ADAPTATION OF AASHO INTERIM GUIDE TO FUNDAMENTAL CONCEPTS

Using 1971 vehicle classification counts and truck weights from nine Kentucky locations, equivalent axleloads (EAL's) were calculated by several methods. Apparent discrepancies led to a review of axleload equivalency factors used to estimate either EWL's (equivalent wheel loads) or EAL's. Axleload equivalencies are determined as the ratio of the number of repetitions of a standard or reference load to the number of equivalent (damage-wise) repetitions of the load in question. The choice of equivalency factors can result in as much as a 40-percent difference in calculated EAL's. Most of Kentucky's traffic is made up of axleloads less than 80 kilonewtons (18 kips). The 1973 Kentucky design guide axleload factors are more severe than either the 1959 Kentucky or 1972 AASHO Interim Guide factors for axleloads less than 80 kilonewtons (18 kips). An extensive effort has been made herein to explain these differences. The AASHO Road Test has provided an independent source of data. In 1970, the Kentucky laboratory CBR of the AASHO Road Test subgrade soil was reported to be 5.2, corresponding to the AASHO soil support value of 3.0. For this study, thickness requirements for a CBR of 5.2 were converted to structural numbers using appropriate AASHO coefficient values. Thus, comparisons of data and designs could easily accomplished. An investigation of the AASHO "design" nomographs and equations indicated values from the equations which produced a line that either passed through the midst of the data points or else had a rather severe rotation and departure from the data. This implies that any pavement thickness design based on that equation has approximately a 50-percent probability of premature fatigue failure. A system for pavement thickness designs shold, however, be predicted upon a line that encompasses a higher percentage of the test data, even as much as 90 to 95 percent of the data points. This would assure a reasonably high probability of successful performance of the pavement during its design life. Kentucky pavement designs have, for years, been predicted on the premise that high volume roads should be thick enough to minimize rutting so as to minimize the costs, inconvenience, and hazards associated with maintenance activities. On the other hand, low volume roads can tolerate considerable rutting and pavement cracking because geometrics generally reduces the allowable speed limit to the point that rutting is not a dangerous factor. These notions of adequate pavement performance can be embodied in the AASHO Road Test concept of Terminal serviceability. Considering an analysis of the combination of equivalencies in repetitions of different axleloads and variable terminal serviceability values, many apparent discrepancies between results of design obtained by the 1959 Kentucky design curves, the 1973 Kentucky design guides, and the AASHO nomographs were acounted for and explained. Several conclusions are noted.

  • Corporate Authors:

    Kentucky Bureau of Highways

    533 South Limestone
    Lexington, KY  USA  40508
  • Authors:
    • Southgate, H F
    • Deen, R C
    • Havens, J H
  • Publication Date: 1974-11

Media Info

  • Features: Figures; Tables;
  • Pagination: 33 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00096814
  • Record Type: Publication
  • Contract Numbers: KYP-56
  • Files: TRIS
  • Created Date: Sep 30 1975 12:00AM