SUPPLEMENTARY CORROSION PROTECTION OF REINFORCING STEEL · PDF file · 2011-05-27SUPPLEMENTARY CORROSION PROTECTION OF REINFORCING STEEL ZUSÄTZLICHE KORROSIONSSCHUTZMASSNAHMEN FÜR

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  • SUPPLEMENTARY CORROSION PROTECTION OF REINFORCING STEEL

    Otto-Graf-Journal Vol. 11, 200077

    SUPPLEMENTARY CORROSION PROTECTION OF REINFORCINGSTEEL

    ZUSTZLICHE KORROSIONSSCHUTZMASSNAHMEN FRBEWEHRUNGSSTAHL

    MESURES SUPPLMENTAIRES POUR LA PROTECTION ANTICOR-ROSION de lacier darmature.

    Ulf Nrnberger

    SUMMARY

    The publication gives a survey of different steel concerning supplementarycorrosion protection measures that prevent or retard corrosion. They areproposed and used for new structures but also as repair measures for existingreinforced concrete structures. Besides the performance characteristics alsoproblems with the application are discussed.

    ZUSAMMENFASSUNG

    Der Bericht gibt einen berblick ber verschiedene, den Bewehrungsstahlbetreffende zustzliche Korrosionsschutzmanahmen, welche Korrosionverhindern oder verzgern. Diese Manahmen wurden entwickelt und werdengentzt in neuen Konstruktionen aber auch bei der Instandsetzung bestehenderStahlbetonkonstruktionen. Neben den Gebrauchseigenschaften werden auchProbleme bei der Anwendung diskutiert.

    RSUM

    Le rapport offre une vue densemble de differentes mesuressupplmentaires qui peuvent eviter ou retarder la corrosion de lacierdarmature. Ces mesures taient dveloppes et sont utilises pour desconstructions modernes, mais galement pour la mise en tat des constructionsexistantes. ct des qualits du maniement, les problmes dapplication sontgalement discuts.

    KEYWORDS: corrosion protection, epoxy coating, galvanizing, reinforcing concrete,stainless steel reinforcement

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    1. INTRODUCTION

    Steel in concrete is usually protected against corrosion by passivation of thesteel arising from the high alkalinity of the pore solutions within the concrete. Astable oxide layer is formed on the steel surface which prevents the anodicdissolution of iron. Loss of durability in reinforced concrete only occurs if thisstable oxide layer is rendered unstable (if depassivation occurs) due to theingress of chlorides to the steel/concrete interface or carbonation of the concretereducing the alkalinity of the pore solution at the steel/concrete interface.Durable reinforced concrete therefore must be designed to resist carbonation andto exclude chlorides from any source [1]. Reinforcing steel should be embeddedin concrete specified in accordance with current standards. In particular the mixdesign and minimum cover must be observed and suited to corrosivity ofenvironment. In many cases this will provide sufficient corrosion protection tothe reinforcing steel, provided that the concrete is correctly placed, compactedand cured.

    Nevertheless there is significant evidence that some of these conditions arenot fulfilled and that problems of steel and concrete deterioration are due eitherto inadequate design or to incorrect site practice. There are circumstances inwhich it is difficult to achieve the specified design life without additionalcorrosion protection measures. Problems arise if

    the concrete cover and the concrete quality is - by design or otherwise -reduced relative to the necessary values for the surrounding environmentalconditions (e. g. by extreme filigree elements);

    special structures have to be erected, e. g. connections between precastand cast in place elements or heat insulated joints between the structureand external structural elements (e. g. balconies);

    non-dense or dense lightweight concrete is designed to reach a requiredthermal insulation as well as low ownweight;

    structures are exposed to high concentrations of chlorides (e. g. in marinestructures and bridge or parking decks due to the use of deicing salts).

    In such cases designers may consider modifications to the concrete mixdesign in order to decrease permeability. Coatings and surface treatments tolimit chloride ingress into the concrete, the use of corrosion protectedreinforcement and of more corrosion resistant materials for the reinforcement (e.g. stainless steels) and addition of inhibitors to the fresh concrete and cathodic

  • SUPPLEMENTARY CORROSION PROTECTION OF REINFORCING STEEL

    Otto-Graf-Journal Vol. 11, 200079

    prevention by impressed current my also be considered. This publication gives asurvey of corrosion protection of reinforcement that prevent or retard corrosionand which might be proposed and used for new structures but also as preventiveand as repair measures for existing reinforced concrete structures.

