White dealing with the durability aspect of concrete, the chloride effect on concrete is the most important factor as it causes the corrosion of reinforcement of concrete. The statistical studies have shown that more than 40% structures have failed due to the corrosion of reinforcement steel.
Due to high alkaline nature of calcium hydroxide (pH value about 13) in concrete a protective oxide film is formed around the surface of the reinforcement. This protection is called passivity. This protective passivity cover or layer may be destroyed due to the carbonation.
This protective cover may also be destroyed or lost due to the presence of chlorides in the presence of oxygen and water. In reality the corrosive action of chloride is more serious than any other cause. Actually the sulphate attack destroys concrete whereas chloride attacks the reinforcement. The volume of corroded reinforcement may go upto six times of its original volume causing cracks and spalling of concrete.
Source of Chlorides:
Chlorides may enter into the concrete from the following sources:
i. From cement of the concrete.
ii. Water mixed in concrete.
iii. Aggregates of the concrete.
iv. Admixtures added to the concrete.
v. Chlorides may also enter by diffusion from atmosphere.
Earlier Bureau of Indian Standards had recommended the maximum content of chloride in cement as 0.05%, but now it has been increased to 0.1%. The revised IS-456-2000 has specified the chloride content as chlorine (CI), in concrete at the time of placing as shown Table 17.11.
The amount of chloride needed for starting or initiating the corrosion of reinforcement partly depends on the pH value of the pore water in the concrete. At pH value less than 11.5, the corrosion may develop even without chlorides. At pH value greater than 11.5, more amount of chloride is required for initiating corrosion.
Out of the total quantity of chloride present in concrete, partly is soluble and partly insoluble type of chloride. The insoluble chloride is chloro-aluminate. The soluble chloride is responsible for initiating corrosion of reinforcement. Table 17.12 below shows, limiting chloride content corresponding to pH value of concrete.
Corrosion of Reinforcement:
Corrosion of steel is an. electro chemical process, which usually takes place when two dissimilar metals are in electrical contact in the presence of moisture and oxygen. The same process takes place in the steel alone due to the difference in electro-chemical potential on the surface which forms anodic and cathodic regions, connected by the electrolyte in the form of the salt solution in the hydrated cement.
The positively charged ferrous ions Fe++ at the anode pass into solution while negatively charged free electrons e pass along the steel into the cathode, where they are absorbed by the constituents of the electrolyte and combine with water and oxygen to form hydroxyl ions (OH–). Then they combine with the ferrous ions to form ferric hydroxide. This hydroxide is converted to rust by further oxidation.
The reactions can be written as follows:
From these equations it can be seen that oxygen is consumed, but water is regenerated and needed only for the process to continue.
Thus it can be noted that in a completely dry atmosphere no corrosion can take place, most probably below 40% relative humidity. It is also noticed that not much corrosion of reinforcement takes place if concrete is fully immersed in water except when water can entrain air. 70 to 80% relative humidity is most-congenial for the development of corrosion of reinforcement. At higher than 80% relative humidity, the diffusion of oxygen is reduced considerably and atmospheric conditions along the steel are more uniform. It has also been observed that the products of corrosion occupy volume about six times the original volume of steel depending upon the oxidation state.
Reaction of Chloride Ions:
Chloride ions present in the cement paste around the reinforcement react at anode, the reaction forms hydrochloric acid (HCl), which destroys the passive protective layer formed around the steel. In this situation the surface of the steel acts as anode and the passive surface coating acts as cathode.
The Chemical reaction may be as follows:
Thus C I̅ is regenerated. The other reactions, specially the cathodic reaction are same as in the absence of chlorides. It should be noted that rust contains no chloride, though ferric chloride is formed at an intermediate stage.
Due to the formation of hydrochloric acid in the pit, once it is formed, the pit remains active and increases in depth. The pitting corrosion takes place at a certain potential, called the process is started, the potential is higher in dry concrete than at high humidities. As soon as a pit forming processes started, the potential of the steel in its neighbourhood drops and no new pit is formed for some times. Eventually there may be large scale spread of the corrosion and general spread of corrosion may take place in the presence of large amount of chlorides.
It is important to remember that in the presence of chlorides as well as in their absence, the electrochemical corrosion takes place only in the presence of water and oxygen. In this reaction only the oxygen is consumed. Even in the presence of large quantities of chlorides there is no corrosion of the dry concrete. It has been noticed that if the total chloride ions content is less than 0.4% by mass of cement, the corrosion of reinforcement is small in ordinary port-land cement concrete. Only soluble chlorides are responsible for reinforcement corrosion. Dry chlorides have no effect on corrosion.
Effect of Corrosion:
The increased volume of rust exerts pressure or thrust on the cover of the concrete, resulting in cracks, spalling or delamination as shown in Fig.17.7.
