Concrete surfaces are prepared prior to CFRP application. On the Coats Lane bridge, sand blasting was used to remove loose particle that cuold hinder adequate bonding.

MoDOT strengthens bridges with FRP composites

A recent survey conducted by the National Cooperative Highway Research Program (NCHRP, Synthesis 249) indicated that sixty-three percent of the North American transportation agencies expect to see the need to increase the live load capacity of existing highway bridges grow as the infrastructure continues to age. Many bridges have exceeded their design life and carry loads heavier than they were originally designed to bear.

Yet budget constraints due to shortage of available funds have forced many state DOTs to post load restrictions on deficient bridges, while professionals search for practical and economic upgrade methods.

One conventional method for improving the live load capacity of bridges involves section enlargement and the use of epoxy-bonded steel plates. Corrosion-related problems and difficulty of application due to the heavy weight of steel plates, however, have limited the use of this technique in the United States.

Recent developments in advanced fiber reinforced polymer (FRP) materials have made externally bonded FRP systems an effective alternative to steel plate bonding. Research studies and field applications documented by ACI Committee 440 show that FRP materials provide an excellent and economical solution for the structural upgrade of bridge components due to their lightweight, corrosion-resistant, and high tensile strength properties.

The most important benefit in using FRP for highway structural repair and strengthening applications is the speed and ease of installation. The higher material cost is typically offset by reduced labor, use of heavy machinery, and shut-down costs, making FRP strengthening systems very competitive with traditional strengthening techniques.

In response to the growing popularity of FRP materials, criteria for evaluation FRP systems are becoming increasingly available to the construction industry. Following are a few examples.

• In the United Kingdom, the Concrete Society has recently released Technical Report No.55 - Design Guidance for Strengthening Concrete Structures Using Fiber Composite Materials;
  

• The American Concrete Institute (ACI) is in the final stages of releasing a similar document to be used as a guide for the design and construction of externally bonded FRP systems for strengthening concrete structures; and
  

• FHWA is currently sponsoring research programs to develop model specifications for repair and strengthening of existing bridges using FRP composites. The progrmas are also designed to ensure the quality and performance of FRP strengthening.

Figure 1: CFRP Strengthening for Coats Lane Bridge

FRP put to the test

In Missouri, the latest FRP upgrade project involves strengthening three off-system bridges located in Boone County. Estimated construction years of these bridges (Brown School Road Bridge, Creasy Springs Bridge, and Coats Lane Bridge) are 1970, 1976, and 1970, respectively. Each bridge consists of a single-span, simple supported deck constructed with precast reinforced concrete (RC) channel sections, 38 inches wide, and 18 to 24 inches deep with a 4-inch thick slab running the entire span of the bridge. Each channel has RC diaphragms, spaced at approximately 6'3", connecting the two stems. The precast channels are tied together with 1-inch diameter steel bolts and fasteners to ensure composite behavior.

In 1986, the lanes of Brown School Road Bridge were widened with two 18-inch thick and 51 inch wide RC slabs, one on each side of the deck. The edge slabs were designed to withstand HS20 truck loading. Creasy Springs and Brown School Road bridges are located on asphalt surface roads and have a high traffic count. Coats Lane Bridge is located on a gravel County road. All three structures were evaluated in 1979, and a 15-ton load limit was determined based on load-posting criteria used at that time plus whatever information was available about the structures.

Approximately 3,600 vehicles per day cross Creasy Springs and Brown School bridges with an estimated 10 percent truck usage. Coats Lane is used by approximately 160 vehicles per day. Due to increasing traffic counts and use of heavier traffic, the County desired to strengthen the bridges to remove the load posting. Upgrading Creasy Springs and Brown School Road bridges would open up the surrounding area to greater accessibility for industry. Due to the weight restrictions, loaded trucks from a local rock quarry and asphalt plant have to avoid these bridges.

Upgrading Coats Lane Bridge would allow emergency vehicle to reach area residents without being delayed by a six-mile detour. Analysis using the current AASHTO code indicated that the RC channels required upgrading of their flexure and shear capacity to carry the new truck loading.

Boone County investigated the possibility of upgrading or replacing these bridges in June 1999. The initial cost estimate of deck strengthening for all three bridges using conventional upgrade methods was approximately $220,000. Replacing these three bridges was not an option. The engineering firm of Harrington and Cortelyou Inc. (H&C) of Kansas City, Missouri, brought the use of externally bonded FRP composites to the attention of the County as a strengthening alternative that could result in cost savings. To ensure proper application and quality control of the FRP system, however, the County Public Works Department decided to award this project as design-build.

The project was awarded to Structural Preservation Systems, Baltimore, a specialty concrete structural repair and upgrade contractor with approximately 10 years of experience in the design and application of FRP strengthening systems. Following a field investigation, condition survey, and a review of bridge plans, the initial estimate for a structural upgrade of the three bridges with FRP composites showed savings in excess of $100,000. To allow for load posting removal, MoDOT requested a full-scale load testing of the bridges before and after strengthening. H&C supervised and coordinated these efforts.

