DE19810530A1 - Post-treatment agent for protecting mortar and concrete structures against carbonation and corrosion of reinforcement - Google Patents

Abstract

A mortar and concrete post-treatment agent, comprising a concentrated aqueous lithium hydroxide solution, is new. An Independent claim is also included for post-treating mortar and concrete structures by applying the above solution and allowing the solution to diffuse into the structures. Preferred Features: The solution is a saturated aqueous solution of 200 g/l lithium hydroxide monohydrate.

Description

The invention relates to an aftertreatment agent for concrete structures and its type application to protect concrete structures.

A known problem that occurs with the age of concrete structures is the so-called carbonation, d. H. the predominant alkali in fresh concrete These conditions are created by the penetration of the coals present in the air acid and there is more and more an undesirable acidic environment by removing the passivation of the reinforcement and thus corrosion can occur.

Various methods of preventing carbonation have been used seeks, but the results have generally been unsatisfactory or indicated applied methods very cumbersome and costly.

It has now been found that the aftertreatment of concrete structures with a novel chemical means a longer lasting alkalization of the concrete structure and thus effectively prevent a drop in the pH value can.

The invention therefore relates to an aftertreatment agent for mortar and Be clay structures obtained from a concentrated aqueous solution of lithium hydroxide consists. Lithium hydroxide is used as an additive for cement preparations such as mortar and concrete, where it is used as a means of preventing negative alkali Aggregate reaction used. From Japanese patent publication Kokai 62- 117957 it is known that lithium hydroxide is used together with a silicate compound used to repair concrete structures. It was observed that prevents corrosion of the reinforcement when using lithium hydroxide will. However, the patent relates primarily to the use of silicate powder bonds for repairing concrete structures, d. i.e., the goal of such a repair is to increase the surface compressive strength and the tightness of the component and thus bring about a reduction in pollutant input. Essentially it was found that with a previous application of an alkaline solution (Li thium hydroxide) the depth of penetration of the silicate compounds increases, a corro sion protection of the reinforcement is achieved and the surface compressive strength of the treated concrete structure increases. It was recognized that during the application ei ner silicate compound together with an alkaline solution the pH of the Be tons increased and thus a corrosion protection of the reinforcement is achieved. It was however, no attempt is made to apply a pure alkaline solution (Li thium hydroxide) to bring about an increase in the pH value, since the mechanism the reaction taking place in the concrete was not recognized. It was not recognized that the application of pure lithium hydroxide increases the pH of the kar bonized concrete and thus repassivation of the non-rusting Reinforcement is achieved. That there is a pro in the carbonation of lithium hydroxide product is created (lithium carbonate), which guarantees a pH value of more than 10.5 de also not recognized. No attempt has been made to date to use lithium hydroxide Use prevention of the lowering of the pH value below 10.5 and only one concentrated simple aqueous solution of lithium hydroxide as an aftertreatment medium to use.

It has now been found that by applying this aqueous solution of lithium hydroxide on the surface of mortar and concrete structures, permanent alkalinization of these structures and thus protection against the lowering of the pH value can be achieved by using the alkaline conditions as they are prevail in fresh mortar and concrete, are strengthened and are maintained even with prolonged exposure to carbonic acid, since the Li ₂ CO ₃ resulting from the carbonation of lithium hydroxide has a constant pH of greater than 10.5.

The aqueous solution of lithium hydroxide used is preferably a concentrated as possible, ie a saturated solution of about 200 g of lithium hydroxide monohydrate per liter of water at 20 ° C. This is painted with known tools on the upper surface of mortar or concrete structures or these structures are flooded with the solution so that they penetrate as deeply as possible and diffuse into the interior of these struc- tures and ensure a continuous alkalization of the entire struc- tures can. The pH of a solution according to the invention is between 12 and 14.7; The pH value of the mortar and concrete structures is thus adjusted to 10.5 to 12.5 and remains at this level for months and years if the treated surface is not exposed to the weather. Even prolonged exposure to carbonic acid has no effect and the mortar and concrete structures that have been treated in this way remain protected against carbonation up to a depth of 15 to 20 mm. Since lithium hydroxide and Li ₂ CO _{3 have} a relatively high solubility, the two substances can be washed out. By sealing or reapplying lithium hydroxide, the pH value of greater than 10 can, however, be guaranteed without any problems.