    2. EPOXY-COATINGS [1-5]

    2.1 Application of coated reinforcement

    Epoxy coating is one of the most widely used techniques for protectingreinforcing bars against corrosion inside the concrete. The effectiveness ofepoxy coating as a corrosion prevention method was first studied in the USAand Japan [6]. Later this method has spread also to Canada, Middle East andEurope. Epoxy-coated rebar has been in frequent use in the United States sincethe mid-1970s. There the main application is in the decks of highway bridgessubject to deicing salts but all over the world the product has also been used asreinforcement in many other fields of concrete constructions e. g. garages,substructures of marine bridges and offshore structures. The consumption ofepoxy-coated reinforcement in USA has increased gradually to about 250.000tons yearly in 1990. In Europe the application concentrates on single projects.

    Standards to epoxy-coating reinforcing steel for example exist in the USA,UK, Germany, Japan and Norway [3].

    2.2 Manufacture of the coating

    There are two types of epoxy-coatings: liquid and powder coatings.Because of better corrosion protection efficiency [7] electrostatic spraying ofepoxy powder to the straight lengths of rebar currently accounts for the majorityof coated rebar. After cleaning the steel by abrasive blasting in electrostaticspraying the electrically charged powder particles are sprayed onto a preheatedsteel surface (+230 C) where they melt to form an even and uniform powderfilm. After a heat catalysed irreversible reaction the powder starts to gel. Afterthe film is solidified the coated bars are cooled in water or air.

    As a result an uniform coating without pores and cracks is the best.Experiences showed that fusion bonded epoxy-coatings render rather eventhicknesses, even across the ribs on ribbed bars.

    With regard to failures in application of epoxy-coated bars in substructureof marine bridges some producer use chromated bars to improve adhesionbetween steel and epoxy.

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    2.3 Mode of action

    The purpose of the coating is to isolate and insulate the steel from thecorrosive environment. The coating act solely as a barrier against theenvironment. The epoxy-coatings used today to protect reinforcing steel containno corrosion inhibitive pigments.

    To provide adequate protection the coatings should have a minimumthickness. Nevertheless it should not be so thick that it empedes flexibility andbonding of the coating between steel surface and concrete: According to USstandard ASTM A 775-81 the thickness of epoxy powder coating in order tofulfil flexibility, bonding and corrosion protection requirements should bebetween 130 m and 300 m.

    If there are defects on the coating through which aggressive agents canpenetrate the barrier, corrosion concentrates on these areas. Integrity of thecoating therefore is essential for effective corrosion protection. The filmtherefore must be free from pores, cracks and damaged areas.

    2.4 Properties of coating

    Owing to their chemical composition epoxy resins exhibit several physicalproperties such as high ductility, small shrinkage in polymerisation, goodabrasion resistance, good heat resistance and outstanding adhesion on metalsurfaces if sufficiently pretreated.

    Epoxy resins normally exhibit good durability against solvents, chemicalsand water. The long-term durability of most epoxy-coatings in concrete aregood. Thin epoxy coatings until 250 m are not completely impermeable tooxygen and moisture, but diffusion ca be reduced by sufficient thickness anddensity. Chloride permeability in a defect free coating is considerably lower thanthat of water vapour and oxygen if a powder epoxy-coating has a thickness of130 - 250 m [7].

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    Otto-Graf-Journal Vol. 11, 200081

    Epoxy-coatings have no electrically but a electrolytical conductivity in thepresence of water and/or increased temperatures. Areas beyond the coating canact as anodes and cathodes of corrosion elements if adhesion is removed. But

    - the epoxy-coating will not soften or deteriorate in the highly alkalineenvironment,

    - it has an excellent adhesion to a well pretreated steel reinforcement, ensuringno delamination as a result of corrosion forces.

    2.5 Corrosion protection behaviour [6-11]

    In numerous accelerated corrosion tests on natural exposure epoxy-coatedand untreated or in other way protected steel bar reinforcement have beencompared. Sound epoxy-coating provided considerable long-term protection tothe steel when exposed in carbonated concrete and concrete with a highconcentration of chloride. The use of epoxy-coatings free of essential defectsguarantees complete protection in carbonated concrete and a significantreduction in the rate of deterioration of reinforced concrete containing highlevels of chloride.

    The corrosion prevention ability of liquid epoxy-coatings is not quite asgood as that of powder epoxy-coatings. Liquid coatings may have manyholidays or are more permeable to water and/or chloride ions.

    Cracks in the concrete did not increase corrosion of epoxy-coated bars withan undamaged coating.

    However, the use of coatings in chloride containing concrete does notprovide complete protection. Corrosion of the ste