AASHTO Standard Specification requirements for design of concrete bridge deck slabs on longitudinal beams are based on distribution of loads in the slab according to Westergaard theory and assume flexural action of the slab. On the basis of these specifications, many states have developed design tables and charts for quick determination of slab thickness and both primary (transverse) and secondary (longitudinal) reinforcement. The main variables in the design of the deck slab are
• Beam spacing
• Concrete strength
• Weight allowance for future paving
• Live load (generally HS 25 or, LRFD, HL-93)
• Continuity factor for dead load
Applying the specifications, the simple dead and live load moments per unit width of slab are calculated. Dead load includes the client-specified future paving allowance, weight of any separate wearing surface, and weight of the deck slab including any monolithic wearing surface. Live load is the wheel load(s) of the client-specified HS or HL truck loading. The simple span moments are calculated for the design slab span length, and are then modified for continuity over the beams. For this factor, most states use 0.8 for both dead and live load, but some states use 1.0 for dead load. The moments are factored, and the slab is designed by the strength method using the slab thickness minus any monolithic wearing surface considered subject to loss due to traffic wear. Effective depth dimension d from the compressive face is usually different for the top and bottom steel, because a minimum cover of 1 in (25 mm) is permissible and generally adequate for the bottom of the slab but much greater cover, up to 3 in (76 mm), is specified for the top steel to provide protection against intrusion of chlorides. The rebar diameter is usually different as well, since most agencies maintain the same spacing of top and bottom steel and vary the bar size. However, practices vary among agencies. For example, the New Jersey DOT keeps the bar size the same for top and bottom reinforcement. A uniform spacing makes bar placement and inspection easier and facilitates concrete placement. Secondary steel is provided in accordance with the specifications, with a lesser amount in the outer quarters compared with that in the middle of the distance between beams. But again, practices vary and some states prefer uniform spacing of secondary steel.
Slab overhang beyond outside beams is limited so that the reinforcement furnished for interior panels is adequate for the overhang, or extra reinforcement is provided if required. Slab overhang is sometimes also limited for construction reasons; the weight of fresh concrete on an excessive overhang, acting through a diagonal brace, can cause local buckling of unstiffened steel girder webs or can damage the web of a prestressed-concrete girder.
Some states such as Ohio DOT require that the top distribution reinforcement is placed above rather than below the primary steel. This practice was adopted in recognition of the fact that most deck slab cracking is transverse, and the distribution steel is more effective in resisting that cracking if placed closest to the surface.
Some states continue to use deck slab design tables developed using the allowable — stress design method, while other states have updated using the LRFD method. In addition, some states assign an allowable concrete stress that is less than AASHTO Standard Specifications would allow on the basis of the required 28-day strength of the specified concrete. These conservative practices reflect the prevalent attitude gained from a common experience of premature and extensive bridge deck deterioration, mostly in the form of spalling due to reinforcing bar corrosion. If the preventive measures now being taken prove to be effective in eliminating or greatly reducing this premature deterioration, those states will be more inclined to adopt less conservative design methods.
The design procedures described above have resulted in safe designs. However, research has determined that significant membrane action is present in interior panels, and actual stresses are considerably lower than design stresses calculated on the basis of flexural action. Following laboratory testing, the province of Ontario and several states have constructed and tested full-scale bridges with so-called orthotropic deck slab reinforcement. In these designs, the reinforcement is the same size and spacing in both directions, and of a reduced total amount compared with designs by AASHTO Standard Specifications. These experimental decks have performed well, in most cases.
