BEHAVIOR OF CARBON STEEL IN SIMULATED CONCRETE PORE SOLUTIONS OF AIR-ENTRAINED AND CONVENTIONAL CONCRETE (EUROCORR 2012 paper 1176)
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BEHAVIOR OF CARBON STEEL IN SIMULATED CONCRETE PORE SOLUTIONS OF AIR-ENTRAINED AND CONVENTIONAL CONCRETE (EUROCORR 2012 paper 1176)

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In Brazil, the air-entrained concrete has been extensively used as structural walls of popular housing units. Some of these units built recently in urban areas were inspected. Upon inspection, a ...

In Brazil, the air-entrained concrete has been extensively used as structural walls of popular housing units. Some of these units built recently in urban areas were inspected. Upon inspection, a significant variation of the potential corrosion measurement was obtained and reddish stains on the surface of the reinforcement, were also observed indicating active state of corrosion. Not always the concrete was fully carbonated and there wasn´t chloride contaminations. In order to understand the occurrence of premature corrosion of the inspected reinforcement, a complementary study was conducted at the laboratory to characterize air-entrained concretes and evaluate the behavior of steel bars immersed in solutions that simulate the water in the pores of these concrete and compare them to the pore solution of an ordinary Portland concrete.

The steel bars were evaluated under three conditions: blasted, corroded and galvanized. The behavior of the bars was monitored by visual examination and by electrochemical measurements. Finally, the corrosion rate was calculated. Tests on concrete specimens were also conducted to validate the results. The characterization tests showed an inferior quality of the air-entrained concretes, having both high concentrations of pores, many of them fully interconnected. This justified the high deep carbonation observed in a short period of time and a variable electrical resistivity detected in the field. The pore solution immersion tests showed the higher corrosion susceptibility of metallic reinforcement in air-entrained concretes. In the studied air-entrained concretes, the corrosion occurred preferentially under the sealant applied on the bar extremities. In one of them, corrosion was also observed on the free surface of the blasted bars. The corrosion was also observed in the air-entrained concrete specimens, confirming the tests solution results.

No Brasil, o concreto com ar incorporado tem sido intensamente utilizado como parede estrutural de unidades habitacionais. Algumas dessas unidades construídas recentemente (inferior a três anos) em regiões urbanas, do norte, sul e sudeste do país, foram inspecionadas. Na inspeção, foram detectadas manchas avermelhadas na superfície das armaduras, indicando um estado ativo de corrosão do aço-carbono. Diante desse fato, foi conduzido um estudo experimental objetivando avaliar o comportamento de barras imersas em soluções que simulam a água presente nos poros do concreto com ar incorporado e do concreto convencional. As barras foram avaliadas em três condições: aço jateado, aço já em processo de corrosão e aço zincado. Em algumas barras foram fixados anéis de material não metálico para estudo da corrosão em frestas. O comportamento das barras foi acompanhado por exame visual e por medidas eletroquímicas. Ao final, a taxa de corrosão foi calculada.

As soluções foram obtidas pela mistura de água destilada e do material resultante da moagem de co

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BEHAVIOR OF CARBON STEEL IN SIMULATED CONCRETE PORE SOLUTIONS OF AIR-ENTRAINED AND CONVENTIONAL CONCRETE (EUROCORR 2012 paper 1176) BEHAVIOR OF CARBON STEEL IN SIMULATED CONCRETE PORE SOLUTIONS OF AIR-ENTRAINED AND CONVENTIONAL CONCRETE (EUROCORR 2012 paper 1176) Presentation Transcript

