflection is
direct measurement of the magnitude of the sub-grade pressure. This pressure
deformation characteristics of rigid pavement lead Westergaard to the define the
term radius of relative stiffness l in cm
 Where E is the modulus of elasticity of cement concrete in kg/cm 2 (3.0×105), µ is
the Poisson’s ratio of concrete (0.15), h is the slab thickness in cm and K is the
modulus of sub-grade reaction.

Critical load positions

Since the pavement slab has finite length and width, either the character or the
intensity of maximum stress induced by the application of a given traffic load is
dependent on the location of the load on the pavement surface. There are three
typical locations namely the interior, edge and corner, where differing conditions
of slab continuity exist. These locations are termed as critical load positions.

Equivalent radius of resisting section

When the interior point is loaded, only a small area of the pavement is resisting the
bending moment of the plate. Westergaard’s gives a relation for equivalent radius of
the resisting section in cm in the equation.

Where a is the radius of the wheel losad distribution in cm and h is the slab thickness
in cm

Wheel load stresses - Westergaard’s stress equation:

The cement concrete slab is assumed to be homogeneous and to have uniform
elastic properties with vertical sub-grade reaction being proportional to the
deflection. Westergaard’s developed relationships for the stress at interior, edge
and corner regions, denoted as σi, σe, σc in kg/cm2 respectively and given by the
equation.

Where h is the slab thickness in cm, P is the wheel load in kg, as is the radius of the wheel
load distribution in cm, L the radius of the relative stiffness in cm and b is the radius of
the resisting section in cm
Temperature stresses
Temperature stresses are developed in cement concrete pavement due to variation
in slab temperature. This is caused by (i) daily variation resulting in a temperature
gradient across the thickness of the slab and (ii) seasonal variation resulting in
overall change in the slab temperature. The former results in warping stresses and
the later in frictional stresses.

Warping stress
The warping stress at the interior, edge and corner regions, denoted as σti , σte ,
σtc in kg/cm2 respectively and given by the equation

where E is the modulus of elasticity of concrete in kg/cm 2 (3×105), s is the

thermal coefficient of concrete per oC (1×10−7) t is the temperature difference
between the top and bottom of the slab, C x and Cy are the coefficient based on Lx/l
in the desired direction and Ly/l right angle to the desired direction, µ is the
Poisson’s ration (0.15), a is the radius of the contact area and l is the radius of the
relative stiffness.

Frictional stresses
The frictional stress σf in kg/cm2 is given by the equation

Where W is the unit weight of concrete in kg/cm 2 (2400), f is the coefficient of sub
grade friction (1.5) and L is the length of the slab in meters.
Combination of stresses
The cumulative effect of the different stress give rise to the following thee critical
cases
 Summer, mid-day: The critical stress is for edge region given by
σ critical = σe + σte − σf
 Winter, mid-day: The critical combination of stress is for the edge
region given by σ critical = σe + σte + σf
 Mid-nights: The critical combination of stress is for the corner region
given by σ critical = σc + σtc
Design of joints
Expansion joints

The purpose of the expansion joint is to allow the expansion of the pavement due to
rise in temperature with respect to construction temperature. The design
considerations are:

 Provided along the longitudinal direction,

 design involves finding the joint spacing for a given expansion joint thickness
(say 2.5 cm specified by IRC) subjected to some maximum spacing (say 140
as per IRC)

Contraction joints

The purpose of the contraction joint is to allow the contraction of the slab due to fall
in slab temperature below the construction temperature. The design
considerations are:

 The movement is restricted by the sub-grade friction

 Design involves the length of the slab given by:

 Where, Sc is the allowable stress in tension in cement concrete and is taken as 0.8
kg/cm2, W is the unit weight of the concrete which can be taken as 2400 kg/cm 3 and
f is the coefficient of sub-grade friction which can be taken as 1.5.

 Steel reinforcements can be use, however with a maximum spacing of 4.5 m as per IRC.

Dowel bars

The purpose of the dowel bar is to effectively transfer the load between two
concrete slabs and to keep the two slabs in same height. The dowel bars are
provided in the direction of the traffic (longitudinal). The design
considerations are:

 Mild steel rounded bars,

 bonded on one side and free on other side

Bradbury’s analysis
Bradbury’s analysis gives load transfer capacity of single dowel bar in shear, bending
and bearing as follows

where, P is the load transfer capacity of a single dowel bar in shear s, bending f and
bearing b, d is the diameter of the bar in cm, Ld is the length of the embedment of
 dowel bar in cm, δ is the joint width in cm, Fs, Ff, Fb are the permissible stress in
shear, bending and bearing for the dowel bar in kg/cm2.

Design procedure:-
Step 1: Find the length of the dowel bar embedded in slab Ld by equating 2 & 3

Step 2: Find the load transfer capacities Ps, Pf, and Pb of single dowel bar with the
Ld

Step 3: Assume load capacity of dowel bar is 40 percent wheel load, find the load
capacity factor f as

Step 4: Spacing of the dowel bars.

 Effective distance upto which effective load transfers take place is given by
1.8 l, where l is the radius of relative stiffness.

 Assume a linear variation of capacity factor of 1.0 under load to 0 at 1.8 l.

 Assume dowel spacing and find the capacity factor of the above spacing.

 Actual capacity factor should be greater than the required capacity factor.

 If not, do one more iteration with new spacing.