Recently there have been instances where NHBC have requested additional justification and calculation for basements waterproofed with a concrete additive (‘waterproof concrete’).
Currently the waterproofing of NHBC warrantied basements is designed in accordance with BS 8102 (2009): Code of practice for protection of below ground structures against water from the ground, and NHBC Chapter 5.4 Waterproofing of basements and other below ground structures.
It is also stipulated in BS 8102 that a ‘waterproofing specialist’ forms part of the design team, while NHBC state that this specialist should hold the CSSW (Certified Surveyor in Structural Waterproofing) qualification.
BS 8102 classifies basement Grades as below:
And the different methods of protection as below:
NHBC Chapter 5.4 states that any Grade 3 space will require two forms of waterproofing. It is usual to specify external tanking (Type A) and waterproof concrete (Type B), taking care to ensure that the warranties are compatible.
NHBC Chapter 5.4 states that for Type B integral protection the concrete elements with an additive (i.e. waterproof concrete) should be designed with maximum flexural crack widths of 0.3mm, and maximum through cracks of 0.2mm.
In recent cases, upon submission of calculations, NHBC have requested that additional crack width calculations are carried out in accordance with BS EN 1992-3 Design of concrete structure: Liquid retaining and containment structures.
There are a number of checks to complete within this code, some which require crack widths to be limited to less than those specified above, and one check (end restraint cracking) which gives very onerous results.
An ICE paper by Imperial College: Cracking in walls with combined base and end restraint, supports the assertion that crack widths calculated via this method are unrealistic. In order to comply with these values, basement slabs and walls would potentially need to increase both section size and reinforcement tonnage to comply.
At this point, until NHBC release clear written guidance, Patrick Parsons recommend:
- For Grade 1 or 2 basements external tanking is to be used (not concrete additive)
- Where areas of Grade 3 are required, external tanking is used throughout as the primary waterproofing, and the additional protection is provided by concrete additive in localised areas as necessary. This limits the area of slab which could potentially be subject to increased depth / reinforcement quantities.
- Where the entire basement is Grade 3, consideration of the above is to be given at initial design stage. It may be prudent to propose a solution to NHBC for their review early in the design process