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UNITED STATES DEPARTMENT OF COMMERCE. Maurice H. Stans, Secretary
NATIONAL BUREAU OF STANDARDS. Lewis M. Branscomb, Director
Natural Weathering of Mineral Stabilized
Natural Weathering of Mineral Stabilized Asphalt
Coatings on Organic Felt
Sidney H. Greenfeld
Sixteen years of outdoor weathering of laboratory-prepared smooth-surface and mineralsurfaced, felt-base roofing specimens has provided information on the effects of mineral additives on the durability of coating-grade roofing asphalts. Six finely divided mineral additives (blue black slate, clay, dolomite, fly ash, mica, and silica) were evaluated at concentrations up to 60 percent in California, Mid Continent and Venezuela asphalts. The mineral-surfaced specimens are all performing satisfactorily, and show only minor degrees of degradation. Of the smoothsurfaced specimens, the Mid-Continent asphalt performed the best and the California asphalt the poorest. The mica and blue black slate increased the durabilities of all three asphalts at all concentrations and two coating thicknesses. Fly ash, clay, dolomite, and silica were beneficial in some combinations, but had little effect in others. In general, these early results from outdoor exposure tend to corroborate the results obtained on these coatings exposed in weatherometers. Key words: Additive; asphalt; durability; felt; stabilizer; weathering.
The most certain way to determine how a product will weather in a given geographical area is to expose the product and observe it over the years. For materials or systems of low life expectancy, this procedure has been proven extremely satisfactory. For building materials, where the life expectancy is of the order of decades, the observed frequently outlives the observers. Those who design the tests, know their background and appreciate the significance of results, pass out of the picture and are replaced by others who find current work of greater interest. In many instances, the current production is no longer strictly comparable to that being observed.
Therefore, it has long been the practice to design rapid methods for evaluating materials and simultaneously exposing the same materials to the weather. The results of the laboratory evaluation become available in a relatively short period of time; those of the simulated-service tests (outdoor exposures) get lost in the shuffle and the validity of the rapid tests is never proven.
This study was designed in 1947 by the Research
Committee of the Asphalt Roofing Manufacturers Association to include a simulated-service study and several degrees of accelerated evaluation of the system: coating asphalt and mineral additives. The laboratory phase of the work was completed in 1954 and a series of reports and publications issued [1, 2, 3] 2. The verification of the conclusions drawn in these articles will be found in the results .obtained for the outdoor tests. This paper is in the nature of a progress report of the first 15-17 years of these outdoor tests.
1.1. Scope of Study
The mineral additive-asphalt systems included in this study consisted of three asphalts and six mineral additives at three concentrations on one grade of shingle felt. The coatings were applied in two film thicknesses, and were exposed unsurfaced and surfaced with roofing granules.
1 Formerly the Asphalt Roofing Industry Bureau.
? Figures in brackets indicate the literature references at the end of this paper.
2. Experimental Details
2.1. Types of Specimens
Three types of specimens were included in this study. The basic specimen was a smooth-surface coated felt. These were exposed with coating thicknesses of 15 and 25 mils,3 were back-coated with the same coating blend, and were dusted with mica retained on a No. 100 sieve.
1 Mil= 0.025 mm (approx).
The first type of specimen was made on 9- X 12-in1 felts and trimmed to 7 X 11 in prior to exposure to remove any edge effects.
The second type of specimen also had a felt base. In addition to being front and back coated, it was surfaced with granules. These, too, were trimmed to 7 x 11 in.
41 in 2.54 cm.
1 Plasticity index 34. Plastic limit = 34. ASTM method D424-39. Isopropyl alcohol displacement.
Low temperature nitrogen adsorption-B.E.T. method.
ASTM D281-31, using a mineral oil and water instead of the specified oil.
• Turns phenolphthalein pink in aqueous solution.
Lerch, W., and Bogue, R. H., Ind. Eng. Chem. 2, 296-300, 1930.
• Sedimentation in isopropyl alcohol.
? Supplier's analyses.