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148 Barry’s Advanced Construction of Buildings
Ground
level
Plinth
Concrete floor
Brick wall
Space, flushed
with mortar
Three-coat
asphalt
Fillet
Strip
foundation
Screed
Loadbearing
brick wall
Asphalt
Concrete floor
Figure 3.69 Mastic asphalt tanking.
When the reinforced concrete walls have been cast in place and have dried, the vertical
asphalt is spread in three coats and fused to the projection of the horizontal asphalt with
an angle fillet. A half brick protective skin of brickwork is then built, leaving a 40 mm gap
between the wall and the asphalt. The gap is filled solidly with mortar, course by course,
as the wall is built. The half brick wall provides protection against damage from backfilling
and the mortar filled gap ensures that the asphalt is firmly sandwiched up to the structural
wall. In Figure 3.68, the asphalt tanking is continued under a paved forecourt. Where vertical
asphalt is carried up on the outside of external walls, it should be carried up at least
150 mm above ground to join a damp-proof course (dpc).
Figure 3.69 is an illustration of mastic asphalt tanking to a concrete floor and loadbearing
brick wall to a substructure. The protective screed to the horizontal asphalt and protecting
outer wall and mortar filled gap to the vertical asphalt serve the same functions as they do
for a concrete substructure. As a key for the vertical asphalt, the horizontal joints in the
external face of the loadbearing wall should be lightly raked out and well brushed when
the mortar has hardened sufficiently.
Where the walls of substructures are on site boundaries and it is not possible to excavate
to provide adequate working space to apply asphalt externally, a system of internal tanking
may be used. The concrete base and structural walls are built, the horizontal asphalt is
spread on the concrete base and a 50 mm protective screed spread over the asphalt. Asphalt
is then spread up the inside of the structural walls and joined to the angle fillet reinforcement
at the junction of horizontal and vertical asphalt. A loading and protective wall,
usually of brick, is then built with a 40 mm mortar filled gap up to the internal vertical
asphalt. The internal protective and loading wall, which has to be sufficiently thick to resist
the pressure of water on the asphalt, is usually one brick thick. A concrete loading slab is
then cast on the protective screed to act against water pressure (also called hydrostatic
pressure) on the horizontal asphalt. An internal asphalt lining is rarely used for new buildings
because of the additional floor and wall construction necessary to resist water pressure