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• Conclusions - Corrosion Resistant Alloys for Reinforced Concrete

Conclusions - Corrosion Resistant Alloys for Reinforced Concrete

5. CONCLUSIONS

Both short-term laboratory experiments in simulated pore solutions and long-term exposure of concrete specimens reinforced with corrosion resistant rebars exposed to chlorides were performed. The reinforcements included UNS-S31603 (Type 316L stainless steel (SS)), UNS-31803 (Type 2205 SS), ASTM A055-98 (Type 2201LDX SS), UNS-S41003 (Type 3Cr12 SS), ASTM A615, Grade 75 (MMFX-II™), two 316 SS clad steels (Stelax and SMI), and ASTM A615 (black bar). The following conclusions were reached based on results during the initial 3 years of this 5-year study:

1. For Accelerated Screening Test-1 (AST-1), which involved measurement of polarization resistance and weight loss of bars exposed to repetitive 1.75 hours wet—4.25 hours dry cycles in synthetic pore solution with chlorides for a total of 84 days, 316L SS solid bars performed best and black bars the worst. Corrosion rate for the other bars was intermediate and shared a common scatter band. Performance of the clad bars was mixed in that data for some specimens without intentional clad defects approached that of the solid 316L SS, whereas with clad defects polarization resistance approached that of the black bars.
2. Accelerated Screening Test-2A (AST-2A) involved exposure of specimens that were potentiostatically polarized to +100 mV_SCE in synthetic pore water to which chlorides were incrementally added. Corrosion initiation was defined as having occurred when current density reached 10 µA/cm². For bars that became active, the average critical Cl‾ concentration for corrosion, [Cl_th‾], was 0.25 weight percent (w/o) Cl‾ (black bar), 0.47 w/o Cl‾ (3Cr12), 0.87 w/o Cl‾ (MMFX-II™), and 1.06 w/o Cl‾ (2201). By comparison of the presently determined [Cl_th‾] values with those from the literature that were acquired using pore water expression from specimens for which companion chloride thresholds in concrete, C_T, were available, C_T of black bar was projected as 0.54 w/o, for 3Cr12 0.80 w/o, for MMFX-II™ 1.10 w/o, and 2201 1.30 w/o (cement weight basis). A comparison between the AST-1 and AST-2A data indicated that rebars with a polarization resistance greater than 6·10⁴ ohm·m² had a [Cl_th‾] greater than 5 w/o, whereas below this [Cl_th‾] was about 1 w/o or less. It was unclear if the [Cl_th‾] could be predicted from the Pitting Resistance Equivalent Number (PREN).
3. Three types of reinforced concrete specimens, simulated deck slabs, three bar columns, and macro-cell slabs, have been under either continuous or cyclic wet-dry ponding with a NaCl solution for over 600 days. Similar to the accelerated tests, the best corrosion resistance was exhibited by the 316L reinforcement (both solid and clad); however, no 2205 SS bars have been tested in concrete. For bars with poor or intermediate performance in the accelerated tests (black bar, 3Cr12, MMFX-II™, and 2201), time-to-corrosion in concrete increased in proportion to [Cl_th‾] as determined in the accelerated AST-2A tests. The ranking (best to worst) was:
   

   (4.2)

   except for the macro-cell slab specimens where the 3CR12 exhibited longer times-to-corrosion than either 2201 or MMFX-II™. This may have been because these bars were wire brushed; however, if this is the explanation, then wire brushing the pickled 3Cr12 resulted in greater corrosion resistance enhancement than wire brushing the as-rolled 2201 or MMFX-II™.

4. In general, time-to-corrosion of the different reinforcements in concrete increased in direct proportion to the chloride threshold that was determined by accelerated testing (AST-2A).
5. Additional data are still being collected for the concrete specimens with various types of reinforcement that remain under test. Final analysis based on non-destructive data and observed condition of dissected specimens may change the ranking which will be documented in the final report.