Current Practices In FRP Composites Technology
FRP Bridge Decks And Superstructures
The purpose of this advisory is to provide guidance on the application of fiber reinforced polymer (FRP) composites for bridge decks & superstructures based on generally accepted practice in the industry today. The use of these materials has been fostered through the Innovative Bridge Research and Construction (IBRC) Program, a funding program administered by the Federal Highway Administration. The following does not imply any endorsement by the FHWA or the author.
FRP composite decks & superstructures have been used in lieu of conventional construction material on approximately 40 bridges nationwide. Noticeably, over half of these are in the states of West Virginia, Ohio, and New York where the use of de-icing salts has led to the premature deterioration of many existing bridges. Though composites have been researched and used extensively in other industries, these bridge projects have been designed and built without the benefit of nationally accepted standards. This document is based on recent construction experience of various DOT's, input from researchers, general state of the practice, and in-service performance.
FRP bridge decks and superstructures are typically made with vinyl ester or polyester resin reinforced with E-glass fiber. They are pre-engineered and pre-fabricated in a shop, then assembled and installed at a bridge site where a wearing surface is added. FRP's are engineered materials with their strength dependant on several factors such as fiber type and percent volume, fiber orientation, resin type, manufacturing method, and the bonding materials used in the final assembly.
There are more than six competitive suppliers of these systems. For a list of known suppliers and completed projects, go to: http://www.fhwa.dot.gov/bridge/frp/frppract.cfm.
- Light weight. A typical 200mm (8") FRP deck with its wearing surface weighs 122 kg/m2 (25 psf) vs. 581 kg/m2 (119 psf) for a standard 240mm (9.5") concrete deck.) See table below. Reduction in dead load on an existing bridge results in an increased capacity for live load with possible elimination of weight restrictions.
|FRP deck||Concrete deck|
|25 psf||119 psf|
A typical bridge superstructure spanning 10m (30') weighs approximately 270 kg/m2 (55psf).
- Resistance to de-icing salts and other chemicals.
- Fast installation due to modular, pre-fab nature and the elimination of time consuming forming and curing necessary for conventional concrete decks.
- Reduced traffic delay which leads to lower user costs, less expense for maintenance and protection of traffic, and better public relations.
- Good durability. The cracking, corrosion of reinforcement, and spalling associated with concrete decks is eliminated.
- Long service life. Many large, non-civil structures have been used in harsh environments for decades. FRP decks are expected to provide a long service life with little maintenance. At this point, the useful life is estimated to be at least 75 years.
- Fatigue resistance.
- Fabrication in a controlled environment is conducive to good quality control.
- Ease of installation. A deck replacement project can be done with a stand-by "where & when" contractor or maintenance forces using standard details. This type of project administration can bring fast relief of a posting problem.
- Cost savings. A bridge rehabilitation using FRP can solve a posting problem for much less than a bridge replacement.
- If a bridge can be rehabilitated rather than replaced because of using lightweight FRP, there will be less environmental impact and fewer permits required.
- Higher initial cost compared to a conventional concrete deck. The unit cost of FRP materials is often more expensive than conventional materials. Sometimes, the added expense is offset by other savings such as maintenance and protection of traffic (MPT).
- FRP's low modulus of elasticity leads to a deflection driven design which does not allow a designer to fully capitalize on the FRP's strength.
- Currently available designs are proprietary. There is no standard manufacturing process.
- Response to thermal change is slightly different than for concrete and steel and requires special consideration when an FRP deck is used on a concrete or steel superstructure or when FRP is used for a superstructure.
- Some past projects have experienced a failure of the wearing surface (i.e. cracking and/or debonding). Research is underway in NY to try to identify the best material for a wearing surface. Conventional asphalt has been used on some bridges.
- A thorough analysis of the material's behavior requires a finite element model. Designs will most likely need to be pre-approved.
- FRP material properties like strength and stiffness naturally degrade over time. The resultant tendency to creep must be addressed in the design. Appropriate strength reduction factors need to be used to insure adequate stiffness over the entire service life of the structure.
- Limited FRP experience within the construction industry.
- Lack of long term performance data. There are few FRP bridges that have been in service for any substantial length of time. The first bridge in the U.S. to carry public traffic was built in November 1996 in Kansas.
- Lack of design standards and conventions for material characterization.
