Corrosion is a chemical process of destruction of materials due to reaction with environmental conditions. The most prominent of the various factors of corrosion is atmospheric corrosion which causes corrosion of steel. Appreciable erosion begins only when the relative humidity of the air exceeds about 65%. There is no risk of corrosion in dry, pure air and below freezing point of water.
Corrosion can be defined as the involuntary destruction of substances such as metals and mineral building materials by the surrounding media.
Corrosion of steel results in reduction of cross-section area of steel and also causes cracks and splits of covered concrete. Due to the reduction in cross-section, the load carrying capacity is reduced, as well as a decrease in the elongation properties and fatigue strength.
Reinforcement corrosion occurs due to the presence of chloride and sulfate over a significant limit in reinforced cement concrete construction and when sufficient alkali is not received within the concrete to maintain the steel in a positive state.
The following factors are responsible for the corrosion of steel reinforcement in concrete structure.
1. quality of concrete
Concrete consists of coarse aggregates, fine aggregates, cement and water. Right quality of material with proper w/c ratio, right mixing, adequate compaction by tamping or vibration and proper curing yields good quality concrete. If any of the above steps are not done in a specified manner, it will result in not so good concrete and there is a possibility of corrosion of the reinforcement.
High strength concrete, i.e. dense concrete is impervious to a large degree and generally resists the carrion of embedded steel.
2. Cover Thickness of Concrete Reinforcement
The reinforcement is protected by a suitable concrete cover over it. The greater the thickness of the cover, the greater the degree of protection against various climatic and other environmental conditions. For different structural members, the thickness of the cover should be different depending on their importance and degree of risk. The evenness of the concrete cover on the reinforcement is also very important for its corrosion protection.
3. condition of reinforcement
The surface condition of steel reinforcement, when placed in concrete, affects its corrosion rate. If the reinforcement is contaminated with salt or is badly corroded, the corrosive action on the reinforcement is promoted rapidly once it is applied to the concrete.
4. Effects of environment and other chemicals
Chemicals are the main source of the spoilage process, either from the environment or from within the material making the concrete. The attack of chemicals causes cracks in the concrete, which is the first sign of deterioration. The effect of chemicals is mainly due to the presence of salt, carbonation, chloride attack and reaction of sulfates with tricalcium aluminate.C3a) is present in cement.
Concrete is an intimate mixture of cement, aggregate and water which is highly alkaline in green state. The hydration of cement develops calcium hydroxide which increases pH value 12.5 . So far, In such alkaline conditions, the reinforcing steel is covered with a film of oxide which protects the steel.
5. porosity of concrete
The penetration of aggressive chemicals is possible due to the permeability or porosity of the concrete. The porosity of concrete depends on the size, distribution and consistency of the capillary pores. It depends on the w/c ratio for a given degree of hydration. Porosity also depends on other factors, such as
- age of concrete
- degree of compactness
- Aggregate Size and Grading
- type of cement
6. effect of high thermal stress
normal concrete 100 . can withstand temperatures up to0C. Beyond this temperature the corrosion of concrete starts. Concrete in industrial plants and power stations requires special protective measures otherwise thermal cracks may develop in the concrete. Cracked concrete structures are affected by environmental chemicals and the process of corrosion is triggered.
7. freezing and thawing conditions
In colder regions, moisture tends to accumulate in the pores of the concrete. Ice formation gives rise to volumetric expansion which can be the pressure of the burst in addition to the surrounding solid mass. This results in the development of cracks in the concrete and can cause corrosion of the reinforcement.
Degradation of concrete can be due to corrosion of concrete/reinforcement steel or the formation of elaborate chemical compounds such as Calcium Silicate Hydrate (CSH) in an aggressive environment. The damage caused by corrosion of steel is enormous. In order to produce durable concrete and resist the harmful effects of aggressive environments, concrete must be produced with extreme care.
The following methods will help protect concrete reinforcement from corrosion.
1. improve the quality of concrete
- by adopting a rich mix: High cement content and low w/c ratio give strong and impervious concrete
- Adopting the best mixing ratio: Impermeable concrete can be produced by designing the most suitable mixing ratio
- Efficient compaction during casting: It gives dense concrete with minimum voids
- leak proof formwork: It reduces the leakage of cement solution during casting of concrete.
- salt free sandThe salt content in the mixture can be reduced by thoroughly rinsing the sand.
- using plasticizers: The use of plasticizer improves the working ability without increasing the water content
- Using sulfate resistant cement and pozzolana cement
2. Increasing depth of concrete cover for reinforcement
The additional cover depth increases the time for the penetration of corrosive agents. Such a measure increases the weight due to additional concrete requiring structural design changes. When surfaces of concrete members are exposed to the action of harmful chemicals, acids, vapours, saline environments, sulfurous fumes, etc., an increased thickness of the cover should be provided.
As per the observation, the increase in the thickness of cover may 15 mm and 40 mm . in between, Total cover thickness 50 mm. should not exceed, Concrete cover of more than 50 mm is not recommended as it may increase the width of the crack which may allow direct penetration of material harmful to the reinforcement.
3. Concrete Coatings and Sealers
When untreated reinforcing bars are used, the best method is to apply protective coatings to the concrete surface to seal the ingress of moisture, carbon dioxide and chloride.
The surface of dry concrete should be roughened with a chisel. Then, a practical mix of 1:3 cement sand mortar To be applied after properly watering the surface by watering the concrete surface to a thickness of 6 mm. surface should be finished with clear cement slurry consisting of water and cement in the ratio 2:1,
In this type of treatment, the zinc itself becomes a sacrificial anode and then protects the bar from corrosion for about five years. This method is used when no better treatment is available.
5. Fusion Bonded Epoxy Coating (FBEC)
Worldwide today, Fusion Bonded Epoxy Coating (FBEC) has proven to be the most effective, reliable and long-term economical method of anti-corrosive treatment for reinforcing bars.
It is applied directly to the reinforcing steel which prevents corrosion by isolating and insulating the steel from the corrosive environment. These coatings protect against external and internal corrosive agents.
6. coating of rebars
The erosion of the bar can be prevented by applying proper coating on the bar. The coating can be one of the following:
- Chemical compound
- Metallic Epoxy Coating
- Fusion Bonded Epoxy
7. Proper storage and stacking of reinforcing steel
Steel reinforcement should be stored in such a way as to avoid deformation and prevent degradation and corrosion. It is desirable to coat the reinforcement with cement wash before stacking to prevent scaling and corrosion.
In case of long storage, reinforcement bars should be placed above ground level at least 150 mm, Also in the coastal zone or in case of long storage; A coat of cement wash will be given to prevent scaling and rusting.
article written by
- Sanjay Kumar
- Virendra Kumar
- M Prasad
Originally published on NSCP – 2001