The Correct Use Of Renovation Plaster WTA

If a material has held its own in the market for over 40 years, it can be described with a clear conscience that it has been tried and tested. Sanierputz-WTA has successfully demonstrated its suitability for damp and salty masonry, if you look at the objects that have been carried out without damage in recent years. Nevertheless, these plasters are not a miracle cure. In the field of masonry renovation, it is essential to deal intensively with the material if you want to achieve good results - no matter how good a product is.

Restoration plaster WTA, the better alternative?

Sanierputz-WTA is able to be used even with heavily salted and damp walls. At first glance, it does not show the viewer what problems the masonry has to do with inside - and that is what most clients expect: a facade that ensures that the plaster surface remains intact for many years without additional effort remains. However, this is not so easy with renovation plaster WTA because you have to deal intensively with your property.

How this is to be done is regulated in the WTA leaflet “Restoration Plastering Systems”, which was published for the first time in 1985 and is available as a revised version this year as version “2-9-20 / D”. This not only specifies the technical parameters for the usual components "spraying", "leveling and pore base plaster" and "renovation plaster" with regard to fresh mortar and solid mortar properties. It is also very important to clarify the causes of the damage and to analyze the moisture and salt load so that the structure and thickness of the plaster can be determined depending on the load situation.

The preliminary examination regulates the process

In the above-mentioned information sheet, different plaster structures are used to react to different salt loads. In contrast to the previous edition, however, the evaluation has become more concrete, since sampling is now dealt with in more detail. Salt concentrations are always higher in old plaster than in the masonry behind. However, since it happens in practice that the examination is only carried out after the plaster has been removed, there are now two separate tables for plaster and masonry samples. The division of the salt load into "low", "medium" and "high" is therefore better adapted to the actual salinity.

"If the salt load consists exclusively of a medium to high sulfate content, the cations calcium, magnesium, sodium and potassium should also be determined for assessment." This note is also new. The reason for this is the property of the salts to absorb moisture from the air and to dissolve in it. Depending on the salt, this process takes place at different air humidities (equilibrium or deliquescent moisture of the respective salt). The more often this process - dissolving and recrystallizing - takes place, the greater the risk that the building structure will be damaged. Is the equilibrium moisture z. B. in the range of over 90 percent or below 40 percent relative humidity, this process takes place less often in our latitudes than between 60 and 70 percent. If the salt then creates a very voluminous crystal, the potential for damage tends to be higher and it is all the more important to be aware of it.

Moisture absorption via the air humidity has an additional effect - salts cannot be recognized because they appear in the clothes of a moisture penetration. Many nitrate salts in particular quickly take moisture from the air and dissolve in it. This may give the impression that a functioning plinth seal alone could fix the damage. If the plastering and sealing work is carried out and a conventional plaster material is used, moisture spots will soon appear on the surface again.

Nevertheless, it is important that the moisture supply is switched off as far as possible or at least reduced. How it is sealed depends on the moisture level and the additional measures to be carried out. A vertical seal according to DIN 18533 can e.g. B. with sealing slurry. Vertical seals ensure that, if used correctly, the masonry can dry out if the moisture is supplied only through the basement wall surfaces. It should be noted that drying out in the masonry also favors the crystallization of the salts. If the masonry is not plastered, there is a greater risk of damage than if the masonry is not sealed. In this case, it is advisable not to attempt to seal the masonry,but only to reduce the ingress of moisture.

When sealing with sealing slurries, make sure that the surface is as level as possible. For this reason, a leveling plaster may need to be included. If mineral slurries are applied in different thicknesses (as would always happen with uneven masonry), the different drying processes lead to more or less fine cracks that would make it possible for water to enter. In the case of bituminous compositions, these leveling plasters are also necessary, although they have flexible properties. This is due to the fact that a dry surface must be available when applying, if sufficient adhesion is to be achieved. As this is usually not the case when the masonry moisture rises, a leveling plaster should be applied. To take the right actionit requires a more detailed examination in advance, so that even small measures should be excavated at certain points.

Restoration plasters have a very high water repellency in order not to let liquid water reach the surface, since this acts as a transporter for the salts. However, in order not to create a barrier, the plaster structure is interspersed with a large number of pores that allow water vapor to pass through. This is to prevent the moisture in the masonry from rising further. Unfortunately, these pores are also the reason that water can be pressed through the plaster with pressure. The water repellency, which only prevents the plaster from vacuuming, cannot prevent this. Restoration plasters are therefore the wrong choice. As a leveling plaster, cement plasters or barrier plasters have proven themselves better below the site.

Processing of renovation plaster WTA

There are a number of things to consider when it comes to processing and post-treatment so that renovation plasters can develop their special properties. Of course, a renovation plaster needs a clean, stable surface. The masonry must be cleaned of old plaster, loose parts and crumbly grout and cleaned as dry as possible (sweep, vacuum). If the masonry is cleaned wet, there is additional moisture penetration, which leads to the further spreading of the salts in the masonry and delays the subsequent drying of the renovation plaster.

