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eCorrosion Resistant Steel Alloys for Reinforced ConcreteFHWA Publications Numbers FHWA-HRT-07-0-39 (July, 2007) and FHWA-HRT-09-020 (May, 2009)

INTRODUCTION

The objective of this research study was to investigate the properties of vari-ous steel alloys compared to black bars in concrete exposed to chlorides, given uncertainties regarding long-term perfor-mance of epoxy-coated reinforcements. This research commenced in 2003 as a joint study by the Florida Department of Transportation (FDOT) and Florida Atlantic University (FAU) under dual sponsorship by the Federal Highway Administration (FHWA) and FDOT. An Interim Report1 was published by FHWA in 2007 and a second Interim Report in 2009.2 The purpose of this Research Note is to describe the ma-terials and test methods employed and to summarize the findings.

STEEL TYPES

Table 1 lists the steel types that were tested. Bar size in all cases was #5 (0.625 inches nominal).

SPECIMEN PREPARATION

Differences in the as-received surface condition, as listed in Table 1, reflect the decision that this would not be specified by the researchers; and tests were performed per the condition provided by the manufac-turer/supplier and subsequently subjected

to acetone degreasing. All tests included straight bars and some also with bars bent 180º with a radius four times the bar diam-eter. Also, testing of the two clad bar types and A1035 included surface damaged con-ditions, which for clad bars consisted of 1) abrasion and 2) surface indentations, where the former did not penetrate the cladding and the latter did. For A1035 specimens, surface indentation alone was employed. Also included were concrete specimens with various combinations of bar crevices, simulated cracks, black bot-tom bars, and top bars sand blasted, pick-led or wire brushed.

TEST AND MEASUREMENT METHODS

Tests involved both 1) short-term ac-celerated screening tests (AST) in aque-ous solutions and 2) long-term exposures in concrete. The former, all of which were conducted at FAU, were comprised of three subsets: AST-1 (Wet-Dry Exposures), AST-

2A (Aqueous Solution Chloride Threshold Determinations), and 3) AST-2B (Pitting Potential Determinations). Particular em-phasis was placed upon AST-2A, since results from this can po-tentially provide data relevant to service life projection modeling. Figure 1(a) shows a photograph of a six-inch-long stainless steel clad specimen with epoxy end sealing and electrical lead as em-ployed for AST-1 and Figure 1(b) shows a photograph of speci-

mens under test. The procedure involved successive 28 day two

hours submerged – four hours in humid air with 3, 9, and 15 wt% NaCl. Corrosion rate was determined both by periodic polariza-tion resistance, RP, measurements(1) and

(1) Polarization resistance or Rp is a parameter derived from electrochemical measurements to which corro-sion rate is inversely proportional.

Table 1: Listing of steel types that were studied. Designation/Specification

Common Designation

As-Received Condition Microstructure

UNS-S31603 Type 316 SS Pickled Austenitic

UNS-S30400 Type 304 SS Pickled Austenitic

UNS-S31803 Type 2205 SS As-Rolled Duplex

ASTM A955-98 Type 2101LDX SS* As-Rolled Lean Duplex

UNS-S31803 Type 2304 SS As-Rolled Duplex

ASTM A1035 MMFX-2™ As-Rolled Microcomposite

n/a Nouvinox Pickled 316 Clad black bar

n/a SMI Pickled 317 Clad black bar

UNS-S41003 Type 3Cr12 SS Pickled Ferritic

ASTM A615 black bar As-Rolled Ferrite/Pearlite

*The 2007 report incorrectly labeled this reinforcement as 2201.

2 Fire Resistance of Reinforced Concrete Buildings [ETN-B-1-16]

Figure 4: Schematic illustration of straight and bent bar MS specimens.

Figure 5: Photograph of MS specimens under test. specimens.

(a) (b)Figure 1: Photographs of (a) an AST 1 test specimen and (b) specimens under test.

(a) (b)Figure 2: Schematic illustration (a) and photograph (b) of the AST-2A test arrangement.

