enced by their coatings. Table 12. 2gives
some capillary water intake coefficients
(w-values) of loam plaster with and without
a variety of treatments:
Making surfaces water-repellent
Water repellents
Several colourless liquids can be used to
impregnate loam surfaces, making them
water-repellent. A given impregnated sur-
face is considered water-repellent if the
wetting angle of contact made by a drop of
water is greater than 90° (12. 3). The water-
repelling agent penetrates into the pores of
the loam without closing them, so that
while capillary water intake is significantly
reduced, vapour diffusion is not. As a rule,
these substances are dissolved in organic
alcohols, hydrocarbons or water.
The following groups of repellents can be
distinguished:
- silane and siloxanes
- polysiloxanes (silicone resins)
- siliconates
- acrylic resins
- silicate ester with hydrophobising addi-
tives
- silicates with hydrophobising additives.
Silane, siloxanes and silicone resins react
chemically with mineral substances in the
loam and are highly weather-resistant; they
reduce water intake by more than 90%.
Vapour diffusion is decreased by only 5%
to 8%.
Silicate ester and acrylic resins show similarly
promising water-repelling effects, but they
reduce vapour diffusion by 15% to 30%.
Since the water repellents found on the
market have different compositions and
varying effects, they should be tested before
use. The water absorption coefficient w of
different loam plasters which were flooded
twice with different water repellents lies
between 0.0 and 0.2 kg/m^2 h0.5(see 12. 2).
Application of water repellents
With the so-called ”flooding“ technique,
water repellents are applied at least twice,
with the solution applied with rollers, so that
the liquid oozes and runs off as the roller is
pulled down over the surface. The second
flooding has to be done before the first is
dry. The loam surface has to be dry, and
neither cooler than 8°C nor warmer than
25°C before being treated. Only silanes and
siloxanes require the ground to be some-
what moist. Normally, this application has to
be repeated every few years owing to the
deteriorating effect of weather on these
repellents.
Testing
A simple method of checking the amount
of water repulsion, used by the BRL, is
shown in 12. 4. Here, the treated test sam-
ples are rotated at the rate of 7.5 rotations/
min on a base and passed under a shower
where water at 36°C is sprayed at a rate
of 12 litres per minute through an ordinary
hand shower. Another more sophisticated
apparatus was described in chapter 2, p. 26.
Lime plasters
Loam plasters used on exterior walls
(described in chapter 11) are only suitable if
they are without cracks and water-resistant.
As a rule, exposed surfaces should not have
loam plasters, the most common alternative
being lime plaster. Cement plasters are not
appropriate, as they are too brittle. They
cannot withstand strong thermic and hygric
forces without cracking, allowing water to
penetrate the loam to cause swelling, which
101 Weather protection
12. 4
12. 2w-values of loam
plasters with coatings
12. 3Drop of water
on a surface that has
been treated with
water repellent (right,
angle larger than 90°)
and on an untreated
surface (left, angle
smaller than 90°)
12. 4Simple spraying
test (BRL)
12. 5Church of San
Francisco de Asis,
Ranchos de Taos, USA
12. 5
