Waterproof concrete

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WU concrete refers to waterproof concrete .

According to DIN 1045-2: 2008-08 and DIN EN 206 (2017-01) (with the additions A1 and A2), hardened concrete with this special property is called “concrete with high water penetration resistance”.

Waterproof concrete is used, for example, in the construction of water towers, water tanks, bathing pools, sewage treatment plants, tubs in the groundwater, pipelines, dam walls and bank reinforcements.

To ensure that the components made of waterproof concrete are also impermeable to water, additional requirements must be met, see white tank .

Until the excess moisture in the fresh concrete has escaped from the so-called drying layer of the concrete component after a few years through diffusion , waterproof concrete will also release moisture.

In order to meet the requirement for high water penetration resistance, the impermeability of the cement paste is crucial. Therefore, the water-cement ratio for components up to 40 cm thick must not exceed 0.6. For components over 40 cm not over 0.7. It is advisable to set the value 0.05 lower due to unavoidable variations on the construction site. Waterproof concrete prevents the passage of liquid water mainly through the impermeability of the cement stone. In the case of a capillary pore space of less than 20% by volume, the capillary pores usually no longer connect continuously, which makes the cement stone impermeable to water. The diffusion of water vapor is also prevented from a component thickness of 200 mm. If waterproof concrete is used against pressing water , for example in the groundwater area, an additional sealing layer can usually be dispensed with.

Moisture transport

Cross section through a cut-off wall . A minimum thickness of the structural waterproofing must be observed so that the capillary area and the drying layer do not overlap .

According to more recent studies, moisture is transported into the concrete in an approx. 70 mm thick capillary area regardless of the hydrostatic pressure, while on the air side the component slowly dries out to equilibrium moisture. As long as the approx. 80 mm thick drying layer in the diffusion area does not overlap with the capillary area, the core area of ​​the component prevents water from being transported from the outside to the inside.

Stress classes

The WU guideline recognizes two stress classes:

  • Stress class 1 (BKL-1), for constant and occasional pressing water, as well as waterproof roofs.
  • Stress class 2 (BKL-2), soil moisture and water running off the wall.

Usage classes

The WU guidelines recognize two usage classes:

  • Usage class A (NKL-A), standard in residential construction, storage rooms with high-quality use, with additional measures also offices. Damp spots on the component surface are not permitted.
  • Usage class B (NKL-B), garages, installation shafts, storage rooms with low requirements. Damp spots with water penetration (water pearls) are permitted.

Design principles

With regard to the requirements for the crack widths, the WU guideline lists design principles A, B and C.

A) Avoidance of cracks, through the choice of suitable constructive, concrete technical and execution measures.

B) Crack distribution - For the BKL-1, the permissible separating crack widths are limited in order to be able to prevent the penetration of water by self-healing of the cracks. The permissible crack widths are based on the ratio of the pressure water level to the component thickness. In any case, the separation crack widths must not exceed 0.2 mm. It is assumed here that water passing through the wall causes a structural remodeling and self-healing of the cracks as long as the concrete has not yet completely set. In order for the cracks to close before the start of use, water must be present from the outside during the shell construction phase. If the water levels are only changing, there may be limited water penetration later. With service class A, one should not rely solely on the self-healing of the cracks.

C) With design principle C, locally larger separation crack widths are permitted, which are subsequently closed with separately implemented sealing measures. To reduce the residual risk, for example, in areas that are difficult to access, the use of fresh concrete composite foils as an additional sealing layer can be considered.

White tub

So-called “white tubs” or “waterproof tubs” are structures that are impermeable to water from all sides and made only of impermeable concrete without additional sealing sheets. It is usually located in the area of ​​the groundwater (basement). A watertight structure needs more than just waterproof concrete. From a static point of view, too, it must be guaranteed that the concrete remains in state 1, i.e. not cracked. Cracks must be mathematically limited to a maximum of 0.2 mm. In addition, construction and expansion joints must be securely sealed.

The installation of the concrete must be carried out carefully and with thorough compaction. Careful post-treatment of the concrete is also very important.

The width of the separation crack is influenced by different measures:

  • Component thickness
  • Component size / distance from joints ( controlled crack joints, construction joints, expansion joints)
  • Constraints from temperature during setting (the concrete becomes warm due to the setting process and then cools down again if it has already hardened)
  • Concrete recipe,
  • Using cement with low heat generation,
  • Constraints from load, amount and type of reinforcement (many thin steels are cheaper than a few thick ones (=> more cracks but smaller), mesh reinforcement is advantageous)
  • Measures for the installation and the post-treatment of the concrete

In Germany, the guideline of the German Committee for Reinforced Concrete (DAfStB) "Water-impermeable structures made of concrete (WU guideline)" (December 2017) regulates structures with corresponding properties. For Austria there is the guideline "Water-impermeable concrete structures - white tubs" (March 2009) of the Austrian Association for Concrete and Construction Technology.

Recommended minimum component thicknesses

The minimum component thicknesses recommended in the WU guideline are based on the exposure class, i.e. the type of water that occurs. Stress class 1 applies to constant and occasional pressing water. Stress class 2 applies to soil moisture and water running off the wall.

The recommended minimum total thicknesses of waterproof concrete components in millimeters can be found in the table:

Component Stress class Execution type
In-situ concrete Element walls or element ceilings with in-situ concrete Prefabricated parts
walls 1 240 240 200
2 200 240 100
Base plate 1 250 - 200
2 150 - 100
Roofs without thermal insulation 1 200 240 180
Roofs with thermal insulation 1 180 220 160

Web links

Individual evidence

  1. ↑ a b R. Beddoe, R. Springenschmid: Moisture transport through components made of concrete. In: Concrete and reinforced concrete construction. 94 (1999) H. 4, pp. 158–166, cited according to regulations and recommendations for water-impermeable (waterproof) concrete structures , see web links.
  2. ↑ M. Fastabend, E. Eßler, B. Schücker, M. Albert: White tubs with high quality use. In: Concrete and reinforced concrete construction. 105 (2010) H. 5, pp. 304-317.
  3. ↑ Prof. Dr.-Ing. Rainer Hohmann: Waterproof structures made of concrete, innovations in the revised WU guidelines , 2019
  4. ↑ a b Dr.-Ing. Christoph Alfes: The new WU guideline of the DAfStb , In: Cemex.de, German Committee for Reinforced Concrete eV, Berlin, February 22, 2018
  5. ↑ Zement-Merkblatt Hochbau H 10 5.2019 - Water-impermeable structures made of concrete , In: Beton.org; accessed in March 2020