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concrete carbonation ~ the construction civil

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HOME CONSTRUCTION GALLERY VIDEOS ADVERTISE HERE LINKS CONTACT US Subscribe to RSS Feed BUILDING MATERIALS CONCRETE IRRIGATION ENGG ENGG TERMS FOUNDATIONS CEMENT FLOORING ADMIXTURES CATEGORIES Admixtures Aggregates Brick Brick Flooring Brick Masonry Buildings Cement Ceramic Tiles Cofferdams Concrete Construction Equipments Cork Flooring Curing Dams Do-It-Yourself Drainage Drawings Fasteners Fencing Ferrocement Finishes Flooring Foundations Glass Floor Irrigation Engineering IS Codes Joints Masonry Mosaic Flooring Pavers Pictures Plumbing Post Tensioning Products Retaining Walls Roofing Rubber Flooring Self Compacting Concrete Shotcrete Shrinkage Site Investigation Softwares Soil Soil Investigation Stone Masonry Carbonation of concrete is a process by which carbon dioxide from the air penetrates into concrete and reacts with calcium hydroxide to form calcium carbonates. The conversion of Ca(OH)2 into CaCO3 by the action of CO2 results in a small shrinkage. CO2 by itself is not reactive. In the presence of moisture, CO2 changes into dilute carbonic acid which attacks the concrete and also reduces alkalinity of concrete. Air contains CO2. The concentration of CO2 in rural air may be about 0.03 per cent by volume. In large cities the content may go up to 0.3 per cent or exceptionally it may go up to even 1.0 per cent. In the tunnel, if not well ventilated the intensity may be much higher. The pH value of pore water in the hardened concrete is generally between 12.5 to 13.5 depending upon the alkali content of cement. The high alkalinity forms a thin passivating layer around steel reinforcement and protect it from action of oxygen and water. As long as steel is placed in a highly alkaline condition, it is not going to corrode. Such condition is known as passivation. In actual practice CO2 present in atmosphere in smaller or greater concentration, permeates into concrete and carbonates the concrete and reduces the alkalinity of concrete. The pH value of pore water in the hardened cement paste which was around 13 will be reduced to around 9.0. When all the Ca(OH)2 has become carbonated, the pH value will reduce upto about 8.3 In such a low pH value, the protective layer gets destroyed and the steel is exposed to corrosion. The carbonation of concrete is one of the main reasons for corrosion of reinforcement. Of course, oxygen and moisture are the other components required for corrosion of embedded steel. The carbonation of concrete is one of the main reasons for corrosion of reinforcement. Of course, oxygen and moisture are the other components required for corrosion of embedded steel. Rate of Carbonation: The rate of carbonation depends on the following factors. 1. The level of pore water i.e., relative humidity. 2. Grade of concrete 3. Permeability of concrete 4. Whether the concrete is protected or not 5. depth of cover 6. Time It is interesting to know that if pore is filled with water the diffusion of CO2 is very slow. But whatever CO2 is diffused into the concrete, is readily formed into dilute carbonic acid reduces the alkalinity. On the other hand if the pores are rather dry, that is at low relative humidity the CO2 remains in gaseous form and does not react with hydrated cement. The moisture penetration from external source is necessary to carbonate the concrete. The highest rate of carbonation occurs at a relative humidity of between 50 and 70 percent.The rate of carbonation depth will be slower in case of stronger concrete for the obvious reason that stronger concrete is much denser with lower W/C ratio. It again indicates that the permeability of the concrete, particularly that of skin concrete is much less at lower W/C and as such the diffusion of CO2 does not take place faster, as in the case of more permeable concrete with higher W/C ratio. Depth of cover plays an important role in protecting the steel from carbonation. Measurement of depth of carbonation: A common and simple method for establishing the extent of carbonation is to treat the freshly broken surface of concrete with a solution of phenophthalein in diluted alcohol. If the Ca(OH) is unaffected by CO2 the colour turns out to be pink. If the concrete is carbonated it will remain uncloured. It should be DO YOU KNOW ? Why you cure concrete ? Why you consider creep ? Whether creep is beneficial? UNIT CONVERTER Category: A B Popular Topics Cement Types Some of the different cement types are listed below, Ordinary Portland Cement Rapid Hardening Cement Extra Rapid Hardening Cement Sulphate... Ads by Google Concrete Testing Color Concrete Fly Ash Concrete Concrete Materials Concrete Durability Tests Micro-cracks in concrete can easily be detected with this NDT device www.maverick-ndt.com Concrete Carbonation Ads by Google Recycled Concrete Concrete Screed Mix Concrete Concrete Mortar Concrete Carbonation ~ The Construction Civil http://www.theconstructioncivil.com/2010/02/concrete-carbonation.html 1 of 2 21/02/2011 02:49 م

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Page 1: Concrete Carbonation ~ The Construction Civil

