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flexible pavment design
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1
Pavement Design
CE 453 Lecture 28
2
Objectives
Understand and complete ESAL calculation
Know variables involved in and be able to calculate required thickness of rigid and flexible pavements
3
AASHTO Pavement Design Method Considerations
Pavement Performance Traffic Roadbed Soil Materials of Construction Environment Drainage Reliability Life-Cycle Costs Shoulder Design
4
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
5
Advantages of Rigid Pavement
Good durability Long service life Withstand repeated flooding and
subsurface water without deterioration
6
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
7
Flexible Pavement Typical Applications
Traffic lanes Auxiliary lanes Ramps Parking areas Frontage roads Shoulders
8
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
9
Disadvantages of Flexible Pavement
Loses some flexibility and cohesion with time
Needs resurfacing sooner than PC concrete
Not normally chosen where water is expected
10
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
11
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
13
Variables included in Nomographs
Effective Modulus of Sub-Grade Reaction, k• Considers:
1. Sub-base type2. Sub-base thickness3. Loss of support4. 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 = [(1+i)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
28
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 > 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
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EXAMPLES
http://training.ce.washington.edu/wsdot/modules/09_pavement_evaluation/09-7_body.htm
http://training.ce.washington.edu/wsdot/modules/09_pavement_evaluation/09-8_body.htm