Effects of Salt and Its Trace Minerals on Aggregate in Bituminous Pavements

Research on the effects of deicing salts and their trace minerals on bituminous highway wearing courses was prompted by early pavement deterioration of federal and state highways in Indiana. Several bituminous surface courses containing slag, slag-dolomite, or dolomite aggregates showed deterioration soon after deicing salts were first applied. Initially, a detailed literature review (Appendix A of report) was conducted. Data on eighteen dolomite sources from a previous study (Bruner, Choi and West, 1995) plus ten additional dolomite sources formed the basis for the brine freeze thaw testing program. Mercury intrusion porosimetry testing conducted on multiple specimens showed that rock texture significantly affects pore size distribution. Petrographic analysis, acid etching of thin sections and electron microprobe studies revealed the distribution of clays on dolomite grains. Additional brine freeze thaw testing on selected dolomite sources, plus one slag sample, were conducted using deicing salts from six Indiana highway districts. Petrographic evaluation of pavement cores from the distressed pavements was accomplished. Brine freeze thaw test results ranged from 0.6 to 27.3% loss for the 28 samples; this test is a good measure of aggregate durability. INDOT has recommended a maximum 30% loss for the brine freeze thaw test. Regression analysis showed that acid insoluble residue content and sodium sulfate loss have the highest correlation with brine freeze thaw loss; elemental magnesium content has a negative correlation with it. It is concluded that dolomite aggregates with a low insoluble residue content have lower sulfate soundness losses, lower brine freeze-thaw losses and greater durability. Evaluation of pavement cores indicated no evidence of deterioration caused by aggregate weathering or degradation. Brine freeze thaw testing is a good measure of freeze thaw resistance of aggregates in bituminous surface courses and no evidence was found indicating that early deterioration of the pavements studied was caused by salt attack