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Rigid pavements have sufficient flexural strength to transmit the wheel load stresses to a wider area below.

**Rigid
pavements**

Rigid pavements have sufficient flexural strength
to transmit the wheel load stresses to a wider area below. Compared to flexible
pavement, rigid pavements are placed either directly on the prepared sub-grade
or on a single layer of granular or stabilized material. Since there is only
one layer of material between the concrete and the sub-grade, this layer can be
called as base or sub-base course.

In rigid pavement, load is distributed by the slab
action, and the pavement behaves like an elastic plate resting on a viscous
medium .Rigid pavements are constructed by Portland cement concrete (PCC) and
should be analyzed by plate theory instead of layer theory, assuming an elastic
plate resting on viscous foundation. Plate theory is a simplified version of
layer theory that assumes the concrete slab as a medium thick plate which is
plane before loading and to remain plane after loading. Bending of the slab due
to wheel load and temperature variation and the resulting tensile and flexural
stress.

**Types of
Rigid Pavements**

Rigid pavements
can be classified
into four types:

Jointed plain
concrete pavement (JPCP),

Jointed reinforced
concrete pavement (JRCP),

Continuous
reinforced concrete pavement (CRCP), and

Pre-stressed
concrete pavement (PCP).

Jointed Plain Concrete Pavement: are plain cement concrete
pavements constructed with closely spaced contraction joints. Dowel bars or
aggregate interlocks are normally used for load transfer across joints. They
normally has a joint spacing of 5 to 10m.

Jointed Reinforced Concrete Pavement: Although reinforcements
do not improve the structural capacity significantly, they can drastically
increase the joint spacing to 10 to 30m. Dowel bars are required for load
transfer. Reinforcements help to keep the slab together even after cracks.

Continuous
Reinforced Concrete Pavement: Complete elimination of joints are achieved by
reinforce-ment.

**Material
characterization for pavement construction**

The following material properties
are important for
both flexible
and rigid pavements.

When pavements are considered as linear elastic, the elastic
moduli and poisson ratio of subgrade and each component layer must be
specified.

If the elastic modulus of a material varies with the time of
loading, then the resilient modulus, which is elastic modulus under repeated
loads, must be selected in accordance with a load duration corresponding to the
vehicle speed.

When a material is considered non-linear elastic, the
constitutive equation relating the resilient modulus to the state of the stress
must be provided.

However, many of these material properties are used in
visco-elastic models which are very complex and in the development stage. This
book covers the layered elastic model which require the modulus of elasticity
and poisson ratio only.

**The
Environmental factors that affect the pavement materials**

Environmental factors
affect the performance
of the pavement
materials and cause various damages. Environ-mental factors
that affect pavement
are of two
types, temperature and
precipitation and they are discussed below:

Temperature

The
effect of temperature on asphalt pavements is different from that of concrete
pavements.

Temperature
affects the resilient modulus of asphalt layers, while it induces curling of
concrete

slab. In
rigid pavements, due to difference in temperatures of top and bottom of slab,
temperature

stresses or
frictional stresses are
developed. While in
flexible pavement, dynamic
modulus of

asphaltic concrete
varies with temperature.
Frost heave causes
differential settlements and

pavement
roughness. Most detrimental effect of frost penetration occurs during the
spring break up

period
when the ice melts and subgrade is a saturated condition.

Precipitation

The
precipitation from rain and snow affects
the quantity of surface water in filtrating

into the subgrade and the depth of ground water table. Poor
drainage may bring lack of shear strength,pumping, loss of support, etc.

**Factors
which affects pavement design**

Traffic and
Loading

There are three different approaches for considering vehicular
and traffic characteristics, which affects pavement design.

Fixed traffic: Thickness of pavement is governed
by single load and number of load repetitions is not considered. The heaviest
wheel load anticipated is used for design purpose. This is an old method and is
rarely used today for pavement design.

Fixed vehicle: In the fixed vehicle procedure, the
thickness is governed by the number of repetitions of a standard axle load. If
the axle load is not a standard one, then it must be converted to an equivalent
axle load by number of repetitions of given axle load and its equivalent axle
load factor.

Variable traffic and vehicle: In this approach,
both traffic and vehicle are considered individually, so there is no need to
assign an equivalent factor for each axle load. The loads can be divided into a
number of groups and the stresses, strains, and deflections under each load
group can be determined separately; and used for design purposes. The traffic
and loading factors to be considered include axle loads, load repetitions, and
tyre contact area.

**Important
factor in the pavement design**

Traffic is the most important factor in the
pavement design. The key factors include contact pressure, wheel load, axle
configuration, moving loads, load, and load repetitions.

Contact pressure: The tyre pressure is an important factor, as
it determine the contact area and the contact pressure between the wheel and
the pavement surface. Even though the shape of the contact area is elliptical,
for sake of simplicity in analysis, a circular area is often considered.

Wheel load: The next important factor is the wheel load which
determines the depth of the pavement required to ensure that the subgrade soil
is not failed. Wheel configuration affect the stress distribution and
deflection within a pavemnet. Many commercial vehicles have dual rear wheels
which ensure that the contact pressure is within the limits. The normal
practice is to convert dual wheel into an equivalent single wheel load so that
the analysis is made simpler.

Axle configuration: The load carrying capacity of the
commercial vehicle is further enhanced by the intro-duction of multiple axles.

Moving loads: The damage to the pavement is much higher if the
vehicle is moving at creep speed. Many studies show that when the speed is
increased from 2 km/hr to 24 km/hr, the stresses and deflection reduced by 40
per cent.

Repetition of Loads: The influence of traffic on pavement not
only depend on the magnitude of the wheel load, but also on the frequency of
the load applications. Each load application causes some deformation and the
total deformation is the summation of all these. Although the pavement
deformation due to single axle load is very small, the cumulative effect of
number of load repetition is significant. Therefore, modern design is based on
total number of standard axle load (usually 80 kN single axle).

Tags : Civil - Highway Planning and Alignment - Design Of Flexible And Rigid Pavements

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