June 2002
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An EVT review | Technology Today

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An EVT review

by Mark S. Graham
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Twenty-five years ago, NRCA adopted the equiviscous temperature (EVT) concept for the application of interply mopping asphalt in built-up roof (BUR) systems. Since then, this concept has become widely accepted and adopted by the U.S. roofing industry. However, it sometimes is incorrectly applied to purposes other than those for which it was intended.

If you install hot-applied, bitumen-based roof systems, it would be useful to review the EVT concept and know when it applies.

Background

In 1977, after more than four years of research and experiments, NRCA adopted the EVT concept and issued its Technical Bulletin #2.

The research revealed that the critical property for mopping asphalt is the flow of the asphalt at the application point. The experiments proved that the ideal flow, adhesion and mopping weight for asphalt for BUR systems are achieved when the asphalt's viscosity is about 125 centistokes (0.125 Pa•s) immediately before application, such as in a mop...



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