Three slightly different correlations between Kentucky CBR's and the AASHO Road Test soil support values were developed. The first relationship was made by assuming a logarithmic scale between Kentucky CBR's of 5.2 and 100, which corresponded to values on the soil support scale of 3 and 10, respectively. The Kentucky CBR of 5.2 was determined by performing tests on the AASHO road subgrade soils. For practical purposes, the AASHO Road Test crushed stone base material was assumed to be a "100 percent CBR material" (this assumption was based on CBR data previously reported by Shook and Fang). The second correlation was obtained by assuming a logarithmic scale between Kentucky CBR's of 5.2 and 90, corresponding to values on the soil support scale of 3 and 10, respectively. The third relationship was constructed through computations using actual traffic data, the Kentucky flexible pavement design curves and the AASHO Design Chart (PT = 2.5). Computed soil support values of 3 and 10 corresponded to Kentucky CBR's of 6 and 90, respectively. Computed values of soil support were plotted to an arithmetic scale and Kentucky CBR's were plotted to a logarithmic scale. In a range of Kentucky CBR's varying from about 4 to 40, the relationship was linear, while from 40 to 90, the curve was concave upward. There was reasonable agreement between a Kentucky CBR of 5.2, determined by tests, and 6, determined through computations.

Comments are made regarding the Kentucky CBR testing procedure. In particular, it is noted that Nomographs C and D in Appendix A of the AASHO Guide which relate Kentucky CBR's and soil support values are not valid because the Kentucky CBR testing procedure does not permit the substitution of dynamic compaction in lieu of static compaction for molding CBR soil specimens.

Several ASTM and Kentucky CBR tests were performed at different molding moisture contents and compactive energies on the AASHO embankment soil, four representative Kentucky soils, and one soil from the state of Ohio. These data were compared to CBR data previously reported by Shook and Fang. For CBR's ranging from about 4 to 12, a relationship was developed between Kentucky and ASTM CBR's. Within this range of values, Kentucky and ASTM CBR's are approximately equal. Molding specimens under the static pressure of 2000 pounds per square inch as used in the Kentucky CBR procedure produced specimens with initial dry densities that averaged about six percent higher than those obtained by AASHO Designation: T99-57. CBR's and axial swell values were also higher. For soil specimens molded at the same initial dry densities, CBR's of statically compacted specimens are distinctively lower than those observed for dynamically compacted specimens. For relatively small decreases in initial dry densities, there were very large decreases in CBR's. This probably accounts for discrepancies that have been observed between field and laboratory CBR's.

Report Date


Report Number

No. 297

Digital Object Identifier



The opinions, findings, and conclusions in this report are not necessarily those of the Department of Highways or the Federal Highway Administration.