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NOTE: “Lifetime Predictions of Exposed Geotextiles and Geomembranes” was delivered by Prof. Robert M. Koerner as a keynote lecture at GeoAmericas 2016 in Miami. The conference was organized by IGS North America. A most frequently asked question regarding all types of geosynthetics is, “How long will they last?” This paper answers the question for exposed geotextiles and geomembranes assuming that they were properly designed and installed. Furthermore, it compares these new results to earlier lifetime prediction results of a covered geomembrane. Nonexposed (or covered) lifetime conditions have been previously evaluated and published on a 1.5 mm thick high density polyethylene (HDPE) geomembrane. Used in the prediction were landfill incubation devices at four elevated temperatures of 85, 75, 65 and 55°C so as to reach 50% of retained strength and elongation. Considering the three stages of (i) depletion of antioxidants, (ii) induction time, and (iii) 50% reduction in mechanical properties, the lifetime extrapolation was made down to 20°C. The halflife for this geomembrane under these conditions was approximately 450 years. Since the laboratory incubation times took 12-years, other nonexposed geosynthetics were not evaluated under the supposition that the covered situation is generally a moot point for most geosynthetics in their customary applications. For exposed (or uncovered) geosynthetics, however, the situation is quite different. Ultraviolet radiation, elevated temperature and full oxygen are available which shortens the service lifetime, but how much? For evaluation of this situation we utilized laboratory ultraviolet fluorescent tube weathering devices per ASTM D7238 for incubation purposes. Seven different geotextiles and five different geomembranes were evaluated. Each material was incubated at 80, 70 and 60°C until 50% reduction of strength and elongation occurred. The data was then extrapolated down to 20°C for laboratory halflife values and for comparison with the nonexposed condition. The ratio of nonexposed to exposed lifetime for HDPE geomembranes is approximately 7.0. The calculations for the twelve exposed geosynthetics then progressed to using site-specific radiation so as to obtain an equivalent field life. Phoenix, Arizona conditions are illustrated although the procedure is applicable worldwide. Halflife predictions for the geotextiles vary from a few months for the needle punched nonwovens to up to 10-years for monofilaments and high antioxidant formulated products. Results for geomembranes vary from 47 to 97 years with HDPE being the highest. These exposed halflife results (which took 14-years of laboratory incubation and are still ongoing at 60°C) are felt to be most interesting and are presented for the first time to an international audience.