Flexible Pavements

Report
PRESENTATION ON
ROAD PAVEMENT DESIGN BY
Engr. Ejaz Ahmad Khan
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Deputy Director
Pakhtunkhwa Highways Authority
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CEE 320
Steve Muench
OUTLINE
Section - 1
•
Pavement Structure
Section - 2
•
Design of Pavement Structure
Section - 3
•
Flexible Pavement Design
•
How to Design
•
Practical Example
Section - 4
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Section - 5
Section - 1
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Pavement Structure
PAVEMENT :
Combination of various
layers between road top
surface / Finished Road
Level (FRL) and the
subgrade is known as
pavement structure.
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Pavement
Structure:
PAVEMENT PURPOSE
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•
•
•
•
•
•
Load support
Skid Resistance
Good ride
Less VOC
Time Saving
Drainage
CHAPPAR - DARBAND ROAD (30 KM) PHASE-I
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PHILOSOPHY OF PAVEMENTS
•
Pavements are subjected to moving traffic loads that are repetitive in
nature.
•
Each traffic load repetition causes a certain amount of damage to the
pavement structure that gradually accumulates over time and
eventually leads to the pavement failure.
•
Thus, pavements are designed to perform for a certain life span before
reaching an unacceptable degree of deterioration.
•
In other words, pavements are designed to fail. Hence, they have a
certain design life.
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PAVEMENT TYPES
•
Flexible Pavement
– Hot mix asphalt (HMA) pavements
– Called "flexible" since the total pavement structure
bends (or flexes) to accommodate traffic loads
– The load transmit to the subgrade through particle to
particle contact.
•
Rigid Pavement
– Portland cement concrete (PCC) pavements
– Called “rigid” since PCC’s high modulus of elasticity does
not allow them to flex appreciably
– The load transmit to subgrade through beam action.
FLEXIBLE PAVEMENT
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• Structure
– Surface course
– Base course
– Subbase course
– Subgrade
RIGID PAVEMENTS
 In rigid pavements the stress is transmitted to
the sub-grade through beam/slab effect. Rigid
pavements contains sufficient beam strength to
be able to bridge over localized sub-grade
failures and areas of inadequate support.
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 Thus in contrast with flexible pavements the
depressions which occur beneath the rigid
pavement are not reflected in their running
surfaces.
RIGID PAVEMENT
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• Structure
– Surface course
– Base course
– Subbase course
– Subgrade
Section - 2
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Design of Pavement Structure
PAVEMENT THICKNESS DESIGN
Pavement Thickness Design is the determination of
required thickness of various pavement layers to protect a
given soil condition for a given wheel load.
Given Wheel Load
150 Psi
Asphalt Concrete Thickness?
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3 Psi
Given In Situ Soil Conditions
DESIGN PARAMETERS
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• Subgrade
• Loads
• Environment
SUBGRADE
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•
Characterized by strength and/or
stiffness
– California Bearing Ratio (CBR)
• Measures shearing resistance
• Units: percent
• Typical values: 0 to 20
– Resilient Modulus (MR)
• Measures stress-strain
relationship
• Units: psi or MPa
• Typical values: 3,000 to 40,000 psi
Picture from University of Tokyo
Geotechnical Engineering Lab
SUBGRADE
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Some Typical Values
Classification
CBR
MR (psi)
Typical Description
Good
≥ 10
20,000
Gravels, crushed stone and sandy soils.
Fair
5–9
10,000
Clayey gravel and clayey sand, fine silt
soils.
Poor
3–5
5,000
Fine silty sands, clays, silts, organic
soils.
TRAFFIC LOADS CHARACTERIZATION
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Cars
Pickups
Buses
Trucks
Trailers
EQUIVALENCY FACTOR
Equivalent
Standard
18000 - Ibs
ESAL
(8.2 tons)
Damage per
Pass = 1
Axle Load
• Axle loads bigger than 8.2 tons cause damage
greater than one per pass
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• Axle loads smaller than 8.2 tons cause damage
less than one per pass
• Load Equivalency Factor (L.E.F) = (? Tons/8.2
tons)4
EXAMPLE
Consider two single axles A and B where:
A-Axle = 16.4 tons
 Damage caused per pass by A -Axle = (16.4/8.2)4 = 16
 This means that A-Axle causes same amount of damage per
pass as caused by 16 passes of standard 8.2 tons axle i.e.,
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=
16.4 Tons
Axle
8.2 Tons
Axle
DAMAGE PER PASS
80
70
60
50
40
30
20
10
0
1.0
1.1
2.3
3.3
4.7
6.5
8.7
11.5
14.9
18.9
23.8
29.5
36.3
44.1
53.1
63.4
75.2
AXLE LOAD & RELATIVE DAMAGE
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8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
SINGLE AXLE LOAD (Tons)
SERVICEABILITY CONCEPT OF PAVEMENT
•
•
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•
Serviceability
Serviceability is the ability of a pavement to serve the
commuters for the desired results for which it has been
constructed within the designed life and without falling the
Terminal level (TSI).
