Enhancing The Reliability Of Evaluating Chloride Ingress .

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Journal of ASTM International, March 2006, Vol. 3, No. 3Paper ID JAI13403Available online at www.astm.orgM. T. Bassuoni,1 M. L. Nehdi,1 and T. R. Greenough1Enhancing the Reliability of Evaluating Chloride Ingress inConcrete Using the ASTM C 1202 Rapid ChloridePenetrability TestABSTRACT: The rapid chloride penetrability test 共RCPT-ASTM C 1202兲 is commonly used to evaluate theresistance of concrete to chloride ions ingress owing to its simplicity and rapidity. However, it has beencriticized for various shortcomings such as giving favorable results to supplementary cementitious materials共e.g., silica fume兲, and bias against calcium nitrite corrosion inhibitors 共CNI兲. Based on the ASTM C 1202induced voltage concept, this study aims at enhancing the reliability of rapidly evaluating concrete resistance to chloride ions ingress. It is proposed that upon test termination, not only the passing charges arerecorded, but the depth of chloride front migrating into concrete is also measured. Concrete mixtures hereinwere prepared incorporating selected materials 共silica fume and CNI兲 that have been known to causemisleading results for the RCPT test. The effects of silica fume and CNI dosages on RCPT results wereinvestigated and correlated to porosity trends evaluated by mercury intrusion porosimetry 共MIP兲. The studyreveals that measuring the migrating chloride front in concrete subsequent to the ASTM C 1202 test caneliminate the bias induced by electrolysis conductivity resulting from silica fume and/or CNI. This improvesthe reliability of assessing concrete resistance to chloride ions penetration using the RCPT procedure.KEYWORDS: rapid chloride penetrability test 共RCPT兲, chloride ions, silica fume, calcium nitrite corrosion inhibitor 共CNI兲, migration coefficient, resistivity, porosityNomenclatureACNICRDdE / dxILMIPPTRCPTRMTtVxxd Cross-sectional area of specimen 共m2兲Calcium nitrite corrosion inhibitorsCorrosion rate 共 m Fe/yr兲Non-steady-state migration coefficient 共 10 12 m2 / s兲Electric field strength 共V/cm兲Current 共A兲Thickness of specimen 共mm, m兲Mercury intrusion porosimetryPorosity 共%兲Average value of initial and final temperatures in anolyte solution 共 C兲Rapid chloride penetrability testRapid migration testTime 共s, h兲Applied voltage 共V兲Distance 共cm兲Average value of penetration depth 共mm兲Ionic mobility 共cm2 / V · s兲Resistivity 共 . m兲Manuscript received March 2, 2005; accepted for publication September 14, 2005; published January 2006.1Department of Civil and Environmental Engineering, The University of Western Ontario, 1151 Richmond Street, London, ON,Canada, N6A 5B9.Copyright 2006 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.

