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better than anticipated from the results of the weatherometer tests. Whereas, in a few of these systems in the weatherometers the coatings with additives were 20 percent less durable than their corresponding straight asphalt coatings, none exposed outdoors was less durable than its corresponding asphalt coating. In the extreme cases, such as 60 percent silica in the 15 mil coatings of all three asphalts, the specimens exposed outdoors were more durable than the corresponding straight asphalt specimens and the specimens in the weatherometers were roughly half as durable. Thus, where the results of the weatherometer tests have not corresponded with those obtained with the exposure outdoors, they have been on the conservative side.
The 25-mil thick coatings were more durable than the 15-mil coatings of the same mixes, but in many instances not 40 percent more durable. Both of these conditions were also apparent in the results of the weatherometer tests.
After 16 and a half years of exposure of smoothsurface and granule-surfaced roofing in Washington, D.C., the following conclusions can be drawn:
(1) All of the granule-surfaced specimens are performing satisfactorily. A few, which failed or were borderline in the granule-loss test, have lost noticeable quantities of granules during
(2) Of the smooth-surface specimens, those with 35 percent mica and 35, 50, and 60 percent blue black slate in all three asphalts have consistently outperformed the corresponding asphalt specimens without additives. The systems containing fly ash, dolomite, clay, and silica gave variable performances; each combination must be considered separately. (3) The thicker coatings are outperforming the thinner coatings.
(4) There were differences in durability among the specimens made with the three asphalts, with the Mid-Continent asphalt being the most durable and the California the least. When mineral additives were mixed with the
asphalts, corresponding differences in durability were observed.
(5) Based on the smooth-surface specimens, the results in the weatherometer tests in most instances predicted the relative durabilities of the coating. Where the weatherometers were different, the difference was in the conservative direction.
This work was sponsored by the Asphalt Roofing Manufacturers Association at the National Bureau of Standards. The author is grateful to the members of the Research Committee for their help in designing and monitoring this program and to William H. Appleton, Robert F. F. Jackson, John Marchi, Shigeru Ishihara, John P. Falzone, Jefferson C. Weeks, Thomas R. Davis, Royce E. Stine, and Thomas Crowe for their help in perparing the exposure specimens and making the periodic inspections during 1623 years of exposure. The author wishes to express his gratitude, also, to the members of the staff of the National Bureau of Standards who helped make this work possible.
 Greenfeld, S. H., A method of preparing uniform films of bituminous materials, ASTM Bul. 193, 50 (1953). Adopted as ASTM Standard Method D 1669-62 and USA Standard A 109.40 - 1965.
 Greenfeld, S. H., Effects of thermal shock on the durability of asphalt coatings under accelerated test, ASTM Bul. 193, 46 (1953).
B3] Greenfeld, S. H., Effects of Mineral Additives on the Durability of Coating-Grade Roofing Asphalts, NBS Building Materials and Structures Report 147, (Sept. 1956).
 ASTM D 225-65, Asphalt Shingles Surfaced with Mineral Granules.
 Pfeiffer, J. P., The Properties of Ashpaltic Bitumen, pp. 157-158 (Elsevier Publishing Co., New York, 1950).
 Abraham, H., Asphalts and Allied Substances, 6th ed., 4, pp. 163-169, (D. Van Nostrand Co., New York, 1962).
 Hoiberg. A., Bituminous Materials 1, 47 (Interscience Publishers, New York, 1964).
[S] Cullen, W. C., Effect of thermal shrinkage on built-up roofing, NBS Monograph 89 (1965).
 Handbook of Physics and Chemistry, 42d edition, 1528, (Chemical Rubber Publishing Co., 1960).  Manual of Test Procedures for Roofing Granules, Asphalt Roofing Industry Bureau, (1956).
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