CHAPTER 6
Summary of Findings and Recommendations for Future Research

6.1 Summary of Findings

This research effort produced proposed language for guide specifications for the design and construction of concrete bridge elements prestressed with stainless steel prestressing strands. Analytical modeling and physical tests were also conducted. The project research team also conducted analytical and physical tests to provide the information required for the development of the design and construction guide specifications. Language for the proposed design and construction guide specifications has been provided to the AASHTO Committee on Bridges and Structures for consideration. The following are selected highlights of the research findings:

• Prestress losses:
  • Test results demonstrated that relaxation loss for stainless steel prestressing strands was approximately the same as for conventional low-relaxation carbon steel prestressing strands.
  • Test results demonstrated that friction loss for stainless steel prestressing strands was approximately the same as for conventional carbon steel prestressing strands.
• Harping of stainless steel prestressing strands:
  • Tests of draped stainless steel and carbon steel prestressing strands indicated that SS strands could be successfully draped using hardware typically used for CS strands.
• Transfer length of stainless steel prestressing strand:
  • Test results demonstrated that the transfer length of stainless steel prestressing strands was similar to those measured for CS strands and could be predicted adequately using current provisions in the LRFD BDS.
• Development length of stainless steel prestressing strand:
  • Test results demonstrated that the development length of stainless steel prestressing strands was similar to those measured for CS strands and could be predicted adequately using current provisions in the LRFD BDS.
• Bond of stainless steel prestressing strand:
  • Test results demonstrated that the bond strength of stainless steel prestressing strands in concrete and grout was similar to those measured for CS strands and that the bond strength satisfied the PCI Strand Bond Task Group specifications (PCI Strand Bond Task Group, 2020).
• Full-scale girder performance and flexural design:
  • Since the likely failure mode for concrete elements prestressed using stainless steel prestressing strands is rupture of strands, the extreme compression fiber does not reach the assumed ultimate compression strain of 0.003. Therefore, it is recommended that factors for the rectangular compression stress block be computed using a general form of the stress block where crushing of the concrete is not assumed as the failure mode.

• • Test results for two fatigue tests of concrete elements prestressed and posttensioned with SS strands demonstrated that fatigue cycles did not have a noticeable effect on the nominal resistance of the sections tested.
• Ductility:
  • Although stainless steel prestressing strands have a reduced ultimate tensile strain compared to conventional CS strands, bridge members prestressed using the strands may still have adequate ductility and provide warning of failure by extensive cracking and deflection.
• Resistance factors:
  • When concrete bridge elements are prestressed using stainless steel prestressing strands, the tension-controlled resistance factor was calibrated to a value of 0.85, which is less than the 1.0 value used for conventional carbon steel prestressing strands.
  • The strain limits defining the transition between compression-controlled and tension-controlled designs were calibrated to values of 0.004 and 0.0075, respectively, for stainless steel prestressing strands.
  • In the posttensioned application, due to the possible failure at the neighboring jacking hardware (chucks) resulting from stress concentration, a more conservative resistance factor of 0.75 is used for both tension-controlled and compression-controlled failure.
• Pretensioning:
  • Reducing stresses at jacking and immediately prior to transfer provides slightly greater load resistance and deformation for a concrete bridge element prestressed with SS strands before the ultimate tensile strain is reached and strands rupture.
  • During strand detensioning operations, stresses in the remaining tensioned stainless steel prestressing strands may prematurely rupture because of the relatively small ultimate tensile strain of stainless steel prestressing strands. (This can be avoided by providing a longer length of free strand or detensioning all strands at the same time.)
• Posttensioning:
  • A test of a beam with multiple single-strand unbonded posttensioning tendons initially exhibited concrete crushing at the point of maximum moment. As loading continued (post failure), strand rupture occurred at the anchorages due to high-stress concentration at the chuck level at a stress level slightly lower than the ultimate measured stress.

6.2 Recommendations for Future Research

The following research topics are recommended for further study to more completely address issues that will enable wider acceptance and use of stainless steel prestressing strands for concrete bridge elements:

• Explore the potential of galvanic corrosion between SS strands and other reinforcement, permanent anchorages, strand or tendon deviation hardware, or other embedments, and to develop strategies to mitigate corrosion, if needed.
• Investigate the potential effects of reduced ultimate tensile strain in SS prestressing strands on the shear resistance of prestressed concrete elements.
• Explore the potential effects of reduced ultimate tensile strain in SS prestressing strands on the design of D-regions using the Strut-and-Tie Model (STM).
• Explore potential effect of reduced ultimate tensile strain on the longitudinal reinforcement requirement for prestressed concrete girders with SS strands.
• Evaluate the effect of simplifying assumptions related to strand strains, specifically the definition of the strain in strands at decompression, or the locked-in strain difference between SS strands and concrete.
• Evaluate design and detailing to allow use of SS strands in elements for which a special level of ductility is required for acceptable performance during extreme events, such as seismic events or vessel collision.

• Assess whether SS prestressing strands can be used for prestressed concrete piles as foundation elements in regions where extreme event designs are required for seismic or vessel impact.
• Evaluate whether concrete cover may be reduced for structural elements with stainless steel prestressing strands because the potential for corrosion is greatly reduced.
• Investigate the initial and long-term costs of using SS prestressing strands for bridge elements, especially in severe corrosive environments.
• Evaluate approaches to reducing the potential for spontaneous detensioning of SS pretensioned strands during element fabrication.
• Further research may be beneficial to evaluate whether moment redistribution can be safely used for elements with SS prestressing strands.
• Investigate the potential for unbonded posttensioning tendons to suffer premature strand breakage at anchorages when SS prestressing strands are used. This may include development of modified posttensioning strand anchorage hardware details to reduce potential strand rupture at the anchorage.
• Evaluate whether the use of SS prestressing strands for posttensioned tendons may not require treatment with corrosion inhibitors or some forms of corrosion protection.
• Assess the performance of SS strands in the bare condition under fatigue loading since the manufacturer of SS strands reports that these strands are not recommended for cable-stayed bridges because of concerns related to fatigue.