What do you have to do to comply with BS8102
You should have a design team. It should include:
- A geotechnical engineer.
- A waterproofing specialist capable of devising solutions.
- All design decisions should be bought to the attention of the waterproofing specialist.
The specialist can be a manufacturer but only if they have experience in all of the aspects of waterproofing. In reality they will not be able to give unbiased advice. You must take into account the repair ability of the chosen system and the feasibility of remedial measures must be assessed.
Maclennan Waterproofing comment.
- We always work with the engineer on the design team.
- Reparability is not possible with Type A Systems. Reparability is only possible with Type B waterproof concrete during construction phase.
- It is for these reasons that you should not have Type A and Type B together but can have A&C or B&C
You have to carry out a risk assessment.
Taking into account The effects of climate change, burst water mains and sewers, adjacent trees, Sulphates , Radon, Methane and other ground gases. Where external drainage is proposed the effects of drawdown, Silting, and Biofouling Even in permeable subsoil groundwater needs time to drain away and this can result in limited pressure periodically coming to bear on the structure.
Maclennan Waterproofing comment.
Taking the above into account you can never consider anything less than full waterproofing from ground level down.
Two Forms of waterproofing:
Two Waterproofing systems combined should be considered where…
- The assessed risks are deemed to be high.
- The consequence of failure to achieve a dry environment is not acceptable.
- Additional vapour checks are necessary for a system where unacceptable water vapour transmission can occur.
Maclennan Waterproofing comment.
Consequence of failure Is always unacceptable in a grade 3 Habitable basement.
Taking into account the requirements of BS8102 and need for a grade 3 basement.
An external system with type B waterproof concrete cannot comply because.
- In the event of a leak neither can be repaired. And so do not comply fully with BS8102 requirements for reparability and feasibility of remedial measures.
An external system must have a fully serviceable external drainage system around the perimeter below the internal floor slab level. This must drain to a soakaway that will work in all groundwater conditions or to pumped drainage. This is also a requirement of Building regulations 2000 basements for dwellings.
Failure to have a fully serviceable drainage system can lead to water pressure build up.
A system without drainage would fall foul of The Outwing Case 1999 and would bring liability for failure to the designer.
It is proven in law that an applied membrane system cannot be detailed defect free. Therefore water pressure has to be removed.
Leaks in type B frequently occur through cracks, honeycombing and joints ( even with Hydrophilic strips )
Waterproof concrete companies employ teams or engage sub contractors ( often MLSE ) to repair these leaks with resin injection, because they happen so frequently.
Once the structure is fitted out and internal finishes are complete. If a leak occurs it cannot be repaired.
Waterproof concrete company guarantees to not extend to consequential damages incurred by the necessity to strip out finishes to carry out repairs. Failures in the above systems are common place and cost hundreds of thousands a year in litigation and disrupted contracts.
The worst combination of systems is Type A and Type B
In the event of a failure the argument from the type B supplier will be that the hydrophilic strips or some other element was not correctly installed. They will also say that the installation of the Type A prevented autogenous healing of the concrete.
The Type A supplier will not accept any liability for failure. They will also point out that damage could have occurred during backfilling and also rely on the Outwing Case to blame design. Which they are right to do as the design should accept that the Type A cannot be defect free.
Final note Hydrophilic strips.
Hydrophilic strips are not permanent. Manufacturers cannot prove that they work beyond a few years. High content Bentonite systems have a finite amount of Bentonite in them which will stop reacting at some stage. High rubber content strips do not react as much as Bentonite. The safest Hydrophilic strip has a small hose in the Hydrophilic strip and it creates a fully serviceable and repairable hydrophilic joint. These can be supplied and serviced by MacLennan.
Type C System
- Complies fully with BS8102 and with Grade 3 construction
- Fully serviceable.
- Guarantees do cover consequential damage
- In 20 years and thousands of designs and installations MLSE have only had 3-4 very minor leaks.
- MacLennan accept design liability on our PI for waterproofing design.
- If the construction is well compacted poured concrete MLSE will advise the cavity drain as a standalone system with no requirement for additional Type A or Type B
- If the construction does show signs of leakage prior to installation of Cavity drain then it can be dealt with locally by injection or applied coatings over the kicker or defects externally.
- The cavity drain system can be designed into any existing proposal without any changes to existing design
- Unlike type A the Type C system can be installed in any weather conditions and so would not affect programme in foul weather or cold conditions.
This is an extract from an expert witness report on a failed Bentonite installation.
- 5.5 Bentonite Clay Membranes.
- 5.5.1 Bentonite clay sheet membranes are one of the several methods used to stop water at the point of entry (Type A structures). By stopping the water flow into a building, hydrostatic pressure often builds up behind the membrane.
- 5.5.2 Bentonite clay has the property of very significant expansion when it becomes wet, and so is capable of sealing small defects.
- 5.5.2 However, the manner in which the material is installed leaves it prone to contamination (concrete, soil, etc) between the laps (particularly at angles and other awkward junctions), and the property of expansion of the Bentonite may not seal those types of defects. Furthermore, unless a full bond with the substrate can be achieved, there is no way of identifying where leakage through defects is occurring, and so remedial work is not feasible.
- 5.5.5 For this reason, if an unbonded Bentonite membrane is to be used then, to comply with BS 8102, it should only be used in conjunction with a properly designed external perimeter land drain, positioned at the base of the floor slab and which must be maintainable.