WO1989006299A1

WO1989006299A1 - Method to stop alkali-aggregate reactions in concrete etc., and means to carry out this method

Info

Publication number: WO1989006299A1
Application number: PCT/NO1989/000001
Authority: WO
Inventors: John B. Miller
Original assignee: Miller John B
Priority date: 1988-01-04
Filing date: 1989-01-02
Publication date: 1989-07-13
Also published as: DK168228B1; GB2223526A; NO171606B; NO880003D0; NO880003L; DK423989D0; NO171606C; DK423989A; GB8918985D0; GB2223526B

Abstract

A method for stopping alkali-aggregate reactions in concrete, where on one side of the concrete structure (11) which is to be treated is placed an electrode (14) in an electrolytic medium (13) (''wet electrode''), and where on the opposite side is placed electrically conductive means (12) in a dry milieu (''dry electrode''). A DC current is impressed between the wet electrode and the dry electrode. The dry electrode can consist of a waterfree film, for example applied as paint containing electrically conductive particles. As electrolytic medium can be used such as strongly retarded gunite or an electrolytic solution which is supplied in liquid form or used soaked up into a porous medium such as mineral wool.

Description

Method to stop al ali-aggregatereactions in concrete etc., and means to carry out this method.

This invention relates to a method to stop alkali aggregate reactions in concrete etc., and a device to carry out this method.

Background of the invention

In some parts of the world, deterioration of concrete constructions owing to so called "alkali-aggregate reactions" is a major problem. The reaction appears when aggregates in concrete contain amorphous silica, such as flint, opal, volcanic glass and related minerals. The silica reacts with the alkalies in cements to form water rich alkali silicates. This reaction lead to an increased volume compared to the original materials. The increase is followed by considerable swelling pressure which can burst the concrete, and thereby lead to loss of bearing capacity and open access to corrosive substances with rebar corrosion as a possible result.

The problems related to alkali-aggreate reactions is well known in many countries. In the Nordic countries, it is frequent in Denmark, Iceland and Greenland. Today, there does not exist any practical repair procedure for such damage, other than replacing of the attacked concrete with new concrete.

It is known from laboratory experiments, that, if the concrete is dried out such that the relative humidity in the air in the concrete pores is reduced to less than 80%, the alkali aggregate reactions stops. However, it is very time- and energy consuming to do this by means of conventional drying based on heating and ventilation. Objecives of the invention

The main objective of the present invention is to develop faster and cheaper methods to stop such disruptive reactions. The invention is based on certain aspects of electro osmosis. In some porous media, where the pores are partly or completely filled with water or aquous solutions, water in contact with the pore walls and the pore walls themselves, assumes opposite electrical charges. If the porous media is exposed to an electric field, the water in the pores will move in a direction towards the electrode which is oppositly charged compared to the water. This phenomenom is called electro osmosis.

Electro osmosis has been used within the building industry for such purposes as foundation reinforcement by reducing the water content in the soil, or by creating favourable flow patterns for subsoil water. This technology has also been used to lower the water content in basement walls in order to remove humidity from rooms in the basement.

Principles of the invention

It appears that favourable results can be achieved by the method as described in the characterizing part of claim 1.

Further favourable features of the invention are disclosed in the other claims. Above the soil, the reaction is stopped in the following way: On one side of the construction part to be treated, an electrode is placed in a suitable electrolytic medium. The electrode may comprise filaments, bars, plates, foils, or films of metal, conductive plastic materials or other electric conductive materials. The electrolyte can be strongly retarded gunite, or electrolytic solutions either in liquid form or sucked up in a porous medium such as mineral wool, cellulose pulp, sawdust, clay, etc or as salt containing gel.

The opposite side of the construction part is painted or covered by an electrically conductive water free material which, for example, can be metal sheets or paint or a paste containing finely dispersed metals or graphite as filler materials, or pigments in sufficient amounts to form a continous electrically conductive film. This film serves as the second electrode in the electro osmosis cell. A rebar layer can, as an option, serve as electrode if the construction part contains such a suitiable layer. It is also possible to make use of conductive electrodes which are drilled into the wall for the purpose. The electrodes in the electrolytic medium, including the electrode comprising a conductive film or a rebar layer, are connected to the poles of the a DC source.

For reinforced concrete constructions below soil level the method is used by impressing a current between the reinforcement or the optional electrodes drilled into the wall, and a buried or rammed metallic body, such as a rail, or a metallic bar. The electrolyte consists then of the soil surrounding the construction and the metal body.

By impressing a current, the water moves towards one of the electrodes, and this results in water being extracted from the concrete and accumulating in the corresponding electrolyte, whose water content thereby increases. The correct polarity must be determined in each case. The water in concrete will normally move towards the negative pole.

