Journal of Building Engineering 34 (2021) 101624
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Journal of Building Engineering
journal homepage: http://www.elsevier.com/locate/jobe
A comprehensive taxonomy for structure and material deficiencies,
preventions and remedies of timber bridges
Maria Rashidi a, *, Azadeh Noori Hoshyar b, Liam Smith c, Bijan Samali a, Rafat Siddique d
a
School of Engineering, Western Sydney University, New South Wales, Australia
Federation University, Brisbane, Queensland, Australia
c
Douglas Partners, New South Wales, Australia
d
Thapar Institute of Engineering & Technology, Patiala, India
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Timber bridge
Deterioration mechanism
Prevention
Remediation
Taxonomy
As timber bridges have become archaic, they are no longer able to effectively service their community. It is
neither practical, nor possible, to replace all existing timber bridges, hence it is of paramount importance to
maintain and extend the service life of those remaining timber bridges. The following discourse intends to
provide an extensive and comprehensive review of the various deterioration mechanisms, the preventive actions
and possible remedial options for management and maintenance of timber bridges. The classified information
has been summarised in a tabular format and presented as a ready-reckoner taxonomy for quick reference. This
taxonomy is purely a re-staetment of the information already covered in the paper, but when presented in the
summary form, reference becomes highly convenient.
1. Introduction
It is estimated that there are currently 43,000 timber bridges in use
across Australia [1], with most being constructed before 1950 [2]. Un­
derstandably, majority of these bridges are now some of the oldest
bridges in transportation networks, and have become dilapidated and
structurally weakened as a result. Though these bridges may have
minimum worth themselves, they have a greater value to the economy
as they are from part of trade routes and links between communities;
their assessed cost under values their net worth. Timber bridges in some
situations are unable to service their community as they are no longer
able to handle modern or increased traffic load or conditions and their
cost of maintenance [3]. However, it is neither possible nor practical,
economically or physically, to replace all timber bridges simultaneously.
Thus, they must be maintained until they can be replaced or made
redundant [4].
The majority of timber bridges that are currently in use are not
designed for current vehicles and traffic loadings, as such these bridges
are exceeding their antiquated design capacity [5]). Local government
asset managers must ensure that their infrastructure corridors are able to
satisfy the demands of their community while on a limited budget,
hence, bridges that are difficult to maintain are usually prioritised for
replacement [1].
The replacement of a bridge cannot be done without justification and
reasoning, as such councils have been known to neglect older timber
bridges so that they deteriorate and can be prioritised for replacement
[1]. This unsafe practice can endanger the local community and cause
devastating disruptions to local commuters and economies. Such an
example of this unsafe bridge collapse is the Somerton bridge collapse.
In 2008 the Somerton timber bridge collapsed after a truck passed over
it. According to ABC reports the local council believed that this bridge
was one of their better maintained and heavily used bridges; and the
collapse was “quite unexpected” [6]. Roads and Maritime Services
(RMS) of New South Wales (NSW) reported that the collapse was due to
improper maintenance. The substructure failure of the bridge involved
the subsidence of the piers leading to loss of deck stability. This bridge
collapse highlights the high dependence that communities and in­
dustries have on bridges and their inability to function properly without
this infrastructure [7].
A commonly held belief is that the timber bridges have a shorter
service life than steel or concrete bridges. This belief can be shown to be
factious in bridges like that of the Bogoda Bridge in Sir Lanka, dating
back to the 13th century, and the 14th century Kapellbrücke bridge in
Switzerland [8]. Even with the possibility of long services lives, timber
bridges are inherently susceptible to a myriad of deterioration mecha­
nism, to which other construction materials are resilient; such as that of
* Corresponding author.
E-mail address: m.rashidi@westernsydney.edu.a (M. Rashidi).
https://doi.org/10.1016/j.jobe.2020.101624
Received 6 September 2019; Received in revised form 19 June 2020; Accepted 26 June 2020
Available online 28 July 2020
2352-7102/Crown Copyright © 2020 Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
M. Rashidi et al.
Journal of Building Engineering 34 (2021) 101624
natural disasters and biological hazards. The way in which these dete­
rioration mechanisms are mitigated and managed will affect the ex­
pected service life of the bridges. Bridges that are poorly maintained
experience dramatically shorter life spans, this is evident in the 2013
Hanlys Bridge replacement in NSW; it was replaced due to severe marine
borer attack on structural members (2013). Whilst the bridge was being
replaced commuters had to travel an additional 17.8 km to cross the
creek. Furthermore, during the replacement it placed some of the local
residence of being at risk of isolation during a flood event as there was
no method of escape.
In this research an extensive literature review is carried out to
investigate the main deterioration mechanisms of timber bridges, causes
of defects, effects on structure, prevention techiniques and remediation
strategies. In the next step, all the gathered information are classifidied
and tabulated in a comprehensive taxonomy which can be further used
for educational and professional purposes.
2. Deterioration mechanisms
Timber is a natural resource that provides material size, strength and
durability that makes it an ideal construction material. However, as it is
biological and porous in nature, it is susceptible to decay and defects.
Each defect has a variety of causes which will be outlined in the paper.
There are two main groups that timber deterioration can fall into:
biological and non-biological. The main biological deterioration mech­
anisms are forms of decay as well as insect attack. Whereas the main
non-biological deterioration mechanisms come from physical decay
through weathering as well as forms of mechanical wear [9].
Within various deterioration mechanisms, there is usually an un­
derlying cause, and the two most common are moisture content and
overloading. Moisture content issues cause a cycle of wetting and drying
which alters the surface and end grains of the timber. The cross-sectional
movement causes the timber to warp and form splits and checks. These
splits and checks along with the high timber moisture content, of above
20%, creates the perfect environment for development of fungi and in­
sect infestation. The high moisture content also causes unprotected
metal components to corrode and rust.
Moisture content is not only a root cause of the biological deterio­
ration mechanisms, when combined with overloading, which is the
application of a load exceeding the current load carrying capacity of
either the element or structure, it becomes the starting point for many
deterioration mechanisms. Overloading causes deck damage, de­
formations like sagging, element crushing and buckling, fractures and
when combined with high moisture content, can play a crucial role in
delamination. The main deterioration mechanisms of timber bridges are
illustrated in Fig. 1.
2.1. Weathering
This deterioration mechanism is mainly due to the environmental
conditions such as moisture content and ultraviolet radiation [10].
Swelling and drying due to saturation, corrosion, warping and ultravi­
olet radiation are the main sub-categories of this mechanism.
Fig. 1. The main deterioration mechanisms of timber bridges.
checks are also results from varying moisture contents and have also
been associated with a loss in strength [12].
Moisture meters can efficiently be utilised in undertaking assess­
ments of timber bridge components. It’s well known that the existence of
moisture is essential for decay to take place in timber. Timber piles need
to be thoroughly examined close to the water-line because waterways
and rivers have fluctuating water levels during the year and from year to
year. Moisture meters use long pins to measure the water content of
timber. Pin style moisture meters calculate the electrical resistance
amongst two pins which are inserted into the timber component.
2.1.1. Swelling and drying due to saturation
Timber has an optimal moisture content of around 15%, depending
upon species, and it is not a problem until the moisture reaches about
20%. The environmental conditions play a large role in this matter.
Moisture content deterioration is when timber reaches the saturation
point free water existing between cell cavities and causes the micro­
structure to swell. The repetitious process of swelling and drying can
cause leaching of heartwood toxins which preserves the timber and
prevent biotic growth [10]. As a result of the constant moisture fluctu­
ation, the timber can also become subject to surface checking. This
defect along with overloading of the member can decrease its strength
[11]. Timber deformations like grain rising, warping, cupping and
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Journal of Building Engineering 34 (2021) 101624
wood surface to become exposed and turn grey in colour. UV radiation is
a very slow process with an estimated rate of 63 mm per 100 years. [16];
states that UV radiation effects the aesthetics of the timber but can also
allow other deterioration mechanisms to occur through minor cracks.
UV degradation, while not causing significant damage besides surface
wear, is a form of weathering and can lead to slow delamination of
certain timber members such as ply and glulam timbers.
Ultraviolet radiation should be inspected visually. Some of the most
noticeable timber deterioration effects come from the action of the ul­
traviolet portion of sunlight, which chemically damages the lignin close
to the surface of timber [17]. Ultraviolet damage normally causes light
timber to darken and dark timber to lighten, however this damage
merely infiltrates a shallow distance below the surface. The damaged
timber is marginally weaker; however the shallow depth of the damage
has only a small effect on its strength, except when constant removal of
damaged timber ultimately results in section loss. Fig. 3 shows the
deterioration of timber by UV radaiation.
2.1.2. Corrosion
The lowest level at which corrosion of metal fastenings occur in
wood is 18%, can produce loose connections. Oxidation occurs when
moisture in the timber causes metal elements (gusset plates, bolts, fas­
teners, etc) to corrode and release ferric ions which deteriorate wood
cells. The high moisture environment associated with corrosion can be
conducive for rot and fungus manifestation [13]. The chemical reaction
between the iron and timber increases oxidation of the wood poly­
saccharides causing a loss of tensile strength due to brittle cellar struc­
ture. Corrosion creates movement between the members and can lead to
rapid wear and high maintenance costs [11].
Corrosion needs to be visually inspected. Timber damage outside the
circumference of bolt holes indicates corrosion of the metal bolts.
Timber damaged in this manner is usually dark and looks soft. In several
timber species, staining is another indication of corrosion [14]. This
takes place when iron (from fasteners) interacts with the heartwood.
Fig. 2 shows the corrosion of the metal support plates and bolts of a
timber bridge.
2.2. Biological
2.1.3. Warping
Timber deforming from its original geometry is known as warping.
The classification of warping depends on the plane in which the timber
has deformed; for example, there is cupping, which is deformation
around the minor axis, while bowing is deformation around the major
axis [10]. Warping can cause not only aesthetic issues but can pull loose
connections and fasteners which will decrease the overall structural
capacity due to the ineffective transfer of loads through the defected
connections.
There are six major types of warp as bow, crook, twist, oval, dia­
mond, and cup. The occurrence of warping is due to two parameters. The
first is sporadic moisture content within the timber as the element is
subjected to wet and dry conditions, this is as a result of the timber cells
constantly differing in size due to swelling and the different rates of
drying throughout the element. Secondly growth stresses play a role as
warping is aggravated by irregular or distorted grain and the presence of
abnormal types of wood, such as juvenile and reaction wood which react
differently when they are subject to wetting and drying, causing the
timber to deform. When inspecting timber bridges, warping is a type of
distortion which can be classified as either bowing, twisting, crooking or
cupping [8].
Timber deterioration is largely effected by the environment and the
biological agents that accompany those conditions. The key concerns in
regards to biological deterioration are insects (termites, borers and
ants), fungi (soft rot, brown rot and white rot) and bacteria [18]. Biotic
deterioration can only occur if the following conditions are present: 1)
The presence of moisture, generally above the saturation point of the
timber though some organisms are able to flourish in dryer environ­
mental conditions, 2) A source of sustenance or food, oxygen (with the
exception of anaerobic organisms) and 3) Appropriate temperatures.
2.2.1. Insect attack
• Termites
Dry-wood termites do not require contact with the ground in order to
survive and as a result can be present within a timber bridge for many
years before visible signs are evident and detected [11]. This form of
termite is more commonly found in damp tropical climates [19]. found
2.1.4. Ultraviolet degradation
When timber is exposed to UV radiation or sunlight, a degenerative
photochemical reaction in the lignin of the timber cells occurs. This
reaction only directly affects the aesthetics of the bridge causing the
Fig. 2. Corrosion of the metal support plates and bolts [15].
Fig. 3. Paint and timber deteriorated by UV radiation [5].
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Journal of Building Engineering 34 (2021) 101624
that the termite damage occurs in two stages. First, there is some ma­
terial lose due to the termites eating the timber and secondly, exposure
to the weather and decay through the material deterioration.
Subterranean termites, as the name suggests, primarily live under­
ground. However, as [11,19] both state, this form of termite will build
shelter tubes and earthen mounds that will connect their nests to the
timber structure. This act does however mean that the detection of the
insect infestation is dramatically easier than their dry-wood relatives.
Any cellulose based material, which timber is, in direct contact with soil
is a target for the subterranean termite [20].