2 JOURNAL OF ASTM INTERNATIONALIntroductionBackgroundCorrosion of embedded reinforcement in concrete structures is a key durability issue. Generally, corrosionrelated costs are estimated to be in the range of 3 to 5 % of the Gross National Product 共GNP兲 in the U.S.,Western Europe, and Japan, 25 % of which can be ascribed to corrosion of steel reinforcement in concretestructures 关1兴. It is well established that chloride ions can destroy the passive layer surrounding steelreinforcement provided by the high alkalinity of cement paste, thus activating electrochemical corrosion关e.g., 关2兴兴.Several approaches have been employed in concrete mixtures to minimize the risk of corrosion ofembedded reinforcing steel, such as incorporating appropriate dosages of supplementary cementing materials 共silica fume, fly ash, slag, etc.兲 and corrosion inhibiting admixtures. Silica fume possesses an ultrafine particle size 共around 0.1 m兲 and produces a pozzolanic reaction with portlandite resulting in arefined interfacial transition zone 共ITZ兲 between cement paste and aggregates. It helps achieving densermicrostructure, reduced permeability and increased electrical resistance of concrete 关3兴. Consequently, oneof the most prominent applications of silica fume is in the protection of embedded steel reinforcementagainst chloride ions ingress 关4兴.Corrosion inhibiting admixtures delay the onset of steel reinforcement corrosion and reduce its rate.There are different types of these admixtures with various chemical and physical inhibition mechanisms.Calcium nitrite-based inhibitors are a common example of inorganic corrosion inhibitors and are oftenreferred to as CNI. They are usually added in dosages within the range of 5 to 30 L / m3 of concretedepending on the severity of chloride ions exposure 关5兴. The nitrite ions function through a healing processto preserve the passive layer surrounding steel reinforcement. When chloride ions attack weak spots offerrous oxide on the passivating layer, corrosion is initiated and corrosion products are generated. Thisexposes pure iron atoms, which quickly turn into the unstable ferrous oxide state. The healing actionoccurs when nitrite ions move rapidly to surround the newly exposed ferrous oxide sites, shielding themfrom chloride ions attack. Also, nitrite ions help in converting ferrous oxide into ferric oxide that forms theprotective passive layer 关6兴. Because of this competition between chloride and nitrite ions over vulnerablespots on steel reinforcement surfaces, the mechanism of most CNIs produced in the market is based on acritical chloride-to-nitrite ratio of 0.92 关5兴.Rapid Chloride Penetrability Test (RCPT)Several techniques have been employed, though some of which remain questionable, for the evaluationand prediction of corrosion damage in reinforced concrete. These techniques include chemical analysis ofchlorides, rapid chloride penetrability test 共RCPT兲, half-cell potential, ponding test, linear polarization,resistivity, and radiography 关7兴. The RCPT is commonly used to evaluate the resistance of concrete tochloride ions ingress through electrical conductivity measurements. In the mid-1970s, because of theextensive use of deicing salts, the Federal Highway Administration 共FHWA兲 was interested in evaluatingthe ingress of chloride ions in the concrete cover towards steel rebars. Construction Technology Laboratories of Skokie, Illinois, was assigned the task of developing an instrument that gives a rapid indicationof the resistance of concrete to the infiltration of chlorides. At that time, there were rapid methods forassessing the permeability of concrete to air and water; however, these results did not correlate well withthat of diffusion of chloride ions in concrete 关8兴.The original concept of the RCPT depended on ionic mobility. Chloride ions are forced into concretethrough the induction of an external direct voltage on the concrete surface, and ionic mobility wascalculated by: where Ionic mobility 共cm2 / V · s兲x Distance 共cm兲x dEt dx共1兲

BASSUONI ET AL. ON CHLORIDE INGRESS 3t Time 共s兲dE / dx Electric field strength 共V/cm兲The results of this applied voltage method agreed well with results of long-term chloride immersiontests, such as the 90-day ponding test 共AASHTO T 259兲 关9兴. In a preliminary phase, it was concluded thatapplying 80 V DC for eight hours was effective in differentiating between low and high chloride ionspenetrability concrete. High currents and high temperatures accompanied extremely permeable samples,and vice versa for low permeability samples 关10兴. In 1981, a final laboratory version of the test wasdeveloped to determine the chloride permeability class of concrete. In this test, a cylindrical plain concretespecimen of approximately 100 mm in diameter and 50 mm in thickness is placed across 3 % sodiumchloride 共NaCl兲 and 0.3 N sodium hydroxide 共NaOH兲 solutions for 6 h, while applying 60 V DC. Thesamples are preconditioned under vacuum for 3 h and submerged in water for 18 h for full saturationbefore testing. The output of this apparatus is the total current integrated over time, or charges passed inCoulombs 关10兴. The test method was issued under AASHTO T 277-83 including a table for determiningthe chloride permeability class 共negligible, very low, low, moderate and high兲. This table was preparedbased on cores from slabs provided by the FHWA without considering the effect of cement type, aggregatetype, and density of concrete. Thus, it was recommended that users may construct their own RCPTclassification tables considering the effect of local materials tested by the RCPT 关9兴. In 1991, the RCPTwas issued under the ASTM C 1202 designation, without major deviations from the AASHTO version关10兴.The RCPT spread widely since 1986 accompanying the extensive use of silica fume, particularly inbridge overlays 关10兴. Also, many concrete standard specifications 共e.g., CSA A23.1 issued by NationalStandards of Canada兲 have used the RCPT passing charge levels as a durability indicator for concretequality. However, the RCPT has since been criticized for various shortcomings including its bias tosupplementary cementing materials such as silica fume and against CNI 关e.g., 关11兴兴. Several researchers关e.g., 关12,13兴兴 questioned the reliability and suitability of this test method to assess chloride ionspenetrability into concrete. It was also suggested to adopt the rapid migration test 共RMT兲 developed byTang and Nilsson 关14兴 and issued under NT BUILD 492 共Nordic Standards兲 as an alternative.Basically, the RMT is similar to the RCPT in the sense that chloride ions are forced to migrate intoconcrete by applying a strong electrical field 关14兴. Under a specific direct voltage, a concrete disk of100 mm in diameter and 50 mm in thickness is placed across 10 % sodium chloride and 0.3 N sodiumhydroxide solutions for certain time duration depending on the initial current. At the end of testing, thespecimen is axially split and sprayed with a silver nitrate solution to determine an average chloridepenetration depth from which a diffusion coefficient is calculated 关15兴. Contrary to RCPT passing chargeresults, RMT chloride penetration measurements are insensitive to pore solution conductivity, and are lessvulnerable to testing variations 关12兴. It should be emphasized that both the RCPT and RMT are acceleratedtests based on ionic mobility under an induced direct voltage, but the measured parameters are different ineach test.Using the ASTM C 1202 concept and procedure, this study aims at enhancing the reliability of rapidlyevaluating concrete resistance to chloride ions ingress by obtaining multiple measurements at the end oftesting. Julio-Betancourt and Hooton 关16兴 have recently proposed modifying the ASTM C 1202 to a1-min conductivity test to eliminate heat effects on ions mobility and final Coulomb values. Nonetheless,based on the conductivity criterion alone, both the conventional and the newly proposed procedure canyield unreliable assessment for chloride ions penetrability due to the incorporation of pozzolanic materialsand/or CNI in concrete mixtures.The current authors believe that there is a need to obtain an additional measurement 共chloridepenetration depth兲 from the RCPT, which better correlates to physical characteristics of the pore structureand in turn chloride ions penetrability. Hence, the reliability of assessing the resistance of concrete to thepenetration of chloride ions, and in turn the risk of reinforcement corrosion, using this acceleratedprocedure 共ASTM C 1202兲 can be improved by relying on multiple measurements that indicate bothchloride ions penetrability and concrete resistivity. These are chief factors in the service life prediction ofstructures exposed to chlorides. The total service life is the sum of time-to-corrosion initiation of steel

