12/31/2013

Admixtures

Corrosion Inhibiting Admixtures

Team 7

Function

They are chemical admixtures use to increase the passivation state of reinforcement and other embedded steel in concrete structure this can inhibit the corrosion process over extended periods when passivation would otherwise have been lost as a result of chloride ingress or carbonation.

Corrosion inhibiting admixture added to concrete during production are called “integral” corrosion inhibitors. Migratory admixtures are also available which can be applied to the hardened concrete but these are not admixtures

The most common cause of corrosion is pitting corrosion due to the ingress of chloride ions through the covering concrete abs subsequent diffusion down to the embedded steel. Although corrosion inhibitors can raise the corrosion threshold of the steel, they are not alternative to producing impermeable, durable concrete which limits chloride diffusion.

Advantages

Significantly reduce maintenance cost of reinforced concrete structure throughout a typical service life span.

It is compatible with all type of cement and blends with other binders such as fly ash and ground granulated blastfurnance slag (ggbs).

Over dosing does not have a significant effect on properties of concrete

Disadvantages

It is costlier.

Calcium nitrate as admixture act as accelerator.

Skilled supervision is required.

Materials

The three most common generic types of corrosion inhibiting admixtures are:

1) Calcium nitrate(normally contain a residual amount of calcium nitrate)2) Amino acid.3) Amino alcohols blended with inorganic inhibiters.

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Dosages

Generally it is depend on the client's expected service lifespan and on range of factors that affect the durability of concrete. These include cement type, W/C ratio, concrete cover, ambient temperature and level of exposure to chlorides.

The dosage of amino acid based corrosion inhibitors is usually between 3 to 4% by weight of content.

For calcium nitrate it is 10 to 30 liters/meter cube but usually used between 10 and 20 liters/meter cube.

Both type can be used with other admixture types and their use with a high range water reducing admixture is usually recommended in order to ensure the quality and durability of the base concrete.

Mechanism

For calcium nitrate

The mechanism by which corrosion inhibitors operate is dependent on their chemical nature. Calcium nitrate base inhibitors converts or oxidize the ferrous oxide sites within protective passive oxide layer into ferric oxide(+3 oxidation state) which is more stable and less reactive than the +2 oxidation state, ferrous oxide, so when chloride ion reaches ferric oxide layer no reaction occurs- the steel is in passive state. It is anodic corrosion site on the steel that are protected against the chloride attack and for this reason nitrates are called anodic inhibitors. It is important that sufficient nitrate is present to counter the chloride ion. The dosages of admixture is therefore based on the predicted level of chloride the steel over the design life of the structure.

For Amino Acid

Amino acid based inhibitors coat the embedded steel with a monomolecular layer that keeps chlorides away from steel. They also inhibit the reaction of oxygen and water at the cathodic sites on steel which are essential part of corrosion process. As a result amino alcohols can be regarded as both anodic and cathodic inhibitors.

Effect on property of concrete

Strength

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Corrosion inhibitors have little effect on strength at later ages but may accelerate early strength development.

Workability

Workability of concrete containing this admixture will not significantly affected provided the water content of admixture is included as a part of mixing water.

Slump loss

Amino alcohol based inhibitors do not produce a significant change in the rate of sump loss.

In case of calcium nitrate based corrosion inhibitors, a faster rate of slump loss can be expected compared to an identical “control” concrete without containing the inhibitor. This is because of the accelerating effect of calcium nitrate. Some commercially available corrosion inhibitors based on calcium nitrate do contain a set retarder to offset the accelerating effect thus reducing the effect on slump loss

Setting time

Calcium based admixture do increase the setting time of concrete over a range of curing temperature unless they are formulated with a set retarder to offset the accelerating effect.

Air entrainment

Neither calcium nor amino acid based corrosion inhibitors cause air entrainment or effect the use of air entrainment admixture.

Bleeding

Neither calcium nor amino acid based corrosion inhibitors affect the rate at which concrete bleeds.

Heat of hydration

Neither calcium nor amino acid based corrosion inhibitors affect the maximum rise in temperature of concrete.

Volume deformation

Neither calcium nor amino acid based corrosion inhibitors significantly affect the creep or dying shrinkage of concrete.

Durability

The function of this admixture is to enhance the long term durability to reinforced concrete by providing enhanced passivation to the cathodic and or anodic areas of embedded steel.

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Structures built in the 1970’s with calcium nitrate have been reported to still contain the original of inhibitors and shows no sign of deterioration.

Some Corrosion Inhibitors produced by CICO are:-

Chorchem 60

Nature :- liquid

Compound :- single

Color :- amber

Self-life : - 1 yr.

Packaging :- 25 liter

Price : - 600 Rs/lt.

Usuage

Industrial plant

Jetties

Highway structure

Bridges

Tunnel

High-rise bldg.

Maritime structure

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Chorchem 210 I

Nature :- Solid

Compound :- Double

Color :- Grey

Self-life : - 1 Yr.

Packaging :- 25 kg

Price : - 450 rs/lt.

Usuage

Coastal structure

Water containment structure

Tank lining

Pipe lining

Deck coating

Corrosive environment

Health and safety of admixtures

Most admixtures are nonhazardous and pose no abnormal health and safety risk but as with all forms of chemicals it is essential that the material safety data sheets are read and understood before use. Mixer should be cleaned before use of and after use. It should at room temperature should not exceed 40 degree Celsius

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Some of the Manufacturers

FOSROC CETEX PERMA

GRM International RUIDA CICO

Some Structure in which this admixtures are used

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