TRANSPORTATION ENGINEERING-II

1
TRANSPORTATION ENGINEERING-II
AASHTO 1993Flexible 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 (ESALs)
• 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.

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)
• ?0.40.081(L1L2)3.23/(1SN)5.19L23.23
• log (p) 5.93 9.36log(SN1)-4.79log (L1L2)
  4.33log(L2)
• Gta logarithmic function of the ratio of the
  loss in serviceability at time t to the potential
  loss taken to a point where pt1.50
• pa function of design and load variables that
  denotes the expected number of axle load
  applications to a pt1.5
• ? a function of design and load variables that
  influence the shape of the p Vs W serviceability
  curve.
• Wtaxle load applications at the end of the time
  t
• L1load 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
• SNa1D1M1a2D2M2a3D3M3
• Where
• D1,D2,D3 thickness in inches respectively of
  surfacing, base and sub-base.
• a1,a2,a3 coefficients of relative strength.

13

• 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

• 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 designers 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
• MrCBR x 1500

15
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((?PSI)/(4.2-1.5))
  /(0.4(1094/(SN1)5.19)2.32x log10(MR)-8.07
• where
• W18predicted number of 80 KN (18,000 lb.) ESALs
  ZRstandard normal deviate
• Socombined standard error of the traffic
  prediction and performance prediction

16

• SNStructural Number (an index that is indicative
  of the total pavement thickness required)
• SNa1D1M1 a2D2m2 a3D3m3...
• ai ith layer coefficient
• di ith layer thickness (inches)
• mi ith layer drainage coefficient
• ? 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 1.33 million 3.46 million

20
Step 2 Get MR Value

• CBR tests along Kailua Road show
• CBR 8
• MR conversion

AASHTO Conversion
NCHRP 1-37A Conversion
21
Step 3 Choose Reliability

• Arterial Road
• AASHTO Recommendations

Functional Classification Recommended Reliability Recommended Reliability
Functional Classification Urban Rural
Interstate/freeways 85 99.9 85 99.9
Principal arterials 80 99 75 95
Collectors 80 95 75 95
Local 50 80 50 80

WSDOT
95
85
75
75
Choose 85
22
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
23
Step 4 Choose ?PSI

• Somewhat arbitrary
• Typical p0 4.5
• Typical pt 1.5 to 3.0
• Typical ?PSI 3.0 down to 1.5

24
Step 5 Calculate Design

• Decide on basic structure

Resilient Modulus (psi) Resilient Modulus (psi)
Layer a Typical Chosen
HMA 0.44 500,000 at 70F 500,000
ACB 0.44 500,000 at 70F 500,000
UTB 0.13 20,000 to 30,000 25,000
Aggregate 0.13 20,000 to 30,000 25,000
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 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