Lawrence Kahn, Professor, Georgia Institute of Technology, Atlanta, Georgia
In order to develop “corrosion-free” precast prestressed concrete bridge piles with design life greater than 100 years, the Georgia Department of Transportation (DOT) initiated research in 2009 at the Georgia Institute of Technology (Georgia Tech) to develop high-strength stainless steel prestressing strand to replace the conventional grade 1080, 270 ksi low-relaxation, seven wire strand. To determine corrosion resistance of conventional and stainless steel prestressing reinforcement, test specimens containing either a single wire or 7-wire prestressing strands were exposed to various simulated concrete pore solutions made with various concentrations of NaCl. The specimens were evaluated using cyclic potentiodynamic polarization (CPP) techniques (ASTM G61) to determine corrosion potentials in marine environments. Cl- concentrations were varied in steps of 0.1 M from 0.0 M to 1.0 M. A 0.5 M solution represents typical seawater which occurs along the Georgia coast from I-95 to the Atlantic Ocean, and a 1.0 M represents concentrated salts which occur in cracks in piles within the tidal zones.
Working with Sumiden Wire Products Corporation, researchers identified six stainless steel alloys for potential application: grades 17-7, 304, 316, 1080, 2101, 2205, and 2304. The performances of all alloys were compared with that of conventional 1080 steel used for standard prestressing strands. When the austenitic stainless steel grades 304 and 316 were cold-drawn to wire diameters for use in ½-in. diameter, 7-wire strand, the ultimate strengths were about 181 ksi, but the structure was changed from principally austenite to ferrite, and their generally excellent corrosion resistance was lost. Cold drawing of the martensitic grade 17-7 also resulted in poor corrosion resistance of that alloy. Of the modern duplex stainless steels, grade 2205 and grade 2304 developed ultimate tensile strengths between 240 ksi and 250 ksi along with good corrosion resistance. However, tensile strength tests of seven-wire strand made using the grade 2304 stainless steel showed that the strand failed in the standard prestressing anchors at loads between 40% and 60% of its ultimate strength due to the material’s notch sensitivity. Therefore, the grade 2304 stainless steel was not used for further production tests. Cold-drawn grade 2101 showed unsatisfactory corrosion resistance. Table 1 shows the corrosion resistance of the various strands in alkaline concrete solutions (pH of 12.5) and in carbonated concrete solutions (pH of 9.5). As anticipated, conventional high-strength steel strand (1080 steel) demonstrated very poor corrosion reistance with extensive pitting corrosion initiating in the grooves between wires.
Induction heating processes were used in the trial manufacture of both 2205 and 2304 strand by Sumiden Wire Products, Dixon, Tennessee. The induction heat treatment worked well and increased the ultimate tensile strength from 225 ksi to between 240 ksi and 250 ksi. Further, relaxation tests of the strands showed that relaxation losses were less than 2.5% after heat treatment, whereas relaxation losses were about 8% before treatment (Schuetz et al., 2012).
Based on the superior strength and corrosion resistance of the 2205 alloy, ½-in. diameter strand was produced and was used to construct three70-ft long, 16-in. square precast prestressed concrete piles. The strand was stressed to 70% of its 250 ksi ultimate strength. The stressing and construction operations were performed at Standard Concrete Products Company, Savannah, Georgia and proceeded identically to those for two companion piles made with conventional grade 1080 strand. Construction using 2205 stainless steel strands and grade 304 stainless steel wire spirals was completed with no difficulties and with no need for any special operations. Grade 304 with yield stress of 50 ksi was used for the spirals so that the small radius bends could be made; the high-strength 2205 wire could not be bent without fracturing. The same 5000 psi design strength concrete was used for all piles (28 day strength was 8100 psi).
Transfer length measurements at each end and on each side of all piles were made using external DEMEC gauges. The average transfer length was 18 in., 36 times the nominal strand diameter. This length is less than the 60 times the diameter maximum specified in the AASHTO LRFD Bridge Design Specifications [2012].
About six months following pile construction, three of the stainless steel reinforced piles and two piles with conventional strands were driven to refusal in the Savannah River. They were then extracted for future flexural, shear, and development length tests. No cracking, spalling or other damage was noted in any pile.
Further tests of the reinforcement and of the piles are being conducted at Georgia Tech. The author’s preliminary conclusion is that high-strength stainless steel strand, wire, and spiral show excellent promise for providing 100-year service life for prestressed concrete piles in marine environments. The Georgia Tech researchers are working with Georgia DOT to standardize specifications and design of corrosion-free piles for use in exposed pile bents along Georgia’s coast.
Acknowledgments
The research reported herein was sponsored by the Georgia DOT through Research Project Number 10-26. For more information, please contact Lawrence F. Kahn, [email protected]
References
- AASHTO LRFD Bridge Design Specifications [2012], American Association of State Highway and Transportation Officials, Washington, D.C.
- ASTM G 61 (2009) Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA Moser, Robert D; Preet M. Singh; Lawrence F. Kahn; and Kimberly E. Kurtis [2012]; “Durability of Precast Prestressed Concrete Piles in Marine Environment, Volume 2: Stainless Steel Prestressing Strand & Wire,” GDOT Research Project Report No. 10-26, Georgia Institute of Technology, Atlanta, Georgia, pp. 342. http://www.dot.ga.gov/doingbusiness/research/projects/Pages/Reports.aspx
- Schuetz, Daniel; L. Kahn; K. Kurtis; P.Singh; and R. Moser [2012]; “Preliminary Studies of the Mechanical Behavior of High-Strength Stainless Steel Prestressing Strands,” Proceedings PCI Convention and National Bridge Conference, Nashville, TN Sept 29-Oct 2, 2012, Paper #70.