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22 Percentage Response FIGURE 14 Observed types of defects in waterproofing membrane systems. a. Lack of adhesion between the waterproofing membrane and the concrete deck b. Lack of adhesion between the waterproofing membrane and the asphalt surface c. Punctured waterproofing membranes d. Membrane blistering e. Horizontal shear failure at the membrane f. Cracks in the waterproofing membrane g. Voids under the waterproofing membrane h. Reinforcement corrosion i. Moisture penetration through the membrane but cause unknown j. Other nations, resistivity readings, and crack measurements. The According to Distlehorst (20 ), Kansas currently uses bridge decks were 14 and 15 years old at the time of their res- asphalt membrane overlays only as a rehabilitation measure toration. Fourteen years after installation, both decks receive on existing bridge decks in very bad condition to extend ratings of "good" from the Kansas Department of Transpor- the service life by 3 to 5 years. This was confirmed by the tation (KDOT) bridge management inspectors. KDOT response to the survey for this synthesis. KDOT also uses asphalt membrane overlays to reduce the added dead These results are consistent with an earlier report (21) load when deck rehabilitation is needed on bridges with total that looked at the condition of six bridge decks with asphalt load limitations (20 ). interlayer membrane overlays installed between 1967 and 1971 after 20 to 25 years in service. Three different types of membranes were used: a preformed coal tar and polypro- COSTS pylene sheeting, a coal tar modified polyurethane elastomer membrane covered with an asphalt roofing sheet, and a non- Kepler et al. (8) compared the life cycle costs of 33 differ- woven polypropylene fabric. All three types of membranes ent corrosion protection systems and concluded that the use were overlaid with hot-mix asphalt. The system using the of hot rubberized asphalt membrane was the second-lowest- nonwoven polypropylene membrane was the most effective. cost strategy, with assumed discount rates of 2% and 4%. At