All Bridge Decks Are Not Created Equal

Editorial by Cliff Freyermuth, Manager, ASBI

The second edition of the ASBI “Durability Survey of Segmental Concrete Bridges” enclosed with this edition of the newsletter again indicates that segmental bridges are performing very well with service lives approaching 30 years. The most recent NBI data did not disclose any structurally deficient segmental superstructures or decks. In contrast, a 1997 compilation of NBI data by FHWA representatives (Fig. 2 in the durability survey) indicates that decks of about 22,000 bridges with steel superstructures were structurally deficient. The superior durability performance of segmental bridge decks is considered to result from the following factors:
• Segmental bridge decks are typically prestressed longitudinally and transversely. Elimination of cracking in segmental bridge decks results in greatly enhanced durability.
• Liveload deflections of segmental superstructures are typically 25 to 40 percent of the specified AASHTO maximum deflections (span/800 or span/1000). Both research and practical experience have demonstrated that superstructure flexibility speeds bridge deck decay.
• There is no material incompatibility between decks of segmental bridges and the remainder of the superstructure. Restraint to shrinkage of deck concrete by the supporting elements in other types of construction often contributes to development of transverse cracks in the deck which have a detrimental effect on durability performance.
• Segmental bridge decks generally have a minimum thickness of 8 or 9 inches with substantial filets or haunches at the intersections of the deck and the webs.
• Precast segmental bridge decks have the acknowledged durability advantages resulting from the superior quality control associated with plant-produced precast elements.

Notwithstanding these advantages, the ASBI Board of Directors recommends additional corrosion protection for all bridge decks exposed to aggressive environments. For segmental bridges, it is recommended that this protection should be provided by addition of 1 inch or 1-1/2 inches of concrete cover placed monolithically with the deck concrete.

In the U.S., Europe, and other parts of the world, segmental concrete bridge decks have exhibited excellent durability over the past fifty years. With use of appropriate technology and reasonable maintenance, a minimum service life of 100 years is considered to be readily obtainable.