Research Article Corrosion of Steel in High-Strength …downloads.hindawi.com/journals/ijc/2014/564163.pdfResearch Article Corrosion of Steel in High-Strength Self-Compacting Concrete

Research ArticleCorrosion of Steel in High-Strength Self-Compacting ConcreteExposed to Saline Environment

Hana A. Yousif,1,2 Farqad F. Al-Hadeethi,3 Bashar Al-Nabilsy,4 and Amani N. Abdelhadi3

1 Building and Construction Engineering Department, University of Technology, Alsinaa Street, 12906 Baghdad, Iraq2 Building Research Center, Royal Scientific Society (RSS), Al-Jubaiha, Amman 11941, Jordan3 Scientific Research Center, Applied Science Sector, Royal Scientific Society, Al-Jubaiha, Amman 11941, Jordan4Conformity and Quality Institutional Company, Abdul Hameed Badees Street 45, Shmeisani, Amman 11121, Jordan

Correspondence should be addressed to Hana A. Yousif; [email protected]

Received 9 January 2014; Accepted 6 April 2014; Published 23 April 2014

Academic Editor: Flavio Deflorian

Copyright © 2014 Hana A. Yousif et al.This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A research work was carried out to investigate the effectiveness of high-strength self-compacting concrete (SF-R) in controllingcorrosion of embedded steel. Reinforced concrete cylinders and plain cubes were subjected to 5%NaCl solution. Slump flow, J-ring,V-funnel, compressive strength, electrical resistance, and electrochemical tests were conducted. Corrosion resisting characteristicsof steel were examined by monitoring corrosion potential, polarization resistance, corrosion currents, and Tafel plots. Therelationship between corrosion current density and corrosion potential was established. Results were compared with characteristicsof a grade 40MPa reference concrete (R) and grade 70MPa conventional self-compacting concrete (SP). Results indicated that at270 days of exposure, the corrosion currents for steel in SF-R were 63- and 16-fold lower compared to those of steel in R and SPconcretes, respectively. This concrete showed a considerable increase in electrical resistance and compressive strength of 96MPaat 28 days of exposure. Relying on corrosion risk classification based on corrosion current densities and corrosion potentials, thesteel in SF-R concrete is definitely in the passive condition. The splendid durability performance of steel in SF-R concrete linked toadorable self-compacting features could furnish numerous opportunities for future structural applications in severe environmentalconditions.

1. Introduction

Corrosion of reinforcing steel in concrete and the high repairand rehabilitation costs of structures associated with mate-rials failures are nowadays a matter of considerable concernand forcing the engineers to become durability conscious. Anincreasing number of incidents of structural problems arisingfrom corrosion of steel associated with chloride intrusionhave been reported fromdifferent parts of theworld [1, 2].Thedamage to concrete resulting from corrosion of steel may leadto a reduction in service life or precocious structural damagedue to loss of bond between steel and concrete and reductionof steel cross-sectional area [3].Therefore, construction engi-neer should focus on producing dense, high-quality concretewith very low permeability which consequently limits theintrusion of the corrosion inducing substances. Buildingelements made of high-strength concrete are usually denselyreinforced. The small distance between reinforcing bars may

lead to defects in concrete. Therefore, suitable compaction isnecessary to ensure that adequate strength and durability areachieved. Insufficient compaction will lead to the inclusionof voids, which not only lead to a reduction in strength butalso strongly influence the natural physical and chemicalprotection of embedded steel reinforcement afforded byconcrete. Accordingly, the utilization of high-strength self-compacting concrete in structural elements exposed to salineenvironment may provide splendid performance in termsof self-compacting features without any external means ofconsolidation [4]. Furthermore, the wonderful flowability,filling ability, and high resistance to segregation of SCClead to consistent and dense microstructure characterizedby the low porosity of the matrix and interfacial transitionzone and consequently enhance the electrical resistance andlimit the transmission properties of concrete. These featuresmay promote the durability characteristics and conservethe reinforcing steel versus corrosion [5]. The low capillary

Hindawi Publishing CorporationInternational Journal of CorrosionVolume 2014, Article ID 564163, 11 pageshttp://dx.doi.org/10.1155/2014/564163

2 International Journal of Corrosion

Table 1: Chemical composition of cement and SF.

Components CaO SiO2 Al2O3 Fe2O3 SO3 MgO K2O Na2O LOI IR C3S C2S C3A C4AFC 63.3 20.8 5.0 3.2 2.8 2.8 0.6 — 0.9 0.8 53.3 19.7 7.9 9.8SF — >90 — — — — — — <3 — — — — —

porosity and high electrical resistance limit the chloridepenetration and impede the electrochemical reaction andtherefore prevent corrosion of steel reinforcement.

Numerous researchers have been investigating variouscorrosion resistance characteristics and related durabilityfeatures of SCC and confirmed to be superior to normalconcrete (NC). Their investigations involved essentially thepermeation properties [6], steel-concrete interface defects[7], chloride penetration [8, 9], oxygen permeability andaccelerated carbonation [8], electrical resistivity, porosity,and transport properties [10], structural performance underfour-point loading after exposing to corrosive environment[11], chloride permeability [12, 13], diffusion coefficient [13],reinforcement corrosion initiation and chloride ion content[14, 15], and corrosion loss of steel [10, 14]. Despite vitalimportance and extensive application of SCC in chlorideladen environments, extremely little research work [7, 12]has been published about some electrochemical behaviors ofsteel in self-compacting concrete exposed to the severe salineenvironment.

The main objective of the current research work is toinvestigate the electrochemical corrosion performance ofreinforcing steel in high-strength self-compacted concrete.The concrete incorporated high performance super plasti-cizer and silica fume as a partial replacement by weight ofcement as well as an addition to cement.The electrochemicalcorrosion resisting characteristics of steel were examined bymonitoring half-cell potentials, linear polarization resistance,corrosion currents, and Tafel plots. The strength of the SCCand the DC electrical resistance of the corrosion carryingmedia were also examined. The results of this study providea vital reference for future structural application and design,particularly for structures exposed to deicing salts and thoselocated in a coastal marine environment.

