Friday, 13 January 2017

Corrosion Protection for Steel in RCC Bridge Structures

Introduction

Corrosion is the wearing of a metal due to a chemical or an electro-chemical reaction by its surroundings that indicates the damage and loss of material due to a chemical attack. In this process, metal is converted into an oxide, salt, or some other compound that may lead to the development of rust. 

The implementation of corrosion protection is a new construction technique to avoid the loss of materials through corrosion. Corrosion protection uses corrosion inhibitors, low-permeability concrete, and coated reinforced steel that considerably reduces the amount of reinforced steel corrosion in new bridges.

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Corrosion Control Measures 

Concrete provides a protective environment for steel to avoid corrosion. Due to high alkalinity, the thin passive film of ferric-oxide (Fe2O3) is automatically formed on the surface of steel. This layer effectively protects the steel from corrosion and will stay effective as long as the material maintains a high alkalinity. Therefore, by preserving an alkaline environment, the corrosion of steel can be effectively prevented, and the durability of the structure can be ensured. The alkaline medium must be maintained for a longer period by making the concrete impermeable. The factors that impact the corrosion of reinforcing steel bars that are embedded in concrete depend on the amount of chloride ions in the concrete, the resistivity of the concrete, temperature, and the concrete microstructure. Structural material characteristics of rebars and cement concrete topics are reviewed in SE exam review courses. 

Corrosion Control Methods

Mechanical Methods: The physical barriers that prevent the access of chlorides, oxygen, and moisture to the reinforcing steel.

Electro-Chemical Methods: This method forces the steel reinforcing bars to be cathodic. It includes chloride extraction and protection.

The above methods are widely used for preventing corrosion; a qualified structural engineer with their structural engineering exam certification will recommend the implementation method for each project.

How to Prevent Corrosion of Steel in Concrete

A major issue of a reinforced concrete structure is deterioration. The cost of repairing deteriorated structures has become a major liability for highway agencies. The primary cause of deteriorations is cracking, spalling, and delamination. To minimize the development of corrosion of steel in concrete, the following steps should be taken: 
  • Avoid heavily congested reinforcements at the intersection of beams and columns 
  • Avoid the use of materials that speed up the process of corrosion 
  • Clean rebars to remove rust scales before placing concrete 
  • Provide cathodic protection to the steel 
  • Use high quality and impermeable concrete, as well as an accurate water-cement ratio 
The above quality control and assurance measures are refreshed in SE exam review courses for engineers who are interested in becoming a certified professional structural engineer.

Techniques Used for Protection of Reinforced Steel Against Corrosion 

Protection techniques are used to improve the adhesion of epoxy coatings to steel. One method involves pretreatment of uncoated steel with zinc chromate. This procedure is used for all epoxy-coated reinforcement. Epoxy-coated reinforcement steel is one of the protection systems for new bridge deck construction. Calcium nitrite [Ca(NO2)2] is the most popular inorganic corrosion inhibitor, and it is used to prevent chloride ion reactions on steel surfaces. The use of a corrosion-inhibitor admixture in the concrete provides adequate corrosion protection.

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