1. Engr. Ejaz Ahmad Khan Deputy Director Pakhtunkhwa Highways Authority
PRESENTATION ON
ROAD PAVEMENT DESIGN BY
Engr. Ejaz Ahmad Khan
Deputy Director
Pakhtunkhwa Highways Authority

2. CEE 320 Steve Muench

3. OUTLINE Pavement Structure Design of Pavement Structure
Section - 1
Pavement Structure
Section - 2
Design of Pavement Structure
Section - 3
Flexible Pavement Design
Section - 4
Section - 5
How to Design
Practical Example

4. Section - 1
Pavement Structure

5. PAVEMENT :
Combination of various layers between road top surface / Finished Road Level (FRL) and the subgrade is known as pavement structure.
Pavement Structure:

6. CHAPPAR - DARBAND ROAD (30 KM) PHASE-I
PAVEMENT PURPOSE
Load support
Skid Resistance
Good ride
Less VOC
Time Saving
Drainage
CHAPPAR - DARBAND ROAD (30 KM) PHASE-I

7. 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.

8. 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.

9. FLEXIBLE PAVEMENT Structure Surface course Base course Subbase course
Subgrade

10. 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.
Thus in contrast with flexible pavements the depressions which occur beneath the rigid pavement are not reflected in their running surfaces.

11. RIGID PAVEMENT Structure Surface course Base course Subbase course
Subgrade

12. Design of Pavement Structure
Section - 2
Design of Pavement Structure

13. 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
3 Psi
Given In Situ Soil Conditions
Asphalt Concrete Thickness?
Base Course Thickness?
Subbase Course Thickness?

14. DESIGN PARAMETERS
Subgrade
Loads
Environment

15. Picture from University of Tokyo Geotechnical Engineering Lab
SUBGRADE
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

16. SUBGRADE 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. 

17. TRAFFIC LOADS CHARACTERIZATION
Pavement Thickness Design Are Developed
To Account For The Entire
Spectrum Of Traffic Loads
Cars
Pickups
Buses
Trucks
Trailers

18. Axle loads bigger than 8.2 tons cause damage greater than one per pass
EQUIVALENCY FACTOR
Equivalent
Standard ESAL
Axle Load
Ibs
(8.2 tons)
Damage per Pass = 1
Axle loads bigger than 8.2 tons cause damage greater than one per pass
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

19. Consider two single axles A and B where:
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.,
16.4 Tons Axle =
8.2 Tons Axle

20. AXLE LOAD & RELATIVE DAMAGE

21. SERVICEABILITY CONCEPT OF PAVEMENT
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.

22. SERVICEABILITY CONCEPT OF PAVEMENT
Defined by users (drivers)
Develop methods to relate physical attributes to driver ratings
Result is usually a numerical scale
From the AASHO Road Test
(1956 – 1961)

23. Present Serviceability Index (PSI)
Values from 0 through 5
Calculated value to match PSR
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
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 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.

24. PSI vs. Time p0
Serviceability (PSI)
p0 - pt pt
Time

25. PAVEMENT THICKNESS DESIGN Comprehensive Definition
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.

26. Section - 3
Flexible Pavement Design

27. Flexible Pavements
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.

28. TYPICAL LOAD & STRESS DISTRIBUTION IN FLEXIBLE PAVEMENTS.
Wheel Load
Bituminous Layer
Vertical stress
Foundation stress
Sub-grade

29. EFFECT OF PAVEMENT THICKNESS ON STRESS DISTRIBUTION

30. BASIC EQUATION OF AASHTO PROCEDURE FOR FLEXIBLE PAVEMENT DESIGN.
The various terms/parameters which are used in the basic equation of AASHTO Procedure for the Design of flexible pavements are:
i). W18 (ESAL): It is the accumulated traffic load converted to 18-kips 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.

31. 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):
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.

32. TABLE FOR REALABLILITY Recommended Level of Reliability ( R )
Functional Classification
Reliability (%)
Urban
Rural
Interstate and Other Freeways
Principal Arterial
80-99
75-95
Feeders
80-95
Local
50-80

33. Standard Normal Deviate (ZR)
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 (%)
Standard Normal Deviate (ZR) 50
0.000 60
-0.253 70
-0.524 75
-0.674 80
-0.841 85
-1.037 90
-1.282 91
-1.340 92
-1.405 93
-1.476 94
-1.555 95
-1.645 96
-1.751 97
-1.881 98
-2.054 99
-2.327
99.9
-3.090
99.99
-3.750

34. Definition of Structural Number
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
Subgrade
Structural Coefficient (a):
a = fnc (MR)
SN = SN1 + SN2 + SN3

35. vi) Loss of Serviceability Index ∆ 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
Serviceability (PSI)
p0 - pt pt
Time

36. vii). Resilient Modulus (MR):
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

37. 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 _______________ (2)
a1, a2, a3 = Layer coefficients representative of surface, base
and subbase courses respectively. It depends upon the modulus of resilient.
D1. D2, D3 = Actual thicknesses (in inches) of surface, base and subbase courses respectively.
m2, m3 = Drainage coefficients for base and subbase layers respectively.

