University of California Pavement Research Center

This technical memorandum summarizes a study that investigated the use of a confined compressive strength (CCS) test to replace the R-value test currently used for some pavement design and for quality control/quality assurance in California. The report covers a literature review, development of an alternative test, and a comparison of the results from the proposed test with R-value results.

The following are conclusions made based on the findings. First, California Test 216 needs revision. The compaction density reported by the test method is lower than the physical density of the compacted specimen in the CT 216 apparatus. The reported CT 216 density is approximately 88% of Modified Proctor density. Second, the proposed specimen compaction procedure, using Modified Proctor as the reference density, produced unbound specimens that could be handled and tested in a triaxial cell. Third, there is no correlation between the R-value test and the CCS test for the materials tested to date because the limited range of R-values produced by the 22 base and one subbase materials is too small to define a relationship with the CCS test. Most of the R-values of the materials tested were just above the minimum specified value of 78 for Class 2 aggregate base. Fourth, the suggested preliminary CCS criteria for replacement of the R-value specification for Class 2 aggregate base are minimum 101 psi peak stress at 5 psi confinement and minimum 144 psi peak stress at 10 psi confinement.

The following interim recommendations are made based on the testing, the documented low test precision of the R-value test, and the limitations of developing a correlation between R-value and the CCS test. First, additional testing of four or five subbase materials with R-values between 40 and 60 should be completed to refine the CCS test criteria. If these materials are not available from producers, they can be manufactured in the UCPRC laboratory by blending available materials. Second, material properties, including gradation, flakiness, crushed faces, sand equivalence, Atterberg limits, and moisture sensitivity (i.e., shape of the optimum moisture content curve) should be considered when analyzing any correlations.