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Topic 6 Asphalt Institute DesignProcedure
Dr. Christos Drakos
University of Florida
Introduction to Pavement Design
1. Introduction
Establish Layer Thicknesses:
To limit distress (acceptable levels)
For anticipated loading & environmental conditions
Using available/selected materials
1.1 Elements to be Defined/Identified for Design
Conditions:
Traffic loading (volume, frequency,magnitude ESALs)
Environment (temperature, moisture)
Material Properties: Subgrade varies w/ season (existing material)
Pavement Structure (engineered materials)
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Introduction to Pavement Design
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
PAVEMENTPERFORMANCE
PERFORMANCERELATION
Introduction to Pavement Design
2. Design Approach
TRIAL
MATERIALS
TRIAL
THICKNESSES
PERFORMANCE
RELATION
NO
YESLIFE-COST
CYCLE
PERFORMANCE
PERFORMANCE
CRITERIA
TRAFFIC
ENVIRONMENT
SUBGRADE
MATERIAL
PROPERTIES
A Pavement Performance Model is an equation that relatessome extrinsic time factor (age, or number of loadapplications) to a combination of intrinsic factors (structuralresponses, drainage, etc) and performance indicators
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Introduction to Pavement Design
3. Empirical Vs Mechanistic-Empirical
Difference is in the nature of Performance Relation
3.1 Empirical
Statistical/Experimental (based on road tests)
Limited conditions/environment
3.2 Mechanistic-Empirical
Relate analytical response to performance:
More reliable/robust than empirical
Integrates the structural aspects of a pavement to thematerial/mix design properties of the pavement layers!!!
Improve the relation by understanding the mechanics
Introduction to Pavement Design
4. Response and Performance
4.1 Response = Reaction to an action
Response = Pavement & Material response to applied loads
(traffic & environment)
AC
BASE
1 &2Pavement Responses
1
2
3
&Material Responses
1
2
What are Pavement & Material Responses?
element
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Introduction to Pavement Design
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:
th = fnc (material, temperature, cooling rate, dimensions)
Introduction to Pavement Design
4.2 Performance
Performance is the measurable adequacy ofSTRUCTURAL&FUNCTIONALservice over a specified design period
Structural Functional (user defined)
Number of loads the pavementcan support before it reachesunacceptable level ofstructural/functional distress
Roughness Ride quality
Friction Geometry Appearance
Surface cracking
Loss of color
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Topic 6 Asphalt Institute Design Procedure
ASPHALT INSTITUTE (AI)
US based association of international asphalt producers thatpromotes 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 traffic in a specific environment 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
Topic 6 Asphalt Institute Design Procedure
Two types of strains are considered critical in design of asphaltpavements:
Horizontal tensile strain, t @ the bottom of AC layer Vertical compressive stain, c @ the top of the subgrade
2. Design Criteria
2.1 Fatigue Cracking
AC t
Basic equation:
32
1
ff
tfEfN
=
Where: Nf= Number of cycles to failure t = Tensile strain @ bottom of AC layer f1 = Field correlation shift factor f2 & f3 = Laboratory determined values
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Topic 6 Asphalt Institute Design Procedure
2.1 Fatigue Cracking (cont)
32
1ff
tf EfN =
Asphalt Institute calibrated the field shift factor using data fromthe AASHO road test
f1 = 0.0796
2.1.1 Fatigue tests
t
Why 3rd-point loading?
To have an even distributionof M; we know the value of
M, no matter where thespecimen failsV
M
Topic 6 Asphalt Institute Design Procedure
2.1.1 Fatigue tests (cont)
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Topic 6 Asphalt Institute Design Procedure
2.1.2 Constant Stress Fatigue Test
Apply constant stress Failure occurs when the material fractures
0
Stress,
Number of Cycles, N
Strain,
Number of Cycles, N
2.1.3 Constant Strain Fatigue Test
Apply constant strain (rate of deformation) Failure occurs when E=E0
Stress,
Number of Cycles, N
Strain,
Number of Cycles, N
0
0
0
0
0
02
1
= ; 021
=
Topic 6 Asphalt Institute Design Procedure
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 @ different temperature
Strain,Logt
Number of Cycles, Log Nf
Which curve has the highest stiffness?
