Continuing Professional Development: Waterproofing below-ground structures
Water ingress below ground can have significant structural and cost impacts on the service life of buildings and structures. Standard BS 8102:2009 offers a guide to the different waterproofing options and combinations that are now available to the construction team. Alex Burman explains.
Waterproofing below-ground structures can only be achieved through careful design, the right choice of materials and professional workmanship with quality control on site. With 75% of building failures attributed to water ingress, it is absolutely crucial to get the below-ground waterproofing of new-build structures correct at the earliest possible stage.
All structures need waterproofing, which is in the long term the most cost-effective way to maintain the value of a building. Defects can occur in any system but risk can be minimised through design, planning, skilled application and a specialist who can offer expert specification advice.
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Standard BS 8102:2009 (Code of Practice for Protection of Below Ground Structures Against Water from the Ground) (see box, right) provides some guidance and information on the three types of waterproofing protection available, but choosing the right strategy can still be difficult. And, as water ingress has far-reaching financial implications, there is a growing trend to incorporate a combination of systems – which can include watertight concrete, sheet membrane and cavity drainage – to optimise protection in the long term.
When designing a basement, you must understand the terms of internal use and external effects on the structure. External effects vary from one site to the next, from the type of soil to the level and type of water table. It is crucial to design a basement that is prepared for water – whether it is present or not.
There are stringent standards on durability, exposure and stress conditions for waterproofing systems for below-ground structures. Owners usually request a service life of 50 years or more for buildings, and up to 120 years for infrastructure. A failure in watertightness will reduce this. Water ingress will lead to physical attack to the concrete and embedded steel of the structure.
Buildings and below-ground structures are subject to exposure conditions, which vary from project to project. They include:
- Different levels of water exposure and pressure – for example, damp soil, percolating water or water under hydrostatic pressure, open water and variable water tables;
- Aggressive groundwater containing chemicals, commonly sulphates and chlorides in solution;
- Unequal static forces caused by load, settlement or uplift;
- Dynamic forces – for example, from settlement or seismic activity;
- Temperature variations;
- Gases in the ground, such as methane and radon;
- Aggressive biological influences, such as plant roots or growth, or fungal or bacterial attack.
All of these may adversely influence the use, watertightness and durability of a basement structure, reducing the service life of the entire structure. Effects range from structural damage and cracking, to loss of thermal insulation and corrosion of the steel reinforcement – reinforcing the need for a high-quality and appropriate waterproofing solution.
The most common method of waterproofing below-ground structures is the barrier system – applied internally or externally – which incorporates the application of a sheet membrane to the outside of the basement or substructure, or cement-based materials to the inside. With a barrier system, performance is wholly dependent on the effectiveness of the waterproofing product specified.
A building should not allow water penetration but this becomes even more critical between the membrane and structure. A widely recognised problem for industry experts, a penetrated conventional membrane will make the whole structure vulnerable.
SikaProof A is an innovative fully bonded sheet membrane system for waterproofing in basement and below-ground structures. With a special sealant grid and non-woven fleece, SikaProof A provides a unique, mechanical-bond effect to the concrete structure and prevents any lateral water underflow between the membrane and the concrete.
Making the grade
Although not UK law, BS 8102:2009 is the document with which designers and specifiers must be familiar when specifying below-ground waterproofing products and systems.
As detailed in BS 8102, there are three grades of protection available:
Grade 1: Basic utility – some seepage and damp areas are tolerable
Grade 2: Better utility – no water penetration but moisture vapour tolerable
Grade 3: Habitable – totally dry environment
Grade 1 offers waterproofing standards that are suitable for use in
car parking facilities, plant rooms (excluding electrical equipment)
Grade 2 delivers protection for such spaces as residential and commercial basements, workshops and retail storage, where water migration is intolerable, but moisture vapour
can be allowed.
Grade 3 is necessary where absolutely no water or moisture vapour can be allowed to penetrate. This includes areas for archives and stores for sensitive or valuable materials and equipment.
