Key Considerations for Type C Waterproofing systems...
Having already written short articles on both Type A & B Waterproofing, this article will focus on Type C Waterproofing. Ahead of the International Structural Waterproofing Conference and following on from the previous articles, this is based on some of the key considerations detailed in the PCA Best Practice Guidance Type C Waterproofing Systems.
The use of cavity drain systems, as a method of waterproofing, has become the most common form of waterproofing system used in retrofit and existing basements and increasingly, is being fitted in new build basements. Their popularity is a result of the fact that systems are relatively quick to install and have a greater margin for defects but may not be suitable for application to convoluted shapes and flat soffits.
What exactly is Type C waterpoofing?
Type C is defined by BS 8102:2009 (Code of Practice for Protection of Below Ground Structures Against Water from the Ground) as drained protection. The structure itself provides primary resistance against water penetration and incorporates a drained cavity within the basement structure to collect any leakage.
Typically, seepage is removed via a mechanical sump pump system, or occasionally, by gravity to low ground or drains externally where properties are formed into sloping sites. Another option is to use pumped back-flow protection devices where connection to drains is not safe. In all cases, consideration should be given to the point at which water discharges, understanding that the effectiveness of the system is reliant on removal of water, so an appraisal of this factor is required.
How do Type C systems work?
The external basement wall must provide enough primary resistance to water ingress to ensure the cavity accepts only a controlled amount of water or dampness. Type C systems do not provide a hydrostatic barrier but provide protection by means of water management.
Type C construction relies on water being resisted by the structure and any water that penetrates is collected in a cavity formed between the external wall and an internal lining/wall. There is permanent reliance on this cavity to collect groundwater seepage and direct it to a suitable discharge point, e.g. drains or a sump for removal by gravity drainage or mechanical pumping. The amount of free water entering the cavity will depend on the size of the structure, on the volume of external water and its hydrostatic pressure, and on the resistance of the structure itself to water ingress.
Maintaining Type C systems
Type C pumped systems should be engineered to cope with worst-case water ingress. If drainage capacity is exceeded, this may result in dampness or flooding. Type C systems are only designed to control and manage minor leakage and seepage into a structure.
The need to service and maintain the drainage elements of a Type C Waterproofing system is paramount to its long term success. As cavity drainage systems rely on free drainage usually in association with sumps, perimeter drains and mechanical pumping devices, it is essential that these elements are regularly maintained to ensure their long term effectiveness.
Immediately after the installation of a drainage system, the drainage channels and sumps must be cleared out and tested. Pumping devices must be checked, tested and properly commissioned.
In accordance with the Code of Practice BS 8102:2009, they need to be maintained. This has been helped significantly with the introduction of perimeter drainage channels and inspection ports, so as to make the drainage aspects maintainable and help to prevent blockages caused by the existence of free lime for example.
Issues of ‘free lime’ in cavity drainage systems
In most new construction, retrofit and also in refurbishment basements (where the floor has been replaced), there is a high risk of free lime and /or mineral salts leaching from the concrete walls and floors. In retrofit this is particularly prevalent where “dry pack” is used at the top of the underpinning.
As free lime leaches from the new construction by groundwater ingress it deposits itself within the drainage cavity, (behind and underneath membranes) and particularly within the sump chamber and around the sump pumps. Thus potentially causing pump failure and therefore failure of the system.
The impact of free lime within the system will greatly increase the frequency of maintenance over the first 3 – 5 years (but especially within the first six months), reducing the interval to weeks in some instances, thus increasing both the costs of maintaining the system and also putting the system under undue risk.
In order to minimise the risk of free lime impacting on the system, an “anti-lime” coating should be applied to the new concrete in all cases.
Type C service intervals
Type C systems should be inspected and serviced at least annually, but in some circumstances the period between services may be considerably shorter. Site conditions, design, materials, machinery used and the implications of any system failure will all play a part when deciding on the frequency and scope of service visits.
It is advisable to schedule the first service visit within three months of the system being commissioned. This will highlight any problems, allow for the removal of debris that may have been deposited in the drainage channels during the construction phase and allow the service engineer to assess the risk posed by free lime and silt that may be washed into the system. Service intervals may be affected by the following factors:
- The volume of water being handled by the waterproofing system
- The prevalence of free lime
- The number of sumps and pumps associated with the system
- The deposition of silt, fines or other particulate matter into the system
- How hard the pumps are working (in some circumstances it may be advisable to install larger pumps)
- The implications of any failure in the drainage system
- The service life of the pump and batteries
If uncertainty exists as to the frequency of service visits then it is always advisable to take a cautious approach. It is better to schedule more visits than may be necessary and prevent a failure rather than leave things too long and risk a flood. If it is decided that visits are occurring too often, then the maintenance schedule can be amended (see note in service and maintenance section above).
Why use a PCA member?
Members of the PCA that specialise in structural waterproofing are the recommended first port of call for practical advice on waterproofing below ground structures. Those who specialise in structural waterproofing are proficient in a number of techniques including Type C Systems that can be utilised to prevent water ingress in underground ground structures.
The PCA’s nationwide list of contractor members are carefully vetted before being awarded membership and are then subjected to rigorous ongoing auditing procedures once admitted to the Association. Members of the PCA can offer insurance backed guarantees for much of the structural work they undertake.
Take advantage of our Knowledge – 2019 Conference
For those wishing to find out more about waterproofing underground structures, the annual forthcoming International Structural Waterproofing Conference takes place on 17th July 2019 at the Slate, University of Warwick. This is the must-attend event for anyone wanting to be at the forefront of the UK construction sector and brings together leading individuals in the structural waterproofing and construction industry to learn and grow knowledge through presentations and debates.
We have already noted the significance of construction joints and this will feature as a prominent topic at the event.
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