As the termites extend their galleries through the structure, moving
fungal spores and moisture about with their bodies. Hence, although
most of the material removed by termites has already lost its structural
strength because of decay, the control of termites remains an important
consideration [21]. Fig. 4 illustrates the deterioration of a timber bridge
affected by termite attack.
A drilling method which has been commercially established is the
resistance micro-drill system. Established in the late 1980s, this system
was initially established for arborists and tree care specialists to examine
tree rings, assess the state of urban trees, find voids and characterise
decay [14]. This system is currently being implemented to detect and
quantify decay, voids, and termite galleries in timber columns, beams,
piles, and poles.
Borer Wood borers are beetles which, at some point, during their
short life, use timber as a method of shelter, food or both. In Australia
there are two main type of borers that affect hardwood timber, pow­
derpost beetle and pinhole borer. For the most part borers are not cause
for alarm as their damage is usually minimal [22]. [23]. outlined the
characteristics of an infestation of both powderpost and pinhole borers.
Powderpost beetles are most commonly found in dry timber. In this
method, the infestation is happened when the female beetle lays its eggs
in the exposed sapwood vessels of hardwood timber. When the eggs
hatch they begin to feast on the starch rich sapwood for their 3–6 week
life. During their short lives the beetles mate and propagate through the
infected timber or structure, and thus infestations can last for genera­
tions. The powderpost beetle’s ability of flight enables it to rapidly
enlarge its area of contamination.
The powder post beetles leave a number of small tunnels behind,
filled with powderlike frass. When the larvae of these beetles tunnel,
they push frass out of the timber. This frass accumulates below the
attacked timber and is a positive indication of powder post infestation.
Pinhole borers prefer to inhabit moist timber. The way in which they
infest the timber is by the female beetle boring through the sapwood
and, in some instances, through the heartwood as well. During this
process the female leaves spores of fungus along the gallery walls which
will germinate and become food for her young when they hatch. At the
end of the tunnel the pinhole beetle will lay her eggs and as she leaves
she will often die at the entrance of the tunnel she has just made to
protect her young eggs from predators and to maintain the humid
environment for the fungus to germinate.
The effect on an element of both borers is reduced strength, due to
removal of material resulting from the boring, as well as making the
timber susceptible to weathering deteriorations by increasing perme­
ability. The main cause or enabling factor surrounding borer infestation
is a high percentage of lyctid susceptible sapwood in hardwoods being
used in timber construction.
The pinhole borer or Ambrosia beetle, usually only attack green
wood [24]. The galleries are free of residue and the adjacent timber is
darkly blemished. Fig. 5 demonstrates an example of borer
deterioration.
Fig. 4. Termite deterioration [21].
Fig. 5. Borer deterioration [21].
• Ants
Ants are insects which often create tunnels and nests in decay cav­
ities in timber structures. They deposit sawdust in gallery openings,
thereby trapping moisture, the result of this is an increase in the rate of
decay of an element [14].
Insect activity is usually recognised by the existence of cavities, frass,
and powder posting. For timber boring insects such as ants, frass is
characterised as the combination of insect feces and hollowed out timber
material from wood components where they are active. The presence of
insects might also signify the presence of decay, as ants frequently
construct tunnels and nests in decay cavities. Ants deposit sawdust in
gallery openings, trapping moisture and increasing the rate of decay of a
timber bridge component.
2.2.2. Bacteria
Bacteria are a single cell organisms and in wet conditions can cause
timber to have an increase permeability and cause the timber surface to
soften. Though bacterial decay is a slow process and has the potential to
deteriorate preservatives and allow organisms with a reduced chemical
threshold to develop. (the ecology of building material).
Softening of the timber exterior indicates that bacterial attack is a
deterioration mechanism which is affecting one or more components in
a timber bridge (Ritter, 1990).
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2.2.3. Fungi
Fungus is an organism that breaks down timber for a source of sus­
tenance, which propagates through timber via threadlike hyphae that
grow through pits or penetrate cell walls. As the fungus infests timber it
secretes enzymes that break down Hemicellulose (cell wall constructed
of short branched carbohydrates made of different monosaccharides);
Cellulose (insoluble substance which is the main constituent of plant cell
walls’ constructed of long unbranched fibrils of glucose) and Lignin.
The ways in which fungus spreads along the structure differ ac­
cording to the species and method of reproduction. There are three
classifications of fungi viz. Mould fungi, stain fungi and decay fungi all
with differing effects on the structure [25].
The variety of fungi organisms can survive and thrive under a
differing environmental conditions. Soft rot is the most resilient fungi
able to tolerate a wide range in humidity, temperature, moisture content
and pH levels. The likes of mould and stain fungi, and brown and white
rot have limited tolerances to these environmental conditions.
While they do not show the quantity or degree of decay, fruiting
bodies give an affirmative indication of fungal attack.
2.2.3.2. Decay fungi. [24] notes that decay fungi is generally the main
cause of decay in timber bridges, it has three classicisation’s based upon
the way in which it appears and manifests itself in the timber which are:
- Brown Rot;
- White Rot;
- Soft Rot,
Usually, moisture contents in timber less than 20% won’t allow
decay to take place in timber. Though, as the moisture rises beyond 20%,
the likelihood for decay to take place rises. Significant decay transpires
only when the water content of untreated timber is greater than 28%–
30%. This ensues when dry timber is open to direct wetting via rain,
moisture penetration or interaction with groundwater or bodies of
water. Timber decay fungi doesn’t attack timber that is completely
saturated with water, however deprived of oxygen. It is also known that
lack of maintenance is a major contributor to timber decay [19]. Decay
relies heavily on a combination of factors. It requires suitable temper­
ature, appropriate moisture levels, oxygen and cellulose in timber [27].
When timber bridges aren’t yet displaying indications of decay,
increment cores should be cultured to detect the presence of decay fungi.
This procedure can detect decay prior to noticeable damage taking place
and offers a method of assessing future risk. The presence of decay fungi
normally means that the timber is in the early or incipient stage of decay
and needs to be remedially treated. Culturing offers an easy way to
evaluate the possible decay hazard and numerous laboratories run
routine culturing services [24]. Since there is a vast array of fungi close
to the exterior of timber, culturing is not suitable for evaluating the
hazard of external decay.
2.2.3.1. Mould and stain fungi. Mould and stain fungi damage occurs
with timber with high moisture contents and the damage persist after
the wood has dried, however this type of damage is small and insignif­
icant in terms of the timber strength. Stain fungi can occur beneath
coatings and eat through them causing problems when trying to seal a
timber structure. If the staining penetrates deep into the timber that can
not be removed by planning.
Moulds can also cause patchy discoloration on the surface of the
timber, ranging from green to black to pink. They most commonly occur
in timber that has a moisture content greater than the fibre saturation
point which is between 28% and 32%. The optimum temperature range
for mould growth is between 24 and 30 ◦ C. The toughness of the wood
can be affected by moulds however have little impact on strength. A
major problem with mould is that it increases the porosity of timber
members which in turn opens the door to decay due to moisture de­
formations. Fig. 6 illustrates a pine timber member showing signs of
mould and stain fungi.
Moulds and stains are said to do little damage to the timber however
do increase the porosity as well as reducing or nullifying the toxicity of
some fungicides [16]. This poses a problem as it inhibits remedial and
maintenance actions used for other deterioration mechanisms. The
surface damage can also be the precursor to other more detrimental
organisms.
Mould and stain fungi needs to be inspected visually. The main
purpose of these fungi is to discolour or blemish the timber. Mould fungi
attack the exterior of timber, producing marks which can usually be
eliminated by scrubbing or planning, however stain fungi cause severe
concerns since they penetrate to a greater depth and stain the timber
[10].
- Brown Rot
Brown rot is a form of decay fungi that is common in timber struc­
tures and can cause severe damage. It has an optimal growth tempera­
ture of 200 Celsius. The methodology of attack for brown rot is the
reason as to why it can be considered the most serious of all the decay
fungi. Brown rot attacks the cellulose and hemicellulose of the cell wall
and alters the remaining lignin, this process can cause weight losses of
up to 70% in the timber element. Due to the fact that brown rot removes
the cellulose, which provides strength to the cell, it can cause strength
reduction in early stages of decay. Brown rot releases enzymes that have
the ability to migrate or defuse far from the area where hyphae are
present; as such losses in strength can be present in areas far from the
visibly affected areas. Of the least important effects of brown rot, it
discolours the timber brown. During advanced stages the rot becomes
brittle and has numerous cross checks and makes the surface of the wood
look charred in appearance. Fig. 7 shows a timber element deteriorated
by brown rot fungi.
Brown rot fungi, as its name suggests, give decayed timber a
brownish appearance in colour. In progressive stages, brown decom­
posed timber is brittle, has a dark colour and has multiple cross checks
alike in appearance to the surface of a cracked and severely charred
timber [24].
- White Rot
In appearance, white rot is a shade of white or tan in colour with dark
streaks present. White rot is not easily detected in the early stages of
development. The way in which it propagates is by releasing enzymes
that remain close to the hyphae, therefore localising infestation. When
the rot has become advanced, it is soft in texture and fibres may peel
individually from the timber. White rot attacks all three components of
the cell wall causing extensive weight losses of up to 97% and thus a
substantial loss in strength [21]. The main environmental factors
causing white rot are high humidity or moisture content and appropriate
temperatures of around 20◦ Celsius [29].
Fig. 6. Pine timber member showing signs of mould and stain fungi [26].
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Journal of Building Engineering 34 (2021) 101624
• Incipient - this occurs where infection is freshest and hard to detect
• Intermediate – discolouration begins and little strength is left in the
timber and the wood becomes soft;
• Advanced – minimal to no strength is left in the timber, voids begin
to appear as the timber is dissolved.
Though the rot can have devastating effecting on a structure it is not
usually associated with structural decay [24].
Hardwoods are more inclined to suffer from soft rot and that it is
usually found in timber that is in contact with the ground.
The Pilodyn is utilised to detect external damage. The Pilodyn is a
spring-loaded pin device which forces a toughened steel pin into the
timber [30]. The depth of pin penetration is utilised as a measure of the
extent of decay. The Pilodyn is utilised frequently in Europe, where soft
rot fungi is more widespread.
2.3. Mechanical wear
Mechanical wear describes deterioration of the timber elements and
their connections as a result of traffic and friction and abrasive damage
that accompanies that as well as the loads that the traffic applies [31].
The common underlying factor in most of the mechanical deterioration
mechanisms is overloading. The forces from the loading cause multiple
defects in the structure, from fractures, loose connections, element
crushing and deformations like buckling and sagging [32].
Fig. 7. Timber element deteriorated by brown rot fungi [28].
2.3.1. Deck wear
Timber bridge decking comes in two common forms. First, there is
regular sawn decking that ranges from 200 mm to 250 mm wide and
125 mm deep and second, LVL or stress Laminated Timber (STL)
decking. The SLT system uses thin (35 mm–50 mm thick) timber lami­
nates of widths from 140 mm up to 290 mm. These placed on edge and
combined together using high strength bars or prestressing strands. This
form a solid slab like structure [5]. In the case of the regular sawn
decking the main deterioration issues are damage from abrasion and
friction as a result of traffic and debris. This wears away the timber or
any coating making the deck vulnerable to both fungal decay and insect
infestation which are the most prevalent forms of deck deterioration
according to the [5]. A specific issue with the SLT system, is that a loose
tie down can cause the deck to deform, causing damage, due to too much
stress. All of these forms also cause section loss in the elements in both
decking systems that has a direct impact on the strength and perfor­
mance of the deck [10]. Fig. 9 shows some forms of deck wear.
Timber bridge decks can be inspected by using the following in­
spection techniques and equipment:
White rot fungi create decay which bears a resemblance to ordinary
timber in appearance, however might be whitish or light tan in colour
with dark streaks. In the progressive stages of decay, infected timber has
a particularly soft surface, and single fibers are able to be peeled from
the timber [24]. This gives a positive indication of white rot. Fig. 8
shows timber substructure with evidence of white rot fungi.
- Soft Rot
Generally soft rots attack the outer wood shell and have exogenous
nuisance to create substantial decay. The detrition method can be
divided into three stages:
1. Visual assessment;
2. Hammer sounding with pick hammer;
3. Awl and level edged probes;
Fig. 8. Timber substructure with evidence of white rot fungi [15].
