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CHAPTER 3

PAVEMENT DESIGN:PAVEMENT DESIGN:Flexible Pavement DesignFlexible Pavement Design

BFC 3042 HIGHWAY ENGINEERING


FACTORS THAT INFLUENCE PAVEMENT DESIGN

(1)Traffic Loading

Magnitude of axle load

Wheel configuration

Volume and composition of axle loads

Tyre pressure and contact area

?

(2) Material Characteristi cs

(3) Climate or Envir onment


Single Axle Tridem Axle

Each tyre has point of load

Tandem Axle


Car 1.5 tonneLorry 9 tonnes

00114.0

16.8

5.1 4

48.1

16.8

9 4

4

S

x

L

L

LS = 80kN, 8.16 tonne,

18,000 lb

Bus 18 tonnes Trailer 26 tonnes67.23

16.8

18 4

07.103

16.8

26 4


/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /

/ / / / / / / / / / / / / / / / / / / / / / / / / / / /

/ / / / / / / / / / / / / / / / / / / / / / / / / / /

Climatic / Environmental Effect

Seepage

from

Seepage through

shoulderSeepage through pavement

highlands

Water content rises in subgradeWater content rises in subgrade

Subgrade looses strength and stabilitySubgrade looses strength and stability

If subgrade is too w eak, pavement will failIf subgrade is too w eak, pavement will fail

Water ponding

FLEXIBLE - MECHANISTIC-EMPIRICAL

METHOD

Mechanics is the science of motion and the action of

forces on bodies.

Thus, a mechanistic approach seeks to explain

phenomena only by reference to physical causes.

, ,

strains and deflections within a pavement structure, and

the physical causes are the loads and material

properties of the pavement structure.

The relationship between these phenomena and their

physical causes is typically described using a

mathematical model. Various mathematical models can

be (and are) used; the most common is a layered elastic

model.

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Along with this mechanistic approach, empirical

elements are used when defining what value ofthe calculated stresses, strains and deflections

result in pavement failure.

The relationship between physical phenomena

and pavement failure is described by empirically

derived equations that compute the number of

loading cycles to failure.

The basic advantages of a mechanistic-empirical

pavement design method

Improvement in reliability of pavement design

The ability to predict types of distress

Feasibility to extrapolate from limited field and laboratory data

It can be used for both existing pavement rehabilitation and new

pavement construction

It accommodates changing load types

It can better characterize materials allowing for:

Better utilization of available materials

Accommodation of new materials

An improved definition of existing layer properties

It uses material properties that relate better to actual pavement

performance

It provides more reliable performance predictions

It better defines the role of construction

It accommodates environmental and aging effects on materials

The benefit of a mechanistic-empirical approach

is its ability to accurately characterize in situ

material (including subgrade and existing

pavement structures).

This is typically done by using a portable device

(like a FWD) to make actual field deflection

measurements on a pavement structure to be

overlaid.

These measurements can then be input into

equations to determine existing pavement

structural support (often called "backcalculation")

and the approximate remaining pavement life.

This allows for a more realistic design for thegiven conditions.

Mechanistic Model

Mechanistic models are used to mathematically

model pavement physics.

There are a number of different types of models

available today (e.g., dynamic, viscoelastic

models and the finite elements model (FEM)) but

this section will present the layered elastic

model, as examples of the types of models

typically used.

This model can easily be run on personal

computers and only require data that can berealistically obtained.

Layered Elastic Model

A layered elastic model can compute stresses, strains

and deflections at any point in a pavement structure

resulting from the application of a surface load.

Layered elastic models assume that each pavement

structural layer is homogeneous, isotropic, and linearly

elastic. In other words, it is the same everywhere and will

rebound to its original form once the load is removed.

The origin of layered elastic theory is credited to V.J.

Boussinesq who published his classic work in 1885.

Today, Boussinesq influence charts are still widely used

in soil mechanics and foundation design.

