Exodermic decks permit rapid bridge redecking.
The advent of the Exodermic bridge deck design has given designers an important tool in bridge rehab. Exodermic deck panels permit rapid-often overnight-deck replacement, while continuing to provide the desirable aspects of reinforced concrete decks: ride quality (including excellent skid resistance), easy and accustomed maintenance, and contractor familiarity.
Exodermic decks also address another important issue. For many rehab projects, it is desirable to increase the liveload rating of the structure. Exodermic decks help achieve this by reducing deadload (an Exodermic deck typically weighs 30 to 40 percent less than a conventional concrete deck). Also, Exodermic decks are typically made composite with the supporting steel beams, significantly increasing their structural capacity.
An Exodermic or "unfilled, composite steel grid" bridge deck is comprised of an unfilled steel grid 3 to 5 in. deep, with a 3 in. to 5 in. reinforced concrete slab on top of it. A portion of the grid extends up into the reinforced concrete slab, making the two composite. The concrete portion of an Exodermic deck can either be cast-in-place or precast.
In a standard reinforced concrete deck, in positive bending, the concrete at the bottom of the deck is considered "cracked" and provides no practical benefit. Thus, the effective depth of the slab is reduced, and the entire bridge-superstructure and substructure-has to carry the dead load of this "cracked" concrete. In an Exodermic deck, essentially all of the concrete is in compression and contributes fully to the section. The main bearing bars of the grid handle the tensile forces at the bottom of deck. Because the materials (steel and concrete) in an Exodermic deck are used more efficiently than in a reinforced concrete slab, an Exodermic design can be substantially lighter without sacrificing stiffness and strength.
In negative bending, the reinforcement in the concrete handles the tensile forces, as it does for a conventional concrete deck. Rebar in the concrete component of an Exodermic deck can be specified to handle the negative moment, where the deck is continuous over supporting beams, or where a cantilever (overhang) is required.
Exodermic decks can be specified to accommodate the particular requirements of a specific bridge. To date, overall deck thickness on different projects has ranged from 6.75 to 10 in. The concrete component of an Exodermic deck can be precast - as it was for the bridges described later in this article - or cast-in-place. In the latter case, the steel grid can be thought of as a super stay-in-place form, where the strength of the steel grid panels permits longer spans, if necessary, and elimination of half the concrete.
During precasting, blockouts are used so that there is no concrete over the grid in the areas that will be over floorbeams and stringers. During erection, headed shear studs are welded through these openings in the deck onto the stringers or floor-beams below. These portions of the deck are filled full depth with rapid-setting concrete which captures the shear connectors (now attached to the superstructure) and the grid and rebar of the deck panels, locking them together and making them composite. No field welding of the grid to supporting beams, or grid panels to each other, is required.
Pitman Creek, Kentucky. The Route 27 bridge, over Pitman Creek-an arm of Lake Cumberland in Pulaski County, Kentucky-carries heavy truck and passenger vehicle traffic. Twenty-five miles west, and parallel to Interstate 75, Route 27 is a major industrial corridor. The closest practical alternate route for trucks would have meant a 90-minute detour. According to Steve Goodpaster, now Director of Bridges for the Kentucky Department of Highways (KDOH), "We didn't need to crunch numbers [on user costs] to know that we'd have to do the work at night."
The bridge is a 700-ft three-span, continuous, Warren deck truss, built in the early 1950s. The main span is 300 ft long, and is supported on 120-ft tall concrete piers. Expansion joints are used at both abutments, with an expansion bearing on top of one of the piers. The existing 33-ft 2-in. wide deck was non-composite, reinforced concrete, and was failing badly.
KDOH determined that the new deck would have to be modular, wider, add to the strength of the floor system, and lighter than the deck being replaced to raise the live load rating. Modular construction would permit installation in segments overnight, so the bridge could be kept fully open during daylight hours. Even at night, one lane of traffic had to be maintained at all times.