Present Serviceability Index (PSI)
Present Serviceability is defined as the adequacy of a
section of pavement in its existing condition to serve its
intended use.
Terminal Serviceability Index (TSI)
It is defined as that stage of the pavement condition after
which it is not acceptable for the road users.
SERVICEABILITY CONCEPT OF PAVEMENT
• Defined by users (drivers)
• Develop methods to relate physical
attributes to driver ratings
• Result is usually a numerical scale
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From the AASHO Road Test
(1956 – 1961)
Present Serviceability Index (PSI)
• Values from 0 through 5
• Calculated value to match PSR


PSI  5.411.80log 1  SV  0.9 C  P
SV = mean of the slope variance in the two wheel paths
(measured with the profile meter)
C, P = measures of cracking and patching in the pavement surface
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C = total linear feet of Class 3 and Class 4 cracks per 1000 ft2 of pavement area.
A Class 3 crack is defined as opened or spilled (at the surface) to a width of
0.25 in. or more over a distance equal to at least one-half the crack length.
A Class 4 is defined as any crack which has been sealed.
P = expressed in terms of ft2 per 1000 ft2 of pavement surfacing.
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Serviceability (PSI)
PSI vs. Time
p0
p0 - pt
pt
Time
PAVEMENT THICKNESS DESIGN
Comprehensive Definition
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Pavement Thickness Design is the determination
of thickness of various pavement layers (various
paving materials) for a given soil condition and
the predicted design traffic in terms of equivalent
standard axle load that will provide the desired
structural and functional performance over the
selected
pavement
design
life
i.e.
the
serviceability may not fall below the TSI.
Section - 3
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Flexible Pavement Design
Flexible Pavements
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A flexible pavement absorbs
the stresses by distributing
the traffic wheel loads over
much larger area, through
the individual layers, until
the stress at the subgrade is
at an acceptably low level.
The
traffic
loads
are
transmitted to the subgrade
by aggregate to aggregate
particle contact. A cone of
distributed loads reduces
and spreads the stresses to
the subgrade.
TYPICAL LOAD & STRESS DISTRIBUTION IN FLEXIBLE PAVEMENTS.
Wheel Load
Bituminous Layer
Vertical stress
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Foundation stress
Sub-grade
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EFFECT OF PAVEMENT THICKNESS ON STRESS DISTRIBUTION
BASIC EQUATION OF AASHTO PROCEDURE FOR FLEXIBLE
PAVEMENT DESIGN.
 PSI 
log10 

4.5  1.5 

log10 W18   Z R  So  9.36 log10 SN  1  0.20 
 2.32 log10 M R   8.07
1094
0.40 
SN  15.19
The various terms/parameters which are used in the basic
equation of AASHTO Procedure for the Design of flexible
pavements are:
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i). W18 (ESAL): It is the accumulated traffic load converted to 18kips or 8.2 tons. This is also known as 18-kips Equivalent Standard
Axle Load (ESAL). That the pavement will experience over its
design life.
ii). Standard Deviation (S0):
Standard deviation accounts for standard variation in materials
and construction, the probable variation in traffic prediction and
variation in pavement performances for a given design traffic
application. The recommended value of S0 for flexible pavement is
0.4 to 0.5.
iii) Reliability (R):
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Design Reliability refers to the degree of certainty that a given
pavement section will last for the entire design period with the
traffic & environmental condition. The recommended level of
reliability for freeways in rural areas varies from 80% to 95%. A
high reliability value will increase the thickness of pavement layer
and will result in expensive construction.
TABLE FOR REALABLILITY
Recommended Level of Reliability ( R )
Reliability (%)
Functional Classification
Urban
Rural
85-99.90
80-99.90
Principal Arterial
80-99
75-95
Feeders
80-95
75-95
Local
50-80
50-80
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Interstate and Other
Freeways
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iv). Standard Normal Deviate
(ZR):
It is defined as the
probability that
serviceability will be
maintained at adequate
levels from a user’s point of
view throughout the design
life of the facility.
This factor estimates the
probability that the
pavement will perform at or
above the TSI level during
the design period and it
accounts for the inherent
uncertainty in design. The
relationship of reliabilities
with ZR is given in the table:
Value of (ZR)
Reliability R (%)
50
60
70
75
80
85
90
91
92
93
94
95
96
97
98
99
99.9
99.99
Standard Normal
Deviate (ZR)
0.000
-0.253
-0.524
-0.674
-0.841
-1.037
-1.282
-1.340
-1.405
-1.476
-1.555
-1.645
-1.751
-1.881
-2.054
-2.327
-3.090
-3.750
v). Structural No (SN):
Structural No is the total structural strength value required to cater for
the cumulative equivalent standard axles load (CESAL) during design
life so that the serviceability may not fall below the Terminal
serviceability Index (TSI)
Definition of Structural Number
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Subgrade
Structural Coefficient (a):
a = fnc (MR)
SN = SN1 + SN2 + SN3
vi) Loss of Serviceability Index ∆ PSI.
