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    • TECHNOLOGY FOR DURABLE REINFORCED CONCRETESTRUCTURES USING CR-BEARING CORROSIONRESISTANT REBARS.H.Tae and T.NoguchiTokyo University, JapanT.Ujiro and O.FurukimiJFE Steel Corporation, JapanAbstractThis study was investigated to corrosion resistance of Cr-bearing corrosion resistantrebars in simulated concrete pore solutions and concrete specimens having chloride ioncontents. Simulated concrete pore solutions were saturated Ca(OH)2 solutionscontaining 0.27%, 1.07%, and 21.4% NaCl. The pH of the test solutions wereadjusted at 12.5, 11, 10, and 9 by HCl. Pitting potentials of the steels in the solutionswere investigated. The results of the study showed that 5Cr and 9Cr steels showedgood corrosion resistance in 1.07% NaCl solutions at ph12.5 and ph10, respectively.In the case of concrete specimens having chloride ion contents, the test specimens weremade by installing 8 types of rebars into concrete specimens having chloride ioncontents of 0.3, 0.6, 1.2, 2.4 and 24kg/m3 each. The corrosion resistance of the Cr-bearing corrosion resistant rebar was examined by measuring corrosion losses andaverage corrosion rates up to 155 cycles of corrosion accelerated curing. The resultsof the study showed that the Cr-bearing corrosion resistant rebar containing a Crcontent greater than 5% and 9% exhibited corrosion resistance when the chloride ioncontents were 1.2 and 2.4kg/m3 respectively.1. IntroductionRecently the durability of steel reinforced concrete structures has drawn large publicattention. The lifetime of the structures has been considered to be semi permanent upto the present. For this reason, many studies have been performed to discover the bestmethod of preventing corrosion in reinforcing bars. However, most of them have been disproportionately concentrated on the improvement of concrete quality such as theincrease in concrete cover thickness, optimization of water-to-cement ratio or theaddition of corrosion resistant materials. In America and Europe, stainless steel rebarswith high corrosion resistance have already been applied to many concrete structures, TAE, Durable Reinforced Concrete Structures, 1/8
    • such as road bridges, elevated highways, tunnels, parking garages, residential buildings,and port facilities [1]. Stainless steel rebars have excellent properties as corrosionresistant rebars, but their higher cost prevents from coming into wide use. If alloycontent (Cr, Ni, Mo, etc.) can be reduced preserving appropriate corrosion resistance,new corrosion resistant rebars (hereinafter referred to as Cr-bearing corrosion resistantrebar) that have good properties and cost performance should be designed.As a fundamental study on the Cr-bearing corrosion resistant rebar with the necessarycorrosion resistance for use in steel reinforced concrete structures under corrosiveenvironments, This study was investigated to corrosion resistance of Cr-bearingcorrosion resistant rebars in simulated concrete pore solutions and concrete specimenshaving chloride ion contents.2. Corrosion resistance in simulated concrete pore solutions2.1 Outline of the experiments and Methods of testing2.1.1 SteelsChemical compositions of steels are given in Table1. The steels containing Cr from 0% to 16% were used to change the corrosion resistance in simulated concrete pore solutions. SD345 steel that is a typical carbon steel rebar and SUS304 steel that is a typical austenitic stainless steel were also used to compare the corrosion resistance to theCr-bearing corrosion resistant rebar. All steels were produced from 100kg vacuum-melted ingots. These ingots were hot forged and hot drawn at 1100-1150℃ to 13mmφ wires. These wires except SUS304 steel were annealed at 700℃for 2hours.SUS304 wires were annealed at 1100℃ for 5minutes. All wires were pickled toremove surface oxides. The wires of the Cr-bearing corrosion resistant rebar andSD345 steel had ferritic microstructure. The wire of SUS304 steel had austeniticmicrostructure. These wires were sliced to 2mm thick plates for corrosion testspecimens. Table1:Chemical compositions of steels used, mass%. Steel C Si Mn P S N Cr Ni SD345 0.2280 0.31 1.34 0.029 0.020 0.005 0.084 0.04 0Cr 0.0120 0.32 0.50 0.031 0.006 0.010 0.005 0.01 5Cr 0.0150 0.28 0.53 0.027 0.006 0.006 5.020 0.01 9Cr 0.0107 0.28 0.53 0.028 0.006 0.010 9.140 0.01 11Cr 0.0117 0.28 0.53 0.028 0.004 0.010 11.00 0.01 13Cr 0.0117 0.28 0.53 0.028 0.004 0.008 13.050 0.01 16Cr 0.0113 0.29 0.53 0.027 0.004 0.009 15.980 0.01 SUS304 0.0630 0.31 1.01 0.026 0.006 0.046 18.360 8.28 TAE, Durable Reinforced Concrete Structures, 2/8