Check: Select a strain level Find the corresponding Nf Higher stiffness will have less
number of cycles to failure
C1
C2
Nf1Nf2
Low
High
From the graph: Stiffness of the material will depend on time of the year (temperature) t depends on the material properties (E) So, the cycles to failure Nfwill also depend on the temperature
Must use cumulative damage approach to evaluate failure
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Topic 6 Asphalt Institute Design Procedure
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, t4E3, t3E2, t2E1, t1Material properties
4321Periods (Seasons)
Dr=Dri i.e. Dr=0.1; Design Life = 1/Dr = 10 years
Nf4Nf3Nf2Nf1
n1 n2 n3 n4
Dr1= n1/Nf1 Dr2= n2/Nf2 Dr3= n3/Nf3 Dr4= n4/Nf4
Topic 6 Asphalt Institute Design Procedure
2.3 Permanent Deformation
Only SUBGRADE rutting considered, as governed bycompressive strain
5
4
f
cd fN
=
477.4910365.1
= cdN
AI calibrated the equationusing AASHO road test data
Consider the following two pavements
E1
E2
E1
E2
cA cBE3A E3B
Similar structure E3A>> E3B Assume cA= cB
Assume cA= cB
BUT:
c @ P =c
E3cA> NdB
So,
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Topic 6 Asphalt Institute Design Procedure
3. Environment
Nf& Nd vary with time of the year because of change inmaterial properties with the weather
64
34
4
11 +
+
++=
zzMM
AP
Where: MP = Mean pavement temperature
MA= Mean monthly air temperature z = Depth below the surface (1/3 of AC layer depth)
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 =
Topic 6 Asphalt Institute Design Procedure
Four distinct periods: Freeze Thaw Recovery Normal
Table 11.9 shows the suggested conditions to represent frosteffects on the subgrade
3.2 Subgrade
Normal MR
Frozen MR
Thaw MR
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Topic 6 Asphalt Institute Design Procedure
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 < in
Fatigue Cracking < 20% of Area
OVER THE DESIGN LIFE (as defined by traffic)
Topic 6 Asphalt Institute Design Procedure
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 Subgrade MR& ESALs Read thickness off the chart
Example: Subgrade MR= 11,000 psi Traffic = 1.1x106 ESAL Thickness = ?
For multiple HMA within a layer usecomposite modulus
( ) ( )
+
+=
BA
BBAA
hh
EhEhE
11
3/1
11
3/1
11
h1A
h1B
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Topic 6 Asphalt Institute Design Procedure
5.2.1 Full-Depth HMA (cont)
Thickness = 8in
Topic 6 Asphalt Institute Design Procedure
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 Dense Graded (Crushed Rock) TYPE II Gap Graded (Rounded Gravel) TYPE III Uniform Graded (Sand Asphalt)
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Topic 6 Asphalt Institute Design Procedure
5.2.2 HMA over Emulsified Asphalt Base (cont)
Minimum HMA thickness required (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
Topic 6 Asphalt Institute Design Procedure
5.2.2 HMA over Emulsified Asphalt Base (cont)
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Topic 6 Asphalt Institute Design Procedure
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 thicknessfor the specific base thickness
Fig 11.15-11.20
Topic 6 Asphalt Institute Design Procedure
5.2.3 HMA over Untreated Aggregate Base (cont)
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Topic 6 Asphalt Institute Design Procedure
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
2HMA Surface
Full Depth HMA
hHMA
Figure 11.11
Emulsified Base
hEMUL
Figure 11.12-11.14
THMA=hHMA-2TEMUL=hEMUL-2
HMA
EMUL
T
TSR =
Topic 6 Asphalt Institute Design Procedure
5.2.4 HMA on Asphalt Emulsion over Untreated Aggregate Base
1. Design pvt using full-depth HMA Fig 11.11 Assume 2 HMA surface Determine THMA
2. Design pvt using Emulsified Asphalt Mix Fig 11.12-11.14 Assume 2 HMA surface Determine TEMUL
3. Calculate SR=TEMUL/THMA4. Design pvt using HMA on Aggregate Base
Select aggregate base thickness Fig 11.15-11.20
5. Determine minimum HMA thickness Table 11.12
6. Determine HMA thickness to be replaced by Emulsified Mix Design thickness (step 4) Min HMA (step 5)
7. Determine thickness of Emulsified Mix Thickness (step 6) * SR (step 3)
First threesteps todetermineSR
Last threesteps toperform thesubstitution
ActualDesign
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Topic 6 Asphalt Institute Design Procedure
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 mixType I base, and 8 aggregate subbase
WORK EXAMPLE ON THE BOARD
Topic 6 Asphalt Institute Design Procedure
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. When funds are insufficient2. When traffic is unpredictable (Utah Olympics example)
3. May detect weak spots during 1st stage (organic peat insubgrade)
Apply successive HMA layers according to predeterminedschedule:
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Topic 6 Asphalt Institute Design Procedure
6.1 Relative Damage
1
1
1N
nDr =
Where: n1 = Actual (predicted) ESALs for Stage 1 N1 = Allowable ESALs for initial thickness h1
Dr = 1 so our pavement will fail at the end of stage 1!BUT, we want to construct the second stage before the first onestarts showing signs of distress
What happens if n1=N1?