HOME CONSTRUCTION GALLERY VIDEOS ADVERTISE HERE LINKS CONTACT US Subscribe to RSS Feed

BUILDING MATERIALS CONCRETE IRRIGATION ENGG ENGG TERMS FOUNDATIONS CEMENT FLOORING ADMIXTURES

CATEGORIES

Admixtures

Aggregates

Brick

Brick Flooring

Brick Masonry

Buildings

Cement

Ceramic Tiles

Cofferdams

Concrete

ConstructionEquipments

Cork Flooring

Curing

Dams

Do-It-Yourself

Drainage

Drawings

Fasteners

Fencing

Ferrocement

Finishes

Flooring

Foundations

Glass Floor

Irrigation Engineering

IS Codes

Joints

Masonry

Mosaic Flooring

Pavers

Pictures

Plumbing

Post Tensioning

Products

Retaining Walls

Roofing

Rubber Flooring

Self CompactingConcrete

Shotcrete

Shrinkage

Site Investigation

Softwares

Soil

Soil Investigation

Stone Masonry

Carbonation ofconcrete is a processby which carbondioxide from the airpenetrates intoconcrete and reactswith calciumhydroxide to formcalcium carbonates.The conversion ofCa(OH)2 into CaCO3by the action of CO2results in a smallshrinkage.

CO2 by itself is notreactive. In thepresence ofmoisture, CO2changes into dilutecarbonic acid whichattacks the concrete and also reduces alkalinity of concrete.

Air contains CO2. The concentration of CO2 in rural air may be about 0.03 percent by volume. In large cities the content may go up to 0.3 per cent orexceptionally it may go up to even 1.0 per cent. In the tunnel, if not wellventilated the intensity may be much higher.

The pH value of pore water in the hardened concrete is generally between12.5 to 13.5 depending upon the alkali content of cement. The high alkalinityforms a thin passivating layer around steel reinforcement and protect it fromaction of oxygen and water. As long as steel is placed in a highly alkalinecondition, it is not going to corrode. Such condition is known as passivation.

In actual practice CO2 present in atmosphere in smaller or greaterconcentration, permeates into concrete and carbonates the concrete andreduces the alkalinity of concrete. The pH value of pore water in the hardenedcement paste which was around 13 will be reduced to around 9.0. When allthe Ca(OH)2 has become carbonated, the pH value will reduce upto about 8.3In such a low pH value, the protective layer gets destroyed and the steel isexposed to corrosion.

The carbonation of concrete is one of the main reasons for corrosion ofreinforcement. Of course, oxygen and moisture are the other componentsrequired for corrosion of embedded steel.

The carbonation of concrete is one of the main reasons for corrosion ofreinforcement. Of course, oxygen and moisture are the other componentsrequired for corrosion of embedded steel.

Rate of Carbonation:

The rate of carbonation depends on the following factors.

1. The level of pore water i.e., relative humidity.2. Grade of concrete3. Permeability of concrete4. Whether the concrete is protected or not5. depth of cover6. Time

It is interesting to know that if pore is filled with water the diffusion of CO2 isvery slow. But whatever CO2 is diffused into the concrete, is readily formedinto dilute carbonic acid reduces the alkalinity. On the other hand if the poresare rather dry, that is at low relative humidity the CO2 remains in gaseousform and does not react with hydrated cement. The moisture penetrationfrom external source is necessary to carbonate the concrete.

The highest rate of carbonation occurs at a relative humidity of between 50and 70 percent.The rate of carbonation depth will be slower in case ofstronger concrete for the obvious reason that stronger concrete is muchdenser with lower W/C ratio. It again indicates that the permeability of theconcrete, particularly that of skin concrete is much less at lower W/C and assuch the diffusion of CO2 does not take place faster, as in the case of morepermeable concrete with higher W/C ratio. Depth of cover plays an importantrole in protecting the steel from carbonation.

Measurement of depth of carbonation:

A common and simple method for establishing the extent of carbonation is totreat the freshly broken surface of concrete with a solution of phenophthaleinin diluted alcohol. If the Ca(OH) is unaffected by CO2 the colour turns out tobe pink. If the concrete is carbonated it will remain uncloured. It should be

DO YOU KNOW ?

Why you cure concrete ?

Why you consider creep ?

Whether creep is beneficial?

UNIT CONVERTER

Category:

A

B

Popular TopicsCement TypesSome of the different cement types arelisted below, Ordinary Portland CementRapid Hardening Cement Extra RapidHardening Cement Sulphate...

Ads by Google

Concrete Testing

Color Concrete

Fly Ash Concrete

Concrete Materials

Concrete Durability TestsMicro-cracks in concrete can easily be detected with this NDT devicewww.maverick-ndt.com

Concrete Carbonation

Ads by Google

Recycled Concrete

Concrete Screed

Mix Concrete

Concrete Mortar

Concrete Carbonation ~ The Construction Civil http://www.theconstructioncivil.com/2010/02/concrete-carbonation.html

1 of 2 م 02:49 21/02/2011

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