INTRODUCTION
Spalling of reinforced-concrete bridge decks due to chloride-induced corrosion is a widely recognized problem. A number of remedial steps to preclude this problem have been implemented by various states. For new bridges, the most common (and generally considered the most effective) measure is the use of epoxy-coated reinforcing steel. Other preventative steps include use of waterproofing membranes, various types of dense or impermeable concretes in bridge decks and overlays, extra concrete cover over reinforcing steel, cathodic polarization of reinforcing steel and various sealing compounds. For new construction, none of those methods has proven either economically competitive or suitably durable to supplant epoxy-coated reinforcing steel.
The Department of Transportation "Standard Specifications for Road and Bridge Construction" requires the use of ASTM A 615 Grade 60 steel for both epoxy coated and uncoated reinforcing bars in bridges.
The present pre-eminence of epoxy-coated reinforcing steel in protecting bridge decks should not preclude consideration of alternative schemes. The cost of epoxy-coated reinforcing steel could rise due to market fluctuations and in such cases, it would be desirable to have optional methods of corrosion prevention. The use of calcium nitrite as a corrosion inhibitor in reinforced concrete is a promising alternative.
Calcium nitrite was originally used as a corrosion inhibitor in Japan. The scarcity of suitable aggregates forced the Japanese to use marine sand in concrete. That sand contained chlorides which promoted corrosion of reinforcing steel. Research by the Japanese led to the use of calcium nitrite which was determined to be an effective corrosion inhibitor that did not adversely effect the properties of concrete.
Concrete normally provides a highly alkaline environment in the vicinity of steel (13.5 to 13.8 pH). That renders reinforcing steel passive in normal environments. Use of deicing salts on bridge decks provides chlorides which permeate the concrete and contact the reinforcing steel. Chloride ions attack the passive nature of reinforcing steel in a manner that is not well understood (1). In bridge decks, the effect of chloride ions is to locally shift the potential of reinforcing steel. That occurs both horizontally across the deck and vertically between the upper and lower reinforcing mats. The effect of the potential shift is to create a "macro-cell" type of galvanic corrosion that accelerates the rate of corrosive damage.
The corrosion resistance of reinforcing steel embedded in concrete is suspected as being due to a passive film formed on the surface of the reinforcement. Chloride ions apparently penetrate that layer and transport ferrous ions away from anodic areas on the surface of the reinforcing steel. Corrosion of the reinforcing steel may be viewed as two basic anodic reactions (2):
Fe
++ + 2QH'
kl-FeO + (H
20)
x (1)
Fe
++ + Cr
k2-{FeCr Complex}
+ + OH'-FeO (H
20)
x + Cl'
Reaction 1 is the normal initial corrosion of reinforcing steel which creates a ferrous-hydroxide layer that prevents further corrosion. Reaction 2 is the breakdown of that layer by chloride ions and formation of ferrous oxide away from the anode site on the reinforcing steel surface.
The inhibiting action of calcium nitrite may be represented by a third anodic reaction:
2 Fe
++ + 20H* + 2N0
2k3^2N0 + Fe
20
3 + H
20 (3)
When a nitrite is present, as shown in reaction 3, the corrosion reactions are competitive.
Nitrite ions prevent the migration of ferrous ions from anodic areas. Ferrous hydroxide (formed in reaction 1) and ferrous ions react to form a hydrated ferrous oxide. The ferrous oxide precipitates and forms a barrier on the reinforcing steel that greatly decreases the corrosion potential for reaction 2. As noted, the three corrosion reactions are competitive. If the amount of nitrite ions is sufficiently high compared to the chloride ions, inhibiting reaction 3 proceeds rapidly and stifles additional chloride-induced corrosion.
Calcium nitrite must be added to the concrete in sufficient quantity to neutralize the effect of chloride ions over the life of a structure. Typical dosage rates range from 2 to 4 percent by weight of cement (for W. R. Grace PCI calcium-nitrite admixture). Calcium nitrite has been shown to be effective for chloride to nitrite ratios exceeding 1.5 to 1.0 percent by weight based on original nitrite content (3). At a dosage of 2 percent by weight of cement, calcium nitrite would inhibit the corrosive action if the chloride content in a deck were 2 to 3 percent by weight of cement. This is significant, since 0.3 to 1 percent of chloride by weight of cement in the deck is sufficient to cause corrosion of reinforcing steel, based upon European experience (op. cit. 1). Protection provided by calcium nitrite has been analyzed experimentally and determined to be effective in preventing corrosion at very high chloride levels, 3.6 to 22.6 lbs of chloride per cubic yard of concrete (approximately 0.8 to 5 percent by weight of cement) (4). The potential corrosion-inhibiting properties of calcium nitrite have warranted construction of experimental bridges incorporating its use. That has been done previously by other state highway agencies including Michigan, New Hampshire, Rhode Island, North Carolina, Pennsylvania, and Illinois.