- Bridge railing systems attached to FRP decks have not been crash tested. Some static tests have been done successfully.
In addition, engineers may fear that a liability issue exists because of the lack of performance history and the need to truly engineer a solution rather than follow established standards.
There are several active research projects that will further the state of the practice. Federal Highway Administration has contracted for studies that may evolve into AASHTO guidelines for the design and construction of composite decks and superstructures, as well as for material standards. In addition, FHWA's IBRC funding program is providing an abundance of data from completed projects. This information is still being collected.
There are several manufacturing methods used for FRP decks. (1. pultrusion e.g. Martin Marietta Composites, 2. vacuum-assisted-resin-transfer-molding (VARTM) e.g. Hardcore Composites, 3. open mold hand lay-up e.g. Kansas Structural Composites). Each has its own merits. The following table is offered as a qualitative comparison:
|Ability To Get Custom Sizes||Adherence To Dimensional Tolerances||Attractive Cost||Ability To Incorporate Special Features E.G. Scuppers||Overall Quality|
|Relative Benefit: H = High, M = Medium, L = Low|
|3. Open mold||H||M||H||M||M|
A special specification for FRP decks & superstructures will be performance-based because the systems use proprietary designs and manufacturing methods. Using performance standards also has additional benefits of encouraging innovation and allowing the refinement of new technology.
Any FRP deck system must appear on the purchasing agency's approved list of materials or meet nationally accepted testing standards if some are eventually established. Design calculations certified by a professional engineer, an installation procedure and working drawings are required to be submitted to the purchasing agency's chief engineer for structures. Wearing surface materials are also to be selected from an approved list if not listed specifically as part of the pre approved decking system. Each supplier is responsible for certification of the finished product as well as quality control (QC) during the manufacturing process. The purchasing agency will verify adherence to the approved QC plan through its own quality assurance (QA) reviews.
Though FRP decks and superstructures are very similar in nature, one important distinction is that an FRP deck is a manufactured product that the supplier designs and manufactures according to stipulated parameters. The manufacturer must certify compliance. An FRP superstructure with a span over 20' is, however, by definition, a bridge. Many states have laws that require that any bridge, either site built or manufactured, be certified by a licensed professional engineer and that a certified load rating analysis be filed.
Though not mandatory, the designer should consider specifying that FRP superstructures be load tested prior to placing the bridge in service. This will serve to verify the theoretical finite element model.
When to consider the use of FRP:
Appropriate projects to consider include:
- A posted bridge that could benefit from a reduction in dead load and subsequent increase in live load rating.
- A bridge needs to be widened without imposing additional loads on the substructure.
- Superstructures under 12 m span (and longer spans as technology evolves).
- A historic structure that must be saved (i.e. rehabilitated instead of replaced) due to its cultural value.
- Moveable spans where the lightweight can save operating expenses.
- A bridge with an existing lightweight deck (e.g. steel grating) that needs another lightweight deck. An FRP deck provides the additional benefit of protecting the superstructure from the elements.
- A bridge needs a deck designed for light duty like those on locally owned structures constructed of timber or corrugated steel pans with asphalt. In these instances, stringer spacing is typically less than three feet.
- A bridge needs an improvement in load rating sooner than can be addressed through a capital improvement program. It is often unacceptable to program work for five years in the future when postings present an immediate economic hardship.
- Accelerated schedule to installing decks or superstructures to reduce the cost of maintenance and protection of traffic and reduce congestion.
Other project selection criteria:
- Traffic volume: Though fatigue test data suggests that there is not a concern about using of FRP decks on high volume roads, the designer should be cautious about the application in an area that would be difficult to monitor or repair. Low volume roads provide an opportunity for easy access so that more can be learned with little risk.
- Skew and grade: Deck manufacturers have been successful in the fabrication and installation of skewed superstructures and the use of FRP decks on skewed steel bridges. At the present time, there are no general restrictions on such use, but caution is advised in these circumstances. The same is true for % grade.
- Span: A particular deck system is acceptable only up to the support span length it has been tested at. There are several deck systems that can be used with steel supports spaced at 2.5m. Superstructures built thus far have been relatively short spans (<14m). This is due to the cost of controlling deflection. Longer spans will most likely require the use of high performance carbon fiber or a hybrid system which utilizes concrete or another material with FRP.
