A small device available commercially uses litmus paper and a spectrophotometer to analyze vapors from hot liquid asphalt binders and mixtures to determine the percentage of antistripping additive present. Approximately 60 five-point additive content-color index count regressions were performed on binders and mixtures to determine how well the device measured additive content. The regressions basically fit the quadratic format used by the manufacturer in the recommended calibration process. The regressions were best when the litmus color index count was calculated by subtracting the initial count of the blank strip from the final count after exposure from the mixtures. Changes to the instrument software and testing temperature were necessary as the investigation progressed to accommodate different grades of binders. After the planned testing was completed, some retesting of the binders was performed with the use of modified equipment and procedures. The changes appeared to improve the consistency of the results. Test results with mixtures were less accurate than for binders; however, if the vapor trap is modified as described, the accuracy for mixtures should be improved substantially. Because the test can be performed quickly, it would be possible to perform multiple tests on a sample. Multiple tests would increase the confidence of the test results.
Abstract