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Engineering Issues Related to Trucking Pavements

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Title: Engineering Issues Related to Trucking Pavements


1
Engineering Issues Related to Trucking - Pavements
  • AASHTO Subcommittee on Highway Transport
  • Annual Meeting
  • June 9-11, 2004

2

Impact of Truck Size _at_ Weight on Highway Pavements
By Roger Green Andrew Williams of ODOT
3
  • MODELS OF PAVEMENT DETERIORATION
  • The American Association of State Highway
    Transportation Officials (AASHTOs) conducted
    research on pavement damage in the 50s
  • The research involved a test of the performance
    of two basic pavement types, flexible and rigid
    under controlled conditions for different types
    of vehicles and loads
  • The most important factors in evaluating the
    effect of changes in vehicle size and weight on
    pavement deterioration are vehicle loadings and
    volume

4
  • Pavements are designed to accommodate a projected
    number of axle repetitions of a specified
    magnitude for a projected service life
  • To simplify design procedures, pavement design
    methods typically express the damage created by
    mixed traffic loads in terms of a base reference
    axle load known as an ESAL

5
ESALS An ESAL factor is a ratio relating the
damage imparted to the pavement by a passing
vehicle of a specified weight to the damage
caused by an 18,000 lb axle load
6
  • The AASHTO Design procedures indicate the effect
    of traffic loads on pavement condition
  • The effect of a single axle on flexible or rigid
    pavement increases as approximately a fourth
    power function of axle load
  • For example A 36,000 lb single axle load is
    only twice as large as an 18,000 lb axle load but
    it causes 17 times more loss in pavement life

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  • SUMMARY
  • Pavement Damage factors come from the AASHTO road
    test project
  • Spreading a load on single axles is more damaging
    to flexible pavements than rigid pavements
  • Changes in truck size and weight regulations,
    which increase axle loads will also increase
    pavement cost
  • Trucks are far more damaging to roads than
    passenger cars (1 80,000 lb semi 5,900 cars on
    flexible pavement in terms of damage.
  • New Mechanistic Design procedures will use load
    spectrum and design off of it

20
Engineering Issues Related to Trucking - Pavements
  • Current Pavement Design Procedures
  • Future Pavement Design Procedures

21
The AASHO Road Test
AASHO RT
(AASHO, 1961)
22
AASHO Road Test Vehicles
(AASHO, 1961)
23
1993 AASHTO Flexible Pavement Design Equation
24
1993 AASHTO Rigid Pavement Design Equation
25
Axle Type and Load Example
  • 600,000 trucks per year - design lane
  • MR Base 30,000 psi
  • MR Subgrade 7,000 psi
  • Reliability 95
  • So 0.49 flexible, 0.39 rigid
  • aAC Base 0.35, aAC 0.44
  • 2S1 and 3S2 vehicles
  • 12,000 lb front axle

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Engineering Issues Related to Trucking - Pavements
  • Current Pavement Design Procedures
  • Future Pavement Design Procedures

28
Empirical vs. Mechanistic-Based Design
Wood Floor Joist Example
P
P
d
d
L
L
(Schwartz, 2001)
29
Inputs Structure Materials Traffic
Climate Both mean and standard deviations inputs
are required
Selection of Trial Design
Structural Responses (s, e, d)
Performance Prediction Distresses Smoothness
Revise trial design
Design Reliability
Design Requirements Satisfied?
Performance Verification Failure criteria
No
Yes
Final Design
30
Traffic Data Requirements for M-E AnalysisAxle
Type
  • For a given vehicle type determine relevant axle
    types
  • Single axle
  • Tandem axle
  • Tridem axle
  • Quad axle
  • Special vehicles axles

31
Traffic Data Requirements for M-E AnalysisAxle
Load Spectra
  • For a given axle type determine load range and
    spectra
  • Single (1,500 to 40,000 Ibs)
  • Tandem (3,000 to 80,000 Ibs)
  • Tridem (6,000 to 120,000 Ibs)
  • Etc.

32
Flexible Pavement Performance Indicators
  • Fatigue Cracking
  • Permanent Deformation (Rutting)
  • HMAC Thermal Cracking
  • Ride Quality (Smoothness)

33
Conventional - HMA Over Granular Base
34
Fatigue Cracking and Rutting
Pavement Structure
Layer
Material Type
Thickness (in)
Modulus (ksi)
Poisson's Ratio
1
HMAC
6
450
0.30
2
DGAB
12
22
0.35
3
Subgrade
180
7
0.45
4
Bedrock
infinite
1000
0.15
35
Asphalt Fatigue Cracking Wheel Load
36
Rutting Tire Load
37
Rigid Pavement Performance Indicators
  • Jointed Plain Concrete Pavements
  • Joint Faulting
  • Transverse Cracking
  • Ride Quality (Smoothness)

38
Faulting (Pumping Mechanism)
EXPULSION OF WATER AND FINES
DIRECTION OF TRAFFIC
PCC SLAB
PCC SLAB
BASE
SUBGRADE
ACCUMULATION OF
INADEQUATE LOAD TRANSFER
ERODED BASE MATERIAL
CAUSING DIFFERENTIAL DEFLECTIONS
39
Faulting Example
  • PCC elastic modulus 4,000,000 psi
  • Poissons ratio 0.15
  • Subgrade k-value 200 psi/in
  • Base type Granular
  • Traffic, ESAL/yr 1,000,000
  • Slab thickness 9 in
  • Joint spacing 15 ft
  • No Dowels
  • Traffic, ESAL/yr 1,000,000

40
Plot of Faulting vs. ESALs
41
Mechanism of Bottom-Up Transverse Cracking
42
Mechanism of Top-Down Transverse Cracking
43
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