STRUCTURAL
DESIGN OF
HIGHWAY
Pavements are among the costliest items
associated with highway construction and
maintenance. Because the pavement and
associated shoulder structures are the most
expensive items to construct and maintain, it is
important for highway engineers to have a basic
understanding of pavement design principles.
Paved surface performs two basic functions:
1. It helps guide drivers by giving them a visual
perspective of the horizontal and vertical
alignment of the traveled path
2. It support vehicle loads.
Difference Between Rigid and Flexible
Pavement
Rigid Pavement ( Concrete )
Flexible Pavement ( Asphalt )
◦ Load Transfer not exists
◦ Load is transferred by grain to grain
◦ High Strength
◦ Low Strength
◦ Life span is more
◦ Low Maintenance Cost
◦ Rolling Surface is not needed
◦ No damage by oils and greases
◦ Low life span
◦ High Maintenance Cost
◦ Rolling Surface is needed
◦ Damaged by oils and certain
chemicals
Functions of Pavement
❑
A multilayer system that distributes the vehicular load over a larger
area as not to damage the subgrade.
❑
Provide all weather access to the vehicles.
❑
Provide soft smooth and comfortable ride
to the road users.
❑
Reduce the wear and tear of the vehicles.
❑
A pavement’s function is to distribute the traffic load stresses to the
soil (subgrade) at a magnitude that will not shear or distort the soil.
Requirements of Pavement Structure
❑
Sufficient thickness to spread loading to the subgrade which can
tolerate the pressure intensity.
❑
Sufficiently strong to carry imposed stress due to traffic load.
❑
Sufficient thickness to prevent the effect of frost susceptible
subgrade.
❑
Pavement material should be impervious to the penetration of
surface water which weakens the subgrade and subsequently the
pavement.
❑
Pavement surface should be skid resistant.
Layers of Rigid Pavement
❑
The components of Rigid Pavement
from consists of
❖
❖
❖
❖
Surface Course – Concrete
Base Course
Subbase Course
Soil Subgrade
Layers of Rigid Pavement
❑
❑
❑
❑
Surface course ( Item 311 ) – pavement of Portland Cement Concrete (
PCC ), with or without reinforcement, constructed on the prepared base in
accordance with the DPWH specification and the lines, grades, thickness
and typical cross section shown on the Plans.
Base Course ( Item 201 ) shall consist of furnishing, placing, compacting an
aggregate base course on a prepared subgrade/subbase in accordance
with the DPWH specification and the lines, grades and cross sections
shown on the plans, or as directed by the engineer.
The Subbase (Item 200 ) shall consists of furnishing, placing and
compacting an aggregate subbase course on a prepared subgrade in
accordance with the DPWH specification and the lines, grades and cross
sections shown on the plans, or as directed by the Engineer.
The Subgrade ( Item 105 ) consist the preparation of the subgrade of the
support of overlying structural layers. It shall extend to full width of the
roadway.
Components of Rigid Pavement
Parameters for Design of Rigid Pavement
❑
Load spectrum of commercial vehicles in terms of single, tandem,
and tridem axels vehicles.
❑
Tyre pressure and CBR.
❑
Lane configuration of the road for deciding slab lane width.
❑
Climatic factors
❑
Design period
Stresses considered for design of Rigid Pavement
❑
The flexural stresses due to simultaneous application of traffic loads
and temperature differentials between top and bottom fibres of
concrete slab is considered for design of pavement thickness
• Bottom up cracking during day time
• Top down cracking during night time
Stresses considered for design of Rigid Pavement
❑
❑
The flexural stress at the bottom layer of the concrete slab is
maximum during mid day because of the positive temperature
gradient.
Figure below illustrates Bottom up cracking (BUC)
Stresses considered for design of Rigid Pavement
❑
❑
During night time top surface is cooler than the bottom surface and
the ends curl up in concave shape. The flexural stress at the top layer
of the concrete leads to the Top down cracking.
Figure below illustrates Top Down cracking (TDC)
Layers of Flexible Pavement
❑
Topmost layer i.e. surface course ( ITEM 307 )is the layer
directly in contact with traffic loads and generally contains
superior quality materials.
❑
Second layer - Binder course ( ITEM 310 ) is the bulk of
asphalt concrete structure to distribute load to base course
with less asphalt and quality as compared to the surface
course.
❑
Third layer - Base course ( ITEM 201 )provides
additional load distribution and
contributes to the sub-surface
drainage.
Layers of Flexible Pavement
❑
Fourth layer – sub-base course ( Item 200 )has primary functions such
as to provide structural support, improve drainage, and reduce the
intrusion of fines from the sub-grade in the pavement structure.
❑
Sub-grade ( Item 105 ) is a layer of natural soil compacted to the
desirable density, near the optimum moisture content.
❑
Tack coat ( Item 302)is an asphalt emulsion to
provide proper bonding between
two layers of binder Course.
❑
Prime coat ( Item 301 )is a low viscous bitumen
Layer over granular bases to provide the better bonding.
Typical Flexible Pavement
Roadway Section
Typical Roadway Section with
Existing Asphalt Overlay
Parameters for Design of Flexible Pavement
❑
The Californian Bearing Ratio (CBR) test is a
penetration test used to evaluate the subgrade
strength of roads and pavements.
❑
Elastic modulus of materials used in various layers.
❑
Lane configuration of the road for distribution factor.
❑
Traffic for the period of the considered pavement life
(CVPD).
❑
Vehicle damage factor, Poisson’s ratio, Wheel load,
Tyre Pressure and Radial distance.