1. TRANSPORTATION ENGINEERING-II
AASHTO 1993 Flexible Pavement Design Equation

2. 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.

3. AASHTO DESIGN METHOD CONSIDERATIONS
Pavement Performance
Traffic
Roadbed Soil
Materials of Construction
Environment
Drainage
Reliability
Life-Cycle Costs
Shoulder Design

4. STEPS FOR DESIGNING The AASHTO 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.
Measuring Standard Axle Load
Predicting Serviceability
Performance
Present Serviceability Rating (PSR)

5. Present Serviceability Index
Terminal Serviceability
Regional Factor
Structural Number
Soil Support
Reliability
Over all Standard Deviation
Resilient Modulus

6. 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.

7. Present Serviceability Rating
To define PSR, the AASHO constituted a panel of drivers belonging to different private and commercial vehicles. They were asked to
Rate the serviceability of different section on a scale of 0-5.
Say whether the sections were acceptable or not.
Very Good
Good
Fair
Poor
Very Poor

8. 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

9. Terminal Serviceability Index (ISI)
“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.

10. 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)= log(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

11. 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)

12. 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.

13. Soil Support Its value depends on the CBR value of the layer.
a1 = 0.2 for road bricks
0.44 for plant mix
0.45 for the sand asphalt
a2 = 0.07 for sandy gravel
0.14 for crushed stone
a3 = 0.11 for sandy gravel
0.50 to 0.10 for sandy soil
M1, M2,M3 = drainage coefficients
M1 = 1 shows good drainage conditions
Soil Support
Its value depends on the CBR value of the layer.

14. Mr = Repeated Axial Stress / Total Recoverable Axial Strain
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

15. AASHTO DESIGN EQUATION
This equation is widely used and has the following form:
Log10(W18)=Zr x So x log10(SN + 1)-0.20+(log10((ΔPSI)/( )) /(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

16. 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)

17. Nomo-graph

18. 1993 AASHTO Structural Design
Step-by-Step

19. Step 1: Traffic Calculation
Total ESALs
Buses + Trucks
2.13 million million = 3.46 million

20. Step 2: Get MR Value CBR tests along Kailua Road show: MR conversion
AASHTO Conversion
NCHRP 1-37A Conversion

21. Step 3: Choose Reliability
Arterial Road
AASHTO Recommendations
Functional Classification
Recommended Reliability
Urban
Rural
Interstate/freeways
85 – 99.9
Principal arterials
80 – 99
75 – 95
Collectors
80 – 95
Local
50 – 80
WSDOT 95 85 75
Choose 85%

22. Step 3: Choose ReliabilityZR
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
Choose S0 = 0.50

23. 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

24. 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
UTB
0.13
20,000 to 30,000
25,000
Aggregate

25. 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

26. 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

27. Comparison Layer California AASHTO HMA Surface 2.5 inches ACB
UTB
6.0 inches
Aggregate subbase