As a part of the biennial routine bridge inspections, visual inspection data is obtained for each of the major elements of a bridge, including the deck. This information, reported in the National Bridge Inventory (NBI), is generally used to assign and prioritize specific actions, ranging from minor repairs to full deck replacement. Bridge deck deterioration, however, occurs below the surface and is not apparent visually until significant damage has occurred. Therefore, this visual inspection data does not provide the important subsurface condition information essential to making cost effective maintenance, rehabilitation, and preservation decisions. Given this limitation, an increasing number of agencies are now using subsurface scanning methods such as Ground Penetrating Radar (GPR) and Infrared Thermography (IR). This paper provides a case study which examines the implications of decisions made solely based on NBI type rating data versus decisions made including subsurface data from deck scanning. The study is based on a group of 57 bridge decks recently scanned in Southwest Michigan. The decks have been analyzed for percent deterioration using either GPR, IR, or both. The deterioration quantities derived from these methods are compared to the NBI ratings. Using this comparison, the study shows that, using NBI data alone, deck condition estimates and resulting rehabilitation decisions would often be inappropriate for the actual condition of the structure, and the resulting life cycle costs are significantly higher.

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