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Photo depicting workers driving in a prestressed pile with Corrosion-Free Carbon Fiber Composite (CFCC) reinforcement at the Nimmo Parkway Bridge site in Virginia Beach 

Fig. 1. Prestressed pile with Corrosion-Free Carbon Fiber Composite (CFCC) reinforcement being driven at the Nimmo Parkway Bridge site in Virginia Beach
            

Piles with Corrosion-Free Carbon Fiber Composite Cable in Virginia
Celik Ozyildirim, PhD, PE, and Stephen Sharp, PhD, PE, Virginia Center for Transportation Innovation and Research, a division of the Virginia Department of Transportation

Introduction

The corrosion of steel reinforcement in concrete structures is a common problem for state departments of transportation (DOTs) due to exposure to the marine environment or deicing salts (see Figures 2 and 3). The Virginia Department of Transportation (VDOT) has been using corrosion resistant reinforcement, such as solid stainless steel reinforcing bars (rebar), in bridge structures to extend their life by mitigating corrosion and thereby reducing concerns associated with maintenance costs, traffic congestion, and public safety. As a continuation of such efforts, VDOT has initiated research into improving prestressing strand materials. Corrosion is of great concern in strands, which are under high stress and used in load-carrying structural elements.

Figure 2 shows a photo of a concrete bridge girder with exposed steel caused by corrosion, and a second photo which is a close up of the exposed steel strands showing corrosion and some section loss. 

Fig. 2. Strands corroding in beams caused by leaking joints.
            
Figure 3 shows 2 photos, both examples of tendon failures caused by strand corrosion in the Varina Enon Bridge. 

Fig. 3. Tendon failure in the Varina Enon Bridge because of strand corrosion.
            

Solution

A possible solution is the use of corrosion-free carbon fiber composite cable (CFCC), which is also known as carbon strand. It was developed and first used on a bridge built in 1988 off the coast of Japan. This bridge, which replaced a 20-year-old structure that failed due to corrosion, continues to show no signs of corrosion. (Enomoto et al., 2012). CFCC does not corrode because it is corrosion-free. Around the world there are examples of CFCC used in prestressed elements. Here in the United States, the same strand material was placed in a bridge structure in Michigan and has undergone continuous monitoring since construction was completed in 2001 (Enomoto et al., 2012). Currently two other bridges in Michigan are being constructed using CFCC strands. Moreover, even the cables of a cable-stayed bridge in Maine were constructed with CFCC (Berube et al., 2008). The rationale behind using CFCC is that even though it costs more per unit length than conventional steel strand, repairs to elements with steel strands are costly and difficult since these elements generally are load-carrying members under the bridge deck.

Nimmo Parkway Project

Construction of Nimmo Parkway in Virginia Beach, Virginia includes two 1600 foot long bridges, which carry in both directions traffic over West Neck Creek and adjacent wetlands. The $58 million project is under construction and is expected to be completed in the summer of 2014. Once completed, it will provide major congestion relief in the vicinity of the Virginia Beach Municipal Center, a vital part of city’s tourist economy.

The Nimmo Parkway bridges have 272 piles supporting the two long bridges. Eighteen of the piles, located adjacent to West Neck Creek, contain CFCC reinforcement. All of the piles have square cross-sections that measure 2 feet by 2 feet. Although the cross-sectional dimensions are the same for each pile, there were two allowable pile reinforcement designs: The CFCC-reinforced piles employed the circular spiral design because it eliminated a sharp corner radius.

Carbon-fiber composite cable is a very strong composite material with an ultimate strength of at least 270 ksi. In design, however, the prestressing stress of this composite material was limited to 65% of the ultimate strength following ACI 440 recommendations rather than up to 75% for the conventional steel strand. Although the same number of strands was used in the CFCC and conventionally reinforced piles, a larger diameter CFCC strand was used compared to the conventional steel strand; for the CFCC 0.6 inch instead of the 0.5 inch used for the conventional piles.

Unlike conventional steel, special end preparation requirements were used to avoid crushing the CFCC at the chucks during stressing. At the ends, CFCC was wrapped with a mesh, and a braided steel grip was slid over the top of the mesh. Long wedges were placed before seating the wedges in the chuck. This chuck was placed in one end of a coupler, and a conventional chuck holding a steel strand was placed in the other end of the coupler. The prestressing force was provided by pulling the steel strand with a traditional jack.

Initially two test piles with CFCC were cast and driven (see Figure 1) to determine the length for the production piles. These concrete test piles were instrumented to determine the behavior during driving. The data indicated that during driving the behavior between the CFCC and the conventional piles was similar. After the successfully completion of the test piles, 16 production piles were cast with no problems.

Because it is important to properly prepare the ends for stressing, technicians from Japan performed the end preparation of the test piles while local workers observed the process. Later, during fabrication of the production piles, local workers prepared the ends at the plant. Further work is underway by the CFCC producer and VDOT to simplify and reduce the time for end preparations.

This project demonstrated that the fabrication and placement of corrosion free piles reinforced with CFCC is possible. VDOT is looking for more applications not only in piles but also other prestressed elements to extend their life and thereby avoid costly repairs, traffic interruption, and unsafe conditions.

For more information please contact Celik Ozyildirim celik@vdot.virginia.gov or Stephen Sharp stephen.sharp@vdot.virginia.gov.

References

  1. Berube, K. A., Lopez-Anido, R., A., and Caccese, V., Integrated Monitoring System for Carbon Composite Strands in Cable-Stayed Bridge,
    Penobscot Narrows, Maine. Transportation Research Record: Journal of the Transportation Research Board. Washington, DC, pp 177-186, 2008
  2. Enomoto, T., Grace, N. F., and Harada, T., 2012: Life Extension Of Prestressed Concrete Bridges Using CFCC Tendons And Reinforcements:
    http://www.iifc-hq.org/proceedings/CICE_2012/13_Field%20Applications%20and%20Case%20Studies/13_476_Enomoto,Grace,%20Harada_
    LIFE%20EXTENSION%20OF%20PRESTRESSED%20CONCRETE%20BRIDGES.pdf