Fiber-reinforced polymers or FRP's are robust materials that are highly resistant to corrosive action, have a high strength to weight ratio and are well suited for assembly line production into modular components that can be rapidly erected. However, FRP material costs are significantly greater than traditional concrete and steel materials. Therefore, cost savings due to either reduced weight, increased speed of construction or lower maintenance and increased life expectancy must offset this higher cost to make sensible use of FRP materials.

Because of the severe environmental conditioning that bridge decks are subject to and the fact that they account for a major percentage of a bridge structures dead load, they are the most suitable bridge application for FRP materials. An 8-inch deep FRP deck weighs approximately 20 lbs./sq. ft. as compared to 100 lbs./sq. ft. for a concrete deck of the same depth. In addition, FRP decks can be constructed faster than conventional cast-in-place decks that take more time due to formwork construction, rebar placement and concrete curing.

Overview of Typical Deck Systems
The majority of the FRP deck systems on the market today utilize glass-reinforcing fibers set in a polyester or vinylester resin matrix. Other FRP material systems that utilize carbon or aramid fibers and epoxy resins offer superior structural performance characteristics but are cost prohibitive for use in bridge deck systems.

The typical deck systems on the market today consist of two principal types: pultruded tubes that are bonded together with adhesive and honeycomb or sandwich core systems that are hand laid-up or utilize vacuum assisted resin transfer molding techniques. An example of a pultruded tube or beam system is shown in Figure 1. Each deck system is factory assembled into deck panels that are sized appropriately for shipment to the site. The panels are then erected and bonded together in the field using high performance adhesives.

Figure 1 - DuraSpan™ deck system by Martin Marietta Composites, Inc.

All of the deck systems require an overlay to provide adequate skid resistance and sufficient geometric tolerances. The overlay system can consist of a conventional latex concrete, micro-silica concrete or high-density concrete; however, these types of overlays do not have comparable stiffness, tensile strength and compressive strength properties as compared to FRP deck systems. This lack of compatibility can lead to debonding and/or cracking of the overlay if the interaction between the deck and overlay is not properly analyzed and accounted for. Thin polymer modified concrete and epoxy overlays are better suited for FRP deck applications.

Hot-applied asphalt has been used as an overlay for FRP decks; however, the temperature of the asphalt typically exceeds the glass transition temperature of the resin. FRP materials begin to lose their rigidity as they approach the glass transition temperature of the polymer and start to exhibit a viscoelastic type behavior. The corresponding effect on the behavior and performance of the deck should be analyzed and tested prior to the use of a hot-applied asphalt overlay.

FRP decks that are supported by beams require a haunch or fillet between the beam and deck to provide adequate tolerance to accommodate geometric imperfections introduced during fabrication or erection of the beams. Either a conventional non-shrink grout or polymer modified grout can be used to form the haunch. Regardless of whether the bridge is designed for composite or noncomposite action under superimposed loads, the deck must be connected to the beams with a nominal number of connectors in order to provide adequate confinement of the haunch. Otherwise, the haunch will break apart over time as the deck rotates over the beam line and separates from the haunch as the deck is subject to unsymmetrical live loading.

The method of connecting FRP decks to beams is one topic that requires further research and testing to optimize the cost and performance of this detail. Welded shear studs contained within grout-filled pockets have been used successfully for connecting FRP decks to steel beams to achieve full composite action. The key to this approach is to ensure adequate strength and confinement of the grout in order to develop the required connection capacity. The advantage to this approach is that it utilizes conventional technology; however, the trade-off is the increased fabrication cost associated with cutting holes in the deck and forming the pocket at each connection location. Figure 2 shows a typical detail for this type of connection.

Cost Effective Applications
The cost of FRP deck systems is approximately two to three times that of conventional cast-in-place reinforced concrete decks. This high initial cost must be offset by either a savings associated with a reduction in life cycle cost or a savings associated with the reduction in dead load. Thus, the two primary market applications for FRP decks are replacement of deteriorated concrete decks on high volume roadways and rehabilitation of weight sensitive structures.

Traffic delays can cost as much as hundreds of thousands of dollars per week in wasted fuel and reduced productivity on high volume roadways or on roadways located in urban areas that are heavily relied upon by motorists and businesses for commerce, safety and mobility. These costs are often referred to as user costs. The actual delay cost is heavily dependent upon the traffic volume and delay time. The benefit of FRP decks is that they result in a system that can be rapidly erected and offer enhanced durability that significantly reduces the need for future rehabilitation.

Life cycle cost analyses conducted by J. Muller International have shown that FRP bridge decks used on conventional multi-beam overpass bridges can reduce the life cycle cost of a bridge anywhere from 10 to 30 percent over a 75 year design life. The major component of the cost savings is a reduction in user costs associated with the increased speed of construction and fewer traffic impacts due to a reduction in maintenance requirements. The user costs were found to account for as much as 80 percent of the life cycle cost of a bridge.

However, transportation officials operate under the constraint of yearly budgets that do not always allow the expenditure of more money today in order to save money in the future. In addition, its is sometimes difficult to quantify the magnitude of the user costs or even to convince people that it is a real cost that should be factored into the decision making process. The fact that the traveling public is becoming increasingly intolerant of traffic impacts caused by bridge construction and maintenance activities may force the industry to make a fundamental change to life cycle cost based decision making.

The second major market area is related to weight sensitive structures including cable-stayed, suspension, arch, moveable and truss bridges. However, the most promising markets seem to be replacement of open grating decks on moveable bridges and replacement of open steel grating or concrete decks on truss bridges. FRP decks offer the advantage of a closed deck system that protects the floor system of the bridge thereby increasing the overall durability of the structure. In addition, there are a significant number of existing truss bridges with concrete decks that are deficient with respect to live load capacity. FRP decks are approximately 20 percent of the weight of a concrete deck. By replacing an existing concrete deck with an FRP deck, the live load carrying capacity of the bridge can be increased without requiring significant rehabilitation or replacement of the main structural members.

Summary
FRP deck systems offer the benefit of a lightweight decking system that can be rapidly erected and provides excellent long-term durability. FRP decks systems are available today as a viable alternative to traditional decks. Nonetheless, further research, development and validation of FRP deck systems is necessary in order to further optimize and standardize these decks systems so that they gain widespread acceptance in our industry. Standard design and construction specifications are also necessary to give engineers and contractors the information necessary to properly design and build FRP decks. We also must make use of the 40-year successful track record of FRP's in the aerospace and boating industries to educate ourselves with respect to the long-term durability characteristics relating to fatigue, freeze-thaw, creep and moisture.

FRP decks will likely never be competitive with conventional deck systems based upon first cost alone. Our industry needs to make a fundamental change in our approach to quantifying costs and making decisions regarding the use of specific structure types and components. Life cycle cost based decision making needs to be standardized and accepted in order to truly begin to appreciate and take advantage of the speed of construction and durability attributes of FRP decks.