The rate of drying, and the degree, depends on the DC voltage used, which again depends on the conductivity of the concrete and the electrolyte being used, the porosity of the concrete, the density of the electrode, and the distance etween the electrodes. For conventional building constructions, the applied voltage is in the range of 10-100 V, depending on the degree of drying and whether drying is being performed during the course of days or weeks. In the case of constructions above the soil, the following applies: After the water content in the concrete is reduced sufficiently such that measurements of relative humidity in closed drill holes, shows that this humidity is less than 80%, the electrolyte is removed by scraping, brushing, blowing, knocking, sucking or by other suitable methods, before interruption of the current. After the current has been interrupted, electrode materials and possible remaining electrolyte are removed.

Finally, the surfaces of the dried construction part which may be exposed to humidity are sealed against new humidity penetration by application of a coating which is impermeable to water and water vapour. This coating, besides being impermeable, must also be sufficiently elastic to resist rupture by minor thermal or load dependent movements in the constrouction part.

Suitable coatings can be such as polyurethane, rubber, acryl or similar. Below soil level the current supply must be permanently connected to keep the humidity continuously down.

Example

In the drawings there are shown schematically an example of an embodiment, where: fig. 1-2 show a vertical section through two wall constructions which extend above soil level and which in different ways are provided with means to carry out the method in accordance with the invention, whilst fig. 3-5 shows corresponding sections for wall constructions which are buried.

In the figures, there is shown a wall 11 made of concrete with aggregate materials as defined previously. Fig. 1 shows a wall 11 including a reinforcement net

12. On that side of the wall 11 which is farthest from the reinforcement net 12 a layer 13 of mineral wool is applied which is soaked by an electrolytic medium. In this layer

13, there is an electrode 14 in the form of a metal net. The metal net 14 is connected to the negative pole of a DC source 15, of which the positive pole is connected to the reinforcmer_t net 12.

UTE SHEET Fig. 2 shows a wall 11 without a reinforcment net, but with an electrically conductive film 16 as dry counterelectrode . Besides that, the arrangement is as shown in fig. 1. Fig. 3 shows a buried wall 11 with a reinforcement net 12 and a metallic bar 17 as the respective electrodes. Fig. 4 shows a buried wall 11 with a metallic rod 17 and one or more embedded metal bars 18 as the respective electrodes. Fig. 5 shows a buried wall 11 with a metallic rod 17 as "dry electrode" and a metal net 14 included in a mineral wool layer 13 as "wet electrode".

The use of electrodes and materials in the examples can be varied as indicated above and as defined in the patent claims.

Claims

Claims :

1. A method to stop alkali aggreagate reactions in concrete, c h a r a c t e r i z e d by that in or juxtaposed to the concrete structure which is to be treated is placed an electrode in an electrolytic medium ("wet electrode"), that in an other area of the concrete is placed an electrically conductive means in a dry miljeu as additional electrode (dry electrode), and that between the wet electrode and the electrically conductive layer is impressed a DC current, thereby extracting water by electro osmosis.

2. Method in accordance with claim 1, c h a r a c t e r i z e d by that the electrically conductive means comprises a waterfree film, for example applied as paint with a content of electrically conductive particles.

3. Method in accordance with claim 1, c h a r a c t e r i z e d by that the electrically conductive means comprises a reinforcment net.

4. Method in accordance with any of the claims 1-3, c h a r a c t e r i z e d by that as an electrolytic medium is used strongly retared gunite.

5. Method in accordance with any of the claims 1-3, c h a r a c t e r i z e d by that as an electrolytic medium is used an electrolytic solution which is supplied in liquid form or is used soaked into a porous medium such as mineral wool, cellulose pulp, sawdust or clay.

6. Method in accordance with any of the claims 1-3, c h a r a c t e r i z e d by that as an electrolytic medium is used a salt containing gel.

7. Method in accordance with any of the claims 1-6, c h a r a c t e r i z e d by that the electrode with the electrolytic medium is placed in surface contact on one side of the concrete construction.

8. Method in accordance with any of the claims 1-2, c h a r a c t e r i z e d by that the electrically conductive means in dry miljeu is coated onto an opposite surface of the concrete structure in relation to the electrode in electrolytic medium.

9. Means for carrying out the method in accordance with claim 1, c h a r a c t e r i z e d by that it comprises an electrode (14) which is placed in a electrolytic miljeu juxtaposed to a wall structure (11) which is to be treated, and a second electrode (12) which is placed in or nearby the wall structure such that it is being placed in a relatively dry miljeu.

10. Means in accordance with claim 9, c h a r a c t e r i z e d by that it comprises electric electrodes (14) in an electrolytic medium, such as an electrolytic solution soaked into mineral wool (13) and on the opposite side of the concrete structure an electrically conductive surface coating, f.ex in the form of paint which containts electrically conductive particles.