2. Research Significance

Intensive research studies were required to examine corro-sion resisting characteristics of steel in SCC. The perfor-mance of reinforced SCC in aggressive environment maydiverge widely depending on the mix formulation.Therefore,investigations addressing the influence of the various typesof powders, their specific surface area, and mix design onthe flowability, resistance to segregation, permeability, anddurability of steel in the SCC are needed to manage theelectrochemical reactions and meet the design life undercertain environmental conditions.

The utilization of high-strength self-compacting concrete(HS-SCC) has increased vastly worldwide, and their appli-cations were expanded to structural members situated insevere environmental conditions. However, the electrochem-ical corrosion which was extremely important concern has

Table 2: Proportion of concrete mixes.

Symbol Mix ingredients, kg/m3w/c or w/cm SP %

C SF FA CAR 550 — 780 850 0.52 —SP 550 — 780 850 0.31 3SF-R 495 55 780 850 0.31 3SF-A 550 55 780 850 0.31 3

not been investigated yet and was very limited for steelin SCC. Moreover, well controlled HS-SCC may produce ahigh durability system, provide lower electronic conductionand high electrical resistance that hinders the flow of thecorrosion currents, maintain high pH level in the vicinity ofsteel, improve corrosion resisting characteristics, and extendthe service life of the structure.

3. Experimental Details

3.1. Materials and Mix Proportions. The cement used in thisstudy was ordinary Portland cement (OPC) that conforms toASTMC 150 [16] Type I. ElkemMicrosilica Grade 940Dwitha typical bulk density of 500–700 kg/m3 conforming toASTMC 1240 [17] was used as a highly active mineral admixturefor the production of HS-SCC. Silica fume (SF) containedgreater than 90% SiO

2and less than 3% carbon.The chemical

composition of cement (C) and SF according to the produceris shown inTable 1. “Flocrete PC260” fromDonConstructionProducts (DCP) Ltd. based on polycarboxylic ether polymerwas used as a high-performance superplasticizer (SP) forreducing water content of the mix. It meets the requirementsof ASTM C 494 Type A and F [18]. A mixture of 40% Swelhsand and 60% Sel sand and crushed coarse aggregate withparticle sizes 12.5 to 4.75mm conforming to ASTM C 33 [19]were used. Table 2 lists the details of the mix proportions.Steel reinforcement with an effective diameter of 15.8mmwascalculated based on unit weight of steel, and the weight of therebar of 305mm in length was used.

Four types of concrete mixes were used; reference con-crete mix (R) was designed to have 28 days characteristiccompressive strength of 40MPa with a slump of 180mm, andconventional self-compacting concrete (SP) incorporatingonly 3% SP was designed to yield 28 days strength of 70MPa.Further, two HS-SCC were used: SF-R and SF-A concretesincorporating 3% SP and 10% SF as a partial replacementby weight of cement for the former and as an addition tothe latter. There is no standard method for the design ofSCCmixes. Mix design often uses volumes as key parametersbecause of the importance of the need to overfill the voidsbetween the aggregate particles [20]. The details of the mixesare presented in Table 2.

International Journal of Corrosion 3

Table 3: Self-compacting features.

Symbol SP % w/c or w/cm Self-compacting featuresTests Actual value

R — 0.52 Slump 180mm

SP 3 0.31

Slump flow𝑇 50 cm

J-ring flow Passingability

V-funnel time

710mm7 sec

700mm13mm8 sec

SF-R 3 0.31


J-Ring flowPassing abilityV-funnel time

683mm7 sec

670mm13mm9 sec

SF-A 3 0.31


J-Ring flowPassing abilityV-funnel time

675mm8 sec

660mm15mm9 sec

3.2. Trial Mixes and Self-Compacting Tests. Self-compactingconcrete must have the ability to flow under its own weight,to pass easily between reinforcing bars or other obstacleswithout segregation during or after casting. Trial mixeswere conducted to address these fresh properties and tomonitor w/c or w/cm materials ratio and superplasticizerdosage. Concretes were mixed using pan mixer. Slump testin accordance with ASTM C 143 [21] was used to determinethe workability of R mix. Slump flow based on ASTM C1611 [22], J-ring according to ASTM C1621 [23], and V-funnel tests based on EFNARC [24] were carried out toexamine the self-compacting features of SCC mixes. Theslump flow test is used to examine the horizontal free flowof self-compacted concrete in the absence of obstructions.The diameter of the concrete circle is a measure of fillingability. The flow rate is influenced by the concrete’s viscosity,and the stability can be observed by visually examining theconcrete patty for evidence of segregation. The J-ring test isused to determine the passing ability in combination withthe slump flow test. The J-ring is placed outside the slumpcone so that the concrete flows through the legs of the ringwhen the cone is lifted. The slump flow with and without theJ-ring is measured. A difference of less than 25mm indicatesa good passing ability. The V-funnel test is used to measurethe ease of flow and filling ability of concrete. The funnel isfilled with concrete and the time taken for it to flow throughthe apparatus ismeasured. Shorter flow time indicates greaterflowability. Target values are slump flow of 550–750mm andV-funnel time of 6 to 12 seconds.The self-compacting featuresare demonstrated in Table 3.