Fig. 9. Forms of deck wear [15].
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Journal of Building Engineering 34 (2021) 101624
4. Moisture meters for exterior surface timber with assumed high water
content;
5. Stress wave timing assessment;
6. Resistance microdrilling of decayed regions.
away from around the piers and abutments of bridges increasing the
effective length of the element and exposing footings [8]. Contraction
scour is caused by the narrowing of the waterway as it approaches the
structure and the accelerated flow that it creates. Local scour is a result
of the interference of the piers and abutments to the flow. In all cases,
due to the increase in the flow speed and volume of water, the incident
of flooding greatly increases the severity of the scour as well as reducing
the time in which said scour will occur.
All of these factors that attribute to the development of element
buckling have been found to timber pile load carrying capacity [28]. In
severe cases, if the buckling causes total failure of the element the
deterioration can cause the entire structure to fail.
2.3.2. Deformation
Deformation is the altering of the shape or direction of the member
as a result of a load or loads being applied. The deformation causes the
movement in the entire structure that can result in damage to other
elements such as the more rigid surface layer [33]. There are two main
causes of chord deformation. The first is sagging of the truss, results in an
increase in stress on the top chord causing in to warp and deform out of
shape. The second cause and perhaps the underlying cause is loading or
overloading of the bridge [34]. attributes loading to long term defor­
mation which can double or even quadruple the elastic deformation.
Sagging is the deformation of an element within the y-axis, where the
element sags down in the middle, and is in itself a method of deforma­
tion. The two main causes of sag are span lengths that are too long for
the elements capacity and an uneven horizontal dispersal of weight
through the deck which causes sagging of timber stringers. Within
timber design, the effect that deformation has on the elements distri­
bution of forces is assumed to be insignificant. However connections of
these deformed elements will show signs of semi-rigid behaviour which
can result in fastener deformation and failure [35]. Any long-term sag
will also increase bending in a headstock and therefore, decrease its
capacity [28].
2.3.5. Delamination
Delamination is the process to separation and deterioration of the
layers of certain timber products such as glue laminated timber,
plywood and laminated veneer lumber or LVL. It occurs when gluedlaminated layers separate as the adhesive that bonds the layers fails
[13]. It can transpire locally in the case of end grains but can also be a
gradual process where layer by layer the timber is deteriorated, each
time revealing new undamaged material that is then subject to
deterioration.
The main causes of delamination occurring involve either weath­
ering and overloading. Transit New [40] attribute the process to
movement and shrinkage of the timber. The movement can be a result of
the warping from moisture or the deflection from overloading. The
Queensland Government Department of Main [28] believe that the
timber bridge location and environment are keys to the deterioration,
with tropical areas and frequent submergence of timber elements are
often the cause. They also outline weathering and UV degradation as
factors influencing delamination which is supported by Ref. [16] who
also attributes UV degradation to the deterioration mechanism, stating
that it does not cause significant damage besides surface wear but can
lead to slow delamination of certain timber members such as ply and
glulam timbers.
The effect that delamination has on the timber element and structure
as a whole was put forward in a paper by Ref. [13]; where he said that
delamination provides openings for decay to begin as moisture can
penetrate and be trapped between layers creating a humid environment,
perfect for fungi and insects. He also attributes a reduction in strength of
the element as the loads cannot be effectively transferred through the
damaged element.
Feeler gages and awls should be utilised to measure the degree of
delamination [14]. Fig. 10 demonstrates and example for delamination
of ply wood.
The following items are a concise list of the areas for the essential
visual assessment of SLT bridges [33]:
2.3.3. Element crushing
Crushing is a deterioration mechanism that occurs when overloading
takes place, either parallel or perpendicular to the grain. When the load
is applied parallel to the grain, it shortens the cells within the element
along their longitudinal axis which causes the micro fibrils of the cell
wall to fold, eventually folding the cell itself. This deforms the cellular
structure creating planes of weakness and instability finally resulting in
visible surface damage [36]. Overloading is not the only cause, over
tightening of the connections and the fixings can also result in crushing.
Crushing causes a loss of strength and can also affect the serviceability of
the element [5]). The damage to the surface as a result of crushing can
cause protective coatings to become ineffective and make the timber
susceptible to biological deterioration like decay, insects and
weathering.
The correct way to inspect element crushing is to locate crushed
regions at bearing points along the cap supporting the superstructure on
the top of a pole or pile which trap water and deteriorate the preserved
timber shell [24].
2.3.4. Element buckling
Element buckling is a deformation of the timber element. A force is
applied that is too high for the element to carry resulting in the element
distorting in a direction that is perpendicular to that of the force being
applied [37]. It has two forms, first Global buckling which is where part
or all of the length deforms longitudinally. The second is where the cross
section of the element deforms. In this case the damage is localised [38].
Buckling can be attributed to many causes depending on the situation,
they include but are not exclusive to, overloading, loose bolts or con­
nections and scour and abrasion.
There are a few factors that can cause buckling. The first is loading or
overloading of the element, most commonly the pile. The pile is unable
to support the axial load and therefore, transfers the force in the only
available direction which is lateral [28]. Furthermore, the element will
buckle when rot or steel corrosion affects the pile connections as these
can either cause the connections to become loose or can cause a loss of
section that will reduce the bracing effectiveness, ultimately resulting in
member buckling. Corrosion of the pile itself, or scour, can also cause a
buckling and vertical failure [39]. Scour is a form of deterioration that is
the result of the flowing water eroding the soil, the material is carried
Fig. 10. Delamination of ply wood [41].
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Journal of Building Engineering 34 (2021) 101624
structural stability. From Ref. [44]; a direct relationship between inef­
fective fasteners and connections and an increase in the peak displace­
ment of the timber member is evident, which could then result in partial
or complete failure of the element or structure.
A good indication of loose connections is when primary timber
bridge members (for example primary deck or slab members) are out of
alignment and are not operating as intended [40].
- The SLT deck must be assessed under traffic loading for excessive
deformation or movements.
- Special consideration must be given to potential slip amongst the
laminates under substantial loads.
- Drainage systems must be assessed for obstructions or debris.
- The wearing surface must be assessed for cracks and deterioration.
- The waterproofing system, including the edge flashing, must be
assessed for deterioration and seepage.
- All regions of exterior surface timber decking must be assessed for
fractures, deterioration and indications of moisture and staining.
- The wood directly beneath the prestressing arrangement must be
visually assessed, giving particular attention to:
- Deterioration of the anchorage protection system;
- Excessive deformation of the anchorage system.
- The deck tie down bolts and deck joint bolts must be assessed for
tightness and a minimum of 5%, but greater than 12, of the tie down
bolts must be physically inspected for tightness.
2.4. Natural element defects
Not all timber deterioration mechanisms are the result of a third
party, some are naturally occurring defects that come about through the
growing of the tree. Knots, checks and splits are the main three natural
element defects.
Knots are defects that arise when a piece of branch or limb that was
growing on the tree has been incorporated into the timber element that
has been milled, they are a natural product of growth [8]. attributes a
reduction in strength and load carrying capacity to the presence of knots
while [1] says that a reduction in mechanical properties results from
knots reducing the effective cross section while causing localised sloping
of grain. Splits and checks are similar in both their cause and effect on
the structure. [17]; outlines checks are a separation of wood occurring
perpendicular to the cross sectional grain or growth rings and splits are a
separation of wood from one surface to another, usually parallel to the
grain. Both of which are a results of the differential shrinkage during
drying or seasoning. The outcome of the two deterioration mechanisms
is a reduction in strength and load carrying capacity as forces cannot
effectively be transferred through the members and structure while also
opening the timber to further weathering and deterioration [13]. Fig. 12
shows some small end checks (lefir photo) and severe through split
(right photo) in some timber pieces.
Probes should be utilised to measure the depth of checks. Level edged
probes such as pocket knives or calibrated feeler gauges are suggested
for utilisation during this process. Stress wave timers and resistance
drills should be utilised to inspect for splits in timber bridge elements
[14]. Feeler gages and awls should be implemented to measure the
extent of splits.
2.3.6. Fractures
Fractures are cracks in timber as a result of beams being under
flexural loading. The fractures are influenced by various mechanical
properties and loading conditions of the timber element such as knots
present within the element as well as the grain of the timber and the
loading in relation to that grain, whether it be parallel or perpendicular
[42]. Elements around shear plates and keys are subject to high amounts
of bearing stress and shear forces when loads are applied, which can
result in fractures in the timber around the plates and keys [33]. Loading
is not however the only cause of fractures [43]. attributes moisture to
producing fractures. The constantly changes in volume throughout the
entire member, as a result of moisture penetration, in combination with
the low strength normal to the grain of the timber can result in the
creation of fractures. Regardless of the cause of the fractures the effect
that they have on the member and structure as a whole is the same and
that is a reduction in strength and a reduced ability to effectively
transfer loads through the member to the supporting elements [42].
Fig. 11 shows some longitudinal fractures on a timber bridge.
2.3.7. Loose connections
Connections are an area of bridges that are subject to many forms of
deterioration as a result of cut ends, fixture material removal and
moisture retention. These all contribute to loosing connections [13].
states vehicle traffic loads crossing the bridge along with weathering
crush the wood around the fasteners due to the repetitive impact. The
loading wears on the connection (fasteners and their holes) causing
them to loosen. Loading, vibration and weathering are the main factors
causing the loosening. The effect this has on the structure is a reduction
in the bridges load carrying capacity while also severely reducing the
3. Inspection, condition assessment and remediation planning
Majority of state bridge authorities use three levels of bridge in­
spection procedures. These are:
Level 1 – Routine maintenance inspection
Fig. 11. Longitudinal fractures [15].
Fig. 12. Left: Small end checks. Right: Severe through split [15].
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Journal of Building Engineering 34 (2021) 101624
to be more accurate than visual inspection, because it does not only
identify deterioration mechanisms, but it also measures the extent of the
damage caused.
In order to help timber bridge inspectors prioritise which timber
bridges need remediation, condition ratings are implemented. Timber
bridge elements are given a condition state from one to four, as shown in
Table 1. A condition state of one is the best condition that a timber
bridge element can be in, while a condition state of four is the worst
condition that a timber bridge element can be in.
Each timber element is quantified with the units of each, meters
squared or linear meters. These quantities are then categorised into the
condition states of one to four as stated above. For example, if there are
large number of quantities of timber elements in poor condition states
(e.g. 3 or 4), this can give a poor overall condition rating to a timber
bridge. Hence, condition ratings can assist in the prioritisation of
remediation of timber bridges [45].
Existing timber bridges are progressively deteriorating and as a
result local governments are constantly trying to find and improve upon
Maintenance, Repair and Rehabilitation (MR&R) strategies in order to
optimally distribute their limited funds.
Maintenance or preventative maintenance is any work that is done to
maintain the current condition level and to reduce future defects.
Currently no decay has begun however the risks are present.
Repair is split into two sub groups, early remedial maintenance and
major maintenance. Early remedial maintenance is carried out when
deterioration has begun however it does not affect the performance of
the structure. Greater decay is forthcoming if corrective steps aren’t
taken. Major maintenance involves corrective actions that reform the
bridge to its original state. Significant deterioration has occurred to
members and repair is needed to maintain the level of service.
Rehabilitation is carried out when the current bridge has deterio­
rated beyond repair and have become structurally incompetent or
outdated. It is often done to increase the load carrying capacity to cope
with the demands of modern traffic conditions.
[4] has outlined a three levelled decision tree for the remediation
courses of action. Each decision tree has sections on preventative
maintenance, rehabilitation, both minor and major, repair and
replacement. However, option “Do Nothing and Monitor” is a vital
addition when dealing with local governments as they are often strug­
gling with limited funds and may not be in a position to act. This allows
them to keep an eye on the structure until funds are found or action must
be taken. Fig. 13 demonstrates a decision tree including the major
remedial strategies.
Level 2 – Bridge condition inspection;
Level 3 – Detailed structural engineering inspection.
