7._AASHTO_Flexible_Pavement_Design_Method (1).pdf

AASHTO Flexible Pavement Design
Method

Design considerations for the
AASHTO Flexible Pavement Design
Pavement performance
Traffic
Roadbed soils (subgrade material)
Materials of construction
Environment
Drainage
Reliability
The following factors are considered in the pavement thickness
design.

Pavement performance
Structural  Cracking, faulting, raveling, etc.
Functional  Riding comfort (measured in terms of
roughness of pavement.)
Serviceability Performance: Measured by PSI  Present
Serviceability Index with scale 0 to 5.
0 “Road closed”
5 “Just constructed”
4.2 Initial PSI (pi)
Terminal PSI (pt)
 2.5 to 3.0 for major highways
 2.0 for lower class highways
 1.5 for very special cases
PSI

Pavement Condition
From the AASHO Road Test
(1956 – 1961)

Typical PSI vs. Time
Time
Serviceability
(PSI)
p0
pt
p0 - pt
Terminal Serviceability Level

Traffic
 In the AASHTO flexible pavement design, traffic is considered in terms of
ESAL for the terminal PSI (Table 20.13 for pt = 2.5.)
 We must assume the structural number of the pavement. So, we must check
if the final SN3 is similar to the assumed SN. Higher SN means stronger
pavement, thus the impact of traffic on pavement deteriorations is less.

Roadbed soils (Subgrade material)
CBR (California Bearing Ratio), R-value (Resistance), and
Mr (Resilient modulus) are used to describe the property of the
subgrade material.
During the structural design, only Mr values are used. The
following conversion formulas are used if either CBR or R-
values are given.
Mr (lb/in2) = 1500 x CBR for fine-grained soils with soaked CBR of 10 or less.
Mr (lb/in2) = 1000 + 555 x (R-value) for R <= 20

Materials of construction (Subbase), a3
Charts are available to convert the properties of pavement
construction materials to structural coefficients: a3, a2, and a1
Structural coefficient
of the subbase, a3
Use CBR, R-value, or Mr
to find a3 values

Materials of construction (Base course), a2
of the base course, a2
Use CBR, R-value, or Mr
to find a2 values

Materials of construction (AC surface), a1
= Resilient modulus, Mr
of the AC surface, a1
0.44

Environment
Temperature and rainfall
affect the level of strength of the
subgrade, reflected on the value
of resilient modulus. AASHTO
developed a chart that helps us
to estimate the effective roadbed
soil resilient modulus using the
serviceability criteria (in terms
of “relative damage, uf.”)
Determine the average uf.
value and obtain Mr from the
chart or the equation of uf. .
 The bar on the right is used
twice: Once to read uf value for
each month’s sample Mr, then to
read annual average Mr using the
average uf value.
Step 1
Step 3
Step 2

Drainage
The effect of drainage on the performance of flexible pavements
is considered with respect to the effect of water on the strength of
the base material and roadbed soil.
This effect is expressed by the drainage coefficient, mi. This
value is dependent on the drainage quality and the percent of time
i.e., the time to which a pavement structure is exposed to moisture
levels approaching saturation.

Definition of drainage quality and finding
recommended mi values
Time required to
drain the
base/subbase layer to
50% saturation.
If “Fair” and 30%
exposure, then mi
is 0.80.
Step 1
Step 2

Reliability
The reliability factor (FR) is computed using:
The Reliability design level (R%), which determines assurance levels
that the pavement section designed using the procedure will survive for its
design period (it is a z-score from the standard normal distribution)
The standard deviation (So) that accounts for the chance variation in the
traffic forecast and the chance variation in actual pavement performance
for a given design period traffic, W18.
o
R
R S
Z
F 

10
log SD, So
Flexible pavements 0.40-0.5
Rigid pavements 0.30-0.40
Functional Classification Recommended Level of Reliability
Urban Rural
Interstate and other freeways 85-99.9 80-99.9
Principal arterial 80-99 75-95
Collectors 80-95 75-95
Local 50-80 50-80

Structural design
The object of the design using the AASHTO method is to determine a flexible
pavement SN adequate to carry the projected design ESAL.
The method discussed in the text applies to ESALs greater than 50,000 for the
performance period. The design for ESALs less than this is usually considered
under low-volume roads.
 
 
 
07
.
8
log
32
.
2
)
1
/(
1094
40
.
0
)
5
.
1
2
.
4
/(
log
20
.
0
1
log
36
.
9
log
10
19
.
5
10
10
18
10











r
o
R
M
SN
PSI
SN
S
Z
W
Simplify this as f(W18) = f(ZRSo) + f(SN)
3
3
3
2
2
2
1
1 m
D
a
m
D
a
D
a
SN 


Where

How to use Fig. 20.20 to get structural
numbers based on Eq. 20.13
For subgrade,
Mr= 9000 lb/in2
SN3= 4.4
For sub-base,
Mr=13500 lb/in2
For base course,
Mr=31000 lb/in2
SN2= 3.8
SN1= 2.6

Once SN value is set, thickness design begins…
3
3
3
2
2
2
1
1 m
D
a
m
D
a
D
a
SN 


1
1
1 D
a
SN 
2
2
2
1
1
2 m
D
a
D
a
SN 

3
3
3
2
2
2
1
1
3 m
D
a
m
D
a
D
a
SN 


Proceed in
this direction
Use Fig.20.15 for a3, Fig.20.16 for a2, Fig.20.17 for a1, and Tab. 20.14 and
20.15 m2 and m3. Find the depth that results in a SN value close to the SN
value obtained from Fig. 20.20.

7._AASHTO_Flexible_Pavement_Design_Method (1).pdf

Equivalent Axle Load Factors, Single axles

Equivalent Axle Load Factors, tandem axles

Equivalent Axle Load Factors, triple axles

7._AASHTO_Flexible_Pavement_Design_Method (1).pdf

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