For the specifier, BS 8102:2009 also gives recommendations on three waterproofing methods, the choice of which should take into consideration all relevant project requirements. The types of waterproofing are: “tanking”, typically using a membrane or cement-based render system; integral protection using an appropriate concrete mix design and admixtures that provide a watertight barrier; and an internal drainage membrane (or water management system) that collects and disposes of water that enters the structure.
The grid pattern offers additional protection: the integrated sealant bonds tightly with the surface of the concrete, forming watertight mini-compartments. So even if the membrane is damaged or pierced, moisture or pressured water cannot spread between the membrane and the concrete structure, as it is contained within the damaged grid square.
Concrete can only be fully protected from water damage by preventing water ingress entirely. An external membrane is the most effective barrier. Installed externally and covering the entire basement structure in contact with the ground, the Sikaplan waterproofing system is divided into compartments with a network of cast-in-place compatible waterstops welded to the membrane. Used extensively in heavy civil engineering projects or in highly aggressive conditions, this type of protection makes it easy to locate the position of a leak. Once identified by the control and injection sockets, remedial action can be taken quickly at any time during the product’s service life to ensure watertightness is not compromised.
As with any waterproofing system, it is crucial to choose a barrier or tanking product that offers proven performance. First used in 1910, the cement-based Sika 1 waterproofing system forms an integral part of the structure, and is designed to last the lifetime of that structure.
Suitable for internal or external applications, it incorporates a specially developed admixture – a liquid with microscopic insoluble particles suspended in it – which is mixed with blended pre-bagged mortars. Once it has been hand applied, the admixture reacts to water by turning into a jelly-like substance, blocking gaps and capillaries, and providing an invisible, impregnable seal. Bonding with the substrate, it essentially becomes one with the structure.
Watertightness can also be incorporated into the build process through integral waterproofing. Integral waterproofing systems have gained popularity because of their inherent buildability, repairability and reduced cost. When relying on concrete for a watertight barrier, the detailing of joints and penetrations, as well as “crack-free” concrete, are key.
Fully bonded membrane system
2. Internal applied waterstops
3. Over-bonding sealing tape system
Design and execution should also consider a limited crack width, correct mix design, an effective curing regime and pour sequencing to avoid shrinkage or cracking. Integral waterproofing may not be suitable as a gas barrier on its own, but can be combined with a membrane to offer methane, radon and carbon dioxide resistance. An integral waterproofing system is not suitable for an application such as a podium deck, where thermal movement and shrinkage may lead to cracking. Additional measures are needed where, for example, radon is an issue. Even “crack-free” concrete will contain micro-cracks that provide a potential path for radon and methane to enter the building.
The Sika Watertight Concrete System offers a solution in the waterproofing of concrete structures. Admixtures are added to the concrete to prevent water penetration. The system is completed with carefully selected waterstops for construction and movement joints.
Prevention of water penetration should always be the first waterproofing method. But when penetration cannot be prevented, such as in challenging renovations, cavity drain membranes work on the principle of allowing water to continue to penetrate the structure, and then managing the water and diverting it into a suitable drainage point.
Used where the substrate is not strong enough to resist stresses caused by water pressure, cavity drains can be used to achieve grades 1-3 in BS 8102. Irrespective of conditions at the design or installation, BS 8102 states that it must be assumed that hydrostatic pressure is going to occur at some time during the lifetime of the basement.
When specifying a cavity drainage system, consideration must be given to the form of construction, groundwater levels, external ground drainage, soil type and ground contamination. Using a mechanical pump to remove the water, the Sika Cavity Drainage System combines the water-removal system with a secondary internal wall and floor to control water after it has penetrated the structure.
Sika Ltd is a wholly owned subsidiary of Sika AG, a global company based in Switzerland that specialises in the manufacture and supply of chemical-based products for construction and industry. A world leader in its field, with subsidiaries in more than 80 countries, 16,000 employees and annual sales of CHF4.8bn (£3.3bn), Sika is also committed to providing quality, service, safety and environmental care.
