This study was conducted to determine the wear characteristics of bridge decks overlayed with Class D (open-graded) asphalt concrete. A total of 74 bridges were inspected and rated during the period May to August, 1984.
A rating system was developed to allow a numerical comparison between bridges, and also to establish levels of tolerable deficiencies. The rating value considers such things as cracking, rutting, raveling, surface texture and break-up. Values for a variety of problem areas were assigned in ascending order, from zero to 27, according to severity. To use the system the bridge deck is inspected and values assigned according to visual observations. For example, if a bridge deck had some raveling, severe rutting and a transverse crack at the expansion joint, a rating of 5 (some raveling) + 6 (severe rutting) + 2 (transverse expansion joint crack) = 13 would be applied.
After comparing the ratings of all 74 bridges, three levels of serviceability were established. Category A represents bridge ratings of five or less and are considered to be in near perfect condition. Category B represents ratings in the range from 6 to 10. These indicate potential problems that may require attention in the near future. Category C represents ratings of 11 and greater. Bridges in this category require immediate action to prevent failure of the deck overlay.
Of the bridges that were evaluated, 86% fell in category A, 8% in category B and 6% in category C. With one exception, all of the bridges in categories B and C are subject to extremely heavy, high volume, high speed traffic. On the other hand, a great many category A bridges are subject to the same conditions.
None of the problems observed could be attributed to the open graded asphalt mix. Rather, most of the problems seem to emanate from the substrate and would be present regardless of the class of asphalt overlay.