Composite Bridge Decking, Final Project Report
APPENDIX A: LESSONS LEARNED–ALLEGANY COUNTY PERSPECTIVE
Owner’s Perspective Before Starting The Project
Allegany County DPW approached participation in the Highways for LIFE Composite Bridge Decking project as an opportunity to perform a complete rehabilitation of a deteriorating local bridge owned by the Town of Bolivar, NY. The selected location was BIN 2215390, Local Bridge # 10-06, Pleasant Street over Little Genesee Creek in the Village of Bolivar, NY. Since 2006, the County had twice performed section loss repairs to multiple girders of the superstructure to address Red and Yellow Structural Flags issued under New York State's bridge inspection program.
DPW is the responsible party or designated point of contact for all County and Town owned bridges in Allegany County, NY. As such, all correspondence pertaining to the NYS Bridge Management and Inspection Program for the 298 locally owned bridges in our County is handled through the Department. DPW provides assistance to the Towns in the form of engineering, repair, and maintenance services of the town owned bridges as they request, with the understanding that they pay for all materials needed. County equipment and labor costs of the work provided are typically absorbed by the County under this arrangement.
This relationship was the basis for the agreement between the Town of Bolivar (bridge owner) and Allegany County for the rehab of LB #10-06. The County agreed to provide the equipment and labor, the HfL project would provide the deck and means to anchor the deck, and the Town would provide the rest of the required materials and hardware. The proof of concept installation afforded the opportunity to put local money into other structural elements of the bridge and extend the scope of the rehab. Instead of just replacing the deck, three of eleven girders were replaced, new steel backwalls were installed, and new crash-worthy, deck mounted bridge railing was installed that was far superior to that which existed prior to the project. Also, approach guide railing was installed where there had been none prior.
LB #10-06 seemed to be a good candidate for the proof-of-concept installation because it was relatively small, it had an open grate steel deck, and it had started to become a bridge that required regular repairs to the girders due to section loss. The high section loss measurements were a direct result of the use of chlorides from winter maintenance getting to the primary structural members through the open grate deck. It was understood from the beginning that the use of a composite deck would provide protection of the main superstructure elements (by being more weather proof) at a near zero increase in deadload supported by said elements. Other improvements were the incorporation of fascia overhangs on the deck for additional protection to the fascia girders that did not exist prior.
Alternatives that Allegany County would normally have considered for a rehab of this type were a transverse glulam deck with asphalt pavement wearing course or corrugated steel bridge decking filled with asphalt pavement. Both of these options would likely have reduced the load rating of the bridge due to the associated increases in deadload. Our experience with these deck types is that they don't completely prevent moisture from making its way from the deck surface to the structural elements of the superstructure either.
Experiences on Project Specific Details
As the owner's engineering representative and installer, Allegany County DPW's preferences and recommendations were considered and incorporated if possible by the project design team when determining the final geometry and anchorage utilized for the proof-of-concept deck. DPW has extensive experience in installing Steel multi-girder simple-span bridges with transverse glulam timber decks as well as simple-span glulam timber stringer with transverse glulam timber deck bridges. Therefore, Allegany County's recommendations for the anchorage used on the composite deck were provided in an attempt to make installation procedures as close as possible to the procedures used for transverse timber decks. The project design team designed and supplied a system that utilized a fabricated clip and threaded expansion anchor for the proof-of-concept installation. This avoided having anchorage penetrations through the top surface of the panels as would normally occur on a timber deck. (This was a steadfast issue for the deck design team, and rightly so. The composite panels are hollow, and water migrating into the cavities from top surface penetrations was deemed detrimental to the long term performance of the system. Also, the top surface of the composite panel was the base of support for the thin, 2 part wearing course. The solid nature (no voids) of a glulam timber deck does not allow for the migration and retention of surface water entering the top surface penetrations. Most water just passes right through and out the bottom of the anchorage hole.)
To provide drainage from the closed surface created by the composite panels, the deck was installed with a 2% cross slope from the centerline to the fascia overhangs. This required the use of a shim system as the existing beams were installed flat to accommodate the open grate steel decking. The shim system was composed of composite coated, planed hardwood planks, each milled to specified thicknesses based on a beam elevation survey. This survey was taken during the first portion of the rehab just after 3 of the girders were replaced. The shims came predrilled to provide a template to follow for drilling the holes in the bottom of the panels to install the deck anchors.
