Concrete structures such as below ground basements normally have to be watertight to prevent damage due to moisture or water ingress. This can be achieved by applying an external waterproofing system either as coatings, membranes or other surface applied systems or by using an integral waterproofing system that renders the structural concrete watertight.

It is well known, that concrete can be designed to be impermeable to water pressure through careful mix design and admixtures modification, but to keep a concrete structure completely watertight, more than just the concrete design has to be taken into consideration.
There are many expressions used worldwide that describe a ‘watertight concrete’. In general, we can differentiate between ‘Waterproof Concrete’, ‘Watertight Concrete Systems’ and ‘White Box’.

Waterproof concrete

Waterproof concrete describes the concrete mixture, which is impermeable to water and is focused on the quality of concrete, which has been modified using concrete admixtures such as superplasticizers and pore blockers. Because this includes only the concrete mix design, the joints and design of a basement are not considered. Therefore waterproof concrete does not indicate the watertightness of a specific concrete structure.

Watertight concrete systems

This term reflects a system consisting of waterproof concrete together with joint sealing solutions to build simple designs of watertight basements. Building a below ground concrete basement will include various working steps that incorporate construction and movement joints as well penetrations.
To ensure that the appropriate level of watertightness can be achieved, general guidance for the concrete mix design, construction and concreting are to be provided.

White Box

The next level of a watertight structure is the white box concept, which has been established, mainly in Central Europe, for many decades. In addition to waterproof concrete the white box concept includes the planning, design and all operations to be undertaken on site during the construction, in order to obtain a watertight basement. The main solution to achieve this goal, is the production of high quality concrete with focus on control crack formation. To achieve this, all cracks in the concrete must be very fine and well dispersed with no separation cracks going through the whole structure that would allow the transmission of water. Various standards for white box construction require a maximum single crack width ≤ 0.2 mm. Important elements that affects the crack formations are:

  • Concrete mix design
    An optimized granulometry and w/c-ratio; the selection of appropriate type of cement; an improved rheology and the use of various admixtures as shrinkage reducer, pore blockers, superplasticizers, etc. result in a limited crack formation within the concrete.
  • Concrete thickness
    A homogenous thickness of the concrete, without any changes of thickness, reduces local stress points. A minimum concrete thickness of ≥ 250 mm for walls and base slabs shows good practice.
  • Grade of steel reinforcement
    This is the key design element to limit crack formation. The amount of steel reinforcement normally is significantly higher than that needed for the structural integrity only. Calculation of the minimum steel grade and distribution should be carried out by a structural engineer who will be familiar with the local standard.
  • Shape and layout
    To reduce stress within the structure, the layout of a ‘White Box’ basement shall be designed at one level and in simple rectangular shape. Offsets or inside corners must be avoided.
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Traditional design of reinforced concrete base slab, not suitable for ‘White Box’ concept

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Appropriate design for ‘White Box’: simple tank shape, homogenous thickness, no offsets. The red part will be designed in high quality concrete (watertight concrete)

 

 

  • Design of joints
    The selection and definition of construction and movement joints has to be carried out according to the shrinkage behavior of concrete and the concreting stages. Joints should be positioned in order to split the base slab into regular square areas to reduce stress. Site conditions such as water pressure, underground and climatic conditions have to be take into consideration. Design requirements are different depending on the method and purpose of use.
    For joint sealing, PVC waterstops (for construction and movement joints), swelling profiles or injection hose systems (both only for construction joints) are mainly used.
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Optimized definition of joint layout on a base slab to reduce cracks.

 

 

 

 

  • Preparation on site
    To reduce the friction between the concrete base slab and the ground, a double layer of plastic foils will be required.
  • Concrete placement
    The structure, shuttering system and reinforcement have to allow good and easy concreting. A proper placing of concrete is required to prevent stresses and leakages and uncompacted or segregated concrete. This can be achieved by pouring each section (from joint to joint) continuously in one step without any breaks. By limiting the drop height to ≤1.0 m and by careful compaction of the fresh concrete, honeycombs can be eliminated.
  • Curing
    An adequate curing for at least three days using plastic foils or curing agents is necessary to prevent cracks due to dry shrinkage.

In addition to these detailed points for crack reduction, there are other points that affect the water tightness of the White Box construction:
Minimum concrete cover (≥30 mm)

  • Use of cementitious steel spacer
  • Correct positioning and sealing of all penetrations

Advantages of White Box concept

Compared to traditional external applied waterproofing system, the White Box concept includes the following advantages:

  • Concomitant static and sealing function
  • Simplified static and constructional design principles
  • Easy and fast application, no additional application of waterproofing layer required (less working steps)
  • Durable and integral waterproofing system
  • No drainage or double walls required
  • Simple excavation and less substrate preparation
  • Relative independence on weather conditions
  • Leakages can be located and repaired more easily
  • Less susceptible to external damages

All these advantages result in a cost effective solution and in addition reduce the complexity of site logistics.