Composite Bridge Decking, Final Project Report
5. CONCLUSIONS AND LESSONS LEARNED
Deck panels were made by combining consistent-quality pultruded subcomponents with a vacuum-infused outer wrap. The strength and stiffness were first determined analytically using finite element methods, then validated independently with extensive full-scale laboratory testing. Details of the installation were demonstrated on a 40-foot-long bridge during August 2012. Load testing further added to the calibration of the finite element model, so there is a high level of confidence that the numerical model is reliable.
The 5-inch-thick composite deck is versatile and can be tailored to be light or especially robust, depending on the need. With working stresses less than 25 percent of the material’s ultimate strength, a catastrophic failure of the deck is virtually impossible. Furthermore, panels purposely overloaded in the laboratory exhibited a pseudo-ductile behavior and had residual strength after failure.
The end result of the project is a robust, high-quality deck suitable for many applications, including moveable bridges, historic trusses, and posted bridges. Although the deck has many desirable attributes, the initial material cost is higher than conventional alternatives. This may mean that future use will be restricted to situations where the rapid installation offsets the cost of maintenance and protection of traffic, or situations where the light weight is especially important, such as on deteriorated or historic structures. Further data and analyses will be necessary to accurately compare the cost of this deck system to alternative lightweight decks on a materials basis, an in-place basis, and a life cycle basis.
The performance of the bridge deck will be monitored for the next 5 years. Inspections will be primarily visual, but opportunities will be sought to gather in-service deflection data with a structural health monitoring system that is accessible via the internet. Condition reports will be made available by the authors upon request.
The following sections highlight some of the most important findings of the project. Appendix A provides lessons learned from Allegany County’s perspective.
Lessons Learned – Design
- From the beginning, the project team and its advisors agreed that the most important design criterion was deck performance (driven by stiffness needed for good durability), even if it came at a higher cost. Evidence abounds that a superior deck has been produced and that it can be expected to serve worry-free for the life of the bridge, but the cost was not driven down as much as the project director had anticipated.
- There is a large factor of safety for strength. The design is driven by the stiffness of the deck under local deformation because of the need to assure the integrity of the wearing surface. Deflection, per se, (rider comfort) is not the issue.
- The pultruded combination tube was developed during this project. In the words of one advisor, “I also like the concept of alternate top and bottom grouted sections. It performs well and looks like it responds well to the local deformation issue. It has the benefit of efficiencies in the manufacturing process by having only one tube section to manufacture.”
- The two-course wearing surface appears to have the potential to eliminate wearing surface problems. Laboratory testing seems to support this argument, but long-term performance in the field will be the key.
- A key benefit of the design is the deck’s versatility. The stiffness can be increased 40 percent over an empty panel when needed for wider stringer spacing by strategically adding grout or modifying the fiber architecture of the outer wrap. Since the proof-of-concept bridge had tightly spaced stingers (24 inches), no grout was needed. Using the deck without grout fill made it extremely light (17 pounds per square foot).
- The county specified test level TL-2 for the railing on the bridge because of the low operating speed and light traffic. The rail post connection provided and tested is sufficient for TL-2 loading without any special treatment. Grout fill will be needed to withstand greater loads when the railing must be mounted on the deck and a higher level of performance is warranted.
- A low-modulus epoxy grout achieved high compressive strengths yet remains compatible with the rest of the deck system.
- Further testing may be necessary to assess the potential for debonding between the grout and FRP tube as a result of thermal cycling.
- Internal shear blocks would diminish concerns about slippage between the FRP tubes and the grout (from either overload or thermal cycles).
- The bond between the cementitious grout and FRP is not satisfactory without some additional measure being taken to ensure load transfer (other than simply sanding).
- Testing showed that direct fire can damage the deck panels by burning off the resin, even when a fire-resistant resin is used. However, the damage was limited to the exposed area, and deterioration of strength (and stiffness) is slow. After 20 minutes of a 1500 °F fire under the deck, the top surface was not much above room temperature and did not appear to suffer any damage. The damaged panel would still be considered serviceable after the fire test.
Lessons Learned – Fabrication
- The use of prefabricated haunches worked well, but machining them with the proper cross slope was more difficult than planned. Perhaps a combination of a rectangular section and a small wedge would simplify this detail.
- When installing the first course of the wearing surface, it is important to be aware of the pot life of the adhesive. There are some deck panels that did not get the proper embedment of stone because the adhesive had set up before the stone was applied. An attempt was made to remove the hardened adhesive and reapply it in the field to make sure course 1 was bonded well, but it was very difficult to do, even with a grinder.
- It may be because of the uniquely difficult economic times that we have lived through (dubbed The Great Recession), but the project team found that there were few FRP fabricators willing to take any financial risk. It was difficult to get quotations when trying to find a variety of sources for the tubes. In the end, the pultruded tube proved to be a good solution, but it would have been better to have more competitive quotes. The size of deck panels and, thus, the number of field joints was constrained by the lifting capacity of the fabrication shop.
- The size of the panels is customizable, so applying the system each bridge is not a problem.
Lessons Learned – Installation
- Installation was fairly straightforward, even for a work crew that was inexperienced at installing FRP decks.
- Drilling holes in the deck for expansion bolts from underneath proved to be more tedious and time-consuming than anticipated. On-site drilling was selected because it would have been hard to align predrilled holes precisely. Perhaps a different type of drill, like one with a magnetic mount, would have facilitated this part of the field installation operation.
- A prefabricated “grout pad” made of recycled high-density polyethylene material simplified and accelerated the post installations.
- Applying course 2 of the wearing surface in the field worked well because it sealed the deck surface with one layer for water tightness. It also served to level out small irregularities in course 1. The resin and stone were spread manually on this project, but this process could be mechanized for a bridge deck that had a large surface area.
- The sloped faces at the edges of panels allowed for a better panel-to-panel joint. It was easy to construct in the field. Based on laboratory testing, the epoxy grout selected for the joint is expected to perform well.
- The stainless steel hardware is excellent quality, but it was relatively expensive. Some parts needed to be custom made because of the variation in dimensions caused by making the cross slope with a haunch. If the weight of the deck system were not as much as a concern, the deck could be applied flat on the stringers and a cross slope created with asphalt paving or a polymer concrete. This would have made the connection hardware more uniform and less expensive.