ASPHALT INSTITUTE METHOD FOR

FLEXIBLE PAVEMENT DESIGN

Dr. Padma Bahadur Shahi

GENERAL DESIGN CONCEPT FOR PAVEMENT DESIGN

TRAFFIC ANALYSIS In pavement design, number of repetition of each

axle load group during the design period. The initial daily traffic in two directions over all traffic

lanes must be multiplied by directional and lane distribution factors to obtain the initial traffic on design lane.

The traffic used for design is the average traffic for design period, s initial traffic must be multiplied by growth factor.

If ni is the number of load repetitions to be used in in design for ith load group, then

))(365)()()()()(()()( 01YLDGATADTFpESAL

m

i ii

TRAFFIC ANALYSIS

(no)i is the initial number of repetitions per day for the ith load group, G is the growth factor, D is the directional distributional factor, L is the lane distribution factor, Y is the design period in years.

If the design is based on the equivalent 80 kN single axle load, then the initial number of repetitions per day for the ith load group can be computed by:

))(()(()( 0) ATADTFpn iiio pi is the percentage of load repetitions for ith load group, Fi is the equivalent axle load factor for the load group , (ADT)o is the average daily traffic at the start of the design period, T is the percentage of truck in the ADT, and A is the average number of axle per truck.

The equivalent axle load for the design lane is

))(365)()()()()(()()( 01YLDGATADTFpESAL

m

i ii

Truck Factor : number of 80 kN single axle load applications per truck , can be expressed as:

)()(1

AFpTm

i iif

Equivalent Single Axle Load

))(365)()()()(()( 0 YLDGTADTTESAL f

AVERAGE DAILY TRUCK TRAFFIC

The minimum traffic information required for the pavement design is the average daily truck traffic (ADTT) at the start of design period. It may be expressed as a percentage of ADT or as an actual value.

The traffic count must be adjusted for daily and seasonal variation to obtain AADT.

If the actual traffic date is not available, distribution of AADT on the different categories of highway can be used from the existing database.

DISTRIBUTION OF TRUCKS IN DIFFERENT CLASS OF HIGHWAY IN USA:

Percentage of trucks

Truck class freeways Principle arterial

Single unit truck:2 axle four tire2 axle 6 tire3 axle or moreAll single units

6612483

6715385

849295

Multiple unit trucks4 axle or less5 axle6 or more axleAll multiple units

513<118

312<115

23<15

All trucks 100 100 100

TRUCK FACTORAxle Load, lb EAL Factor Number of

axleESAL

Single axleUnder 30003000-69997000-79998000-1199912000-1599916000-29999

.002

.005

.032

.087

.3605.3890

016144161

0.005.19212.5285.7605.389

Tandem axle

Under 60006000-1199912000-1799918000-2399924000-2999930000-3200032001-3250032501-3399934000-35999

0.01000.010.0440.1480.4260.7530.88501.0021.230

014214442442110143

0.14.924.51217.89233.13218.585101.20252.890

ESAL for all trucks weighed 255.151

Truck Factor (255.151/165) 1.5464

1 lb=4.45 N

TOTAL GROWTH FACTOR Asphalt Institute method total growth factor

r

rYGGF

n 1)1()(*)(

To determine annual growth rate following factors should be considered: Attracted or diverted traffic: due to improvement of

existing pavement Normal traffic growth: due to increase number or use of

motor vehicles Generated traffic: due to vehicle trips that would not

have been made if new facility had not been constructed

Development traffic: due to changes in landuse as a result of new facility

EXAMPLE:

For annual growth rate 3.5% and a design period of 30 years, compute the growth factors:

LANE DISTRIBUTION FACTOR For two lane highways, the lane in each direction is the

design lane, so that lane distribution factor is 100%. Truck distribution for multiple lane highway: table

TRUCK DISTRIBUTION FOR MULTILANE HIGHWAY

Two lanes in each direction

Three or more lanes in each direction

One way ADT

Inner Outer Inner Central outer

2000 6 94 6 12 82

4000 12 88 6 18 76

6000 15 85 7 21 72

8000 18 82 7 23 70

20000 25 75 7 30 63

60000 34 66 8 39 53

SUMMARY OF PERCENTAGE OF TOTAL TRUCK TRAFFIC IN DESIGN LANE

Number of traffic lanes in two direction

% of trucks in design lane

2 50

4 45

6 or more 40

Number of traffic lanes in each direction

% of 80 kN ESAL in design lane

1 100

2 80-100

3 60-80

4 50-75

ASPHALT INSTITUTE METHOD It has pavement thickness design manual which assumes: Asphalt pavement are multilayered elastic system. the wheel load W, is transmitted to the pavement surface

through the tire at a uniform vertical pressure Po. The stress are then spread through the pavement structure to produce a reduced maximum vertical stress P1, at the sub-grade surface.

The wheel load W, causes the pavement structure to deflect, creating both compressive and tensile stresses in the pavement structure.