This section covers the basic assumptions, inputs and

outputs from a typical layered elastic model.

Assumpt ions

The layered elastic approach works with

relatively simple mathematical models and

thus, requires some basic

.

Pavement layers extend infinitely in the

horizontal direction.The bottom layer

(usually the subgrade) extends infinitely

downward.Materials are not stressed

beyond their elastic ranges.

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Inputs

A layered elastic model requires a minimum number of

inputs to adequately characterize a pavement structure

and its response to loading. These inputs are:

Material properties of each layer Modulus of elasticity

Poisson's ratio

avemen ayer c nesses

Loading conditions

Magnitude. The total force (P) applied to the pavement surface

Geometry. Usually specified as being a circle of a given radius

(r or a), or the radius computed knowing the contact pressure of

the load (p) and the magnitude of the load (P).

Repetitions. Multiple loads on a pavement surface can be

accommodated by summing the effects of individual loads. This

can be done because we are assuming that the materials are not

being stressed beyond their elastic ranges.

Layered Elastic Inputs

Output

The outputs of a layered elastic model are the stresses,

strains, and deflections in the pavement:

Stress. The intensity of internally distributed forces

experienced within the pavement structure at various

points. Stress has units of force per unit area (N/m2, Pa

orpsi).

Strain. The unit displacement due to stress, usually

expressed as a ratio of the change in dimension to the

original dimension (mm/mm or in/in). Since the strains in

pavements are very small, they are normally expressed

in terms of microstrain (10 -6).

Deflection. The linear change in a dimension.Deflection is expressed in units of length (mm or km or

inches or mils).

Critical Analysis Locations in a Pavement Structure

Critical Analysis Locations in a Pavement Structure


JKR ARAHAN TEKNIK (JALAN) 5/85 DESIGN METHOD

PROCEDURE:

1. Design life is usually taken as 10 years.

2. Traffic Estimation:

Initial Annual Commercial Vehicle Traffic per direction, Vo

where ADT = average daily traffic

Pc = percentage of commercial vehicles

D = directional distribution (usually 0.50)

L = lane distribution (usually 1.00)

LD365100

ADTV co

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BFC 3042 HIGHWAY ENGINEERING L e ct u re r: Mr. Basil David Daniel

Table 3.9: Structural Layer Coefficient

ComponentType of Layer Property Coefficient

Wearing and Binder

CourseAsphalt Concrete 1.00

Dense BituminousMacadam

Type 1 : Stability> 400 kg

0.8

Type 2: Stability

> 300 kg0.55

Base CourseCement Stabilized

Unconfined compressive

strength (7 days) 30 -40kg/m2

0.45

Mechanically

Stabilized crushedaggregate

80% 0.32

Subbase

Sand, Laterite etc 20% 0.23

Crushed aggregate 30% 0.25

Cement Stabilized 60% 0.28


Table 3.10: Structural Layer Coefficient

Type of LayerMinimum

ThicknessWearing Course 4 cm

Binder Course 5 cm

Bituminous 5 cm

Base Course

Wet Mix 10 cm

Cement Treated 10 cm

SubbaseGranular 10 cm

Cement Treated 15 cm


Table 3-11: Standard and Construction Layer Thickness

Type of LayerStandard

ThicknessOne layer lift

Wearing Course 4-5 cm 4-5 cm

Binder Course 5-10 cm 5-10 cm

Base Course

Bi tu mino us 5-20 c m 5-15 cm

Wet Mix 10-20 cm 10-15 cm

Cement Treated 10-20 cm 10-20 cm

SubbaseGranular 10-30 cm 10-20 cm

Cement Treated 15-20 cm 15-20 cm

Table 3.12: Minimum Thickness of

Bituminous LayerTA Total thick of

bituminous layer

< 17.5 cm17.5 22.5 cm

23.0 29.5 cm> 30.0 cm

5.0 cm10.0 cm

15.0 cm17.5 cm


bfc 31802 chapter 3a

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