The renovation plaster should be used up to at least 80 cm beyond the visible damage zone. This "safety zone" helps to prevent damage due to the salt and moisture distributions inside the masonry that are not visible from the outside. The moisture damage that shows on the plaster surface does not always reflect what is happening in the masonry. In order to be able to make a clear statement as to how high subsequent renovation plaster work is to be carried out, one has to take a closer look. The harder and therefore denser a plaster is, the less precise the masonry's moisture horizon is reflected in the plaster. The strength of the plaster should therefore already be taken into account during a preliminary examination. If this is very high, you can use electronic help to estimate the substrate moisture very quickly and with sufficient accuracy. Comparative measurements on definitely dry surfaces show in which areas the display is located and thus serve as a benchmark. Once you have gained an overview, the exact state of the masonry moisture and salinity can be determined in the laboratory by sampling.

A pre-spray is indispensable to improve adhesion - especially on dry stone masonry. However, in any case, you should always attach importance to a not fully opaque way of working, so as not to unnecessarily hinder the transport of moisture from the masonry into the renovation plaster. A spraying WTA may be applied fully covering. But it is also absolutely forbidden to compensate for the thick layers of the masonry, as it prevents the transport of moisture.

Mix the renovation plaster WTA correctly

If renovation plaster-WTA is mixed with the plastering machine, there are usually no special features to consider. You have to be more careful when mixing your hands. If you mix the plaster for too long, too many air pores develop. The resulting light and creamy material can be processed very well in this condition, but the surface tears open more than usual when it is removed. The development of strength is problematic. The foam-like structure on the wall may not reach the minimum strength even with optimal post-treatment and must be removed again. Mixing in slows down the drying of the wall and increases the strength of the plaster, which in turn can lead to cracks.

Plaster thicknesses - plaster layers - plaster materials

In the WTA leaflet 2-9-20 / D, the thickness of the renovation plaster and the assignment in one or two-layer processing is specified based on the degree of salinization. If you find an uneven surface, you should never work in one layer for safety reasons. Even if the salinization permits this, there is always the risk that the covering on protruding bricks is too low and the required plaster thickness of at least 20 mm is not achieved. With two layers, the layer thicknesses can be better adhered to in terms of processing technology. Even if drying is difficult to achieve (e.g. in cellars or vaults), you should choose this processing variant for safety reasons.

In old buildings in particular, the thickness of the old plaster does not allow such a high layer of plaster. This requires everyone to work together beforehand so that there are no unpleasant delays in execution. In such cases, the importance of the different requirements must be weighed up. If you do not want direct sales or a layer of plaster that slowly runs to the thinner level, all that remains is a single-layer renovation plaster. However, since this cannot be applied in the necessary thickness, compromises must be made with regard to durability.

If the surface is so uneven that the total plaster thickness exceeds four centimeters, only a pore primer WTA may be used as a plaster. This material has a slightly higher capillary conductivity than renovation plaster WTA and thus improves the drying speed. However, this is also the reason why renovation plaster WTA must be applied as a top plaster with a minimum thickness of 15 millimeters. If pore base plaster is used consistently, salt penetration to the surface occurs much faster, since salts are also transported via the capillary moisture transport.

Otherwise, the concealed installation is also carried out with renovation plaster WTA. In addition to easier handling on the construction site, there is another point in favor of this procedure: pore base plaster must always be firmer than renovation plaster. However, if the surface is already more damaged or, due to its composition or age, is not as solid on its own, a harder, more tension-rich plaster can lead to cracks. If you take renovation plaster throughout, this can be prevented under certain circumstances.

With regard to the design options, there are hardly any limits with renovation plasters, since there are many variations from coarse structures to fine surfaces. However, some surfaces cannot be created with a plaster layer that is at least 10 millimeters thick, which is required by Sanierputz-WTA. In this case, another material must be used as an additional finishing coat. Mineral plaster is particularly suitable here.

Restoration plastering systems have their application limits and their durability depends on the load in the subsurface and the quality of the workmanship, as well as the fact whether the actual causes of damage could be eliminated. These systems do not do that. They do not eliminate the causes of damage, but "react" to the stress. Over time, they are practically used up and then salts and moisture can be seen on the plaster surface. The time until this happens is many times longer than with conventional lime or lime-cement plasters. But also compared to moisture regulation plasters, in which the focus is on moisture transport and the general service life and not on the optically perfect surface.

In any case, despite all recurring work, the use of a renovation plaster WTA remains an individual matter for every property, but with a good preliminary examination, preliminary planning and correct processing, you can make a safe choice of materials.

Author: Olaf Janotte / Photos: Baumit

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