    • 2.1.2 Pitting potentialsPitting potentials were measured to evaluate the resistance to corrosion initiation. Theplate specimens were polished to #800 finish and degreased with acetone. Thesespecimens were coated by silicon sealant except a measuring area (10 mm in diameter).Simulated concrete pore solutions were saturated Ca(OH)2 solutions containing 0.27%,1.07%, and 21.4% NaCl. These NaCl concentrations correspond to the Cl-concentrations in our research work [2] that investigated the corrosion resistance of Cr-bearing corrosion resistant rebars in concrete with high chloride content. In theresearch work, the concrete contained chloride ion (0.3, 1.2, and 24 kg/m3), water (185kg/m3), and other stuffs (cement, sand, coarse aggregate, and air entraining agent).0.27%, 1.07%, and 21.4% NaCl correspond to the concentration of 0.3, 1.2, and 24kg/m3 of Cl- in 185 kg/m3 of water, respectively. The pH of the test solutions wereadjusted at 12.5, 11, 10, and 9 by HCl. Pitting potentials in deaerated these solutionsat 30℃ were measured using a potentiostat. Specimens were polarized in an anodicdirection at the rate of 20 mV/min after 10 min immersion in the solutions. Pittingpotentials were determined as the most noble potential at which the anodic current wasover 100µA.2.2 Results and discussionFigure1 shows the effect of pH on the pitting potential. In the case of 1.07% NaClsolutions, pitting potentials decreased largely between pH 12.5 and 11. The decreaseof pitting potentials with pH drop was almost saturated in the range of less than pH 11.5Cr steel showed considerably better pitting potential than that of 0Cr steel at pH 12.5.However, its advantage became small at pH 11 as long as corrosion initiation likepitting potentials was concerned. The steels containing 9%Cr and more showed goodcorrosion resistance even in the range of less than pH 11. In the case of 21.4% NaClsolutions, pitting potentials of all steels decreased gradually with the drop in pH.16Cr steel showed good corrosion resistance even in 21.4% NaCl solutions at 8 0 0 8 0 0 (a) 1.0 7 % N a C l (b ) 2 1.4 % N a C l 6 0 0 6 0 0 Pitting potential (mV vs Ag/AgCl) Pitting potential (mV vs Ag/AgCl) 4 0 0 4 0 0 1 6 C r 2 0 0 2 0 0 1 6 C r 0 5 C r 0 9 C r 9 C r - 2 0 0 - 2 0 0 0 C r 5 C r - 4 0 0 - 4 0 0 0 C r - 6 0 0 - 6 0 0 8 9 1 0 1 1 1 2 1 3 8 9 1 0 1 1 1 2 1 3 p H p H Figure1:Effect of pH on the pitting potentials of 0Cr, 5Cr, 9Cr, and 16Cr steels in simulated concrete pore solutions containing. TAE, Durable Reinforced Concrete Structures, 3/8
    • 1 6neutralized condition. In our research work [2], 1 4open circuit potentials of rebars in concrete weremeasured. It was showed that the potentials could 1 2rise up to 0V Ag/AgCl (~ -0.1 V CSE). Therefore, 1 0 S afe ar e aif the pitting potentials of the steel are nobler than Cr (%) 80V Ag/AgCl, the steel has few risks of corrosion. 6Figure2 shows a safe area and a risk area in Cr 4 Ris k ar e acontent vs. NaCl concentration map judging from 2pitting potentials in pH 12.5 solutions. Corrosion 0resistance of rebars should be discussed in terms of 0.1 1 1 0 1 0 2corrosion initiation and corrosion rate. Figure2 is N a C l (%)just a guideline for corrosion initiation. However, ifcorrosion does not initiate in a corrosion condition Figure2:Guideline map of Cr-of steels, the steels are safe for use in the condition. bearing corrosion resistant reThe steels containing few percent of Cr have bars in simulated concretepotential as corrosion resistant rebars in salt- pore solutions at pH 12.5.damage environments.3.Corrosion resistance in concrete having chloride ion contents3.1 Outline of the experiments and Methods of testingTable1 indicates the basic components of the reinforcing bars. Table2 shows the mixproportion of concrete. Five levels of chloride ion contents, being 0.3, 0.6, 1.2, 2.4and 24 kg/m3 of concrete were applied, which were adjusted using NaCl (first classreagent). In this study, test specimens were made to form micro cell corrosion bypouring concrete with particular chloride ion content after laying each rebar with20mm concrete cover thickness as shown in Figure3. In the specimen, two types ofrebars were arranged at the left and right sides in two layers, with one type of rebar ineach layer. The specimen was sealed and cured