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additives, the thicker films are performing better and the asphalts are tending to fall in the same order of durability. It is also apparent, with one exception, that coatings with higher concentrations of blue black slate are deteriorating more slowly than those with lower concentrations.
For materials with other than those with platelike particle shapes, the response to mineral additives cannot be discussed in general terms . The other additives used tended to have blocky to spherical shapes; their performances were varied. in the three asphalts.
Fly ash was the next best performer in that it increased the durability of some coatings as much as 170 percent and in no instance were coatings containing fly ash less durable than the asphalt with which they were made. The higher concentrations again produced the best performers and the thicker films lasted longer than the corresponding thinner ones. Most of the Mid-Continent coatings with fly
California asphalt without additives. Thus, the pattern established by the unstabilized asphalts has been carried through to the coatings containing fly ash.
The effects of clay, dolomite, and silica on the durability of the smooth-surface specimens were more variable than those of blue black slate, fly ash, and mica. The silica was most beneficial in the California asphalt, for it increased its durability (25 percent coating spalled) as much as 80 percent. In all instances it increased the durability by at least 10 percent. Similarly, in the Mid-Continent asphalt it also increased the durability of the asphalt by at least 10 percent; the 25-mil coatings with 50 and 60 percent silica have not failed after 200 months, or a 15 percent greater time than the straight asphalt specimens. The specimens made from the Venezuela asphalt-silica system again were in between those with Mid-Continent and California asphalts, but the inprovement in durability by the silica was not as consistent as in the other two asphalt systems. Three of the coatings performed only as well as the asphalt alone, one not quite as well, one a little better, and the one with 60 percent silica 25 mils thick is still performing well after 200 months.
In only a few combinations containing clay or dolomite were improvements in durability of more than 20 percent observed. Fifty percent clay increased the durability of the California asphalt in both thicknesses by 60 percent. Fifty percent dolomite improved the thin films of both Mid-Continent and Venezuela asphalt by 30 percent. However, the other combinations performed within 20 percent of the durability of the straight asphalts.
Thus, in summary, the Mid-Continent asphalt was more durable than the Venezuela asphalt, which was more durable than the California asphalt. The mineral additives either improved the durability or had little effect. Mica and blue black slate always improved the durability of the three asphalts. Fly
ash, silica, dolomite, and clay improved the durability to a lesser extent and only in specific combinations of composition and coatings thickness.
Figure 3 is a photograph of some of the specimens made with Mid-Continent asphalt after 161⁄2 years of exposure. The test specimens are the five in each column under the identification tags, as identified in figure 3.
The variation of performance with composition and coating thickness is evident in this photograph.
5.2. Granule-Surfaced, Felt-Base Specimens
Prior to exposure, samples of all of the granulesurfaced specimens were subjected to granule adhesion tests. The procedure used was ARIB Index No. 1.119 , using the electrically operated equipment. As recommended in the procedure, the average weight loss for 10 2-X 10-in specimens when abraded dry must be in the range of 0.2 to 0.7 g. The only composition that produced specimens that did not fall within this range was 60 percent blue black slate in Venezuela asphalt. A second set of specimens was made, but it also did not pass the test. Both sets of specimens were exposed and both experienced large losses of granules early during their exposure. The California specimens containing clay passed the test marginally. They, also, lost some granules early during exposure. However, none of these specimens has failed by the established criteria.
Figures 4 and 5 are photographs of California and Venezuela specimens after 15 years of exposure. The unstabilized asphalt coatings show noticeable losses of granules and cracking of the coatings. Cracking of the coatings is also evident in the California coatings containing clay. However, all of the granule-surfaced specimens are still performing well after 15 years and should give many more years of protection from the weather.
Correlation With Exposures in Accelerated Weathering Machine
Because most asphalt-prepared roofing is exposed as granule-surfaced material, the ultimate correlation must be between the granule-surfaced specimens exposed outdoors and the specimens exposed in the accelerated weathering test. Because none of the granule-surfaced specimens has failed by any of the established criteria for failure, a discussion of correlation would be meaningless. However, many of the smooth-surface, felt-base specimens have failed. While no quantitative correlation can be made until more specimens fail, certain parallels can be seen in figure 6 between the performances of these specimens and their corresponding aluminumbase specimens exposed in the weatherometer. The dots represent the average durability of four specimens on aluminum panels exposed in the weatherometers and the bars represent the average durability of two felt-base specimens exposed outdoors. Based on those specimens exposed outdoors that
have failed, it is quite apparent that many of the findings in the weatherometer test  can be correlated with the exposures outdoors.
The California asphalt (Asphalt I in reference ) produced the least durable specimens. The Venezuela asphalt (Asphalt III in ref.  produced the next more durable specimens, and the MidContinent asphalt (Asphalt II in ref.  )the most durable specimens. However, for some systems there was little or no difference in the durabilities of corresponding coatings made from the latter two asphalts.
For all three asphalts, mica (shown in tables 6 and 7, but not in fig. 6) made the greatest improvement in performance. Blue-black slate was also beneficial to all three asphalts. And dolomite, just as in the weatherometer tests, did not appreciably affect the durability of the three asphalts. On the other hand, clay, fly ash, and silica all are performing