3.3. Specimens Preparation and Exposure Condition. Fourcylindrical series were prepared for electrochemical corro-sion measurements. Specimens were 100mm in diameterand 200mm in length, reinforced with deformed mild steelrebar of 16mm in diameter. The rebar has an effective coverof 42mm and was protruding from the upper surface of

the cylinder. The rust on the rebar was cleaned by steel brushand SiC sand paper. The specimens for electrical resistanceand compressive strength measurements were 100mm cubescast also in four series. Concrete poured into the molds inthree layers for cylindrical specimens and in two layers forcubes, and compaction was applied using vibrating table forR mixes, while SP and HS-SCC mixes neither needed anyvibration nor surface leveling. After casting, specimens werecoveredwith nylon sheets for 24 hours to prevent evaporationand then removed from the molds and cured for 27 days in acontrolled environment of 25 ± 2∘C temperature and about95% relative humidity. During this period, the projectedportions of the rebar were wrapped with sealed tape toprevent corrosion. At the end of the curing period, specimenswere placed in laboratory air for three days then partiallyimmersed in a 5% NaCl solution in vertical position toaccelerate the deterioration process. The level of the solutionwas adjusted so that half the depth of the specimens was insolution continuously. Measurements were conducted at agesof 7, 60, 90, 120, 150, 180, and 270 days of exposure to 5%NaClsolution. The compressive strength tests for the four serieswere carried out in accordance with ASTM C39 [25] usinga digitally controlled compression machine with a maximumload capacity of 2000KN. For each measurement, a set of sixspecimens was used for HS-SCC and a set of three specimensfor R and SP specimens.

3.4. Half-Cell Potential (HCP) Measurements. The half-cellpotential has been greatly used in the laboratory and in thefield for the assessment of the corrosion potential of steelreinforcement in concrete. It is nonintrusive test and providesa rapid and inexpensive means of identifying zones of activecorrosion in reinforced concrete structures that need furtheranalysis and rehabilitation [3]. The test was performed basedon ASTM C876 [26] to determine the corrosion activity andthe time to corrosion initiation. The principal concernedwith this technique is basically a measurement of corrosion


Figure 1: Half-cell potential setup.

potential of the working electrode (steel reinforcement inconcrete) as the voltage difference between the steel and astandard reference electrode close to concrete surface [27–29]. Silver/silver chloride (Ag/AgCl) was used as a standardreference electrode, Figure 1. Electrically, the reinforcing steelhas to be connected to the positive terminal of the voltmeterand the half-cell to the negative (ground) terminal of thevoltmeter. Precaution measures were taken to maintain thehalf-cell in proper conditions during the testing period. Half-cell potential measurements were taken at scheduled periods.The potentials recorded in the half cell readings can beused to indicate the probability of reinforcement corrosion.If the potentials are less negative than −0.119VAg/AgCl,there is greater than 90% probability that no reinforcing steelcorrosion was occurring at the time of measurement. If thepotentials are in the range of −0.119 to −0.269VAg/AgClcorrosion activities are uncertain. If the potentials are morenegative than −0.269VAg/AgCl there is greater than 90%risk of corrosion [26].

3.5. Corrosion Current Measurements. Corrosion currentswere determined by Tafel extrapolation methods. Threeelectrodes were used: reinforcing steel as a working elec-trode, Ag/AgCl electrode as standard reference electrode andcopper plate measuring 60 × 30 × 3mm as the auxiliaryelectrode.The electrical connection of the reinforcement wassecured by pin connected to the projected part of the bar.The experimental setup is shown in Figure 2. Measurementswere carried out to determine the polarization resistance (𝑅

𝑝)

using small potential scan within ±20mV from 𝐸corr at thescan rate of 0.167mV/s.The amount of polarization shift (Δ𝐸)was selected within linear Stern-Geary range of 10–30mV[30] to avoid electrochemical disturbance of the polarizedelectrode during the test. In the present work, 𝑅

𝑝was

obtained directly using the “𝑅𝑝fit graphic tool” associated

with Software techniques.The fundamentals of this techniqueare based on the following Stern-Geary equation [30]:

𝑅𝑝=

Δ𝐸

Δ𝐼

. (1)

Tafel extrapolation is used to determine the polarizationcurve. The anodic and cathodic Tafel constants 𝛽𝑎 and 𝛽𝑐were determined from Tafel plots. The Tafel slopes 𝛽𝑎 and𝛽𝑐 are the voltage change per one logarithmic cycle of

Potentiostat

CE

PC

RE WE

Figure 2: Corrosion current measurement setup.

current (volts per decade) in the positive (Nobel) andnegative(Active) direction, respectively. The back extrapolation linesintersect at a point on line of corrosion potential, 𝐸corr,and the projection of this point on the current scale givesthe value of the 𝐼corr. In the present work, the potentio-stat instrument and general-purpose electrochemical system(GPES) software package were used to plot polarizationcurves and determine corrosion rates, 𝛽𝑎 and 𝛽𝑐 with Tafelfits. This technique is designed for plotting polarizationcurves and determining 𝐼corr through potential steps of±100mVaround the corrosion potential.Work has been donein correlating 𝑖corr to the condition of the rebar [27]; where𝑖corr < 0.1 𝜇A/cm2, steel is in a passive condition, 𝑖corr 0.1to 0.5 𝜇A/cm2 corrosion risk is low to moderate, 𝑖corr 0.5to 1 𝜇A/cm2 corrosion risk is moderate to high, and 𝑖corr> 1 𝜇A/cm2 corrosion risk is high. The surface area (𝐴) ofthe steel reinforcement that has been polarized needs to beaccurately known for determination of the corrosion currentdensity (𝑖corr) as in the following equation:

𝑖corr = 𝐼corr𝐴

. (2)

The corrosion rate (CR) in a millimeter per year (mmpy) iscalculated according to the next equation:

CR = 𝐼corr ⋅ 𝐾 ⋅ EW𝑑 ⋅ 𝐴

, (3)

𝐼corr is corrosion current,𝐾 is constant that defines the units of the corrosionrate (3272mm/(A⋅cm⋅year),EW is equivalent weight of steel (atomicweight/valence of the reacting electrode (iron)27.9225 g/eq),𝑑 is density, 7.8 g/cm3,

𝐴 is sample surface area, 78.4 cm2.


+

VA

Concrete

Cement

Steel plate

−

Figure 3: Electrical resistance measuring setup.

