Waterproof sheet membranes are well known for keeping the water out or “tanking” below-ground structures. However, in recent times there has been significant product evolution in this field, with manufacturers using technology to overcome historic failings in these types of system. Richard Crossley of Newton Waterproofing Systems reports.
The BS 8102:2009 Code of Practice for the protection of below-ground structures against water from the ground is the predominant consideration when designing and installing waterproofing systems, and refers to waterproofing membranes as ‘Type A’ systems.
Overall, the Code of Practice outlines three system types:
- Type A (barrier protection)
- Type B (structurally integral protection)
- Type C (drained protection)
Examples of Type A Systems
Type A systems can be applied either externally or internally, as shown in Figure 2.a) from BS 8102:2009:
- An external membrane, as in key 1, will resist hydrostatic pressure that pushes the membrane against the structure (positive pressure)
- A membrane applied internally must be designed so that it resists the negative or counter-thrust pressure
- Sheet membranes applied internally, usually by an adhesive bond, have to be loaded by further construction, shown here by key 4 as ‘sandwiched waterproofing’
- Alternatively if a material can be applied internally so that it achieves a full bond, as in key 6, it can resist counterthrust pressure sufficiently without further construction
The Importance of Achieving a Full Bond
Regardless of the membrane being internal or external, it is recommended that a full bond is achieved instead of the partial bond that occurs with adhesive sheet systems.
Figure 6 from BS 8102:2009 explains the role of a fully bonded membrane in eliminating water tracking from a defect. For example, if water ingresses though a defect in a partially bonded membrane, the water will track between the membrane and structure to a crack or joint in the supporting wall.
Designing to Consider Defects
The Code of Practice also advises that waterproofing design should allow for defects owing to poor workmanship, the inappropriate use of materials, or defects owing to the specific properties of the materials used. It is therefore essential that the construction methods and materials utilised are such that:
- Potential defects owing to poor workmanship or the inappropriate use of materials are avoided
- Potential defects owing to the specific properties of the materials used should be recognised and catered for in the design
With all systems however, although the products themselves are often tested and certified, it is the installation that lets them down, and it is widely acknowledged that 100% perfection in installation is not possible.
This judgment is reflected by this overview from the landmark High Court case, Outwing vs. Weatherald, which states that, “It is not reasonable or realistic to expect a bonded sheet membrane to be applied without any defects at all”
Defects in Type C systems include; poorly sealed and badly detailed laps which do not weather against the possible flow of water, systems which are not maintainable and can clog up with lime and silt, and poorly installed/designed pumping and back-up systems.
Defects in Type B systems include; poor compaction leaving voids (honeycombing), poor/high water-cement ratios leaving voids in the form of open capillaries, poorly installed construction joint accessories, and the many forms of cracking.
Defects in Type A systems include; poorly sealed laps, damage during application, and penetrations caused by ongoing trades. With external membranes that are subject to hydrostatic pressure, water can ingress at such a defect and, in historic membranes, will track between the concrete and the membrane to find further ingress at joints, therefore making leak detection almost impossible.
The Evolution of the Type A Membrane
Pre-applied membranes were developed as a solution to such issues, with several being on the market for quite some time. Whilst some of these membranes achieve an adhesive bond, others form a full mechanical bond, and whilst some are passive membranes, others are active, i.e., they swell when hydrated.
The ability to bridge cracks and other defects in type B construction is also an essential requirement for the evolution in Type A membranes, otherwise two compromised systems can potentially align. For example, while a passive system relies on its adhesion or mechanical bond to withstand the pressures of water tracking, an active membrane will swell in order to resist water ingress, using its mechanical bond to suitably confine it. Currently however, the materials used in active systems require full confinement, otherwise, should they lose their restricted swell pressure, water would be allowed to percolate through.
The evolution of cutting-edge hydrophilic polymer membranes is designed to minimise this risk. Newton 403 HydroBond is one such membrane, with a locking fleece to one side and a polymer hydrophilic coating to the other. Pre-applied to the over-site substrate, formwork, or property line construction (such as steel or concrete piled walls), the fleece fully engages with the cast-in-place concrete via a mechanical bond, and is therefore suitable for all below-ground and earth-retained structures.
The full mechanical bond restricts water tracking from potential defects, and works in conjunction with the self-healing hydrophilic coating, thus eliminating water tracking and sealing penetrations. Tests have shown that such hydrophilic polymer membranes are capable of bridging cracks of up to 4mm wide.
Finally, the hydrophilic element is inseparably linked to the parent material, with the result being that there is no loss in swell, even when unconfined. When this membrane is coupled with defects in Type B construction or even at open laps, the material should therefore still form an effective barrier to water.
Considering Combined Waterproofing Systems
BS 8102:2009 advises that a Combined Waterproofing System be considered where the likelihood of leaking is high, or the consequence of leakage is unacceptable.
At Newton Waterproofing Systems we always recommend that a combination waterproofing system is designed, and that a Waterproofing Specialist is involved as early as possible, preferably as the Design Specialist, as recommended by BS 8102:2009. Newton Specialist Basement Contractors (NSBC) can fulfil this role.
This article featured in Concrete Magazine, July 2015 – Download Article