  • • introduction: overview of the results obtained infield inspections that motivatedthe present study;o objectives of the study;o methodology;o results;o conclusions.
  • Recently, the air-entrained concrete has beenextensively used as structural walls in Brazilianpopular housing units.One of the biggest unit constructions, 18 thousand houses.
  • Some houses and their construction systemHouse formwork Reinforcement and concrete placement
  • ACI concrete terminology: causes thedevelopment of a system of microscopic airbubbles in concrete, usually to increase theconcrete workability and frost resistance.The concrete is deliveredto the site (in a truck agitator).The air-entraining admixtureis incorporated at thatpoint.Carbon steel weld wiremesh is used asreinforcement(Ø 3.6 mm - 150 mm x 150 mm)In the housing units:This admixture is used to increase theconcrete workability (self-compactingconcrete) and thermal and acustıccomfort of the houses.Corroded bars(outdoor storage)
  • • urban atmosphere (mostly in tropical regions);• built by different constructers (less than 3 years ago).Visual inspection (concrete and bars surface) Potential and electric resistivityConcrete coverChloride ions profile Carbonation frontSome houses (6 units) were inspected, all were:The usual tests were performed during the inspection:
  • Insıgnıfıcant chlorideions contentVariations inconcrete coverand porosotyIn all the units inspected were detected:High carbonation front(it reaches the steel in some walls)Intensereddish stainsand corrosionproducts onthe surface ofthe barsThey were presentin carbonatedand integrityconcrete!
  • high presence of air bubbles was visualized (by nakedeyes and SEM images) in:Concerning the porosity:Soylev, T.A.; Francois, R. (2003): the porosity and the density variation of the concrete enhance the corrosion of steelembedded due the lack of chemical and physical protection supplied. Jingak, N. et al (2005): the corrosion initiatedpreferentially at an air void along the steel-concrete interface. Glass, G.K. et al (2000): the absence of calciumhydroxide at the location of the entrapped air voids in steel-concrete interface is a dominant effect in initiate acorrosion process by chlorides. Page, C. L. (1975): local heterogeneities in the zone adjacent to steel are liable toinfluence its corrosion behavior in the presence of aggressive agents. Lambert et al (1991): the interface macro-defectdisrupts the integrity of the cement hydration-product layer.Concrete-steel interfaceConcrete bulkand at the surfaceThe entrapped air voidsdisrupt the integrity of thebuffering layer of cementhydration products. Thismay result in a missingpassivation for parts of thesteel and the prematurecorrosion in the defectareas (crevices).The literature reports the interface defects and its impact on the corrosion:
  • Dispersed areasWire tiesDispersed areasConcerning the corrosion in the integrity concrete:The stains and the corrosion products were visualized (naked eyes) indispersed areas of the bars’ surface and in wire ties and weld areas.
  •  Ecorr: mostly more positive than -250 mVCSE, mostpotential gradients were less than 200 mVCSE;GONZÁLEZ et al. (2004) low levels of wetness can confuse active and passive based only in Ecorr measurement. Themost value Ecorr will be that one obtained when the concrete resistivity is less than 20 kΩ.cm. The electric resistivitybetween 100-50 kΩ.cm indicate low risk of corrosion, between 50-20 kΩ· cm there is a high risk and less than 20 kΩ·cm very high risk. ASTM C876 (2009) not considers normally applied for carbonated concrete the criteria (i.e.: 90 %probability of corrosion for Ecorr values ≥ -350 mV, CSE). LIVECON D3.1 (2003) considers significant potential gradient≥ 200 mV (in 1 m). The gradient has more weight then the absolute potential value. BROOMFIEL (1991) considers thatthe Ecorr measured by a reference electrode is not the true value, but a mixed potential of an unknown area of therebars. GONZÁLZ et al. (1979) considering the Nernst formula, the concrete pH reduction (~7-8) by carbonatedreactions mean a 250 mV to -300 mV increase. ELSENER et al. (1990) when surface potentials are taken, they areessentially remote from the reinforcement due the concrete cover. The potential thereby measured are, in fact,mixed potentials. It can not be interpreted in a straightforward manner.The measurement results were not in agreement with theresults obtained from the visual examination. The smalldiameter bars embedded in high depth must haveaffected the measurements, as well as the surface wetnessvariation and its impregnation with paint product.…the literature mentions restrictions in the measurements:In all the units inspected: Resistivity: usually above 50 KΩ· cm.