An overlay was to be placed after deck replacement was complete. Widening the deck would permit the overlay to be placed in three strips, maintaining two lanes of traffic throughout the overlay installation. Wider shoulders after completion of construction was an added benefit; the new deck was to be 35 ft 4 in. wide. Thus, the new deck had to be able to handle the additional cantilever without expensive and time consuming additions to the superstructure.
The new deck had to contribute substantially to the strength of the floorbeams and stringers with which it would be made composite, to achieve the desired HS-25 load rating. And the new deck needed to be lightweight, to permit the deck widening while reducing dead load significantly, thus increasing live load capacity.
KDOH decided on an Exodermic deck. The contractor purchased the steel grid needed from an Exodermic licensee, the L.B. Foster Company. Four inches of reinforced concrete was precast on top of the steel grid, giving an overall deck thickness of 8.25 in. The new Exodermic panels weighed approximately 64 lb per sq ft. Installed weight, including concrete haunches and connections between panels was about 73 lb per sq ft.
In December 1992, the contract was awarded to Apex Contracting. Beginning in July 1993, deck erection got underway. On a typical night, the erection sequence proceeded as follows:
* Traffic was diverted to share one lane, using signal lights at both ends of the bridge to control traffic flow.
* The concrete parapet was removed, and a 25-ft length of lane, the spacing between floor beams, was sawcut into three pieces and hauled away. Temporary wooden work platforms were dropped into place for safety and ease of access. Headed shear studs were welded to the stringers and floorbeams using an automatic stud welding "gun."
* The Exodermic panels were then positioned on the bridge and quickly adjusted for elevation using leveling bolts built into the steel grid during fabrication. A 20-ft length of precast Jersey barrier was dropped into place and bolted to the deck.
* Rapid-setting concrete was placed full depth in the areas over stringers and floorbeams, and transversely between deck panels (precast with a double female keyway).
* The rapid-setting concrete was given a broom finish to provide traction for the few months before the application of the overlay.
* In about two hours after pouring concrete, the bridge was fully reopened to traffic.
In the first week of November 1993, the bridge redecking project was completed. Both the state and the contractor consider the project a major success.
Milton Madison Bridge Over the Ohio River. A $7.9-million contract for the rehabilitation of the US 421 bridge over the Ohio River between Milton, Kentucky and Madison, Indiana was awarded in spring 1996 to contractor Jim Skaggs, Inc. The 3,200-ft bridge is a major link in the area, with the next closest crossing 25 miles up river. A precast Exodermic deck was chosen by KDOH and its consultant, Hazelet & Erdal/Dames & Moore.
KDOH's Goodpaster figures that this approach is "probably saving six months to a year over a cast-in-place concrete deck."
The Milton-Madison bridge had been slated for rehabilitation for a number of years, and the current contract specifies a significant amount of steel repairs to the trusses and floor system. The bridge is comprised of 19 spans of deck truss, through truss, and deck girder construction. The four main spans of the bridge are 600-ft, 600-ft, 727-ft, and 254-ft in length. Because of the unacceptably long detour that closing the bridge would entail, KDOH determined that it would need to do the work in stages, maintaining at least one lane of traffic on the two-lane structure at all times. Because the bridge is so narrow (less than 23-ft between trusses), because there are no center stringers on this bridge, and because of the desire to get the work done as quickly as possible, a precast, panelized deck system was required.
IKG Greulich of Cheswick, Pennsylvania is fabricating the steel grid component of the Exodermic deck. Precasting is being handled by Thorn Orwick of Corydon, Indiana. Deck replacement is expected to be complete by the middle of 1997.
With only an 11-ft lane width to work in, with traffic moving in the adjacent lane at virtually all times, the contractor came up with an innovative deck erection plan. After removing the deck from a stage (typically 300 ft to 1,000 ft long), foam strips supplied by Williams Products of Troy, Michigan are placed on the edges of the stringers to form the edges of the new haunch. Temporary compression strips are also placed to keep the haunch forming foam from being permanently crushed during deck erection.
