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Engr. Ejaz Ahmad Khan

Deputy DirectorPakhtunkhwa Highways Authority

PRESENTATION ON

ROAD PAVEMENT DESIGN BY

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CEE 320Steve Muench

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OUTLINE

• Pavement Structure

Section - 1

Section - 2

• Design of Pavement Structure

Section - 3

• Flexible Pavement DesignSection - 4

• How to DesignSection - 5

• Practical Example

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Section - 1

Pavement Structure

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Combination of various layers between road top surface / Finished Road Level (FRL) and the subgrade is known as pavement

structure. Pavement

Structure:

PAVEMENT :

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PAVEMENT PURPOSE

• Load support• Skid Resistance• Good ride• Less VOC• Time Saving• Drainage

CHAPPAR - DARBAND ROAD (30 KM) PHASE-I

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• 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.

PHILOSOPHY OF PAVEMENTS

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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.

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FLEXIBLE PAVEMENT

• Structure– Surface course– Base course– Subbase course– Subgrade

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RIGID PAVEMENTS

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

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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RIGID PAVEMENT

• Structure– Surface course– Base course– Subbase course– Subgrade

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Section - 2

Design of Pavement Structure

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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?

PAVEMENT THICKNESS DESIGN

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DESIGN PARAMETERS

• Subgrade• Loads• Environment

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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 TokyoGeotechnical Engineering Lab

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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. 

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06TRAFFIC LOADS

CHARACTERIZATION

Pavement Thickness Design Are Developed To Account For The Entire Spectrum Of Traffic Loads

Cars Pickups Buses Trucks Trailers

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Equivalent

Standard ESAL

Axle Load

18000 - 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

EQUIVALENCY FACTOR

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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.,

8.2 Tons Axle

16.4 Tons Axle

=

EXAMPLE

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1.0

1.1 2.3 3.3

4.7 6.5 8.

711

.5 14.9 18

.9 23.8 29

.5 36.3 44

.1 53.1

63.4 75

.2

0

10

20

30

40

50

60

70

80D

AM

AG

E P

ER

PA

SS

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

SINGLE AXLE LOAD (Tons)

AXLE LOAD & RELATIVE DAMAGE

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• ServiceabilityServiceability 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

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• 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)

SERVICEABILITY CONCEPT OF PAVEMENT

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Present Serviceability Index (PSI)

• Values from 0 through 5• Calculated value to match PSR

PCSVPSI 9.01log80.141.5

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 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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PSI vs. Time

Time

Ser

vice

abil

ity

(PS

I) p0

pt

p0 - pt

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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.

PAVEMENT THICKNESS DESIGN Comprehensive Definition

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Section - 3

Flexible Pavement Design

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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.

SHOULDER7.30 m2.00 m 2.00 m

SHOULDERCARRIAGE WAY

cLOF ROAD

A C WEARING COURSE

WATER BOUND MACADAM

SUB BASE COURSE

N.S.L.N.S.L.

2% 4%4% 2%

TYPICAL CROSS-SECTION OF FLEXIBLE PAVEMENT

SUB GRADE / COMMON MATERIAL

COMPACTEDEMBANKMENT

COMPACTEDEMBANKMENT

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Vertical stress

Foundation stress

TYPICAL LOAD & STRESS DISTRIBUTION IN FLEXIBLE PAVEMENTS.

Bituminous Layer

Wheel Load

Sub-grade

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EFFECT OF PAVEMENT THICKNESS ON STRESS DISTRIBUTION

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BASIC EQUATION OF AASHTO PROCEDURE FOR FLEXIBLE PAVEMENT DESIGN.

07.8log32.2

1

109440.0

5.15.4log

20.01log36.9log 10

19.5

10

101810

RoR M

SN

PSI

SNSZW

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.

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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.

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Recommended Level of Reliability ( R )

Functional Classification

Reliability (%)

Urban Rural

Interstate and Other Freeways 85-99.90 80-99.90

Principal Arterial 80-99 75-95

Feeders 80-95 75-95

Local 50-80 50-80

TABLE FOR REALABLILITY

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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 (%)Standard Normal

Deviate (ZR)

50 0.00060 -0.25370 -0.52475 -0.67480 -0.84185 -1.03790 -1.28291 -1.34092 -1.40593 -1.47694 -1.55595 -1.64596 -1.75197 -1.88198 -2.05499 -2.327

99.9 -3.09099.99 -3.750

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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

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vi) Loss of Serviceability Index ∆ PSI.

∆ PSI = Initial Serviceability Index – Terminal Serviceability IndexThe 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

Time

Serv

iceab

ilit

y (P

SI) p0

pt

p0 - pt

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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

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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.

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 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.

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NOMOGRAPH

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Section - 4

How to Design

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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 > 10OR

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.

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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%

Growth Rate

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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

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.

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Vehicle Type ADTAnnual 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 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

Calculation of CESAL

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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 WherePn = 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.

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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 = 1.7R = 90%So = 0.45ZR = -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.

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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

TABLE FOR QULALITY OF DRAINAGE

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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%

Excellent1.40 - 1.35 1.35 - 1.30 1.30 - 1.20 1.20

Good1.35 - 1.25 1.25 - 1.15 1.15 - 1.00 1.00

Fair1.25 - 1.15 1.15 - 1.05 1.00 - 0.80 0.85

Poor1.15 - 1.05 1.05 - 0.80 0.80 - 0.60 0.60

Very Poor1.05 - 0.95 0.95 - 0.75 0.75 - 0.40 0.40

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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.

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Section - 5

Practical Example

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Practical Example• 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

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THE CESAL IS WORKED OUT AS FOLLOW:-

Vehicle Type ADTAnnua

l 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

1,348,67

5 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 4,718,925ESAL by taking 100 % of directional factor 4718925.35 = 4.719millionESAL 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=1.7Using Nomograph to work out the SN

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• 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+0.11*103.35≈3.38

Hence the Design thickness areACWC= 5cmWBM=25cm Granular sub base=25cm

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Thank You