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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

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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:

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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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 > 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