The normal crown 2% cross slope dictated the use of a longitudinal centerline joint along the full length of the bridge. This was because of the nature of the straight, pultruded, trapezoidal shaped tubes bonded together to form each panel. It also provided a means to get better bearing to all the shims and beams from such a stiff deck. A full width panel may have caused some bridging action to occur over one or more beams if the crown wasn't perfect. Also, the number of panels was dictated by the size and weight handling capabilities of the composites manufacturer who was contracted to assemble the panels. It took 12 panels to complete the 40' long deck, so attention needed to be given to the panel-to-panel field joint detailing to ensure a watertight deck.
Selection of the panel anchorage method was influenced by the various project budgets. Allegany County DPW forces were participating under the constraint of no cash outlay from the County. The County was restricted to contributing manpower, equipment, and materials that were already in County inventory only. (Inventory materials were billed to the Town of Bolivar.) The Town of Bolivar was also participating under a similar constraint after they had finished purchasing the steel and hardware for the rehab items not associated with installing the composite deck. Also, it was understood that the HfL project budget was nearly exhausted by the time the project reached the proof-of-concept stage. A different method of shimming (pressure grouting?) the panels may have required the use of rented equipment or work by contract, which none of the three parties involved could afford. Use of prefabricated HDPE haunches were investigated, but were not selected due to cost.
The use of the clip and expansion anchor bolt detail to secure the deck down to the beams posed the greatest challenge to the County staff installing the deck. The deck panels could not be pre-drilled prior to shipment due to the inability to ensure that the pre-drilled anchorage holes in the shims would line up with pre-drilled holes in the underside of the deck during placement in the field. This required the installation crew to drill all of the required anchorage holes (approx. 264) in the field, over their heads, in a very narrow work space. Each hole was drilled with a 15/16 dia. conventional drill bit driven by a ½ inch drill motor. Each hole was required to progress through two layers of glass reinforced composite to accommodate the correct expansion anchor bolt embedment. Two men were required to progress the drill. The first held alignment and the trigger. The other advanced the drill with the use of two scrap 2x4 pieces of lumber positioned as a fulcrum and lever propped under the drill motor. Most of the time the drill crew was a three person team, each member rotating between the two positions on the drill and resting. Employees of Allegany County DPW, The Town of Bolivar, and Bridge Composites, LLC, all had their turns working this operation. Everyone involved shared in the misery each night due to the drill cuttings (glass fiber dust) getting on exposed arms and necks. It took nearly a month to complete this operation. In fact, this operation continued on below the deck at the same time work progressed above on the panel joints and the second course of the 2 part wearing surface.
Panel-to-panel joints in the deck were filled with a two-part resin concrete. This operation went as expected and very similar to methods of joint repair and replacement that Allegany County staff were already familiar with. The installation of the joint material went quickly and was accomplished from the top, while monitoring the bottom side to stop any potential leakage.
The second course for the wearing surface was a two-part resin layer with dry aggregate broadcast by hand before the resin cured. This operation was also accomplished easily. Significant guidance was provided by Bridge Composites, LLC during this activity. A few days after this operation was completed, excess aggregate was removed by push brooms and a leaf blower. Any blemishes that appeared where the aggregate did not bond were quickly addressed and repaired by Bridge Composites, LLC.
A deck overhang was included in the panel width at the request of Allegany County. This offered protection to the fascia girders from further deterioration due to winter maintenance operations. The existing bridge railing posts had been mounted directly to the fascia girders of the bridge prior to the start of the rehab. The new deck configuration required the use of a deck mounted bridge railing system installed on the fascia overhang.
The bridge railing type was chosen because Allegany County had in storage a quantity of bridge railing posts and box beam railing that it acquired free of charge from the NYS Department of Transportation. These posts and rail came from an existing bridge on I-86 that NYSDOT removed over an abandoned railroad grade. The County provided the posts and railing for the two-rail system to the project at no cost. The Town of Bolivar bought and provided new anchorage and assembly hardware for the railing as part of the rehab materials. Because of the very low traffic volumes on the bridge and its location near the end of a dead end street, a Test Level of TL-2 was deemed adequate by Allegany County DPW for the completed railing installation. The loads associated with TL-2 were tested on a sample panel and post at Penn State during design development. The anchorage configuration used on the proof of concept installation was found adequate.
