DEVELOPMENT OF A FLEXIBLE PAVEMENT FATIGUE MODEL FOR WASHINGTON STATE (OVERLAY, DEFLECTION, ASPHALT CONCRETE STIFFNESS, SEASONAL VARIATION, EQUIVELENCY TRAFFIC FACTORS)

Flexible pavements comprise 94 percent of the Washington State highway system. Each year a large percentage of this network is rehabilitated or reconstructed.;Fatigue cracking is the primary type of failure in Washington state. In addition, it has been found that the major cause for overlaying a pavement is most often severe cracking and patching.;There are at least four methods to define the fatigue relationship. These methods are: theoretical analysis of existing design curves, analysis of the performance of in-service pavements, Laboratory fatigue testing and a combination of these three methods. The development of the fatigue model in this study was based on comparing the performance of the in-service pavements with a laboratory fatigue model.;As in the WSDOT Pavement Management System, pavement sections with known traffic records and performance history offered a good way of establishing the failure criteria. Accuracy was improved by determining the material properties through deflection testing and by limiting the analysis to those sections which are known to have failed by fatigue.;The aging highway system, increased truck weight, increased traffic volume, and the increased cost of pavement construction and rehabilitation make it imperative to have a rational method of evaluating the pavement conditions and design the overlay thickness required to upgrade the pavement.;Ten sites were selected for the fatigue analysis. The common criterion between these sections is that they are original construction or reconstructed asphalt concrete pavements. Deflections were measured using the Falling Weight Deflectometer.;The BISDEF Computer program was used to backcalculate the resilient moduli of the pavement structures. The maximum tensile strains at the bottom of the asphalt layers were determined using the PSA2D2A Computer program.;The strains and the corresponding resilient moduli were used to calculate the number of load repetitions until failure for each season. Then the actual number of 18-kip load repetition for each site was determined. The ratio of the actual number of load repetition to the calculated number of load repetitions is defined as the shift factor or the relationship between distress and performance.