TO: All Design Section Staff FROM: J. Kapur DATE: October 2, 2002 SUBJECT: Concrete Deck Slab Details
The following detailing practice is intended to minimize the bridge deck cracking in both transverse and longitudinal directions:
·The minimum amount of reinforcement in each direction shall be 0.18 in2/ft for the top layer and 0.27 in2/ft for the bottom layer. The amount of longitudinal reinforcement in the bottom of slabs shall not be less than percent of the primary reinforcement for positive moment as specified in AASHTO LRFD 220.127.116.11.
·Top and bottom reinforcement in longitudinal direction of deck slab shall be staggered to allow better flow of concrete between the reinforcing bars.
·The maximum bar spacing in transverse and longitudinal directions for the top mat, and transverse direction of the bottom mat shall not exceed 12 inches. The maximum bar spacing for bottom longitudinal within the effective length, as specified in LRFD article 18.104.22.168, shall not exceed the deck thickness.
These recommendations are for concrete or steel beam-slab bridges with main reinforcement perpendicular to traffic, and removable formwork.
Recent inspections of relatively new bridge decks indicate increasing transverse shrinkage cracking on the bottom of slabs. The concrete on top of girder flange is restrained while the concrete between girder flanges is free to shrink. This differential shrinkage within the deck slab could be, among other reasons, one of the reasons for transverse cracking. These are typical shrinkage cracks that may be reduced by better distribution of reinforcement as shown in the figure.
If you have any questions regarding these issues, please contact Bijan Khaleghi at 705-7181.
cc: J. Weigel, Bridge and Structures - 47340
F. Higgins, Bridge and Structures - 47340