THE CONCRETE BRIDGE MAGAZINE

SPRING 2018

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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CONCRETE BRIDGE TECHNOLOGY Production of Prestressed Concrete Piles Using Stainless Steel Strand by Dr. Krista Brown Although stainless steel strand is not a new technology, it has rarely been used for precast, prestressed concrete components in transportation structures. There are rumors and misconceptions about special requirements and equipment needed for its incorporation into the prestressed concrete fabrication process. This article focuses on the production process of full-size prestressed concrete piles using stainless steel strand. The article “Structural Design Using Stainless Steel Strands” in this issue of ASPIRE® addresses designing prestressed concrete components using stainless steel strand. Background According to Mark Bucci, bridge design manager at the Louisiana Department of Transportation and Development (LaDOTD), the use of noncarbon steel strand in the precast, prestressed concrete piles on one of three bridges on the Bayou Thunder Overflow Project is an effort to increase the service life of the structure from 50 to 100 years. Corrosion-resistant strand (stainless steel or carbon fiber reinforced polymer) was specified for the forty-two 24-in.-square, 78-ft-long prestressed concrete piles on one of the three bridges of the project. Low-permeability concrete was used in these piles to limit saltwater and chloride intrusion and new detailing for the pile tip and head was also incorporated. The cost of the piles with the stainless steel strands and other detailing measures to increase service life was more than 200% higher than that of piles using conventional carbon steel strands. The result was a $590,000 increase to the cost of the project. Gulf Coast Pre-Stress (GCP) in Pass Christian, Miss., bid the project based on the stainless steel option and was awarded the contract to produce the piles. GCP then began the journey to fabricate the first prestressed concrete piles using stainless steel strand to be used in an LaDOTD structure. Materials For the project, LaDOTD specified that the 7-wire, ½-in.-diameter stainless steel strand meet the chemical requirements of ASTM A276 and the mechanical and dimensional requirements of ASTM A416. Stainless steel alloy 2205 meets these requirements. In addition to the material testing certificates provided by the strand supplier, LaDOTD performed its own tests on strand samples that GCP took from the strand packs for this project. Compared with its carbon steel counterpart, 2205 stainless steel strand has a lower tensile strength and a lower elastic modulus, as shown in the table below. The stainless steel strands were also stressed to a lower fraction of the tensile strength (70% instead of 75%) as designed by LaDOTD, so the force in each strand was reduced. Therefore, to achieve the same level of precompression in the piles, twentyeight ½-in.-diameter stainless steel strands were required instead of the 24 carbon steel strands typically used in a 24-in.- square pile. Another reason to increase the number of strands by four was to keep the strand pattern symmetrical. It should be noted that the LaDOTD design for the piles was based on a 240 ksi ultimate strength for the stainless steel strands, which was lower than the 250 ksi for the strands that were used. To obtain the LaDOTD specified prestressing force, the strand was stressed to a fraction slightly less than 70% of the tensile strength. Strand was not the only item in the concrete piles that was stainless steel. LaDOTD specified that the W4.5 wire spirals be Type 304 or 316 annealed stainless steel. The tie wire, the strand used for lift loops, and the reinforcing bar template that GCP uses to ensure strand placement in the middle of the long piles, were also required to be stainless steel. In 2016, LaDOTD in t roduced perfomance-based concrete specifications1 that included lowpermeability concrete mixtures. GCP had already received approval for a 6-ksi concrete mixture with a minimum surface resistivity of 22 kohm-cm at 28 days. The concrete mixture did not require any modifications because of the stainless steel strand. Long Lead Times for Materials Stainless steel strand is not a stock item, especially when domestically produced strand is required, as on this project. Currently, suppliers only produce the strand when a sufficient number of orders have been placed to warrant production. Sumiden Wire was willing to guarantee delivery within six months for the ½-in.- diameter stainless steel strand, which is produced in its Dickson, Tenn., facility. Lead times for the stainless wire for the spirals and preformed tie wire were six and two weeks, respectively. Production In November 2017, GCP cast the 78-ft-long, 24-in.-square prestressed concrete piles. In the typical production cycle, four piles were cast in each of two adjacent 420-ft-long casting beds, for a total of eight piles. The use of the stainless steel strand did not require GCP to change

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