ELECTROCHEMICAL RE-ALKALIZATION OF CONCRETE

The present invention concerns a method of re-alkalizing carbonized zones in concrete, and the like, in accordance with that stated in the introduction to Claim of Patent 1.

Reinforcement and other steel objects which are cast in

5 concrete, cements or calcareous materials, such as mortar, plaster or gunite etc., are normally protected against corrosion because of the alkaline environment in the mass. Gradually however, the alkaline concentration is reduced by the absorption dioxide and sulphur trioxide. This absorption

10 leads to a gradual neutralization of the alkaline environment. When the pH-value of the mass reaches 9,5 (approx.) the steel is no longer protected and corrosion is initiated.

The most well-known neutralization reaction is caused by the absorption of carbon dioxide, and is known as

15 "carbonization".

Carbonization is already a serious problem today as is evidenced by the structural damage to buildings caused by the corrosion of reinforced steel, which in turn reduces the cross-section of the steel, and leads to the penetration of

20 the concrete overlay as well as reductions in strength.

Damage caused by carbonization easily becomes a serious matter and is both difficult and expensive to repair. Traditionally this type of damage is repaired by chopping away the carbonized zone, sandblasting the exposed steel, and then

25 concreting or using gunite. Sealing combined with elutriation or a filler is another approach. The result is seldom

satisfactory as far as durability and load-carrying capability are concerned.

The purpose of the present invention is to find a means of repairing carbonized zones in concrete etc. which produce better results than the methods already in existence.

One of the characteristics of carbonization is that the damage is usually restricted to the surface zones of the structure, i.e., in the zone down to the first layer of reinforcement. This is normally a thin zone in relation to the remainder of the concrete cross-section, which is non-carbonized and has a surplus of alkalic matter.

The present invention concerns a method for the realkalization of carbonized zones in accordance with that stated in the characterizing part of Claim of Patent 1. The carbonized zone is supplied with alkaline either from the fresh interior of the concrete etc., or from an external electrolytic medium by employing electricity. The re-alkalization from this alkalic matter re-establishes the corrosive protection of the steel. The procedure is based on the following principle:

When a base electrolytic solution which contains calcium, sodium and potassium hydroxide, for example, is exposed to an electrical field between two electrodes, ions will be transported between the electrodes, which will make the area in the immediate vicinity of one of the electrodes highly alkaline. This alkalic content remains after the removal of the electrical field.

This procedure is executed in practice in the foiH-Mέrng way: 1. If the concrete etc., contains a double layer of reinforcement, where one layer is in carbonized concrete arixl the other is in fresh concrete, the reinforcement in the carbonized zone is connected to one pole on a rectifier or battery, whilst supplied, the alkalic hydroxyl-ions migrate to the reinforcement existing methods, such as pH-sensitive tracers or pH-electrodes. When the desired pH value is reached (usually over 12), the current can be disconnected.

2. If the concrete contains one layer of reinforcement, or possibly a second layer which is unaccessible, or if the distance between the two layers is too great for the technique described in point 1 to be feasible, an external electrode is located in a suitable electrolytic medium on the surface of the structure. This electrode can consist of wires, cords, plates, foil or sheet metal, conducting plastics or other conductive materials. The electrolyte can be an aqueous solution of calcium, sodium and/or calcium salts either in a liquid or absorbed in a porous medium such as rock wool, cellulose, sawdust, sand, clay and the like, or it can be strongly retarded concrete, mortar, cement grout, lime paεJ≥e etc. When a cement-based concrete, mortar or paste is used, aa strongly retarding substance such as sucrose is added to prevent the mass from setting, thus once the treatment is completed, this mass can be removed by scraping or flushing. The external electrode is connected to one pole of the rectifier or battery, whilst the reinforcement in the carbonized zone is connected to the other pole in the manner described in point 1, above.

The speed of the re-alkalization process depends on the direct voltage which is applied, which is in turn dependent cm the conductivity of the concrete and electrolyte, the density of the electrodes, and the distance between them. For normal building structures this voltage should be between 6 - 20 V, which will ensure that the re-alkalization is completed within a matter of days or weeks, depending on the overall condittixπrε and the degree and depth of carbonization.

The method is illustrated in the enclosed drawings, __h__re: Fig. 1 shows a section through part of a concrete structure, where electricity is applied to the reinforcement in fresh and carbonized concrete, respectively, whilst

Fig. 2 shows an equivalent section through a concrete structure where electricity is supplied by means of an external electrode in an electrolytic medium and part of the reinforcement in carbonized concrete.

The procedure will be described in more detail with

reference the enclosed illustrations. In Fig. 1, there is a layer of reinforcement 1 in an area with carbonized concrete and a layer of reinforcement 2 is in fresh concrete. The reinforcement 1 is connected to one pole of a rectifier or battery 3, and the reinforcement 2 connected to the other pole. When the current is supplied the alkalic hydroxyl-ions migrate to the reinforcement 1 in the carbonized zone.

In Fig. 2, the reinforcement 4 is in a carbonized zone in the concrete. A suitable electrolytic medium 5 is applied to the surface of the structure. An electrode 6 has been located in this medium 5. The electrode 6 is connected to one pole of the battery or rectifier 7, whilst the reinforcement 4 in:the carbonized zone is connected to the other pole. When the current is supplied, the alkalic hydroxyl-ions will migrate from an external medium 5 to the reinforcement 4 in the carbonized zone.

As a rule the polarity is selected so that the reinforcement in the carbonized concrete is connected to the negative pole of the battery. In special cases it could be an advantage to reverse the polarity (this is dependent on factors such as the type of salt in the electrolytic medium).