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| FREQUENTLY ASKED QUESTIONS Why use an Exodermic™ deck? An Exodermic™ deck typically weighs 35% to 50% less than a reinforced concrete deck that would be specified for the same span. Reducing the deadload on a structure can often mean increasing the liveload rating. The efficient use of materials in an Exodermic™ deck means the deck can be much lighter without sacrificing strength, stiffness, ride quality, or expected life. Precast Exodermic™ decks can be
erected during a short, nighttime work window, allowing a bridge
to be kept fully open to traffic during the busy daytime hours. An Exodermic™ deck is easily maintained with standard materials and techniques, since the top portion of an Exodermic™ deck is essentially the same as the top half of a standard reinforced concrete deck. If desired, any overlay compatible with concrete can be used, including latex modified concrete, polymer concrete, microsilica concrete, or a membrane with asphaltic concrete overlay. What loads can Exodermic™ decks be designed for? Most projects have been designed for HS-20 or HS-25 loading. Other loading can be analyzed as needed. What Specifications cover Exodermic™ design? The Exodermic™ design is specifically covered in Section 9.8.2.4 of the LRFD code as “Unfilled Grid Deck Composite with Reinforced Concrete Slab”. Significant changes to this and related sections, and to LRFD coverage of fully and partially filled steel grid decks, were approved at the main AASHTO Bridge Subcommittee meeting in May, 2002 in Atlantic City. Exodermic™ design has traditionally been done with conservative provisions of the Standard Specification. See Design section. What spans can the Exodermic™ design handle? With standard grid and rebar layouts, up to 18 feet. With deeper main grid bars and heavier reinforcing, longer spans are possible. Please contact D. S. Brown for assistance. What are the range of deck thicknesses available? Total deck thickness can range from approximately 6.5” to over 9.5” (165 mm to 240 mm) with standard designs, depending on girder spacing and desired concrete cover over rebar. Custom, deeper sections may be required for longer spans. What is the typical depth of the concrete? The concrete component of an Exodermic™ deck is typically 4” to 4.5” (102 mm to 114 mm) thick, with top cover of 2” to 2.5”. Less concrete has been specified for sidewalk applications, and where a separate overlay is to be placed. Are overlays used with Exodermic™ decks? The top portion of an Exodermic™ deck is essentially the top half of a standard reinforced concrete slab, and is therefore compatible with any overlay types used with concrete bridge decks. Overlays are recommended for use on precast Exodermic™ decks, but are not essential. For precast decks, an overlay helps to protect the field-placed closure pours and cold joints from water intrusion and freeze/thaw damage. Overlays are rarely used on cast-in-place Exodermic™ deck applications, as weight is generally a concern when Exodermic™ decks are chosen. What overlays can be used with Exodermic™ decks? Since the top portion of an Exodermic™ deck is essentially the top half of a standard reinforced concrete deck, overlays can be whatever would normally be used for a concrete deck. Overlays used on Exodermic™ decks to date include: membrane and asphaltic concrete; latex modified concrete (1" to 1.5" overlays); microsilica overlays; polymer overlays such as PolyCarb's Flexogrid (3/8” (9.5 mm) used on the Tappan Zee Bridge).
50 to 75 psf Reason for using cast-in-place vs. precast? A cast-in-place Exodermic™ design eliminates cold joints. On smaller projects, cast-in-place projects will generally be less expensive because no precast form work has to be fabricated (for the edges of the panels and for the blockouts over stringers). This generally reverses on larger projects, particularly ones where the bridge is located in a high labor cost area. Since the grid portion of an Exodermic™ deck takes the place of form work in the field, cast-in-place construction is fairly fast, though not as fast as with precast Exodermic™ panels, of course, where the work can be done overnight if needed. Can lightweight concrete be used? Lightweight concrete helps to reduce deck weight even further. With the Exodermic™ design, this can result in deck weights of 50 psf or even less in some cases. What mix designs are recommended for use with light weight concrete? Because some light weight aggregates are more porous than standard aggregates (this is very dependent on the specific material available), we recommend using a high performance concrete such as one that uses silica fume (microsilica) and fly ash as specified by NYSDOT, or ground granulated blast furnace slag, as specified by Florida DOT. What are some examples of projects that have used Exodermic™ decks with a light weight concrete component? Florida has used lightweight concrete on two large bascule bridge projects with Exodermic™ decks, and has had excellent results (17th Street Causeway Bridge in Ft. Lauderdale and the bridge at Hallandale Beach). Another large bascule bridge with an Exodermic™ deck with lightweight concrete is under construction in West Palm Beach, and two other similar projects are in design. In New York, the Route 378 (“Troy-Menands”) Bridge over the Hudson River near Albany, the 4th Street Bridge in Waterford, NY, and two bridges over the Mohawk River/Barge Canal have used a light weight high performance concrete. What is the in-place cost of a typical Exodermic™ deck? Each project will be different. There are many factors: project size; project location; union versus non-union labor; precast versus cast-in-place; distance between supports; HS-20 versus HS-25; etc. We can provide you with representative project cost information for a range of different types of projects. Except in a few areas such as New York City, or where there are unusually difficult restraints on how the work gets done (i.e. very short work windows), a rough estimate of cost would be in the mid $30’s per square foot, including steel grid, concrete reinforcing steel, and erection. |
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