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Asphalt Institute Design Procedure

Dr. Antonis Michael

Frederick University

Notes Courtesy of Dr. Christos Drakos

University of Florida

Introduction to Pavement Design

1. Introduction

• Establish Layer Thicknesses:

– To limit

– For anticipated

– Using available/selected materials

1.1 Elements to be Defined/Identified for Design

• Conditions:

– Traffic loading (volume,

–

• Material Properties:

– Subgrade

– Pavement Structure (engineered materials)


1.1 Elements to be Defined/Identified for Design (cont.)

• Performance Criteria:

– Conditions that define failure

• Performance Relationship

• TRAFFIC• ENVIRONMENT

• SUBGRADE• MATL PROPERTIES

• LAYER THICKNESSES

PAVEMENT PERFORMANCE

PERFORMANCERELATION


2. Design Approach

TRIALMATERIALS

TRIALTHICKNESSES

PERFORMANCERELATION

NO

YESLIFE-COST

CYCLE

PERFORMANCE

PERFORMANCECRITERIA

� TRAFFIC� ENVIRONMENT� SUBGRADE� MATERIAL

PROPERTIES

A Pavement Performance Model is an equation that relates some extrinsic ‘time factor’ (age, or number of load applications) to a combination of intrinsic factors (structural responses, drainage, etc) and performance indicators


3. Empirical Vs Mechanistic-Empirical

Difference is in the nature of Performance Relation

3.1 Empirical

•

• Limited

3.2 Mechanistic-Empirical

• Relate analytical response to performance:

– More reliable/robust than empirical

– Integrates the structural aspects of a pavement to

Improve the relation by understanding the mechanics


4. Response and Performance

4.1 Response = “Reaction to an action”

Response = Pavement & Material response to applied loads

(traffic & environment)

AC

BASE

What are Pavement & Material Responses?

element


4.1 Response = “Reaction to an action”

Predict load responses with structural response models:

• Vary in sophistication:

– Linear Elastic

– Non-linear Elastic

– Viscoelastic

– … etc

Predict temperature responses with thermal response models:

• σth = fnc (material, temperature, cooling rate, dimensions)


4.2 Performance

Performance is the measurable adequacy of STRUCTURAL & FUNCTIONAL service over a specified design period

Structural Functional (user defined)

Number of loads the pavement can support before it reaches unacceptable

• Roughness–

• Friction • Geometry• Appearance

––

Topic 6 – Asphalt Institute Design Procedure

ASPHALT INSTITUTE (AI)

US based association of international asphalt producers that promotes the use of petroleum asphalt products

• http://www.asphaltinstitute.org/

Design method based on computer model DAMA • Computes amount of damage (cracking & rutting) based

on • Multilayer elastic theory; used correction factors to

account for base non-linearity• Used three temperature regimes; representing three

climatic regions in the US – NY(45), SC(60) & AZ(75)• Developed design charts from the results

1. Development


Two types of strains are considered critical in design of asphalt pavements:

• Horizontal tensile strain, εt @ the bottom of AC layer• Vertical compressive stain, εc @ the top of the subgrade

2. Design Criteria

2.1 Fatigue Cracking

AC εt

Basic equation:

32

1ff

tf EfN −− ⋅⋅= εWhere:• Nf = Number of cycles to failure• εt = Tensile strain @ bottom of AC layer• f1 = Field correlation shift factor• f2 & f3 = Laboratory determined values


2.1 Fatigue Cracking (cont)

32

1ff

tf EfN −− ⋅⋅= εAsphalt Institute calibrated the field shift factor using data from the AASHO road test

• f1 = 0.0796

2.1.1 Fatigue tests

εt

Why 3rd-point loading?

V

M


2.1.1 Fatigue tests (cont)


2.1.2 Constant Stress Fatigue Test

• Apply constant stress• Failure occurs when

Stress, σ

Number of Cycles, N

Strain, ε

Number of Cycles, N

2.1.3 Constant Strain Fatigue Test

• Apply constant strain (rate of deformation)• Failure occurs when

Stress, σ

Number of Cycles, N

Strain, ε

Number of Cycles, N


2.1.4 Fatigue Test Analysis

• Plot the strain Vs number of repetitions to failure on log scales• C1 & C2 curves for the same material @

Strain, Log ε

t

Number of Cycles, Log Nf

Which curve has the highest stiffness?

