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T W E N T Y T H I R D E D I T I O N
T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 6
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PROFESS IONAL SERV I CES
Just as important is the pressure difference on
either side of the meniscus: in ‘capillaries’ (fine
interlinked pores, cracks, or thin gaps between
flat materials) this is easily strong enough to
move water, and even lift it against gravity.
The mechanisms behind ‘capillary transfer’
were first understood by WTThompson
(later Lord Kelvin), who built on Young’s and
Laplace’s work on menisci. The curvature of a
meniscus depends on the forces between the
water and the surface it touches, so where these
forces are high, a drop of water on a surface
is quite flat, whereas on material with which
water does not interact the droplet will be
sharply curved.
It was Kelvin’s great insight that the Vp
of the air above the meniscus will depend on
this curvature – convex menisci raising the
Vp and concave menisci lowering it – and that
this could explain the behaviour of water in a
straw. The interaction with the straw’s walls
forms a concave meniscus that, for a thin straw,
reduces the Vp enough to lift the level of the
water considerably (it will rise until its weight
balances the force of the pressure difference).
The Vp above the meniscus will be lowered still
more by evaporation at the top of the straw (air
flow is one of the principal drivers of moisture
movement).
Capillary transfer is the main mechanism
by which trees bring water and nutrients
through their roots and up to their leaves,
and is often cited as a reason for ‘rising damp’
in buildings, but in fact permeable building
materials are very unlike trees: their pores are
not smooth but rough and chemically active,
and the pore structure is not ‘bundled straws’,
but interconnected, convoluted, broken, and
with many ‘mouths’ at the surface through
which water can evaporate (capillary forces
conduct water not just vertically but sideways
as well). Water therefore rises only a very small
distance (often no more than the first course of
bricks, since joints of all kinds are a barrier to
movement). For substantial capillary rise, a very
strong and persistent source of water is needed
(such as a broken water main or sewer), and this
must usually be coupled with a coating or finish
that slows or prevents evaporation (such as a
cement render).
WHAT HAPPENS IN THE PORES?
All these forces draw water into permeable
materials, and move them around. Inside
the pores, movement is also affected by
other many forces, notably electrochemical
reactions with the pore walls, and friction
with rough and convoluted surfaces.
Although it is impossible to observe exactly
what happens inside a permeable material,
we are fairly confident about the basic
processes that must be involved. As liquid
water or water vapour is drawn in, films
form on the surfaces of certain pores, and
these curved liquid surfaces reduce the
vapour pressure (Vp), drawing in yet more
water. If enough liquid collects, it begins to
flow through to the neighbouring pores.
Almost everything happening inside
the pores decreases the Vp, drawing in
moisture, and this is why ‘waterproof’
coatings can fail so spectacularly: they
cannot entirely stop water being pulled
in, but they do reduce evaporation, so
over time moisture levels build up.
Some forces resist the uptake of water,
most importantly the pressure of the air
in the pores. Some simple experiments on
bricks have shown that the pressure of the
air pushed through the pores ahead of water
rising by capillary action alone is around
1 bar. Air pressure has been shown to be the
cause of pipes bursting in cold weather, and
it seems reasonable to suspect that a similar
mechanism is involved in freeze-thaw damage,
and perhaps also in some of the damage
related to coatings and salt crystallisation.
Water will not be distributed evenly
throughout the pores: some capillaries may
fill, while others stay almost empty. Although
an analysis of a drilled core sample might
demonstrate that the fabric contains very
little water, what moisture there is might
be concentrated as a liquid in capillaries or
cracks through which it can move easily and
very quickly, carrying with it contaminants
such as salts. Moreover, the situation will
be dynamic, and can change very rapidly in
response to air temperature and humidity
changes, or air flow across the surface.
This explains the failure of many standard
laboratory tests – such as those for freeze-
thaw – to mimic the deterioration observable
in the field. Most tests require saturating the
sample, but in buildings complete saturation
is extremely rare. In fact, materials are
demonstrably much more at risk from salt
decay and freezing temperatures if their pore
structure contains air as well as water.
Water vapour in pores does not behave
like liquid water. Individual water molecules
will bounce around randomly, exactly as
they do in the air, and will not be affected
by gravity, or indeed the environmental
conditions outside the material. Collisions
with the pore walls will quickly lead to
condensation, so it is virtually impossible for a
Water will interact strongly with most surfaces. Left,
a droplet of water on a surface that is ‘hydrophilic’
(“water-loving”, so reacting with water molecules); right,
a droplet on a ‘hydrophobic’ surface.
‘Rising damp’ is an excellent example of how many different sources of moisture can produce very similar symptoms. At the right, this wall shows the classic symptoms often
attributed to capillary rise from groundwater: a zone of salt damage and staining, separating a dry area above from a wet area below. Closer examination shows that the majority
of the wall is fine – which it would not be if groundwater were the issue – and that the source is in fact run-off from a glass roof behind, saturating the wall and percolating
downwards. Other common causes of this damage pattern include splash-back and plumbing leaks.
For the same reason that a sponge must be slightly wet
if it is to soak up a spill, dry building materials resist
absorbing water. On the other hand, wet materials
will draw water in quickly: the ‘moisture history’ of the
material is therefore of paramount importance.