3.6. Electrical Resistance Measurements. Direct electric cur-rent (DC) technique was exercised for electrical resistancemeasurements of the concrete cubes. A constant electricfield was applied between two external electrodes, and theresulting current was measured. The electrical resistancemeasuring setup includes DC power supply, two high per-formance digital multi-meters, used as voltage and currentmeasuring devices and two stainless steel plates, measuring100 × 100 × 3mm were used as external electrodes. Steelplates were cleaned thoroughly using SiC paper to ensuregood conductivity. The two plates were lasted to the oppositesides of the specimen using cement paste with a w/c or w/cmmaterials ratio analogous to those of the test specimen toavoid a significant change in resistance due to the polarizationeffect. The electrical circuit for DC resistance measurementis shown in Figure 3. The specimens were investigated over awide range of output voltages. At each selected voltage, thesynchronous value of current was recorded. The electricalresistance of concrete specimens was determined by the slopeof the voltage-current plot. The average of three specimens,measured just after removal from the solution was adoptedat the scheduled ages. This ensured continuous electricalconnectivity across the gap separating the electrodes. Theresistance of concrete was determined by the slope of the lineof the voltage current plots. The resistivity then is calculatedfrom the following equation:

𝜌 = 𝐴 ⋅

𝑅

𝐿

(4)

𝑅: resistance (Ohm)𝜌: resistivity (Ohm⋅cm)𝐴: cross-sectional area the current flows through =10 × 10 = 100 cm2

𝐿: the gap separating the electrodes.

4. Results and Discussions

4.1. Half-Cell Potential (HCP). The HCP data presented inFigure 4 indicated that at the end of 28 days curing in tapwater the steel potentials in concretes were in the range of−124mV to −360mVAg/AgCl. According to Arup [31] in

−600

−500

−400

−300

−200

−100

00 50 100 150 200 250 300

RSP

SF-RSF-A

Age days

HCP

ver

sus A

g/A

gCl (

mV

)

Threshold value −269 mV

Figure 4: Half-cell potentials (HSPs) with age.

the absence of chlorides, the reinforcing steel in concreteis in the passive state brought about by the high pH andthe availability of oxygen. The range of passive potential,however, is very wide from +200mV to –700mV CSE ormore negative against Ag/AgCl at pH 13. Consequently, inthis state, the potentials more negative than ASTM C-876threshold value (−269mVAg/AgCl) do not indicate, with anydegree of accuracy that corrosion of steel reinforcement isoccurring at the end of 28 days curing. At an early stageof immersion in 5% NaCl solution the reinforcing steelpotentials, in general, rapidly changed to values less negativethan the initial potentials. This is clearly indicative of achange in the electrochemical status due to the presenceof chloride ions in the solution. Thereafter, the reinforcingsteel in SF-R and SF-A concretes exhibited gradual variationin potentials toward more negative values up to 60 days ofexposure and then varied continuously toward less negativepotentials up to the end of the testing period, Figure 4. Inthose specimens, a threshold value of −269mVAg/AgCl wasnot exceeded and corrosion activity of reinforcing steel isuncertain based on ASTM C876 [26]. Furthermore, the steelin R and SP concretes showed rapid and great variationstoward more negative potentials and a threshold value of−269mVAg/AgCl based on ASTM C876 was exceeded after70 days of exposure. This is indicating corrosion initiationaccording to ASTM C876 in R and SP specimens at 70 daysof exposure, Figure 4, due to penetration of chloride ions andbreakdown of the passive protective layer.

The measurements are too susceptible to the humiditystanding on the concrete, oxygen availability in the proximityof the steel and micro cracks in concrete. More negativepotentials result for steel in concrete with a higher degreeof saturation, lack of oxygen in the vicinity of the reinforce-ment and the presence of localized micro cracks [3]. Thesuperior performance of steel in a system of lower corrosionactivity, SF-R and SF-A specimens, may be explained interms of pores size and continuity, hence, the penetrabilityof the solution aggressive species. Therefore, these systems


Table 4: Polarization resistance and proportionality constant.

Age—d 𝑅𝑝—Ohm 𝛽—mV/decade

R SP SF-R SF-A R SP SF-R SF-A7 139 237 1973 1700 31 36 82 6260 211 312 2715 2523 66 58 77 70120 232 381 2900 2100 68 56 68 58150 265 481 3064 2023 73 55 62 46180 623 745 6167 5989 112 98 77 120270 1074 1268 9592 8095 150 119 90 154

have a higher electrical resistance and superior internalintegrity, thus, reducing the likelihood for the intrusion ofaggressive chloride ions, that is, the factors that managethe electrochemical corrosion activity. Attempts, however, todescribe the electrochemical process in terms of potentialmeasurements is irritating and not adequate as a criterion.Accordingly, may result in improper conclusions, since theyare affected by many factors such as polarization causedby limited propagation of oxygen [26–28], the existence ofhighly resistive layers and concrete porosity [32]. Further,deceptive conclusions regarding corrosion riskmay result in astructure where portions are exposed to dry conditions whileothers remain moist [3]. Thus, potentials readings should beinterpreted together with corrosion rate data as considered inthis work.

4.2. Corrosion Current. Polarization resistance and Tafelextrapolation techniques were applied to all series. Threespecimens were tested at each scheduled age. Examinationof the Tafel data indicated that the corrosion process ofsteel reinforcement embedded in all types of concrete iscathodically controlled, since the cathodic Tafel slope (𝛽𝑐)is greater than the anodic slope (𝛽𝑎). This means thatcorrosion is determined by polarization of the cathodiccontrolling reactions, that is, the electrode is acceleratedby moving the potential in the negative (active) direction.Figure 5 represents typical Tafel plots at 7 days of exposureto NaCl solution. The proportionality constants (𝛽), werein the range of 31–150mV/decade for steel in R concrete,36–119mV/decade in SP concrete, 62–90mV/decade in SF-R concrete and 46–154mV/decade in SF-A concrete. Theaverage 𝑅

𝑝and proportionality constant determined from

Tafel plots (TP) at scheduled ages are shown in Table 4 andthe variation of the corrosion current with exposure timeis shown in Figure 6. Results reveal that, in general thecorrosion current of steel in SF-R concrete exhibited a gradualreductionwith exposure time, while in SF-A concrete showeda slight increase in the corrosion current as compared tosteel in SF-R concrete after 150 days of exposure. On theother hand, the steel in R concrete showed a rapid increasein corrosion current and in SP concrete exhibited, a gradualincrease after 60 days indicating a breakdown in the passivefilm as a result of chloride ion ingress. However, the increasein the corrosion current is more pronounced for steel in Rconcrete compared to steel in SP concrete.