  • The present study complements the presentedresults by: characterization of air-entrained concrete; Evaluation of the behavior of steel barsimmersed in solutions that simulate the waterin the pores of the air-entrained concrete.
  • Concrete bulk analysis: calculation of quantity anddiameter of the air bubbles in petrographic samplesprepared according to ASTM C856 (2011) and observed in SEM(associated with Scandium program);Physical and mechanical tests: performed accordingto Brazilian standards.2 air-entrained concretes - AC1 and AC2(specimens were casted in the field, in 2 different units)1 ordinary concrete (reference) – OC(specimens were casted in the laboratory)
  • The pore solutions were obtained from the mixturesprepared with distilled water and the powder obtained bygrinding the concrete specimens (AC1, AC2 and OC).3 different conditions of the bar weredefined(deformed, Ø 3.6 mm):• blasted (BS),• corroded - open air exposition (CS);• chromate treated galvanized (GS).Cupper wire - electric contactSealant over epoxy paint in thebars extremityo-ring wrapped aroundthe bar (middle of theexposed length)1 bar to simulate crevice corrosion3 replicate bar +Experimental set-up (bar immersedin the supernatant of the solution)
  • Cylindrical concrete specimens were used (Ø 5 cm x 10 cm).They were cured in laboratory atmosphere.1 blasted bar + 2 corrode barsTo validate the results in pore solution, tests on air-entrained concrete specimens (AC1 and AC2)were also conductedBar embedded in aconcrete specimen
  • Potentiostat (Solartron), usingthree-electrode cell(saturated calomel electrode, SCE).The Stern-Geary equationwas applied.IR drop compensation forconcrete specimens measures.After 1 day and 10 days of the bars immersed in the solutions(AC1, AC2 and OC) and after 28 days of the concrete specimenscure (AC1 and AC2), the bars were assessed by means ofcorrosion potential (Ecorr) and linear polarization resistancemeasurements of instantaneous corrosion current density(icorr) and visual (naked eyes) examination. Ecorr was monitoringduring in the immersion tests.… the usual electrochemical procedure for concrete studies was followed:
  • MaterialsAC1 AC2Ratio Features Ratio FeaturesDatasupliersWater 185 l W/C 0.66 185 l W/C 0.71Admixture 0.5 lAir-entrainingbased on pitchsoap, 0.18 % byweight of blinder0.5 lAir-entrainingbased on pitchsoap, 0.19 % byweight of blinderFine aggregates 943 kg - 839 kg -Coarse aggregates 725 kg - 796 kg -Portland cement 280 kgCP IV 32(pozzolanic-modified cement)260 kgCP II E 40(slag-modifiedcement)Fiber 0.3 kgPolypropylenemicrofiber0.2 kgPolypropyleneMicrofiberTestsresultsCompressivestrengh6.5 MPa 6.2 MPaVoids content 37.5 % 36.8 %Density 1730 kg/m³ 1650 kg/m³Water absorption 22.7 % 22.4 %The tests showed the inferior quality of the air-entrained concretes. Bothdo not fulfill the Brazilian standard (design of structural concrete).The poor quality must affect the performanceOCRatio Features166 l W/C 0.552.11 kgPolycarboxylate,0.7% by weight ofblinder257 kg -284 kg -301 kg CP II E 32- -38.8 MPa11.2 %2280 kg/m³4.8 %Soylev e Francois (2003): the corrosion rate is a function of concrete quality andit increases as the class of concrete strength decreases
  •  high presence of air bubbles withvariation of size and distribution; proximity of the air bubbles,some of them communicating; trend of concentration of the airbubbles surrounding theaggregates in AC2.OCAC2AC1The AC1 and AC2 poor quality wasconfirmed by microstructureanalysis, being detected:20 % 35 %Ø 40 to 60 µm10 %Bubble content 18 % Bubble content 4.5 %Bubble content 17 %OCAR1 AR 2Ø 120 to 140 µmAmountDiameter
  • Solution Calcium (Ca2+)Inorganic compounds (mg/l) pH Temperature (oC)Chloride (Cl-) Sulfate (SO42-) Initial Final Initial FinalPSAC1 0.21 ± 0.01 4.5 ± 0.2 12.0 ± 1.0 12.4 11.7 23 23PSAC2 0.11 ± 0.01 6.7 ± 0.4 5.9 ± 0.2 12.2 11.7 22 19PSOC 0.25 ± 0.01 2.4 ± 0,2 25.7 ± 0.6 12.2 11.7 21 21Blasted surface - BS(some corrosion areas) Corroded surface - CSGalvanized layer- GS(bubbles and irregularities inthe layer and cracks at thesurface)In the 3 solutions, the chloride content is not significant.During the tests, the pH is found to be similar.Pretest (SEM image plus EDS) showed the presence ofcracks in the surface of GS bars and bubbles andirregularities in its layer.
