Corrosion of Concrete Reinforcement

Corrosion is a chemical process of destruction of material because of its reaction with the environmental conditions. The most predominant among various factors of corrosion is the atmospheric corrosion which causes the rusting of steel. Appreciable corrosion only starts when the relative humidity of the air exceeds around 65%. In dry, pure air and below freezing point of water there is no danger of the corrosion.

Corrosion may be defined as the involuntary destruction of substances such as metal and mineral building materials by surrounding media.

Corrosion of steel results in reduction of cross-section area of steel and also causes cracks and splitting of cover concrete. Due to reduction of cross-section, the load carrying capacity is reduced, in addition to reduction of elongation properties and fatigue strength.

Factors Influencing Corrosion of Reinforcement

In reinforced cement concrete construction the corrosion of reinforcement takes place due to the presence of chlorides and sulphates beyond a critical limit and when sufficient alkalies is not obtained within the concrete to maintain steel in a positive condition.

The following factors are responsible for corrosion of steel reinforcement in concrete structure.

1. Quality of Concrete

Concrete consists of coarse aggregate, fine aggregate, cement and water. The right quality of materials with proper w/c ratio, correct mixing, adequate compaction by tamping or vibration and proper curing results in good quality concrete. If any of the above mentioned steps are not done in a specified manner, then that will result in a not so good concrete and there is a chance of corrosion of 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 suitable concrete covering over it. The greater the cover thickness more is the degree of protection against the various climatic and other environmental conditions. For various structural members, the cover thickness should be different depending upon their importance and degree of exposure. Evenness of concrete cover over the reinforcement is also very important for its corrosion protection.

3. Condition of Reinforcement

The surface condition of the steel reinforcement, at the time of its placing in concrete, affects its corrosion rate. If the reinforcement is contaminated with salt or badly corroded, the corrosive action on reinforcement after placement in concrete is promoted rapidly.

4. Effect of Environmental and Other Chemicals

Chemical either from environmental or from within the concrete making materials are the main source of deterioration process. Due to attack of chemicals, the concrete develops cracks, which is the first sign of deterioration. The effect of chemicals is mainly due to presence of salt, carbonation, chloride attack and reaction of sulphates with tricalcium aluminate (C₃A) present in cement.

Concrete is an intimate mixture of cement, aggregate and water which in the green state is highly alkaline. The hydration of cement develops calcium hydroxide which increases the pH value up to 12.5. 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 permeability or porosity of concrete. the porosity of concrete depends on size, distribution and continuity of capillary pores. This depends upon the w/c ratio for given degree of hydration. The porosity also depends upon other factors, such as

• Age of concrete
• Degree of compactness
• The size and grading of aggregate
• Type of cement

6. Effect of High Thermal Stress

Normal concrete can withstand temperature upto 100⁰C. Beyond this temperature the deterioration of concrete starts. The concrete in industrial plants and power stations required special protective measures otherwise the concrete may develop thermal cracks. Cracked concrete structures are consequently affected by the environmental chemical and the process of corrosion starts.

7. Freezing and Thawing Condition

In cold regions, the moisture in the pores of concrete freezes. The ice formation gives rise to volumetric expansion which may excess bursting pressure of surrounding concrete mass. This results in development of cracks in concrete and can lead to corrosion of reinforcement.

Remedial Measures to Protect Reinforcement from Corrosion

The deterioration of concrete may be due to either corrosion of concrete / reinforcement steel or formation of expansive chemical compounds such as calcium silicates hydrate (C-S-H) in aggressive environments. The loss due to corrosion of steel is heavy. To produce durable concrete and resist the harmful effects of aggressive environment, the concrete should be produced with utmost care.

The following methods will help to protect concrete reinforcement from corrosion.

1. Improving the Quality of Concrete

• By adopting the rich mix: Higher cement content and lower w/c ratio give stronger and impermeable concrete
• Adopting the best mix proportion: By designing the best suitable mix proportion the impermeable concrete can be produced
• Efficient compaction during casting: This gives dense concrete with minimum voids
• Leak proof formwork: This reduces the leakage of cement slurry during casting of concrete.
• Adopting salt free sand: The salt content of mix can be reduced by washing the sand properly.
• Using plasticizers: The use of plasticizers improves the workability without increasing the water content
• Using sulphate resisting cement and Pozzolana cement

2. Increasing Depth of Concrete Cover to Reinforcement

Extra cover depth lengthens the time for ingress of corroding agents. Such a remedy increases weight due to additional concrete requiring changes in structural design. Increased cover thickness should be provided when surfaces of concrete members are exposed to the action of harmful chemicals, acids, vapors, saline atmosphere, sulphurous smoke etc.

As per observation, the increase in cover thickness may be between 15 mm and 40 mm, the total cover thickness should not exceed 50 mm. concrete cover more than 50 mm is not recommended as it may give rise to increase crack widths which may further allow direct ingress of deleterious materials to the reinforcement.

3. Concrete Coating and Sealers

When untreated reinforcing bar is used, the best method is to apply protective coatings to concrete surface to seal entry of moisture, carbon dioxide and chlorides.

The dry concrete surface should be roughened by chiseling. Then, a workable mixture of 1:3 cement sand mortar should be applied on the concrete surface after watering over the surface properly by trowelling to a thickness of 6 mm. The surface should be finished with neat cement slurry consisting of water and cement in the ratio 2:1.

4. Galvanizing

In this type of treatment, Zinc itself becomes a sacrificial anode and then protects the bar from corrosion for five years approximately. This method is used when no superior treatment is available.

5. Fusion Bonded Epoxy Coating (FBEC)

Today the world over, fusion bonded epoxy coating (FBEC) has proved to be most effective, reliable and long-term economical method of anti corrosive treatment for reinforcing bar.

It is applied directly on 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 corrosion of rebars can be prevented by applying proper coating to rebars. The coating can be one of the following:

• Paint
• Chemical compound
• Metallic epoxy coating
• Fusion bonded epoxy

7. Proper Storage & Stacking of Reinforcing Steel

Steel reinforcement should be stored in such a way as to avoid distortion and to prevent deterioration and corrosion. It is desirable to coat reinforcement with cement wash before stacking to prevent scaling and rusting.

In case of long storage, reinforcement bars should be stacked above ground level by at least 150 mm. Also in coastal area or in case of long storage; a coat of cement wash shall be given to prevent scaling and rusting.

Article written by

• Sanjay Kumar
• Virendra Kumar
• M. Prasad

Originally published on NSCP – 2001