−700

−600

−500

−400

−300

−200

−100

0

100

200

300

0.01 1 100 10000

Pote

ntia

l ver

sus A

g/A

gCl (

mV

)

RSF-A

SF-RSP

Current (𝜇A)

Figure 5: Polarization curves at 7 days of exposure.

Examination of the Tafel plots and data in Figure 6 forsteel in SF-R concrete indicated that at the end of the testingperiod the corrosion currents are 63 and 16 fold lower thanthose for steel in R, and SP concretes respectively. Thecorresponding values for steel in SF-A concrete are 17 and4 fold lower. Superior corrosion resistance performance ofsteel in SF-R concrete over the R as well as SP concrete isattributed to many reasons: The utilization of high surfacearea silica fume leads to progressive improvement in theinternal microstructural system and the transition zonethrough the pore size and grain size refinement processes,hence blocking the continuity of the voids [3]. The use ofhigh performance super plasticizer produced concrete mix ofexcellent flowability and high resistance to segregation, witha 40% water reduction. The well dispersion of the cementi-tious particles by the action of superplasticizer produces ahomogeneous and dense microstructure. Furthermore, thetight microstructure can impede the movement of electronsfrom the anode to the cathode zone, thus, tends to hinderthe propagation of the corrosion process. These features asa parcel contribute positively to reduce corrosion current ofSF-R. Based on corrosion current density’s data presented inTable 5 and according to Rodriguez et al. [27] corrosion risk


0

50

100

150

200

250

300

350

0 100 200 300Exposure time-days

Cor

rosio

n cu

rren

t (𝜇

A)

RSF-A

SF-RSP

Figure 6: Corrosion currents versus time of exposure.

Table 5: Corrosion current density.

Age—d Corrosion current density—𝜇A/cm2

R SP SF-R SF-A7 0.83 0.69 0.02 0.1160 0.79 0.23 0.09 0.1290 1.38 0.51 0.08 0.07120 1.94 0.79 0.07 0.07150 2.52 0.79 0.06 0.07180 2.96 0.87 0.08 0.19270 4.24 1.09 0.07 0.25

classification, the steel in SF-R was absolutely in passive con-ditions, whereas, corrosion risk is low to moderate for steelin SF-A concrete, moderate to high for steel in SP concrete,and high in R concrete. It is worth mentioning that, after 150days of exposure, the steel in SF-R concrete elucidates betterperformance as compared to steel in SF-A concrete in spite ofhigher cement content of the latter concrete. This behavioris mainly associated with the formation of larger size andconcentration of crystalline calcium hydroxide crystals withdistinctive hexagonal-prism morphology in the interfacialtransition zone [3]. These crystals are the weakest link in thesystem causing cracks in the direction perpendicular to the𝑐-axis. Such effects explicate lower strength of the transitionzone than the bulk cement paste in concrete leading to highercorrosion current.

4.3. Corrosion Current versus Corrosion Potential. Com-monly, it is known that the corrosion rate is higher at morenegative corrosion potential (𝐸corr). However, no pragmaticconclusions can be drawn from 𝐸corr without great hazardof error. In the deprivation of superficial signs of corrosionof the embedded steel, dependable qualitative assessmentusing potentials data counts on a convenient foreknowledge

−700

−600

−500

−400

−300

−200

−100

00 2 4 6

RSP

SF-RSF-A

Ecor

r ver

sus A

g/A

gCl (

mV

)

Corrosion current density (𝜇A/cm2)

Figure 7: Corrosion current density versus 𝐸corr.

of the electrochemical history of the structure.The corrosioncurrent densities versus 𝐸corr. plots based on experimentaldata, Figure 7, indicate different slopes for different types ofconcrete and were increasing in the following order: R, SP,SF-A, and SF-R. Consequently, weak correlation is observedfor steel in a higher corrosion activity system which is steelin R concrete, and the most obvious trend is associated withsteel in lower corrosion activity systems, thus, SF-R specimen.Figure 8 shows the corrosion current density versus corrosionpotential data for the whole specimens. Irrespective of thetype of concrete, the embedded steel identifies a regularincrease in values of 𝐼corr as 𝐸corr moves toward a morenegative value. The points possessed lower corrosion currentdensities and less negative potentials located in the upper leftportion of the curve are related to steel in SF-R concrete,whereas, the points with higher corrosion current densitiesand more negative potentials are linked with steel in R con-crete. The improved corrosion resistance features of steel inhigh-Strength self-compacting concretes are attributed to thedensification ofmicrostructure providing an intensive barrieragainst the transmission of the intrusive agents. Moreover,the excellent flowability and high resistance to segregationauthorized well distribution of cementitious particles, henceproduce a consistent and dense microstructure by well inter-meshing of the hydration products and provided improvedadhesion to steel reinforcement. Similar linear relationships,of negative slope, between 𝐼corr and 𝐸corr has been reportedby Page and Havdahl [33] for steel in various microsilica con-tent cement pastes admixed with chloride and Lambert et al.[34] for steel in conventional concrete exposed to chlorideingress.

