TRANSPORTATION ENGINEERING-II
AASHTO 1993
Flexible Pavement Design Equation
AASHTO DESIGN
METHOD
• The basic objective of this test was to
determine significant relationship
between the no. of repetition of
specified axle loads (of different
magnitude and arrangement) and the
performance of different thickness of
pavement layers.
AASHTO DESIGN METHOD
CONSIDERATIONS
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Pavement Performance
Traffic
Roadbed Soil
Materials of Construction
Environment
Drainage
Reliability
Life-Cycle Costs
Shoulder Design
STEPS FOR DESIGNING
• The AASHO design method states that:
• “The function of any road is to carry the
vehicular traffic safely and smoothly from one
place to another”.
• Following are the different steps
followed in AASHTO design method
while designing the pavement.
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Measuring Standard Axle Load
Predicting Serviceability
Performance
Present Serviceability Rating (PSR)
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Present Serviceability Index
Terminal Serviceability
Regional Factor
Structural Number
Soil Support
Reliability
Over all Standard Deviation
Resilient Modulus
Standard Axle Load (ESAL’s)
• “An axle carrying a load of 18Kips and causing
a damaging effect of unity is known as
Standard Axle Load”.
Serviceability
• “Ability of a pavement to serve the traffic for
which it is designed”.
Performance
• “Ability of a pavement to serve the traffic for a
period of time”. Performance is interpreted as
trend of serviceability with time.
Present Serviceability Rating
• To define PSR, the AASHO
constituted a panel of drivers
belonging to different private
and commercial vehicles.
They were asked to
Very Good
Good
• Rate the serviceability of
different section on a scale of
0-5.
• Say whether the sections were
acceptable or not.
Fair
Poor
Very Poor
Present Serviceability Index (ISI)
• The prediction of PSR from these physical
measurements is known as PSI and defined as
“Ability of a pavement to serve the traffic for
which it is designed”. Normally the value is
taken as 4.
• PSI value depends on the following factors;
• Measurement of longitudinal surface irregularities
• Degree of cracking
• Depth of rutting in the wheel paths
Terminal Serviceability Index (ISI)
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“The lowest serviceability that will be tolerated
on the road at the end of the traffic analysis
period before resurfacing or reconstruction is
warned”.
Its usual value is 2 for roads of lesser traffic
volume and 2.5 for major highways.
Basic design equation for Terminal Serviceability is
Pt= Gt-{log (Wt)-log (p)}
• =0.4+{0.081(L1+L2)3.23}/{(1+SN)5.19+L23.23}
• log (p)= 5.93 + 9.36log(SN+1)-4.79log (L1+L2)+
4.33log(L2)
• Gt=a logarithmic function of the ratio of the loss in
serviceability at time t to the potential loss taken to a point
where pt=1.50
• p=a function of design and load variables that denotes the
expected number of axle load applications to a pt=1.5
• = a function of design and load variables that influence
the shape of the p Vs W serviceability curve.
• Wt=axle load applications at the end of the time t
• L1=load on one single axle or on one tendon axle set, in
kg
• SN= Structural Number of pavement
• Regional factor
It is a factor which helps the use of the
basic equations in a climatic condition
other than the ones prevailing during
the road test. Its values are:
• Road bed material frozen to a depth of 5 in
or more (winter)
• Road bed material dry (Summer and fall)
• Road bed material wet (spring thaw)
• Structural Number
An index number that represents the overall
pavement system structural requirements
needed to sustain the design traffic loading for
the design period. Analytically, the SN is given
by:
SN=a1D1M1+a2D2M2+a3D3M3
Where
• D1,D2,D3 = thickness in inches respectively of
surfacing, base and sub-base.
• a1,a2,a3 = coefficients of relative strength.
a1
=
a2
=
a3
=
M1, M2,M3 =
M1
=
0.2 for road bricks
0.44 for plant mix
0.45 for the sand asphalt
0.07 for sandy gravel
0.14 for crushed stone
0.11 for sandy gravel
0.50 to 0.10 for sandy soil
drainage coefficients
1 shows good drainage conditions
Soil Support
• Its value depends on the CBR value of the
layer.
