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UNIT 5 REPAIRING MATERIALS FOR
DEFECTS IN TIMBER
Structure
5.1 Introduction
- -
, Objectives
5.2 Preservation of Timber
5.3 Qualities and Requirements of a Good Preservative
5.4 Types of Preservatives
5.4.1 ASCU Treatment
5.4.2 Chemical Salts
5.4.3 Coal Tar
5.4.4 Creosote Oil
5.4.5 Oil Paints
5.4.6 Solignum Paints
5.5 Application
5.5.1 Brushing
5.5.2 Chaning
5.5.3 Dipping and Steeping
5.5.4 Hot and Cold Open Tank Treatment
5.5.5 Injecting Under Pressure
5.5.6 Spraying
5.6 Fire Resistance of Timber
5.6.1 Application of Special Chemicals
5.6.2 Sir
Abel's Process
5.7 Seasoning of Timber
5.7.1 Free Moisture and Bound Moisture
5.7.2 Determination of Moisture Content
5.7.3 Purposes of Seasoning
5.7.4 Methods of Seasoning
5.7.5 Classification of Timbers with Respect to Seasoning
5.8 Inspection of Timber Members
5.9 Case Study
5.10 Summary
5.1 1 Answers to SAQs
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I 5.1 INTRODUCTION
We have studied the defects in timber. You know when the timber is defective, it is not
very much useful for engineering purposes. At the same time, all defects cannot be
repaired. However, there are many methods and treatments available to rninimise the
defects and there are certain treatments developed to repair the defects. So let us now study
as preservation of timber.
the repairing materials or treatment of timber which can be called
Objectives
After studying this unit, you should be able to:
a know methods adopted for the preservation of timber, to increase the life of
timber structures,
a understand the methods to make the timber structures durable, and
know how to protect the timber structures from the attack of destroying
agencies such as fungi, insects, etc.
- -
Repairing Materials
Preservatives are used to protect timber against deterioration due to fungi and attacks of
termites, borers and marine organisms. Most efficient means of preserving timber are, good
L seasoning and free circulation of air. Protection against moisture is afforded by oil-paint
provided that the timber is perfectly dry when first painted; otherwise the
filling up of the
outer pores only confines the moisture and causes rot. A prerequisite for satisfactory
treatment is that the timber shall be seasoned so that the outer layers have a moisture content
of less than 20 per cent. For exposed timber, the only remedy at present available is
impregnation by substances poisonous to fungi, these substances being either of the oil or
chemical types.
Thepreservatives used, fall into two categories-Barrier Preservatives and Diffusing
preservatives. The former are generally copper based oils which may be applied by brush or
spray or in emulsion form. The latter are generally copper fluorohoron gels and solid rods.
# '
In the latest chemical products developed, copper a traditional timber barrier preservative is
deleted as it is considered that it reduces the ability of the active constituents to diffuse.
These diffusing preservatives are introduced by drilling holes (generally 16mm dia) in the
timber at
predetermined locations, installing the gel or rod and inserting a stopper which
may be removed for renewal of the preservative.
Barrier preservatives are applied to end grains of all structural members which are subject to
wetting and drying through rain. Spacing of holes in critical locations should not exceed
300mm. Holes are usually located only where decay is suspected.
Although the chemicals used, have low toxicity, contact with the eyes can be painful and
skin contact can produce burning or rashes. Also some chemicals can stain or possibly
damage clothing.
Another problem involves the dripping of the preservatives. Alternative water based
products to be used are copper chrome arsenic preservatives.
A product by name PRESCHEM ROD is with active ingredients Boron and fluorine. The
rod is hygroscopic and swells to fill the drill hole tightly. Diffusion is slower but no free
liquid is involved so that dripping does not occur.
Another problem is the incompatibility of the preservative with the paint systems. Barrier
preservatives are applied once paint is removed and paint will not adhere to the surface once
the preservative is applied.
In the case of the preservative
B.P.P.G (BUSAN POLE PRESERVATIVE GEL) a product
based on (THIOCYANOMETHYLTHIO) BENZOTHIAZOLE, it was noticed that it
virtually acted as a paint stripper from within. The problem was overcome using a solid rod
form of diffusing preservative like PRECHEM rod.
In the U.K., the two principal types of preservatives used for new building components:
1) The water borne salt types are the formulations based on solutions of copper sulphate,
sodium dichromate and arsenic pentoxide, they are applied by pressure and
2) Organic solvent based types. The solutions consist of one or more organic fungicides in
an organic solvent such as white spirit or a more refined solvent. Common fungicides
are Pentachlorophenol, Tributyl
Tin Oxide, Zinc Napthenale and Pentachlorophenyl
Laurate. There are many others. In addition an insecticide should be added if
resistance to insect attack is required. It is quite normal to apply organic solvents by a
double vacuum process in a pressure tank although some species can be treated for
certain applications by immersion in an organic solvent solution.
Use of tar oil type preservative e.g., creosote gives strong smell and can contaminate.
Hence this is usually relevant to the preservation of fencing.
