Corrosion of reinforcement in hvfa concrete
• Concrete is a complex material of construction that enables the high compressive
• strength of natural stone to be used in any configuration. In tension, however,
• concrete can be no stronger than the bond between the cured cement and the surfaces
• of the aggregate. This is generally much lower than the compressive strength of the
• concrete. Concrete is therefore frequently reinforced, usually with steel. When a
• system of steel bars or a steel mesh is incorporated in the concrete structure in such a
• way that the steel can support most of the tensile stresses and leave the immediately
• surrounding concrete comparatively free of tensile stress, then the complex is known
• as “reinforced concrete”.
• Use in construction
• Concrete is reinforced to give it extra tensile strength; without reinforcement, many concrete buildings would not have been possible.
• Reinforced concrete can encompass many types of structures and components, including slabs, walls, beams,columns, foundations, frames and more.
• Reinforced concrete can be classified as precast or cast in-situ concrete.
• Much of the focus on reinforcing concrete is placed on floor systems. Designing and implementing the most efficient floor system is key to creating optimal building structures. Small changes in the design of a floor system can have significant impact on material costs, construction schedule, ultimate strength, operating costs, occupancy levels and end use of a building
• Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen. Formation of an oxide of iron due to oxidation of the iron atoms in solid solution is a well-known example of electrochemical corrosion, commonly known as rusting. This type of damage typically producesoxide(s) and/or salt(s) of the original metal. Corrosion can also refer to other materials than metals, such as ceramics or polymers, although in this context, the term degradation is more common.
• In other words, corrosion is the wearing away of metals due to a chemical reaction.
• Many structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances (see below). Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such aspassivation and chromate-conversion, can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable.
COMMON CORROSION TYPES
• Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the
• micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas).
• Oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal. Oxygen
• content of liquid at the mouth of the crevice which is exposed to the air is greater, so a local cell
• develops in which the anode, or area being attacked, is the surface in contact with the oxygen-depleted
• 2) Pitting
• Theories of passivity fall into two general categories, one based on adsorption and
• the other on presence of a thin oxide film. Pitting in the former case arises as
• detrimental or activator species, such as Cl-, compete with O2 or OHat
• surface sites. By the oxide film theory, detrimental species become incorporated
• into the passive film, leading to its local dissolution or to development of
• conductive paths. Once initiated, pits propagate auto-catalytically according to the
• generalized reaction,
• M+n + nH2O + nCl- → M(OH)n + nHCl, resulting in acidification of the active region
• and corrosion at an accelerated rate (M+n
• and M are the ionic and metallic forms of
• the corroding metal).
REASONS OF CORROSION
• The two most common causes of reinforcement corrosion are (i) localized
• breakdown of the passive film on the steel by chloride ions and (ii) general
• breakdown of passivity by neutralization of the concrete, predominantly by reaction
• with atmospheric carbon dioxide. Sound concrete is an ideal environment for steel
• but the increased use of deicing salts and the increased concentration of carbon
• dioxide in modern environments principally due to industrial pollution, has resulted
• in corrosion of the rebar becoming the primary cause of failure of this material. The
• scale of this problem has reached alarming proportions in various parts of the
• world. Following are the contributing factors leading to corrosion :
1) Loss of Alkanity due to Carbonation
• It is well known that if bright steel is left unprotected in the atmosphere a brown oxide rust quickly
• forms and will continue to grow until a scale flakes from the surface. This corrosion process will
• continue unless some external means is provided to prevent it. One method is to surround the steel with
• an alkaline environment having a pH value within the range 9.5 to 13. At this pH value a passive film
• forms on the steel that reduces the rate of corrosion to a very low and harmless value. Thus, concrete
• cover provides chemical as well as physical protection to the steel. However, alkalinity can be lost as a
• result of:
• (a) Reaction with acidic gases (such as carbon dioxide) in the atmosphere.
• (b) Leaching by water from the surface.
• Concrete is permeable and allows the slow ingress of the atmosphere; the acidic gases react with the
• alkalis (usually calcium, sodium and potassium hydroxides), neutralising them by forming carbonates
• and sulphates, and at the same time reducing the pH value. If the carbonated front penetrates sufficiently
• deeply into the concrete to intersect with the concrete reinforcement interface, protection is lost and,
• since both oxygen and moisture are available, the steel is likely to corrode. The extent of the advance of
• the carbonation front depends, to a considerable extent, on the porosity and permeability of the concrete
• and on the conditions of the exposure.
• In the case of carbonation, atmospheric carbon dioxide (CO2) reacts with pore water alkali according to
• the generalized reaction,
• Ca(OH)2 + CO2