Corrosion-Free Carbon Fiber Reinforced Polymer for Prestressed Piles

About the report:

This study investigated the feasibility of replacing traditional steel strands and spiral reinforcement in bridge piles with carbon fiber reinforced polymer (CFRP) strands and spiral reinforcement.  CFRP is a corrosion-free material, whereas the conventional steel reinforcement used in a traditional pile is prone to corrosion when exposed to chlorides.  The Virginia Department of Transportation (VDOT) placed CFRP in 18 piles in two bents of the Nimmo Parkway Bridge over West Neck Creek in Virginia Beach.

Initially, two CFRP-reinforced concrete test piles were cast for the project. To fabricate these piles, a buffer material was used to protect the ends of the strands prior to placing them in the couplers used for prestressing.  The couplers had CFRP strands in chucks on one end and the steel strands in chucks on the other end.  Prestressing force was applied through the steel strands.  Concretes used in the piles had conventional slump.  To minimize damage during consolidation, rubber tipped vibrators were used.  Concretes were steam cured under insulating blankets; however, the couplers used for prestressing the CFRP strands were protected from high heat by keeping the area exposed to the environment.  Heat above 122 °F was thought to cause slipping of the strands in the couplers.  After the concrete had cured sufficiently, the piles were detensioned and removed from the forms.  When the contractor was ready for driving the piles, the CFRP piles were shipped to the job site, instrumented, and successfully driven at one end of each of the two bents. Since the fabrication and driving operation with the test piles were successful, the remaining 16 CFRP-reinforced piles were cast and driven in two of the bents.

This project provided VDOT with the ability to implement the use of corrosion-free reinforcement in pre-stressed piles where corrosion is a concern, such as those exposed to brackish or saltwater.  To guide VDOT in selecting projects where premium reinforcement such as CFRP would be economically justified, a life-cycle cost analysis was performed using the actual costs of the Nimmo Parkway contract and maintenance plans contained in consultant reports created for another VDOT structure: the Hampton Roads Bridge-Tunnel.  These reports provided pile maintenance plans designed to extend the service life of a heavily trafficked VDOT structure in an aggressively corrosive environment. Although user costs of maintenance activities are potentially significant factors in VDOT decisions when VDOT’s costs between alternatives are similar, the life-cycle cost analysis in this study focused on VDOT costs alone because in exchange for high construction costs for piles with corrosion-free reinforcement, user costs for corrosion mitigation in those piles is nil.  Thus, in an aggressively corrosive environment, if agency costs for 100 years of service from premium-reinforced piles can be shown to be competitive with discounted construction and corrosion mitigation costs of conventional piles, user costs add no substance to the decision criteria.

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