In the UK, Sika provides market-leading solutions for concrete, waterproofing, roofing, flooring, refurbishment, sealing and bonding, and industry. It has plants in Welwyn Garden City, Preston and Leeds, 700 employees and annual turnover of £190m.
Using a high-density polypropylene internal drainage membrane, the Cavity Drainage System is installed loose laid in flooring applications, and attached to the wall with surface plugs in vertical installations. The system directs penetrating water into a drainage system and a collection sump before using a pump to discharge water from the building.
While each system has its own advantages and suitability for specific projects and applications, most manufacturers only offer one type. However, as projects become increasingly complex, specifiers are combining different solutions to create a highly robust waterproofing strategy.
When the assessed risks are deemed to be high or the consequences of failure to achieve the required internal environment are too great, BS 8102: 2009 recommends a combination of systems. This approach could use the application of tanking and watertight concrete to provide an impenetrable structure.
External waterproofing membranes are the most widely used in the UK and many other countries. Often they perform a dual purpose: waterproofing as well as offering an additional protection for the concrete in aggressive ground conditions.
For combined refurbishment and new-build projects, different systems could be specified – for example, render systems for retro-installation as part of the refurbishment and watertight concrete, or membranes for the new-build element.
In heavy civil engineering and tunnelling applications, or aggressive ground conditions such as those found in the Middle East, membrane strength, durability and performance are key in the specification process. For example, a flexible polyolefin (FPO) membrane can be expected to have a service life of more than 75 years, while a bitumen membrane of a similar thickness is expected to last about 25 years. Flexible materials such as polyvinyl chloride (PVC-P) and FPO have superior crack-bridging ability and stress-crack resistance compared with high-density polyethylene (HDPE).
In BS 8102:2009, the new design team code states that early involvement and good communication are essential for the success of any project involving below-ground structures. The code also recommends “a waterproofing specialist should be included as part of the design team so that an integrated waterproofing solution is created”.
It is implied that the specialist should have knowledge of the waterproofing systems available, the requirements of BS 8102, and structural and geotechnical knowledge. The Certificated Surveyor in Structural Waterproofing (CSSW) qualification is the highest accreditation in the structural waterproofing industry.
It gives the client assurance that the specialist has passed the CSSW examination, and legal and health and safety examination.
As the waterproofing specialist, the product manufacturer or supplier must be made aware of any design decisions or specifications that may affect waterproofing performance. It must also approve final decisions and recommendations.
To ensure buildings are impenetrable to water while meeting the building standards of today and the future, a considered approach to the selection of a product needs to be taken. A single solution may not be right. The NHBC has also upped the ante with higher standards of waterproofing, most notably in terms of habitable below-ground accommodation, insisting that a combined approach to waterproofing should be used. CM
Alex Burman is product manager – waterproofing, at Sika
Floor to sealing at Farringdon Tube
This access shaft at London’s Farringdon Tube station was formed by the construction of concrete piles around the perimeter, followed by the excavation of the enclosed earth from within. The pile rings and base slab were then constructed.
A safe and reliable waterproofing solution was required to prevent any water ingress through and around the base slab, which is of circular section, around 200 sq m in area, and situated 20 metres below ground level. Dense steel reinforcement within the base slab, and the complicated detail at the intersection between the cast slab and the sprayed concrete lining, meant that traditional rear-guard waterbars were not an option to seal the joint.
The fully bonded waterproofing membrane system SikaProof A-12 was selected to prevent water ingress through the base slab. It is a completely engineered solution, which produces a mechanical bond with the concrete, and incorporates a unique grid pattern of sealant that prevents any lateral water underflow and migration between the membrane and concrete.
Sika Tricosal Metal Waterstop, SikaSwell A-2010 and SikaFuko VT 1 jointing solutions were installed around the perimeter of the slab to protect against any water ingress at the interception of the slab and the walls.