A Level 1 Inspection (Routine Maintenance Inspection) is the most
basic of the three levels of inspection. The procedure simply involves a
visual inspection for deterioration mechanisms which might be affecting
elements of the timber bridge. The main purpose of a Level 1 inspection
is to ensure the safety of motorists and any pedestrians that may be using
timber bridges. Level 2 inspections (Bridge Condition Inspections) are
the medium level of inspections. They involve using condition state ta­
bles under the heading of “Condition Ratings”. There are commonly four
condition states in a condition state table (Please see Table 1), although
there are different classifications for condition states between different
bridge inspection manuals. Level 2 Inspections can give timber bridges
an overall condition rating and can therefore help timber bridge in­
spectors prioritise with regards to the remediation of timber bridges.
A level 3 inspection consists of two components, either a structural
engineering investigation or a structural engineering inspection. The
purpose of a structural engineering investigation is to better understand
the timber bridge and be able to manage it. While on the other hand, a
structural engineering inspection is a very detailed inspection which
includes the use of advanced inspection equipment and structural
analysis of timber bridges. The structural analysis of timber bridges can
determine many degrees of freedom. These include deflections at certain
points of the bridge, the angles of rotation of the bridge near its supports,
the stress and strain of bridge elements, the axial forces of bridge ele­
ments (either tension or compression), shear forces and bending mo­
ments at points along the bridge.
Inspecting for deterioration mechanisms is an important aspect
regarding the lifecycle management of timber bridges. Whether it is
visual inspection or inspecting bridge components with equipment or
apparatus, it is essential to know what type of deterioration mechanism
is affecting the bridge in order to know which kind of preventative or
remedial methods should be utilised.
Visual inspection of bridge elements is usually undertaken when
deterioration mechanisms are affecting the exterior surface to the
bridge. This commonly can be the first step of inspection, followed by
the use of equipment and measuring devices to measure the extent of the
deterioration. While visual inspection can be much less accurate with
relation to indicating the correct deterioration mechanism, it is an
adequate inspection procedure for a number of mechanisms.
Inspection equipment is used to detect, measure, assess and quantify
deterioration mechanisms in timber bridges. Equipment has been found
4. Prevention strategies
Table 1
Element Ratings [5]]).
Condition
State
Description
1
The timber is in good condition with no evidence of decay. There
may be cracks, splits and checks having no effect on strength or
serviceability.
All connections are in good condition and bolts are tight.
Minor decay, insect infestation, splitting, cracking, checking or
crushing may exist but none is sufficiently advanced to affect
serviceability.
Joint connections may be slightly loose but does not affect the
serviceability.
Medium decay, insect infestation, splitting, cracking or crushing has
produced loss of strength of the element but not of a sufficient
magnitude to affect the serviceability of the bridge.
Joint connections may be slightly loose but the serviceability of the
bridge is not significantly affected.
Advanced deterioration. Heavy decay, insect infestation, splits,
cracks or crushing has produced loss of strength that affects the
serviceability of the bridge.
Connections are very loose causing large movements, bolts are
corroded and ineffective or missing and the serviceability of the
bridge is affected.
2
3
4
Generally the service life of a bridge can be sub-divided into four
different phases [45,47]:
Fig. 13. Remediation decision tree [46].
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Journal of Building Engineering 34 (2021) 101624
Phase A – design and construction
Phase B –deterioration has not yet started but initiation processes are
underway
Phase C – damage propagation has just begun
Phase D – extensive damage is occurring
unable to expand with absorbed water.
Heat treated timber also exhibits resistance to rot and fungus as the
fungi is unable to recognise the timber as a source of sustenance. The
chemical reactions during the heating process cause molecules such as
furfural to react with lignin, thus changing the substratum which the
fungal enzymes recognise. Further to this the altered moisture content
lowers the fibre saturation point to levels lower than what is usually
conducive for fungal decay [48].
Though heat treating is able to many positive effects that prevent
decay and defects, it also causes changes to the mechanical properties of
the wood. Once timber has been heat treated it becomes more brittle,
decreasing the dynamic & static strengths and also of tensile strength. It
is believed that the degradation of the hemicellulose is responsible for
the decreased strength along with the crystallisation of the amorphous
cellulose [49].
According to the Law of Fives, 1 dollar spent in Phase.
A equals 5 dollars spent in Phase B; 25 dollars in Phase.
C and 125 dollars in Phase D, implying this law is the corner stone of
any asset management decision-making.
From this rule it can be deduced that implementing preventive
maintenance must become of paramount importance to avoid further
deterioration and achieve structural longevity and long term economic
benefit.
4.1. Asphalt/Bitumen
4.3. Flashings
Asphalt is a dark viscous liquid which is created as a by-product of
distilling petroleum; though it can also be a naturally occurring product
as the prelude to petroleum. This primordial sludge is able to preserve
fossils for palaeontologist and the decayed remanence of prehistoric
organisms can act as a wonderful hydrophobic membrane for timber.
Coating elements of the bridge in asphalt, namely the deck and pene­
trations, provide a physical barrier between the timber elements and
wear form traffic or the elements [28]. Furthermore, this physical bar­
rier insulates timber from changing moisture content and subsequent
conditions which are conducive to biological decay such as cycling
dimensional loading which can cause splits, checks and warping in
elements.
A flashing according to the Penguin Civil Engineering Dictionary, is
“a strip used to seal a junction between two surfaces to exclude rain­
water”. Flashings are of most use when elements of the structure will be
constantly exposed to precipitation and UV radiation i.e. the top of truss
chords, hand rails, beams and of upmost importance on the end grain of
timber. In such situations flashings minimise the risk of deterioration by
preventing water pooling on elements which are exposed to the natural
elements such as rain or sun.
It should be noted that the flashing is raised off the timber element to
allow for ventilation. If ventilation is not present between the flashing
and element, water will become stagnant and soaking into the element
creating condition suitable for decay. Further, it should also be observed
the material employed for a flashing has electrolytic compatibility with
the timber; typically, thin metal plates are used [17].
4.2. Heat treated timber
Heat treating timber is by no means a new method of wood modi­
fication, though it is also far from being antiquated. In 1920 heat treated
timber was shown to be effective for dimensional stabile of timber and
subsequent research in the prevailing decades has also furthered this
observation. Though the specifics of heat treatment for heat treatment
differ from manufacture to manufacture, the method and principles
employed are the same [48]. Thermowood, retailer of heat treated
timber in Europe, has the following methodology:
4.4. Paint and stains
Paints and stains work in similar ways to prevent timber decay; both
paints and stains act as a sacrificial layer to the structure to create a
protective coating. This protective coating is a physical barrier that
prevents decay agents such as ultra violet radiation, moisture, fungal
spores and insects, form reaching the surface of the timber. This barrier
also prevents moisture egress from the timber element, creating
dimensional stability. The dimensional stability provided by paint pre­
cludes incurrence or further development, of splits and checks [43]. If
paint is improperly applied or is in need of maintenance it can be
detrimental to the structure as it allows a method of ingress for insects
and moisture, the painted surface then provides shelter from the sun and
will decay from the inside out [33].
1) The lumber must be placed in a humid atmosphere for 2–10 h at
temperatures exceeding 150Co to obtain a mass loss of 3%.
2) A vapour with a treatment of 100 ◦ C is then applied as the oven
temperature is slowly increased to 130 ◦ C with almost no humidity.
3) The temperature is then raised again to 185–230 ◦ C for two to 3 h to
complete the treatment.
The above process has profound effects on timber viz. cell and mo­
lecular changes, increased durability, dimension stability, mass loss, and
altered mechanical properties.
The main effect of heat treatment is the reduction to the moisture
equilibrium with a subsequent stability in shrinking and swelling. The
degree to which the equilibrium is adjusted depends on several factors
such as, species of timber, temperature of treatment and duration. The
main reason for decrease in the moisture equilibrium is that less water is
able to be absorbed into the cells due to the reduction in hydroxyl groups
and other chemical changes in the timber cells as a result of the treat­
ment process. Conversely, it has also been noted that the crystallisation
of cellulose as a result of the treatment could cause hydroxyl groups to
be inaccessible to water molecules [49]. As stated above, the decrease in
hygroscopicity results in dimensional stability; it is believed that the
polymers formed from sugars during treatment have less hygroscopicity
than the hemicelluloses. Further, other chemical changes cause the
lignin to become more reactive with crosslinks in the lignin, the increase
in crosslinks makes the molecule inelastic; thus the micro-fibrils are
4.5. Design
The design of a timber bridge will extend the service life of a timber
bridge just as much as paints, treated timber and regular maintenance. A
properly designed and well maintained timber bridge will able to last a
minimum of 70 years. There are several considerations which must be
taken into account when a bridge is being designed or upgraded, which
are:
- Timber species and section size;
- Design detailing.
The most important part of timber design is to direct water and
moisture away from the structure, thus much of the design detailing is to
about ensuring there are minimal moisture traps.
4.5.1. Timber species and section size
Durability of timber varies with the species of timber, as a general
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Journal of Building Engineering 34 (2021) 101624
rule of thumb, hard woods tend to be more durable than softwoods.
Australian standard AS6504 provides a guideline to the resistance of
different species of timber to biological hazards and fire [50]. The
standard gives a guide to the expected life of untreated timber to in
ground and above ground use and a resistance rating against hazards of
termites and lyctid attack.
Hence it can be determined that if more durable timber is imple­
mented throughout the structure, it will fare better against deteriora­
tion. AS5604 is only able to provide a guide to the expected durability of
timber, as timber is a biotic material there are variations from tree to
tree caused by conditions during growth. Further, the environment in
which the timber is situated will affect the rate or sustainably of decay;
such an example is that of marine borer attack, in AS5604 durability is
based on samples in southern waters which are not as hazardous to
attack as northern waters. The natural durability of timber is owed to
extractives formed when sapwood metamorphoses into heartwood
(AS5604). When this transformation occurs tannins and other sub­
stances are contained within the parenchyma cells, which often become
toxic and have reduced porosity [30]. Incidentally, the reduced porosity
increases the timbers resistance to shinkring and swelling, and thus
decay relating to dimensional stability. These extracts are also respon­
sible for the darker hue that can be observed in durable timber; though
this is not always the case. Hence, it can be noted that heartwood is
responsible for the primary durability of each timber species, and should
be used during construction as much as possible.
A timber bridge can be designed with inevitable deterioration in
mind. If the timber members are oversized, it will increase the initial
load capacity of the bridge. However, as the members start to deteriorate
they will approach the designed maximum load limit.
mechanical properties. This process is sought after in areas where
environmental impacts are considered of high importance.
Within the polymeric structure of timber cells viz. cellulose, hemi­
cellulose and lignin, are hydroxyl groups which are responsible for the
interactions between water and timber. When water molecules are
present in the timber polymers (when the wood gets wet), they form a
hydrogen bond with the hydroxyl groups.
During the reaction, the acetic anhydride hydroxyl groups in the
polymer are converted into acetyl groups that are hydrophobic. Addi­
tionally, these acetyl groups are considerably larger and heavier than
their acetic anhydride hydroxyl counterparts [44].
The enlargement of molecules within the timber polymer cause the
treated timber to be in permanently swollen state, thus increasing its
dimensions, and have increased mass; the degree to which mass gain is
measured as weight percentage gain and it indicates the extent of
Acetylation. One of the advantages of acetylation over other treatment
processes for biotic decay is that chemicals which are not beneficial to
the environment, and can leach out of the timber over time, are not
required.
Acetylation has profound effects on the durability of the timber. Due
to the hydrophobia of the altered wood polymers the equilibrium
moisture content, hygroscopicity, and saturation point are reduced;
becoming, and remaining, too dry to sustain biological organisms such
as mould and Fungi. Also, due to being dimensionally stable, it is not
subject to internal stress that occur from swelling and shrinking that
cause splits, checks, cracking and warping, nor does it convey stresses
onto external coatings, such as paint, causing them to crack and require
resurfacing [8].
4.7. Physical barriers
4.5.2. Design detailing
Design detailing is mainly about drainage which keeps the timber dry
and prevents a myriad of deterioration. Moisture traps are often most
prevalent in timber connections, where two or three elements meet, such
as post connections or where halfing joints are used, as it is an area
where moisture is able to seep in and remain stagnant.