Stage 1 = X-amount of years. So, n1 is the predicted traffic forthe specific location for X-amount of years
N1 is the DESIGN life ESALs for h1. Meaning that the pavementwill fail (20% cracking / rutting) after N1 applications of loads
Topic 6 Asphalt Institute Design Procedure
Stage 1X-years & n1 (actual) loads
Remaininglife
Dr0 0.6 1
Design Life for N1 loads
1. Define a relative damage for the end of the first stage (AIsuggests 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 itslife span
3. By dividing n1 with 0.6 we get a design N1 that allows somuch traffic, that by the end of Stage 1 we reach a damageratio of 0.6
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Topic 6 Asphalt Institute Design Procedure
6.2 Planned Stage Procedure (cont.)
Stage 1:Purpose is to select an initial thickness that will have someremaining life after the initial applied (n1) ESALs
Specify Dr1 after Stage 1(AI suggests Dr1 = 0.6)
1
1
1Dr
nN =
Use N1 to obtain thickness h1 that will provide sufficientprotection, so that after n1 loads the relative damage will beequal to 0.6
N1 = allowable ESALs for Stage 1
Topic 6 Asphalt Institute Design Procedure
6.2 Planned Stage Procedure (cont.)
Stage 2:For the 2nd stage design we need to consider the existingstructure from Stage 1; the remaining life that carries over tothe 2nd stage is Dr2
Use N2 to obtain thickness h2 that will provide sufficientprotection for the expected traffic, n2
hoverlay = h2 h1
For the 2nd stage we expect to have n2 ESALs over Y-amountof years
)1( 1
2
2Dr
nN
=
Dr2 = 1-Dr1
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Topic 6 Asphalt Institute Design Procedure
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)
Topic 6 Asphalt Institute Design Procedure
7. Material Characterization
Calculate subgrade MR(Topic 5): Confining stress: 1=2=2 psi Deviator stress: d=6 psi
8. Variability/Reliability
Subgrade MRvalues WILL vary within a design unit (segment) If material and test method remain the same, we may
assume that MR is normally distributed with mean MR(avg)
MR(avg)MR(max)MR(min)
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Topic 6 Asphalt Institute Design Procedure
1. 104 ESAL or less, design using MR60 60% probability that MR>MR60
2. 104-106, design using MR75 75% probability that MR>MR75
3. 106 or more, design using MR87 87.5% probability that MR>MR87
8.1 Three Levels of Reliability
MR(avg)M
R
(max)MR
(min)MR60MR87 MR75
50% of values greater than MRAVG50% of values less than MRAVG
Topic 6 Asphalt Institute Design Procedure
8.2 Variability/Reliability Method
1. Need to get at least eight subgrade samples
x2x
xx
xxxx
2. Evaluate the samples and rank in descending MRorder3. Calculate percent equal or greater than
C2C1 C3
2C3C =valuesof#
100%
C1= MRvalues indescending order
C2= # of values equalto or greater than
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Topic 6 Asphalt Institute Design Procedure
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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