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Job Site Evaluation of Corrosion Resistant Alloys

IBRC PROJECTS INVOLVING CORROSION-RESISTANT REINFORCEMENT

General

Task I was accomplished in conjunction with a companion research project and resulted in Federal Highway Administration (FHWA) publication.⁽¹⁴⁾ Table 1 lists information that was made available to the project team regarding approved State IBRC projects involving innovative reinforcement (Task II). This shows that 27 State projects were approved; and of these, 20 were either completed as planned or with an alternate innovative reinforcement. These completed projects include seven different types of innovative reinforcements, as listed in table 2.² A dual listing is given for clad stainless steel since two very different production methods are involved. Likewise, table 3 lists the number of projects, both as-planned and as-completed, that employed each specific reinforcement type. Those involving ECR and black bar reflect instances where the supplier was unable to meet schedule in providing the specified innovative reinforcement, and so these were used as the fallbacks. Delivery was particularly a problem in the case of stainless clad reinforcement: one producer (Stelax, Inc., steel designated below as “Source 1”) went into receivership during the project time frame; the other (CMC Steel Group, designated as “Source 2”) experienced unexpected technical production difficulties. This was disappointing since stainless clad rebar has the potential of providing excellent corrosion resistance at relatively low unit cost. These two companies are addressing their respective difficulties, and one is now producing again and the second hopes to be in production in 2007. In many of the instances where a specified alternative reinforcement could not be delivered on schedule, MMFX-II served as the replacement. This reinforcement was consistently delivered in a timely manner even though the lead time was sometimes short.

Figure 1 shows the number of projects in each of the table 1 footnote classes. This indicates that the project team visited five of the projects (Note 1 designation) and in each case, acquired samples of the innovative reinforcement. For two additional projects (FHWA Project Numbers MO-00-01 and SC-00-01), one of which was completed prior to the present study, sufficient information was provided by the respective State DOT personnel that a report was prepared. Each of these seven reports is included below as appendixes A–G. In instances where samples of the innovative reinforcement were acquired from a job site, composition and mechanical properties were determined; and in some cases accelerated corrosion tests were performed.

Figure 1. Distribution of information acquisition and analysis for the IBRC projects.

Results and Discussion

Hallmark Projects

Two of the projects for which reports were written (MT-01-01 andSC-00-01) merit special comment because of their unique nature. The first involved a replacement bridge across the Middle Fork of the Flathead River on U.S. 2 in Flathead County, MT. Permitting and closure for repair issues are such that it was desirable to have this bridge in uninterrupted service for as long as possible. With regard to permitting, one end of the bridge terminates on land owned by Glacier National Park and the other on land administered by Flathead National Forest. At the same time, the Flathead River is under jurisdiction of the United States Fish and Wildlife Service and contains several threatened or endangered species. Permitting for this project was complicated because these entities, as well as the U.S. Army Corps of Engineers and various State agencies, were also involved. Consequently, it was reasoned by the Montana Department of Transportation that any future repairs, rehabilitations, or replacement would be complex and difficult. In addition, because of the rural setting and mountainous surroundings, closure of this bridge results in a 480-kilometer (km) (300-mile (mi)) detour for motor traffic. For these reasons, the added initial cost of corrosion-resistant reinforcement was particularly justified.

An additional, particularly noteworthy issue arose in conjunction with projectMT-00-01. The specification called for either Type 2205 or 316LN stainless as the reinforcement. It was assumed that the latter would be delivered because it generally is less expensive; however, the bridge engineer subsequently identified the bars as Type 2205 stainless steel. While both materials met specification and were acceptable, this situation points out a potential problem in that different stainless grades are generally not visually distinguishable. Consequently, where stainless reinforcement is employed, an independent determination should be made to confirm that the delivered product conforms to what was specified.

The second project, SC-00-01, was particularly noteworthy because it incorporated five different reinforcement scenarios, (1) black bar with discrete Galvashield XP™ embedded galvanic anodes, (2) black bar without anodes, (3) Type 2205 stainless steel, (4) Type 316 clad black bar (Source 2), and (5) MMFX-II. Individual spans were constructed using one of these five alternatives. It was initially intended that the black bar without anodes span would use 316 clad stainless steel from Source 1; however, the delivery delays discussed above precluded this. As constructed, this bridge affords an excellent opportunity for side-by-side comparison of a variety of reinforcing steel corrosion control alternatives.

A number of other projects also provide the opportunity for future side-by-side comparisons but in these cases between the corrosion-resistant reinforcement and ECR. Thus, in instances of a divided highway, one bridge commonly used ECR and the second, an innovative reinforcement.

Compositional Analyses of Innovative Reinforcements from Job Sites

Chemical analysis was performed on samples of bars from six job sites, as reported in table 4. The results indicate that composition for all MMFX-II bars is within the specified range for that material. For the Source 2 cladding (SC-00-01), carbon concentration exceeds the upper limit for some 316 grades and is at the upper limit for others. Bars of this composition should not be welded unless special precautions are taken. The MT-01-01 bars are within the specified composition range for 2205 stainless.

Mechanical Properties of Innovative Reinforcement Samples from Job Sites

Mechanical properties of samples of the same six corrosion-resistant reinforcements that were chemically analyzed (table 4) were determined, and the results are listed in table 5. All bars were #5 and qualified as Grade 60, although the MMFX-II is of considerably higher strength than is normally experienced here. All bars met their applicable standard specification, where one exists.