Serviceability (PSI)
∆ PSI = Initial Serviceability Index – Terminal Serviceability Index
The recommended value for initial serviceability index is 4.2 while for
terminal serviceability index it is to 2 to 2.5.
∆ PSI = 4.2 – 2.5 = 1.7
p0
p0 - pt
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pt
Time
vii). Resilient Modulus (MR):
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It is defined as repetitive or cyclic stress divided by recoverable
strain. Resilient Modulus is a measure of stiffness of the soil.
MR =
Repetitive stress / recoverable strain
MR
can be determined from the resilient modules test
in the
laboratory or from the following equations:
MR = 1500 * CBR for CBR < 10 %
MR = 2555 (CBR)0.63 for any value of CBR
viii). Computation of Required Pavement Thicknesses
The structure Number (SN) requirement as determined through
adoption of design parameters as discussed above is balanced by providing
adequate pavement structure. Under AASHTO design procedure the
following equation provides for the means for converting the structural
number into actual thickness
of surfacing, base and sub base materials.
SN = a1D1 + a2D2m2 + a3D3m3
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a1, a2, a3 =
D1. D2, D3
=
m 2, m 3
=
_______________ (2)
Layer coefficients representative of surface, base
and subbase courses respectively. It depends upon
the modulus of resilient.
Actual thicknesses (in inches) of surface, base and
subbase courses respectively.
Drainage coefficients for base and subbase layers
respectively.
This equation does not have a single unique solution. There are
many combinations of layer thicknesses that can be adopted to achieve a
given structural number. There are, however, several design, construction
and cost constraints that may be applied to reduce the number of possible
layer thicknesses combinations and to avoid the possibility of constructing an
impractical design. According to this approach, minimum thickness of each
layer is specified to protect the under lying layers from shear deformation.
ix). Recommended Value of Layer Coefficients
Asphaltic Wearing Course, a1
=
0.44/inch (0.1732/cm)
Asphaltic Base Course, a1
=
0.40/inch (0.1575/cm)
Water Bound Macadam, a2
=
0.14/inch (0.0551/cm)
Granular Subbase, a3
=
0.11/inch (0.0433/cm)
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OR Nomograph can be used to work out SN.
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NOMOGRAPH
Section - 4
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How to Design
How to Design
Step 1. Fix the design life of the pavement.
Step 2. Work out MR value of the subgrade
MR = 1500 CBR for CBR <10%
OR
MR = 2555 (CBR)0.63 for CBR > 10
OR
Work out MR in the laboratory.
Step 3. Conduct 7-days traffic count.
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Step 4. Classify the traffic and consider the commercial vehicles i.e. Bus,
Tractor , Trolley, 2-Axle, 3-Axle, 4-Axle, 5-Axle and 6-Axle Trucks.
Step 5. Take Growth rate from the table on the next slide.
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Growth Rate
S. No
Vehicle Class
Growth Rate
1
Bus
8.4%
2
Tractor Trolley
7.9%
3
Mini Truck
7.9%
4
2-Axle
7.0%
5
3-Axle (Single)
7.0%
6
3-Axle (Tandem)
7.0%
7
4-Axle
7.0%
8
5-Axle
7.0%
9
6-Axle
7.0%
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CONVERT THE TRAFFIC TO EQUIVALENT STANDARD AXLE LOAD.
ESAL = TRAFFIC X EQUILLANCY FACTOR , EQUIVALENCY FACTOR FOR
VARIOUS CLASSES OF VEHICLES ARE GIVEN IN THE FOLLOWING TABLE.
S. No
Vehicle Class
Equivalency Factor
(Empty)
Equivalency Factor
(Loaded)
1
Bus
0
0.939
2
Tractor Trolley
0.1
1.19
3
Mini Truck
0.0172
2.596
4
2-Axle
0.043
6.49
5
3-Axle (Single)
0.072
16.62
6
3-Axle (Tandem)
0.072
18.48
7
4-Axle
0.206
19.00
8
5-Axle
0.084
19.59
9
6-Axle
0.165
27.96
Calculation of CESAL
ESA Factor
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Vehicle Type
ADT
Annual
Traffic
Growth
Rate %
Growth
Factor
Total
Traffic for
10 Years
80%
20%
Loaded
Empty
CESAL for
10 Years
Buses
20
7300
6
13.18
96,214
0.939
0
72,276
Tractor Trolly
139
50735
6
13.18
668,687
1.19
0.1
649,964
Trucks2XL
500
182500
6
13.18
2,405,350
6.49
0.043
12,509,263
Trucks 3XL
250
91250
6
13.18
1202675
18.48
0.072
17,797,666
Cumulate the future traffic throughout the design life with the
help of the selected growth rate. Following is the simple relation
to project the traffic to any selected year.
