Can some one explain the root cause of this defects and rectification procedure pls?
- Thread starter Srinivasan
- Start date
You have not mentioned what part of the structure it is...!!
Looking at the pattern of cracking it may be termed as CARBONATION type of corrosion.
Concrete is alkaline, and when it has just been poured its alkalinity approaches pH 13.
It is this high alkalinity that protects the steel from oxygen and water by forming a thin oxide layer on the steel, thus preventing the metal atoms from dissolving, ie corroding. This protection is known as the passive layer.
This passive layer does not actually stop corrosion; it reduces the corrosion rate to an insignificant level. For steel in concrete, the passive corrosion rate is typically 0.1µm per year. Without the passive film, the steel would corrode at rates at least 1,000 times higher.
Rust has a lower density than metal, so it expands as it forms. As it does, it cracks and damages the surrounding concrete, further damaging the surrounding passive layer.
Corrosion can still occur however when the passive layer breaks down and becomes compromised. The deterioration of the protection occurs when the alkalinity of the concrete is reduced or when the chloride concentration in concrete is increased to a certain level. This is mainly caused by the natural process of the gases in the air.
CARBONATION PROCESS:
The carbonation of concrete is one of the main defects in concrete, causing the reinforcement to corrode. Along with the breakdown of the passive layer, oxygen and moisture are the other components required for corrosion of embedded steel.
Carbon dioxide (CO2), which is a natural gas present in the air, and sulphur dioxide (SO2), which is increased by industrial production, combine with the moisture (CO2) in the atmosphere and react with the calcium hydroxide in the concrete producing calcium carbonate and calcium sulphate (gypsum).
This chemical process and the reaction of the material attacks the concrete and reduces the natural alkalinity. The layer of affected concrete is known as the carbonated layer, and when the steel reinforcement falls within this, it loses its passive layer.
The oxygen and moisture then penetrate the concrete and react with the steel causing it to corrode. In actual practice, CO2 present in the atmosphere, permeates into concrete and carbonates it naturally, reducing the alkalinity of the concrete. The pH value of pore water in the hardened cement paste, which was around pH 13, will be reduced to around pH 9.0. When all the Ca(OH)2 has become carbonated, the pH value will reduce to about pH 8.3, at which level the passive film on the steel is not stable. The loss of the passivity usually occurs around pH 11.
Water is essential for carbonation, and relative humidity ranging from about 50% to 70% creates an ideal environment for the reactions to occur. Concrete with a relative humidity lower than 40% is less susceptible to carbonation because there is insufficient water to dissolve carbon dioxide. At a relative humidity in excess of 90%, when pores are filled with water, carbonic acid is unable to penetrate the saturated pores and diffuse throughout the concrete, again preventing carbonation.
A pH reduction in steel-reinforced concrete is more troublesome than in non-steel-reinforced concrete. The highly alkaline environment of concrete, which usually has a pH in excess of 12, creates a protective, passivating oxide layer around steel, protecting the reinforcement from corrosion.
TESTS FOR CARBONATION
The first time that carbonation is evident is usually when the concrete around the reinforcement spalls and cracks. Over time this will worsen and expose the corroded reinforcement.
If the depth of carbonation needs to be established, then a simple method to test this is to treat the concrete with phenolphthalein. Phenolphthalein comes in powder form, and will need to be dissolved in alcohol for use in this test.
In order to undertake the test, a section of concrete should be broken away and the fresh surface sprayed with the solution. Alternatively, it can be applied to split cores, the powder from drill holes, or allowed to fall on indicator-impregnated paper.
Phenolphthalein is an acid base indicator, which makes it extremely useful for testing for signs of carbonation. The solution will turn bright pink if it interacts with an alkaline and will even show up to a pH 9.5. As unaffected concrete has a high pH, it will turn pink.
But if concrete is carbonated, it will remain uncoloured. It should be noted that the pink colour indicates that enough Ca(OH)2 is present but it may have been carbonated to a lesser extent.
TREATMENT:
Treatment will consist of following procedure:
PROTECTING CONCRETE IN THE FUTURE
A long term solution to inhibit further carbonation would be to restrict the entry of carbon dioxide by applying a coating to the concrete that will act as a protective barrier. The coating should prevent the ingress of liquid water but allow water vapor to pass through.
There are various products on the market specifically designed for this purpose. The necessary surface preparation has to be undertaken, which will usually consist of cleaning the surface back to a sound finish.
The coating treatment should then be checked for its compatibility with the concrete, and the number of coats and total thickness required also considered.
(Monopol®-456 is a concrete specific coating system that provides protection against chloride ion penetration, carbonation, UV radiations and weathering effects. Monopol®-456 has excellent crack bridging ability and excellent biological resistance. )
Hope I have addressed your question Sirji...
Looking at the pattern of cracking it may be termed as CARBONATION type of corrosion.
Concrete is alkaline, and when it has just been poured its alkalinity approaches pH 13.
It is this high alkalinity that protects the steel from oxygen and water by forming a thin oxide layer on the steel, thus preventing the metal atoms from dissolving, ie corroding. This protection is known as the passive layer.
