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Federal Highway Administration > Publications > Research > Structures > A Laboratory and Field Study of Composite Piles for Bridge Substructures |
Publication Number: FHWA-HRT-04-043 |
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Composite materials have attractive features such as high resistance to corrosion, high strength-to-weight ratios, and the ability to be used in infrastructure projects with little or no maintenance requirements (Meiarashi, et al., 2002). Recent studies have found that the lower weight of bridge decks made of FRP composite materials results in greater cost effectiveness due to shorter construction time and decreased cost of the bridge superstructure (Ehlen 1999). The economical competitiveness of composite piles needs to be assessed. To help provide data for such assessments, this chapter provides cost information for the composite piles and prestressed concrete piles used in this research project.
The cost per unit length of the concrete-filled FRP composite piles used at the Route 40 Bridge project was $312/m ($95/ft). This unit cost includes the materials (GFRP tube and concrete infill), the manufacturing costs (including FRP tube and concrete casting), and shipping to the job site. The cost of driving the composite piles was $66/m ($20/ft). This results in a total cost of $378/m ($115/ft) for the installed composite pile. On the other hand, the total cost of the installed 508-mm (20-inch) square prestressed concrete pile was $213/m ($65/ft). Therefore, the initial cost comparison indicates about 77 percent higher unit cost for the composite pile. No instrumentation was installed in these piles.
No significant time differences were observed in the installation time required for these two types of piles. The unit weights of the two piles were similar due to the concrete infill; therefore, the high strength-to-weight ratio of the composite material was not utilized. The composite piles were not easy to lift or handle, and required special arrangements and equipment. For example, the slickness of the FRP pile's outer surface made it difficult to use conventional slings to lift the piles with the crane. In addition, the number of pick-up points had to be increased due to the lower cracking moment of these piles compared to the prestressed concrete pile. The difficulties encountered by the contractor in handling these piles also may have been associated with lack of experience in dealing with these piles. However, such problems are expected to occur in other projects until the use of composite piles becomes more common.
The cost information for the three types of piles tested at the Route 351 Bridge project is provided below. These test piles were heavily instrumented, and the costs provided below do not include the instrumentation or its installation. However, pile fabrication and handling costs may have been increased due to the presence of the instrumentation.
The cost for the 24-inch FRP tubes was $165/m ($50/ft). This cost is freight-on-board (FOB) at the Hardcore plant in New Castle, DE. The cost associated with steel reinforcement, concrete infill, and transportation was about $541/m ($165/ft). The resulting total unit cost for the pile delivered on site is $705/m ($215/ft). The cost of installation of the test piles at this project was about $82/m ($25/ft). Therefore, the initial cost of the installed FRP composite pile is $787/m ($240/ft).
The ordinary unit cost for this type of pile FOB at the Plastic Piling, Inc., (PPI) plant in Rialto, CA is $410/m ($125/ft). The cost for shipment of this test pile from Rialto, CA, to Hampton, VA, was $209/m ($64/ft). The resulting total unit cost for the pile delivered on site was $619/m ($189/ft). The cost of installation for the test piles at this project was about $82/m ($25/ft). Therefore, the initial cost of the installed PPI composite pile was $701/m ($214/ft).
The total cost of the installed 610-mm (24-inch) square prestressed concrete pile was about $180/m ($55/ft).
The initial cost information given above indicates that the unit costs for the PPI plastic composite pile and the Hardcore composite pile are 289 percent and 337 percent higher than for the prestressed concrete pile, respectively.
No significant installation time differences were observed during handling and installation of the composite piles compared to the prestressed concrete pile. The handling and lifting of the FRP composite piles was the most difficult due to the slickness of the outer surface of the FRP shell. Special arrangements were required to handle this pile, including using steel anchors to keep the lifting slings from sliding and rolling. The location and number of pickup points for handling this pile had to be modified with respect to conventional practice due to the lower cracking moment of these piles compared to the prestressed concrete piles. These kinds of complications were not encountered during the handling and installation of the plastic composite pile.
The initial costs of the composite piles studied in this project were higher than the initial unit costs for prestressed concrete piles. The initial unit costs of the installed composite piles at the Route 40 Bridge were about 77 percent higher than the unit costs for the prestressed concrete piles. The initial unit costs for the composite piles installed at the Route 351 Bridge were higher than the cost of the prestressed concrete piles by about 289 percent and 337 percent for the plastic and FRP piles, respectively.
The cost effectiveness of the composite piles is expected to improve with economies of scale as production volume increases. The low maintenance requirements of these composite piles also increase their cost effectiveness. Life cycle cost analyses should consider the pile lifespan, the annualized maintenance costs, and the replacement costs of composite piles compared to prestressed concrete piles. The number of years of use that should be factored into the life cycle cost analyses may not be related only to the lifespan of the piles, but may also be governed by the actual lifespan of the bridge superstructure, which may be related to deterioration or increased traffic demands. A life cycle cost analysis was not performed for this study due to lack of maintenance cost and frequency information.