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June 15 - June 21, 2015

What is the best way to prevent corrosion of reinforcing bar in concrete?


Selected Answers

From Joe Miller of NextGen Green Building Products, Inc. dba Direct2Contractors.com on July 1, 2015:
Andrew, of course not all sealers are alike. Many developed years ago and based on silane or siloxane resins are very hazardous and toxic to mechanics, animals, plants and aquatic life. But they were "state of the art" then. I know I brought several of them to market years ago. Today, the "state of the art" formulations are safe, penetrating, non-toxic and do not pollute the run-off water. These formulations penetrate into the pores and capillaries where they form crystals that expand and contract in response to moisture vapor or liquid water to block entry of them into the concrete.They work below the surface so are almost completely unaffected by wear or abrasion at the surfaces. They are permanent protection. Of course, concrete permeated by CO2 still undergoes carbonation, shrinkage and loss of pH. Therefore, even liquid crystalline penetrants should be re-applied as the concrete ages and continues to shrink.Shrinkage still produces micro-cracks under the tensile forces it creates. So I would recommend using the latest, patented, safe liquid crystalline formulations and discard the old silane and siloxane products that we used to call clear sealers.

From Dana Stiles of concreteplus on June 28, 2015:
The relationship between steel and concrete is a “perfect marriage” as long as the pH levels maintain passivation. Cover, density, hydrophobicity, and elastomeric coatings all help to slow down the loss of pH that occurs with carbonation and chloride ion ingress. I think every respondent is "right" in one way or another. The EN1504 standard addresses many of these issues for repairs, and the addition of anodic protection certainly helps to maintain the electron potential and avoid the so-called "halo effect."

From Andrew Sedor of AECOM on June 25, 2015:
This question just begs for more information. Best way for you, me, or my client? Where is this concrete being used? How is this concrete being used? In the manner of your use of the concrete, are there any limitations to using any of the methods mentioned here? If you dismiss these limitations, as well as cost, practicality and opinion, maybe take every suggestion here and all other methods not mentioned. And use them all. Wouldn't a combination of sealer and CP be better than sealer alone?

From Mark Puckett of Orfanos Contractors, Inc. on June 18, 2015:
Use epoxy-coated rebar. You can also use siloxane sealers to greatly minimize chloride ion passage.

From Joe Miller of NextGen Green Building Products, Inc. dba Direct2Contractors.com on June 18, 2015:
David, your comments about epoxy-coated rebar sound reassuring; however, I recall an extensive study of the practice conducted by WJE in Chicago many years ago. As I recall, they were testing blocks of steel-reinforced concrete whose rebars had been bent and formed at site to try to determine if the bending or forming process created cracks in the powder-coat epoxy that then allowed corrosion cell development at those cracks. Do you have any knowledge of this testing results? Of course, coating rebars with epoxy (either powder-coated or wet coatings) would preclude the use of impressed current or sacrificial anodes at a future date.

From Mario Colica of Colimet srl on June 17, 2015:
One possible soluton is to spray zinc on the concrete surface. The zinc particles will penetrate the pores of the concrete, acting first as a barrier and then as anodic protection when rain or other contaminants try to penetrate.  Zinc can be applied on both new and existing concrete.

From David McDonald of Epoxy Interest Group of CRSI on June 17, 2015:
Corrosion of steel in concrete is caused by either chloride ions from deicing salts or marine waters reaching the bar in sufficient quantities or from carbonation of the concrete due to reactions of the cement with the atmosphere. While there are many solutions to corrosion, high quality concrete and appropriate concrete cover are the first and least costly steps to take. Epoxy-coated reinforcing steel (FBECR) was found to be the next most commonly specified method to reduce damage due to corrosion, and in North America approximately 10 percent of all reinforcing steel is coated (Russell 2004 NCHRP 333). This product has been used for over 40 years and is in use in over 80,000 bridge decks and many other structures in the USA alone. Design lives over 75 years are expected from the older structures and even longer lives with the newer structures that utilized improved concrete. Significant quantities of epoxy-coated reinforcing steel are also being used in the Middle East and Asia.

From Joe Miller of NextGen Green Building Products, Inc. dba Direct2Contractors.com on June 16, 2015:
Yes, Carl, alkalinity of fresh concrete is well known. Less well known is that, as concrete ages, it gets permeated by carbon dioxide gas. As CO2 passes through concrete, it changes the composition of the cement paste into another substance that occupies less volume, is harder and, worst of all, reduces the alkalinity down to near neutral pH of 7. The protective alkalinity is then gone. Actually, the protective alkalinity values are probably lost at a pH of around 8 or 9. The loss of alkalinity, of course, begins at the surfaces that the CO2 permeates into the concrete. For concrete elements exposed to CO2 such as balconies, bridge decks and columns and elevated walkways, the loss is occurring on all exposed surfaces. Rate of loss and depth of permeation, of course, is greater with micro-cracks so that permeation can go all the way down to rebar if the micro-cracks form even earlier in the concrete elements. So effective blocking of micro-cracks becomes even more critical for these elements. Soluble salts must not get to the bars since they initiate and accelerate corrosion cell formations. Protecting steel rebar in concrete is complex more so as point loads on these elements induce flexing, as well.

From Carl Havemann of www.corrosioneducation.co.za on June 16, 2015:
Because concrete is alkaline,  the rebar should not corrode, but corrosion will take place when water penetrates the concrete cover to the rebar. Therefore, make sure there is sufficient cover over the rebar and seal the concrete to fill any micro cracks.

From Joe Miller of NextGen Green Building Products, Inc. dba Direct2Contractors.com on June 15, 2015:
Without a context for consideration, this question is almost unanswerable.  Corrosion of steel  requires an anode, a cathode, an electrolyte and oxygen to occur. Removing oxygen is almost impossible, so the only remaining choice is to eliminate the electrolyte. Given the fact that most concrete elements have water in their pore structures, it comes down to reducing or eliminating soluble salts within the liquid or severely restricting their entry into the pore structures. So---waterproofing. One proven way to do that is to apply a liquid, crystalline penetrating sealer that effectively blocks water and moisture vapor from gaining access to the pore structures. Hope this helps.

From luiz de miranda of ECOPROTEC on June 15, 2015:
Cathodic protection by means of sacrificial flexible anodes is welcome, since hydrogen embrittlement can be avoided. Electrode potential must be greater than -700 mV by report of a copper sulphate reference electrode

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Tagged categories: Asia Pacific; Corrosion; EMEA (Europe, Middle East and Africa); Latin America; North America; Quality Control; Rebar


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