for 7days and then with the formsremoved, it was cured in the air for one week in an area having constant temperatureand constant humidity (20±3℃, 50±5%). Following the above-mentioned curingprocess, the acceleration of corrosion for each specimen was achieved by repeated highand low temperature curing as well as wet and dry curing. Each curing cycleconsisted of one-day high temperature/humidity (temp. 60, humidity 95) curing andone-day low temperature/humidity (temp. 30, humidity 50) curing.The measurement of the weight loss by corrosion was performed in accordance withthe “method and criteria for testing concrete structures; and evaluation of corrosion ofrebars in concrete” published by the Japan Concrete Engineering Association [3].Rust was removed from the rebars taken from the specimen concrete by dipping SD345,0Cr and 5Cr rebars in a 10% aqueous solution of diammonium citrate and the rebarwith Cr contents exceeding 5% in 30% nitric acid, at which point their weights were TAE, Durable Reinforced Concrete Structures, 4/8
    • measured in grams up to the second decimal place (0.01g) using an electronic balance.After that, the weight loss was calculated using an equation (1). ∆W = (Wo − W ) − Ws × 100 (1) Wo Where: ∆W = Weight Loss (%) Wo = Reinforcing bar mass before the rust removing process (g) W = Reinforcing bar mass after the rust removing process (g) Ws = The non-corroded part of the rebar dissolution (g) Table2:Mix proportion of concrete. W/C Water Cement Sand Coarse s/a Air entraining Cl- Aggregate agent (%) (kg/m3) (kg/m3) (kg/m3) (kg/m3) (%) (g/m3) (kg/m3) 0.3, 0.6 , 1.2 , 60 185 308 810 970 46 20 2.4, 24 Placing Epoxy Resin a 20 100 Chloride addition concrete 20 20 20 10 a’ 10 100 370 a-a’ Cross section *Unit;mm Figure3:Details of test specimen.3.2 Results and discussion3.2.1 Weight LossesFigure4 shows the change in weight losses of various types of rebars by chloride ion content with an elapse of time. According to Figure4, as the age of corrosion accelerated curing increased, the corrosion loss showed a tendency to increase for all typesof rebars, and when the chloride ion content was the same, the corrosion loss of therebar shows a tendency to decrease as the Cr content of the rebar increased. In addition, regardless of the rebar type, the corrosion loss increased as the chloride ion conte TAE, Durable Reinforced Concrete Structures, 5/8
    • nt increased. Such tendency was conspicuous in the SD345 rebar and 0Cr rebar.And the corrosion loss, which was less than 3% when the chloride ion contents were0.3kg/m3 and 0.6kg/m3, increased rapidly as the chloride ion content increased. Suchphenomenon became conspicuous as the cycle of the corrosion accelerated curing increased. In the case of the concrete having a chloride ion content of 1.2kg/m3, whichis the limit of chloride content for corrosion of the carbon steel rebar set out by theJapan Society of Civil Engineers [4], while the corrosion loss ratio of the SD345 rebar exceeded 3% at the 155cycle of the corrosion acceleration curing, that of the 5Cr rebar was approximately 0.3%, which is very minute. When the chloride ion content is 2.4kg/m3, even for the 5Cr rebar the corrosion loss showed a tendency to slowly increase as the cycle of corrosion accelerated curing increased. However, forthe Cr-bearing corrosion resistant rebar with a Cr content of more than 9%, the corrosion loss measured was very small. In addition, when the chloride ion contentis 2.4kg/m3, for the Cr-bearing corrosion resistant rebar with a Cr content of morethan 11%, the corrosion loss measured was less than 1%. 9 9 Cl-:0.3kg/m3 Cl-:0.6kg/m3 Weight Loss (%) Weight Loss (%) 6 6 3 3 0 0 0 50 155 0 50 65 155 9 9 Cl-:1.2kg/m3 Cl-:2.4kg/m3 Weight Loss (%) Weight Loss (%) 6 6 3 3 0 0 0 50 65 155 0 50 65 155 9 Cycles of Wetting & Drying Cl-:24kg/m3 Weight Loss (%) 6 SD345 0Cr 5Cr 9Cr 3 11Cr 13Cr 16Cr SUS304 0 0 50 65 155 Cycles of Wetting & Drying Figure4:Change in Weight Loss. TAE, Durable Reinforced Concrete Structures, 6/8