Material Characteristics

The original bridge plans indicated that the design concrete strength is 3,000 psi. However, a field test conducted using a Schmidt-Hammer yielded a concrete strength of approximately 9,000 psi. A decision was made to use a concrete strength of 5,000 psi for deck analysis and strengthening design. Steel yield strength of 40 ksi was also used. Visual inspection of the bridges revealed no significant deterioration of the concrete channels apart from some hairline cracks.

Structural Capacity

The capacity of the three bridges was calculated according to AASHTO specifications. Summaries of the flexural and shear strength and design requirements, based on HS20 truck loading for each bridge, are given in Tables 1 and 2. The tables also give the required level of strengthening to achieve the design strength.

Table 1: Summary of Flexural Requirement

Table 2: Summary of Shear Requirment

Design and application

The design of externally bonded FRP srengthening was achieved using carbon FRP (CFRP) reinforcement. The CFRP material used on all three bridges has a design strength of 550 ksi, a modulus of 33,000 ksi, and an ultimate strain at failure of 0.017 inch/inch.

Flexural and shear strengthening was based on the design approach proposed by ACI Committee 440. Table 3 provides a summary of the CFRP reuirements to correct the flexural deficiency of a single bridge channel, while Table 4 provides a summary of requirements to correct its shear deficiency.

Table 3: Flexural CFRP Reinforcement

Table 4: Shear CFRP Reinforcement

Although strength calculations indicated that Coats Lane Bridge did not need flexural strengthening, a decision was made to apply CFRP strengthening to the bridge to ensure flexural strength reserve. (See figure 1.)

The CFRP strengthening system was applied by means of the wet lay-up procedure using unidirectional CFRP sheets of 0.0065-inch thickness. Wet lay-up systems are saturated in place and cured in place and, in this sense, are analogous to cast-in-place concrete. A saturating resin, along with the compatible primer and putty, is used to bond the FRP sheets to the concrete surface.

The surface of the concrete was prepared prior to CFRP application by sand blasting it to remove loose concrete and other particles that could hinder the development of an adequate bond. Because bonding is the main shear transfer mechanism between the concrete and the CFRP system, achieving a composite behavior in the upgraded member is very sensitive to surface preparation and application of the system. It is advisable for a contractor with a record of success with similar projects to apply the FRP strengthening systems.

Elastic in-situ load testing

To assess service-load performance before and after strengthening, two load tests each were scheduled for Coats Lane Bridge and Brown School Bridge. The load test did not seek to evaluate the safety or the ultimate load-carrying capacity of the entire structure. Rather, it was used to confirm the expected performance of the bridges. The in-situ load testing procedure involved applying vehicular loading to the bridges using two H20 trucks. The response of each bridge was monitored during the test.

After strengthening of the Coats Lane bridge, the load posting was removed.

Each of the four load tests was performed in a similar manner. Linear Variable Differential Transducers (LVDT) were installed on the bridge RC channels to measure the strain of concrete and steel reinforcement. Strain gauges were installed on the surface of the FRP reinforcement to monitor their performance during the tests. LVDTs were also used to measure the vertical displacement of the bridge channels.

The load tests prior to strengthening were performed on July 18, 2001, and those following strengthening on August 4, 2001. In each test, two trucks were driven side-by-side along the length of the span, making two passes on the bridge. The trucks were stopped with their rear (heaviest) axle positioned at five locations along the span of each of the bridges.

The measured response of the bridges clearly indicated the improved deflection behavior due to FRP strengthening. The deflection at mid-span was reduced by approximately 20 percent after strengthening with CFRP composites. This behavior indicated that CFRP composites are contributing to the stiffness of the structure and should be carrying a portion of the applied load.

It also indicates that, for the same load level, the internal stress in the original members have been reduced after strengthening, thereby increasing the members load carrying capacity. The moment calculated from the strain readings indicated that the maximum computed moment after strengthening is approximately 40 percent less than that computed during the test performed before strengthening. For any given load, the deflection decreased after CFRP strengthening was applied.

For the three Boone County bridges, externally bonded carbon FRP composites provided the most economical solution for flexure and shear upgrade. The lower upgrade cost resulted from speed and ease of composite system application. Each bridge was closed for approximately one week, which resulted in minimum disruption to traffic. Due to its light weight, installation of the CFRP system was achieved by a crew of four men and did not involve the use of any heavy machinery. Results of in-situ load testing of the strengthened bridges suggest that the CFRP application improved the stiffness and the strength of the bridge deck.

Based on the results of the analytical calculations and the validation by load testing, a recommendation to remove the load posting was substantiated. Moreover, the design-build approach that was used on this project, along with the quality control measures of the specialized contractor were essential to achieve a successful upgrading with externally bonded FRP reinforcement.