The brown tank is a type of waterproofing for components in contact with the ground. It consists of a water-impermeable supporting structure made of reinforced concrete (WU concrete) and an additional waterproofing layer based on sodium bentonite applied to the water-bearing side. The brownish hue of the natural material sodium bentonite, which makes up the main component of the water-side waterproofing membrane, led to the name "brown tub".
Bentonite waterproofing as a component of the brown tank is still largely unregulated and is therefore considered an additional waterproofing measure of a waterproof construction of the DAfStb guideline (WU Directive).
In the construction industry, the brown tub is a surface waterproofing system that has an additive effect on a supporting structure made of waterproof concrete in the area in contact with the ground. Both components, both the external waterproofing layer and the waterproofing of the waterproof concrete, together ensure safe component sealing. The cost of a brown tub is usually between that of a white tub and that of a black tub.
The technology of the brown tank is mainly used in civil engineering, earthworks and tunnelling, but is increasingly also used in the basement area of building buildings. It is suitable for sealing components in contact with the ground, which must be secured in particular against accumulating seepage water and pressing groundwater.
The sealing effect of the brown tub is largely based on the swelling effect of the natural material bentonite, which is installed in granular form in geotextile sheets. Ideally, a composite effect of the bentonite mat with the fresh concrete unfolds via needling or via textile fibres that are integrated directly into the concrete. In this way, the supporting structure is firmly connected to the bentonite mat. When it comes into contact with moisture, bentonite absorbs 5 to 7 times as much liquid and swells to 12 to 15 times its original volume. With sufficient contact pressure, the sealing effect of the material can fully develop and rear running can be prevented. Bentonite waterproofing therefore only works if there is sufficient additional load (e.g. under floor slabs) or sufficient earth pressure (e.g. on backfilled basement walls). The brown tank can therefore not be led up to the top of the ground (see also "Contact pressure" in the section Design), but always requires an alternative seal in this area, e.g. as a black tank.
Material: Bentonites are natural clay minerals that were formed from the weathering of volcanic ash. They are used in construction in the form of sodium bentonites for waterproofing purposes. The material is characterized by a high swelling capacity, and as soon as it comes into contact with moisture, the material seals due to the swelling pressure that occurs. The permeability coefficient of the bentonites used for waterproofing buildings in mat form is in the range of Kf = 5 x 10-11 m/s or less when installed. Hydration remains even if the immediate environment (e.g. the soil) dries out.
Self-healing effect: The persistent swelling pressure causes a certain self-healing effect of the material, even after it has been installed and activated. This means that minor damage, e.g. due to contact with sharp objects, closes again by itself without repair work.
Regulations: Products containing bentonite for surface building waterproofing are still not regulated in the German standard system. However, bentonite mats have been included in the Building Rules List Part C as an additional sealing measure for components that have a high water penetration resistance. Since the brown tub is classified as a composite construction of waterproof concrete and bentonite mats, it also falls within the scope of the DAfStb Directive, also known as the WU Directive, in Germany. With this interpretation, general building authority approval is not mandatory.
Cost-effectiveness: Since the Braune Wanne works as a "double" waterproofing, primarily as a bentonite waterproofing and secondarily as a waterproof waterproofing, the crack width restriction for the waterproof concrete can be reduced in certain cases and depending on the respective load case of the water exposure, which would otherwise be the sole waterproofing measures of white tanks. This means that, in consultation with the structural engineer, larger crack widths of the waterproof concrete can be accepted, as the upstream bentonite waterproofing can seal cracks up to a width of approx. 0.3 mm. In this way, the reinforcement of the waterproof concrete can be reduced, which, in addition to economic advantages, also facilitates the mechanical compaction of the concrete and thus increases its quality.
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Thermal insulation: For insulated basements, as in the black tub system, external perimeter insulation is advisable to avoid condensation inside the building. The perimeter insulation is subsequently glued to the waterproofing level from the outside. Under floor slabs, the bentonite mat is laid directly on the insulation layer and then concreted. The concrete surface and bentonite mat must always be in direct contact, and the insulation layer must not be in between under any circumstances.
Joints and penetrations: All manufacturers offer special products that are recommended to complement the system in vulnerable areas. The majority of these products are also based on the swellable material bentonite. These include, in particular, coated joint plates, joint tapes and swelling tapes, which serve as additional protection in the area of working and structural joints. Coves that tend to be hind can also be sprinkled with bentonite granules. Anchor holes or penetrations of installation pipes are closed with bentonite fillers or similar products.
Radon: Radon is a radioactive noble gas that occurs everywhere in nature and escapes from rocks and soils. Its concentration varies greatly locally and is imperceptible to humans. The risk of penetration into the building exists especially in the part of the building in contact with the ground via joints, pores and cracks in the outer shell.
Sodium bentonite is considered a diffusion-permeable building material which, like concrete, has a braking effect against gases, but cannot be regarded as radon protection. In areas with a high radon concentration, a multi-layer structure of the bentonite waterproofing with additional layers of plastic is therefore advisable. See also encyclopedia article ►Radon radiation.
: During storage, protection against moisture is required to prevent premature swelling. The swelling pressure required for sealing must only build up after installation. Manufacturer's instructions on this must be observed.
Weather influences: The installation of bentonite waterproofing is largely independent of weather and temperature and can also be carried out in winter sub-zero temperatures or hot temperatures in summer. However, excessive exposure to moisture, for example due to heavy rain during paving, should be avoided, especially with mats without an additional PE layer.
Installation: Bentonite sheets are generally not welded. As a rule, they are loosely overlapped according to the manufacturer's specifications, or attached to the formwork by clamps or nails in the case of vertical installation with fresh concrete composite . Multi-layer systems with an additional PE layer can be glued with special adhesive tapes after overlapping. In the case of horizontal bentonite sheets under floor slabs, it is important to protect them until the concreting process. Early installation of the sub-reinforcement can prevent damage caused by walking. Stresses caused by material deposits on the waterproofing membranes must be avoided.
Contact pressure: The contact pressure or the pressure due to the load is necessary so that the sealing swelling pressure can build up and rear flow can be ruled out. It is an essential part of the Brown Tank system. This means that bentonite mats can only be used on the basement wall up to an area of approx. 40 to 80 cm below the ground surface, as the earth pressure is too low above that. The maximum possible upper edge of the brown tank must be determined in consultation with the respective manufacturer.
Protection after installation: Even though the self-healing function of the bentonite mats offers a certain degree of security against minor damage, the waterproofing should also be mechanically protected with a non-woven fabric. In the case of insulated buildings, the protective function can also be taken over by the perimeter insulation. In order to achieve the necessary external contact pressure at the waterproofing membrane, the excavation pit should be filled in layers and with careful compaction.
Note: Since the brown tank is not yet subject to its own regulation, the standards of the DIN 18195 series may be of interest with regard to the assessment of the effect of water on a building.
DIN 18195-1, Building Waterproofing, Part 1: Principles, Definitions, Assignment of Waterproofing Types
DIN 18195-9, Building Waterproofing, Part 9: Penetrations, Transitions, Connections and Terminations
WU Directive (DAfStb Guideline for Waterproof Concrete Structures), Publisher: German Committee for Reinforced Concrete – DafStb at DIN German Institute for Standardization e. V.
Source: bauwion
