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    A Theory for Blistering and Alligatoring in Asphalt Bitumen Built-Up Roofs

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    Liquid asphalt entrains air which should escape. The rate of escape depends, in part, on the thickness and viscosity of the asphalt. The latter varies with the crude. Some asphalts require a higher application temperature or a longer period of time at high temperature to permit the buoyancy of an air bubble to overcome the viscosity and burst through the surface. Too much asphalt can contain more air than can escape before surface cooling to the point where the viscosity is too great. Entrapped air migrates to form larger bubbles. These, in turn, absorb more air and water vapor from the heating-cooling action of the sun. The bubbles grow to the point where the asphalt membrane between bubbles becomes too thin for strength and felt separation can occur.

    The surface tension of the asphalt within the pour coat will strive to form a series of spheres each of which has the minimum surface area for volume. Cracks developing into fissures provide the material which grows thicker and creates rounded corners. The result is alligatoring.


    building materials, durability, asphalt, membrane, air entrainment, viscosity, surface tension, vapor pressures, blistering, alligatoring

    Author Information:

    Macnaughton, AC
    Consulting engineer, Toronto, Ont.

    Committee/Subcommittee: E06.57

    DOI: 10.1520/STP36109S