(a) (b)Figure 3: Schematic illustration of an SDS specimen (a) and photo of specimens under test (b).

CRSI Technical Note 3

also weight loss upon specimen termination. A total of 267 specimens were tested.

Figure 2, on the other hand, provides a schematic illus-tration (a) and photograph (b) of the AST-2A experimen-tal setup. The same specimen type was employed as

for AST-1. Individual specimens and a counter electrode were positioned about the container perimeter with a reference electrode at the center. The electrolyte was a simulated pore solution consisting of NaOH and KOH. Specimens were potentiostatically polarized to +100 mV (SCE) and current periodically recorded via a data acqui-sition system. Chloride was incrementally added; and the concentration that corresponded to a current density of 10 μA/cm2 was taken as the critical value for passive film breakdown, CT.

AST-2B experiments, on the other hand, involved po-tentiodynamic polarization scans in saturated Ca(OH)2 solutions with 0-6.07 wt% Cl-. Both longitudinal and transverse cross-section specimens were employed. Testing was limited to A1035 and stainless steel types 3Cr12, 2101, and 316.

Four different types of reinforced concrete specimens were employed for longer term exposures, including 1) simulated deck slabs (SDS), as shown in Figure 3, 2) macro-cell slabs (MS) as seen in Figures 4 and 5, 3) two different types of three bar columns (designations S3BC and 3BTC – Figure 6), and 4) field columns (FC – Figure 7). All were fabricated by the Florida Department of Transportation State Materials Office (FDOT-SMO). The SDS and MS specimen types were intended to simu-late bridge decks and the S3BC, 3BTC, and FC partially submerged bridge substructure elements. Three con-crete mix designs, designated STD1 (5 bags cement and 0.50 w/c) which yielded a high permeability concrete, 2) STD2 (7 bags cement and 0.41 w/c) which resulted in moderate permeability, and 3) STD3 (7 bags of cement and 0.50 w/c) which provided intermediate permeability, were employed. Exposure of SDS specimens took place at FAU, while S3BC and 3BTC ones were at FDOT-SMO. Field Column exposures, on the other hand, were posi-tioned in the tidal zone at an Intracoastal Waterway site near Crescent Beach, FL (Figure 8).

MEASUREMENT RESULTS

Example AST-1 results are shown in Figure 9 as a plot of polarization resistance, RP, and chloride concentration versus time. Considering that corrosion rate is inversely proportional to RP, the data indicate a one to two or-der of magnitude lower corrosion rate for A1035 and 3Cr12 compared to black bars. Likewise, corrosion rate of 316SS and 2205SS is one to two orders of magnitude less than for A1035 and 3Cr12.

Figure 10 illustrates example AST-2A results for ten A1035 specimens. The data show that the assumed 10 µA/cm2 threshold for active corrosion (see above) was reached between 230 to 335 days. Figure 11 summa-rizes these results for three steel alloys and black bars as a plot of the number of active specimens versus chlo-ride concentration. Based upon these and other results, bars were classified as either “Improved Performers” or “High Performers”, where the former initiated corrosion

Figure 6: Schematic illustration of straight, bent, and elevated 3BTC type

Figure 7: Photograph of S3BC and 3BTC specimens under test.

Figure 8: Photograph of FC specimens under test.

4 Fire Resistance of Reinforced Concrete Buildings [ETN-B-1-16]

within the project time frame, albeit at greater times than for black bars, and the latter did not, at least for speci-mens of the standard configuration (all bars straight and

no crevices or simulated concrete cracks). Results such as shown here led to the con-cept that CT is not a specific value but con-forms to a distribution. This principle was adapted as well in evaluating the results for reinforced concrete specimens.

