Technical Questions

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Embedded metals other than reinforcing steel

Q. What metals other than reinforcing steel are used inside concrete? And what are the concerns of having these metals inside concrete?

 

A. According to ACI ACI 222.3R, “Guide to Design and Construction Practices to Mitigate Corrosion of Reinforcement in Concrete Structures”, Metals other than steel are occasionally used in concrete. These metals include aluminum, lead, copper and copper alloys, zinc, cadmium, Monel metal, stellite (cobalt-chromium-tungsten alloys), silver, and tin. Galvanized steel and special alloys of steel, such as stainless steels and chrome-nickel steels, have also been used. Zinc and cadmium are used as coatings on steel.

 

Aluminum: Corrosion of aluminum embedded in concrete can crack the concrete. Conditions conducive to corrosion are created if the concrete contains steel in contact with the aluminum, chlorides are present in appreciable concentrations, or the cement is high in alkali content. When the metals are coupled, increasing ratios of steel area, particularly in the presence of appreciable chloride concentrations, increase corrosion of the aluminum.

Additionally, hydrogen gas evolution may occur when fresh concrete contacts aluminum. This may increase the porosity of the concrete and, therefore, the penetration of future corrosive agents. Some aluminum alloys are more susceptible to this problem than others. Corrosion inhibitors, such as calcium nitrite, have been shown to improve the corrosion resistance of aluminum in concrete.

 

Lead: Lead in damp concrete will be attacked by the CaOH2 in the concrete, and may be destroyed in a few years. Contact of lead with reinforcing steel can accelerate the attack. Lead should be isolated from the concrete by protective plastic, or other materials that are unaffected by damp concrete. Corrosion of embedded lead is not likely to damage the concrete.

 

Copper and copper alloys: Copper is not normally corroded by concrete, as is evidenced by the widespread and successful use of copper waterstops and the embedment of copper pipes in concrete for many years. Corrosion of copper pipes, however, has been reported where ammonia is present. Also, there have been reports that small amounts of ammonia, and possibly of nitrates, can cause stress corrosion cracking of embedded copper. Galvanic corrosion of steel will occur if the steel is connected to the copper.

 

Zinc: Zinc reacts with alkaline materials, such as those found in concrete. Zinc in the form of a galvanizing coating on reinforcing steel, however, is sometimes intentionally embedded in concrete. Available data are conflicting as to the benefit, if any, of this coating. A chromate dip on the galvanized bars or the use of 400 ppm of chromate in the mixing water is recommended to prevent hydrogen evolution in the fresh concrete. Use caution when using chromium salts because of possible skin allergies. Additionally, users are cautioned against permitting galvanized and black steel to come in contact with each other in a structure because the use of dissimilar metals can cause galvanic corrosion. Corrosion inhibitors, such as calcium nitrite, have been shown to improve the corrosion resistance of zinc in concrete.

 

Other metals: Chromium and nickel alloys generally have good resistance to corrosion in concrete, as do silver and tin. The corrosion resistance of some of these metals may be adversely affected by the presence of soluble chlorides from seawater or deicing salts. Use of stainless steel may be economically justified in some high chloride environments where the higher initial cost is offset by reduced cost in service over the life cycle. Examples would be marine locations and heavily deiced bridge decks. The 300 Series stainless steels, however, are susceptible to stress corrosion cracking when the temperature is over 140°F (60°C) and chloride solutions are in contact with the steel material. Embedded natural-weathering steels generally do not perform well in concrete containing moisture and chloride. Weathering steels adjoining concrete may discharge rust and cause staining of concrete surfaces.

 

References: ACI 222.3R-11; ACI 201.2R-16

Topics in Concrete: Corrosion; Reinforcement; Materials

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