AN ANALYTICAL INVESTIGATION OF AASHTO LOAD EQUIVALENCIES

Abstract

An objective of this study was to develop procedures and/or refined relationships between Kentucky ESALs and AASHTO ESALs. Kentucky load equivalency relationships are the result of mechanistic analyses based on elastic theory. AASHTO load equivalency relationships were developed from recorded empirical data collected at the AASHO Road Test. Comparison of Kentucky and AASHTO ESALs necessarily in depth analyses of AASHTO load equivalency equations C-19, D-19, and their developmental equations given in the 1972 AASHTO Interm Guide. These equations evolved from the basic format used in analyzing AASHO Road Test data. In this investigation, the repetitions reported in Appendix A of AASHO Road Test Special Report 61E were converted to ESALs using Equation C-16. For Loop 3 (12-kip (53kN) single axieload and 24-kip (108-kN) tandem axieload), the ESALs at service facilities of 3.0, 2.5, 2.0 exceeded the ESALs at failure (Pt = 1.5). The AASHTO design equation, C-13, was used yo calculate the design ESALs for each of the AASHO Road Test pavement sections. The ratio of ESALs at a given P, to ESALs at failure and the ratio of repetitions at the same Pt to repetitions at failure were calculated. Direct correlations of the averages of these calculated ratios occurred for Lane 1 of Loops 5 and 6 and Lane 2 of Loop 6. This suggests that the AASHTO load equivalency relationships correlate best for loads greater than the legal limits. From recorded Kentucky loadometer data collected at stations located on Interstate routes, over 95% of all single and tandem axleloads are less than legal limits. This suggests that the AASHTO load equivalency relationships are not as appropriate to actual traffic loads. Ratios of AASHTO ESALs may be calculated using Equation 6 for flexible pavements and Equation 7 for rigid pavements to estimate combinations of Pt and SN, or Pt and slab thickness. AASHTO load equivalencies are based on pavement serviceability and structural number for flexible pavements, or slab thickness for rigid pavements respectively. Pavement serviceability is based upon measurements of surface roughness, cracking, patching, and rut depth. Pavement fatigue is an inherent parameter. In the Kentucky system, load equivalencies are based upon strain-repositions relationships developed from laboratory tests and matched with theoretical calculated strains based on elastic theory. Inherently included in the Kentucky system in the assumption that surface roughness will increase with traffic, cracking may develop, patches may be constructed, and ruts may develop. The common factor between the two systems is traffic. Measured parameters in on system are inherent in the other system and vice versa.