This passive layer does not actually stop corrosion; it reduces the corrosion rate to an insignificant level. For steel in concrete, the passive corrosion rate is typically 0.1µm per year. Without the passive film, the steel would corrode at rates at least 1,000 times higher.
Rust has a lower density than metal, so it expands as it forms. As it does, it cracks and damages the surrounding concrete, further damaging the surrounding passive layer.
Corrosion can still occur however when the passive layer breaks down and becomes compromised. The deterioration of the protection occurs when the alkalinity of the concrete is reduced or when the chloride concentration in concrete is increased to a certain level. This is mainly caused by the natural process of the gases in the air.
CARBONATION PROCESS:
The carbonation of concrete is one of the main defects in concrete, causing the reinforcement to corrode. Along with the breakdown of the passive layer, oxygen and moisture are the other components required for corrosion of embedded steel.
Carbon dioxide (CO2), which is a natural gas present in the air, and sulphur dioxide (SO2), which is increased by industrial production, combine with the moisture (CO2) in the atmosphere and react with the calcium hydroxide in the concrete producing calcium carbonate and calcium sulphate (gypsum).
This chemical process and the reaction of the material attacks the concrete and reduces the natural alkalinity. The layer of affected concrete is known as the carbonated layer, and when the steel reinforcement falls within this, it loses its passive layer.
The oxygen and moisture then penetrate the concrete and react with the steel causing it to corrode. In actual practice, CO2 present in the atmosphere, permeates into concrete and carbonates it naturally, reducing the alkalinity of the concrete. The pH value of pore water in the hardened cement paste, which was around pH 13, will be reduced to around pH 9.0. When all the Ca(OH)2 has become carbonated, the pH value will reduce to about pH 8.3, at which level the passive film on the steel is not stable. The loss of the passivity usually occurs around pH 11.
Water is essential for carbonation, and relative humidity ranging from about 50% to 70% creates an ideal environment for the reactions to occur. Concrete with a relative humidity lower than 40% is less susceptible to carbonation because there is insufficient water to dissolve carbon dioxide. At a relative humidity in excess of 90%, when pores are filled with water, carbonic acid is unable to penetrate the saturated pores and diffuse throughout the concrete, again preventing carbonation.
A pH reduction in steel-reinforced concrete is more troublesome than in non-steel-reinforced concrete. The highly alkaline environment of concrete, which usually has a pH in excess of 12, creates a protective, passivating oxide layer around steel, protecting the reinforcement from corrosion.
TESTS FOR CARBONATION
The first time that carbonation is evident is usually when the concrete around the reinforcement spalls and cracks. Over time this will worsen and expose the corroded reinforcement.
If the depth of carbonation needs to be established, then a simple method to test this is to treat the concrete with phenolphthalein. Phenolphthalein comes in powder form, and will need to be dissolved in alcohol for use in this test.
In order to undertake the test, a section of concrete should be broken away and the fresh surface sprayed with the solution. Alternatively, it can be applied to split cores, the powder from drill holes, or allowed to fall on indicator-impregnated paper.
Phenolphthalein is an acid base indicator, which makes it extremely useful for testing for signs of carbonation. The solution will turn bright pink if it interacts with an alkaline and will even show up to a pH 9.5. As unaffected concrete has a high pH, it will turn pink.
But if concrete is carbonated, it will remain uncoloured. It should be noted that the pink colour indicates that enough Ca(OH)2 is present but it may have been carbonated to a lesser extent.
TREATMENT:
Treatment will consist of following procedure:
- Undertake a hammer test to establish all loose areas of concrete.
- Hack off loose concrete from around the exposed steel bars.
- Grit/Sandblast to remove corrosion and other deposits from the exposed steel reinforcement
- Apply an alkaline rust converter (FEOVERT- Krishna Conchem) a proprietary bar primer to the surface of the reinforcement. This could either be alkali based; encapsulating, such as epoxy resin; or sacrificial, normally zinc-rich paints.
- Apply 2 coats of (IPNET-RB - Krishna Conchem) to the steel reinforcement.
- Apply by spraying CORROSION INHIBITOR (KP-100 - K.C) over the total area which is spalled and to be repaired.
- Patch repair areas of concrete using a cementitious material and sand, and a polymer dispersion.either you can use ( MONOBOND or MOLITH.PMM)
- If concrete has spalled beyond the core and hammer tests or UPV tests indicate loss of strength of concrete then FIBRE-WRAPPING can be undertaken to address the same.
PROTECTING CONCRETE IN THE FUTURE
A long term solution to inhibit further carbonation would be to restrict the entry of carbon dioxide by applying a coating to the concrete that will act as a protective barrier. The coating should prevent the ingress of liquid water but allow water vapor to pass through.
There are various products on the market specifically designed for this purpose. The necessary surface preparation has to be undertaken, which will usually consist of cleaning the surface back to a sound finish.
The coating treatment should then be checked for its compatibility with the concrete, and the number of coats and total thickness required also considered.
(Monopol®-456 is a concrete specific coating system that provides protection against chloride ion penetration, carbonation, UV radiations and weathering effects. Monopol®-456 has excellent crack bridging ability and excellent biological resistance. )
Hope I have addressed your question Sirji...
