| There
are a number of causes for fissures and cracks in timber. The two
principal reasons are:
- Over-stressing of beams and in
some cases, columns, causes splitting of what was sound timber.
Structural repair methods for these situations are considered
elsewhere.
- Fissures and cracks are also occasioned
by drying and shrinkage of a timber component.
Often timber components for structural
use are also those timbers acting as architectural features. They
may be installed with relatively high moisture content compared
with what will be the eventual service class. The outline characteristics
for each of the service classes are indicated below:
Service Class 1
Characterised by a moisture content
in the materials corresponding to a temperature of 20ºC and
the relative humidity of the surrounding air only exceeding 65%
for a few weeks per year. In such conditions most timber will attain
an average moisture content not exceeding 12%.
Service Class 2
Characterised by a moisture content
in the materials corresponding to a temperature of 20ºC and
the relative humidity of the surrounding air only exceeding 85%
for a few weeks per year. In such moisture conditions most timber
will attain an average moisture content not exceeding 20%.
Service Class 3
Characterised by climatic conditions
leading to moisture contents higher than Class 2. Timber should
be installed at a moisture content close to that which it will attain
in service. Strength graded timber to be used in Service Class 1
and 2 should be assessed for fissures and distortion at a moisture
content of 20% or lower, with no reading to exceed 24%. These recommendations
do not apply to timber with a target thickness of 100mm or more.
Timber supplied in a dry condition should be adequately protected.
Wood-based panels are manufactured with a low consistent moisture
content; if variation in moisture condition during service is likely
then this situation should be considered and addressed before installation.
Often softwood featured exposed columns
and beams, ie for structural or pseudo-timber framing, exhibit cracks
and fissures on a continuing basis as the timber dries. Quite frequently,
the timber is in an external position, when the fissures will be
affected by intermittent wet and dry and often open and close seasonally.
In order to minimise this affect, the following action has proved
successful over a number of years:
1. Establish the date
when the timber was installed and, identifying its moisture content,
take a number of regular readings of a minimum of a 2-month period
or through a seasonal variation - all in order to check both variability
of crack width and depth and moisture content.
1.1 Measurement
of the fissures is most appropriately carried out by using the
Avonguard tell-tale system. These movable tell-tale units can
be fixed across the fissure in the timber and regular recordings
of the movements made on the standard sheet. The resulting information
will enable a decision to be made regarding the most appropriate
method of repairing the fissures.
2. Ensure that the
timber is below 22% moisture.
2.1 Where necessary,
enlarge the existing fissures using a jig saw or a router. The
aim is to ensure that the fissures which are to be in-filled are
regularised in such a way that the vertical sides are parallel
as apposed to natural scalene triangle effect which is apparent
in normal timber shrinkage.
3. Remove all dust
and other foreign material.
4. Prepare over-size
timber slips of the same species and moisture content as the timber
component to be treated. (Variation in moisture content not to exceed
1%)
5. If the length of
the fissure may be irregular, these slips will be required to be
made in a number of lengths. The final visual appearance will be
dependent upon the quality of the carpentry coupled with the type
of final finish.
6. It is possible
in some cases that the regularising and removal of the timber does
not remove the full depth of the fissures, i.e. there will be continuing
fissures on the bottom surface of the freshly exposed timber. The
procedure then will be to treat these small fissures with the injection
technique described below. Once this work is completed, the insert
timber can be installed as follows:
- Apply the CB10T SS lubricant adhesive
to the sides and base of the regularised fissure.
- Apply sufficient CB10T SS to 3 faces
of the timber fillet to ensure complete wetting.
- Place the pre-coated fillet(s) into
the appropriate slots and firmly fix with a soft mallet.
Vertical fissure
prepared by router technique
Vertical column
fissure routed to 100mm wide 60mm deep
7. In
the case of relatively small section timbers 150 by 150 mm that
are heart wood centred, usually purlins, and where fissures exist
on two faces especially at right angles, care must be taken not
to apply undue pressure with the mallet, otherwise splitting may
occur. When working on alternate faces slips should be inserted
alternately from face to face rather than one series of slips down
the whole length of an individual face.
8. Allow
a minimum of 2 to 3 days at a temerature at least 18°C for cure
of the adhesive.
8.1 Sand or plane
down the surfaces as required.
8.2 Stain or apply
suitable protective coating as required.
Completed and re-stained
beams and columns.
References
1. Timber Engineering Fact Sheets 1-6 - Rotafix
document FCST0093/1
2. BS 5268 Part II, 1996 - Sections 1 and 2
3. CB10T SLOW SET data sheet - Product Code 3552
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