Will galvanized steel rust in concrete?

The total life of galvanized steel in concrete is made up of the time taken for the zinc to depassivate, plus the time taken for consumption of the zinc coating, as it sacrificially protects the underlying steel. Only after the coating has been fully consumed in a region of the bar will localized steel corrosion begin.

galvanizeit.org - In Concrete | American Galvanizers Association

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Home » Hot-Dip Galvanizing (HDG) » How Long Does HDG Last? » In Concrete

In Concrete

Concrete is an extremely complex material. The use of various types of concrete in construction has made the chemical, physical, and mechanical properties of concrete and its relationship to metals a topic of ongoing studies. Reinforcing steel bars (rebar) are embedded in concrete to provide strength, and are critical to the integrity and performance of the structure throughout its life. As concrete is a porous material, corrosive elements such as water, chloride ions, oxygen, carbon dioxide, and other gases travel into the concrete matrix, eventually reaching the rebar. Once the concentration of these corrosive elements surpasses steels corrosion threshold, the rebar starts to corrode. As the rebar corrodes, pressure builds around the bar leading to cracking, staining, and eventually spalling of the concrete. Because failure of the rebar leads to compromised or failing structural capacity, protecting against premature rebar failure is key. Similar to in the atmosphere, galvanized rebar extends the life of the steel in concrete. The corrosion mechanisms in concrete are quite different than atmospheric exposure, and one of the biggest factors is chloride concentration. Galvanized rebar can withstand chloride concentration at least four to five times higher than black steel, and remains passivated at lower pH levels, slowing the rate of corrosion. In addition to the higher chloride tolerance, once zinc corrosion products are formed from the galvanized rebar, they are less voluminous than iron oxide and actually migrate away from the bar. The less voluminous zinc particles migrate away from the bar (galvanized coating) and into the pores of the concrete matrix. This migration prevents the pressure buildup and spalling caused by iron oxide particles.

Galvanized rebar in concrete can protect against premature failure

Corrosion Resistance of Galvanized Rebar in Concrete

Spalling Concrete and Bare Steel

The corrosion mechanisms and performance of black and hot-dip galvanized steel in concrete are different than when exposed to atmospheric conditions. Steel embedded in concrete is exposed to a highly alkaline environment. Black steel is passive in alkaline concrete until the chloride level exceeds approximately 1 lb/yd 3 , when steel becomes depassivated and starts to corrode. Zinc, on the other hand, can withstand chloride concentration at least four to five times higher than black steel, and coupled with its impervious barrier protection, delays the onset of chloride corrosion on galvanized rebar. Chlorides penetrate the concrete through small pores and cracks that form on the surface through use and weathering. While black steel in concrete typically depassivates below a pH of 11.5, galvanized reinforcement can remain passivated at a lower pH, thereby offering substantial protection against the effects of concrete carbonation. In addition to the higher chloride tolerance, once the zinc coating does start to depassivate, the zinc corrosion products formed are less voluminous than iron oxides and actually migrate away from the galvanized bar into the matrix of the concrete. Unlike the development of iron oxide, the migration of the zinc corrosion products from the rebar prevents the pressure buildup and eventual concrete spalling. The total life of a galvanized coating in concrete is made up of the time taken for the zinc to depassivate (which takes longer than black steel because of its higher tolerance to chloride ions), plus the time taken for the consumption of the zinc coating as it sacrificially protects the underlying steel. Only after the coating has been fully consumed in a region of the bar will localized corrosion of the steel begin. Bond Strength Good bonding between reinforcing steel and concrete is essential for the reliable performance of reinforced concrete structures. When protective coatings on steel are used, it is essential to ensure they do not reduce bond strength. Studies on the bonding of galvanized and black steel bars to Portland Cement concrete have been investigated. The results of these studies indicate:

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Development of the bond between black or galvanized steel and concrete depends upon cure time and environmental factors. In some cases, the full bond for galvanized rebar may take longer to form than for uncoated steel, depending on the zincate/cement reaction. As reported by Stephen Yeomans in GalvanizedSteel Reinforcement in Concrete, there are a number of studies that have concluded the fully developed bond strength of galvanized rebar has no significant difference from black rebar bond strength. A study by C. Andrade in Spain monitored bond strength of galvanized rebar samples over 10 years immersed in seawater and found no deleterious effects on bond strength over that time. Bond Strength Test The bond of the hot-dip galvanized reinforcing bar to the concrete can be tested according to ASTM A944, Test Methods for Comparing Bond Strength of Steel Reinforcing Bars to Concrete Using Beam-End Specimens. The bond strength relies heavily on the deformation of the bar and not as much on the actual bond between the zinc and the concrete. For plain bars with no deformation, the bond between the zinc and the concrete becomes very important. Pullout strength of hot-dip galvanized reinforcing steel has been tested many times, and the values of bond strength are equivalent to, or better than, black steel bond strength.

Bond strength to concrete: black vs. galvanized reinforcing steel

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Zinc Reaction in Concrete

During curing, the galvanized surface of steel reinforcement reacts with the alkaline cement paste to form stable, insoluble zinc salts accompanied by hydrogen evolution. This has raised the concern of the possibility of steel embrittlement due to hydrogen absorption. Laboratory studies indicate liberated hydrogen does not permeate the galvanized coating to the underlying steel and the reaction ceases as soon as the concrete hardens. ASTM A767 requires hot-dip galvanized reinforcement be chromate passivated after galvanizing. Many cement mixtures contain small amounts of chromate that may serve the same purpose as chromate passivating the zinc coating. The reaction between the alkaline cement paste and the zinc coating is dependent on the amount of zinc-coated surface in the concrete with the potential for reaction increasing with more zinc metal in contact with the concrete. Removal of Forms The intermetallic layers of the galvanized coating are harder than the base steel, so galvanized rebar and other embeds are extremely resistant to damage from abrasion.

galvanizeit.org - In Concrete | American Galvanizers Association

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