Corrosion-resistant rebar extends service life of concrete bridge structures

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2 Corrosion-resistant rebar extends service life of concrete bridge structures A common cause of reinforced concrete bridge deterioration is corrosion of the reinforcing steel bars due to exposure to chloride ions from deicing salts and marine environments that permeate the concrete. In the absence of chloride ions, concrete s higher ph promotes the passivation of the reinforcing steel, where a passive oxide flm is formed on the steel s surface that protects the metal from corroding. When chloride ions, in combination with moisture and oxygen, penetrate the concrete and interact with the reinforcing steel, its protective passive flm is compromised and the steel is susceptible to corrosion. To combat rebar corrosion and lengthen the service life of bridges in the United States, several state departments of transportation have evaluated the use of corrosion-resistant reinforcing (CRR) steel bars for concrete bridge decks and The new Huguenot Bridge over the James River in Richmond, Virginia is being constructed with Class I CRR microcomposite reinforcing steel bars. Photo courtesy of VDOT. Continued on page 18 NACE International, Vol. 52, No. 6 June 2013 MATERIALS PERFORMANCE 17

3 MATERIAL MATTERS Continued from page 17 The Class I CRR microcomposite reinforcing steel bars are being used to construct the deck and railing of the Route 675 bridge (Beulah Road) in Fairfax County, Virginia. Photos courtesy of VDOT. other bridge structures. The Structure and Bridge Division of the Virginia Department of Transportation (VDOT) (Richmond, Virginia) made a decision to discontinue the use of epoxy-coated re- inforcing (ECR) steel bars and galvanized reinforcing steel bars based on research completed by the Virginia Transportation Research Council and Virginia Tech. 1 Field observations indicated coating failures in <20 years and section loss of reinforcing steel bars of up to 100%. Since September 2010, VDOT has designed all bridge projects with CRR steel bars, including new construction, widening, superstructure replacement, repairs, and rehabilitation, to achieve a 75-year or longer service life for its bridges. In 2012, VDOT reported that it used ~8.8 million lb (4 million kg) of CRR steel bars in the past two years, and 20% of the CRR steel bars used was stainless steel (SS). Although the quality of concrete has improved over the years and high-performance concretes today resist salt intrusion, the concrete is going to crack on a bridge deck at some stage and saltwater will intrude, says Julius Volgyi, assistant state structure and bridge engineer for VDOT s Structure and Bridge Division. If the reinforcing steel bars are black carbon steel (CS) or ECR, he says, corrosion will begin, and the resulting ferrous oxide (FeO), because it has a larger volume than the steel reinforcing bars, will put additional pressure on the concrete deck and cause further cracking that allows even more chloride intrusion. This repetitive, cyclical process will continue until no reinforcing steel is left, Volgyi comments, noting that the move to CRR steel bars is expected to extend the life of the concrete bridge deck. To us, an 18 MATERIALS PERFORMANCE June 2013 NACE International, Vol. 52, No. 6

