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Vermont Demonstration Project: Route 2 – East Montpelier Bridge Reconstruction
The Vermont HfL project includes the replacement of the US 2 Bridge near East Montpelier. The new bridge was opened to traffic on November 19, 2009. These types of smaller rural bridges form a significant portion of the national bridge inventory. The innovative approaches used in the delivery of this project, if successful, could therefore have a wider deployment potential nationally.
The project is located on US Route 2 over the Winooski River, about 1.1 mi east of the US Route 2 & Vermont 14 intersection, in the town East Montpelier, Washington County, Vermont. This portion of Route 2 is part of the NHS and is classified as a Principal Arterial. The bridge is located on a key access route to Montpelier and Barre. The following subsections highlight the innovative features of this project.
Figure 1. Map. General project location. (source: Google Maps).
The purpose of the project was to replace a functionally obsolete bridge. The average daily traffic (ADT) on the bridge is 8,500 vehicles per day with 5.6 percent trucks. The old bridge (Figures 2 and 3) was narrow, with 20 ft clearance between rail faces, and was the site of regular minor collisions and “near misses” representing a significant safety concern to VTrans and the agency’s stakeholders.
Figure 2. The functionally obsolete existing structure.
Figure 3. Deteriorated bridge deck.
The replaced structure was a three span bridge built in 1930. The superstructure was concrete T beams with a concrete deck. The total span was 140 ft. The concrete beams showed significant deterioration, with large areas of spalling and pop-outs that had exposed the reinforcing steel. Some areas showed substantial section loss. The deck joints at each of the intermediate piers had failed and allowed saturation of the bearings and substructure below.
The substructure consisted of concrete abutments and two intermediate piers. Although the substructure was in fair condition, its capacity could not be increased to accommodate any roadway improvements. The July 2001 scoping report referenced the inspection report that recommended that the structure be rehabilitated or replaced. In addition to the structural deficiencies, the bridge was supported on timber piles, and cofferdams had been installed to reduce scour (see Figure 4).
Notes of a November 8, 1999, Local Concerns Meeting showed concern over the narrow width of the structure and adjacent landowners citing examples of broken mirrors due to vehicles sideswiping on the bridge.
Figure 4. Photos. Scour protection at the existing structure.
VTrans considered both replacement and rehabilitation options and decided against the latter because:
Having made the decision to replace, not rehabilitate, VTrans considered three replacement alternatives:
Alternatives 1 and 2 consisted of maintaining the existing bridge for traffic during construction of the new bridge south or north of the current location with roadway alignments modifications included. The new bridge would be built at the same grade as the existing bridge. Both alternatives were discarded because of potential environmental/cultural impacts including agricultural lands, wildlife, wetlands, floodplains, and archeological (if determined). Alternative 2 further required reverse curves to connect to existing roadway. The reverse curve would have required a speed reduction along US Route 2.
VTrans' consultant, Earth Tech, recommended Alternative 3 because the existing alignment was good, with less permanent impacts, and had the support of the public. VTrans accepted the recommendation and considered the following options to maintain traffic:
Option 1 required a detour of 8 miles (see Figure 5) through a small village and required intersection upgrades. Furthermore, a bridge on the detour route was structurally deficient. Concerned with the extended commute and traffic jams, the public was opposed to this option.
Figure 5. Map. Detour route for option 1.
Option 2 required the bridge to be constructed in sections, with the left and right edges constructed first and then traffic shifted to newly constructed sections allowing the center to be completed (Figure 6 and Figure 7). The concerns with phased construction included a wider bridge than necessary and longer span for maintenance of traffic. Furthermore, the project would have required two construction seasons and would entail higher construction costs.
Figure 6. Diagram. Phase I construction.
Figure 7. Diagram. Phase II construction.
Option 3 was developed with a temporary bridge located to the south side of the structure. The simple span bridge would be 122 ft long. Concrete deck with a membrane and pavement, weathering steel girders and conventional abutments on piles would be used. The conventional superstructure is shown in Figures 8 and 9.
Figure 8. Diagram Temporary bridge with conventional structure, option 3
Figure 9. Diagram. Bridge elevation, option 3.
A typical timeframe for the removal and replacement of this type and size bridge is about 6 months using a cast-in-place substructure with footings founded on bedrock.
