Waterproof concrete structures
In structures made of waterproof concrete, which also includes the white tank, the concrete not only has structural load-bearing tasks, but also sealing functions. Waterproof concrete is used, for example, in the presence of pressurised water, for base plates or basement walls. The construction is usually carried out using in-situ concrete, but it is also possible to erect waterproof structures using prefabricated concrete elements, for example element walls as triple walls made of concrete with high water penetration resistance.
Precise instructions on the design of white tanks are laid down in the waterproof concrete guideline of the DAfStB (German Committee for Reinforced Concrete). In addition to the waterproofing guideline, the general regulations also apply. DIN 18195, “Building waterproofing”, is not applicable as it does not contain any regulations for waterproofing structures.
Concrete with high water penetration resistance according to DIN 1045 and DIN EN 206-1 is used for the construction of waterproof structures. The minimum compressive strength class is C25 / 30, the cement content must be at least 280 kg per m³ of waterproof concrete. Higher strength classes can result, for example, from static requirements of the structures. A defined water-cement ratio ensures the impermeability of the concrete.
The requirements for ensuring the durability of the concrete are defined via the exposure classes and the concrete composition is adapted accordingly. For the installation consistency of waterproof concrete as in-situ concrete, at least consistency class F3 or softer is recommended.
Further specifications result, for example, for the crack width limitation or possible deformations due to hydration heat or drying shrinkage.
A perfectly planned white tank is designed to create a structure that is permanently functional and is not affected by the effects of moisture or water ingress. In order to ensure this, building physics principles must be considered and structural design criteria fulfilled as early as the planning stage. This is the only way to produce waterproof concrete structures without defects. The Water Reclamation Guideline provides precise information on this. For planning and execution:
- In the first step, the stress class and the design water level are determined. The BWS can be initially measured on the basis of groundwater level maps.
- Determination of the usage class on the basis of the planned building project and thus also the permitted tolerances.
- Building physics requirements resulting from the use, for example thermal insulation, moisture protection, heating and ventilation.
- Determination of the minimum wall thicknesses from the stress class and other requirements.
- Determination of pressure gradient and calculated crack width
- Optimisation of the design, in particular with regard to constraint loads
- Determine joint layout and sealing requirements
- Plan and consider installation parts and penetrations
Two stress classes are defined for white bathtubs in the waterproofing guideline. The stress class depends on the type of water present.
- Stress class 1 applies to pressing and non-pressing water as well as temporarily accumulating seepage water that can accumulate on low-permeability soil layers without drainage.
- Stress class 2 includes non-accumulating seepage water on highly permeable soils and on less permeable soils with drainage according to DIN 4095 as well as soil moisture with capillary-bound water.
- Depending on which function the finished watertight structure is to have, there are two different use classes for which different tolerances apply with regard to watertightness. A distinction is made between
- Use class A as standard in residential construction as well as for storage rooms with high-quality use
- Use class B for individual and underground garages, installation and supply shafts as well as storage rooms with low requirements.
In service class A, water is not permitted; no damp spots may form on the surface. The formation of condensation is generally permissible, but can be excluded by special additional requirements for the room climate and building physics, depending on the agreement. In use class B, on the other hand, damp spots and discolouration or even water beads are tolerated. Water penetration is not permitted here either. Special agreements in the construction contract may include special regulations on the structure (structural waterproofing), such as a WU concrete basement (basement waterproofing), over and above the defined use classes.
Even if the requirement for a white tank is to create a structure that is as monolithic as possible and thus impermeable to water, construction joints or expansion joints cannot be avoided. Here, suitable systems for waterproofing with waterproof concrete must be installed, such as can be found in the B.T. waterproofing products. Suitable and approved for waterproofing in the area of waterproof concrete constructions is the product SynkoElast, which is counted among the regulated waterstops.
Which joint seal is used depends on the expected water pressure. The guideline further differentiates between two types of joint sealing:
- Uncoated metal waterstops for interior use and waterstops for interior and exterior use are among the regulated joint sealants.
- Non-regulated joint seals include injection and grouting hoses, swelling profiles, sealing pipes, coated metal waterstops or combination solutions. General building authority test certificates (ABP) must be available as proof of usability for these waterproofing variants.
All types of penetrations such as pipe penetrations, cable ducts or anchor sleeves from the formwork bracing also place high demands on the sealing of a white tank. In order to minimise the sealing effort, care should be taken during the planning phase to ensure as few penetrations as possible or otherwise lead through the walls at right angles.