FEATURES
Continued from page 36
“The moisture content in wood is defined as the ratio of the
mass of water that can be removed from the wood to the mass
of the dry wood,” she explains.
The processing and harvesting of wood dries it from the
green state to a moisture content that is in equilibrium with
the relative humidity and temperature of the surroundings.
Harte explains this as equilibrium moisture content (EMC).
Harte further explains that as the wood dried up, the free
water in the cell cavities is lost first. Then the bound water in
the cell walls is lost. “The moisture content at which all of the
free water has been removed and the cell walls are still
saturated is known as the fibre saturation point (FSP).”
In the article Harte explains that the moisture content can
change and any changes in the content below the FSP can
result in swelling and shrinkage of the timber. Moistures
induced deformations which are prevented in a structure will
lead to the development of stress. “For this reason, it is
important that, before installation, structural timber is dried to
a moisture content close to that which it will experience in
service – normally its EMC.”
Swelling and shrinkage is another property of timber. In her
article Harte explains that below the fibre saturation point,
wood shrinks and swells as its moisture content falls and rises.
The rate of the shrinkage or swelling is never constant as it
changes with direction.
Durability is one of the physical properties of timber. There
are various organisms and insects that can attack the timber.
Such an attack compromises the strength and durability of the
timber. According to SAWPA, wood destroying fungi can be
subdivided into three groups namely brown rots, white rots
and soft rots. The association further explains that fungi feed
on the compounds of the cell wall and consequently weakens
the structure of the wood to such an extent that the wood
breaks and crumbles away. In addition to fungi, insects such as
wood borers and termites also have the potential to attack and
cause severe damage to timber. Because of these and other
factors such as the weather, it becomes essential to
preservative treat and protect timber.
THE TIMBER TREATMENT PROCESS
It’s clear that timber ranks highly among the building material
favourites. Before any non-durable timber can be used in
construction, it has to be preservative treated first.
It is also worth noting that the effectiveness of all wood
preservatives depends on the penetration achieved and the
retention of preservative in the treatable and permeable
zones. It is important that the correct methods are used in the
application of preservatives to cater for a wide variation in
timber absorption characteristics.
SAWPA outlines the main processes of preservations
as follows:
FULL CELL PROCESS OR BETHELL PROCESS
The timber is placed in a cylinder which is then sealed. A
vacuum is drawn on the timber for a predetermined period of
38 APRIL / MAY 2019 //
CCA treated with information on anti-split end plate.
Creosote treated timber poles labelled with 25mm end markers.
time and the cylinder flooded with preservative while
maintaining the vacuum.
When flooding is complete, the preservative pressure
in the cylinder is raised and held until the timber refuses
to absorb further preservative or until the required
retention has been obtained.
The pressure is then released, the preservative pumped
back to the holding tank and a final vacuum drawn to
remove excess preservative. The complete treatment cycle
can vary from 1½ to five hours, depending on timber
species permeability.
Water soluble preservatives are applied at ambient
temperature while oil types such as creosote are
introduced into the cylinder at temperatures from 80 to
90°C. A modification of the process, developed in South
Africa, is the application of the initial vacuum once the
cylinder is flooded to about 90% with water soluble
preservative. A series of modified Bethell cycles is
employed by the industry to provide better control of the
treatment process, reduce post treatment drip and
facilitate faster post treatment drying where applicable.
The process is called the full cell process as the cell
cavities are filled with preservatives.
EMPTY CELL PROCESS
The two basic procedures are the Lowry process and the
Rueping process, and both require that the timber be
See more on page 40
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