Pn = (1 + r)n – 1
Where
Pn = Projected traffic for nth year
r = Growth rate
n = year of consideration
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Add all the yearly traffic from base year to the last year of the
design life.
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Step 6.
Fix the parameter like R, ZR, So, ∆ PSI etc.
The generally taken value of the above parameters is
listed below:
∆ PSI = 1.7
R
= 90%
So
= 0.45
ZR
= -1.282
Step 7.
Put these values in equation 1 and use trial & error
method or Nomograph to work out the SN
SN = a1D1+a2D2m2+a3D3m3
Step 8.
Take the value of m2 and m3 from the table on the
next slide.
TABLE FOR QULALITY OF DRAINAGE
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QUALITY OF DRAINAGE
Quality of Drainage
Water Removed within
Excellent
2 hours
Good
1 day
Fair
1 week
Poor
1 month
Very Poor
water will not drain
Percent of Time Pavement Structure is exposed to Moisture
Levels Approaching Saturation
Quality of
Drainage
Excellent
Good
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Fair
Poor
Very Poor
Less Than
Greater Than
1%
1 - 5%
2 - 25%
25%
1.40 - 1.35
1.35 - 1.30
1.30 - 1.20
1.20
1.35 - 1.25
1.25 - 1.15
1.15 - 1.00
1.00
1.25 - 1.15
1.15 - 1.05
1.00 - 0.80
0.85
1.15 - 1.05
1.05 - 0.80
0.80 - 0.60
0.60
1.05 - 0.95
0.95 - 0.75
0.75 - 0.40
0.40
Put the above values in equation at step No. 07, to find out the
various combination of thicknesses, keeping in view the
minimum thicknesses requirements as mentioned below:
–
–
–
–
Minimum Asphalt wearing course thickness
Minimum asphaltic base course thickness
Minimum unbound base course thickness
Minimum unbound sub base thickness
= 5 Cm
= 7.5 Cm
= 15 Cm
= 15 Cm
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Select the most appropriate and economical combination of
thicknesses.
Section - 5
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Practical Example
Practical Example
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• Let us work out the thicknesses of various
layers for the Pavement of Topi Bypass
Road.
1. The ADT is given as follow.
Vehicle Type
ADT
COASTER/ FLYING COACH
250
BUSSES
25
Tractor Trolley
36
Trucks2XL
110
Trucks 3XL
2
Trucks 4XL
5
THE CESAL IS WORKED OUT AS FOLLOW:-
Vehicle Type
ESA Factor
Total
Annual Growth Growth Traffic
ADT
Traffic Rate % Factor for 10
Years
80%
20%
Loaded
Empty
COASTER/FLYING
COACH
250
91250
8.4
14.78
1,348,675
0.939
0
1,013,125
BUSSES
25
9125
8.4
14.78
134,868
0.939
0
101,312
Tractor Trolly
36
13140
7.9
14.423
189,518
1.19
0.1
184,212
Trucks2XL
110
40150
7
13.82
554,873
6.49
0.043
2,885,673
Trucks 3XL
2
730
7
13.82
10088.6
18.48
0.072
149,295
Trucks 4XL
5
1825
7
13.82
25221.5
19
0.385
385,309
Total
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ESAL for 10
Years
4,718,925
ESAL by taking 100 % of directional factor
4718925.35
=
4.719million
ESAL by taking 80 % lane factor
3775140.28
=
3.775million
CESAL =3.775 million
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• California Bearing Ratio (CBR)= 30 % at 95%
MDD
• MR= 2555 (CBR)0.63 for CBR > 10
Putting the value CBR , MR= 18000 Psi
• Keeping the value of various parameters as
follow.
R= 90%
So=0.45
∆Psi=1.7
Using Nomograph to work out the
SN
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Required SN = 3.35
54
•
Using the following the equation
•
SN = a1D1+a2D2m2+a3D3m3
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• Given Data:
• a1=0.44,
• a2=0.14 ,
• a3=0.11 and
• m2,m3=1
• from Nomograph SN= 3.35
• Putting these values and assuming d1=2 inch, d2=10 inch and
d3=10 inch
3.35=0.44*2+0.14*10+0.11*10
3.35≈3.38
Hence the Design thickness are
ACWC= 5cm
WBM=25cm
Granular sub base=25cm
CEE 320
Winter 2006
Thank You

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