How flood resistant is autoclaved aerated concrete (AAC)?

AAC, such as Ytong, absorbs water slowly and is therefore well-equipped to withstand floods.
Up to 90 percent of the blocks are made of two main types of pores: micropores and macropores. The micropores (about 30-40% by volume), often referred to as capillary pores, are absorbent and can quickly absorb and transport water, while the macropores (about 40-50% by volume) are non-absorbent or have very limited absorption.
But the release of water also takes time: "Even though autoclaved aerated concrete may look dry, depending on the thickness of the wall and the duration of the flooding, it can take one to two years for the water that has penetrated to be completely released back into the air in the room," says Torsten Schoch, CEO of Xella Technologie- und Forschungsgesellschaft mbH (T&F). That may sound like a long time, but compared to other building materials, the structure of the building remains intact and does not need to be replaced.

How do calcium-silicate units and other materials react to flooding?

Calcium silicate blocks generally behave similarly to autoclaved aerated concrete (German Flood Protection Guide, p. 46). Calcium silicate blocks absorb water primarily through capillary pores; there are no macropores. The absorbency of the surface is slightly higher than that of autoclaved aerated concrete. Compared to autoclaved aerated concrete, calcium silicate blocks can absorb about half the volume of water.

The situation is quite different for other building materials:

• Wooden structures are not very resistant to flooding and are particularly susceptible to mold and rot. Wood-based materials such as particle board, OSB or plywood become permanently deformed when wet and usually need to be replaced (German Flood Protection Guide, p. 49).
• Clay blocks with a lower density and many pores and holes, i.e. porous material and absorbent capillary pores, absorb a lot of water when flooded. However, this has little effect on their strength (German Flood Protection Guide, p. 44).

How does plaster affect the drying process?

Depending on the material used to plaster the AAC or other walls, the extra layer can be extremely detrimental to the drying process. Gypsum and gypsum boards are challenging. They must be completely removed after a flood to allow the walls to dry and preserve the masonry. Gypsum board is common in buildings constructed in the last ten to 30 years. Mineral plaster based on lime or cement, on the other hand, which was used primarily in the 1960s and 1980s, does not interfere with the AAC as it dries and can be left in place.

What happens to furniture and wallpaper during the drying process?

Because furniture on the wall acts as an additional heat transfer resistance, lowering the temperature of the wall surface, the relative humidity in these areas can rise quickly. For this reason, furniture should be moved several centimeters away from the wall over an extended period of time. Wallpaper should be removed from affected walls as soon as possible. New wallpaper should not be applied until the surfaces are dry and the moisture content of the wall material is close to the normal delivery moisture content. However, there are many plasters that contain color pigments to aid in drying. Please consult the suppliers of such plasters.

What can be done about mold?

"Mold does not grow during a flood, but only afterward - because it thrives at relative humidity on the surface of more than 80 percent," explains Torsten Schoch. This should be closely monitored after a flood. Targeted ventilation or the use of ventilation systems can help. A coat of lime plaster can also significantly reduce the risk of mold growth. Other mineral plasters such as cement or a combination of the two are also effective.

Can the drying process be accelerated?

Attempts to speed up the process, such as using heat radiation, can even harm the surface structure of of the material due to the high temperatures. It is advisable to wait for the natural drying time of one to two years.

Why are oil heaters a major hazard during floods?

The danger in flooded areas is not necessarily the damp walls, but what is washed away. Whether in your own basement or your neighbor's, heating oil is a major problem for the environment and your home when the boiler room overflows. The heating oil penetrates the walls – "it can quickly turn them into hazardous waste, which also smells very unpleasant," says T&F Managing Director Torsten Schoch.

In some cases, it helps remove several centimeters of the material and fill it with plaster or mortar thin slabs of autoclaved aerated concrete and/or calcium-silicate blocks to the existing wall. Since heating oil can quickly seep into the groundwater during flooding, gas heating systems or oil storage areas should be secured and sealed accordingly.

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The German Federal Ministry of Housing, Urban Development and Construction provides further useful information in a clear and compact form in the Flood Protection Guide (only in German).