Bridge Product Gateway
Vehicular Bridge Decks

Introduction

Today’s bridge owners are faced with unique challenges as a result of a severely deteriorating infrastructure, insufficient funding and a demanding public.  A recently released study (Report FHWA-RD-01-156) funded by the FHWA, entitled “Corrosion Costs and Preventative Strategies in the United States”, estimates the annual direct cost of corrosion for highway bridges to be $6.43 billion to $10.15 billion.  This includes $3.79 billion to replace structurally deficient bridges over the next 10 years and $1.07 billion to $2.93 billion for maintenance and cost of capital for concrete bridge decks.  In addition to these direct costs, the study’s life-cycle analysis estimates indirect costs to the user due to traffic delays and lost productivity at more than 10 times the direct cost of corrosion. Although almost all bridge owners in the United States continue to make decisions based on lowest initial cost, it has become extremely clear that this approach does not work and in the near future more money will be spent maintaining existing structures than building new ones.  The public has become intolerant of construction delays and is demanding structures that will last longer and provide greater value for their tax dollars.  As a result, there are tremendous opportunities for FRP bridge decks that are corrosion resistant, lightweight, and can be rapidly installed.

Features and Benefits

FRP composite bridge decks deliver viable solutions to meet critical needs for rehabilitation of existing bridges and construction of new bridges.  FRP bridge decks have only been used in the United States since the mid-1990’s but FRP materials have been used successfully for over 50 years in a variety of demanding applications, including the aerospace, marine, and sporting good industries.  Primary benefits of FRP decks include:

• Durability (highly resistant to corrosion and fatigue)

• Lightweight

• High strength

• Rapid Installation

• Lower or competitive life-cycle cost

• High quality manufacturing processes under controlled environments

Compared with cast-in-place concrete decks, FRP bridge decks typically weigh 80% less, can be erected twice as fast and have service lives that can be two to three times greater.  Compared to steel grating, FRP bridge decks are comparable or lighter in weight while providing a solid surface deck (protects support structure from corrosion and environment from pollutants), higher skid resistance (safer), reduced noise, significantly lower maintenance and a service life that can be two to three times greater.  FRP bridge deck types may be either self-supporting structures or panels supported by stringers.

Structural Characteristics

FRP bridge decks are anisotropic, meaning the mechanical properties of the laminates vary with the volume and orientation of the fiber reinforcement (similar to the reinforcing steel in concrete).  As a general rule of thumb for structural FRP applications, design strains are typically kept below 20% of ultimate capacity.  However, bridge deck applications are typically stiffness-driven, many times resulting in strains well below that level.  As a result of such low levels of strain, fatigue and creep are not an issue when properly designed and fabricated.

Design Issues/Considerations
 

• Deflection - Allowable live load deflection in FRP (as well as timber and steel) has been much debated.  The main purpose for deflection limitations should be to prevent local or global deformations under a wheel load that may cause delamination or cracking of the overlay.  Initially FRP decks were held to the same stiffness as concrete decks although there are no deflection requirements in AASHTO for concrete decks.  The thought was that since deflection was not a problem in concrete decks the conservative approach would be to match concrete.  This approach was ultra conservative and resulted in FRP decks that were extremely over designed and too expensive to be viable.  The deflection criteria was later relaxed to L/800 in many cases, but this criteria was based on the global bending of an entire superstructure and was clearly not applicable to FRP or any other types of decks.  As the market has become familiar with FRP materials and realize how over designed these decks are for ultimate strength, deflection criteria has recently settled into the L/300 to L/500 range, which is consistent with the provisions in the current AASHTO LRFD Code provisions for orthotropic steel and timber decks.            

• Deck-to-Girder Connections – FRP bridge decks have been installed on steel, concrete, and FRP girders and connection types vary with each deck type and application.  Connection options include but are not limited to all-adhesive, mechanical fasteners, and conventional shear studs.  Composite bending action between the deck and support girders is possible but the ability to provide this will depend on each specific deck type and manufacturer.

• Barrier Rail Connections - Many types of conventional railing systems can and have been used with FRP bridge decks.  The connections will vary with the deck type but will typically be bolted through the deck or anchors embedded inside the deck

• Overlay - All of the FRP bridge deck systems require an overlay to provide adequate skid resistance as well as abrasion protection.  The type of overlay is generally the owner’s choice and options include but are not limited to conventional asphalt, polymer-modified asphalt, polymer concrete, and micro-silica modified concrete.

• Specifications - Numerous attempts are currently under way from various private and governmental agencies to develop material specifications for FRP materials to be used for bridge applications.  One of the advantages of FRP materials is the ability to choose from a wide range of constituent materials and manufacturing processes.  However, trying to develop a prescriptive-based specification to cover all materials and applications has proved to be a daunting task and the owners and industry continue to struggle with how to best address this issue.  Historically a performance-based specification has been successfully used for FRP materials and it appears for the short term this approach will be continued for bridge applications.

Cost/Current Market Limitations

Based on initial in-place material cost, FRP bridge decks typically cost 2-3 times that of a conventional deck.  FRP bridge decks will probably always have a higher initial material cost when compared to conventional materials, but many times there are no other alternatives than continually replacing and maintaining the existing deck types or complete replacement of the structure.  Eventually life-cycle costs (particularly user costs more so than material cost) will become part of the decision making process in the United States, but until that time FRP bridge decks have to compete in niche markets where initial costs are comparable or where there are limited or no other alternatives.  In these markets, lightweight and rapid installation tend to be the features that can justify the higher material cost.

