Pavement Design CE 453 Lecture 28

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

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

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

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

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

Flexible Pavement Typical Applications Traffic lanes Auxiliary lanes Ramps Parking areas Frontage roads Shoulders

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

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

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

Basic AASHTO Rigid Pavement Design Method Select terminal serviceability Determine number of ESALs Determine the modulus of sub-grade reaction Determine the slab thickness

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

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)

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

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

Economic Analysis Different treatments results in different designs Evaluate cost of different alternatives

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

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

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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