Reliability
It is defined as “probability that serviceability will be
maintained at adequate levels from a user point of view,
through out the design life of the facility”
• Overall Standard Deviation
It takes in to account the designer’s ability to estimate the
variation in 18K Equivalent Standard Axle Load.
• Resilient Modulus
It is defined as
Mr = Repeated Axial Stress / Total Recoverable Axial
Strain
Mr=CBR x 1500
AASHTO DESIGN
EQUATION
This equation is widely used and has the following
form:
Log10(W18)=Zr x So+ 9.36 x log10(SN + 1)0.20+(log10((ΔPSI)/(4.2-1.5))
/(0.4+(1094/(SN+1)5.19)+2.32x log10(MR)-8.07
where:
W18=predicted number of 80 KN (18,000 lb.) ESAL’s
ZR=standard normal deviate
So=combined standard error of the traffic prediction and
performance prediction
SN=Structural Number (an index that is
indicative of the total pavement thickness
required)
SN=a1D1M1 + a2D2m2 + a3D3m3+...
ai
=ith layer coefficient
di
=ith layer thickness (inches)
mi
=ith layer drainage coefficient
Δ PSI =difference between the initial design
serviceability index, po, and the design
terminal serviceability index, pt
MR =sub-grade resilient modulus (in psi)
Nomo-graph
1993 AASHTO Structural Design
Step-by-Step
Step 1: Traffic Calculation
Total ESALs
• Buses + Trucks
• 2.13 million + 1.33 million = 3.46 million
Step 2: Get MR Value
• CBR tests along Kailua Road show:
– CBR ≈ 8
• MR conversion
AASHTO Conversion
M R  1500CBR  15008  12,000 psi
NCHRP 1-37A Conversion
M R  2555CBR
0.64
 25558
0.64
 9,669 psi
Step 3: Choose Reliability
Arterial Road
• AASHTO Recommendations
Functional Classification
Recommended Reliability
Urban
Rural
WSDOT
85 – 99.9
85 – 99.9
95
Principal arterials
80 – 99
75 – 95
85
Collectors
80 – 95
75 – 95
75
Local
50 – 80
50 – 80
75
Interstate/freeways
Choose 85%
Step 3: Choose Reliability
Reliability
ZR
99.9
-3.090
99
-2.327
95
-1.645
90
-1.282
85
-1.037
80
-0.841
75
-0.674
70
-0.524
50
0
Choose S0 = 0.50
Step 4: Choose ΔPSI
Somewhat arbitrary
• Typical p0 = 4.5
• Typical pt = 1.5 to 3.0
• Typical ΔPSI = 3.0 down to 1.5
Step 5: Calculate Design
Decide on basic structure
Resilient Modulus (psi)
Layer
a
Typical
Chosen
HMA
0.44
500,000 at 70°F
500,000
ACB
0.44
500,000 at 70°F
500,000
UTB
0.13
20,000 to 30,000
25,000
Aggregate
0.13
20,000 to 30,000
25,000
Step 5: Calculate Design
Step 5: Calculate Design
Preliminary Results
• Total Required SN = 3.995
• HMA/ACB
• Required SN = 2.74
• Required depth = 6.5 inches
• UTB and aggregate
• Required SN = 1.13
• Required depth = 9 inches
Step 5: Calculate Design
Apply HDOT rules and common sense
• HMA/ACB
• Required depth = 6.5 inches
• 2.5 inches Mix IV (½ inch Superpave)
• 4 inches ACB (¾ inch Superpave)
• UTB and aggregate
• Required depth = 9 inches
• Minimum depths = 6 inches each
– 6 inches UTB
– 6 inches aggregate subbase
Comparison
Layer
California
AASHTO
HMA Surface
2.5 inches
2.5 inches
ACB
7.0 inches
4.0 inches
UTB
6.0 inches
6.0 inches
Aggregate subbase
6.0 inches
6.0 inches