Recalibration of the asphalt layer coefficient
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Recalibration of the Asphalt Layer Coefficient. Dr. David H. Timm, P.E. Mrs. Kendra Peters-Davis. Overview. Current ALDOT pavement design based on AASHO Road Test Structural coefficients (a i ) are key inputs Express relative “strength” of component layers

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Recalibration of the Asphalt Layer Coefficient

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Recalibration of the asphalt layer coefficient

Recalibration of the Asphalt Layer Coefficient

Dr. David H. Timm, P.E.

Mrs. Kendra Peters-Davis


Overview

Overview

  • Current ALDOT pavement design based on AASHO Road Test

  • Structural coefficients (ai) are key inputs

    • Express relative “strength” of component layers

    • Used to determine required thicknesses of layers

  • Current ALDOT asphalt coefficients were officially set in 1990

    • No changes since then


Structural coefficient in design

SN3 SN2 SN1

D1 = SN1/a1

Structural Coefficient in Design

SN1 = a1D1

SN2 = a1D1 + a2D2

SN3 = a1D1 + a2D2 + a3D3


Aashto design equation

Structure

Structure

AASHTO Design Equation

Performance

Soil Strength

Reliability

& Variability

Traffic


Aasho hma coefficients

AASHO HMA Coefficients


Current aldot asphalt coefficients

Current ALDOT Asphalt Coefficients


Problem statement

Problem Statement

  • Given new advances in mixture technology (Superpave, SMA, polymer-modification), there is a need to update the structural coefficient to reflect actual performance in Alabama


Objectives

Objectives

  • Quantify sensitivity of design equation

  • Recalibrate equation to match observed performance


Scope of work

Scope of Work

  • Literature Review

    • Past recalibration efforts

  • Sensitivity Analysis

    • Rank variables from most to least important

  • Recalibration using NCAT Test Track performance data

    • 2003 and 2006 Test Sections


Past recalibration efforts

Past Recalibration Efforts

  • Many studies, few changes

  • Most studies focus on computing a1 from deflection data

  • Previous values range from 0.44 to 0.60

  • Previous Test Track study found 0.59 using very thick sections from 2000 experiment

    • Calibrated to deflection not performance


Sensitivity analysis

Sensitivity Analysis

  • 3-layer pavement (HMA, aggregate base, soil)

  • 5,120 thicknesses calculated

  • Determined correlation coefficients between HMA thickness and input parameters


Results of sensitivity analysis

Results of Sensitivity Analysis


Recalibration procedure

Actual Performance

(weekly IRI measurements)

Actual Traffic

(Loads, Repetitions)

AASHTO ESAL

Equation

a1

pt

DPSI

Measured Traffic

SN

AASHTO Design

Equation

Calibrated

Predicted Traffic

Uncalibrated

Recalibration Procedure

Predicted Traffic

Measured Traffic


2003 test sections

2003 Test Sections


2006 test sections

2006 Test Sections


N1 psi vs date

N1 PSI vs Date

DPSI


N3 psi vs date

Actual

Modeled

N3 PSI vs. Date


N1 predicted and measured traffic

N1 – Predicted and Measured Traffic

a1 = 0.44 (R2 = 0.08)

a1 = 0.55 (R2 = 0.74)


A 1 summary

a1 Summary


Uncalibrated

UnCalibrated


Calibrated

Calibrated


Effect on pavement design

Effect on Pavement Design

18.5% Thinner


Minimum thickness

Minimum Thickness

  • Not calibrated for thicknesses < 5”

  • Need recommendation for thinner sections

  • Lower volume recommendation

    • If new coefficient (0.54) results in thickness < 5”, use old coefficient (0.44)

      • If resulting thickness > 5”; use 5”


Conclusions

Conclusions

  • New advances in mix design technology warrants recalibrating structural coefficient of HMA

  • Structural coefficient has greatest impact of all design variables on pavement thickness

  • Recalibration using NCAT Test Track data resulted in average a1 = 0.54

    • Believed to be conservative estimate

  • Using 0.54 instead of 0.44 yields 18.5% reduction in HMA thickness


Structural coefficient status

Structural Coefficient Status

  • ALDOT has implemented new coefficient for rehabilitation/overlay design

  • ALDOT will soon implement new coefficient on all new construction


Acknowledgements

Acknowledgements


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