Pavement Design

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Pavement Design

• CE 453 Lecture 28

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Objectives

• Understand and complete ESAL calculation
• Know variables involved in and be able to
  calculate required thickness of rigid and
  flexible pavements

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AASHTO Pavement Design Method Considerations

• Pavement Performance
• Traffic
• Roadbed Soil
• Materials of Construction
• Environment
• Drainage
• Reliability
• Life-Cycle Costs
• Shoulder Design

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• Two Categories of Roadway Pavements
• Rigid Pavement
• Flexible Pavement
• Rigid Pavement Typical Applications
• High volume traffic lanes
• Freeway to freeway connections
• Exit ramps with heavy traffic

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• Advantages of Rigid Pavement
• Good durability
• Long service life
• Withstand repeated flooding and subsurface water
  without deterioration

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• Disadvantages of Rigid Pavement
• May lose non-skid surface with time
• Needs even sub-grade with uniform settling
• May fault at transverse joints
• Requires frequent joint maintenance

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Flexible Pavement Typical Applications

• Traffic lanes
• Auxiliary lanes
• Ramps
• Parking areas
• Frontage roads
• Shoulders

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• Advantages to Flexible Pavement
• Adjusts to limited differential settlement
• Easily repaired
• Additional thickness added any time
• Non-skid properties do not deteriorate
• Quieter and smoother
• Tolerates a greater range of temperatures

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• Disadvantages of Flexible Pavement
• Loses some flexibility and cohesion with time
• Needs resurfacing sooner than PC concrete
• Not normally chosen where water is expected

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Basic AASHTO Flexible Pavement Design Method

• Determine the desired terminal serviceability, pt
• Convert traffic volumes to number of equivalent
  18-kip single axle loads (ESAL)
• Determine the structural number, SN
• Determine the layer coefficients, ai
• Solve layer thickness equations for individual
  layer thickness

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Basic AASHTO Rigid Pavement Design Method

• Select terminal serviceability
• Determine number of ESALs
• Determine the modulus of sub-grade reaction
• Determine the slab thickness

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Variables included in Nomographs

• Reliability, R
• Incorporates a degree of certainty into design
  process
• Ensures various design alternatives will last the
  analysis period
• Resilient Modulus for Roadbed Soil, MR
• Generally obtained from laboratory testing

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Variables included in Nomographs

• Effective Modulus of Sub-Grade Reaction, k
• Considers
• Sub-base type
• Sub-base thickness
• Loss of support
• Depth to rigid foundation
• Drainage Coefficient, mi
• Use in layer thickness determination
• Applies only to base and sub-base
• See Tables 20.15 (flexible) and 21.9 (rigid)

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Other Growth Rates

• Multiple payment compound amount factor, with i
  growth rate
• G (1i)n-1/i

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Flexible Pavement Design

• Pavement structure is a multi-layered elastic
  system, material is characterized by certain
  properties
• Modulus of elasticity
• Resilient modulus
• Poisson ratio
• Wheel load causes stress distribution (fig 20.2)
• Horizontal tensile or compressive
• Vertical maximum are compressive, decrease with
  depth
• Temperature distribution affects magnitude of
  stresses

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Components
Sub-grade (roadbed) course natural material
that serves as the foundation of the pavement
structure Sub-base course above the sub-grade,
superior to sub-grade course Base course above
the sub base, granular materials such as crushed
stone, crushed or uncrushed slag, gravel, and
sand Surface course upper course of the road
pavement, should withstand tire pressures,
resistant to abrasive forces of traffic, provide
skid-resistant driving surface, prevent
penetration of surface water 3 inches to gt 6
inches
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Economic Analysis

• Different treatments results in different designs
• Evaluate cost of different alternatives

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Sensitivity Analysis

• Input different values of traffic volume
• Compare resulting differences in pavement
• Fairly significant differences in ADT do not
  yield equally significant differences in pavement
  thickness

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OTHER ISSUES

• Drainage
• Joints
• Grooving (noise vs. hydroplaning)
• Rumble strips
• Climate
• Level and type of usage

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FAILURE EXAMPLES

• Primarily related to design or life-cycle, not
  construction
• All images from Distress Identification Manual
  for the Long-Term Pavement Performance Program,
  Publication No. FHWA-RD-03-031, June 2003

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FATIGUE CRACKING
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RUTTING
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SHOVING
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PUMPING