The present investigation it to study the corrosion performance of fly ash blended cement concrete in chloride-contaminated

environments by various electrochemical and non-electrochemical techniques. The chloride binding ability at the optimum

replacement level of fly ash was also studied

Muralidharan, S.

Palaniswamy, N.

Parande, A.K.

Saraswathy, V.

Srinivasan, S.

IR@CECRI

Studies on the Corrosion Performance for Steel Embedded in Fly AshBlended ConcreteS.Muralidharan*, V.Saraswathy, Anand Kumar Parande, S.Srinivasan and N.PalaniswamyCentral Electrochemical Research Institute, Karaikudi 630 006, Tamilnadu, IndiaReceived:10 November 2003 Accepted:30 November 2004AbstractThe present investigation it to study the corrosion performance of fly ash blended cement concrete in chloride-contaminatedenvironments by various electrochemical and non-electrochemical techniques. The chloride binding ability at the optimumreplacement level of fly ash was also studied.Keywords: Blended cements, Flay ash, Corrosion performanceIntroductionMineral admixtures are finely divided siliceous materials,which are added to concrete in relatively large amounts,generally in the range of 20 to 100% by weight of Portlandcement.Power generation units using coal as fuel and metallurgicalfurnaces producing cast iron, silicon metal and ferrosiliconalloys are the major source of by products namely fly ash(FA), blast furnace slag (BFC) and silica fume (SF) respectively;which are being produced at the rate of millions of tons everyyear in many industrial countries. Dumping away these byproducts represents a waste of the material and causes seriousenvironmental pollution problems. Industrial countries suchas the United States, Russia, France, Germany, Japan and theUnited Kingdom are among the largest producers of fly ash,volatilized silica and granulated blast furnace slag. In additionto these materials, china and India have the potential for makinglarge amounts for Rice Husk Ash (RHA).Properties and ApplicationsFly ashes are results of coal combustion in power plants.* Corresponding author: S.Muralidharan0036 - 0678 - 00 / 40 - 1 - 14 $ 3.50 © 2005 SAESTIn the United States fly ash is classified by chemicalcomposition. ASTM classifies fly ashes into two categoriesnamely class-C and class-F. The differences between thesetwo categories are as follows:Class – C Class - F More than 10% calcium content  Less than 10% calcium content  Produced from sub bituminous coal Produced from bituminous coat Sum of the three components namely SiO2, Al2O3 and Fe2O3 greater than 50% Sum of the three ingredients namely SiO2, Al2O3 and Fe2O3 is 70% or greater Glassy material is often more reactive than the glass in Class -F Glassy material is reactive  Principal active ingredient is calcium alumino silicate glass Active ingredient is siliceous or alumino silicate glass Contains additional reactive components such as free lime, anhydrate, C3A, C3S, etc.  ---  Transactionsof theSAESTTransactions of the SAEST 40 (2005) 14-18Typically 15% to 30% replacement of cements by fly ash isutilized for pavements and bridges. The performance of ablend of fly ash and Portland cement depends on:• The performance of the Portland cement• The ratio of Portland cement to fly ash• Interaction specific to the fly ashThe effects of fly ash on the properties of fresh andhardened concrete are:• Improvement in the fresh concrete workability• Reduction in the rate of hardening and strength gain• Reduction in heat generated in concrete• Improvement in the concrete sulphat resistance• Reduction in concrete permeabilityThe improved performance of concretewith fly ash makesthem suitable for:• Effective cost savings• Massive placements• Improving resistance to sulphate attack• Improving the durability of reinforced concrete inhigh chloride environments• Preventing alkali – silica reaction in concreteCorrosion Performance of Fly Ash Blended CementsAshes from combustion of coal are the major industrialby-products that are suitable for use as mineral admixtures inPortland cement concretes. The utilization of fly in theconstruction industry is growing fast not only as blendedcements but also to minimize the pollution of the environment.Earlier work reported that fly ashes accelerate reinforcementcorrosion due to the presence of unburnt carbon and sulphur.Since the usage of fly ashes in the presence of unburnt carbonand sulphur. Since the usage of fly ashes in reinforcements arestill questionable. In order to produce fly ashes inreinforcements are still questionable. In order to produce flyash with stable reinforcements are still questionable. In orderto produce fly ash with stable properties and adequate quality,many power plants have implemented its own