In 1961, the department initiated a study to determine the uniformity of asphalt-concrete top course mix, and the resulting information was used to establish realistic gradation control specifications. During the years between 1961 and 1964 research crews visited 55 asphalt plants where they obtained 868 hot-bin samples and 682 mix samples. Data were processed and analyzed by electronic computer and from the results it is concluded that the mix gradation depends on the method of testing (i.e., hot-bin analysis or extraction test). Neither method is totally superior to the other, but they complement each other. The hot-bin method is more meaningful when related to coarse aggregate than when related to fine aggregate, while the reverse is true with the extraction test. Job-mix formula tolerances developed from this study are realistic and fair to both the producer and the department, and are now being used on a statewide basis. /author
Abstract