Image-1: Corrosion in reinforced concrete.

Corrosion In Reinforced Concrete || Causes Of Corrosion And Remedial Measures

Reinforced concrete is widely used for construction work in transport infrastructure i.e. Bridges, Tunnels and harbor structures, etc. Reinforced concrete is also used for offshore platforms and a wide range of private and public building structures. Any reinforced concrete structure shall be high strength and durable for long serviceable life. But High durability in reinforced concrete structures is not achieved in practice. In case of adverse environment such as marine environment, chemically affected water body, etc. Corrosion is the main cause of deterioration in a reinforced concrete structure.

Definition of Corrosion in Reinforced Concrete:

It can be defined as deterioration of reinforced concrete due to chemical reaction between steel reinforcements and its environments i.e. carbon dioxide, moisture, chloride, etc. Appearing red-brown color iron oxide and iron chloride or iron salt on steel reinforcements due to breaking protective layer and chemical reaction of iron.

Cause of Corrosion in Reinforced Concrete:

The main causes of steel reinforcement corrosion in concrete are:

Breaking of the passive film on steel reinforcements by chloride ions and
Carbonation Of Concrete (Breaking of passivity by neutralization of the concrete):

1. Breaking of the passive film on steel reinforcements by chloride ions:

Chloride attack on reinforced concrete is the main reason for the corrosion of reinforcements in concrete.

Steel, like most metals (except gold and platinum) is thermodynamically unstable under normal atmospheric conditions.

Iron reacts with water (H2o) and oxygen (O2), produce iron hydroxide (Fe(OH)2).

i. 2Fe+2H2O+O2=2Fe(OH)2

This initially precipitated iron hydroxide is not also stable and tends to react with oxygen (O2) and form higher iron oxides.

ii. 4Fe(OH)2+O2=2Fe2O3+4H2o

Concrete contains large amounts of calcium (Ca++) and a small percentage of sodium ions(Na+) and potassium ions (K+). These ions maintain a high alkalinity level and PH value-12 to 13 in concrete.

Iron oxide and hydroxide create a thin Passive Layer on the outer surface of steel reinforcement and prevent metal atoms from dissolving at high alkalinity leveL (at high-level PH value).

This passive layer does not actually stop corrosion. But passive layer reduces the corrosion rate to an insignificant level. The corrosion rate of steel in concrete is typically 0.1 µm per year in presence of passive layer at high-level PH value. The steel material would corrode at rates at least 1,000 times higher Without the passive layer.

Due to chloride attack, the PH value (alkalinity level) will be reduced and passive layer on the surface of steel will be broke down. After breaking of passive layer, iron atoms can further react with water and oxygen and produce reddish-brown color iron oxide Fe2O3. Iron oxide precipitated around the steel bar surfaces. The volume of the rusted and corroded steel bar is higher than the steel bar. Increased volume of steel bars or the diameter of steel bars leads to internal stress within the concrete. Further, it develops cracking and spalling at the concrete clear cover.

Chloride may exist in reinforced concrete structures or maybe enter from the environment into reinforced concrete.

Existence of chloride in concrete due to

a. Use of seawater during concrete mixing process

b. Use of calcium chloride as an additive to increase the setting time

c. Use of aggregates that contained chlorides that were not washed properly.

d. Aggregates with chloride content more than the limit stated in the specification

Entrance of chloride into reinforced concrete from the exterior environment due to

a. Presence of reinforced concrete structure in marine water or chemical affected water body.

b. Use of salt during ice melting.

c. Exposure condition of reinforced concrete in an adverse environment.

2. Carbonation Of Concrete (Breaking of passivity by neutralization of the concrete):

During hydration of cement concrete, calcium hydroxide Ca(OH)2 makes a protective layer around the steel reinforcement bars and protect the steel bars from rust and corrosion.

Atmospheric Carbon dioxide can interact with calcium hydroxide Ca(OH)2 due to insufficient clear cover or porosity/void concrete. Form calcium carbonate due to the reaction of calcium hydroxide Ca(OH)2 and carbon dioxide (Co2) which make shrinkage cracks in concrete. This process is known as carbonation. This process reduces the alkalinity in concrete and breaks the protective layer around steel reinforcements. without protective layer, iron reacts with moisture and oxygen and produces iron oxides or rust. Hence carbonation leads to corrosion in concrete.