With end grain having open vessels, it has the ability to rapidly
absorb, store and transfer water through the element; as this the rema­
nence of the nurturance transport system of the once living tree. Hence,
end grain should be one of the first items to be considered when prevent
moisture intrusion into timber elements; this is often carried out with
implementation of flashings. Moreover, the bridge must be designed to
enable ventilation. Air movement around the structure will increase
evaporation rates which will dry timber quicker once it has become wet
[17]. Fig. 14 presents an eight years old footbridge detailed for
durability.
With the exception of treated timber, there are usually two other
ways in which termites are prevented physical barriers and chemical
treatment of foundations. However, the latter has the potential to have
adverse effects on the environment and must be implemented in
accordance with AS3660.
Stainless steel meshing and finely divided granite barriers are the
two ubiquitous physical barriers which are employed to prevent the
manifestation of termites in timber structures. The ideology behind both
methods differs, however, both create a barrier which termites find
difficult to traverse. Stainless steel meshing is simply a mesh which is so
fine that termites are unable to penetrate [23]; in Australia the
maximum aperture sized used is 0.66 × 0.45 mm (with the exception of
northern Australia where 0.4 × 0.4 mm is used). The mesh must be made
out of stainless steel to prevent corrosion from the soil and environ­
mental conditions in the soil. For the most part mesh is usually placed
around the footing of the structure.
A granite barrier is a layer of graded basalt upon which the footing
rests. The theory behind this barrier is that the particles are larger and
heavier than the termites are able to move; therefore, preventing them
from entering the structure. Like many inert barriers they are able to be
circumvented and the inspections must be paramount [51].
4.6. Acetylated timber
Acetylation is a timber modification process whereby the substraight of the timber is altered to provide the desired durable and
4.8. Treated soils
Treated soils are intended to either deter or adversely affect the
termites that pass through the impregnated area, depending on which
chemical is utilised. Traditionally chlorinated hydrocarbon insecticides
have been used to treat soils for termites, which have an efficacy of
30–40 years (in undisturbed conditions i.e under slabs). However, dur­
ing the 1980s many of these were banned as they had considerable
impacts on the environment, and were replaced with other commer­
cially available chemicals; with services lives of 5–10 years.
For treated soils to be the most effective they must be treated before
construction [23] to ensure that the perimeter of the in ground element
has been properly covered. Chemical barriers are prone to failure as
Fig. 14. An eight years old footbridge detailed for durability [5].
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Journal of Building Engineering 34 (2021) 101624
conditions around the element change and cause leaching soil redistri­
bution and allow termite impregnation [51].
effective and durable method of preserving timber it has been found to
leach out over time [17]. The rate at which leaching occurs is dependent
on many factors such as timber age, acidity of rain and or soil, original
amount of CCA applied [52]. A recommendation from the review by the
Australian Pesticides & Veterinary Medicines Authority (APVMA) in
2005 said that CCA treated timber should not be used for handrails or
other areas which are in common contact with humans.
4.9. Chemical preservatives
There are two methods of application for chemical preservatives,
these are pressurised and non-pressurised; both methods, however,
require the timber to be seasoned so that the majority of water is
removed from the timber cells. With a variety of chemical preservatives
available for a myriad of potential maladies, there are three categories
into which they can be classified: Oil Based, Water Soluble, Organic
Solvents. Though the method of application and makeup of the preser­
vative may change, they all intend to achieve the following:
4.9.3. Organic solvents
Organic solvents consist of active chemicals, generally less than 10%,
which have been dissolved in an organic solvent like that of petroleum
distillate. The common active preserving agent in organic solvents are:
Pentachlorophenol, Lindane, Dieldrin, Tributyl tin oxide (TBTO), Cop­
per 8-quinolinolate and Copper napthenate.
Each of these preservatives have varying effects in their preservative
ability, they have been known to leach out of the timber over time.
Usually being highly viscous, these preservative treatments are applied
using through brushing, spraying or immersion.
Repelant or toxic to wood destroying organisms;
Ability to be retained in the timber;
Harmless to timber and non-corrosive to metals;
Have minimal effect on the aesthetics of the timber and still be able
to be workable, glueable and paintable;
• Economic and widely available.
•
•
•
•
4.10. Load testing
For older timber bridges overloading is a highly probable and serious
deterioration mechanism. Before overloading causes structural failure, it
can invite a wide variety of other decay agents to manifest themselves.
The simplest way to ensure that timber bridges are not being overloaded
is to perform a level three engineering inspection to determine the safe
loading capacity of the bridge. It is recommended that Dynamic Fre­
quency Analysis (DFA) or hammer testing is utilised rather than the
traditional load test. This is due to the fact that ultrasonic testing is able
to determine load capacities without further stressing overloaded
members and connections [2].
In general, soft wood timbers absorb treatments better than
hardwoods.
4.9.1. Oil Based
The most common and traditional form of oil protection against
fungus, termites, splits and checks is creosote oil. Creosote oil is distilled
from coal tar [30], and contains over 300 different substances, unfor­
tunately, creosote treated timber has a rather pungent odour. Despite its
smell creosote oil is able to provide timber with long services lives, there
have been documented cases of treated timber having service lives of
100 years. Unlike CCA creosote oil does not easy evaporate or leach out
of timber. The treatment process is applied in two ways; full cell and
empty cell. In full cell treatment, the timber is placed in a vacuum
chamber where the preservative is injected, the pressure is then
increased and the preservative is forced into voids which become filled
with oil [30]. A vacuum state is then applied again to remove some of
the preservative. The major difference between empty cell and full cell
treatment is the final vacuum period at the end of the treatment. In
empty cell treatment the timber is kept in the vacuum longer to remove
more of the preservative, so that the oil is only coating the timber cells.
5. Remedial options
The main goal of any remediation strategy is to provide a sufficient
level of reliability with a bridge network at the lowest cost to life-cycle
maintenance. The different remediation work can not only extend the
life span of the bridge but can also improve the quality and reliability of
the bridge as it ages and increase the safety of the structures for the
public [53].
5.1. Tightening of bolts and other fixings
4.9.2. Fixed and non- fixed water soluble preservatives
There are two forms of water soluble preservatives, fixed and nonfixed [30]. Fixed soluble preservatives are those which usually contain
arsenic, copper and chromium salts, while the most common non-fixed
water soluble preservatives are boron compounds while boric acid, so­
dium fluoride, mercuric chloride, sodium pentachlorophenate, copper
sulphate and zinc sulphate are other commonly used compounds.
However, as water soluble preservatives have a tendancy to leach out,
and cannot be used in contact with ground they will not be discussed at
any further depth.
Copper Chrome Arsenic (CCA), is the most ubiquitous of fixed water
soluble preservative and has been a traditional softwood timber treat­
ment to prevent fungus and decay. The ratio of the components is
generally as follows [52]:
When it comes to loose ineffective connections and fasteners, the
most appropriate remedial strategy is to simply retighten the bolts back
to the specified torque [54]. This has the benefit of not only stopping
overstressing on the connection but also minimises water penetration
into the timber member [33]. Transit New [40] not only supports
tightening but also seal of the bolts that provides an increase in water­
proofing. This is vital as many of the holes used for the connections
extend through the entire length of the timber element and the moisture
ingress could cause internal corrosion which is difficult to detect.
5.2. Removal, repair and resealing
The method of removal of the deteriorated section of an element
varies depending upon the element and the deterioration mechanism.
For metal fasteners that are not so severely corroded as to be needing
replacement, removal of rust should be carried out using a wire brush
and rust remover if needed, then painted to prevent further corrosion
[17]. It is a similar process for mould and stain fungi removal. First spot
cleaning and scrapping is one to remove the defect before the applica­
tion of a paint or seal is applied to prevent further damage. As for decay,
cutting away of the affected area is undertaken and should include an
extra 60 cm of surrounding timber in the direction of the grain [55].
To repair the element once the affected area is removed, an epoxy
- Copper 23–25%;
- Chrome 38–45%;
- Arsenic 30–37%.
Copper cations preserve the cellulose while chromium anions pre­
serve the lignin. Chromium is kept within the mixture as it prevents the
preservative from leaching out of the timber. Though this treatment is
free of odours, the copper compound gives the timber a greenish hue
which can be finished with paints and varnishes. Though CCA is a cost
12
M. Rashidi et al.
Journal of Building Engineering 34 (2021) 101624
polymer should be applied to fill any holes left from debris removal [25].
The resin can be applied under pressure or by hand using a putty or gel.
Packing the void with an epoxy, like a copper napthanate paste, restores
any loss of section and stops moisture and other debris from deterio­
rating the element [28]. Another repair that may be needed is the repair
of broken laminates in SLT decking. This should however be carried with
engineering support as the SLT decking is under prestress pressure and
needs to be destressed prior to repair [33].
Any repairs to the members should be sealed with an appropriate
preservative to decrease the risk of moisture entry. This can be achieved
through the use of water repellant paint or stain that preferably contains
a fungicide. This will keep a reasonably constant moisture content
within the timber through creation of a physical barrier from the
weathering mechanisms. Both an undercoat and top coats are to be
applied to achieve the most optimal protection and durability. Laying
asphalt over the repaired decking is another method of sealing the
timber elements from the weather as well as from traffic wear [7].
overloading, one of which is deformation. Strengthening of the element
can be achieved by bracing the member along the length, while bracing
can also be used to assist in relieving permanent forces on cross mem­
bers, chords and piles [14]. explain how bracing can be used to remedy
buckling as it reduces the chance of further deterioration while
increasing the strength and capacity closer to its original state.
Another remedial method for strengthening deteriorated structures
is the use of sister members. A sister member is a member that is able to
support the loans that were applied to the previous member and is
placed next to the existing deteriorated element. The sister member can
also increase the capacity to remedy deterioration such as sag in an
element. Queensland Government Department of Main [28] also points
out that the supplementary member can be used to remedy strength loss
from overloading and the splitting that it causes [14].
5.6. Member augmentation and meachanical repair
The process of member augmentation and mechanical repair is
centered around the use of extra material, such as timber, steel, concrete
and metal fasteners, in order to reinforce and strengthen deteriorated
elements which all aim to increase the effective section and therefore
load capacity.
The use of metal or timber plates as reinforcement is known as
splicing or scabbing [17]. The plates are placed on either side of the
deteriorated area and bolted or screwed together increasing the effective
area around the localised damage [33]. This method is also known as
clamping and stitching and is used in the remediation of deterioration
mechanisms from overloading, like delamination, deformation and
localised crushing [28]. Concrete jacketing around the timber piles of
the bridge is where the pile is wrapped with corrugated metal, often with
reinforcement with in it, then concrete poured into the sleeve encasing
the pile. This can be applied after deformation or buckling has occurred
and is effective at restoring the strength and load carrying capacity of
the pile [25].
In terms of mechanical repair, there are some remedial options that
involve the use of metal fasteners and other elements to fix damage as a
result of overloading and weathering. Steel banding uses metal straps
that are secured over or around deterioration such as splits and longi­
tudinal fractures. This helps to prevent further deterioration, assists in
minimising any buckling in discrete sections of split piles and restores
some strength to the affected member [28]. Other notable mechanical
remedies include anti-split bolts, which are used when splitting has just
occurred and work by securing either side of the split and preventing
further separation to occur [14]. Finally, metal shims, which are effec­
tively packers or spacers, are used to elevate the decking in line where
headstock sag is less than 50 mm [28].
5.3. Replacement of deteriorated elements
Replacement of an element is often the last resort, when either the
extent of deterioration is too severe or if it is believed to be more cost
effective than other remedial strategies. In regards to the metal com­
ponents [7], states that metallic elements should be replaced with ma­
terials which are resistant to corrosion like galvanised or stainless steel.
[54]; the Queensland Government Department of Main [28] and Transit
New [40] believe that bolts that have been damaged, whether from
overloading or corrosion, should be replaced with non-corrosive
elements.
Replacement is carried out on the timber members themselves for
various reasons. Replacement of members is undertaken when they are
severely deteriorated and new timber should be one that is treated with
a preservative [29]. The criteria for replacement of the top or bottom
chord is when the chord is outside half of its own width from center line
[33]. Some elements require special procedures in order to effectively
replace the deteriorated member like in the case of SLT decking, which is
under pre-stress pressure, destressing is needed prior to replacement of
damaged laminates [33]. Timber used as the replacement should be
either a preservative treated softwood or a naturally durable hardwood
to ensure deterioration mechanisms do not affect the new element [40].