4 investment in CRR steel bars is going to pay off in the long run. We won t get spalling associated with the rusting of rebar, he says. According to Michael Sprinkel, associate director of the Virginia Center for Transportation Innovation and Research (Charlottesville, Virginia), CRR steel bars are available in many formulations that affect performance, service life, and cost. VDOT uses three types of deformed CRR steel bars, which are categorized into three classes based on corrosion performance data. Class I, improved corrosion resistance steel bars, has the lowest cost and lowest anticipated service life (60-plus years) and includes low-carbon chromium steel reinforcing bars that meet ASTM A1035/ A1035M 2 and UNS S32101 solid duplex SS reinforcing bars that meet ASTM A955/A955M. 3 Class II, moderate corrosion resistance steel bars, includes clad SS reinforcing bars that meet AASHTO M 329M/M and UNS S24100 SS reinforcing bars that meet ASTM A955/A955M. Class III, high corrosion resistance steel bars, has the highest cost and longest anticipated service life (100- plus years) and includes UNS S24000, S30400, S31653, S31603, S31803, and S32304 solid SS reinforcing bars that meet ASTM A955/A955M. The class of CRR steel bars specifed for bridge structures constructed for VDOT depends on the structure s functional classifcation. Sprinkel notes that bridges on interstate highways and primary roads typically have the highest volume of traffc, including truck loads, and consequently are exposed to more deicing applications and stress than typical bridges on lower-volume roads. Usually, the rural bridges on the roads with lowest traffc volume are subject to the least amount of deicing applications and stress. For high-traffc roads, including interstate highways, freeways, and principal arterial roads in both rural and urban areas, highly corrosion-resistant SS reinforcing bars (Class III) are used. VDOT specifes that Class II reinforcing steel bars be used for minor arterial roads and Class I reinforcing steel bars be used for local rural and urban roads with lower traffc use. Spending more for CRR steel bars is easily justifed for all functional classifcations of roadways, says Sprinkel, because the cost to redo one bridge deck can exceed the initial extra cost to use CRR SS bars. However, he adds, given that lane closure and user costs will be the least on low-volume roads, the use of Class I CRR steel bars can be justifed based on lower initial costs. Also, because of less traffc stress and fewer applications of deicing chemicals on lower-volume roads, one bridge deck overlay in 100 years may not be required for bridge decks constructed with Class I CRR steel bars. The risk of spending more money on low-volume roads by not using stain- Continued on page 20 NACE International, Vol. 52, No. 6 June 2013 MATERIALS PERFORMANCE 19

5 MATERIAL MATTERS Continued from page 19 less is low, and the risk of spending more money on high-volume roads by not using stainless is high, Sprinkel comments. Bridge components constructed with CRR steel bars include concrete deck slabs, parapets, rails, raised medians, terminal walls, prestressed concrete slabs, and reinforced concrete slab spans, says Volgyi. He notes that for all functional classifcations, Class I CRR steel bars are used exclusively for abutment neat work, which applies to all abutment types, and for all prestressed concrete beam continuity reinforcement bars and other reinforcement that extends into the concrete deck slab. The strands inside the prestressed concrete beam, however, are black CS. Over the past few years, several studies were conducted on the corrosion resistance of a high-strength, low-carbon chromium microcomposite steel reinforcing bar commercialized by MMFX Technologies Corp. (Irvine, California), which conforms to the requirements of ASTM A1035/A1035M and is accepted as a Class I CRR steel bar by VDOT. This alloy s composition and manufacturing process was designed to provide improved corrosion resistance and mechanical properties as compared to conventional CS (e.g., ASTM A615 5 and ASTM A706 6 ) when fabricated as a reinforcing steel bar product. The microcomposite has a minimum yield strength of 100 ksi (690 MPa) and minimum tensile strength of 150 ksi (1,030 MPa). The nanotechnology used in the alloy s manufacturing process, which is based on 25 years of research at the University of California, Berkeley and patented by MMFX, manipulates the structure of the steel at the nanoscale to create a microstructure without carbides and secondary particles. Typical CS comprises a matrix of carbides and ferrites at the grain boundaries that are chemically dissimilar. When exposed to moisture, the carbides and ferrites in the steel form microgalvanic cells that initiate galvanic corrosion, explains Salem Faza, vice president of engineering and specifcation for MMFX. The low-carbon chromium steel alloy contains a maximum carbon content of 0.15%, a chromium content of ~9.5%, and low amounts of other carbide-forming elements such as tungsten, molybdenum, vanadium, titanium, niobium, tantalum, and zirconium. During the cooling step in the alloy s fabrication process, a fne lath martensite microstructure is formed where the presence of carbides is almost eliminated, says Faza. This is possible, he notes, because the alloy s low-carbon content reduces the excess carbon available in the matrix, which can combine with the chromium present in the alloy to form chromium carbides and reduce the corrosion resistance of the alloy by depleting the chromium from the matrix. Additionally, the low-carbon content combined with the 20 MATERIALS PERFORMANCE June 2013 NACE International, Vol. 52, No. 6