sophisticatedquality control measures. The property improvement of flyash blended cements were extensively studied and reportedthat the physical and mechanical properties wee matched tothat of ordinary Portland cement (OPC).Although all the studies are focused on the improvementof physical and mechanical properties of concrete. So far asystematic and detailed studies on the corrosion performanceof fly ash cements are very scarce. Therefore, a realisticassessment of  the corrosion-resistance properties of fly ashcement is essentially needed in addition to the importantphysical and mechanical properties of concrete. Macro cellcorrosion studies are reported to be a very aggressive techniqueto assess the corrosion of steel in concrete.Recently the effect of fly ash on the corrosion of reinforcingthick and impermeable, it will normally provide adequateprotection against corrosion. The protective effect of theconcrete cover is of both a physical and a chemical nature andfunctions in three ways.1.   It provides an alkaline in  the  immediate vicinity of thesteel surface.2.   It offers a physical and chemical barrier to the ingress ofmoisture, oxygen, carbon-di-oxide, chlorides and otheraggressive agents.3.   It  provides  an  electrically  resistive  medium around thesteel members.Under alkaline conditions (pH higher than ~ 11.5), aprotective oxide film will form on a steel surface, rendering itpassive to further corrosion.When concrete carbonates and the depth of carbonationreaches the steel concrete boundary, passivation may bereduced and corrosion may occur if sufficient oxygen andmoisture reach the metal surface. Chlorides or other ions mayalso undermine the protective effect of passivation andencourage corrosion.RILEM technical committee (5) on corrosion of steel inconcrete made the following statements, which give perspectiveto this issue:•    The efficiency of the (concrete) cover in preventingcorrosion is dependent on many factors, which collectivelyone referred to as in ‘quality’. In this context the ‘quality’implies impermeability and a high reserve of alkalinity,which satisfies both the physical needs, and chemicalrequirements of the concrete cover. If the concrete ispermeable to atmospheric gases or lean in cement,corrosion of the reinforcement can be anticipated and goodprotection should be attempted by the use of dense aggregateand a well compacted mix with a reasonably low w/c ratio.•    If chloride is expected, it is now usually agreed thatcarbonation of concrete cover is the essential conditionfor corrosion of reinforcement.However the carbonation of fly ash concrete is not a matterof concern, provided attention is paid to obtaining adequateimpermeability in the concrete mass.The sulphur containing components of fly ash couldcorrode the reinforcing steel in fly ash concrete. The most ofthe sulphur in fly ash in present as sulphate and thereforewould have an effect similar to the sulphate components in S.Muralidharan et al. /  Transactions of the SAEST 40  (2005) 15Portland cement. Further, the corrosion of steel is greatlyaffected by pH at the high pH prevailing in concrete; corrosionrates would be expected to be slow.•     Sulphur compounds in fly ash are usually so limited byspecifications that they are not materially different in theconcrete, whether fly ash is used or not. Moreover, thealkaline condition in the concrete is unfavorable to asulphate attack on steel.•     Carbon  in  fly  ash  would  appear  by  theoreticalconsiderations  to be much more significant in concretethan is sulphur. The actual effect should be investigated.However, if it is kept under 3% in the fly ash, its percentagein the concrete becomes so small that if it is well dispersed,its effect on the electrical conductivity of the concrete,and therefore upon the corrosion of steel, should be quiteminor (6).These conclusions seem to be generally acceptable in thelight of reported research that has shown that fly ash concretedoes not decrease the corrosion protection of steelreinforcement compared with normal concrete.RILEM technical committee on corrosion of steel inconcrete, fly ash may influence both permeability and thealkalinity of the system.The properly proportioned fly ash concrete subjected toadequate curing should in general be less permeable at laterages than the corresponding plain concrete. The danger ofpermeability lies in the premature exposure of fly ash concreteto agents, as a result of either inadequate proportioning incomplete curing or poor fly ash quality.Experimental ProcedureMaterials usedThe following materials were used for the present study.Ordinary Portland Cement : Conforming to  IS 456 – 2000(i) Graded fine aggregates : Local clean river sand (fineness(ii) modulus of medium sand equal to 2.6) conforming to gradingzone III of IS-383 – 1970 was used as fine aggregates.