There are many reasons why replacement might be the most appropriate
remedial strategy. However they all have the same goal, which is to
either restore the structure to its original load carrying capacity or up­
grade the structure to support new demands.
5.4. Insect remediation
The main procedure to deal with termite attack on the surface or in
the pipe comes with two scenarios. The first is if the affected area of the
timber member is more than 35%, the timber should be removed and
replaced with a new treated piece. The second scenario is when the
termite attack has damaged less than 35%, in this case, the remediation
method is to treat the area and surrounding members with a termicide to
eradicate the termite infestation.
Borer eradication, revolves around marine borers and it outlines its
main strategy in the eradication of them to reduce the oxygen content of
the water surrounding the piles. This is reliant on the fact that the
damage done has not sufficiently affected the strength of the timber piles
or piers [8].
5.7. Fumigants
Fumigants, like vapam and chloropicrin, are a type of chemical
preservative that come in the form of either a liquid or solid. They are
placed into predrilled holes in order to stop any internal decay and in­
sects [40]. The method in which they work is the fumigants volatise into
a gas that permeates the timber killing any decay fungi or insect. They
can diffuse almost 2.5 m, from the point of origin, in the direction of the
grain and can remain effective from between 10 and 15 years. Fungi­
cides perform in a similar way to fumigants, however, they are usually a
gel or viscous liquid and most commonly based on either fluorine,
copper or boron salts. They also require about 6 weeks under an
impervious wrap immediately after application in order to effectively
diffuse into the timber [40].
5.5. Bracing and sister members
Sometimes it isn’t feasible to replace deteriorated elements due to
their position in the structure. This is when the use of bracing or sister
members is used to reinforce the structure [40]. Bracing assists in
remedying many deterioration mechanisms, often associated with
5.8. Fibre reinforced polymer (FRP)
There are two common ways in which fiber reinforced polymer (FRP)
is used in the remediation of timber elements, wrapping and rods. In the
13
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Journal of Building Engineering 34 (2021) 101624
be seen in Appendix A. The taxonomy has been classified based on the
main deterioration mechanisms as below:
case of wrapping, the FRP is bonded to the tension side of the member
which increases the strength and stiffness of the timber. The fiber
reinforcement also causes the failure mode to change, from brittle to
ductile, becoming safer [8]. By wrapping members in carbon fiber,
which is bonded to the timber, it creates stress sharing between the two
materials and areas of low stiffness because natural defects are taken up
by the carbon fiber. Wrapping members in carbon fiber fabric increases
the horizontal shear (36–68%), bending strength (17–27%) and stiffness
(17–27%). The increase of these strength factors decreases the effect that
natural defects such as knots, checks and splits have on the strength
carrying capacity of the member while also improving the structures
load carrying limit [56].
As for fiber reinforced polymer rods, they are pre-made spikes of FRP
and are inserted into the affected member longitudinally throughout,
they are then covered with epoxy to seal the hole from moisture and
other deterioration mechanisms [57]. The FRP rods increase the stiff­
ness, strain and ultimate flexural strength of the element and are used to
prevent any increase of fractures and splits and to restore the load ca­
pacity of the effected element [58].
-
Weathering;
Biological;
Mechanical Wear;
Natural Defects.
Contained within each of these categories is an inventory of deteri­
oration mechanisms. Each of the deterioration mechanisms is accom­
panied with a description which details the method in which the
mechanism deteriorates the structure. From this description, the type of
deterioration can be identified, and the reason as to why deterioration
has occurred. Once the root cause of the deterioration mechanism has
been identified, an appropriate preventive measure can be taken. For
each deterioration mechanism, there are corresponding remediation and
preventative measures which are specialised to particular forms of
deterioration. Thus from the taxonomy, not only the method of deteri­
oration and root cause can be determined, a series of rectification and
preventive measures can also be identified.
5.9. Composite structures
7. Conclusion
Composite timber structures describe any structure that uses timber
in combination with another material, whether that be concrete, steel or
modified wood products [40]. An example of this is hybrid
composite-timber that is created using laminated veneer lumber (LVL)
layers with a reinforced core material added between layers giving this
form the characteristics of exceptional hardwood [8]. The most com­
mon, and effective, form of composite timber structure is the composite
concrete-timber. This is where a reinforced concrete slab is poured over
the top of an existing timber bridge. It is bonded using non-slip joints
and gluing steel connectors (shear spikes) into the timber [8,34]. The
combination of concrete and engineered wood products can improve the
load capacity by 3 fold. (Makippuro et al., 1996). This increase makes
timber composite structures a great remedial option for deterioration
from overloading like fractures, buckling, crushing and other de­
formations. The only thing to consider is the additional dead load that
the concrete will incur [59].
Timber bridges play a critical role specially in rural transportation
networks, and any disruption in their function may lead to huge eco­
nomic losses. Globally, aging bridges are becoming troublesome to
maintain, particularly for timber bridges that still form part of trans­
portation networks. These remaining timber bridges are often prioritised
for replacement as they are no-longer able to meet the demands of the
communities which they serve and their high cost of maintenance due to
their age. Moreover, as timber bridges are considered archaic and to be
replaced in the near future, knowledge about how to maintain these
structures is becoming obsolete. Hence, a taxonomy table has been
developed to assist in prolonging the life of the remaining timber bridges
so they can continue to serve their communities until they can be
replaced.
Declaration of competing interest
6. Taxonomy
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
All the information that has been collected throughout the research
and presented above, has been collated into a taxonomy table which can
Appendix B. Supplementary data
Supplementary data related to this article can be found at https://doi.org/10.1016/j.jobe.2020.101624.
Appendix A. Taxonomy Table
Number
Deterioration
Mechanism
1
1.1
Weathering
Swelling and
drying due to
Saturation
Description of Deterioration
Mechanism and Main Cause
Effect on Structure
Remediation
Strategy
Prevention
Prevention Description
When timber reaches the
saturation point, free water
existing between cell cavities
causes the microstructure to
swell. The repetitious process
of swelling and drying can
cause leaching of heartwood
toxins which preserves the
timber and also prevents
biotic growth. Further free
water enables fungi to
deteriorate the timber.
Warping
Cupping
Checks
Loss in strength
Element
Replacement
Tightening of all
bolts and
connections
Improper/
degenerated
painting
Bitumen
The utilisation of grease and a
film of bitumen at interaction
faces of wooden elements is
suggested to decrease the
likelihood of water pockets.
Due to heat treatment
altering the fibre moisture
equilibrium, the fibre
saturation point is reduced
leading to mould and fungi
resistance. This resistance is
due to dryer timber is not
Heat Treated Timber
(continued on next page)
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Journal of Building Engineering 34 (2021) 101624
(continued )
Number
Deterioration
Mechanism
Description of Deterioration
Mechanism and Main Cause
Effect on Structure
Remediation
Strategy
Main Cause:
Rain, submerged timber,
environmental conditions,
poorly installed flashings,
removal of protective coating.
Prevention
Flashings
Sealing of Penetrations
Paint
Ventilation
Use of acetylated timber
1.2
1.3
Corrosion
Warping
Moisture in the timber causes
metal elements (gusset plates,
bolts, fasteners) to corrode
and release ferric ions which
deteriorate wood cells. The
high moisture environment
associated with corrosion
encourage rot and fungus
growth. The chemical reaction
between the iron and timber
increases oxidation of the
wood polysaccharides causing
a loss of tensile strength due to
brittle cellar structure.
Main cause:
Salt in timber, moisture in
timber, abundance of
excessive water
Timber deforming from its
original geometry is known as
‘warping.’ The classification
of warping depends on the
plane in which the timber has
-Reduction in connection
strength
-Decay of surrounding
timber
Cleaning and
painting of the
metal components
Replacement of
corroded fixings
Material choice
Decreased structural
capacity
Bracing warped
members
Member
augmentation
Paint, stain, seal
Ventilate members
Paint
Prevention Description
conducive to biological
growth
Installation of flashings over
the end grain of timber and
connections. Flashings are
suitable to areas where high
amounts of air flow occur and
sections where water will
permeate the timber surface
regardless of the preventative
measures taken. If, however,
flashings are improperly
installed or deteriorated it
allows water ingress into the
timber and retains moisture
close to the timber surface
allowing fungal infestations.
Penetrations through timber
members should be sealed
with an appropriate
preservative decreasing the
risk of moisture ingress
through connection holes.
Microporous water repellent
paint or pigment stain (with
fungicide recommended)
maintain a relatively constant
timber moisture content. This
is achieved by creating a
barrier between the surface of
the timber and weathering
mechanisms (precipitation,
heat, UV radiation). This
barrier prevents ingress or
egress into the timber
element.
Ventilation incorporated into
the design of the structure
and allows air to flow
through the structure,
minimising the level of
saturation.
Aceytylation is a process
whereby the chemical
composition is altered by
converting the acetyl groups
in the cell into acetic acid.
The conversion causes the
wood to swell. The
acrytylation process causes
the timber to become more
dimensionally stable
The non-corrosive metals like
galvanised or stainless steel
fixings and non-corrosive
metal components.
Microporous water repellent
paint creates a physical
barrier between the metallic
surface and oxygen
subsequently preventing
oxidisation.
The protective coatings along
with ventilation reduce the
wetting and drying effects
from the environment, which
cause the warping of the
(continued on next page)
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Journal of Building Engineering 34 (2021) 101624
(continued )
Number
Deterioration
Mechanism
Description of Deterioration
Mechanism and Main Cause
Effect on Structure
Remediation
Strategy
deformed i.e., cupping,
around the minor axis,
bowing, the major axis.
Main cause:
Sporadic moisture content
within the timber, growth
stresses
Prevention
Element
replacement
Use of acetylated timber
1.4
Ultra Violet
Radiation (UV)
When timber is exposed to UV
radiation, a degenerative
photochemical reaction in the
lignin of the timber cells
occurs. This reaction only
directly affects the aesthetics
of the bridge causing the wood
surface to become exposed
and turn grey in colour. UV
radiation is a very slow
process with an estimated rate
of 63 mm per 100 years.
Main cause:
Exposure to sunlight
Affects the aesthetics of the
timber
Can allow other
deterioration mechanisms
to occur through minor
cracks caused by UV
radiation
Paint, stain or seal
effected members
Replacement
Paint, stain or seal
Use of acetylated timber
Cladding
2
2.1
2.1.1
Biological
Insect attack
Termite
Well-established termite
attack normally damages
wood rapidly, however it is
uncommon for termite attack
to take place in durable
hardwoods usually utilised in
bridge assembly without some
pre-existing fungal decay. The
decay accelerates when the
termites extend their galleries
throughout the bridge,
moving fungal spores and
moisture around with their
bodies. Therefore, while the
Reduced strength and
structural capacity
Replacement
Construction
detailing
Fumigants
Termite guards
Termicide
Site clearance
Detailing
Prevention Description
timber.
Microporous water repellent
paint or pigment stain (with
fungicide recommended)
maintain a relatively constant
timber moisture content, this
is achieved by creating a
barrier between the surface of
the timber and weathering
mechanisms (precipitation,
heat, UV radiation). This
barrier prevents ingress or
egress of moisture from the
timber element. Consistency
in moisture content reduces
the probability of checks,
splits & fracture by
preventing regular swelling
and shrinking.
Acetylation is a process
whereby the chemical
composition is altered by
converting the acetyl groups
in the cell into acetic acid.
The conversion causes the
wood to swell. The
acrytylation process allows
the timber to become more
dimensionally stable.
Microporous water replant
paint or pigment stain (with
fungicide recommended)
maintain and provide a
physical barrier between
incoming UV radiation.
Acetylation is a process
whereby the chemical
composition is altered by
converting the acetyl groups
in the cell into acetic acid.
The conversion causes the
wood to swell. The
acetylation process allows the
timber to become more
dimensionally stable.
Cladding prevents critical
elements from being exposed
to direct sunlight and thus
reducing UV degradation on
critical elements. Cladding
acts as a sacrificial layer to
the structure by reflecting
and absorbing UV radiation.
It is paramount that cladding
elements are well maintained
to prevent them from
becoming a source of
deterioration on the
structure.