  • BS - solution results:BS4 - PSAC2Ecorr 5 day was lessthan -300 mVPSAC1, PSAC2 and PSOCMost values of Ecorr remained morepositive than -300 mV, except toASTMC876(2009)PSAC1icorr ≤ 0.15 µA/cm2PSAC2icorr ≤ 0.10 µA/cm2PSOCicorr ≤ 0.11 µA/cm2PSAC1icorr ≤ 0.11 µA/cm2PSAC2icorr ≤ 0.24 µA/cm2PSOCicorr ≤ 0.25 µA/cm2BS2BS410The corrosion levels werelow* in all the solution(icorr ≤ 0.5 µA/cm2) after1 and 10 days of immersion.1BS2 29 µA/cm2* RILEM TC 154 (2004)BS3 1.6 µA/cm2BS2 – PSAC2Ecorr 5-10 days wasless than -300 mVThe exceptions were...
  • All bars presentedcorrosion - spotsunder the sealantand the o-ring.PSAC1BS – visual analysis: PSAC2BS2 and BS4 presentedsevere corrosion:initiated under the sealantand propagated towardthe free surface.BS1 and BS3 presentedcorrosion under thesealant.BS2 BS4BS2Ecorr max.: -498 mVSCEBS4Ecorr max.: -329 mVSCEBS4BS2BS2Ecorr max.: -282 mVSCEBS4Ecorr max.: -211 mVSCEPSOCOnly BS4 presentedcorrosion - underthe sealant and theo-ring.BS4Ecorr max.: -202 mVSCEBS4
  • CS - solution results: PSAC1, PSAC2 and PSOCMost values of Ecorr remained morepositives than -150 mV, except to101Although the values of Ecorr indicates the passive state of the bars in allthe solutions, high corrosion levels were obtained (higher than 3 µA/cm2)CS2 – PSAC2Ecorr = -168.89 mVThe no change in the surface of the bars confirmed the passive state.CS1 – PSAC2Ecorr = -174.22 mVRILEM TC 154 (2004): the Rp values cannot be correctly determinate if thesteel is passive.
  • GS - solution results:The values of Ecorrwere morenegative than-600 mV in thebeginning andwent up later(passivationprocess).The values of Ecorr remainedmore positive than -600 mVPSAC1icorr ≤ 1.4 µA/cm2PSAC2icorr ≤ 0.8 µA/cm2PSAC1Icorr ≤ 3.7 µA/cm2PSAC2icorr ≤ 2.1 µA/cm2PSOCicorr ≤ 0.3 µA/cm210In the solution PSAC1 and PSAC2, thecorrosion levels were moderate to highafter 1 and 10 days of immersion .In the PSOC, the corrosion levels were lowafter 1 and 10 days1PSAC1 and PSAC2The values of Ecorr had a tendency todrop (depassivation process)PSOC
  • -1200-1100-1000-900-800-700-600-500-400-300-2000 2 4 6 8 10 12 14 16 18 20Tempo (dias)PSAC1 PSAC2 PSOC3The solution tests for GS bars was repeated …And the tendency of the bars bedepassivated in PSAC1 and PSAC2It also showed the tendency of theGS bars be passivated in PSOC16days2 hPSAC2PSAC1PSAC2PSAC1PSOCPSOCLocalizedcorrosion takesplace (dark spotsand H2 bubbles at thesurface)pH ~ 12.2pH ~ 11.5White corrosionproducts at thesurface2 hThe SEM images showed theformation of crystals on thesurface of coating at the PSOC.Similar crystals are found inother studies. They areformatted during the zincpassivation.
  • BS1 - AC1Ecorr = -520 mVSCE icorr = 0.8 µA/cm2CS1 - AC1Ecorr = +58 mVSCE icorr = 5.6 µA/cm2CS2 – AC1Ecorr = +54 mVSCE icorr = 8.9 µA/cm2CS1 – AC2Ecorr = -123 mVSCE icorr = 9.7 µA/cm2BS1 – AC2Ecorr = -160 mVSCE icorr = 3.2 µA/cm2CS2 - AC2Ecorr = +83 mVSCE icorr = 4.5 µA/cm2The results obtained in the concrete specimens were inagreement with the pore solution tests: In the BS bars therewas crevice corrosion under the sealant. For the CS barsthe potential indicated a passive state, being obtained ahigh corrosion level.
  • The characterization tests showed the low quality ofthe air-entrained concretes and the pore solution testsindicated the greatest susceptibility of steel corrosion inthese concretes. The obtained results are in agreementwith the inferior performance of the air-entrainedconcretes in the field in relation to the usually observedin a variety of ordinary concretes.The air-entrained concrete AC2 showed greatersusceptibility to corrosion. This was clearly observed inthe pore solution tests with blasted bar (BS) andconfirmed by concrete specimen tests.
  • The crevice corrosion was detected in the solutions. Thıscorrosion takes place under the sealant and the o-ringin the blasted bars (BS) in the 3 solutions. In the AC2solution the corrosion propagated towards the freesurface of the blasted bars.A tendency to depassivation of the galvanized bars(GS) was detected in the air-entrained concretesolutions. Localized corrosion was observed in the 3solutions, being probably related to the surfacedefects.It´s important to conduct detailed studies in the air-entrained concrete. These have to consider the bareand galvanized steel and a new system for protection ofthe bars extremities.
  • Thank youfor your attention!aaraujo@ipt.br