(iii) Graded coarse aggregates : Locally available well gradedaggregates of normal size greater than 4.75 mm and less than10 mm having fineness modulus of 2.72 was used as coarseaggregates.The composition of OPC used in this study was reportedin Table 1. The mix proportions used in the study werereported in Table 2.Table 1. Compositions of OPC used in this study Constituents OPC (%) SiO2 21.04 Fe2O3 3.35 Al2O3 9.93 CaO 60.68 MgO 1.30 Na2O 0.20 K2O 0.90 Loss on Ignition  2.60   Techniques usedpH measurementsThe pH values measured for plain cement extracts andcement extracts containing various fly ash replacement levelsnamely 10%, 20%, 30%, 40% and 50%. The pH was measuredusing a standard calibrated pH meter.Anodic PolarizationMild steel rods were embedded cylindrical mortal (1:3)specimens of size 58mm dia and 60mm height using w/c ratio0.45. The mortar specimens only with ordinary Portlandcement (control) and OPC replaced by fly ash at 10%, 20%,30%, 40% and 50% replacement levels were subjected toanodic polarization studies using a three-electrode system.The embedded steel act as a working electrode, cylindricalstainless steel act as counter electrode with respect to saturatedcalomel electrodes (SCE) as reference electrode. The currentflowing at +300 mV and +600 mV vs. SCE from the OCPvalues recorded for all the specimens at ambient temperatureof 32± 1oC.Weight loss measurementsConcrete cube of size 10 x 10 x 10 cm were cast from OPCand OPC containing various fly ash at 10, 20, 30, 40 and 50%16  S.Muralidharan et al. /  Transactions of the SAEST 40  (2005)Table 2. The mix design used for casting was as follows:1:3.3:6.9, W/C = 50 Constituents Quantity used (kg/m3) Ordinary Portland cement 215 Coarse aggregates 710 Fine aggregates 1485 Fly ash 10%, 20%, 30%, 40% and 50%  replacement level  expressed in terms of parts per million (ppm) on the basis ofweight of sample taken for analysis.Results and discussionpH measurementsThe pH values measured for plain cement extracts andvarious fly ash replacement levels are given in Table 3. Fromthis Table, it was found that pH of the plain cement extractwas 13.8. The pH values measured for various fly ashreplacement levels were in the range from 13.07 to 10.10.There was no After that, there was a slight reduction in pHvalues at 30% fly ash replacement level. Above 40% fly ashreplacement level, pH value was drastically reduced to 10.10.replacement levels. Mild steel rod of 1.2 cm diameter and 6cm long was embedded centrally. Initially the mild steel rebarswere cleaned in the following acid solutionHCl: 100%, SnCl2: 35 gms, Sb2O3: 25 gmsdegreased with acetone and washed with double distilled waterand dried. Water and dried. The initial weight of the rebar wastaken before casting using metler balance for gravimetric weightloss measurements.Concrete cube specimens were prepared using 1:3:3:6.9mix with a w/c ratio of 0.50. The specimens wee mechanicallyvibrated. After 24 hours of curing, the specimens weredemoulded and cured for 28 days in distilled water in order toavoid any contamination. After the curing period was over, allthe specimens were completely immersed in 3% NaClsolution. The specimens were maintained in the same conditionfor 15 days and then subjected to drying for another 15 days.One alternate wetting and drying cycle consists of 15 daysimmersion with 3% NaCl solution and 15 days drying in openair at room temperature. In order to induce the acceleratedcorrosion 3% NaCl solution was used.All the concrete specimens were subjected to 3 completecycles of test period. Tests were conducted on a minimum ofsix replicate specimens and the average values were reported.At the end of the exposure period the concrete specimenswere split open and visual observation data on corrosion weremade and expressed in terms of percentage of area rusted. Thecorrosion rate of mild steel anodes measured by gravimetricweight loss method and expressed in millimeter per year(mmpy). The corrosion rate is calculated using the followingequation: S.Muralidharan et al. /  Transactions of the SAEST 40  (2005)                 Corrosion rate : 87.6 x w/(DAT)WhereW= weight loss in gm        D= Density material usedA=Area of the specimen (Cm2)   T=Time duration in hoursChemical analysis for alkalinity and free chloridecontents The core samples collected near he anodes were crushedmechanically and powdered. Then 100 gm of powdered samplewas shaken with 100 ml of double distilled water in a conicalflask using microid flask shaker for one hour. The extract wasthen filtered through a whatman filter paper No.42. The extractprepared from the powdered sample was then analyzed forfree chloride contents as per the standard procedures. 