Next, a large forklift truck is used to bring out the precast panels, which are approximately 11-ft by 8-ft, and include precast curb and railing anchorages. Each panel is placed in approximate position and the forklift truck heads back for the new panel. The design of the Exodermic panels permits the construction traffic before the shear studs are placed and the closure pours are made.
When enough panels are in place, adjusted to proper grade with built-in leveling screws, and shear studs have been welded in place, concrete is placed over the stringers and at the shear keys between panels to complete the new deck installation.
When all of the 73,000 sq ft of bridge deck has been replaced, 1/4 in. of the new deck will be milled off, and 1.5 in. of latex modified concrete will be placed as a wearing surface.
Troy-Menands Bridge. The Route 378 bridge over the Hudson River between Troy, New York and Menands carries an ADT volume of 36,000 vehicles in four lanes. The conventional approach would have been to close half of the former lift bridge at a time, remove the old concrete deck, and form and cast a new deck in-place. Traffic would have had to make do with only two lanes.
Instead, Richard Maitino, Regional Director for the New York State Department of Transportation's busy New York City region, determined that a lightweight, modular bridge deck should be used, erected during the nighttime hours only. The low bidder, Barry, Bette & Led Duke (BBL), chose an Exodermic deck system.
The light weight of the system was especially important to both the DOT and to the contractor. For the DOT, a lightweight deck meant that the floorbeams would rate higher for live load. For the contractor, the lightweight Exodermic panels allowed use of a low boom angle on the crane dictated by the limited overhead clearance of the through truss spans. The Exodermic deck weighed 52 lb/sf (lift span) to 55 lb/sf (side spans)-63 lb/sf to 66 lb/sf installed weight including full depth haunches, etc.-versus 113 lb/sf to 150 lb/sf for the old deck. Lightweight concrete was specified for the Exodermic panels to keep weight as low as possible.
The Troy-Menands Bridge is 1,070-ft long (excluding the western approaches), and consists of a 348-ft former vertical lift span, now fixed; three 224-ft through truss spans; and a 57-ft span framed with girders and floorbeams only. On the latter span, the deck spans 9.5 ft between floor-beams.
BBL's crew quickly settled into an efficient routine. The typical work sequence started with setting up signs, cones, and barrier sections. While work was in progress, traffic shared a single lane controlled by flaggers at either end of the bridge. After sawcutting and removing the old deck, the top flanges of stringers and floorbeams were sandblasted. Following attachment of shear studs, (quickly done with a stud gun), haunches were formed using adhesive backed foam set directly on the edges of the stringer flanges.
Placement of the Exodermic panels took only minutes, with panel height set rapidly by adjusting the built-in leveling bolts. Finally, rapid-setting concrete was placed at the stringers and floorbeams, capturing the shear studs, grid bars, and reinforcing steel, and making the deck composite with the floor system. Rapid-setting concrete was also placed in transverse, double-female shear keys between panels. After the rapid-setting concrete reached 2,500 psi compressive strength (typically in under two hours), the bridge was reopened fully to traffic.
The contractor purchased the steel grid from one of two active Exodermic deck licensees, IKG Greulich of Cheswick, Pennsylvania. Galvanized grid panels were delivered directly to the precaster, Oldcastle/Spancrete Northeast, Inc. Spancrete attached joints and scuppers, and precast 4 in. of lightweight concrete on top of the steel grid panels.
The contractor finished the redecking on November 9, 1995. In three months, the 47,500 sq ft of bridge deck was redecked, with all work completed at night.
Mr. Robert A. Bettigole, P.E. is President, Exodermic Deck Institute, Inc., Scarsdale, New York. Portions of this article were presented in a paper entitled, "Nighttime Re-decking with Precast Exodermic Bridge Deck" at the 13th Annual International Bridge Conference in Pittsburgh, Pennsylvania, June 1996, and are used with permission.
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|Author:||Bettigole, Robert A.|
|Date:||Jun 1, 1997|
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