Civil Engineering Reference
In-Depth Information
homogeneous, and salts. The
influence of the surface can be
avoided by drilling holes and using
shielded probes to measure at some
depth in the wall, but even at depth it
may be possible for salt
concentrations to be sufficiently high
to produce a high meter reading in a
wall that has a low moisture content.
Commercial resistance moisture
meters are useful on timber, and will
indicate when a wall which consists
of other materials is dry, but a
reading on masonry will not
distinguish between soluble salts
present or actual moisture, and care
needs to be taken with understanding
the readings from the instrument.
Plumbing defects
Condensation
Rain penetration
Hygroscopic salts in
the wall from
previous damp
problems
Leaking gutters or
RWPs or leaking
drains through wall
or airbricks
depending on
ground or paving
levels
Drilled samples by the gravimetric
method
The basis of this method is to drill out
damp masonry or mortar and measure
both moisture content and
hygroscopic moisture content. On-
site moisture content can be
established by using a carbide meter,
a commercial piece of equipment
where the damp drillings and carbide
are mixed in a pressure vessel.
A gauge measures the pressure
generated and the calibration is
directly related to moisture content.
Alternatively, laboratory weighing
will achieve the same result.
To extract the samples from the
wall, a low speed drill with a
masonry bit of about 9 mm diameter
is used. Mortar is preferred as in
some cases the bricks will have a
lower moisture content than the
mortar. The sample is collected in a
stoppered bottle for subsequent
laboratory tests. About two grams is
sufficient but if the carbide meter is
to be used then six grams will be
needed. If successive samples are
taken up the height of the wall then a
graph can be plotted of moisture
content against height.
The heat produced by a sharp drill
bit does not cause evaporation of
water from the sample, providing, of
course, the material is not too hard.
The advantages of the method are:
Trapped construction
water
Figure 1.31
Alternative causes of dampness
a moisture profile can be
established by drilling in stages
the ground or paving being at a
higher level than the damp proof
course. The level should be
lowered and any bridging material
removed
●
the use of an unsuitable mortar as a
pointing to hide the edge of the
dampproof course from sight. Such
pointing should be removed. It is
preferable not to point over the
edge of a dampproof course, but if
it must be done, a mastic should be
used
●
renderings that have been taken
down below DPC level
●
internal plaster, occurring for
example when a suspended timber
floor has been replaced by a
concrete floor, and measures taken
to link the underfloor dampproof
membrane with the dampproof
course in the wall have been
inadequate
●
●
measurements are made from only
one side of the wall
●
it can be used on a wall where
previous preparations, such as
building in of probes is not
possible
●
the equipment is inexpensive and
may be readilyavailable (eg a
chemical balance and dessicator);
alternatively a carbide meter can
be purchased
●
The main disadvantage is that the
method is semi-destructive, at the
minimum requiring a series of 9 mm
holes, or, if the wall is plastered,
then a chase cut out to locate the
mortar joints.
If rising damp has not been found,
the trouble may be due to
condensation or rain penetration. If
hygroscopic salts are the cause, then
replacing the plaster in the affected
area will be sufficient to overcome
this problem if no other source of
moisture is present.
If rising damp has been established
it does not follow that any existing
dampproofing course must have
failed. It may have been by-passed.
The commonest ways in which by-
passing occurs are:
Even when the cause of the by-
passing has been removed, the
damp conditions will not disappear
if hygroscopic salts have been
brought up the wall by the rising
moisture. In these circumstances it
will be necessary to replace the
affected plaster.
it is independent of salts
●
it measures moisture content
within the material rather than only
in the surface layer
●
