Stock material or miscellaneous articles – Nonparticulate element embedded or inlaid in substrate and...
Patent
1990-10-19
1993-02-02
Ryan, Patrick J.
Stock material or miscellaneous articles
Nonparticulate element embedded or inlaid in substrate and...
428 76, 428209, 405214, 405216, 204147, 204196, B32B 302
Patent
active
051836947
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method for inhibiting corrosion of reinforcement in structures of masonry or cementitious material, especially (but not only) concrete, and includes apparatus for doing so, methods of installing such apparatus, assemblies for use in such apparatus, and methods of inhibiting corrosion using such apparatus.
Masonry or cementitious materials usually have compressive strength, but little tensile strength It is therefore necessary when using concrete, for example, as a structural member to incorporate reinforcing members (usually metal, preferably steel) to impart the required tensile strength. The reinforcing members may be placed under tension to form "pre-stressed" or "post-tensioned" concrete structures.
If the reinforcement corrodes, it may expand and cause internal stresses within the concrete. This ultimately leads to cracking of the concrete which is then liable to start breaking up. The cracking also results in the reinforcement being further exposed to water and atmospheric oxygen, accelerating the corrosion process.
Rusting is a complex electrolytic process whereby ferrous metal is oxidised to the corresponding oxides and hydroxides of iron, generally by atmospheric oxygen in the presence of water. In fresh
concrete the pH is typically in the range of 12-14, this high alkalinity being a result of the formation of hydroxides of sodium, potassium and calcium, upon the hydration of cement.
Under such conditions steel is passive, this passivation leading to long term stability and protection of the reinforcing member from corrosion.
However, when such passivated reinforcement is exposed to either a strong Lewis base e.g. chloride ions and/or a loss of pH to below 10, the passivation can be disrupted and the steel reinforcement subjected to corrosion. An example of the former is the introduction of chlorides into the concrete matrix by either road salt usage, exposure to the marine environment, or the use of salt-contaminated aggregate or calcium chloride hardening accelerators in the original mix.
These agents have caused widespread corrosion-related damage in many structures including bridge piers, decks, and crossheads, marine piers and harbor structures, and many pre-1970's pre-cast concrete building elements in which chloride-based accelerators were used to improve the economics of their manufacture.
Another cause of corrosion is carbonation of the concrete by penetration of carbon dioxide from the atmosphere which reacts with the pore water in the concrete to form carbonic acid which in turn neutralises the soluble hydroxides and reacts with the calcium hydroxide present depositing calcium carbonate: additional CO.sub.2 made available by diffusion from the atmosphere to react with further calcium hydroxide in the cement.
Carbonated concrete exhibits a sufficiently low pH at which point steel passivation is not stable.
Carbonation takes place at a rate which depends upon a variety of factors such as the original cement content of the concrete, the water:cement ratio of the concrete, aggregate type and grading, density and compaction, and the aspect and degree of protection afforded to the concrete surface. The net result of the carbonation process is however that ultimately the alkalinity which protects the metal from rusting is reduced and the rusting process will start. The reduction in alkalinity is further compounded by acid pollutants in the air such as oxides of sulphur and nitrogen.
In good quality concrete with correct placement of the reinforcement, problems of corrosion resulting from carbonation may not occur for fifty years or even longer. This period can be extended by the use of anti-carbonation coatings on the outer surface of the concrete which help to exclude carbon dioxide and other acidic atmospheric pollutants. There are however many structures where metal corrosion is inevitable and cannot easily be checked. Surface coatings which restrict the entry of carbon dioxide are not as effective if the carbonation depth prior to the application of the coating
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Bahta Abraham
Ryan Patrick J.
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