Lessons Learned and Opportunities for Improvement
Future installations of this particular deck system should eliminate the field drilling requirement associated with the deck anchorage because of the time and effort that was required to do it in the field. Therefore, a different method for shimming the panels to achieve cross-slope should be used (being that it was the pre-drilled shims that necessitated the field drilling of the panels.) Perhaps a method that uses leveling bolts already installed in the bottom of the panels and pre-installed threaded anchorage studs already protruding from the underside of the panels at the correct offsets to straddle the existing girders. When the panels are set in place to the correct elevation, shimming could then be done by some sort of grouting procedure, be it cementitious or synthetic material. This change may be easier if done outside the realm of a research project. A capital budget could be planned to accommodate the use of specialty contractors or special tool/equipment rental that didn't exist within the constraints of this project.
Deck panel sizes should be based on a maximum handling weight provided in the prospective owner's bid specification, especially if the owner knows which piece of equipment will be used to set the panels. In Allegany County's case, a maximum allowable panel weight of 28,000 pounds could have been specified. This is an achievable pick for the County Bridge Crew's crane to make at midspan of a 40' long bridge if the crane is set up in the approach. In reality the proof-of-concept deck could have been set with only 2 panels instead of 12, and just a single longitudinal joint instead of the 5 transverse joints that were also necessary. Having fewer field joints would not only require less time and materials needed to make the joints, it would also reduce the potential for problems in the future.
The design team's requirement of no top surface penetrations in the deck panels precluded the use of embedded anchors in the top to lift and set the individual panels. This required the crew assigned to the proof-of-concept installation to use polyester rigging straps slung around the underside of the panels during installation. This posed no problem at the fascia end of the panel. However, at the centerline end of the panel, where the longitudinal joint occurred over the center girder, the panels had to be propped with pry-bars to remove the strap. A lifting solution might be developed where high tensile synthetic strand lifting loops cast into the top of the panels could be cut off flush with the top of the deck prior to the installation of the second course of the wearing surface. A solution similar to one used by prestressed beam manufacturers who install lifting loops in the beams made of looped prestressing strand protruding from the top surface of the beam.
In Service Observances
At the time of this writing, the deck has been in service for approximately 7 months, including one winter maintenance season. The wearing course has no observable deterioration due to snow plowing operations and has appeared to stand up well in service. The polymeric-concrete joints at the begin and end of the deck have shown to have sufficient durability to withstand plowing operations, especially considering their exposure due to the adjusted roadway profile over the bridge.
There seems to be one location on the deck where the top side outer wrap of one of the panels has separated from the transverse trapezoidal tubes within. This was brought up by the Bolivar Town Highway Superintendent and observed in the field on May 6, 2013. This occurrence is isolated to panel No. 9. The condition is observable under foot or tire as an audible crunch is emitted by the deck. The condition is also felt as a soft spot under foot, as if you are displacing an air pocket when you press down forcing the outer wrap back down onto the tubes. Even with this condition, the wearing surface shows no visible signs of distress or cracking. The rest of the deck appears to be in excellent condition, nearly silent under traffic. Bridge Composites, LLC is aware of the issue, but there hasn't been sufficient time to diagnose exactly what is happening yet. Bridge Composite's initial recommendation is to have the Town and County monitor the condition as a repair attempt may be more detrimental than beneficial. Plans are in place to rerun the FE model for the panel design with the outer wrap modeled in a debonded condition.
Procurement Outside the Realm of a Demonstration Project
Without knowing what the actual costs were for the deck that was installed, it is hard to suggest items to consider if a prospective owner is interested in using this technology. Assuming costs are still very high, Allegany County would have limited use for such a product in the immediate future. The concept of the HfL project originally focused on moveable bridges. This would probably be a strong market to continue trying to break into, but Allegany County has none in its inventory. A possible use for rural municipalities might be federally funded bridge rehabilitations where State regulatory agencies deem the superstructure of the existing bridge to be historic. Otherwise a rehabilitation that allows the owner to replace the superstructure in its entirety is probably more cost effective. This would especially be true for many single-span, locally owned structures where span ranges are less than 65 feet or so.
The specification provided in Appendix D of this report should form the basis of a prospective owner or owner's representative construction specification.
Jeremy D. Ferris, P.E.
Allegany County Department of Public Works
May 13, 2013