- SPECIALIST GEOSTRUCTURAL
- NEW BUILD FOUNDATIONS
At Abbey Pynford Basement Engineering Specialists we recognise the importance for clients to have assurance that any basement structure is completed with the appropriate protection against water ingress from the ground. Accordingly we have invested in training our staff and offer the services of Certified Surveyors of Structural Waterproofing.
We can construct the basement shell to conform to all three types of basement waterproofing as set out in BS8102 "the code of practice for the protection of below ground structures against water from the ground." We have the capability to undertake external Type A lining, Type B water resistant concrete additives or Type 3 internal cavity drainage systems.
Basement waterproofing measures can be applied to the internal or external side of the basement structure as well as embedded within the basement shell. In particular circumstances where the local ground conditions and hydrology increase the risk of water penetrating the basement. a composite solution combining several systems are recommended to provide enhanced protection.
Tanking is a generic term used to describe traditional methods of waterproofing where water resistance is achieved by waterproofing products applied either externally, internally or sandwiched between two elements of the structure. These are not suitable in areas where the water table is likely to exert hydrostatic pressures and therefore, should not be used where the water table is either variable or high.
Type A measures ordinarily consist of a rendered water resistant layers or membrane systems, consisting of dense embedment of hydrophilic or hydrophobic material.
It is usually not possible to effectively utilise these systems externally for retrofit basements or those with piled walls as access to the external face of the basement is not possible. Furthermore in retro-fit basements it is not possible to undertake maintenance or repairs to the installed system as it is not accessible.
We are happy to utilise specified concrete additive but for workability and quality control reasons advise crystalline additives such as Kryton are utilised wherever possible.
Type B systems rely entirely on the concrete structure to prevent water ingress. This system is not suitable without additional measures if the concrete design is to BS 8110 “British Standard for the design and construction of reinforced and prestressed concrete structures”and the water table is either variable or high. Where the water table is high or variable the concrete structure can be augmented by another waterproofing system such as water resistant concrete additives. Alternatively, the structure can be designed to BS 8007 “Code of practice for design of concrete structures for retaining aqueous liquids”.
The principal difference between structures designed to BS 8110 and BS 8007 is that BS8110 structures are not designed to retain liquids (as they are under BS 8007) and the design crack width is limited to 0.3mm. This in a high water table circumstance can lead to water ingress. BS8007 calls for additional steel reinforcement to limit crack widths to between 0.2mm and 0.05mm. In many instances, a reinforced concrete structure designed to BS8007 is considerably more time consuming to construct than that designed to BS8110. It is therefore preferable to adopt an additional waterproofing measure to augment a BS8110 designed structure rather than design to BS8007. There are many products available in this regard, the most notable of which are water resistant concrete additives.
These products perform their function by operating within the concrete structure itself and are applied either mixed into the concrete or to the concrete surface.
A considerable advantage of these products is that they offer increased durability, as they do not rely upon a membrane or thin layer of protection that is susceptible to puncture, ripping etc. Furthermore, they also require significantly less labour time as they are added directly to the concrete prior to placement in the structure.
These additives are broadly divided into two groups; unreactive or hydrophobic and reactive or hydrophyllic products.
function by preventing the passage of moisture by blocking pores, capillaries etc. thereby reducing the permeability of the structure. These products are usually only active during the curing process and therefore, do not protect the structure where cracks occur in joints or in the event that later cracking occurs (settlement associated for instance). These products are not therefore generally warrantied when used in suspended structures such as roofs or intermediate floors etc.
are additives in concrete that react to the occurrence of moisture by causing a chemical reaction that forms new compounds; these develop and seal cracks, pores and capillaries in the concrete. Reactive products continue to retain this capacity once the structure has cured and can reactivate later during the life of the structure. These products are predominantly crystalline in nature.
The basic premise of crystalline additives is that a chemical reaction is created in the concrete causing crystals to grow blocking capillaries, pores and hair line cracks. Once the concrete has cured, these chemicals lie dormant unless another crack develops whereupon the reaction shall reoccur and the concrete shall effectively self-heal. Manufacturers of crystalline additives claim sealing of cracks greater than 0.5mm and most issue warranties for 0.5mm.
The benefit of these products is that the useable area of the basement is maximised as subsequent layers of waterproofing are not required. Moreover, the additional time and associated cost of these works is avoided weather external or internally applied. The products also form integrally to the concrete matrix and therefore are not vulnerable to debonding as Type A systems are. Buildability is improved as the system is integrated with the concrete structure and complicated detailing is not necessary.
Reactive products are preferred to unreactive ones as they have additional benefits: as the additive retains the capacity to reactivate; the durability and longevity of the structure is increased as the concrete has continued protection from water ingress. The systems also usually have construction joints of the same additive creating a homogeneous structure reducing the likelihood of defect or debonding. These systems thus have a considerably reduced likelihood for future maintenance or repair being necessary. Moreover they are warrantied in a wider set of circumstances such as roofs and suspended floors.
Internally applied systems are best used when working within existing structures or new basements where access to the outside of the structure is not possible i.e. when close to adjacent structures or when contiguous/secant/sheet piling is used to retain the adjacent soils.
When constructing retro fit basements we recommend an internal cavity drainage system is always installed.
Cavity Drainage Membranes are plastic (polyethylene most commonly) sheet materials applied directly to structural walls. These are impervious to water and can be vapour proof. These systems achieve water resistance by trapping any penetrating water between the structure and the internal cavity system. The water drains under gravity to a sump and is either pumped from the basement or connected to an existing gravity sewer.
Where a drained cavity is used, the principal means of excluding water from the basement is the structure. The cavity drainage system is intended to cater for the remaining water ingress which may occur through the structure.
Internal finishes including insulation and plaster (plasterboard/render) can be applied directly to the face of the cavity drainage membrane thereby expediting follow on works.
Cavity drainage systems can also be installed externally with protective geotextiles and drainage measures to prevent hydrostatic pressures from developing.
Commissioning and on-going routine maintenance of the system must be undertaken by approved competent contractors to ensure that the system is operating correctly.
Care and consideration should be taken to ensure that the system is installed with a sufficient pumping capacity to cater for the total yield of the basement. There are a variety of pump options available and it is advisable that as standard a double pump unit is installed with a battery back up and alarm. In the event that one pump fails, a second ensures continued protection. A battery ensures continued operation during a power cut and alarm ensures any problems apparent immediately.
Type C cavity drainage systems are regarded as the lowest risk in basement waterproofing. This system is also regarded as the easiest to maintain and repair.