• Removal of the nest (by
either direct chemical action
or by isolation of the colony
from its nourishing moisture)
• Implementation of chemical
and physical barricades to
prevent termites from
attacking a timber bridge or
damaging wood interacting
with the ground
Attacks can be prevented
through the strategic
placement of high risk
members. By providing
(continued on next page)
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Journal of Building Engineering 34 (2021) 101624
(continued )
Number
Deterioration
Mechanism
Description of Deterioration
Mechanism and Main Cause
Effect on Structure
Remediation
Strategy
majority of the material
eliminated by termites has by
this time lost its structural
strength due to decay, the
control of termites is still a
significant concern.
Main cause:
–
-Timber bridges being situated
in humid regions.
-Pre-existing fungal decay in
timber bridges.
-Moist conditions provided by
improperly installed flashings
Prevention
Treat soil
Placement of stainless
steel mesh around
footings
Strip shielding
Timber selection
Use of acetylated timber
2.1.2
Borer
In general, wood borers are
beetle which at some point,
during their short life, use
timber as a method of shelter,
food or both.
Reduced strength and
structural capacity
Reduced strength and
structural capacity
Replacement
Construction
detailing
Fumigants
Timber selection
Detailing
Prevention Description
adequate clearances,
ventiliation, and physical
protection, the risk of attack
is minimised.
Soils in contact or in close
proximity to timber elements
can be treated. Treated soils
deter termites from entering
the timber through the
ground.
The placement of fine
stainless steel mesh around
the footings of the bridge for
elements in close contact
with the ground deter
termites from entering the
element through the ground.
The openings within the
mesh are too small for
termites to pass through and
thus prevent termite
infestation.
Though strip shielding,
otherwise known as ant
capping, do not prevent
termite infestation, they
provide a method of identify
termite infestation. When
strip shields are installed
properly termites must
construct mud tubs over them
from to enter the structure
which can be observed during
inspections. These caps
should be installed on top of
elements which are in contact
with the ground and have
timber elements on top of
them.
Use of timber with a high
natural resistance to termite
to be selected for areas at
high risk.
Acetylated timber has been
found to be highly durable
against Mastotermes
darwiniensis (Australia’s
most aggressive termite).
Field tests of the acetylated
timber have proven it to be
more resistance to termite
attack than other naturally
durable timber such as white
American oak heartwood and
Western Red Cedar
heartwood.
Use of timber with a high
natural resistance to termite
to be selected for areas at
high risk. Australian
Standard AS 5604 natural
durability class 1 or 2
specifies these timber
selections. Areas that are
particularly scriptable to the
powder post beetle must be
avoid timbers with rich
starch sapwood.
Attacks can be prevented
through the strategic
placement of high risk
members. By providing
adequate clearances,
ventiliation, and physical
(continued on next page)
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M. Rashidi et al.
Journal of Building Engineering 34 (2021) 101624
(continued )
Number
Deterioration
Mechanism
Description of Deterioration
Mechanism and Main Cause
Effect on Structure
Remediation
Strategy
Prevention
Metallic Salts
Use of acetylated timber
2.1.3
Ants
Ants are insects that
frequently build passageways
and nests in decay cavities in
timber structures.
Main cause:
Ants deposit sawdust in
gallery openings, thus
trapping moisture
Increases the rate of decay
of a timber component.
Fumigants
Construction
detailing
Metallic salts
Prevention Description
protection, the risk of attack
is minimised.
Copper (23–25%), chrome
(38–45%) and arsenic
(30–37%) (CCA) have been
the traditional method of
preventing timber fungal
infection in timber. The
copper compound in CCA
causes the timber to have a
greenish colour. Boron
compounds are another form
of chemical preservatives.
CCA has been reported to
leach out of timber over time;
the rate at which is
dependent on may factors
such as timber age, acidity of
rain and or soil, original
amount of CCA applied. Due
to this, many countries are
outlawing the use of such
treatments.
Aceytylation is a process
whereby the chemical
composition is altered by
converting the acetyl groups
in the cell into acetic acid.
The conversion causes the
wood to swell due to the
larger acetic molecules with
are present in the timber and
thus cause the treated timber
to have higher strength,
hardness and bending.
Aceytylated timber has be
found to display a high
durability against many
wood feasting organisms
such as marine borers,
teredinids, limnoriids and
shipworms in both field and
laboratory testing. This
increased resistance is marine
bores and other biological
organism is unclear, however
it is hypothesised that
changes to the timber such as:
• Hardening of the cell wall
• blocking of cell wall
micropores
• non recognition of the
enzymes in the altered
timber
Copper (23–25%), chrome
(38–45%) and arsenic
(30–37%) (CCA) have been
the traditional method of
preventing timber fungal
infection in timber. The
copper compound in CCA
causes the timber to have a
greenish colour. Boron
compounds are another form
of chemical preservatives.
CCA has been reported to
leach out of timber over time;
the rate at which is
dependent on may factors
such as timber age, acidity of
rain and or soil, original
amount of CCA applied. Due
to this, many countries are
outlawing the use of such
treatments.
(continued on next page)
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M. Rashidi et al.
Journal of Building Engineering 34 (2021) 101624
(continued )
Number
2.2
Deterioration
Mechanism
Bacteria
Description of Deterioration
Mechanism and Main Cause
Bacteria are single cell
organisms and in wet
conditions. Though bacterial
decay is a slow process, it has
the potential to deteriorate
preservatives and allow
organisms with a reduced
chemical threshold to
develop.
Effect on Structure
Remediation
Strategy
It can cause timber to have
an increase permeability
and cause the timber
surface to soften.
Prevention
Prevention Description
Detailing
Attacks can be prevented
through the strategic
placement of high risk
members. By providing
adequate clearances,
ventiliation, and physical
protection, the risk of attack
is minimised.
Due to heat treatment
altering the fibre moisture
equilibrium, the fibre
saturation point is reduced.
This leads to mould and fungi
resistance as dryer timber is
not conducive to biological
growth
Attacks can be prevented
through the strategic
placement of high risk
members. By providing
adequate clearances,
ventiliation, and physical
protection, the risk of attack
is minimised.
Copper (23–25%), chrome
(38–45%) and arsenic
(30–37%) (CCA) have been
the traditional method of
preventing timber fungal
infection in timber. The
copper compound in CCA
causes the timber to have a
greenish colour. Boron
compounds are another form
of chemical preservatives.
CCA has been reported to
leach out of timber over time;
the rate at which is
dependent on may factors
such as timber age, acidity of
rain and or soil, original
amount of CCA applied. Due
to this, many countries are
outlawing the use of such
treatments.
Aceytylation is a process
whereby the chemical
composition is altered by
converting the acetyl groups
in the cell into acetic acid.
The conversion causes the
wood to swell due to the
larger acetic molecules with
are present in the timber and
thus cause the treated timber
to have higher strength,
hardness and bending.
Further the acrytylation
process cases the timber to
become more dimensionally
stable as it not as sensitive to
swelling and shrinking.
Aceytylatied wood cells are
also protected from UV
radiation.
Ventilation pathways
between timbers and their
supports provide airflow
which prevents the
occurrence of mould and
stains.
Use of timber with a high
natural resistance to fungal
attached should be selected
for areas at high risk.
Heat treated timber
Construction
detailing
Detailing
Replacement
Metallic salts
Use of acetylated timber
2.3
Fungi
Fungus is an organism that
breaks down timber for a
source of sustenance and
propagates through timber via
threadlike hyphae that grow
through pits or penetrate cell
walls.
The way in which fungus
spreads along the structure
differ according to the species
Decreased structural
capacity
Potential for other
biological detrition
Aesthetic appearance
affected
Risk of multiple elements
being effected by
organisms
Replacement
Ventilation
Fumigants
Timber selection
(continued on next page)
19
M. Rashidi et al.
Journal of Building Engineering 34 (2021) 101624
(continued )
Number
Deterioration
Mechanism
Description of Deterioration
Mechanism and Main Cause
Effect on Structure
Remediation
Strategy
and method of reproduction.
There are three classifications
of fungi viz. mould fungi, stain
fungi and decay fungi all with
differing effects on the
structure.
Main cause:
Environmental conditions
2.3.1
Mould & Stain
Fungi
Generally, cause blemishes on
the surface of the timber and
affects the aesthetic qualities
of timber. This form of fungus
uses the contents of the wood
cell for sustenance and do not
affect the cell wall thus not
effecting the strength of
timber.
Main cause:
High moisture content
Under suitable conditions
timber degrade causing
reduced toughness and
increased permeability.
Can be the precursor to
other more detrimental
organisms
Brushing and
scrapping
Prevention
Prevention Description
Use of acetylated timber
Acetylation is a process
whereby the chemical
composition is altered by
converting the acetyl groups
in the cell into acetic acid.
The conversion causes the
wood to swell. The
acrytylation process allows
the timber to become more
dimensionally stable.
Acetylated wood cells are
also protected from UV
radiation.
Please see section 2.1 for
more information.
Prevention of excessive
timber moisture content
or stagnant water on
structure
Ventilation
Paint, stain or seal
Heat treated timber
Epoxy resin
Timber selection
Use of acetylated timber
Prevention of excessive
timber moisture content
or stagnant water on
structure
Metallic salts
Ventilation pathways
between timbers and their
supports provide airflow
which prevents the
occurrence of mould and
stains.
As stated above organisms
require three elements to
survive: water, oxygen, and
sustenance. The physical
barrier of paint prevents
fungus spores from reaching
the surface of the timber and
gaining sustenance to
survive. Further, painting
prevents excessive moisture
content and the likelihood of
fungal infestation.
Due to heat treatment
altering the fibre moisture
equilibrium, the fibre
saturation point is reduced.
This leads to mould and fungi
resistance as minimal free
water in the timber fibres are
available for biological
growth.
It should be noted that heat
treatment does little effect on
fungal attack when timber is
in contact with the ground.
Use of timber with a high
natural resistance to fungal
attack should be selected for
areas at high risk.
Acetylation is a process
whereby the chemical
composition is altered by
converting the acetyl groups
in the cell into acetic acid.
The conversion causes the
wood to swell. The
acrytylation process allows
the timber to become more
dimensionally stable.
Acetylated wood cells are
also protected from UV
radiation.
See moisture content for
more information.
Copper (23–25%), chrome
(38–45%) and arsenic
(30–37%) (CCA) have been
the traditional method of
preventing timber fungal
infection in timber. The
(continued on next page)
20
M. Rashidi et al.
Journal of Building Engineering 34 (2021) 101624
(continued )
Number
2.3.2
2.3.2a
2.3.2b
Deterioration
Mechanism
Decay fungi
Brown Rot
White Rot
Description of Deterioration
Mechanism and Main Cause
Effect on Structure
Remediation
Strategy
Decay fungi is generally the
main cause of decay in timber
bridges, it has three
classicisations based upon the
way in which it appears and
manifests itself in the timber
which are
• Brown Rot
• White Rot
• Soft Rot
Main cause:
Environmental conditions
Checks and splits can grow
to a substantial depth in
the internal untreated
wood.
Of the least important effects
of brown rot it discolours the
timber brown. During
advanced stages the rot
becomes brittle and has
numerous cross checks and
makes the surface of the wood
look charred in appearance.
Brown rot attacks the cellulose
and hemicellulose of the cell
wall and alters the remaining
lignin, this process can cause
weight losses of up to 70%.
Due to the fact the brown rot
removes the cellulose, which
provides strength to the cell, it
can cause strength reduction
in early stages of decay. The
methodology of attack for
brown rot is the reason as to
why it can be considered the
most serious of all the decay
fungi. Brown rot releases
enzymes that have the ability
to migrate or defuse far from
the area where hyphae are
present; as such losses in
strength can be present in
areas far from the visibly
affected areas.
Main cause:
Environmental conditions
Aesthetics
In appearance, white rot is a
shade of white or tan in colour
with dark streaks present.
During early stages white rot
is not as easily detected as the
early stages of decay
Texture
When the rot has become
advanced it is soft in texture
and fibres may peel
individually from the timber.
