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144 Barry’s Advanced Construction of Buildings
The design of a basement is dependent on use, site conditions, construction conditions
and waterproofing system. Table 3.3 provides a brief summary of the types of construction
that are suitable for different basements.
Waterstops to concrete walls and floors
Dense concrete, which is practically impermeable to water, would by itself effectively
exclude groundwater (Figure 3.63). However, in some situations, it is difficult to prevent
movement and the formation of cracks caused by shrinkage, structural, thermal and moisture
movement. As concrete dries out and sets, it shrinks, and this inevitable drying shrinkage
causes cracks, particularly at construction joints, through which groundwater will
penetrate.
Waterstops
As a barrier to the penetration of water through construction joints and movement
joints in concrete floors and walls underground, it is usual practice to either cast 4 (PVC)
waterstops against and across joints or to cast rubber waterstops into the thickness of
concrete. The first method is generally used where water pressure is low, and the alternative,
second method where water pressure is high. The first method is the most economical as
it merely involves fixing the PVC stops to the formwork. Movement joints are formed right
across and up the whole height of large buildings and filled with an elastic material that
can accommodate the movement due to structural, thermal and moisture changes. These
movement joints are formed at intervals of not more than 30 m.
Movement joints are formed in the main to accommodate thermal movement due to
expansion and contraction of long lengths of solid structure and at angles and intersections
right across the width and up the whole height of buildings including floors and roofs. In
effect, movement joints create separate structures each side of the joint. In framed structures,
movement joints are usually formed between a pair of columns and pairs of associated
beams.
PVC waterstops, illustrated in Figure 3.66, are fixed to the inside face of the timber
formwork to the outside face of walls and to the concrete base under reinforced concrete
floors so that the projecting dumbbells are cast into the concrete floors and walls. The large
dumbbell in the centre of the waterstops for movement joints is designed to accommodate
the larger movement likely at these joints. Provided the concrete is solidly consolidated up
to the stops, this system will effectively act as a waterstop. At the right-angled joints of
waterstops, preformed cross-over sections of stops are heat welded to the ends of straight
lengths of stop. Rubber waterstops are cast into the thickness of concrete walls and floors,
as illustrated in Figure 3.67. Plain web stops are cast in at construction joints and centre
bulb stops at expansion joints. These stops must be firmly fixed in place and supported
with timber edging to one side of the stop so that concrete can be placed and compacted
around the other half of the stop without moving it out of place. At the junction of the
joints hot vulcanising joins the stops. Hot vulcanising is where a hot iron heats the PVC,
and as the PVC melts the two ends merge together.
For waterstops to be effective, concrete must be placed and firmly compacted up to the
stops, and the stops must be secured in place to avoid them being displaced during placing
and compacting of concrete. Waterstops will be effective in preventing penetration of water