Corrosion Testing of Job Site Bars

Type 2201 stainless samples acquired from the job site of Project FL-01-01 and clad bar samples (Source 2), which were from the same production run as those used in Project SC-00-01, were subjected to corrosion testing in conjunction with a companion research project.⁽¹⁵⁾ Several different surface preparations (as-received (rolled), carbon steel shot blasted, silica sand blasted, and stainless steel shot blasted) were used for the former alloy (2201) as a part of an FDOT program to identify the most appropriate condition. Based upon that program, silica sand blasted 2201 was qualified for the project. Accelerated corrosion testing of MMFX-II bars from three job sites (PR-02, OK-01-01, and DE-00-01) as well as the Type 2205 bars from MT-01-01 were also tested.

The accelerated test procedure was modeled after that from an earlier program⁽¹⁶⁾ and involved exposure of triplicate specimens to repetitive cycles of 1.75 hours wet and 4.25 hours dry, for a total of 84 days. The test solution was 0.3N KOH-0.05N NaOH (pH ~ 13.40) simulated pore water with 3.00, 9.00, and 15.00 weight percent NaCl (1.82, 5.46, and 9.10 weight percent Cl^-) for each of three successive 28-day periods. Polarization resistance (inversely proportional to corrosion rate) measurements were made periodically during the exposures using a Gamry CMS100 potentiostat with a scan rate of 0.333 millivolts per second (mV/sec) and polarizations of +/-0.020 V referenced to the free corrosion potential. Prior to scanning, potential was monitored for 300 seconds or to a time lapse until any variations were less than 0.1 mV/sec.

Figure 2 shows a plot of polarization resistance (Rp) as a function of exposure time for the various Type 2201 stainless specimens along with data for black bar and Type 316 stainless for comparison. Specimens labeled according to the four surface conditions were provided directly by FDOT (see Appendix C1), whereas the specimens designated “Jensen Beach” (these were silica sand blasted) were acquired directly from the job site, where they had been stored uncovered about one kilometer inland for approximately six weeks. The data show that Rp for the Type 2201 specimens occupy a band about 1–2 orders of magnitude above that for black bar and 1–2 orders of magnitude below the Type 316. Scatter of Rp for the different categories of Type 2201 specimens is about one order of magnitude, with the silica sand and stainless steel blasted materials occupying the upper range. Also, there is a tendency beyond about 50 days for Rp to decrease with time (increasing corrosion rate).

Figure 2. Accelerated testing data for Type 2201 stainless steelspecimens.

Figure 3 shows a plot of Rp for MMFX-II specimens from three of the job sites compared to data for straight and bent bar specimens of this same steel (labeled “Lab”) that were provided directly to the project by MMFX Steel Corporation of America. Specimens designated MMFX (DE), MMFX (OK), and MMFX (PR) are from project numbers DE-01-01, OK-01-01, and PR-02, respectively (see table 1). The results indicate general consistency between the different job site and lab MMFX-II specimens with Rp for these being 5–10 times greater than for black bar.

Figure 3. Accelerated testing data for MMFX-II steel specimens

Figure 4 shows Rp versus time data for specimens prepared from project number MT-01-01 job site bars (Type 2205) and clad bars from the same heat as project number SC-00-01 (not actually from the job site). Data for the Type 316 and Type 2205 stainless that was provided directly to the project by a supplier are shown for comparison. Polarization resistance for the SC-00-01 clad bars varies from the lower range to an order of magnitude below that for the solid 316 (higher corrosion rate for the former). Results for the MT-01-01 specimens fall 3–10 times below those for the laboratory Type 2205 specimens. Thus, while data for laboratory received and job site MMFX-II bars are comparable, corrosion rate for the more corrosion-resistant job site bars was higher than for the laboratory received counterpart. These differences are being evaluated in conjunction with the companion activity.⁽¹⁵⁾

Conformance of Innovative Reinforcement to Specification

Mechanical properties of specimens prepared from the corrosion-resistant reinforcement samples that were acquired from job sites (table 5) were compared with those listed in the relevant specifications (reference 17 for Type 2205 stainless steel, reference 18 for MMFX-II,” and reference 19 for clad stainless steel). All properties of the stainlesses, both solid and clad, conformed to the applicable specification (ASTM A 955/A 955M-06a⁽¹⁶⁾ and AASHTO Designation MP 13M/MP 13-04,⁽¹⁷⁾ respectively). The same applies to MMFX-II (ASTM A 1035/ A 1035M-05⁽¹⁸⁾) with the exception of elongation, where 6 percent was measured for project

Figure 4. Accelerated testing data for stainless steel job site bars.

PA (not an IBRC project) bars (see tables 4 and 5) and 5+ percent for SC-00-01 but with the specification value being 7 percent. It should be pointed out, however, that the ASTM specification pertaining to MMFX-II was only issued in 2004, and the bars in question were produced prior to that date.

Reinforcement Costs

Economics are an important component of any construction materials evaluation. For the reason of evaluating this within the context of the present study, reinforcement costs were acquired for projects for which reports were issued and are presented in figure 5. This shows that the average cost for the 316 and 2205 stainlesses was $5.34/kilogram (kg) and for the MMFX-II $1.46/kg. These values may be misleading, however, for the following reasons:

1. Reinforcement costs can be expected to decline if the materials become more common.

2. For some unknown reason, the cost for MMFX-II employed in conjunction with project FL-01-01 is unrealistically low (less than for black bar). On the other hand, the MMFX–II™ for DE-00-01 and OK-01-01 was provided on short notice because the originally specified clad bar could not be delivered in a timely manner. This could have elevated the cost.

   

3. The listed unit cost is not necessarily the lowest that was available, as material costs were often lumped into the contractor’s bid. In the case of project MT-01-01, the bid unit costs ranged from $4.10 to $5.27/kg. Apparently, by going with the lowest overall contractor bid, a premium was paid for the reinforcing steel.

Figure 5. Cost comparison of the various reinforcements.

² All photographs courtesy of Mr. John Wenzlick, MoDOT.