    • 3.2.2 Average Corrosion Rates Figure5 shows the relationship between the chloride ion content and the average corrosion rate by corrosion for different types of rebars. The average corrosion rate is the average value during 50, 65 and 155 cycles of corrosion accelerated curing. According to Figure5, the average corrosion rate increased along with the increase of chloride ion content irrelevant to the types of rebars. Such tendency was conspicuous in the SD345 rebar and 0Cr rebar having no Cr content. And the average corrosion rate occurred to the SD345 and 0Cr rebar in the concrete containing a chloride ion content of 0.3kg/m3. As well, the weight loss by corrosion did not increase remarkably while the chloride ion content showed an increase up to 1.2kg/m3. However, rapid increase in the average corrosion rates was observed when the chloride ion content had increased to 24kg/m3 from 1.2kg/m3, and the measured average corrosion rates for the SD345 rebar, which contains much carbon, was as high as that of 0Cr rebar. On the other hand, very minute average corrosion rate occurred to the 5Cr rebar when the chloride ion content of the concrete was 1.2kg/m3, and the average corrosion rate showed a tendency to slowly increase as the chloride ion content had increased to 24kg/m3. However, for the rebar with Cr content exceeding 9%, infinitesimal average corrosion rates was observed even in the concrete with chloride ion content of 24kg/m3. This is due to the strong passive film that had formed on the rebar surface by the reaction between chromium (Cr) and oxygen (O2) [5]. Generally, it is known that Cr alloy having Cr content greater than 10% has a higher corrosion resistance than carbon steel [6]. Moreover, Figure6 shows the effect of the chloride ion content and Cr-content to the average corrosion rate of a rebar. According to Figure6, the higher the Cr content, the less the average corrosion rate regardless of the chloride ion content. Therefore, according to the results of above weight losses and average corrosion rates, it can be considered that the rebar with Cr contents above 5% and 9% have corrosion resistant characteristics against concrete with chloride ion contents of 1.2kg/m3 and 2.4kg/m3, respectively. 10 16 4 Corrosion Rate (mg/cm /month) SD345 0Cr 5Cr 9Cr Average Corrosion RateAverage Corrosion Rate 8 11Cr 13Cr 16Cr SUS304 13 (mg/cm2/month) (mg/cm /month) 3 2 Cr content (%) 6 11 2 2 4 9 1 2 5 0 0 0 0.1 0.1 1.0 1.0 10.0 10.0 100.0 100.0 0.3 0.6 1.2 2.4 24.0 Concentration of Chloride Ion (kg/m3 ) 3) Concentration of Chloride Ion (kg/m Concentration of Chloride Ion (kg/m3) Figure5:Change in Average Corrosion Rate. Figure6:Effect of the chloride ion content and Cr content to the Average Corrosion Rate. TAE, Durable Reinforced Concrete Structures, 7/8
    • 4. ConclusionFor the purpose of developing a Cr-bearing corrosion resistant rebar that can be usedunder corrosive environments, experiments were conducted on the corrosion resistanceof Cr-bearing corrosion resistant rebars in simulated concrete pore solutions andconcrete specimens having chloride ion contents.The following is the knowledge obstained from these experiments:1. Corrosion resistance of the steels in simulated concrete pore solutions was increased with Cr content.2. Increase of chloride content and drop in pH of the solutions deteriorated the pitting potentials of the steels. Decrease of pitting potentials in the solutions with the drop in pH was large between pH 12.5 and 11.3. 5Cr and 9Cr steels showed good corrosion resistance in 1.07% NaCl solutions at pH 12.5 and pH 10, respectively. Corrosion rate of 5Cr steel was smaller than that of 0Cr steel in 1.07% NaCl solutions at pH 10.4. 16Cr steel showed as good corrosion resistance as SUS304 steel even in 21.4% NaCl solutions at pH 10.5. Irrelevant of the chloride content, the average corrosion rate was low when the Cr content was high. From this it is considered that the corrosion resistance against the chloride has improved due to the formation of passive film by chromium.6. For the chloride ion contents, the Cr contents required for corrosion prevention are as follows: Cl-(kg/m3) Chloride ion contents(%more) 1.2 5 2.4 9 24 115.Reference1. F. N. Smith and M. Tullmin, “Materials Performance”, May, 72, 1999.2. S. H. Tae, T. Noguchi, M. Kanematsu and T. Ujiro, “FIB2002 Osaka Congress”, Sess ion 8, p.155, 2002.3. Japan Concrete Institute, “Examination method and criterion about corrosion, corrosi on resistance of a concrete construction (plan), JCI - SC1 -corrosion evaluation method of steel materials in concrete-”, pp. 1-2.4. Japan Society of Civil Engineers, “Concrete standard specification (construction), -du rability collation type-”, 2000.5. Stainless steel association, “Basics knowledge of stainless steel”, p.2, 1984.6. H.H. Uhlig, “Corrosion Handbook”, p.150, 1953. TAE, Durable Reinforced Concrete Structures, 8/8