Check:• Select a •• Higher stiffness

C1

C2

From the graph:• Stiffness of the material will depend on • εt depends on the material properties (E)• So, the cycles to failure Nf


2.2 Damage Ratio

Dr=Actual # of Load Repetitions

Allowable # of Load RepetitionsPavement has ‘failed’ if Dr=1

∑∑= =

=p

i

m

j ji

ji

N

nDr

1 1 ,

, Where:m = no. of load types = 1 for AIp = no. of periods in analysis = 12 for a year

2.2.1 Damage ratio example

Damage Ratio

Actual Traffic

Allowable Traffic

E4, εt4E3, εt3E2, εt2E1, εt1Material properties

4321Periods (Seasons)

Nf4Nf3Nf2Nf1

n1 n2 n3 n4

Dr1= n1/Nf1 Dr2= n2/Nf2 Dr3= n3/Nf3 Dr4= n4/Nf4


2.3 Permanent Deformation

Only

5

4f

cd fN −⋅= ε477.4910365.1 −− ⋅×= cdN ε

AI calibrated the equation using AASHO road test data

Consider the following two pavements

E1

E2

E1

E2

E3A E3B

• Similar structure• E3A >> E3B• Assume σcA = σcB

Assume σcA = σcB

BUT:

εc @ P =


3. Environment

• Nf & Nd vary with time of the year because of change in material properties with the weather

64

34

4

11 +

+−

++⋅=

zzMM AP

• Where:– MP = Mean – MA = Mean – z = Depth below the surface (

• AI procedure considers the environment based on:– Mean monthly temperature– Monthly variable material modulus

3.1 Asphalt Concrete

• Then we can use: ( ) ( )PMELog 01.048.61 −=


• Four distinct periods:– Freeze– Thaw– Recovery– Normal

• Table 11.9 shows the suggested conditions to represent frost effects on the subgrade

3.2 Subgrade

Normal MR

Frozen MR

Thaw MR


4. Traffic

Calculate design ESALs (Topic 4)

5. Design Procedure

5.1 Objective

DETERMINE THE REQUIRED STRUCTURAL THICKNESS FOR

EXPECTED TRAFFIC, SUBGRADE CONDITIONS, AND

ENVIRONMENT SUCH THAT:

• Rutting

• Fatigue Cracking

OVER THE DESIGN LIFE (as defined by traffic)


5.2 Pavement Types

5.2.1 Full-Depth HMA

• Pavement constructed completely from HMA• Figure 11.11; includes both surface and base course thickness

HMA BASE

HMA SURFACE

Thickness• Use • Read

Example:• Subgrade MR = 11,000 psi• Traffic = 1.1x106 ESAL• Thickness = ?

For multiple HMA within a layer use composite modulus

( ) ( )

+⋅+⋅=

BA

BBAA

hh

EhEhE

11

3/111

3/111

h1A

h1B


5.2.1 Full-Depth HMA (cont)


5.2.2 HMA over Emulsified Asphalt Base

Emulsified Asphalt:• Mixture of asphalt cement, water and emulsifying agent• Run through a colloid mill that produces asphalt droplets (5-10 microns)• Suspended in in the mixture by electrical charge• Upon contact with aggregate it ‘sets’ or ‘breaks’; water is squeezed out or

evaporated• Anionic emulsified asphalts – Negatively charged; compatible with

aggregate with positive charge (limestone)• Cationic emulsified asphalts – Positively charged; compatible with

aggregate with negative charge (siliceous aggregates)• Rapid, Medium and Slow setting

Emulsified Base:• TYPE I –• TYPE II –• TYPE III –


5.2.2 HMA over Emulsified Asphalt Base (cont)

Minimum HMA thickness required• ƒ(ESAL & Base Type) Table 11.12

HMA SURFACE hHMA

EMULSIFIED BASEhEMUL

• TYPE I – Fig 11.12• TYPE II – Fig 11.13• TYPE III – Fig 11.14

• hEMUL from the graph• Determine hHMA


5.2.2 HMA over Emulsified Asphalt Base (cont)


5.2.3 HMA over Untreated Aggregate Base

• Select the thickness of the aggregate base first• Figures 11.15-11.20 – design charts for HMA surface courses

on aggregate base courses of 4,6,8,10,12 and 18 in

HMA SURFACE hHMA

AGGREGATE BASE (known)

