PAVEMENT: FLEXIBLE VS. RIGID

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Provisions of Rigid, Semi Rigid and Flexible
Pavements as Rural Roads
In near future, the cost of bitumen will go on
increasing. So, various alternates to construct
the roads are to be explored. Though concrete
roads are one of the good alternates, but still
their use is limited. This paper discusses the
merits and demerits of all types of pavements
construction and proposes their optimum use.

Introduction

Development of a country depends on the
connectivity of various places with adequate
road network. Roads are the major channel of
transportation for carrying goods and
passengers. They play a significant role in
improving the socio-economic standards of a
region. Roads constitute the most important
mode of communication in areas where
railways have not developed much and form
the basic infra structure for the development
and economic growth of the country. The
benefits from the investment in road sector
are indirect, long-term and not immediately
visible. Roads are important assets for any
nation. However, merely creating these assets
is not enough, it has to be planned carefully
and a pavement which is not designed
properly deteriorates fast. India is a large
country having huge resource of materials. If
these local materials are used properly, the
cost of construction can be reduced. There are
various type of pavements which differ in their
suitability in different environments. Each type
of pavement has it’s own merits and demerits.
Despite a large number of seminars and
conference, still in India, 98% roads are having
flexible pavements. A lot of research has been
made on use of Waste materials but the role
of these materials is still limited. So there is
need to take a holistic approach and mark the
areas where these are most suitable
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Types of Pavements

There are various type of pavements
depending upon the materials used. A briefs
description of all types is given here.
Flexible Pavements
Bitumen has been widely used in the
construction of flexible pavements for a long
time. This is the most convenient and simple
type of construction. The cost of construction
of single lane bituminous pavement varies
from 2000000 to 3000000 ksh per km in plain areas. In
some applications, however, the performance
of conventional bitumen may not be
considered satisfactory because of the
following reasons:
In summer season, due to high temperature,
the bitumen becomes soft resulting in
bleeding, rutting and segregation finally
leading to failure of pavement.
In cold season, due to low temperature, the
bitumen becomes brittle resulting in cracking,
raveling and unevenness which makes the
pavement unsuitable for use.
In rainy season, water enters the pavement
resulting into pot holes and sometimes total
removal of bituminous layer.
In hilly areas, due to sub zero temperature, the
freeze thaw and heave cycle takes place. Due
to freezing and melting of ice in bituminous
voids, volume expansion and contraction
occur. This leads to pavements failure.
The cost of bitumen has been rising
continuously. In near future, there will be
scarcity of bitumen and it will be impossible
to procure bitumen at very high costs.
Recently, a large number investigations have
demonstrated that bitumen properties (eg.
viscoelsticity and temperature susceptibility)
can be improved using an additive or a
chemical reaction modification.
The use of polymer modified bitumen’s (PMBs)
to achieve better asphalt pavement
performance has been observed for a long
time. The improved functional properties
include permanent deformation, fatigue and
low temperature cracking. The properties of
PMVs are dependent on the polymer
characteristics and content and bitumen
nature, as well as the blending process.
Despite the large number of polymeric
products, there are relatively few types which
are suitable for bitumen modification (2). The
polymers that are used for bitumen
modification can be divided onto two broad
categories, namely plastomers and elastomers.
Elastomers have a characteristically high
elastic response and, therefore, resist
permanent deformation by stretching and
recovering their initial shape. Plastomers from
a tough, rigid, three dimensional network to
resist deformation. The thermoplastic rubber,
styrene butadiene-styrene (SBS), is an
example of an elastomer and the
thermoplastic polymer, ethylene vinyl acetate
(EVA), is an example of a plastomer. One of
the principal plastomers used in pavement
applications is the semi-crystalline copolymer,
ethylene vinyl acetate (EVA). EVA polymers
have been use in road construction for more
than 20 years in order to improve both the
workability of the asphalt during construction
and its deformation resistance in service.

Semi Rigid Pavements
The pavements constructed using the waste
materials, which are more strong the
traditional aggregates may be treated as
Semi-Rigid Pavement. A lot of research work
has been done in this direction. But the work
in terms of real construction is not visible.
Prior to
1991, a major portion of GBFS was being used
by the cement manufacturing industries
located in the nearby areas but its utilization
in this industry has been decreasing gradually.
This material has, therefore, been piling up
gradually due to increased production as a
waste in the plant area an posing serious
problem for its disposal

In a project example, Conventional moorum,
gravel, sand or lime/cement stabilised local
soil were used in subbase layer of a road
pavement. In order to compare the structural
performance of these materials with the steel
industry wastes, a small test track was
constructed . The selection of
different test sections was made on the basis
of laboratory test results as discussed in the
previous sections. The details of the test
sections are as follow:
In order to structurally evaluate the different
specifications/test sections, plate load tests
were conducted on each section using a 30cm
diameter plate. The load deflection values
were recorded by applying incremental load.
Plate load test was also carried out on
subgrade soil. Since with an ordinary truck,
only limited magnitude of reaction can be
obtained, a heavy 35 tonnes dumper was used
for carrying out place load test. Based on
Burmister’s two layer theory, the modulus of
elasticity for different specifications were
worked out and are given in Table-2. The
ratings based on load carrying capacity of
different sections are also indicated in the
same table.
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Rigid Pavements
Rigid pavements, though costly in initial
investment, are cheap in long run because of
low maintenance costs. There are various
merits in the use of Rigid pavements (Concrete
pavements) are summarized below:
Bitumen is derived from petroleum crude,
which is in short supply globally and the price
of which has been rising steeply. India imports
nearly 70% of the petroleum crude. The
demand for bitumen in the coming years is
likely to grow steeply, far outstripping the
availability. Hence it will be in India’s interest
to explore alternative binders. Cement is
available in sufficient quantity in Kenya, and its
availability in the future is also assured. Thus
cement concrete roads should be the obvious
choice in future road programmes.
Besides the easy available of cement, concrete
roads have a long life and are practically
maintenance-free.
Another major advantage of concrete roads is
the savings in fuel by commercial vehicles to
an extent of 14-20%. The fuel savings
themselves can support a large programme of
concreting.
Cement concrete roads save a substantial
quantity of stone aggregates and this factor
must be considered when a choice pavements
is made,
Concrete roads can withstand extreme weather
conditions – wide ranging temperatures, heavy
rainfall and water logging.
Though cement concrete roads may cost
slightly more than a flexible pavement
initially, they are economical when whole-life-
costing is considered.
Reduction in the cost of concrete pavements
can be brought about by developing semi-self-
compact