Timber preserved by a diffusion process e.g., the boron process must be carried out on
timber before it is dried. Sevqral preservatives are formulated and designed for remedial
!
work. I
Table 5.1: Water Soluble Preservatives Repairing Materials For
Defects in Timber
SI. No. Name of Preservative Composition
1) Acid - Cupric - chromate composition 1.68 parts of chromic acid (Cr03) 50 parts
of copper sulphate (CuS04, 5Hz0) and
47.5 parts of sodium dichromate
(NazCr207,2H~O)
2) Chromated - Zinc chloride composition 1 part of zinc chloride (ZnClz) and 1 part
of sodium dichromate or potassium
dichromate
(NazCrz07, 2H20 or KzCrz07)
3) Copper-chrome-boric composition 1.5 parts of boric acid (H3B03). 3 parts of
copper sulphate
(CuS04, 5Hz0) and 4
parts
of sodium dicllormate or potassium
- dichromate (NazCrz07, 2H20 or KzCrz07)
4) Zinc-ci~rorne-boric composition 1 part of boric acid (H3Bo3), 3 parts of
(ZnCla), and 4 parts of
zinc chloride
-- ... sodium dichromate (NazCrz*, 2Hz0)
5) Zinc-meta-arscnite composition 3 parts of arsenious trioxide (AszOg),
2 parts of zinc oxide (ZnO) and acetic acid
just enough to make the preservative
workable.
5.4.2 Chemical Salts
They are water borne preservatives and they are mostly salts dissolved in water. The usual
salq used are copper sulphate, mercury chloride, sodium fluoride and zinc chloride.
are applied on timber surface. These
Solutions are prepared from these salts and they
preservatives are odourless and non-inflammable. The treated surface can be painted or
varnished after drying. These preservatives have good penetration and timbers treated with
thesz preservatives will show an immediate increase in weight of 240 to 480kglcum. After
drying, the net increase in wight will come down to about 5 to 30 kglcum.
5.4.3 Coal Tar -
Timber surface is coated with hot coal tar with the help of brush. Coal tar becomes
lcnown as tarring. Coal tar has unpleasant smell and
workable when heated. The process is
vsuitable for painting. Hence tarring is adopted for frames of
appearance. It makes timber
doors
and windows, rough timber work, etc. and it is found to be most useful for parts
embedded
iri ground or masonry because of its cheapness and effective resistance. Coal Lar
is fie-resistant.
5.4.4 Creosote Oil
In this case, timber surface is coated with creosote oil. The process is known as creosoting.
Creosote oil is obtained by the distillation of tar. Creosoting is carried out as follows:
i) Timber is thoroughly seasoned and dried.
chamber.
ii) It is then placed in an airtight
iii) Air is pumped out from the chamber.
iv) Creosote oil is then pumped under a high pressure of about 7 to lOkg/cum and
! at a temperature of about 5O0C.
V) After a period of about 1 to 2 hours, when timber has sufficiently absorbed
creosote oil, it is taken out of chamber.
Creosote oil is one of the best antiseptic, i.e. substance poisonous for wood attacking fungi.
It is a black or brow11 liquid, weakly affected by water, neither volatile nor hygroscopic,
harmless to wood or metal, inflammable, with an unpleasaut odour and having low
wood-penetrating ability to the extent of lrnrn to 2mn1 only.
Cre/osotuig pr;lclicJly doubles the life of timber and it is generally adopted for piles, poles,
railbay sleepers, etc. Depending upon the net retention and lype of timber, creosote treated
timber will normally increase in weight by 80 to 320 kglcum. Creosote oil IS highly toxic in
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Repairing Male~i AS nature arid gives out highly unpleasant smell. Creosote oil should not be used for interior
surfaces of dwelling houses, fwd stuff storage premises, in underground installations and
near inflammable surfaces.
5.4.5 Oil Paints
Timber surface is coated with two or three coats of oil paint. Wood should be seasoned.
Otlierwise sap will be confined and it will lead to decay of timber. Oil paints preserve
timber from moisture and thus make it durable.
5.4.6 Solignum Paints
These paints preserve timber from white ants as they are highly toxic in nature. They can be
iluxed with colour pigments arid applied in hot state with the help of brush. Timber surface
may, therefore. be given the desired colour or appearance.
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5.5 APPLICATION
- - - - - - -
Following are the application methods adopted for preservation of timber :
a) Brushing
b) Charring
c) Dipping and steeping
d) Hot and cold open tank treatment
e) Injecting under pressure
f) Spraying
5.5.1 Brushing
Tlie solution prepared from preservative is applied on timber surface by good quality
brushes. This is simplest method and it is generally adopted for seasonal timber. The
cracks should be filled up before the application of preservative. For better penetration, oil
type preservatives may be applied hot and the preservative should be liberally used on the
surface. Several coats of preservatives may be applied and enough interval of time should
be kept between successive coats for absorption of preservative.
5.5.2 Charring
This method of charring is rather very old and as such, no preservative is used in this
method in recent times.
5.5.3 Dipping and Steeping
In this method, timber to be given preservative treatment is dipped or soaked for a short
period in the solution of preservative. This method gives slightly better penetration of
preserv;itive than in case vf brushing or spraying. Instead of dipping, steeping or wetting of
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