CPD: Substructure waterproofing
Successful strategies for preventing water ingress below ground require a clear understanding of BS 8102, says Greg Austin.
Water ingress is the primary cause of building failure so developing a robust and strategic approach to structural waterproofing is vitally important for the longevity of any building. With many post-tenancy problems attributable to moisture damage, the consequences of failure are numerous: dispute costs, legal fees, damage liability, loss of rent, business disruption and operational delays.
Substructure waterproofing is particularly challenging, especially as below-ground structures are increasingly used as habitable spaces – often in urban areas where there is a shortage of buildable land. But if the designer or contractor doesn’t have a complete understanding of BS 8102, the British Standards Institute Code of Practice for Protection of Below Ground Structures Against Water from the Ground, the client could be left with a basement that is code compliant but not fit for its planned use.
According to Reducing the Risk of Leaking Substructure, a clients’ guide produced by the Institution of Civil Engineers (ICE): “If the construction is below the water table, the groundwater is under pressure and will flow through the path of least resistance.” Simply put, water will pass through any cracks in the substructure into the basement.
A further complication is that the substructure cannot be constructed in one piece and construction joints can act as water paths. So the waterproofing system needs to be designed and installed to ensure the basement remains dry throughout its lifetime, but continues to comply with Building Regulations through any potential changes of use.
Underground spaces can only be fully protected from water damage by preventing water ingress entirely, so the structural and foundation waterproofing system is crucial. Specifying the right material solutions early in the design stage is important; it is difficult and costly to address leakages later. Technical challenges can arise from soil conditions, water tables, groundwater conditions and environmental changes, improperly cured concrete, inadequately designed foundations or poor workmanship.
Complying with BS 8102
To ensure the highest standards are met, below-ground waterproofing solutions must adhere to BS 8102, which covers the use of waterproofing barrier materials applied to the structure, structurally integral watertight construction and drained cavity construction. It provides recommendations and guidance on methods of preventing or dealing with the entry of water into a below-ground structure.
The standard applies to all structures which extend below ground level and those on sloping sites, and covers the evaluation of groundwater conditions, risk assessment and options for drainage outside the structure.
A waterproofing strategy from design stage allows for a more successful and cost-effective conclusion. To develop a robust design for protecting a structure against groundwater, BS 8102 requires the following factors be assessed:
- All risks from external environment, water, gas and contaminants;
- Adoption of a waterproofing strategy capable of achieving the client’s required internal environment;
- Early inclusion of a waterproofing specialist;
- Clear understanding of client requirements and expectations;
- The selection of a suitable primary waterproofing system as part of robust design.
Preprufe Plus features improved overlap bonding and faster installation
A waterproofing design specialist can, in theory, be anybody who is able to provide expertise in structural waterproofing. The specialist should:
- Be suitably experienced;
- Be capable of devising solutions that accommodate various project constraints and needs;
- Provide the design team with information and guidance that assists with and influences the design, installation and future maintenance of the waterproofed structure.
The specialist will need to classify the water table. It is classified as “high” – and therefore a serious risk – if it is “perched” or is permanently above the underside of the base slab. Where the water table fluctuates, the classification is “variable”, and only if the water table or a perched water table is permanently below the underside of the base slab is it classified as “low”.
BS 8102 places great emphasis on a site evaluation which includes the completion of a desk study:
- to assess the geology and hydrogeology, including soil permeabilities, flood risk, radon, methane and other ground gases and contaminants (eg chlorides and acids);
- to assess the topography of the surrounding ground in relation to the below ground structure;
- to establish the likely highest level of the water table and the potential for the occurrence of a perched water table;
- to identify any missing ground and groundwater information, which should then be obtained by undertaking a site investigation in accordance with BS 5930 Code of Practice for site investigations and BS EN 1997 – Geotechnical design
In section 5.1.2 it states that the risk assessment should also consider long-term water pressures, the effects of surface water infiltration, any use of external drainage, the effects of climate change, burst water main sewers and the effects of drainage on existing neighbouring structures.
During the lifetime of any structure, some degree of groundwater pressure is likely against the waterproofing system. Where cracks or joints can provide potential water paths, water ingress can occur. For water resistance, if no detailed geological/hydrological assessment is available, or there are inconclusive soil investigations and drainage measures, the waterproofing strategy is designed on the basis of water to the full height of the retained ground.
Understanding client requirements
As well as ground conditions and other external influences, the waterproofing strategy must take into account the requirements of the client. For example, does the client understand the difference between “wet”, “damp” and “dry”? Have the costs associated with achieving a “dry” basement been properly considered? What are the consequences of failure and has the future use of the building been discussed?
BS 8102 details the three grades of protection available:
Grade 1: Basic utility (wet) – some seepage and damp patches allowed
Grade 2: Better utility (damp) – no water penetration but water vapour allowed
Grade 3: Habitable (dry) – totally dry environment.