Figure 12 shows typical time-to-corrosion (Ti) results for two Improved Performer steel types compared to black bars in the case of MS specimens with the STD2 concrete mix. The results indicate about a 20-fold Ti increase for 2101SS compared to black bars and 25-fold for A1035. These increases were greater than for the STD1 mix, suggesting that Ti enhancement for these steel alloys increases with increasing concrete quality. Table 2 summarizes these CT data for the case of Improved Performer steel alloys with the STD1 concrete mix de-

sign at 2, 10, and 20 percent active. As noted above, thresholds are expected to be greater for better quality concrete. Likewise, Figure 13 plots potential and macro-cell current versus time for 316SS MS specimens with the STD-1 concrete mix. These data show that poten-tial remained relatively positive and macro-cell current nil except for momentary excursions. Apparently, these resulted from localized passive film disruption events fol-lowed by repassivation. Such excursions were greater for 304SS than for 316SS.

Chloride analyses were also performed at the bar depth of SDS High Performer specimens after expo-sure times as long as 1,726 days, although these bars remained passive, with results being shown in Table 3. As such, the values provide a chloride concentration that CT exceeds.

Improved performers were 3CR12, A1035, and 2101SS, whereas high performers were 304SS, 316SS, 2304SS and the two clad bar types.(2) Black bar results were included with the Improved Performer data for the purpose of comparison.

CHLORIDE ANALYSES

Chloride analyses were performed upon, first, cores acquired from companion non-reinforced concrete spec-imens and, second, powdered concrete from milling along the upper bar trace of reinforced specimens away from active corrosion sites upon autopsy, both shortly after active potentials were noted. The latter concentra-tions were invariably greater than the former because of the bar obstruction effect. These data served to quantify CT. Figure 14 shows a cumulative distribution function plot of CT, and Table 2 summarizes these for the case of Improved Performer steel alloys at 2, 10, and 20 percent

(2) 2205SS is not included as a high performer simply because it was not tested in concrete.

Figure 9: Plot of RP and chloride concentration as a function of time for 316SS, 2205SS, A1035, and 3Cr12SS.

Figure 10: Plot of current density for ten A1035 AST-2A specimens and chloride concentration (heavy dashed line) as a function of time.

Figure 11: Plot of the number of active specimens ver-sus the chloride concentration, CT, for Improved Performer reinforcements.

CRSI Technical Note 5

active. These data are for the STD1 concrete mix design and, as noted above, may be greater for better quality concrete.

The FC specimens with Improved Performer and black bars typically initi-ated corrosion within the first several days of exposure. This is thought to have resulted because of poor con-crete quality and possibly cracks that provided direct water access to the reinforcement. With one exception, the High Performer reinforcements remained passive. The exception was a 316SS reinforced column that was damaged during installation.

AUTOPSY RESULTS

Dissection of specimens shortly af-ter a significant negative potential shift and a corresponding large macrocell current increase were recorded typi-cally revealed one or more small areas of corrosion products upon the bar or bars in question. Figure 15 shows an example where a small area of corro-sion product is seen on the bar trace of a black bar SDS specimen.

CONCLUSIONS

This study classified bars as ei-ther Improved Performers or High Performers. Overall, ranking of the Improved Performers according to Ti was black bars < 2101SS < 3Cr12 < A1035. Based upon the CT and macro-cell current data, the study concluded that the intended service life of major reinforced concrete structures (75-100 years) can confidently be achieved with

Figure 13: Plot of potential and macro-cell current for three 316 SS specimens.

Figure 12: Cumulative probability of corrosion initiation for MS specimens versus exposure time.

Table 2: Listing of CT at different percentages of active bars and ratios compared to black bars for ImprovedPerformer bars.

Percent Active

CT, wt% cement CT (steel alloy)/CT(black bar)

BB 3Cr12 A1035 2101SS 3Cr12 A1035 2101SS

2 0.34 1.40 1.50 1.30 4.12 4.41 3.82

10 0.53 2.05 2.25 1.80 3.87 4.25 3.40

20 0.65 2.40 2.50 2.05 3.69 3.85 3.15

Table 3: Listing of lower limit CT values for High Performer reinforcements.

