PRE CAST TEES AND SLABS BASIC INFORMATION AND TUTORIALS

Precast slabs are available in hollow, cored, and solid varieties for use on floors, walls, and roofs. For short spans, various types of panel and channel slabs with reinforcing bars are available in both concrete and gypsum. Longer spans and heavy loads most commonly involve cored units with prestressed wire.

The solid panel and channel slabs are available in heavyweight and lightweight aggregates. The thicknesses and widths available vary considerably, but the maximum span is generally limited to about 10 feet.

Some slabs are available tongue-and-grooved and some with metal-edged tongue and- groove. These types of slabs use reinforcing bars or reinforcing mesh for added tension strengths.

These lightweight, easy-to-handle nail, drill, and saw pieces are easily installed on the job over the supporting members. A clip or other special fastener should be used in placing the slabs.

Cored units with prestressed wire are used on roof spans up to about 44 feet. Thicknesses available range from 4 to 16 inches with various widths available, 40 and 48 inches being the most common.

Each manufacturer must be contacted to determine the structural limitations of each product. The units generally have high fire resistance ratings and are available with an acoustical finish. Some types are available with exposed aggregate finishes for walls.

Specifications.

The type of material used and the manufacturer specified are the first items to be checked. The materials used to manufacture the plank, type and size of reinforcing, and required fire rating and finish must be checked. The estimator should also note who cuts the required holes in the planks and who caulks the joints, and the type of caulking.

PRECAST AND PRESTRESSED DECK SLABS BASIC AND TUTORIALS

The deck is usually the first element in a bridge to deteriorate and to require funds for rehabilitation. In situations where traffic volumes are high, it is often necessary to rehabilitate or replace the deck in sections during off-peak periods.

Because of the time required for site-cast concrete to cure, a number of replacement strategies have been developed using prefabricated deck slabs (Issa et al., 1995a,b). Most of the systems involve a transverse segment (Figure 33.11) connected to the supporting beams with a rapid-curing polymer or hydraulic cement concrete.

FIGURE 33.11 Prestressed deck slabs. (From Sprinkel, M.M., Prefabricated Bridge Elements and Systems, NCHRP Synthesis 119, Transportation Research Board, Washington, D.C., 1985.)


Shear transfer between adjacent slabs is achieved through the use of grouted keyways, site-cast concrete, and post-tensioning. Composite action is achieved through the use of studs on steel beams that extend into voided areas in the slabs that are then filled with polymer or hydraulic cement concrete.

Precast deck slabs can behave in a full-composite manner when connected to steel stringers with studs and epoxy mortar and when keyways are grouted with epoxy mortar (Osegueda et al., 1989).

An earlier study identified some suitable connection details and concluded that the deck slabs are more economical than site cast concrete because of the structural efficiency provided by post-tensioning and prestressing and because of the reduced construction time (Berger, 1983).

Improved connection details for the use of panels on steel beams and prestressed concrete beams have been developed (Tadros and Baishya, 1998).

More recently, a special loop bar reinforcement detail has been developed to provide live load distribution across transverse and longitudinal joints (see FHWA, 2004). A new full-depth precast prestressed concrete bridge deck slab system has been developed that includes stemmed slabs, transverse grouted joints, longitudinal post-tensioning, and welded threaded and headless studs (Tadros and Baishya, 1998).

The deck slabs are thinner and lighter than a conventional deck and can be constructed faster. Prestressed deck slabs typically have been used on major bridge deck replacement projects (Figure 33.12) such as the Woodrow Wilson Bridge (Lutz and Scalia, 1984).

FIGURE 33.12 Prestressed post-tensioned deck slabs were installed at night to replace the deck of the Woodrow Wilson Bridge.


Also, most replacements have involved the use of transverse slabs. The decks on the George Washington Memorial Parkway were replaced using precast longitudinally post-tensioned transverse deck slabs (Jakovich and Alvarez, 2002).

A latex-modified concrete overlay was placed over the slabs. The truss spans of the deck on I-95 in Richmond, Virginia, were recently replaced with night lane closures using the full-depth transverse deck slabs (Figure 33.13).

FIGURE 33.13 Special loop bar connection detail for deck slabs. (From FHA, Prefabricated Bridge Elements and Systems in Japan and Europe, Summary Report, International Technology Exchange Programs, Federal Highway Administration, Washington, D.C., 2004)


The slabs were also used to replace the deck on Route 50 in Fairfax County, Virginia (Babaei et al., 2001). The Virginia Department of Transportation first used transverse precast deck slabs to replace a deck on Route 235 over Dogue Creek in Fairfax County in 1981 (Sprinkel, 1982).

Longitudinal slabs were successfully used to rehabilitate the Freemont Street Bridge (Smyers, 1984), and a new bridge was built in Thailand (Zeyher, 2003).

Longitudinal, partial-depth, or full-depth deck slabs that that are precast on one or more concrete or steel beams have also been used successfully (FHWA, 2004). The superstructure elements are set next to each other and are typically connected by transverse post-tensioning in the deck and diaphragms between the beams.

Keyways in the deck are grouted. The deck on I-95 in Richmond, Virginia, was recently replaced with night lane closures using the full-depth deck slabs on steel beam superstructure elements. When partial depth deck superstructure elements are set next to each other, reinforced site-cast concrete facilitates the connection of the elements.