• Determine the required HMA thickness for the specific base thickness

• Fig 11.15-11.20


5.2.3 HMA over Untreated Aggregate Base (cont)


5.2.4 HMA on Asphalt Emulsion over Untreated Aggregate Base

• Design charts do not exist• Have to determine substitution ratio between HMA &

emulsified asphalt base

Substitution Ratio (SR)• Thickness of emulsified asphalt base required to substitute a unit

thickness of HMA

HMA Surface 2”

2”HMA Surface

Full Depth HMA

hHMA

Figure 11.11

Emulsified Base

hEMUL

Figure 11.12-11.14


5.2.4 HMA on Asphalt Emulsion over Untreated Aggregate Base

1. Design pvt using full-depth HMA •••

2. Design pvt using Emulsified Asphalt Mix •••

3. Calculate SR=TEMUL/THMA

4. Design pvt using HMA on Aggregate Base••

5. Determine minimum HMA thickness•

6. Determine HMA thickness to be replaced by Emulsified Mix•

7. Determine thickness of Emulsified Mix •

First three steps to determine

SR

Last three steps to perform the

substitution

Actual Design


5.2.5 Combined Design Example

Given:• ESUB = 10,000 psi• Design ESAL = 1,000,000

Need to design a pavement with HMA surface, emulsified mix Type I base, and 8” aggregate subbase

WORK EXAMPLE ON THE BOARD


6. Planned Stage Construction

• Based on the concept of remaining life• Second stage constructed before first shows significant

distress

Why? What are the advantages/reasons for planned stage?

1.2.3.

Apply successive HMA layers according to predetermined schedule:


6.1 Relative Damage

1

11 N

nDr =

Where:• n1 = • N1 =

Dr = 1 … so our pavement will fail at the end of stage 1! BUT, we want to construct the second stage before the first one starts showing signs of distress

What happens if n1=N1?

Stage 1 = X-amount of years. So, n1 is the predicted traffic for the specific location for X-amount of years

N1 is the DESIGN life ESALs for h1. Meaning that the pavement will fail (20% cracking / ½” rutting) after N1 applications of loads


Dr0 0.6 1

1. Define a relative damage for the end of the first stage (AI suggests 0.6)

6.2 Planned Stage Procedure

2. Assume Dr1 = 0.6 � at the end of the 1st stage (after X-amount of years & n1 loads) the pavement reached 60% of its life span

3. By dividing n1 with 0.6 we get a design N1 that allows so much traffic,


6.2 Planned Stage Procedure (cont.)

Stage 1:Purpose is to select an initial thickness that will have some remaining life after the initial applied (n1) ESALs

• Specify Dr1 after Stage 1(AI suggests Dr1 = 0.6)

1

11 Dr

nN =

• Use N1 to obtain thickness h1 that will provide

• N1 = allowable ESALs for Stage 1


6.2 Planned Stage Procedure (cont.)

Stage 2:For the 2nd stage design we need to consider the existing structure from Stage 1; the remaining life that carries over to the 2nd stage is Dr2

• Use N2 to obtain thickness h2 that will provide sufficient protection

• hoverlay =

• For the 2nd stage we expect to have n2 ESALs over Y-amount of years

• Dr2 =


6.2.1 Planned Stage Construction Example

Given:• Full-depth HMA pavement to undergo two-stage construction• ESUB=10,000 psi & Dr1=0.6• First Stage: 5 years, n1=150,000 ESAL• Second Stage: 15 years, n2=850,000 ESAL

WORK EXAMPLE ON THE BOARD

Determine h1 & h2 (hoverlay)


7. Material Characterization

Calculate subgrade MR (Topic 5):• Confining stress: σ1=σ2=2 psi• Deviator stress: σd=6 psi

8. Variability/Reliability

• Subgrade MR values • If material and test method remain the same, we may

assume that MR is

MR(avg)MR(max)MR(min)


1. 104 ESAL or less, design using MR60•

2. 104-106, design using MR75•

3. 106 or more, design using MR87•

8.1 Three Levels of Reliability

MR(avg)MR(max)MR(min)

MR60MR87 MR75


8.2 Variability/Reliability Method

1. Need to get at least eight subgrade samples

x2’ xx

xx

xxx

2. Evaluate the samples and rank in descending MR order3. Calculate percent equal or greater than

C2C1 C3


8.2 Variability/Reliability Method (cont)

4. Plot Percent Greater/Equal Than Vs Resilient Modulus

Which value is the most conservative estimate?

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