John M. Holt, P.E., Texas Department of Transportation, Amy Smith, P.E. Texas Department of Transportation
The most structurally taxed element in a bridge is its deck. A typical bridge deck receives constant pounding from heavy truck wheels, is the element most exposed to the environment, and in some regions treated with corrosive de-icing chemicals several months of the year. If a bridge needs rehabilitation work, it is highly likely that the deck’s condition is the cause.
As a result, decks consume much time from owners before, during and after their construction – specifying quality materials, holding pre-pour meetings, ensuring adequate construction and curing, monitoring and inspecting for cracks and corrosion, and placing and replacing waterproofing and overlays – all in an effort to ensure a long deck life. It should be no surprise, then, that decks have been the subject of numerous research studies and reports, many of which focus on cracking and reinforcement corrosion.
Role of a Deck’s Top Mat of Reinforcement
The top mat serves three primary purposes:
- provide adequate strength for wheel loads at the strength limit state,
- provide strength in the overhangs for traffic rails and barriers subjected to vehicular impacts at the extreme event limit state, and
- control crack widths for long-term serviceability.
The second and third purposes are the top mat’s most important, as research has shown conventional top mat reinforcement in conventional decks does not approach yield when a deck reaches its ultimate resistance to wheel loads since the failure mechanism typically is punching shear1. Conventional decks on multi-girder superstructures typically do not behave in flexure at the strength limit state as some design specifications might imply, and as such, do not require the amount of reinforcement such a design methodology leads to.
Since a deck’s top mat of reinforcement is the mat most exposed to de-icing chemicals and most likely to exhibit corrosion problems, it deserves great attention from designers and owners.
Optimized Reinforcement in Top Mat
In recognition of a top mat’s true behavior, TxDOT is migrating from a reinforcement scheme based on the traditional design of AASHTO LRFD Specifications Article 9.7.32 to one more representative of an empirical design. This new design uses less steel in the top mat, although not as little as allowed by AASHTO (Article 9.7.2), with the idea that the amount of steel is optimized to control crack widths and is reduced in volume in order to reduce future corrosion potential.
Previously, the top mat typically used No. 5 bars at 6-in spacing in the transverse direction and No. 4 bars at 9-in spacing in the longitudinal direction in both 8-in and 8.5-in-thick decks. The weight of this top mat is 3.0 lbs/SF. The new top mat has No. 4 bars at 9-in spacing in each direction, supplemented with short No. 5 bars at 9-in spacing in the overhang portions to ensure an adequate foundation for traffic railings. This new top mat weighs 1.8 lbs/SF, a reduction of 40 percent. See Figure 1 for a comparison between the old and new TxDOT deck configurations.
The selection of 9-in spacing for the top mat was based on inspections and observations of in-service decks which found adequate crack control was being obtained by the No. 4 bars at 9-in spacing in the longitudinal direction. This amount of reinforcement is 50 percent more than the minimum required by AASHTO – 0.18 sq in/ft – for an empirical deck design.
TxDOT treats transverse deck edges with much more reinforcement, decreasing the bar spacing from 9-in to 3.5-in in the last 4-ft of the bridge deck, perpendicular to the edge. This isolated densification at joints is combined with a 2-in thickening of the deck over a 4-ft width to provide adequate deck strength without the need for diaphragms or other means of deck support and helps control cracking perpendicular to the joint.3
A small number of bridges were built with an empirical deck design in Texas in the early to mid-1980s. These bridge decks were fully cast -in-place (CIP) and used an empirical deck design. Recent inspections of these decks found them performing comparably to decks with traditional reinforcement patterns.
TxDOT is using this optimized top mat on its bridges in conjunction with prestressed concrete sub-deck panels. These sub-deck panels are preferred by contractors and are used on the vast majority of Texas’ bridges. AASHTO LRFD Specifications disallow an empirical deck design with stay-in-place concrete formwork. However, TxDOT-funded research4, 5, 6 demonstrated that the empirical deck system with prestressed sub-deck panels performs as well as, if not better than, fully CIP decks. TxDOT’s prestressed sub-deck panels are an excellent example of prefabricated bridge elements and provide a stiff, crack-free bottom half of deck.
In another departure from past practice, TxDOT is placing the longitudinal bars closer to the deck surface than the transverse bars (See Figure 1 for reinforcing placement comparison). This recognizes the predominance of transverse cracking in bridge decks. Having the longitudinal bars closer to the deck surface engages these bars in crack control sooner.7
References
- Beal, D.B., Strength of Concrete Bridge Decks, Research Report 89, New York State Department of Transportation, July 1981.
- AASHTO LRFD Bridge Design Specifications, 6th Edition, 2012.
- Coselli, C.J., E.M. Griffith, J.L. Ryan, O. Bayrak, J.O. Jirsa, J.E. Breen, and R.E. Klingner. 2004. Bridge Slab Behavior at Expansion Joints, FHWA/TX-05/0-4418-1, University of Texas, Austin, TX.
- Fang, I.-K., J.A. Worley, N.H. Burns, and R.E. Klingner. 1986. Behavior of Ontario-Type Bridge Decks on Steel Girders, FHWA/TX-86/78+350-1, University of Texas, Austin, TX.
- Tsui, C.K., N.H. Burns, and R.E. Klingner. 1986. Behavior of Ontario-Type Bridge Deck on Steel Girders: Negative Moment Region and Load Capacity, FHWA/TX-86/80+350-3, University of Texas, Austin, TX.
- Kim, K.H., J.M. Dominguez, R.E. Klinger, and N.H. Burns. 1988. Behavior of Ontario-Type Bridge Decks on Steel Girders, FHWA/TX-88+350-4F, University of Texas, Austin, TX.
- Durability of Concrete Bridge Decks: A Cooperative Study, 1970, Portland Cement Association.