INTRODUCTION
Spalling of reinforced-concrete bridge decks due to chloride-induced corrosion is a widely recognized problem. A number of remedial steps to preclude this problem have been implemented by various states. For new bridges, the most common (and generally considered the most effective) measure is the use of epoxy-coated reinforcing steel. Other preventative steps include use of waterproofing membranes, various types of dense or impermeable concretes in bridge decks and overlays, extra concrete cover over reinforcing steel, cathodic polarization of reinforcing steel and various sealing compounds. For new construction, none of those methods has proven either economically competitive or suitably durable to supplant epoxy-coated reinforcing steel.
The Department of Transportation "Standard Specifications for Road and Bridge Construction" requires the use of ASTM A 615 Grade 60 steel for both epoxy coated and uncoated reinforcing bars in bridges.
The present pre-eminence of epoxy-coated reinforcing steel in protecting bridge decks should not preclude consideration of alternative schemes. The cost of epoxy-coated reinforcing steel could rise due to market fluctuations and in such cases, it would be desirable to have optional methods of corrosion prevention. The use of calcium nitrite as a corrosion inhibitor in reinforced concrete is a promising alternative.
Calcium nitrite was originally used as a corrosion inhibitor in Japan. The scarcity of suitable aggregates forced the Japanese to use marine sand in concrete. That sand contained chlorides which promoted corrosion of reinforcing steel. Research by the Japanese led to the use of calcium nitrite which was determined to be an effective corrosion inhibitor that did not adversely effect the properties of concrete.
Concrete normally provides a highly alkaline environment in the vicinity of steel (13.5 to 13.8 pH). That renders reinforcing steel passive in normal environments. Use of deicing salts on bridge decks provides chlorides which permeate the concrete and contact the reinforcing steel. Chloride ions attack the passive nature of reinforcing steel in a manner that is not well understood (1). In bridge decks, the effect of chloride ions is to locally shift the potential of reinforcing steel. That occurs both horizontally across the deck and vertically between the upper and lower reinforcing mats. The effect of the potential shift is to create a "macro-cell" type of galvanic corrosion that accelerates the rate of corrosive damage.
The corrosion resistance of reinforcing steel embedded in concrete is suspected as being due to a passive film formed on the surface of the reinforcement. Chloride ions apparently penetrate that layer and transport ferrous ions away from anodic areas on the surface of the reinforcing steel. Corrosion of the reinforcing steel may be viewed as two basic anodic reactions (2):
Fe
++ + 2QH'
kl-FeO + (H
20)
x (1)
Fe
++ + Cr
k2-{FeCr Complex}
+ + OH'-FeO (H
20)
x + Cl'
Reaction 1 is the normal initial corrosion of reinforcing steel which creates a ferrous-hydroxide layer that prevents further corrosion. Reaction 2 is the breakdown of that layer by chloride ions and formation of ferrous oxide away from the anode site on the reinforcing steel surface.
The inhibiting action of calcium nitrite may be represented by a third anodic reaction:
2 Fe
++ + 20H* + 2N0
2k3^2N0 + Fe
20
3 + H
20 (3)
When a nitrite is present, as shown in reaction 3, the corrosion reactions are competitive.
Nitrite ions prevent the migration of ferrous ions from anodic areas. Ferrous hydroxide (formed in reaction 1) and ferrous ions react to form a hydrated ferrous oxide. The ferrous oxide precipitates and forms a barrier on the reinforcing steel that greatly decreases the corrosion potential for reaction 2. As noted, the three corrosion reactions are competitive. If the amount of nitrite ions is sufficiently high compared to the chloride ions, inhibiting reaction 3 proceeds rapidly and stifles additional chloride-induced corrosion.
Calcium nitrite must be added to the concrete in sufficient quantity to neutralize the effect of chloride ions over the life of a structure. Typical dosage rates range from 2 to 4 percent by weight of cement (for W. R. Grace PCI calcium-nitrite admixture). Calcium nitrite has been shown to be effective for chloride to nitrite ratios exceeding 1.5 to 1.0 percent by weight based on original nitrite content (3). At a dosage of 2 percent by weight of cement, calcium nitrite would inhibit the corrosive action if the chloride content in a deck were 2 to 3 percent by weight of cement. This is significant, since 0.3 to 1 percent of chloride by weight of cement in the deck is sufficient to cause corrosion of reinforcing steel, based upon European experience (op. cit. 1). Protection provided by calcium nitrite has been analyzed experimentally and determined to be effective in preventing corrosion at very high chloride levels, 3.6 to 22.6 lbs of chloride per cubic yard of concrete (approximately 0.8 to 5 percent by weight of cement) (4). The potential corrosion-inhibiting properties of calcium nitrite have warranted construction of experimental bridges incorporating its use. That has been done previously by other state highway agencies including Michigan, New Hampshire, Rhode Island, North Carolina, Pennsylvania, and Illinois.