- Depth: FRP decks are commonly available as a 200mm (8") deep section, although custom depths are available. During preliminary design, a rule of thumb can be used for the depth of a FRP superstructure: i.e. one inch for each foot of span. If desired, a slimmer section can be obtained by asking the manufacturer to engineer the fabric architecture accordingly.
Since the price of an FRP deck or superstructure typically sells for a premium to steel and concrete, part of the scoping process must involve an evaluation of all costs associated with a project. FRP may offer direct monetary and user cost savings due to speed of construction, shortening of the project, elimination of a detour structure, etc. In certain circumstances, however, they can offer significant advantages that result in a project with net savings over one using conventional construction practice. For example, capitalizing on their modular nature can reduce the duration of a project and subsequent user costs. Similar benefits may also be available from using prestressed concrete systems, precast structures, exodermic decks, etc., so those should also be investigated as alternatives. A cost comparison should be made considering all available options. Since cost can be influenced by a supplier's schedule and product demand, manufacturers should be contacted for an estimate of current prices and lead-time. Recent price ranges are:
- Decks: $700 - $860 /SM ($65-$80 /SF) of deck area
- Superstructure: $1500 - $1615 /SM ($140-150 /SF) of deck area
Specific design criteria are:
- To avoid long term creep, predicted strains under design load shall be less than 20% of the FRP composite's minimum guaranteed ultimate strength. The ultimate strength is based on coupon testing and noted in the approved plans.
- An environmental durability factor (knockdown factor) of 0.65 may be applied to the material properties to account for degradation of properties over time.
- Because of the material's typical low modulus of elasticity, most designs will be driven by deflection limitations and not strength requirements.
- Although the criterion for deflection is somewhat arbitrary, it is typically be kept at 1/800 of the supporting span length.
- In cases where the deck is expected to fail in a manner other than tension of the laminate, a factor of safety of five should be applied. As reliability factors are obtained for FRP materials, a load and resistance factor design (LRFD) methodology will be developed by AASHTO.
- Protect exposed surfaces from ultraviolet (UV) exposure using suitable paint.
- If an FRP deck is being installed on a steel superstructure, careful consideration must be given to the method of attachment. Composite action between the FRP and steel is not to be assumed in capacity calculations. However, the connection itself must detailed to prevent failure when load is inevitably transferred between the two. The design of the interface should either insure composite action or provide a slip mechanism to prevent it.
- FRP can heat up rapidly when exposed to direct sunlight. A dark wearing surface exacerbates the phenomenon. Experience has shown that a thermal gradient will occur between the top and bottom of an FRP deck when surface temperatures change fast but are not able to be conducted through the FRP immediately. This problem is especially likely to occur in early spring or late fall. The design shall assume a 100 degree F temperature difference between the top and bottom of the deck and account for any subsequent internal thermal stresses. Since this temperature gradient can lead to an arching action as the top surface expands faster than the bottom, it may also affect the selection of bearing and anchorage details. It is important to note that thermal stresses may be as large as stresses resulting from live loads.
- Each manufacturing method has its own benefits. In general, pultruded sections produce a good quality, consistent product with tight dimensional tolerances; however, prices tend to be slightly higher than for other methods. The more custom methods (VARTM and hand lay-up) allow the depth of the section to be easily changed, and to accommodate a cross slope, inclusion of scuppers, or custom "knockouts". The designer needs to decide if the application necessitates a particular method of fabrication.
- Acceptable wearing surfaces will be designated for each particular manufacturer.
- A bonded FRP curb may be used if and only if it is protected from impact by bridge rail. Concrete curbs have also been attached to FRP decks.
- In a decking application, a joint over the bridge seat can be eliminated by carrying the FRP deck over the abutment stem to terminate on the approach side of the bridge. This allows any joint leakage to weep into the fill rather than onto the seat.
- Pourable silicone joint material has performed well and is easily repairable.
Details of the FRP structure will be shown in the approved drawings, which were submitted to the owner or certifying agency for listing on an approved list. In the project plans, the designer will need to show outside limits of the deck, (the interior core of the system will be as per approved shop drawings), abutment and bearing details with elevations, and final elevation of the deck surface with wearing surface. Railing treatment will also need to be detailed by the designer.
Since FRP decks are relatively new, consultation with experienced engineers is strongly recommended.
Jerry O'Connor, formerly of NYSDOT Region 6.
Resource Center (Baltimore)