Associate Professor, University of Illinois at Chicago, Chicago, IL
Design engineer, Alfred Benesch & Company, Chicago, IL
In the Strategic Highway Research Program (SHRP) a test method for identifying D-cracking susceptible aggregates in approximately eight days was developed to replace the widely used but time-consuming rapid freezing-and-thawing test using ASTM C 666 test methods. The basic assumption in this new method is that the hydraulic pressure expected in concrete aggregates during freezing and thawing can be simulated by subjecting aggregates, submerged in water, to high pressures and the extreme rapid release of the pressure. The percent fracture, percent mass loss, and hydraulic fracture index are the parameters calculated as a result of the test, which is commonly called the Washington Hydraulic Fracture Test (WHFT). A number of states have conducted tests using this apparatus on the same aggregate source and have found a scatter in the results. These reports prompted research on a newer modified apparatus that accounts for this variability by allowing for a larger specimen and/or sample size and also introducing a computer interface. The WHFT 97 is completely automated in terms of controlling the testing procedure for each respective ten cycles of operation, and requires minimal manual labor.
Twenty-one different aggregate sources, with different degrees of freezing and thawing susceptibility, were tested according to the WHFT test procedure on both machines (the original WHFT 94 and the WHFT 97) to establish a correlation, if possible, between the results of the proposed WHFT test and those reported for the ASTM C 666 Method B as modified by the Illinois Department of Transportation (ILDOT). In addition, a petrographic analysis was conducted on each aggregate source in order to determine its percentage of air voids, pore size, and pore size distribution. A direct correlation was not established; hence, the test data were compared using the pass/fail criteria to establish the potential of the UIC Modified WHFT 97 as a screening test. The failure criteria presented by the percent fracture (2%) from the WHFT 97 test and 0.060% maximum allowable expansion from the ILDOT freezing and thawing test were used. This comparison indicated that 67% of the test results on the WHFT 94 and 76% on the WHFT 97 were identified correctly based on the ILDOT criteria. All the aggregate types (dolomite, gravel, ACBF slag) were successfully classified using these failure modes with the exception of one aggregate type, limestone. It appears, based on this comparison, that the WHFT 97 test may have the potential to be used as a screening test prior to the ASTM freezing and thawing test. In addition, the petrographic investigation showed potential in rating limestone aggregate as well as determining if other testing is required to identify D-cracking susceptible aggregate.
Paper ID: CCA10471J