Detrition method
White rot attacks all three
High reduction in wright
loss and strength
Can affect multiple section
of the structure once the
decay process has
commenced with little or
no sign of decay
Prevention
Removal of
effected area
Paint stains & seal
Fumigants
Timber selection
End caps
Ventilation
Heat treated timber
Use of acetylated timber
Metallic salts
Fumigants
Paint, stain, seal
End caps
Replacement
Use of acetylated timber
Ventilation
Heat treated timber
Metallic salts
Extensive reduction in
wright loss and strength
Can affect multiple section
of the structure once the
decay
Replacement
Paint, stain, seal
Ventilation
Heat treated timber
Use of acetylated timber
Fumigants
Metallic salts
Prevention Description
copper compound in CCA
causes the timber to have a
greenish colour. Boron
compounds are another form
of chemical preservatives.
CCA has been reported to
leach out of timber over time;
the rate at which is
dependent on may factors
such as timber age, acidity of
rain and or soil, original
amount of CCA applied. Due
to this, many countries are
outlawing the use of such
treatments.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section 2.3 for
more iformation.
Please see section
more iformation.
Please see section
more iformation.
Please see section
more iformation.
Please see section
more iformation.
Please see section
more iformation.
2.3 for
2.3 for
2.3 for
2.3 for
2.3 for
(continued on next page)
21
M. Rashidi et al.
Journal of Building Engineering 34 (2021) 101624
(continued )
Number
2.3.2c
Deterioration
Mechanism
Soft Rot
3.
3.1
Mechanical Wear
Deck
Damage
3.1b
LVL & stress
Laminated
Timber (STL)
decking damage
Description of Deterioration
Mechanism and Main Cause
components of the cell wall
causing extensive weight
losses of up to 97% and thus a
substantial loss in strength.
Propagation
Enzymes released by the rot
remain close to the hyphae
therefore localising
infestation.
Main cause:
Environmental conditions
Detrition method
Generally soft rots attack the
outer wood shell and have
exogenous nuisance to create
substantial decay. The
detrition method can be
divided into three stages:
• Incipient - this where
infection is freshest and
hard to detect
• Intermediate –
discolouration begins and
little strength is left in the
timber and the wood
becomes soft
• Advanced – minimal to no
strength is left in the
timber, voids begin to
appear as the timber is
dissolved
Though the rot can have
devastating effecting on a
structure it is not usually
associated with structural
decay.
Main cause:
Continuous wetting or
changing moisture content of
timber
Presents of additional
sustenance for fungi
Effect on Structure
Remediation
Strategy
Prevention
Prevention Description
Structural deterioration
Replacement
Fumigants
Paint, stain, seal
Please see section
more iformation.
Please see section
more iformation.
Please see section
more iformation.
Please see section
more iformation.
Please see section
more iformation.
Please see section
more iformation.
Please see section
more iformation.
End Caps
Ventilation
Heat treated timber
Use of acetylated timber
Metallic salts
Timber selection
Narrowly spaced sawn
timbers up to 125 mm in depth
(200 mm–250 mm in width)
supported on beams.
Main cause:
The main causes of
deterioration in timber
decking are decay and insect
damage.
When combined with the
multiple bolt holes through
the deck, the decking
elements are positioned in
an extremely high danger
environment for decay and
insect attack, therefore
resulting in section loss.
SAPA Decking
Asphalt
Pressure
treatment
Cross Laminated
Timber
The deck can be fastened
to bolting strips, and then
these bolting strips, or the
deck itself, be fixed to the
beams using one of the
techniques summarised in
"Prevention Description".
SLT (Stress Laminated
Timber) decking is a system
which uses thin wooden
laminates. The laminates are
positioned on edge (vertical)
and pressured together using
high strength bars or
prestressing strands to make a
firm structural slab.
Main cause:
A loose tie down system may
cause a state of overstress and
increased deck deformations
leading to timber
deterioration.
One laminate in every few
hundred might be weaker
than the load it is placed
under, therefore that
particular laminate will
have an effect on the
strength and functionality
of the SLT decking.
Rehabilitate or
replace damaged
wooden
laminates.
Preservative treatment.
Protection against direct
moisture ingress.
2.3 for
2.3 for
2.3 for
2.3 for
2.3 for
2.3 for
2.3 for
Technique 1: Utilise steel
cross members below the
beams, where the cross
member is fastened utilising
curved threaded rods bent
over the beams.
Technique 2: Position the
bolting strips near the beams
and then implement straps or
alike curved rods to fix the
deck to the beam.
SLT decking protection
consists of the preservative
treatment of softwood
timbers and the sapwood in
some hardwood.
Wood needs to be protected
against direct moisture
ingress. This protection is
provided on the sides, ends
and top of the SLT deck. The
top surfaces of SLT decks are
protected with an
impermeable film which can
comprise of either an actual
physical film (e.g. Wolfin) or
a rubberised bitumen
wearing surface. The ends
and sides of SLT decks are
fixed with flashing to help
keep the unprotected wood
dry. This flashing can
(continued on next page)
22
M. Rashidi et al.
Journal of Building Engineering 34 (2021) 101624
(continued )
Number
3.2
3.2a
Deterioration
Mechanism
Deformation
Chord
Deformation
3.2b
Sagging
3.3
Element
Crushing
3.4
Element
buckling
Description of Deterioration
Mechanism and Main Cause
Effect on Structure
Remediation
Strategy
Prevention
Prevention Description
comprise of galvanised metal
or a rubber style material
which can be situated
beneath the anchorages.
Chord deformation is the
altering of the shape or
direction of the member as a
result of a load or loads being
applied.
The deformation causes the
movement in the entire
structure that can result in
damage to other elements
such as the more rigid surface
layer.
Main cause:
Sagging of the truss,
deterioration of deck
components, overloading
Sagging is the deformation
where the middle of the
element bends downward and
is the result of the application
of weight or pressure being
applied.
Main cause:
Long span length, uneven
horizontal dispersal of weight
through the deck (causes
sagging of timber stringers),
settlement of piles (causes
sagging of crossheads)
Crushing is a deterioration
mechanism that occurs when
overloading takes place, either
parallel or perpendicular to
the grain. When the laod is
applied parallel to the grain, it
shortens the cell within the
element along their
longitudinal axis which causes
the micro fibrils of the cell
wall to fold, eventually folding
the cell itself. This deforms the
cellular structure creating
planes of weakness and
instability finally resulting in
visible surface damage.
Main cause:
Over tightened connections,
bridge loading
Timber girders are specifically
vulnerable to crushing when
substantial loss of section has
taken place.
Element buckling has two
forms, first Global buckling
which is where part or all of
the length deforms
longitudinally. The second is
where the cross section of the
element deforms. In this case
the damage is localised.
Buckling can be attributed to
many causes depending on the
situation, they include but are
not exclusive to, overloading,
loose bolts or connections and
scour and abrasion.
Main cause:
Buckling is caused by wooden
piles being unable to support
axial load.
If bolted connections come to
Deformation can cause
subsequent damage to the
surface layer resulting in
further deterioration.
Splicing
Strengthening the
bracing
Replacement
FRP Rods
Prevent moisture content
changes
Fibre reinforced polymer rods
are inserted into the affected
member longitudinally
throughout, then covered
with epoxy, increasing the
stiffness, strain and ultimate
flexural strength. This helps
prevent deformation and
warping from occurring.
Please see section 2.1 for
more information.
Any long-term sag will
increase bending in a
headstock and therefore
decrease its capacity.
Metal shims
Reseating
Replacement
Sister member
Sag rods
Relieving arch
Sag rods are steel members
that are under tension and are
combined with diagonal
bracing members to reduce
sag an increase the overall
redundancy of the member.
They work by providing
tension throughout the
member and focus on the
weak properties of strength,
both tension perpendicular to
the grain and shear strength.
Causes loss of strength and
affects serviceability of
timber components.
Decay
Crushing will deteriorate
wooden piles, normally at
or close to the waterline.
Replacement
Strengthening
Supplementary
member
Bolts
Bolts are fixed through the
ends of vulnerable
components with the aim of
preventing crushing.
Anti-crush plates are used at
the connections of structures
that carry large loads and
reduce crushing at those sites.
The way they achieve this is
by making the bearing with
larger, this in turn makes the
bearing capacity larger.
Decreases wooden pile
capacity.
Strapping
Replacing
Concrete Jacket
FRP wrapping
Anti-crush plate
Bracing
Tightening of bolts
Gabions
The elements are wrapped in
a layer of fibre reinforcement
polymer that increases the
modulus of rupture and
ultimate strength and load
capacity, significantly
reducing the chance of
element buckling.
Bracing of the pile can greatly
reduce the chance of the
element buckling.
When bolts or other fixings
are found to be loose,
ineffective or missing they
should be replaced or
tightened back to
specification in ordered to
stop overstressing
(continued on next page)
23
M. Rashidi et al.
Journal of Building Engineering 34 (2021) 101624
(continued )
Number
3.5
3.6
3.7
4.0
4.1
4.2
4.3
Deterioration
Mechanism
Description of Deterioration
Mechanism and Main Cause
be loose because of decay in
wooden piles or if there is a
substantial loss of section
because of steel corrosion, the
efficiency and function of
bracing components will be
lost, causing the component to
buckle.
Vertical failure of a bridge can
also be caused by corrosion of
the pile or scour, due to
moving water.
Delamination
-Delamination is the
separation of layers in timber
from plywood to glulam. It
occurs when moisture
penetrates the ply or when
glued-laminated layers
separate as the adhesive that
bonds the layers fails
Main cause:
Glued laminated timber being
situated in humid regions, or
wherever submergence is
frequent.
Plywood sheet ends being
unprotected from ultraviolet
radiation and weathering.
Fractures
Beams under flexural loading
can exhibit factures which are
influenced by various
mechanical properties and
loading conditions of the
timber element.
Main cause:
Mechanical properties of
timber and loading
conditions.
As the moisture content
throughout timber is not
uniform, it causes sporadic
volume change over the
course of the member. This
volume change combined
with low strength normal to
the timber grain cause cracks
to develop.
Loose connection
Vehicle traffic loads across the
bridge along with weathering
crush the wood around the
fasteners due to the repetitive
impact. The loading wears on
the connection (fasteners and
their holes) causing them to
loosen.
Main cause:
Traffic loading, vibration and
weathering
Natural Element Defects
Knots
Knots are a piece of branch or
limb that has been
incorporated into the timber
member
Main cause:
Natural product of tree growth
Checks
Checks are a separation of
wood occurring perpendicular
to the cross sectional grain or
growth rings
Main cause:
Seasoning, weathering
Split
Splits are a separation of wood
from one surface to another,
usually parallel to the grain
Main cause:
Seasoning, weathering
Effect on Structure
Remediation
Strategy
Prevention
Prevention Description
Gabions are cages filled with
rocks that are placed around
the piles. The use of gabions
in order to protect the piles
from scour as well as abrasion
from debris.
They provide openings for
decay to begin and may
cause a reduction in
strength
Clamping and
stitching
Composite
concrete timber
structure
Element
replacement
Prevent moisture content
changes
Use sealant, for example
bitumen, to the unprotected
exterior surface of plywood
decking, to prevent
delamination of laminates.
Reduced Strength
Steel banding
FRP rods
Replacement
Prevent moisture content
changes
FRP rods
Please see section 2.1 for
more information.
Fibre reinforced polymer rods
are inserted into the affected
member longitudinally
throughout, then covered
with epoxy, increasing the
stiffness, strain and ultimate
flexural strength. This helps
prevent the development on
cracks and fractures
occurring.
The beams are wrapped in a
layer of fibre reinforcement
polymer that increases the
modulus of rupture and
ultimate strength,
significantly reducing the
chance of fracture
Routine tightening is the
scheduled activity of
examining and maintaining
the current level of service of
a bridges connection.
FRP wrapping
Loose connections can
reduce the bridge’s loadcarrying capacity
Replace damaged
fixings
Tighten fixings
Routine tightening of
fixings
Reduce strength and load
carrying capacity
Wrap member in
carbon fibre fabric
Prevent moisture content
changes
Please see section 2.1 for
more information
Reduce strength and load
carrying capacity
Opens the timber to further
weathering and
deterioration
Epoxy fill
Element
replacement
Prevent moisture content
changes
Please see section 2.1 for
more information
Reduce strength and load
carrying capacity
Opens the timber to further
weathering and
deterioration
Split resistant
bolts
Steel banding
Element
replacement
Prevent moisture content
changes
Please see section 2.1 for
more information
24
M. Rashidi et al.
Journal of Building Engineering 34 (2021) 101624
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