Translucency of mineral aggregrates for built-up roofs

The investigation involved the application of ASTM Method D 1866-64 by subjecting individual pieces of aggregate to an ultraviolet light source. Using the same light source and the same method of photograhic interpretation but using graded aggregate layers, it was noted that unreasonable layer thicknesses would be required to prevent penetration of light through void spaces. The method was further modified by replacing the photographic interpretation with a phototube to obtain a quantitative measure of transmitted light. Aggregate types and thicknesses influenced phototube current and consequent voltage drops were measured by a sensitive voltmeter. Results show that only a small amount of ultraviolet light is transmitted through thicknesses normally used in roofing application with an aproximate 0.03 per cent difference between white marble and gravel. Results suggest that the advantage of white aggregate in reducing surface temperature may outweigh any disadvantages from its translucency.