Key Considerations for Type B Waterproofing Systems

Ahead of the 2019 International Structural Waterproofing Conference on 17th July 2019 and following on from the article I wrote recently on LinkedIn regarding Type A Waterproofing, it only seemed fair to give other types of waterproofing their fair share of attention! So this article will look at Type B Waterproofing, or waterproof concrete as it is perhaps more commonly known.

The subject of concrete is extremely vast!

The subject of concrete is extremely vast and this short article will certainly not scratch the surface. It is again based on some of the key considerations of Type B waterproofing as detailed in the PCA Best Practice Guidance Type B Waterproofing Systems.

Economic viability and availability means concrete is the most common material used in modern basement construction and there are a variety of methods in which concrete is used, including:

  • In-situ concrete
  • Insulating Concrete Formwork (ICF)
  • Precast concrete
  • Twin Wall

Waterproofing tightness of Type B – needs good design

Type B waterproofing protection, as defined by BS8102:2009 (Code of Practice for Protection of Below Ground Structures Against Water from the Ground), is where the structure itself is constructed as an integral water resistant shell. Invariably built of reinforced concrete, the basement structure must be designed within certain strict parameters to ensure it is water resistant. Given that concrete is normally part of the makeup of modern basement structures, it is logical to try and incorporate a waterproof element to prevent ground water ingress.

The water tightness of the Type B waterproofing construction is reliant upon good design and construction. Using a concrete of low permeability and joint detailing, will mean defects can be minimised by correct specification and design and, by careful construction.

Making concrete waterproof

Concrete by its very nature is water resistant but there are essentially two methods of making it waterproof. The first is the use of steel reinforcement bars to control cracking to a point that any cracking will not allow the passage of water. The second method is the use of admixtures which, “act to reverse the capillary or ‘sucking’’ action of the tiny capillaries on the concrete surface and to effectively block the pores within the concrete when subjected to hydrostatic pressure.”

Making construction joints waterproof

Construction joints need particular attention as these are the areas most commonly associated with leaks and will be a prominent feature at the forthcoming International Structural Waterproofing Conference on 17th July 2019.

Joints between components, movement joints and day work joints should be durable, watertight and include appropriate water stops or hydrophilic protection. Kickers, usually cast as part of the slab, should also be “watertight concrete” and are used to form the joint with the walls.

The construction of a ‘kicker’ after pouring the floor slab should not be encouraged as it is difficult to construct without defects. Therefore, kickers should be cast with the slab using appropriate edge formwork, but will require careful construction to obtain full compaction. Modern types of formwork and kicker less construction techniques mean that kickers no longer need be part of the construction process.

Consideration of water stops

Water stops should be used to provide enhanced resistance to water transmission at joints in the concrete structure. e.g. at construction or day work joints, services or other penetrations.  The principal types of water stops can be classified as the following:

  • Passive sections:
    – rubber or flexible polyvinyl chloride (PVC) extruded profiles cast into the concrete on both sides of the joint, either at the concrete surface or mid-depth of the concrete section, to form a physical obstruction to water transmission                                              – metal water bar strips placed mid-depth of the concrete section to form a physical obstruction to water transmission.
  • Active or hydrophilic strips or crystallization slurries:                                                           – Preformed profiles or sealant composition of materials applied to the concrete joint at depth in the section.
  • Permeable hose or other sections that are fixed to the construction joint surface before casting the second pour, to facilitate the injection of a specialist sealing resin into the joint, when required.

Water tables and concrete

With a high water table, minor defects in the concrete usually result in only small amounts of water penetration. Stopping these is usually fairly straightforward. Remedial action may, depending on the form of construction, be carried out from the inside thus avoiding the need for external excavation.

Variable water tables present a reduced problem unless the water table stays high for a long time. In a free-draining site, it is rare for a defect to be so serious that the water comes through by capillary action. The water and water vapour resistance of Type B protection relies on the materials incorporated into the external shell of the structure itself and will be a function of the section thickness. Defects are not always identified during the construction stage and only become evident after completion.

One of the most regular causes of failed waterproofing systems is defects in the installation. Typically Type B Waterproofing Systems will usually require that they are installed ‘under license’ of the supplier/manufacturer. However in practice, this does not always happen and it is not uncommon for waterproofing systems to be installed by operatives who have no relevant qualifications and/or no previous experience of installing waterproofing systems. This should not happen and in the main is the responsibility of the main contractor and installations should be undertaken by suitably trained operatives.

Types of waterproofing defects in concrete

The most common defects are:

  • Permeable concrete
  • Honeycombing through lack of compaction
  • Contamination of or cold joints
  • Cracks due to thermal contraction and shrinkage
  • Poor and inadequate placement of water bars, hydrophilic strips and joints

The most common defect is “honeycombing/lack of compaction. The main cause is the wrong choice of consistency. Either the consistency was too low making compaction difficult or the concrete had extra water added and segregation took place.

Cracking in the concrete – who is to blame?

Plastic and long-term drying shrinkage, thermal cracks, induced cracks and cracks caused by restraint are normally associated with either Designer and/or Contractor issues.  It is NOT normally the fault of the material however, this can happen on the rare occasion. In terms of the cracks that typically happen:

  • Thermal Cracks: These are caused by temperature differentials, particularly in mass concrete due to the heat of hydration. As the interior concrete increases in temperature and expands, the surface may be cooler and contracting thus causing tensile stresses that may result in thermal cracks at the surface.
  • Drying Shrinkage Cracks: As most concrete mixes contain more water than actually required for the hydration process, the remaining water evaporates over time causing the concrete to shrink. Restraint to shrinkage causes tensile stresses to develop in the hardened concrete. Restraint to drying shrinkage cracking is a common cause of cracking in concrete.
  • Plastic Shrinkage Cracks: These are caused by the evaporation of water from the surface of freshly placed concrete faster than it is replaced by bleed water, thus causing the surface to shrink. Due to restraint from the underlying concrete, tensile stresses develop in the weak plastic surface of the concrete, resulting in shallow cracks of varying widths and depths.

Why use PCA members for type b waterproofing?

Members of the PCA that specialise in structural waterproofing are the recommended first port of call for practical advice on waterproofing below ground structures. Those who specialise in structural waterproofing are proficient in a number of techniques including Type B Systems that can be utilised to prevent water ingress in underground ground structures.

The PCA’s nationwide list of contractor members are carefully vetted before being awarded membership and are then subjected to rigorous ongoing auditing procedures once admitted to the Association. Members of the PCA can offer insurance backed guarantees for much of the structural work they undertake.

Take advantage of our Knowledge – 2019 Conference

For those wishing to find out more about waterproofing underground structures, the annual forthcoming International Structural Waterproofing Conference takes place on 17th July 2019 at the Slate, University of Warwick. This is the must-attend event for anyone wanting to be at the forefront of the UK construction sector and brings together leading individuals in the structural waterproofing and construction industry to learn and grow knowledge through presentations and debates.

We have already noted the significance of construction joints and this will feature as a prominent topic at the event.

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