The present invention relates to a process for the treatment of concrete, of the kind recited in the introduction to claim 1.

The use of reinforced concrete, both in bridges and buildings, has increased significantly in recent years. It has become usual to employ steadily slenderer constructions and thereby a better utilisation of material. In this way more reasonable and stronger constructions can be obtained, but they are more exposed to corrosion problems, especially at locations close to salt water and locations where the roads are salted in the winter. This leads to a strong increase in the concentration of chloride ions, which in its turn involves the formation of hydrochloric acid and corrosion problems.

Several approaches have been attempted to reduce the problems. One method is to seal the concrete with a surface coating which will prevent penetration of the corrosion-promoting substances, such as chloride ions, sulphate ions, carbon dioxide and oxygen. However it is difficult to obtain a completely sealed coating, and besides this will be broken down with time and thereby access again given to said substances. At places where corrosion has already begun it is not possible to interrupt the corrosion process with such a sealing coating.

Another way of meeting the problem is by way of cathodic protection with applied voltage. A general problem with this method is regulating the voltage. A particular problem is that the method may not be employed in prestressed constructions.

By the fact that corrosion is an electrochemical process it is obvious to make use of electrolysis in order to remove chloride ions. However electrolysis of reinforced concrete, where the steel reinforcement is the one electrode, leads to the reduction of the ion concentration being greatest at the steel, while steadily smaller effects are obtained the greater the distance is to the steel reinforcement. With large centre distances between the steel reinforcements electrolysis will be able to remove ions from the concrete close to the reinforcement and from the concrete between reinforcement and anode. Thereby an incomplete removal of the ions from the concrete is obtained, only a shifting within the concrete and thereby danger of a new attack on the reinforcing steel when the electrolysis is con¬ cluded, and time has passed again.

Tests have also been made with water of varying tempera¬ tures, see R. Fray and B. Weber, T,Z-Fachberichte, Vol. 106, No. 6, 1982, p. 402-406, so as to leach out the corrosion promoting ions from the concrete. But it is clear that being able to supply hot water in large quantities to for example a bridge will be problematical, and besides in many instances it will be difficult to achieve satisfactory penetration of water within a reasonable time. Finally it must be mentioned that repairs are also carried out, and it is actually the most usual, by removing "sick" con¬ crete and casting a new cover on the reinforcing steel. Such a method is very time consuming and expensive.

Thus there is a constant need for better methods and appara¬ tus for reducing these problems, and the object of the present invention is precisely to provide a process which makes it possible to remove ions from concrete in a more rapid, reasonable and reliable manner than has been possible with known processes.

The process according to the invention is characterised by the features which are recited in the characterising portion of claim 1.

As the electrolyte in the electrolysis calcium hydroxide has shown itself to be very well suited. The concentration will usually be 1 molar.

A suitable anode material is nickel, which however is rela¬ tively expensive. Alternatively there is used for example rust- free steel. The anode preferably has the form of a fine-meshed cloth, which ought to cover the surface of the concrete as much as possible.

If there is little moisture in the concrete the first step in the treatment will be moistening with water, for example by irrigation with rising temperatures to at least 80°C in the moistening period. Thereby water-soluble salts, especially chlorides, on or near the surface of the concrete will go into solution, while the water dissolves the salts deeper within the concrete to a lesser degree. The salt (and thereby ion) concen¬ tration in the water will even out, that is to say ions will migrate towards and on the surface of the concrete. The duration and here moistening will vary with several factors, for example temperature and the quality of the concrete. In tests moistening for 1/2 - 3/4 hour has been employed. The water on the surface of the concrete will evaporate, and according to the invention the evaporation is accelerated by feeding hot air _; whereby salts which are in solution will migrate towards that location where the heat is supplied and the evaporation effected, namely the surface of the concrete. Thereby salts are leached out of the concrete, and by moistening again the water penetrates further down in the concrete and dissolves salts. By drying again the ions which have passed into solution are drawn towards the surface of the concrete.

The moistening/drying cycle can be repeated a number of times which is also dependent upon several parameters such as temperature, the quality of the concrete and the covering of the reinforcing steel. Moistening and drying 2-3 times appears to be appropriate in practice.

After the moistening of the concrete it is advantageous to wash the surface of the concrete with water in order to remove salts.

There are limits how far into the concrete one can effect migration of ions by moistening/drying of concrete in the course of a practially acceptable time period. For that reason said cycle is combined with electrolysis. After the concrete is dried after the cycle with moistening/drying is finished, direct current is applied to the concrete, whereby as a rule the reinforcement is used as the cathode. On electrolysis the ions will migrate in the applied electrical field away from the reinforcement in a direction towards the anode which when a relatively flat surface is involved can be found on the one or both sides relative to the reinforcement. As electrolyte calcium hydroxide has proved suitable. Preferably it contains an ion exchange resin, which can "catch" ions before they reach the anode where chloride ions will be able to cause the generation of chlorine gas. On electrolysis voltages of over 50 volts are used. After conclusion of the elctrolysis the concrete is washed and dried after which it is sealed if necessary with a material knov/n per se.

An embodiment of the process according to the invention is shown schematically in the accompanying drawing.

First the concrete is moistened with water at temperatures rising uniformly to about 80° over 1/2 - 3/4 hour, after which it is dried with hot air in approximately the same time period. Moistening/drying is repeated two or three times. Thereafter the concrete is dried again over an equally long time, after which it is subjected to electrolysis with the reinforcement as cathode. After conclusion of the electrolysis the moistening/drying is repeated if necessary. Thereafter the concrete is dried and sealed.

If the concrete is relatively moist the process can be begun with drying and thus the first moistening can be omitted.

With the process according to the invention there is achieved both in laboratory and field tests considerable reduction in the chloride content of concrete.