20 cc offiltered solution was taken and the free chloride content wasestimated by standard silver nitrate solution using potassiumchromate as an indicator. The amount of chloride present wasAnodic polarization techniqueThe results of anodic polarization data for steel embeddedin 1:3 mortar specimens in 3% NaCl solution are given inTable 3. It was inferred from this Table that, in the case ofTable 3. Effect of different factors on fly ash replacement LevelsS.No System pH Current at + 300 mV vs. SCE (mA) Current at +600 mV vs. SCE (mA) Corrosion rate (mmpy) Percentage of area rusted (%) Free Chloride Contents (ppm) 1 OPC 13.80 0.43 1.04 0.0024 No rust 3240 2 OPC + 10% FA 13.07 0.24 0.55 0.0025 No rust 4000 3 OPC + 20% FA 12.86 0.37 0.75 0.0025 No rust 4720 4 OPC + 30% FA 12.80 0.44 1.09 0.0049 5 4800 5 OPC + 40% FA 11.28 0.49 1.18 0.0055 5 5000 6 OPC + 50% FA 10.10 1.26 2.50 0.0074 12 5100   OPC the anodic current measured was found to be 0.43 and1.04 mV respectively at +300 mV and  +600 mV vs. SECE.On the other hand, for fly ash systems, upto 20% replacementlevel the anodic current measured was lesser than OPCindicating the superior performance of the system with better17corrosion resistance properties. Above 30% level, thepassivity was destroyed, as a result large anodic currents inthe range 0.44 to 1.26 mA at +300 mV vs. SCE amd 1.09 to2.5 mA at +600 mV vs. SCE were measured. These resultsconfirmed that 20% is the optimum level of replacement withbetter corrosion resistance properties.Weight loss measurementsThe corrosion rates calculated in mmpy for mild steelanodes embedded in various systems studied are reported inTable 3. From the table it is observed that, the corrosion ratefor OPC system was found to be 0.0024 mmpy. Systemsshowing corrosion rate for less than 0.0024 mmpy may beconsidered as improving the corrosion resistance of steel inconcrete. In the case of fly ash system upto 20% replacementlevel comparable results were obtained with OPC. But beyond 30%, inferior properties were observed. These resultswere concordant with the results obtained form othertechniques.Chemical analysis for free chloride contentsAt the end of the test period, steel anode embedded inOPC and various fly ash system were subjected to qualitativeestimations. The data calculated were reported in Table 3.From this Table, it revealed that in 100% OPC system no rustwas observed at any of the surface of steel rebars. For fly ashsystem, upto 20% negligible red rust (less than 1%) wasobserved. Beyond 30% more than 5% red rust was observed.The free chloride contents estimated were reported inTable 3. 6. Interestingly upto 20% replacement level, thepenetration of chloride ion was found to be in the range 3000to 4000 ppm. After that, the fly ash replacement level increasesthe penetration of chloride 9 on is also increased. The resultswere concordant with results obtained from anodic polarizationand weight loss measurements.ConclusionsThe following main conclusions are drawn from the presentinvestigations. Fly ash up to 20% does not affect the pH ofthe concrete. Comparable results on corrosion rates with OPCwas obtained upto 20% fly ash replacement level from weightloss measurements. Anodic polarization data showed that,fly ash upto 20%, lesser anodic currents were observed whencompared to OPC. Fly ash is not very effective beyond 30%for chloride binding capacity.The overall conclusion is that fly ash has both positiveand negative effects on the different factors governing thesteel reinforcement corrosion. However, the positive factorsare dominating by far over the negative factors.Fly ash has a positive effect on the risk of corrosion andrate of corrosion, provided that normal good workmanship iscarried out and that fly ash is not used in very low concretequalities to reduce cement.AcknowledgementsAuthors thank Director, CECRI for kind permission topublish this paper.References1. V.Saraswathy, S.Muralidharan, R.Mkalyanasundaram,K.Thangavel and S.Srinivasan, Cem. Concr. Res., 31(2001) 7892. S.Muralidharan, V.Saraswathy, K.Thangavel andS.Srinivasan,  J. Appl. Electrochem., 30 (2000) 12553. V.Saraswathy, S.Muralidharan, K.Thangavel and        S.Srinivasan, Advances in Cement Res., 14 (2002) 94. V.Saraswathy, S.Muralidharan, K.Thangavel andS.Srinivasan, Cement and Concrete Composites, 25 (7)(2003) 6735. RILEM Technical committee, 12-CRC 1974, Materialde Construction, 9, 1876. Fly ash in concrete, (Edt), V.M.Malhotra andA.A.ramezanianpour, 1994, CANMET, Canada18  S.Muralidharan et al. /  Transactions of the SAEST 40  (2005)

Studies on the Corrosion Performance for Steel Embedded in Fly Ash Blended Concrete

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