The benefits of a layered approach

The right choice of waterproofing for below ground structures can help avoid the financial implications of water ingress. Alex Burman of Sika looks at the options for architects and specifiers.

With the primary cause of building failure being water ingress, the importance of developing a robust waterproofing strategy is vital to the longevity of any building.

Faced with the consequences of failure from business disruption to operational delays, loss of rent to disputes costs, the best defence is to get the specification right first time with an appropriate and robust waterproofing solution. But are specifiers best advised to take a single system approach or is an integrated, dual system the way to reduce risk and ensure a dry internal environment?    

When designing a below ground waterproofing specification, it is important to first understand the terms of internal use and external effects on the structure. External effects vary from one structure to the next, from the type of soil to water table level and the type of water table. With this information, it is crucial that the below ground structure is fit for the future and prepared for water – whether it’s present or not.

The recommendations given in BS 8102:2009 provide the specifier with an outline of the three different waterproofing methods and include Type A – external or internal “tanking” typically using a membrane or cementitious render system; Type B – integral protection using an appropriate concrete mix design and admixtures providing a watertight barrier; and Type C – an internal drainage membrane (or water management) that collects and disposes of water that enters the structure.

The complexity of so many construction projects, or when the assessed risks are high, has led to specifiers adopting a more integrated approach, one where different solutions are combined and used to their strengths, to create a highly robust waterproofing strategy.

The right combination

The British Standard suggests consideration be given to the use of dual systems in projects where the assessed risks or the consequences of failure are too high. This approach to, say, a multi-million pound inner city development could see the application of tanking and watertight concrete to provide an impenetrable structure.

The most widely used waterproofing system in the UK and many other countries are external waterproofing membranes. Often they perform a dual purpose – waterproofing as well as offering an additional protection measure (APM) for the concrete where aggressive ground conditions prevail.

In the case of projects which combine both refurbishment and new build, different systems could be specified for different applications – for example, render systems for retro installation as part of the refurbishment and watertight concrete and/or membranes for the new build element.

Membrane strength, durability and performance are key considerations in the specification process of waterproofing in heavy civil engineering applications or aggressive ground conditions. For example, a flexible Polyofin (FPO) membrane can reasonably be expected to have a service life of more than 75 years, whereas a bitumen membrane of a similar thickness is expected to last approximately 25 years in service. Flexible materials such as PVC-P and FPO have superior crack bridging ability and stress crack resistance, when compared to HDPE.

Integral waterproofing

Offering buildability, repairability and reduced cost, integral waterproofing systems have gained in popularity. When relying on the concrete to provide a watertight barrier, the detailing of joints and penetrations, as well as achieving a “crack free” concrete are important. Design and execution should also consider a limited crack width, correct mix design, an effective curing regime and pour sequencing in order to avoid shrinkage or cracking.   

It’s important for specifiers to be aware that an integral waterproofing system is not suitable for an application such as a podium deck, where thermal movement and shrinkage may lead to cracking beyond the normally accepted levels. Additional measures are necessary where, for example, Radon is an issue. Even ‘crack free’ concrete will contain micro-cracks which are a potential path for Radon and Methane to enter the building.

The Sika Watertight Concrete System is an example of a guaranteed system which complies with Type B construction, using advanced admixtures added to the concrete to prevent water penetration. Carefully selected waterstops for construction and movement joints complete the system, producing a waterproof structure.

Water management system

Where it isn’t possible to keep water out completely, water management is an effective solution. Cavity drainage 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.

These Type C systems effectively allow water ingress, so the potential effect on the durability of the structure must be considered. Ongoing maintenance costs for sumps and pumps must also be taken into account, as well as any effect these extra design details have on the building’s design and useable space.

These systems are well suited to refurbishment projects but perhaps should be used as a backup in combination with a tanking or integral system for new build projects.

Go with the flow

To ensure buildings remain impenetrable to water, whilst meeting the building standards of today and the future, a measured and evaluative approach to the product selection needs to be taken.

The NHBC, along with other insurers, has also upped the ante having raised standards of waterproofing most notably in terms of habitable below ground accommodation, insisting that a combined approach to waterproofing should be used for Grade 3 environments.

Careful consideration must be given to the selection of the product, or products, as a single solution may not be right for the project. With below ground waterproofing, it is certainly not a case of ‘one size fits all’. In the light of this, an all-round solutions provider has the ability to  to consider the entire waterproofing process to benefit clients.

Alex Burman is product manager at Sika Waterproofing