| EFFICACY:
Unlike a physical
damp-proof course these injected damp-proof courses do not form a 'discrete impermeable
plane', but more of a 'diffuse band'.
When fluids are injected into a heterogeneous
substrate such as brick/mortar they do not totally fill up the porous structures
and neither do they completely push out the water in front of the advancing injection
fluid as is so often claimed. Instead, the fluid tends to 'finger' within the
substrate, a process known as 'viscous fingering' (figure , left). The fingers
of the injected material form when the fluid takes the lines of least resistance
such as the larger pores and cracks. Unfortunately, such pathways are not the
most important elements in the conductance of water up the wall. Furthermore,
the damper the substrate the greater this fingering is likely to be, especially
with solvent based systems since these are not miscible with the resident moisture.
Fingering is also increased by injection at high pressure. Reduction of the phenomenon
is obtained by low pressure injection or, better still, by gravity diffusion of
the dpc fluid.
The
result of the damp-proofing fluids forming fingers give rise to non-impregnated
'pools' within the wall through which water can continue to rise. In the case
of pressure injection damp-proof courses, this suggests that it is unlikely that
the diffuse band of the damp-proofing agent will be totally complete. The resultant
chemical damp-proof course may therefore not stop rising dampness by causing an
immediate cut-off of rising water above the damp-proof course like that effected
by a physical damp-proof course. Instead, a relatively rapid decline in the moisture
gradient should occur above the inserted chemical damp-proof course due to the
'control' exerted. Thus, in practice, the rising ground water should be reduced
to such a level that, in association with specialist replastering, it should no
longer cause decorative spoiling or damage.
The
efficacy of the water repellent damp-proofing systems can be affected where there
are detergents (surfactants) impregnated into the wall by, for example, past leakage
from sink waste pipes. A similar problem may occur when walls are sterilised against
dry rot infection by biocide formulations containing surfactants.
The overall
effectiveness of a remedial damp-proof course can be investigated by examining
the relationship between the distribution of free moisture (water due to rising
dampness or other source of active water ingress) and contaminant salts (chloride
and nitrate). Where rising dampness is still active capillary moisture will be
found to the full height of salts (figure, left). The absence of capillary moisture
in the presence of salts arising from rising dampness indicates that drying back
has occurred (Figure centre) and that the damp-proof course is effective. Intermediary
stages are also found which demonstrate different degrees of control of the rising
dampness.
If chloride
and nitrate are not detected in a sampled profile it is possible that the ingress
of moisture is due to recently developed rising dampness or more likely through
rainwater penetration, condensation, plumbing defect or other sources.
When
evaluating the efficacy of remedial damp-proof courses care must be taken not
to misinterpret electrical moisture meter readings; high readings might not indicate
that the damp-proof course itself has failed. They may reflect a number of possibilities
including contaminated or inadequate plasterwork. Thus, an accurate assessment
of the efficacy of a damp-proof course can only be undertaken by determining full
moisture profiles linked with analysis for contaminant salts. It is also important
to give consideration to the expected performance and limitations of the installed
system as described above.
NOTE:
It is identified in BS 6576:1985 that where timber suspended floors are encountered
the damp-proof course must be injected, where possible, below joist level; this
is to protect the embedded timbers from dampness and the risk of fungal decay.
However, given the likely efficacy of injection systems the embedded joist ends
could still remain in contact with damp masonry even if above the injected damp-proof
course and may therefore remain at risk to fungal decay. It would be considered
prudent that in all cases where a damp-proof course is installed in relation to
a timber suspended floor, action is taken to protect any embedded timbers just
above and certainly below the injected damp-proof course to prevent potential
decay!
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