In practice, the aftertreatment agent according to the invention is used as follows:
Realkalization with lithium hydroxide monohydrate is used for components in which the carbonation front has reached the first layer of the reinforcing iron and the progress of corrosion is low, or for components in which a lowering of the pH value and depassivation of the reinforcement is prevented shall be.

Corroding rebars must be exposed, derusted and re-profiled. The saturated lithium hydroxide monohydrate solution is applied by means of soaking or flooding (spraying, rolling or similar types of application) of the relevant component. The duration of the application, ie the number of work steps, depends on the suction capacity of the existing concrete. Penetration depths of more than 20 mm should be achieved by means of the capillary suction force. In the case of repeated application, a climate with high humidity (up to 100% RH) must be created between the individual work steps in order to prevent the lithium hydroxide monohydrate from crystallizing and thus clogging the pores. In order to prevent the lithium hydroxide monohydrate from being washed out during application, it must be ensured that no water gets onto the surface. Since lithium hydroxide monohydrate has a higher solubility than calcium hydroxide, a sealant can be applied to the treated surface so that long-term washing can be prevented and a high pH value of the treated concrete is maintained. _{The Li 2} CO ₃ formed during the carbonation of lithium hydroxide monohydrate ensures an alkaline environment with a pH of greater than 10.5.

example

A building in Zurich was constructed in 1930. The facades consist of one part brick and partly concrete walls, all of which were plastered. Since the plaster was heavily soiled and cracked, a status report was made carried out.

The following measurements were carried out on the object:

• - Measure the embedment depth of the reinforcement with the "Profometer 3".
  The embedment depth of the reinforcement was determined by electromagnetic means using the "Profometer 3".
  It was between 40 and 50 mm, whereby the average plaster thickness of approx. 25 mm must be deducted.
• - Determine the pH value of mortar and concrete at different depths with fluids sigem universal indicator. The pH on the surface was around 6.0, at the Be currency by 8.0.
• - Determination of the carbonation depth with thymolphthalein.
  The plaster was completely carbonated. The total carbonation depth was more than 50 mm. Here, too, the average plaster thickness of 25 mm must be deducted.
• - Detection of the corrosion status of the reinforcement by free-cutting the reinforcement.
  The reinforcement embedded 50 mm deep (∼25 mm in the concrete) corroded moderately, the one at 40 mm (∼15 mm in the concrete) corroded heavily. It partly showed leaf rust and caused the layers above to flake off.
• - Measuring the surface compressive strengths of the materials with the "Schmidt rebound hammer".
  The surface compressive strength of the plaster was between 36 and 40 N / mm ² , that of the underlying concrete was 50 N / mm ² .

Procedure for the application

After removing the plaster and the cement skin using maximum pressure those rebars with a hydrodynamic taper that had leaf rust.

These areas were then reprofiled with mineral mortar.

After the drying time (concrete moisture (4%), the facade was covered ten times a saturated solution of 200 g lithium hydroxide monohydrate per liter of water flooded.

Care had to be taken between the individual work steps that the Be clay surface did not dry off. On the one hand, care was taken that during the Flood the facade surface was always slightly damp. The other was during at night ensured that the relative humidity near the facade was 100% wore.

Measurements after application

Twelve hours after application, the concrete was hardened to a depth of 70 mm Sharpened and sprayed with a 2% thymolphthalein solution. This helps a pH of 9.5 from colorless to purple around.

The depth of penetration of the lithium hydroxide made visible in this way was measured and statistically evaluated.

The following values were determined:

Weighted mean 10.6 mm minimum 4.1 mm maximum 17.0 mm Standard deviation 2.7 mm Sample quantity 22.0 mm

Measurements taken 36 hours after application showed penetration depths of over 20 mm can be determined.

Claims (4)

1. Post-treatment agent for mortar and concrete structures, consisting of a con centered aqueous solution of lithium hydroxide. 2. Aftertreatment agent according to claim 1, consisting of a saturated Lö Solution of 200 g lithium hydroxide monohydrate per liter of water. 3. Process for the aftertreatment of mortar and concrete structures, thereby marked records that a concentrated aqueous solution of lithium hydroxide is on applies the surface of these structures and allows it to diffuse into them. 4. Use of a concentrated aqueous solution of lithium hydroxide for Post-treatment of mortar and concrete structures for the purpose of prevention the lowering of the pH value below 10.5.