4 JOURNAL OF ASTM INTERNATIONALTABLE 1—Chemical composition of OPC and silica fume.Percent byMassSiO2CaOAl2O3Fe2O3MgOK 2OSO3Na2OOPC19.561. Fume92. and corrosion propagation time until the occurrence of concrete cracking and spalling. Concretecover and chloride diffusivity govern the corrosion initiation stage, while concrete resistivity maydetermine the corrosion propagation rate 关e.g., 关17兴兴.Experimental ProgramMaterialsSeventeen concrete mixtures with water-to-cementitious materials ratios 共w/cm兲 of 0.30, 0.45, and 0.60were prepared incorporating selected materials that have been known to cause misleading RCPT results.The mixtures were divided into two groups: the first group incorporated CSA Type 10 共ASTM Type I兲ordinary portland cement 共OPC兲 and silica fume with and without CNI. The second group involved onlyOPC with and without CNI.OPC with a specific gravity of 3.17 and a specific surface area 共Blaine Fineness兲 of 412 m2 / kg wasused, and densified silica fume with an average particle size of 0.1 m, a specific surface area of19530 m2 / kg, and a specific gravity of 2.12 was used. The total cementitious materials content was400 kg/ m3. The oxide analysis of the cement and silica fume is presented in Table 1. The fine aggregatewas natural siliceous sand with a fineness modulus of 2.80, a saturated surface dry specific gravity of 2.65and absorption of 1.5 %. Crushed stone with a maximum nominal size of 19 mm, a saturated surface dryspecific gravity of 2.68 and absorption of 0.8 % was also used. An ASTM C 494 Type F high-range waterreducing admixture was employed, and its dosage was adjusted to maintain a slump range of160 to 190 mm. An inorganic corrosion inhibitor based on 30 % solution of calcium nitrite 共CNI兲 with adensity of 1.26 kg/ L was used at three dosages of 10, 20 and 30 L / m3 of concrete.ProceduresConstituent materials were mixed in a mechanical mixer in accordance with the ASTM C 192 standardprocedure. Table 2 shows the proportions of the tested mixtures. All specimens were demolded at 24 h andmoist cured at 23 C and 95 % RH for 27 days. Concrete disks of 100 mm in diameter and 50 mm inthickness were sliced from the top portion of 100 mm 200 mm cylinders for the RCPT 关18兴, simulatingconcrete above the rebar level. Three replicates were prepared for each mixture. Samples were preconditioned

procedure ASTM C 1202 can be improved by relying on multiple measurements that indicate both chloride ions penetrability and concrete resistivity. These are chief factors in the service life .

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