Galvanic Corrosion Potential and Material and Bond Properties of Titanium Reinforcing Bars

The use of titanium reinforcing bars for concrete has been proposed. The primary advantage of titanium is its corrosion resistance enhancing concrete element performance and permitting reduced cover in near-surface mounted applications. Like steel, titanium exhibits an elastic behaviour to a proportional limit, a definable yield value, and exhibits a great deal of ductility. Grade 5 titanium exhibits yield strength approximately twice that of ASTM A615 steel and an extensional modulus about 55% of steel.
In this study, the effects of coupling titanium and A615 steel in concrete are investigated considering the potential for galvanic corrosion. This study provides evidence that there are no apparent galvanic effects of coupling titanium and A615 reinforcing bars. The study goes on to characterize geometric and material and fatigue properties of titanium reinforcing bars. Bond characteristics of the titanium bars were assessed using ASTM D7913 pull-out tests, ASTM A944 beam-end tests and concrete prism tension tests. The nature of reinforcing bar bond to concrete is such that deformed bars exhibit very similar patterns of bond stress-slip behaviour. Provided adequate deformations are provided, the bond-slip relationship is dominated by concrete behaviour. The bond performance of the titanium bars was similar to that of A615 bars and, as expected, affected by the rib ratio. The bond behaviour of titanium is similar to that of steel bars and the calculated development lengths are essentially in the ratio of yield strengths of the materials. Crack widths are proportional to modular ratio, while spacing is inversely proportional to the stiffness of the initial bond-slip response. Therefore, a lower modulus bar will exhibit larger crack widths unless bond characteristics are improved proportionally.
An analytical study is presented to evaluate current design paradigms in relation the properties of titanium reinforcing bars. A combination of ACI 318 provisions for strength and ACI 440.1R provisions for serviceability are necessary when designing with titanium. The resulting hybrid design approach is illustrated in a series of benchmark flexural designs. Recommendations and limitations for the design of titanium reinforced flexural members are presented.