38. 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)
OR Nomograph can be used to work out SN.

39. NOMOGRAPH

40. Section - 4
How to Design

41. How to Design
Step 1. Fix the design life of the pavement.
Step 2. Work out MR value of the subgrade
MR = CBR for CBR <10% OR
MR = (CBR)0.63 for CBR > 10
Work out MR in the laboratory.
Step 3. Conduct 7-days traffic count.
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.

42. Growth Rate S. No Vehicle Class Growth Rate 1 Bus 8.4% 2
Tractor Trolley
7.9% 3
Mini Truck 4
2-Axle
7.0% 5
3-Axle (Single) 6
3-Axle (Tandem) 7
4-Axle 8
5-Axle 9
6-Axle

43. 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
0.939 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)
18.48 7
4-Axle
0.206
19.00 8
5-Axle
0.084
19.59 9
6-Axle
0.165
27.96

44. Total Traffic for 10 Years
Calculation of CESAL
Vehicle Type
ADT
Annual Traffic
Growth Rate %
Growth Factor
Total Traffic for 10 Years
ESA Factor
CESAL for 10 Years
80%
20%
Loaded
Empty
Buses 20
7300 6
13.18
96,214
0.939
72,276
Tractor Trolly
139
50735
668,687
1.19
0.1
649,964
Trucks2XL
500
182500
2,405,350
6.49
0.043
12,509,263
Trucks 3XL
250
91250
18.48
0.072
17,797,666

45. 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
Add all the yearly traffic from base year to the last year of the design life.

46. 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 =
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.

47. TABLE FOR QULALITY OF DRAINAGE
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

48. Quality of Drainage
Percent of Time Pavement Structure is exposed to Moisture Levels Approaching Saturation
Less Than
Greater Than 1%
1 - 5%
2 - 25%
25%
Excellent
1.20
Good
1.00
Fair
0.85
Poor
0.60
Very Poor
0.40

49. Select the most appropriate and economical combination of thicknesses.
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 = 5 Cm
Minimum asphaltic base course thickness = 7.5 Cm
Minimum unbound base course thickness = 15 Cm
Minimum unbound sub base thickness = 15 Cm
Select the most appropriate and economical combination of thicknesses.

50. Section - 5
Practical Example

51. Practical Example
Let us work out the thicknesses of various layers for the Pavement of Topi Bypass Road.
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

52. THE CESAL IS WORKED OUT AS FOLLOW:-
Vehicle Type
ADT
Annual Traffic
Growth Rate %
Growth Factor
Total Traffic for 10 Years
ESA Factor
ESAL for 10 Years
80%
20%
Loaded
Empty
COASTER/FLYING COACH
250
91250
8.4
14.78
1,348,675
0.939
1,013,125
BUSSES 25
9125
134,868
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
18.48
0.072
149,295
Trucks 4XL 5
1825 19
0.385
385,309
Total
4,718,925
ESAL by taking 100 % of directional factor =
4.719
million
ESAL by taking 80 % lane factor
3.775
CESAL =3.775 million

53. California Bearing Ratio (CBR)= 30 % at 95% MDD
MR= (CBR)0.63 for CBR > 10
Putting the value CBR , MR= Psi
Keeping the value of various parameters as follow.
R= 90%
So=0.45
∆Psi=1.7
Using Nomograph to work out the SN

54. Required SN = 3.35

55. Using the following the equation
SN = a1D1+a2D2m2+a3D3m3
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
3.35≈3.38
Hence the Design thickness are
ACWC= 5cm
WBM=25cm
Granular sub base=25cm

56. Thank You

57. SERVICEABILITY AND PERFORMANCE CONCEPT
• AASHO Road Test performance based on user assessment:
– Difficult to quantify (subjective)
– Highly variable
– Present Serviceability Rating (PSR)
Performance Measurements
0-1 – V. Poor
1-2 – Poor A panel of experts drove around in standard
2-3 – Fair vehicles and gave a rating for the pavement
3-4 – Good
4-5 – V. Good
• Measurable characteristics (performance indicators):
– Visible distress (cracking & rutting)
– Surface friction
– Roughness (slope variance)