INTRODUCTION
Spalling of reinforced-concrete bridge decks due to chloride-induced corrosion is a widely recognized problem. A number of remedial steps to preclude this problem have been implemented by various states. For new bridges, the most common (and generally considered the most effective) measure is the use of epoxy-coated reinforcing steel. Other preventative steps include use of waterproofing membranes, various types of dense or impermeable concretes in bridge decks and overlays, extra concrete cover over reinforcing steel, cathodic polarization of reinforcing steel and various sealing compounds. For new construction, none of those methods has proven either economically competitive or suitably durable to supplant epoxy-coated reinforcing steel.
The Department of Transportation "Standard Specifications for Road and Bridge Construction" requires the use of ASTM A 615 Grade 60 steel for both epoxy coated and uncoated reinforcing bars in bridges.
The present pre-eminence of epoxy-coated reinforcing steel in protecting bridge decks should not preclude consideration of alternative schemes. The cost of epoxy-coated reinforcing steel could rise due to market fluctuations and in such cases, it would be desirable to have optional methods of corrosion prevention. The use of calcium nitrite as a corrosion inhibitor in reinforced concrete is a promising alternative.
Calcium nitrite was originally used as a corrosion inhibitor in Japan. The scarcity of suitable aggregates forced the Japanese to use marine sand in concrete. That sand contained chlorides which promoted corrosion of reinforcing steel. Research by the Japanese led to the use of calcium nitrite which was determined to be an effective corrosion inhibitor that did not adversely effect the properties of concrete.
Concrete normally provides a highly alkaline environment in the vicinity of steel (13.5 to 13.8 pH). That renders reinforcing steel passive in normal environments. Use of deicing salts on bridge decks provides chlorides which permeate the concrete and contact the reinforcing steel. Chloride ions attack the passive nature of reinforcing steel in a manner that is not well understood (1). In bridge decks, the effect of chloride ions is to locally shift the potential of reinforcing steel. That occurs both horizontally across the deck and vertically between the upper and lower reinforcing mats. The effect of the potential shift is to create a "macro-cell" type of galvanic corrosion that accelerates the rate of corrosive damage.
The corrosion resistance of reinforcing steel embedded in concrete is suspected as being due to a passive film formed on the surface of the reinforcement. Chloride ions apparently penetrate that layer and transport ferrous ions away from anodic areas on the surface of the reinforcing steel. Corrosion of the reinforcing steel may be viewed as two basic anodic reactions (2):
Fe
++ + 2QH'
kl-FeO + (H
20)
x (1)
Fe
++ + Cr
k2-{FeCr Complex}
+ + OH'-FeO (H
20)
x + Cl'
Reaction 1 is the normal initial corrosion of reinforcing steel which creates a ferrous-hydroxide layer that prevents further corrosion. Reaction 2 is the breakdown of that layer by chloride ions and formation of ferrous oxide away from the anode site on the reinforcing steel surface.
The inhibiting action of calcium nitrite may be represented by a third anodic reaction:
2 Fe
++ + 20H* + 2N0
2k3^2N0 + Fe
20
3 + H
20 (3)
When a nitrite is present, as shown in reaction 3, the corrosion reactions are competitive.
Nitrite ions prevent the migration of ferrous ions from anodic areas. Ferrous hydroxide (formed in reaction 1) and ferrous ions react to form a hydrated ferrous oxide. The ferrous oxide precipitates and forms a barrier on the reinforcing steel that greatly decreases the corrosion potential for reaction 2. As noted, the three corrosion reactions are competitive. If the amount of nitrite ions is sufficiently high compared to the chloride ions, inhibiting reaction 3 proceeds rapidly and stifles additional chloride-induced corrosion.
Calcium nitrite must be added to the concrete in sufficient quantity to neutralize the effect of chloride ions over the life of a structure. Typical dosage rates range from 2 to 4 percent by weight of cement (for W. R. Grace PCI calcium-nitrite admixture). Calcium nitrite has been shown to be effective for chloride to nitrite ratios exceeding 1.5 to 1.0 percent by weight based on original nitrite content (3). At a dosage of 2 percent by weight of cement, calcium nitrite would inhibit the corrosive action if the chloride content in a deck were 2 to 3 percent by weight of cement. This is significant, since 0.3 to 1 percent of chloride by weight of cement in the deck is sufficient to cause corrosion of reinforcing steel, based upon European experience (op. cit. 1). Protection provided by calcium nitrite has been analyzed experimentally and determined to be effective in preventing corrosion at very high chloride levels, 3.6 to 22.6 lbs of chloride per cubic yard of concrete (approximately 0.8 to 5 percent by weight of cement) (4). The potential corrosion-inhibiting properties of calcium nitrite have warranted construction of experimental bridges incorporating its use. That has been done previously by other state highway agencies including Michigan, New Hampshire, Rhode Island, North Carolina, Pennsylvania, and Illinois.
