According to some studies rust formation in first stage increases the bond between concrete and steel. But due to change of Alkaline atmosphere around rebar's in concrete created by concrete itself as passivating protection and change of P H value from ideal 12.00 to 8.60 due to various chemical actions. Due to the presence of chlorides, sulphates and various ingredients present in atmosphere /environment, water used for mixing, concrete materials, conditions of curing and initial hydration, it is seen that the rusted steel used in concrete in any stage of corrosions including pitting seems unsafe and affect the durability and longavity of structure in its life span.
Hence I recommend totally to avoid rusted steel in any manner and in exceptional cases do reinforcement protection / preventing procedure and use for embedment as main reinforcement, stirrups or as tiles Generally the following factors influences the corrosion of reinforcement in concrete such as P H value of concrete, chlorides, sulphates, Moisture, Oxygen ambient temperature, carbonation of concrete and relative humidity. Besides, the conditions of exposure, quality of construction materials, permeability of concrete , cover to reinforcement, initial curing conditions, formation of cracks, high carbon content in reinforcement, high stress levels inadequate grouting of stressed tendons, use of rusted reinforcement prior to embedment, alkali- aggregate reaction, potential difference associated with liquid contact of other materials, stray current and absence of periodic repairs. Corrosion (Rusting of rebar's) is the destruction of materials due to chemical reaction with the environment, which leads to loss of steel due to formation of rust. Which reduces the alkalinity of concrete through carbonation? Hazards and economic loss occurs due to premature deterioration of steel on buildings and Civil engineering structures. Corrosion deteriorates concrete due to formation of new products - ferric oxide which occupies more volume than steel and exerts and bursting stress on surrounding concrete. Which may (formation of rust) may lead to staining, cracking and spalling of concrete. This process of corrosion is in geometric progression with respect to time. Due to the process of corrosion the cross sectional area of steel reduces.. Due to corrosion structural distress occurs due to a) Loss of bond between concrete and steel. b) Cracking and spalling of concrete c) Reduces the cross sections of steel. In pre stressed concrete the loss of minor area may possibly lead to tendon failure. The Process of failure is slow and can be repaired/ strengthened on notice of defects. The increase in volume of oxidized compound in concrete results in tensile force leading to cracks in concrete around steel reinforcement and increases the corrosive environment. (Secondary corrosion which is speedy) The corrosion protections of reinforcement is maintained by the alkalinity of concrete. Which leads to passivation of steel. The reduction of P H value of concrete leads to corrosion of reinforcement. Carbon dioxide and environmental influences reduces the P H value of concrete from 12.6 to 8.00, which removes the protections given by alkalinity around steel by concrete. The Reinforcement in concrete have a gamma ferric oxide passivating layer. It has got its on repair mechanism to protect the passivating layer. For corrosion to occur and to continue oxygen and water are required. The corrosion is higher due to increase in relative humidity. Corroded/ Rusted steel should be avoided fully and use only after taking precaution against corrosions in steel. Damages caused by corrosion are. 1) Formation of white patches Which is due to carbonation and white patches occurs over concrete surface 2) Brown patches along reinforcement This is due to formation of ferric oxide and brown lines are seen on slabs, columns and beams as brown lines 3) Occurrences of Cracks Due to increases in volume of steel area due to corrosion products which exerts bursting pressure on surrounding concrete due to other cracks are developed on concrete along the directions of reinforcement and hair line cracks on surface of concrete indicates rusting process 4) Formation of Multiple Cracks Due to formation of multiple layer of rust on rebar's, the bond between concrete and steel is considerably reduced and sounds hollow on light hammering. It is seen that cracks in the form of spider net on surface of concrete which is secondary and serious. 5) Spalling of cover of concrete Loss of bond between rebar's and concrete and reduction of size of rebar's results in multiple cracks and peeling off of concrete cover. Here the concrete spallen down and corrosions speeds up due entry of air, humidity and full loss of alkaline atmosphere. 6) Snapping of bars Continued reduction of size of bars, results in snapping. This occurs mainly in ties and stirrups first. 7) Buckling of bars & Bulging of concrete Spalling of concrete and snapping of ties cause main bars to buckle which may lead to failure of concrete in the region. Which may lead to structural failure. 8) Preventive Measures in New Concrete ? To prevent corrosion, control the water cement ratio, introduction of high - Strength concrete, adoption of higher minimum cement content, strict adherence of thicker concrete cover, proper structural detailing of reinforcement will better up the condition and reduces the corrosion. Reinforcement protections can be done at executions stage itself 1) Cement based coating 2) Galvanizing zinc based paint 3) Epoxy coating 4) Bitumen based paint 5) Phosphatic coating The coating to rebar's have to satisfy the following a) Ensure uniform coating on the deformed surface configuration of the bars. b) Be flexible enough to allow post- coated bending of the bars. c) Be Mechanically stable to sustain handling, transportation and fabrications of reinforcement. d) Provide the facility of easy application e) Resist corrosion DISCLAIMER: Notice : This e-mail and any attachments may contain information which is confidential to the addressee and may also be privileged. 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