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Recalibration of the Asphalt Layer Coefficient Dr. David H. Timm, P.E. Mrs. Kendra Peters-Davis

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Overview Current ALDOT pavement design based on AASHO Road Test Structural coefficients (a i ) 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

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Structural Coefficient in Design SN 3 SN 2 SN 1 SN 1 = a 1 D 1 SN 2 = a 1 D 1 + a 2 D 2 SN3 = a 1 D 1 + a 2 D 2 + a 3 D 3 D 1 = SN 1 /a 1

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AASHTO Design Equation Traffic Reliability & Variability Structure Performance Soil Strength

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AASHO HMA Coefficients LoopLayer Coefficient (a 1 ) Test Sections R2R2 20.83440.80 30.44600.83 40.44600.90 50.47 60 0.92 60.33 60 0.81

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Current ALDOT Asphalt Coefficients Pavement MaterialStructural Coefficient Hot Mix Asphalt0.44 Sand Asphalt0.40 Road Mix (Low Stability)0.20 Limestone Agg. Base0.14 Granite Agg. Base0.12

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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

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Objectives 1.Quantify sensitivity of design equation 2.Recalibrate equation to match observed performance

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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

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Past Recalibration Efforts Many studies, few changes Most studies focus on computing a 1 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

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Sensitivity Analysis ParameterRange Layer coefficient (a 1 )0.20 – 0.60 Traffic level (W 18 )1e6 – 1e9 ESALs Resilient modulus (M R )3,000 – 30,000 psi Reliability (R)50% – 99% Change in serviceability (ΔPSI)1 – 2.5 Variability (S o )0.20 – 0.60 3-layer pavement (HMA, aggregate base, soil) 5,120 thicknesses calculated Determined correlation coefficients between HMA thickness and input parameters

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Results of Sensitivity Analysis ParameterCorrelation Coefficient Layer coefficient (a 1 )-0.518 Traffic level (W 18 )0.483 Resilient modulus (M R )-0.425 Reliability (R)0.157 Change in serviceability (ΔPSI)-0.141 Variability (S o )0.083

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Recalibration Procedure Actual Traffic (Loads, Repetitions) Actual Performance (weekly IRI measurements) SN a1a1 PSI ptpt AASHTO Design Equation Predicted Traffic AASHTO ESAL Equation Measured Traffic Predicted Traffic Uncalibrated Calibrated

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2003 Test Sections

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2006 Test Sections

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N1 PSI vs Date PSI

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N3 PSI vs. Date Actual Modeled

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N1 – Predicted and Measured Traffic a 1 = 0.44 (R 2 = 0.08) a 1 = 0.55 (R 2 = 0.74)

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a 1 Summary

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UnCalibrated

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Calibrated

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Effect on Pavement Design 18.5% Thinner

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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”

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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 a 1 = 0.54 –Believed to be conservative estimate Using 0.54 instead of 0.44 yields 18.5% reduction in HMA thickness

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Structural Coefficient Status ALDOT has implemented new coefficient for rehabilitation/overlay design ALDOT will soon implement new coefficient on all new construction

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Acknowledgements

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