At about the turn of the century, highway agencies across the nation started increasing emphasis on minimizing inconvenience to the traveling public caused by highway construction and maintenance operations. The slogan “Get In, Get Out, Stay Out” resonated with stakeholders in the highway industry. This concern for inconvenience prompted VTrans to take a fresh perspective on this project. The desired outcome was to have a bridge that could be constructed in less time than a traditional approach, would result in a durable product requiring minimal maintenance, and with reduced disruptions to the traveling public.
To achieve the goal of 100-year design life and minimal maintenance, the project team considered designing a single span integral abutment bridge. The superstructure would use weathering steel girders with a bare high performance concrete (HPC) deck reinforced with solid stainless steel, topped with a curbless, pedestal mounted rail. The result would be a bridge of simple design, with no membrane and pavement that can rut and pothole; no joints that can fall into disrepair and leak; no scuppers that can clog; no curbs that will retain salt-laden runoff and accelerate deck deterioration; no bearings that can corrode and freeze; and no beam paint system that can fail. Figures 10 through 14 show conditions that the project team attempted to avoid.
This approach is a departure from the norm for VTrans in many ways. Vermont traditionally has been a pavement and membrane State (Figure 15). The standard treatment is to use a torch welded membrane with a 2 ¾-in pavement overlay on bridge decks. Vermont has been re-evaluating the pavement and membrane policy and considering the use of “bare” decks for some of its bridges, as many neighboring States do for high-quality decks.
10. Failed membrane and pavement condition.
Figure 11. Failed deck joint condition.
Figure 12. Salt-laden runoff at curb.
Figure 13. Corroded bearing condition.
Figure 14. Failed paint system condition.
Figure 15. Conventional construction detail showing deck membrane and curb mounted rail system.
VTrans currently specifies epoxy-coated reinforcing steel for bridge decks. The agency hosted the FHWA seminar prior to this project on high performance reinforcing bars and was interested in trying improved reinforcing technologies for its bridge decks. They decided to use solid stainless steel rebars for deck reinforcement for the first time in the State.
Again, the norm for VTrans is to use the NETC two-rail curb mounted steel bridge rail. The use of curbs adds one more construction requirement and requires scuppers in the bridge deck. Instead, the project team decided to utilize the New York three-rail flush-mounted rail to simplify the construction and eliminate the need for the through-deck scuppers (Figure 16).
Figure 16. Construction detail of innovative bare deck design with flush-mounted rail.
The new structure also would use weathering steel plate girders and have an HPC deck. The deck and integral abutment stem would be reinforced using solid stainless steel. At each end of the bridge there would be at-grade approach slabs.
The innovative components of this bridge result in a simplified design with high performing materials that reduces the timeframe for construction and will result in fewer maintenance activities over the life of the structure.
The following subsections highlight the key features of this project that are either innovative to VTrans or have been implemented only recently.
Integral abutment bridges are designed without any expansion joints in the bridge deck. Bridges can be single span or multiple span continuous deck type structures with each abutment monolithically connected to the superstructure and supported by a single row of flexible piles. The primary difference between an integral abutment bridge and a conventional bridge is the manner in which movement is accommodated. A conventional bridge accommodates movement by means of sliding bearing surfaces. An integral abutment bridge accommodates movement by designing each abutment to move unrestricted as a result of longitudinal loading effects with less induced stress, thus permitting the use of lighter and smaller abutments. Accordingly, with the elimination of bearings and joints and with smaller abutments, these bridges require less time to build, cost less, and require less maintenance than equivalent bridges with expansion joints.
VTrans built its first integral abutment project about 10 years ago, and currently it is VTrans policy to consider integral abutment construction as a first option for all slab and slab-on-stringer bridges.
High Performance Concrete
The American Concrete Institute (ACI) defines HPC as, “concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely when using conventional constituents and normal mixing, placing and curing practices.” Important characteristics of HPC include freeze-thaw resistance, alkali-silica reactivity, permeability, compressive strength, and resistance to early age shrinkage cracking. The project called for air content of 5.5 to 8.5 percent. Laboratory testing was conducted to optimize mix design, and 30 percent of the cementitious material was fly ash. Through the mix design process, the agency found that just fly ash can be used, no silica fume, to optimize the performance of the HPC on this project.
The minimum compressive strength for the concrete was 4,000 psi, and to prevent early age shrinkage cracking, cement was limited to 611 lb/yd3 versus 660 lb/yd3 for Class A concrete. The maximum water/cement ratio was 0.44. The concrete was to be wet cured immediately (within 10 minutes of finishing), and uninterrupted 10-day wet cure was specified. The special provision for HPC is shown in Appendix A.