To achieve these levels of water-tightness, BS 8102 provides an outline of the three different waterproofing methods. These are:
Type A – a physical barrier system typically using a membrane or cementitious render system
Type B – integral structural protection including using admixtures to provide a watertight barrier
Type C – cavity drain protection which uses preformed cavity formers that collect and dispose of water that enters the structure.
BS 8102 states that consideration should be given to the use of combined protection (ie Type A and Type B, Type A and Type C, or Type B and Type C) where in a single system:
- the assessed risks are deemed to be high;
- the consequences of failure to achieve the required internal environment are too high; or
- additional vapour checks are necessary for a system where unacceptable water vapour transmission can occur.
Structures with Type B protection are designed to be water resistant, however, additional waterproofing systems may be applied internally or externally to control water vapour movement where appropriate. An in-situ “liner” wall designed to provide Type B protection can be cast inside an embedded retaining wall to provide combined protection. In some cases, a fully bonded barrier can be provided between the two elements.
Although structures with Type C protection are designed to control and manage seepage into a structure, where this is unacceptably high the water resistance of the structure should be improved prior to the installation of the Type C protection, by the application of either Type A or Type B protection.
The compatibility of the different protection types should be assessed and preferably single-sourced in order to minimise the risks – with a fully bonded, adhesively sealed membrane a crucial component in the waterproofing strategy.
While each waterproofing system has its own advantages and suitability, the complexity of many construction projects or when the assessed risks are deemed too high has meant that specifiers are increasingly adopting an approach which incorporates a combination of these systems. The use of combined protection systems ultimately reduces risk.
Typical Type A solutions include cementitious coatings, bonded sheet membranes and pre-applied bonded membranes. However, once a membrane is breached, the whole concrete foundation will be vulnerable to the presence of migrating groundwater. This can also lead to corrosion of the reinforcing steel if chlorides are present in the groundwater. When a leak becomes visible, it is often a considerable distance from the actual source, which makes leak detection challenging and remediation expensive.
When using a membrane, either alone or as part of a combination of systems, it is vital to use a fully bonded waterproofing barrier that prevents water migration and ensures continuity with the above ground damp proof course/building envelope. It is also advisable to avoid penetrations and prevent the incorporation of movement joints unless unavoidable. Get the waterproofing specification right and water migration can be prevented with one relatively simple application.
Workmanship is a major factor in obtaining good quality and durable structures and the need for concreting and membrane installation to be supervised by qualified and experienced personnel is of primary importance.
Many membrane systems can work well in test conditions but can be complex to install under site conditions with a wide range of ancillary products necessary to complete the details which mean hidden costs, longer installation time and the risk of mistakes.
To be truly effective, a concrete and waterproofing solution requires not only a good concrete design and mix, permanent and fully bonded waterproofing membrane system but protection for joints and connections where the majority of leaks occur. To substantially reduce the risk of leaks and preserve structural integrity a combined protection approach incorporating a high quality fully bonded Type A waterproofing system such as Preprufe will combat the harmful effects of water migration and prevent concrete deterioration before it can begin.
A well-thought-out and carefully considered waterproofing strategy that adopts a simple approach, involves the waterproofing specialist and considers buildability will succeed if implemented correctly. So the waterproofing team must consider the consequences of failure and the additional costs this will bring, before making a very simple waterproofing decision – get it right now or pay for it later.
Greg Austin is global marketing manager – waterproofing at Grace
A proven track record
As Europe’s largest construction project, Crossrail is transforming rail transport in London by increasing capacity by 10%, supporting regeneration and cutting journey times.
The Crossrail Liverpool Street C502 site incorporates Liverpool Street station, Moorgate station and an area known as Blomfield box (above), and spans more than 0.4 km in one of the busiest parts of the UK capital.
As one of the main stations serving the City of London, it required a reliable waterproofing solution to prevent water ingress around the base slab.
Situated 30 m below ground, waterproofing of the dense steel reinforcement within the base slab and the complicated detail at the intersection between the cast slab and the sprayed concrete lining was a crucial consideration.
With both gas and waterproofing protection required in sections of the project, Preprufe proved the perfect system for engineers Mott MacDonald and Arup, and main contractor Laing O’Rourke.
Following onsite training in the application of Grace waterproofing products, approximately 4,000 sq m of Preprufe 300R/160R waterproofing membranes, 1,500 sq m of Preprufe 800PA damp proof and waterproof membrane, and 250 sq m of Silcor liquid waterproofing was installed to provide exceptional protection to the tunnel shafts, piled walls, base slabs and roof. Construction of the Crossrail line began in May 2009. Grace waterproofing products have been used on all projects to date.