Niche applications where weight is critical include movable, truss, arch and potentially suspension bridges.  In many of these applications as well as other types, the bridge may also be classified as a historic structure, in which case there is a desire to preserve rather than replace.  Many of these older structures were not designed for current traffic loads, and as a result traffic may be restricted on these bridges.  The weight savings gained by replacing an existing concrete deck with a lightweight FRP bridge deck can result in additional live load capacity and possibly allow restrictions or postings of these bridges to be removed.

Off-site manufacturing and modular construction of FRP bridge decks allows for rapid installation in the field and is ideal for urban installations or other bridges with high traffic counts that are sensitive to closure times or detours.  When compared to a conventional cast-in-place concrete deck, FRP bridge deck installations can possibly cut construction time of the deck in half.  Applications for rapid installation include all types of bridges.

Materials and Manufacturing

Materials used in the FRP bridge decks consist of highly engineered glass fiber reinforcements with various resins including iso-polyester, vinyl ester and epoxy. These materials are selected because of there strength and inherit corrosion resistance. FRP bridge decks are designed to be modular to allow flexibility for assembly, shipping and installation. Several of the FRP deck panels are designed with different sandwich cores providing stiffness. These cores are characterized as fiber-reinforced foam, cellular core honeycomb, or fiber-reinforced polymer honeycomb.

Manufacturing processes used to fabricate FRP bridge decks include pultrusion, vacuum-assisted resin transfer molding (VA-RTM), and hand layup/contact molding.

Pultrusion

In the pultrusion process, rolls of reinforcements are strategically aligned and pulled into the tooling and folding process. The dry fabrics and fibers are pulled through a shaping tooling that brings them together to form a closer shape of the final product. The reinforcements are wet-out by pulling them through a resin bath. The saturated reinforcements enter a heated die where the composite is cured to form a hard part. A translating saw is used to cut the continuous structural profiles into the desired lengths.

Vacuum Assisted Resin Transfer Molding (VARTM)

VARTM uses the negative pressure of vacuum to infuse resin into dry reinforcement fibers and fabrics that are placed in a mold and sealed in an airtight chamber. A nominal vacuum of 28 in. of mercury is used to evacuate the mold of air. After the air is removed, the assembly is infused with resin and allowed to cure under vacuum. VARTM has been used to fabricate yachts, aircraft components, railcars, subway car body panels, marine piling and fenders (bumpers), components for naval berthing, concrete formwork and bridges. This manufacturing process allows structural components of virtually any size or geometry to be fabricated.

Hand Layup/Contact Molding

Hand layup techniques are used to fabricate the unique sandwich panels comprised of a core and face skins for bridge decks. The face skins are produced by hand layup on a flat surface where resin is manually applied to chopped strand mat glass fiber reinforcement using common paint rollers. Grooved metal rollers are used to remove air bubbles from the laminate. The corrugated sheets that form the core are produced in a similar fashion to the face skins but have a defined shape called flutes. Assembly of the precured flat sheet is then placed on top of the wet corrugated sheet to produce a bond as sandwich panel cures. After cure, the combined corrugation/flat assemblies are trimmed to the proper width. This determines the core thickness of the sandwich panel. Adhesive resin is applied to the fluted side of the strip before it is mated to the flat of an adjacent strip.

Assembly and Installation

FRP bridge decks can be manufactured as a one-piece construction or shipped in separate panels with minimal assembly onsite.  Depending on the type of deck used, lifting of deck panels is accomplished by using integrated lift rings or by using standard lifting straps. Depending on the design, these pre-engineered systems have standard connections that use mechanical fasteners or adhesive bonding for panel-to-panel, panel-to-stringer, or barrier rail-to-panel interface.

Quality

Quality control in manufacturing of FRP bridge decks is assured by each manufacturer. The FRP bridge deck is designed and fabricated in the factory by engineers and personnel having expertise in FRP technology. Prefabrication of the deck offsite results in quality assurance and takes deck construction off the critical path of construction. Quality assurance begins by inspecting incoming raw materials. Each shipment of resin is inspected for gel time, temperature, viscosity, acid value and solids content and the values are compared against the certificate of analysis from the manufacturer. Samples of glass fibers reinforcements including roving, continuous strand mat, and stitched fabrics are inspected for material weight, moisture and binder content and the values are compared to the manufactures’ certificate of analysis or material specifications.

Conclusion

FRP bridge decks are available today as a viable alternative to conventional decks.  In recent years, as owners and bridge engineers have become more familiar with FRP bridge decks, the industry is gradually exiting the demonstration phase and entering an era of acceptance and use that takes advantage of the benefits of composites.  Rehabilitation or new construction with FRP bridge decks can and will flourish when used in applications that utilize their benefits over conventional materials.

Vehicular Bridge Deck Suppliers

American Composites Manufacturers Association   1010 North Glebe Road, Arlington, VA  22201
P: 703-525-0511  F: 703-525-0743  E: info@acmanet.org
New York Office  600 Mamaroneck Avenue, Suite 429  Harrison, NY  10528 
P: 914-381-3572   F: 914-381-1253