CORROSION PROTECTION OF STEEL REINFORCEMENT IN CONCRETE

Abstract: The invention is concerned with protection of steel bars (10, 11, 12) embedded in concrete (17) against corrosion. The bars, prior to embedding, are coated with a physical barrier, typically epoxy resin. Discontinuities occur inevitably in such barriers. Cathodic protection against corrosion is provided at the discontinuities by an anode (18) connected to the bars such that the bars become cathodic at the discontinuities.

Description: CORROSION PROTECTION OF STEEL REINFORCEMENT IN CONCRETE The invention relates to corrosion protection of steel reinforcement in concrete and in particular to a method of applying such corrosion protection and an associated corrosion protected structure. A concrete environment normally provides protection against corrosion for steel reinforcement. If the environment changes so that protection is lost, corrosion tends to arise at the surface of the steel reinforcement and the corrosion products occupy a larger volume than the original materials. Eventually this leads to delamination or spelling (i.e. falling away) of the surface concrete. In some circumstances, the risk of falling concrete or of delamination cannot be tolerated. Another possible problem is that without remedial measures the corrosion can lead to reduction of steel cross section and associated loss of strength. Even in situations where the initial effects of damage can be tolerated, repair costs to make good the structure can be very high. One established procedure for protection against corrosion is to pre-coat the steel with a physical barrier, normally of an organic material. The established commercial product of this kind is a steel bar or rod coated with epoxy resin by fusion bonding. The product is known as fusion bonded epoxy coated rebar or FBECR. As with all forms of corrosion protection, their effectiveness or otherwise may not become apparent until the system has been in use for many years. Some concrete structures incorporating FBECR have corroded significantly before the end of their predicted life. The problem with FBECR arises due to inherent but inadvertent discontinuities in the coating which arise during application of the coating and during fixing of FBECR on site. Attempts to improve the integrity of the coating and to develop practices tending to avoid damage on site have been only partially successful and the coating is inevitably subject to some discontinuities. These discontinuities are a potential location for initiation and propagation of corrosion. Once corrosion has commenced at such a discontinuity, the presence of the coating adjacent to the discontinuity can be a disadvantage because in some circumstances it leads to concentration of the corrosion process at the discontinuity. Cathodic protection of steel reinforcement in concrete is known as a completely separate form of corrosion protection. In cathodic protection, corrosion of the steel reinforcement is prevented by making it the cathode of an electrolytic cell. Cathodic protection can be provided by an impressed current or by means of a sacrificial anode of a metal such as zinc or aluminium connected electrically to the steel reinforcement. Cathodic protection can be very effective but may require frequent replacement or maintenance (in terms of the life of a concrete structure) of the anode. An objective of the invention is to provide an improved and economical means of corrosion protection of steel reinforcement in concrete. In accordance with one aspect of the present invention a method of protecting steel reinforcement embedded in concrete against corrosion includes the steps of coating the steel with a physical barrier subject to discontinuities and applying cathodic corrosion protection to the steel to provide corrosion protection at the discontinuities. The invention also extends to a reinforced concrete structure incorporating corrosion protection provided by the above specified method. The invention is based on the discovery that with a coated bar subject only to inadvertent discontinuities, a very small degree of cathodic protection, which is automatically concentrated at the discontinuities, provides very effective protection against the original occurrence of any significant level of corrosion. An embodiment of the invention will now be described by way of example only with reference to the accompanying drawing which is a diagrammatic representation of part of a reinforced concrete structure. The structure shown in the drawing incorporates steel reinforcement constituted primarily by bars 10, 1 1 and 12. Bars 1 1 and 12 are interconnected by cross members such as 13 and 14 and connected to the bars. Also, at the right hand end of the drawing, it can be seen that bars 1 1 and 12 are in fact constituted by a single bar which has been bent at 15 and 16 to form in effect the two bars 1 1 and 12. Bar 10 is shown as not being in physical contact with bars 1 1 and 12 or any other bars. The bars have been coated with an epoxy resin by fusion bonding the resin to the surface of the bar. Cross members 13 and 14 are connected to bars 1 1 and 12 in such a way as to provide electrical continuity. The steel structure is embedded in concrete 17 in a conventional manner. Although it is intended that the epoxy coating should be as complete as possible, it is inevitable that discontinuities do occur, either during application of the coating or due to placement of the concrete or due to other mechanical damage. In order to guard against the corrosion at discontinuities in the coatings, a sacrificial anode 18 is embedded in the concrete. Of course, the anode is installed before the concrete is poured round the steel reinforcement. Aluminium anodes are particularly suitable but zinc anodes could be used. The anode 18 is electrically connected through lead 19 to bar 12. Because bar 12 has electrical continuity with bar 1 1 and bars 13 and 14, only a single connection is needed for these bars. A separate electrical connection 21 from the anode 18, or in an alternative construction from another anode, leads to the separate bar 10. It is important that all bars or interconnected structures of bars which require protection are electrically connected to an anode. Instead of simply embedding the anode 18 in the concrete 17, it is encapsulated in a capsule 22. The capsule may for example be constituted by expanded foam material contained within a cotton fabric outer skin and treated with a conduction enhancing product such as sodium sulphate in solution. An alternative encapsulation material is a gypsum-bentonite mixture, again treated with sodium sulphate. Other forms of anode encapsulant may be employed. In use an electrolytic cell is created by the anode, the steel reinforcement as the cathode and the concrete as the electrolyte. Current flow from the anode through the concrete to the steel reinforcement is concentrated at the reinforcement in the areas of discontinuities. Throughout the main part of the surface area of the reinforcement, the insulating epoxy coating prevents significant current flow. Because current flow into the steel is over only a very restricted proportion of the surface area of the steel, total current flow is low and the sacrificial consumption of the anode is correspondingly slow. It is for this reason that an embedded sacrificial anode is an effective long term possibility for protection of coated bar. The encapsulation is such as to allow current flow from the anode itself into the concrete and also to accommodate corrosion products generated at the anode. Accelerated tests suggest that a corrosion protection system as described above will provide effective long term protection against corrosion. Protection by means of a sacrificial anode has the advantage that it is self -regulating and avoids the known problems of over-protection, in particular embrittlement associated with hydrogen evolution. This is predicted to provide a long and effective maintenance free life for the structure.

Claims: CLAIMS 1 . A method of protecting steel reinforcement embedded in concrete against corrosion including the steps of coating the whole of the steel with a physical barrier subject to discontinuities and applying cathodic corrosion protection to the steel to provide corrosion protection at the discontinuities. 2. A method as claimed in claim 1 wherein the coating is of organic material. 3. A method as claimed in claim 1 or claim 2 wherein the cathodic protection is provided by a sacrificial anode. 4. A method as claimed in claim 3 wherein the sacrificial anode is embedded in the concrete. 5. A method as claimed in claim 3 or claim 4 wherein the anode is encapsulated in a matrix of material capable of accommodating corrosion products at the anode without incurring local concrete damage. 6. A method as claimed in any one of claims 3 to 5 wherein the material of the anode is zinc or aluminium. 7. A method of protecting steel reinforcement embedded in concrete as claimed in claim 1 and substantially as herein described with reference to the accompanying drawing. 8. A steel reinforced concrete structure incorporating corrosion protection provided by a method according to any one of the preceding claims.

Inventors: THEOPHILUS JOHN PETER, THEOPHILUS, JOHN, PETER
Applicants: THEOPHILUS JOHN PETER, W LIMITED AS, A.S.W. LIMITED, THEOPHILUS, JOHN, PETER
Application year: 1995
Main classification: C23F13
IPC codes: C23F13