6 low content of other carbide-forming elements prevents the formation of secondary particles in the microstructure that can initiate microgalvanic cells and galvanic corrosion. When exposed to chloride ions in concrete, the corrosion-resistant properties of the low-carbon, chromium microcomposite steel reinforcing bar are twofold, Faza says. The chromium in the steel facilitates the formation of a surface oxide flm that is more resistant to chloride ions and has a higher chloride threshold level the chloride concentration level that initiates the breakdown of the passive film on the steel reinforcing bars than an uncoated CS reinforcing bar. If the chloride threshold level is reached, the microcomposite steel reinforcing bar can start corroding; however, the corrosion rate will be lower because of the carbide-free microstructure. A report prepared for MMFX in 2006 by AMEC Earth & Environmental (Burnaby, British Columbia, Canada) 7 examines the results of 10 different studies by universities and state transportation departments on the performance of the microcomposite steel reinforcing bar compared to other reinforcing steel bars in accelerated corrosion tests. According to the report, the test results reviewed indicate that the low-carbon chromium microcomposite steel provides better corrosion resistance in chloride environments than conventional uncoated CS, although the relative amount of improvement varies based on the accelerated test method used in each study. The test results also show that most types of SS provide better corrosion resistance than the low-carbon chromium microcomposite steel. Data from a corrosion study conducted by the Virginia Transportation Research Council, as noted in the AMEC report, show that a bent microcomposite steel bar has a chloride threshold of 2,700 to 2,730 ppm, which compares to a chloride threshold of 460 to 580 ppm for CS bars; 1,550 to 1,560 ppm for a UNS S32101 duplex SS bar; >4,630 ppm for a zinc-sprayed, epoxy-coated CS bar; and >5,630 ppm for UNS S30400, S31653, and S32205 solid SS bars and a SS-clad CS bar. Results of an accelerated chloride threshold test done at Texas A & M University indicated that the mean critical chloride threshold values were 4.6 kg/ m 3 for the microcomposite steel vs kg/m 3 for black CS (ASTM A706); 5.0 kg/m 3 for Type 304 SS (UNS S30400); and 10.8 kg/m 3 for Type 316LN SS (UNS S37653). Results of a study by the University of Kansas concluded that the corrosion rate for a conventional CS bar (ASTM A615) was 3.6 to 4.4 times higher than the corrosion rate for the microcomposite steel bar. Contact Julius Volgyi, VDOT Julius.Volgyi@VDOT.Virginia.gov; Michael Continued on page 22 NACE International, Vol. 52, No. 6 June 2013 MATERIALS PERFORMANCE 21

7 MATERIAL MATTERS Continued from page 21 Sprinkel, Virginia Center for Transportation Innovation and Research Michael. and Salem Faza, MMFX mmfx.com. References 1 Corrosion Resistant Reinforcing Steels (CRR), Virginia Dept. of Transportation, Structure and Bridge Division, IIM- S&B-81.5, August ASTM A1035/A1035M-11, Standard Specifcation for Deformed and Plain, Low-carbon, Chromium, Steel Bars for Concrete Reinforcement (West Conshohocken, PA: ASTM). 3 ASTM A955/A955M-12e1, Standard Specification for Deformed and Plain Stainless-Steel Bars for Concrete Reinforcement (West Conshohocken, PA: ASTM). 4 AASHTO M 329M/M , Standard Specification for Stainless Clad Deformed and Plain Round Steel Bars for Concrete Reinforcement (Washington, DC: AASHTO, 2011). 5 ASTM A615/A615M-12, Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement (West Conshohocken, PA: ASTM). 6 ASTM A706/A706M-09b, Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement (West Conshohocken, PA: ASTM). 7 Comparative Performance of MMFX Microcomposite Reinforcing Steel and Other Types of Steel with Respect to Corrosion Resistance and Service Life Prediction in Reinforced Concrete Structures, AMEC Earth & Environmental, AMEC VA06451, February K.R. Larsen MP welcomes news submissions and leads for the Material Matters department. Contact MP Associate Editor Kathy Riggs Larsen at phone: , fax: , or kathy.larsen@nace.org. 22 MATERIALS PERFORMANCE June 2013 NACE International, Vol. 52, No. 6