Renovation plaster

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Restoration plaster is a plaster with special properties that is suitable for plastering salt-contaminated and damp masonry, as it stores building material-damaging salts in the plaster and thus keeps them away from the plaster surface . Restoration plaster has high porosity and water vapor diffusion as well as reduced capillary conductivity.

Working principle

Restoration plasters are a suitable means of combating damage to plaster caused by moisture and salt. Salts in the masonry dissolve in the water and can thus be transported to the surface via the capillaries. If the water evaporates, the salts crystallize. Efflorescence often occurs, and the crystallization is also accompanied by a change in volume that creates tension. If the voltage is too high, coatings and plasters will be destroyed over a long period of time. When there is no more plastering material, the salts also attack the masonry. This is particularly problematic in the case of architectural monuments if the historical substance is irretrievably destroyed. The salts that are harmful to the structure, mostly sulfate , chloride and nitrate compounds , also increase the moisture content in the structure through hygroscopic effects. This also increases the risk of frost damage.

Since it is very difficult to completely desalinate and dehumidify masonry, restoration plasters use a different mode of action. The high porosity gives the salts space to crystallize out without damage. The evaporation and salt deposition zone is shifted from the plaster surface into the cross section of the restoration plaster. This means that no efflorescence can be seen from the outside and damage can be avoided in the long term. In theory, restoration plaster works until the salt deposits become so large that they clog the pores. The capillary conductivity of the plaster is greatly reduced by hydrophobic (water-repellent) additives, and this ensures that the salt solutions cannot reach the plaster surface. Nevertheless, moisture can escape in the form of water vapor due to the high porosity.

Restoration plasters are not barrier plasters: the moisture can diffuse out of the component due to the high porosity. Restoration plasters are also not sacrificial plasters whose aim is to suck the salt and moisture out of the masonry via capillaries. With renovation plaster, the salt content in the masonry will only decrease slightly.

Manufacturing

When producing restoration plasters, the leaflet 2-9-05 / D "Restoration plaster systems" of the scientific-technical working group for building preservation and monument preservation (WTA) must be observed. Restoration plasters that meet the requirements of the data sheet are also called "Restoration Plaster WTA".

The properties of restoration plasters depend crucially on their composition and homogeneity. Because the quality cannot be guaranteed with site mixes , restoration plasters are only produced as pre-mixed dry mortar in accordance with DIN EN 998-1.

In addition to restoration plasters, which are applied in a single layer, there are also restoration plaster systems made up of several layers. Usually, a spray grout is first applied to the surface, which improves the adhesion. This is followed by a base plaster WTA and then a renovation plaster WTA. The base plaster can either act as a leveling plaster or as a pore base plaster. As a pore base plaster it has a minimum pore volume of 45% by volume and is mainly used to store the salts. The aim is to prevent the salts from migrating into the actual restoration plaster. If necessary, a finishing render and a (colored) coat of paint can be applied to the restoration render. However, it must be ensured that the water vapor diffusion is guaranteed.

Only hydraulically setting binders can be used as binders. The aggregate must be inert in order to be resistant to salt. In some cases, lightweight aggregates such as pumice or perlite, and sometimes expanded glass, are also used.

The renovation plaster must meet the following requirements:

  • Slump (fresh mortar consistency) 170 ± 5 mm
  • Water retention capacity over 85%
  • Air content over 25% by volume
  • capillary water absorption after 24 under 0.3 kg / m².
  • Porosity over 40% by volume
  • Coefficient of water vapor permeability μ <12

Application limits and problems

The first renovation plaster was offered in 1975. Compared to other plasters, renovation plaster is still relatively young. In the meantime it has proven itself and the problems and application limits when using it have been better investigated.

Restoration plasters are not suitable for use against pressure or backwater. In this case, better sealing measures, for example sealing plasters (barrier plasters), are required. Horizontal seals must always be installed against rising ground moisture. Another problem arises when the dew point lies within the renovation plaster cross-section for a long time. Then condensation water can condense in the pores despite the hydrophobization. Moisture penetration of the restoration plaster must be prevented. For this reason, water-saturated masonry must be dried or sealed before applying a restoration plaster.

Furthermore, there are some processing errors; this includes improper application of the spray grout, only one-layer instead of two-layer work on fissured surfaces, insufficient plaster thickness and insufficient pore volume due to the short mixing time.

It is important to ensure that the relative humidity is below 65% so that the restoration plaster can dry out properly. Otherwise, especially in basement rooms in summer, the renovation plaster may harden but not dry out, which means that the water repellent does not work properly. Then salts can get to the surface and bloom there, which the restoration plaster should actually prevent.

As a rule, the spray grout must not be applied over the entire surface in order not to create a barrier layer. The coverage ratio should be below 50%. In addition, it must be sufficiently salt-resistant.

Overall, renovation measures must always be tailored to the respective building; therefore, preliminary examinations are essential.

literature

  • Tanja Dettmering and Helmut Kollmann: Plasters in building renovation and monument preservation . 2012, ISBN 978-3-410-21929-3 .
  • Hermann G. Meier: Restoration plasters - an important part of building repairs . 1999, ISBN 3-8169-1547-7 .