It is meritorious to point out that there is no general cor-relation between the corrosion rates andHCPs [2] since thosevariables respond distinctly to the specific related parametersessentially, moisture, presence of oxygen, concrete resistanceand temperature [35]. FromFigures 7 and 8 it can be seen that,for steel in SF-R concrete the values of𝐸corr were in the range


−600

−500

−400

−300

−200

−100

00 1 2 3 4 5

HCP

ver

sus A

g/A

gCl (

mV

)

Threshold potential −269 mV

Corrosion current density (𝜇A/cm2)

y = −7.9242x3 + 86.813x2 − 330.09x − 106.8

R2 = 0.9308

Figure 8: Corrosion current density versus HCP.

of −190mVAg/AgCl, measured at early ages and approachedto −90mVAg/AgCl at the end of the testing period, and thecorresponding values of corrosion current densities were 0.02and 0.08 𝜇A/cm2.This condition clearly indicates passivity ofsteel in SF-R specimens under NaCl solution. On the otherhand, for steel in R specimens, the values of 𝐸corr fall inbetween −211 to −550mVAg/AgCl and the correspondingvalues of corrosion current densities were between 0.79–4.24 𝜇A/cm2. These values reveal the high probability ofdestruction of the passive protective layer on the surfaceof steel reinforcement and hence high corrosion activity inmost of these specimens at the time of the measurements.Figure 8 plotted for the whole series confirms the correlationbetween HCPs and corrosion rate for SF-R specimens whichall located fully in the passive zone to the left of vertical linerepresenting active corrosion threshold value (0.1 𝜇A/cm2)and above the horizontal line (threshold potential, −269mVversus Ag/AgCl). Accordingly, it is probable that the effect ofmicrosilica in lowering the pH values and hydroxide ion con-centration of the pore solution phase were being greatly offsetby densification of themicrostructure of the hydrated cementmatrix and excellent intermeshing of the hydration productswhich tended to stabilize the passivity of steel reinforcement.Moreover, the correlation can bewell applied to SF-R, and SF-A specimens.The relationship between the corrosion rate andHCPs, Figure 8, indicate active corrosion conditions for steelin R, SP and SF-A concretes located in the active zone to theright of the vertical line (0.1 𝜇A/cm2). However, for the somecases the corresponding potentials are less negative than−269mVAg/AgCl. Consequently, HCP measurements needto be supported by other techniques because wide variationsin corrosion rate are linked with a narrow range of potentialsthat are nobler than −269mVAg/AgCl.

4.4. Electrical Resistance. The DC electrical resistance forvarious types of concretes after 60 and 90 days of exposureto chloride solution are presented in Figures 9 and 10,respectively. These values were determined from current-voltage plots, and a typical plot is shown in Figure 11.Each data point represents the average of three specimens.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

Elec

tric

al re

sista

nce (

Ohm

)

RSP

SF-RSF-A

Figure 9: Electrical resistance at 60 days.

Results indicated that all types of concrete exhibited analmost continuous increase in electrical resistance with timeof exposure. The observed trend may be attributed to thefact that the conduction and electrical current in moistconcrete are essentially electrolytic in nature. Therefore, theelectrical resistance of concrete will be mainly controlled bythe types and concentration of ions of evaporable water inthe capillary pores of the cement paste. Since the volumeof evaporable water is reduced as hydration proceeds, theelectrical resistance is expected to increase with time ofexposure. However, the rate of resistance development ismore pronounced in SF-R and SF-A concretes as comparedto R and SP concretes. The average rates of resistancedevelopment over onemonth of exposure to chloride solutionare 345, 340, 1443, and 1471Ω/day for R, SP, SF-R, and SF-Aconcrete, respectively. The differences in the age dependencyof various types of concrete may be attributed to differencesin the hydration process and the subsequent nature and rateof change in the pore system with time of exposure. Thisprocess directly influences resistance controlling factors suchas the microstructural features of hydrated cement pastein concrete, nature of the transition zone, total pore watercontent, its composition, and the pore size distribution.

SF-R and SF-A concretes showed significant improve-ment in resistance as compared to their R, and SP concretes.After 90 days of exposure, the electrical resistance of SF-R is 5.2-fold and 5.1-fold greater than that of R and SPconcretes, respectively. The corresponding values for SF-Aconcrete are 5.3-fold and 5.2-fold greater. The significantincreases in the resistances of SF-R and SF-A concretes aremainly related to the very low w/cm ratio and extremely finepore structures generated through the pore size and grainsize refinement processes, hence maintaining a high level ofinternal integrity, which minimizes the ingress of aggressiveions into those concretes leading to a reduced conductivityand increased electrical resistance. The results are consistentwith the Safiuddin et al. findings [10] on the electricalresistivity of self-consolidating concrete.They found that this


RSP

SF-RSF-A

0

10000

20000

30000

40000

50000

60000

70000

Elec

tric

al re

sista

nce (

Ohm

)

Figure 10: Electrical resistance at 90 days.

0

5

10

15

20

25

30

35

0 0.0002 0.0004 0.0006Current (A)

Volta

ge (V

)

Figure 11: Typical V-I plot for SF-R concrete.

concrete gives considerably much higher electrical resistancecompared to reference concrete.

4.5. Compressive Strength of Concrete. SF-R concrete con-taining 10% silica fume as a partial replacement by weightof cement showed a compressive strength of 96MPa and104MPa at 28 and 270 days of exposure, respectively, whereasthe corresponding values for SF-A concrete incorporated 10%SF as an addition were 93MPa and 100MPa. The variationof strength with time is shown in Figure 12. Conventionalself-compacting concrete compromised high performancesuperplasticizer (SP) exhibited a considerable increase incompressive strength at all ages compared to the correspond-ing reference concretes, Figure 12. The strength at 28 dayswas 83.5MPa compared to 45.8MPa for reference concrete.This behavior is mainly due to a self-compacting featureand significant water reduction caused by SP in concreteleading to the reduction in capillary porosity system. The SP

0

20

40

60

80

100

120

0 50 100 150 200 250 300Exposure time-days

Com

pres

sive s

treng

th (M

Pa)

RSF-A

SF-RSP

Figure 12: Compressive strength of concretes.

surfactants are adsorbed on the surface of cement particlesand impart a strong negative charge which tends to dispersethe cement agglomerates into individual particles. Therefore,with continuous hydration there is a greater statistical oppor-tunity of intermeshing the products of hydration with fineand coarse aggregate to produce a system of highest integrityand hence of higher strength [36]. In addition, the highrepulsing effect caused by SP surfactants can deflocculate thewatery ettringite shell around the cement particles, and thesmall ettringite needles are freed from their watery shell bythe adsorption of the SPmolecules [37] thus providing amoreconsistent structure of hydrated cement paste.