The foremost corrosion control method is the production of good quality concrete and good construction practices. Here other than production of good quality concrete measures have been discussed in brief.
These measures are as follows:
i. Metallurgical methods
ii. Use of corrosive preventive materials
iii. Coatings to reinforcement
iv. Coatings to concrete
v. Cathode protection
vi. Design and detailing.
i. Metallurgical Methods:
To make the steel more corrosion resistant, its structure can be changed by metallurgical processes. This can be achieved by rapid quenching of the hot bars by series of water jets or keeping the hot steel bars for a short period in water bath. Many other methods may also be applied to improve the corrosion and other mechanical properties of the steel.
ii. Use of Corrosive Preventive Materials or Inhibitors:
Corrosion can be prevented or delayed by the use of certain corrosion preventive materials such as phosphates, nitrites, Benzonates etc. Out of the available materials, a admixture based on calcium nitrite is most widely used. It is added to the concrete during mixing process. The dosage of 10 to 30 litres per cubic metre of concrete has been found quite effective depending upon the quantity of chloride present in concrete.
In the presence of high PH in concrete, a protective layer of ferric oxide is formed around the steel which protects it from carbonation. However this protective layer also contains some ferrous oxide, which can start corroding steel when the chloride ions reach the steel. The nitrite ions present in corrosion preventing admixtures oxidise the ferrous oxide to ferric oxide, stabilizing the protective layer even in the presence of chlorides. However the concentration of nitrite must be sufficient to neutralize the diffusing chloride ions.
Calcium nitrite, a corrosion preventive material is available in liquid form containing about 30% calcium nitrite solids by weight. Higher the quantity of corrosion preventive material added to the solution, longer the period after which the corrosion of steel starts. For structures in chloride environment, the quantity of calcium nitrite in solution should not be less than 18 litres/m3 of concrete. As in such situations the level of 7 kg chloride ions per cubic metre of concrete reaches during the service life of the structure.
iii. Coating of Reinforcement:
The object of coating a steel bar is to provide it a durable barrier against the aggressive materials such as chlorides or oxygen. The coatings should be strong enough to withstand the effects of pouring of concrete, its compaction and finishing and fabrication of reinforcement cage etc. A simple cement slurry coating is a cheap method for temporary protection against rusting of steel.
Galvanising the Reinforcement:
In the galvanising process the steel bars are dipped in molten zinc. A thin zinc coating takes place around the steel. The zinc surface reacts with the calcium hydroxide present in the concrete to form a passive or protective layer and prevents corrosion of steel.
iv. Coating of Concrete:
In the past, concrete was considered a durable material by itself needing no maintenance. But now due to the environmental pollution, industrial fumes, contamination of ground water etc. concrete structures also need extra maintenance. The coating of concrete members not only protects them from harmful effects of aggressive materials, but it also gives it an aesthetic look. Now providing protective coatings to the major concrete structures such as long bridges, flyovers, industrial buildings and chimneys have been made mandatory.
Massive structure members such as bridge piers and girders are of considerable dimensions. Freshly concrete made such members contain plenty of water in the pores structure. It takes a long time to dry. Hence freshly made such concrete structures should not be coated with such materials which will seal off their pores and prevent the internal moisture to go out as per prevailing atmospheric conditions.
The moisture trapped in the body of the concrete can do unimaginable harm to the durability of the concrete and will damage the coating also. For better durability of concrete, the protective coating should be such which will allow the water vapours from inside to outside and from outside to inside. But water as such should not be allowed to enter from outside.
v. Cathodic Protection:
In this method an electrode is embedded in the concrete above the reinforcement steel. A D.C. current is passed through this electrode. This electrode serves as anode and the reinforcement steel which is connected to the negative terminal of the D.C. source acts as cathode. In this process the external anode is subjected to corrosion and the cathodic reinforcement is protected from corrosion, hence the name cathodic protection.
This method of reinforcement steel protection is found very effective. It is well known and extensively used method for reinforcement corrosion prevention in advanced countries. In India it is not used due to its high cost and long term monitoring.
vi. Design and Detailing:
The structural designer should take all necessary precautions in the designing and detailing with respect to spacing between reinforcement bars, to facilitate vibration of concrete for proper compaction, and give proper cover to reinforcement and to restrict the crack width etc.
Nominal Cover to Reinforcement:
The nominal cover is applicable to all reinforcement:
(a) Columns. In a column, for longitudinal reinforcement, the nominal cover should not be less than 40 mm or less than the diameter of the bar.
(b) In case of columns where minimum dimension is 200 mm or less, and the diameter of the reinforcing bar does not exceed 12 mm, the nominal cover of 25 mm should be provided.
(c) For footings a minimum cover of 50 mm should be provided.
The nominal cover to all reinforcements including stripes or links should be provided after taking into account the specified period of fire resistance upto 4 hours.
The nominal cover suggested by IS 456-2000 is shown in table 17.13 below: