Foundations for tanking below ground can be constructed of reinforced concrete, plain concrete or brickwork and should be designed in accordance with Code of Practice BS 8102
The concrete slab on which mastic asphalt tanking is applied should be designed and laid in a manner to ensure that any superimposed loads, such as protective screeds, concrete loading slabs and any plant, equipment or machinery subsequently used or installed, can be supported without deflection or other movement which could induce cracking in the mastic asphalt tanking.
The part of the structure on the inner side of the asphalt membrane must be capable of withstanding the water pressure to which it may be subjected from outside after the building is completed. (The pressure of water kN per m2 at any depth is 9.8 times the number of metres below water level.) In addition earth pressure must be allowed for.
Horizontal surfaces to which mastic asphalt tanking is to be applied should be level and free from irregularities such as ridges, dips, fins or concrete or mortar droppings. The surface of the concrete should, therefore, be given a wood –floated finish and be laid plain and true to allow the specified thickness of mastic asphalt to be applied uniformly.
Brickwork and concrete surfaces provided by timber shuttering are usually sufficiently rough to provide an adequate key for vertical asphalt. Smooth surfaces do not give a good key and if these cannot be avoided technical advice should be sought on the appropriate treatment.
External angles of concrete or brickwork must be rounded off to allow the full thickness of asphalt to be applied.
Chases must be provided for turning to the top of vertical asphalt unless the asphalt if being continued horizontally or as a damp – proof course.
Continuity of Asphalt Membrane
The structure must be designed so that the asphalt membrane forms a tank up to damp-course level without any breaks for stanchion bases, staircases, doorways of other openings.
Any openings for the passage of service pipes, drains, etc. May allow the penetration of water since such openings are not easily made watertight. If features of this kind cannot be avoided, provision must be made for all pipes to receive a sleeve of asphalt in two coats, before they are fixed in place by the main contractor to enable the asphalter to form fillets round pipes (Fig 2.5).
It is essential that the site should be kept dry until the basement structure is completed. For this purpose, dewatering or pumping from carefully arranged sumps with appropriate drainage channels should be continuous whilst the laying of the mastic asphalt membrane is in progress and until all loading coats have hardened and the structure has developed sufficient strength to resist the full water pressure.