Figure 12 shows that SF-R and SF-A have superior per-formance to SP concrete. This is mainly due to the fact thatwith a highly active silica fume, the pozzolanic reaction maystart as soon as Ca(OH)

2are available from the hydration

of Portland cement compounds and thereby strengthen theconcrete through the pore size and grain size refinementprocesses. In the first process, the large capillary voidsare filled with a microporous low density calcium silicatehydrates, formed as a secondary hydration product duringthe pozzolanic reaction. In the second process, nucleationof calcium hydroxide around the fine and well distributedparticles of pozzolanwill have the effect of replacing the largerand oriented crystals of calcium hydroxide with numerous,small, and fewer oriented crystals plus poorly crystallinereaction products [3]. Mehta and Monteiro suggested thatboth processes strengthen the cement paste and transitionzone thus reducing the microcracking and permeability ofconcrete [3].

SF-A concrete showed slightly lower strength than SF-Rconcrete, mainly associated with larger size and concentra-tion of Ca(OH)

2crystals liberated during the hydration of

cement in concrete of higher cement content. These crystalsoffer the feeblest link in the system causing cracks in thedirection perpendicular to the 𝑐-axis. Such effects account for


lower strength of the transition zone than the bulk cementpaste in concrete leading to lower compressive strength.However, the strength is relatively much higher compared toSP concrete Figure 12.

5. Conclusions

The following conclusion can be made from the data devel-oped in the present study.

(i) The half-cell potentials (HCPs) of steel in SF-Rconcrete were considerably nobler than −269mVAg/AgCl indicating a state of passive potential. Theinclusion of 10%SF byweight of cement as an additionto themix in SF-A concrete changed theHCPs of steelto more negative potentials compared to steel in SF-Rconcrete throughout the testing period. In R concretethe HCPs of steel drops suddenly to more negativethan −269mVAg/AgCl at 70 days of exposure andthe steel in SP concrete exhibited fluctuations inpotentials during the testing period.

(ii) The corrosion process of steel in all types of con-crete was cathodically controlled. Based on Tafelextrapolation technique, the corrosion currents forsteel in SF-R concrete were, respectively, 63- and16-fold lower than R and SP concretes at the endof the testing period. The corresponding values forsteel in SF-A concrete were 17- and 4-fold lower.The superior durability performance of steel in SF-R concrete linked to terrific self-compacting featuresmay significantly expand the service life of structuralmembers and provide vast opportunities for futurestructural application under severe environmentalconditions.

(iii) Corrosion risk classification based on corrosion cur-rent densities and HCPs, detected that the steel in SF-R is definitely in the passive condition, while, corro-sion risk is low tomoderate for steel in SF-A,moderateto high for steel in SP concrete and high for steel inR concrete. Moreover, large distinctions in corrosionrate are associated with a narrow range of potentialsthat are less negative than −269mVAg/AgCl. Thisclearly indicates that HCP measurements may yielddeceptive results if considered separately thus, shouldbe always boosted by other techniques. Corrosioncurrent density-Ecorr. plots indicated different slopesfor different types of concrete and were increasing inthe following order: R, SP, SF-A, and SF-R concrete.

(iv) SF-R and SF-A concretes elucidate significant increasein electrical resistance and compressive strength com-pared toR and SP concretes.The average rates of resis-tance development over one month of exposure were345, 340, 1443, and 1471Ω/day for R, SP, SF-R, and SF-A concrete, respectively. The compressive strength ofSF-R concretes was 104MPa at 270 days of exposure,whereas the corresponding value for SF-A concretewas 100MPa. Conventional self-compacting concrete

(SP) exhibited compressive strength of 83.5MPa com-pared to 45.8MPa for R concrete.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgments

The authors would like to express deep thanks and apprecia-tion to the Royal Scientific Society as well as the Institute ofInternational Education Scholar Rescue Fund for providingthe needed support to conduct this research work.

References

[1] M. Raupach and P. Schiesbl, “Macrocell sensor systems formonitoring of the corrosion risk of the reinforcement inconcrete structures,” NDT & E International, vol. 34, no. 6, pp.435–442, 2001.

[2] J. P. Broomfield, Corrosion of Steel in Concrete: Understanding,Investigation and Repair, Taylor & Francis, London, UK, 2ndedition, 2007.

[3] P. K. Mehta and P. J. Monteiro, Concrete: Microstructure,Properties, and Materials, McGraw-Hill, New York, NY, USA,3rd edition, 2006.

[4] K. H. Khayat, “Workability, testing, and performance of self-consolidating concrete,” ACI Materials Journal, vol. 96, no. 3,pp. 346–353, 1999.

[5] ACI Committee 222, “Corrosion of metals in concrete,” ACIManual of Concrete Practice, Part 1, American Concrete Insti-tute, Farmington Hills, Mich, USA, 2000.

[6] W. Zhu and P. J. M. Bartos, “Permeation properties of self-compacting concrete,” Cement and Concrete Research, vol. 33,no. 6, pp. 921–926, 2003.

[7] T. A. Saylev and R. Francois, “Quality of steel-concrete inter-face and corrosion of reinforcing steel,” Cement and ConcreteResearch, vol. 33, no. 9, pp. 1407–1415, 2003.

[8] S. Assie, G. Escadeillas, and V. Waller, “Estimates of self-compacting concrete “potential” durability,” Construction andBuilding Materials, vol. 21, no. 10, pp. 1909–1917, 2007.

[9] H. Yazici, “The effect of silica fume and high-volume class C flyash on mechanical properties, chloride penetration and freeze–thaw resistance of self-compacting concrete,” Construction andBuilding Materials, vol. 22, no. 4, pp. 456–462, 2008.