Reinforcement Exposure Time, days CT, wt% cement (Lower Limit)

316SS 1,726 5.02

304SS 440 3.68

Stelax !,726 5.02

SMI 944 4.51

Figure 14: Cumulative distribution Function plot of CT for Improved Performer and black bar.

Contributors: The primary contributors to this publication are: (2007) W.H. Hartt, R.G. Powers, D.K. Lysogorski, V. Liroux, Y.P. Virmani and (2009) W.H. Hartt, R.G. Powers, F. Presuel Marino, M. Parades, R. Simmons, H. Yu, R. Himiob, Y.P. Virmani.

Keywords: corrosion resistance, reinforcing bar, alloys, testing.

Reference: Concrete Reinforcing Steel Institute – CRSI [2016], “Corrosion Resistant Alloys for Rein-forced Concrete”, CRSI Research Note RN-2016-1, Schaumburg, Illinois, 6pp.

Historical: None. New Research Note.

Note: This publication is intended for the use of professionals competent to evaluate the significance and limitations of its contents and who will accept responsibility for the application of the material it contains. The Concrete Reinforcing Steel Institute reports the foregoing material as a matter of infor-mation and, therefore, disclaims any and all responsibility for application of the stated principles or for the accuracy of the sources other than material developed by the Institute.

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the solid High Performer reinforcements that were in-vestigated, namely 304SS, 316SS, and 2304SS. Note that 2205SS is not included as a high performer simply because it was not tested in concrete in this study. This may be the case also for the clad reinforcements pro-vided, first, there is adequate control of surface defects and, second, bar ends are protected. This same service life may result also with the Improved Performer bars provided design and construction quality control are good and concrete cracking is minimal but with a lesser degree of confidence and margin for error compared to the high performance reinforcements.

Several follow-on publications based upon the findings of this study have resulted.3, 4, 5, 6 Also, the FDOT and FAU have continued the exposures and monitoring of con-crete specimens that were not autopsied, and a report pertaining to some of the results has been issued.7

REFERENCES1. W.H. Hartt, R.G. Powers, D.K. Lysogorski, V. Liroux, Y.P. Virmani, Corrosion Resistant Alloys for reinforced Concrete, Report No. FH-WA-HRT-07-039, Federal Highway Administration, Washington, DC, July, 2007.

2. W.H. Hartt, R.G. Powers, F. Presuel Marino, M Parades, R. Sim-mons, H. Yu, R. Himiob, Y.P. Virmani, Corrosion Resistant Alloys for re-inforced Concrete, Report No. FHWA-HRT-09-020, Federal Highway Administration, Washington, DC, May, 2009.

3. W.H. Hartt, “Protocol for Projecting Time-to-Corrosion of Reinforc-ing Steel in Concrete Exposed to Chlorides,” Corrosion, Vol. 66, 2010, p. 086002.

4. W.H. Hartt, “Corrosion Initiation Projection for Reinforced Con-crete Exposed to Chlorides: Part 1 – Black Bars, Corrosion, Vol. 67, 2011, p. 086002-1.

5. W.H. Hartt, “Corrosion Initiation Projection for Reinforced Con-crete Exposed to Chlorides: Part 2 – Corrosion Resistant Bars, Corro-sion, Vol. 67, 2011, p. 086003-1.

6. W.H. Hartt, “Service Life Projection for Chloride Exposed Concrete Reinforced with Black and Corrosion Resistant Bars, Corrosion, Vol. 68, 2012, p. 754.

7. F. Presuel-Moreno, F. Gutierrez, J. Zielske, V. Casas, and Y. Wu, “Analysis and Estimation of Service Life of Corrosion Prevention Ma-terials Using Diffusion, Resistivity and Accelerated Curing for New Bridge Structures,” Final Report BDK79-977-02, Volume 1, “Corrosion Prevention Materials - Monitoring and Forensic Examination,” Chap-ter 4; Florida Atlantic University, Boca Raton, FL., 2013.

Figure 15. Example of corrosion products upon dissection of a specimen.