Alex Burman of Sika discusses the key guidance, and technology options, for specifiers looking to achieve watertight concrete basement constructions
When beginning a basement construction or renovation, the waterproofing standard of BS 8102:2009 is key. It recommends and provides guidance on methods of dealing with and preventing the entry of water from surrounding ground into below-ground structures. Waterproofing systems are categorised into three types:
- Type A – waterproofing barriers
- Type B – watertight concrete
- Type C – water management systems
For habitable areas, such as residential basements or areas containing plant or other technology, a high degree of waterproofing is required. A structure such as a below ground car park where some seepage and damp areas are tolerable, requires a less onerous system.
Type B watertight concrete and admixtures
In modern constructions, concrete is the material of choice in basement structures, thanks to its proven durability and strength. Regular reinforced concrete designed to BS EN 1992 still allows limited crack widths to form, even though concrete appears to be a solid material. Microscopic capillaries are left behind by excess water (aiding workability when the concrete is wet). By reducing the amount of mixing water, the quantity of capillaries can be reduced. To achieve a workable consistency with less water, adding a ‘superplasticiser’ into the mix as well as capillary blocking admixtures will allow a lower water/cement ratio and waterproofing of the remaining paths for water. The hydrophobic pore blocking technology method blends a watertight concrete powder with a liquid admixture, creating a polymer barrier inside pores during the hydration process. The hydrophobic layer that is created within the capillaries performs well when there is variable water pressure, common in below-ground and basement construction. Alternatively, the crystalline technology method creates a non-soluble crystalline structure in unwanted capillaries. This structure is produced by chemicals reacting in the presence of calcium hydroxide and moisture. It fills and blocks the capillary structure, as well as small voids and micro-cracks. The reaction is reactivated in the presence of moisture or water, offering high performance in conditions where water is permanently present. Furthermore, this method is beneficial as it enhances the concrete’s ability to self-heal cracks, further reducing permeability, providing a fully watertight structure. Test data has demonstrated a significant reduction in water penetration depth using both the hydrophobic pore blocking method and the crystalline method.
Basement sealing
Consideration must also be made when selecting products and methods for sealing the joints between concrete sections in the basement. A PVC waterbar is one option that will create a fully watertight barrier for construction and movement joints. Other joint sealing products for construction joints can make the construction process easier because they are simple, versatile and reliable. These hydrophilic strips allow the sealant to expand when it is exposed to water, thereby halting the potential ingress of water.
Dual/multiple systems
BS 8102:2009 suggests consideration be given to the use of dual systems where the assessed risks are deemed to be high or the consequences of a failure to achieve the required internal environment are too high. A common solution would be to combine a typical Type B solution of watertight concrete with either a waterproof membrane (Type A), or a cavity drain system (Type C). Any mistakes made during the installation process can be costly to put right, so considerations for reparability and remediation should be made from the outset to avoid any unnecessary outlay of time or cost. Basements are at risk of being exposed to underground gases such as radon, methane and carbon dioxide. This is particularly the case in construction projects being delivered on brownfield sites due to the lack of land availability across the country. These gases are more harmful than water because they can be a silent killer. The application of Type A membranes can contribute to reduce the risk of various gases entering into the building.
Summary
The simplest component when constructing any basement or underground structure, is watertight concrete. Consideration needs to be paid to the selection of the correct dual waterproofing system for the project, and ultimately the finished environment. Defects can occur in any waterproofing system, but the risk can be minimised through design, planning and a waterproofing specialist who can offer expert specification advice and a high level of site support throughout construction.
Alex Burman is product manager of waterproofing at Sika