[10] M. Safiuddin, J. S. West, and K.A. Soudki, “Durability per-formance of self-consolidating concrete,” Journal of AppliedSciences Research, vol. 4, no. 12, pp. 1834–1840, 2008.

[11] M. A. Safan, “Performance of beams made of low-cost self-compacting concrete in an aggressive environment,” Acta Poly-technica, vol. 51, no. 5, pp. 120–130, 2011.

[12] H. F. Dehwah, “Corrosion resistance of self-compacting con-crete incorporated quarry dust powder, silica fume and fly ash,”Construction and Building Materials, vol. 37, pp. 277–282, 2012.

[13] M. Uysal, K. Yilmaz, and M. Ipek, “The effect of mineraladmixtures onmechanical properties, chloride ionpermeabilityand impermeability of self-compacting concrete,” Constructionand Building Materials, vol. 27, no. 1, pp. 263–270, 2012.


[14] A. Hassan, M. Hossain, and M. Lachemi, “Corrosion resistanceof self-consolidating concrete in full-scale reinforced beams,”Cement and Concrete Composites, vol. 31, no. 1, pp. 29–38, 2009.

[15] H. Yu, X. Shi, W. H. Hartt, and B. Lu, “Laboratory investigationof reinforcement corrosion initiation and chloride thresholdcontent for self-compacting concrete,” Cement and ConcreteResearch, vol. 40, no. 10, pp. 1507–1516, 2010.

[16] American Society for Testing and Materials, ASTM C150: Stan-dard Specification for PortlandCement, ASTM, Philadelphia, Pa,USA, 2002.

[17] American Society for Testing and Materials, ASTM C1240:Standard Specification for Silica Fume Used in CementitiousMixtures, ASTM, Philadelphia, Pa, USA, 2005.

[18] American Society for Testing and Materials, ASTM C494:Standard Specification for Chemical Admixture for Concrete,ASTM, Philadelphia, Pa, USA, 2001.

[19] American Society for Testing and Materials, ASTM C33: Stan-dard Specification for Concrete Aggregate, ASTM, Philadelphia,Pa, USA, 2003.

[20] The European Guidelines for Self Compacting Concrete, Spec-ification, Production and Use, 2005.

[21] American Society for Testing and Materials, ASTM C143:Standard Test Method for Slump of Hydraulic Cement Concrete,ASTM, Philadelphia, Pa, USA, 2000.

[22] American Society for Testing and Materials, ASTM C1611:Standard Test Method for Slump Flow of Self-ConsolidatingConcrete, ASTM, Philadelphia, Pa, USA, 2005.

[23] American Society for Testing and Materials, ASTM C1621:Standard Test Method for Passing Ability of Self-ConsolidatingConcrete by J-Ring, ASTM, Philadelphia, Pa, USA, 2009.

[24] EFNARC, “Specification and guidelines for self-compactingconcrete,” European Federation Dedicated to Specialist Con-struction Chemicals and Concrete Systems, 2002.

[25] American Society for Testing and Materials, ASTM C39/C39M:Standard Test Method for Compressive Strength of CylindricalConcrete Specimens, ASTM, Philadelphia, Pa, USA, 2001.

[26] American Society for Testing and Materials, ASTM C876: Stan-dard TestMethod forHalf-Cell Potentials of Uncoated ReinforcingSteel in Concrete, ASTM, Philadelphia, Pa, USA, 1999.

[27] P. Rodriguez, E. Ramirez, and J. A. Gonzalez, “Methods forstudying corrosion in reinforced concrete,” Magazine of Con-crete Research, vol. 46, no. 167, pp. 81–90, 1994.

[28] C. Andrade, J. Gulikers, R. Polder, and M. Raupach, “Half-cell potential measurements-potential mapping on reinforcedconcrete structures,” Materials and Structures, vol. 36, pp. 461–471, 2003.

[29] C. Andrade and C. Alonso, “Corrosion rate monitoring in thelaboratory and on-site,” Construction and Building Materials,vol. 10, no. 5, pp. 315–328, 1996.

[30] M. Stern and A. Geary, “Electrochemical polarization-theoretical analysis of the shape of polarization curves,” Journalof the Electrochemical Society, vol. 104, no. 1, pp. 56–63, 1957.

[31] H. Arup, Corrosion of Reinforcement in Concrete Construction,Society of Chemical Industry, London, UK, 1983.

[32] R. D. Browne, M. P. Geoghegan, and A. F. Baker, “Analysis ofstructural condition from durability results,” in Corrosion ofReinforcement in Concrete Construction, A. P. Crane, Ed., EllisHorwood, London, UK, 1983.

[33] C. L. Page and J. Havdahl, “Electrochemical monitoring ofcorrosion of steel in microsilica cement pastes,” Materials andStructures, vol. 18, no. 1, pp. 41–47, 1985.

[34] P. Lambert, C. L. Page, and P. R. W. Vassie, “Investigations ofreinforcement corrosion. 2. Electrochemicalmonitoring of steelin chloride-contaminated concrete,” Materials and Structures,vol. 24, no. 5, pp. 351–358, 1991.

[35] J. A. Grubb, J. Blunt, C. P. Ostertag, and T. M. Devine, “Effect ofsteel microfibers on corrosion of steel reinforcing bars,” Cementand Concrete Research, vol. 37, no. 7, pp. 1115–1126, 2007.

[36] M. R. Rixom, Concrete Admixtures: Use and Applications,Construction Press, Lancaster, UK, 1977.

[37] P. C. Kreiger, “Plasticizers and dispersing admixtures,” inProceedings of the Concrete International Admixtures (CI ’80),pp. 1–17, London, UK, 1980.

Submit your manuscripts athttp://www.hindawi.com

ScientificaHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation http://www.hindawi.com Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials

Documents

Research Article Corrosion of Steel in High-Strength …downloads.hindawi.com/journals/ijc/2014/564163.pdfResearch Article Corrosion of Steel in High-Strength Self-Compacting Concrete