Stainless Steel Reinforcement
The special provision for stainless steel reinforcement is shown in Appendix A. The contract required that the stainless steel meet the requirements of ASTM A955 M and its designated grade, either 420 or 520, and the requirements of ASTM A 276 UNS 31653 or UNS 32304, since both grades show in testing that a service life of 100 years can be achieved. The contractor opted to go with 32304 based on cost, and the laboratory tests met both strength and elongation criteria. The price of stainless steel was $3.64 per lb, versus epoxy coated steel that normally is priced at about $1.25 per lb. VTrans used the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) guidance for empirical deck design to determine steel requirements and specified:
This resulted in a total weight of reinforcing steel of 23,000 lb.
There were five bidders on this project. The winning and low bid came from Winterset, Inc. at $2,369,907. This amount included $560,000 for the temporary two-way bridge, $95,000 for removal of the old structure, $178,000 for mobilization/demobilization, $60,000 for traffic control and $143,500 for the Superpave asphalt mix. Unit prices for unique special provision items are shown below:
The supplier/fabricator information for the stainless steel, guardrail, and HPC is provided in Appendix B.
The contractor's schedule shown in Appendix C estimated that the new bridge would be open to traffic on October 9, 2009.
Initially, the construction progressed on schedule. Figure 17 shows the construction of the temporary bridge. After the traffic was switched to the temporary bridge, work began on demolition of the old bridge (Figure 18). This was followed by pile driving and abutment construction. Instrumentation was installed in both abutments to monitor their movement as part of ongoing VTrans research to improve understanding of integral abutment behavior.
The next major step in the construction process was the placement of girders made of weathering steel (Figure 19), followed by forming deck, installation of studs, and tying of stainless steel deck reinforcement, which was scheduled the week of August 20 through 24, 2009.
Figure 17. Construction of two-way temporary bypass bridge.
Figure 18. Demolition of the existing bridge.
Figure 19. Placement of weathering steel girders.
It was at this juncture that the project team experienced its first significant unanticipated challenge. The stainless steel rebars that arrived to the site were not straight, with the deformations twisted around the bar. (See Figures 20 through 22.) The cause for the problem was identified as rebar steel being produced in large coils, which the fabricator then cut to desired lengths. The resident engineer rejected the material for straightness, or lack thereof, as the contractor was unable to place the bars within specified tolerance of ¼ inch on cover and 1 inch on spacing.
The stainless steel rebars that arrived at the site also showed signs of rust. Since stainless steel does not rust, the “rust” on the rebars was contamination caused by any or all of the following:
Contract specifications required that in-place stainless steel be free from contamination. The contractor was allowed to use stainless steel wire brush to remove contaminants. The contamination removal criteria were as follows:
Figure 20. Condition of stainless steel rebar upon arrival.
Figure 21. Close-up of stainless steel rebar condition.
Figure 22. Curved condition of stainless steel rebar.
The laboratory test results showed compliance with the strength and elongation requirements of the specifications.
A significant amount of reinforcement steel was returned to the fabricator for straightening. The returned steel was corrected and shipped to the site. It was again found to be unacceptable and sent back to the fabricator. The fabricator made significant improvement the second time. Figure 23 shows the condition of the bars prior to placement in the deck. The correction delayed the project by about 4 weeks.
Figure 23. Stainless steel reinforcing bars prior to placement.
The stainless steel reinforcement as placed in the deck is shown in Figures 24 and 25, and the anchor bolt detail is shown in Figure 26. The contractor had considerable difficulty in threading the stainless steel anchor bolts for the deck rail. VTrans has decided to specify galvanized anchor bolts in the future.
Deck pour was scheduled to occur on August 27, 2009. However, because of the delay with the stainless steel, it did not occur until September 25. During the deck pour, the exterior girders began to rotate, causing the screed rails to drop, which compromised the thickness of the deck in the center of the bridge. The compromised portion was removed, and the contractor's temporary lateral bracing was replaced by a permanent bracing that solved the problem. The deck was re-poured, and the bridge was opened to traffic on November 19, 2009—41 days behind schedule. Figures 27 shows traffic moving on the new, wider bridge. Figures 28 and 29 show the completed bridge.
Figure 24. View of deck reinforcement.
Figure 25. Close-up of deck reinforcement.
Figure 26. Anchor bolt assembly detail.
Figure 27. View of the new, wider bridge.
Figure 28. View of the completed bridge.
Figure 29. Roadside view of the completed bridge.