corrosion of steel structure (group 4)

Corrosion of Steel Reinforcement in Concrete Presented by : Group 4

Overview• Introduction• Mechanisms of Steel Corrosion• Control of Corrosion

Introduction• Corrosion is one of the principal causes of concrete

deterioration• The damage is especially large in the structures

exposed to marine environment , contaminated ground water, or deicing chemicals.

• Crores of rupees spent each year for maintenance and repair of steel structures deteriorated due to corrosion.

Cracking of concrete

• Heat of hydration• Alkali-aggregate reactivity• Carbonation• Sulfate attack• Acid and chemicals• Reinforcement corrosion

Reinforcement corrosion• Passivity– High pH leading to formation of passive layer– Chemical binding of chlorides– Dense and impermeable structure of concrete

• Depassivation– Chloride ingress– Carbonation

Mechanisms of reinforcement corrosion

Factors affecting reinforcement corrosion• Depassivation of steel• Atmospheric pollutants• Availability of oxygen and moisture• Electrical resistivity of concrete– Moisture– Chloride and sulfate contamination

Mechanisms of Steel Corrosion• Corrosion of steel in concrete is an electrochemical

process.

• The electrochemical potentials to form the corrosion cells may be generated in two ways:

1. Two dissimilar metals are embedded in concrete, such as steel rebars and aluminum conduit pipes, or when significant variations exist in surface characteristics of the steel.

1. Differences in the concentration of dissolved ions, such as alkalies and chlorides.

Anodic and Cathodic Reactions

Anode: Fe 2e- + Fe2+

(metallic iron) Cathode: (½) O2 + H2O + 2e- 2(OH)-

air water

• FeO (H2O)x rust

Steel Passivity

• Ordinary iron and steel products are normally covered by a thin iron oxide film that becomes impermeable and strongly adherent to the steel surface in an alkaline environment, thus making the steel passive to corrosion.

• This means that metallic iron is not available for the anodic reaction until the passivity of steel has been destroyed.

Sources of Chloride in Concrete

• Admixtures,

• Salt-contaminated aggregate,

• Penetration of seawater, groundwater, or

deicing salt solutions.

Corrosion of the Steel Reinforced Concrete Structures

MARINE STRUCTURES BURIED UTILITIES

FOUNDATIONS BRIDGES & CULVERTS

Control of Corrosion

• Permeability of concrete is the key to control the various processes involved in the phenomena.

– Concrete mixture parameters to ensure low permeability, e.g., low water-cement ratio, adequate cement content, control of aggregate size and grading, and use of mineral admixtures.

Control of Corrosion

Minimum concrete cover should be maintained.Current practice for coastal structures requires a minimum 50 mm of cover on conventional reinforcement, and 70 mm on prestressing steel.

Control of Corrosion

• Waterproof membranes• Overlay of watertight concrete• Protective coatings for reinforcing steel