In developing the design procedure, AI engineers calculated induced horizontal tensile strains, t, at the bottom of the asphalt layer and vertical compressive strain c, at the top of the sub-grade.

Computer program DAMA or SW1 for calculating thiskness using these criteria.

SPREAD OF WHEEL LOAD PRESSURE THROUGH PAVEMENT STRUCTURE

PAVEMENT DEFLECTION RESULT IN TENSILE AND COMPRESSIVE STRESSES

SUB-GRADE EVALUATION

CBR Resilient Modulus

RESILIENT MODULUS Resilience is the property of a material to absorb

energy when it is deformed elastically and then, upon unloading to have this energy recovered. In other words, it is the maximum energy per unit volume that can be elastically stored. It is represented by the area under the curve in the elastic region in the Stress-Strain diagram.

The Resilient Modulus (MR) is a subgrade material stiffness test.  A material's resilient modulus is actually an estimate of its modulus of elasticity (E).  While the modulus of elasticity is stress divided by strain (e.g., the slope of the stress-strain diagram the linear elastic range) for a slowly applied load, resilient modulus is stress divided by strain for rapidly applied loads – like those experienced by pavements. 

RESILIENT MODULUSSubgrade Strength Evaluation The characteristic material property of subgrade soils used for pavement

design is the resilient modulus (MR). The resilient modulus is defined as being a measure of the elastic property of a soil recognizing selected non-linear characteristics. Methods for the determination of MR are described in AASHTO

T294-92 test method. For many years, standard California Bearing Ratio (CBR) tests were utilized to measure the subgrade strength parameter as a design input.

For roadbed materials, the AASHTO Guide [AASHTO 93] recommends that the resilient modulus be established based on laboratory testing of representative samples in stress and moisture conditions simulating the primary moisture seasons. Alternatively, the seasonal resilient modulus values may be determined based on correlations with soil properties.

Since the resilient modulus test equipment is currently not present in many laboratories, researchers have developed correlations to converting CBR values to approximate MR values. The correlation considered reasonable for fine grained soils with a soaked CBR of 10 or less is:

MR (MPa) = 10.3 * ( CBR) [AASHTO 93]

INTRODUCTION

It is the process of determining the thickness of pavement structures consisting of Asphalt Concrete Surface, Emulsified Asphalt Concrete (with surface treatment), Asphalt concrete base, emulsified asphalt concrete base and untreated aggregate base or sub base.

DESIGN PROCEDURE1. Input data:

Traffic loading, ESA Sub-grade resilient modulus, Mr. Surface and base type

2. Determine design thickness for the specific conditions described by the input data.

3. Prepare stage construction design4. Make economic analysis of various solutions.5. Select the final design

FLOW DIAGRAM

Determine the initial traffic and expected

growth rate converted to ESA for design period

Measure of Estimate Sub-grade resilient

modulus

Select materials

Determine Design thickness combinations

Stage construction Options

Economic analysis

Final Design

AI METHOD FOR PAVEMENT DESIGN

MINIMUM THICKNESS OF ASPHALT CONCRETE OVER UNTREATED AGGREGATE BASE

Traffic, ESA Traffic conditions Minimumthickness,

10,000 Light traffic parking lots, light traffic roads

75 mm

10, 000 – 100, 000 Medium truck traffic 100 mm

More than 100, 000 Medium to heavy traffic 125 mm

EQUIVALENT DEPTH RATION OF ASPHALT CONCRETE AND BASE

3/1

b

ac

ac

b

E

E

t

t

EXAMPLE The result of the sub-grade soil in CBR at seven locations

obtained in a certain stretch of the road are given below. We are required to adopt CBR values as 85 percentile.

S/N

Chainage Test CBR result, %

1 0+050 11

2 0+350 7

3 0+500 7.5

4 0+650 8

5 0+800 4

6 0+950 6

7 1+500 5

EXAMPLE

Four lane/ two lane highway to be designed: Annual Growth rate 6% design period 12 years Construction period 2 years Modulus of elasticity of asphalt concrete in 2000MPa Modulus of untreated crushed aggregate base is 250 Mpa Modulus of untreated granular sub-base is 100 MPa

Truck Bus

3-Axle

2-Axle Mini Large Mini Micro

Car/Jeep/Taxi

Utility

Auto Rikshaw

Tractor

Power triller

Motor Cycle

Animal cart

Rikshaw Bicycle

124 243 48 342 98 249 210 45 114 166 8 1440 39 92 2101

3-axle truck 2-axle truck Mini truck Large bus Minibus

10.54 4.5 .97 1.38 .38

Axle load factors for ESA

TRAFFIC ANALYSIS

Growth factor calculation ESA for base period Expected cumulative ESA for design period

85 PERCENTILE CBR DETERMINATION

3 4 5 6 7 8 9 10 11 120

20

40

60

80

100

120

Sub-grade CBR, %

Perc

en

tag

e e

qu

al to

gre

ate

r th

an

in

%