A comprehensive taxonomy for structure and material deficiencies, preventions and remedies of timber bridges

A comprehensive taxonomy for structure and material deficiencies, preventions and remedies of timber bridges

Journal Pre-proof A comprehensive taxonomy for structure and material deficiencies, preventions and remedies of timber bridges Maria Rashidi, Azadeh N...

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Journal Pre-proof A comprehensive taxonomy for structure and material deficiencies, preventions and remedies of timber bridges Maria Rashidi, Azadeh Noori Hoshyar, Liam Smith, Bijan samali, Rafat Siddique PII:

S2352-7102(19)31812-1

DOI:

https://doi.org/10.1016/j.jobe.2020.101624

Reference:

JOBE 101624

To appear in:

Journal of Building Engineering

Received Date: 6 September 2019 Revised Date:

19 June 2020

Accepted Date: 26 June 2020

Please cite this article as: M. Rashidi, A.N. Hoshyar, L. Smith, Bijan samali, R. Siddique, A comprehensive taxonomy for structure and material deficiencies, preventions and remedies of timber bridges, Journal of Building Engineering (2020), doi: https://doi.org/10.1016/j.jobe.2020.101624. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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63 64 65 66 67 68 69 70 Maria Rashidi1, Azadeh Noori Hoshyar2, Liam Smith3, Bijan 71 samali1, Rafat Siddique4 1 Centre for Infrastrsucture Engineering, Western Sydney 72 73 University, New South Wales, Australia 2 Federation University, Brisbane, Queensland, Australia 74 3 Douglas Partners, New South Wales, Australia 75 4 Thapar Institute of Engineering & Technology, Patiala, 76 India 77 78 Corresponding Author: Maria Rashidi ([email protected]) 79 80 Abstract 81 82 As timber bridges have become archaic, they are no longer 83 able to effectively service their community. It is neither 84 practical, nor possible, to replace all existing timber bridges, 85 hence it is of paramount importance to maintain and extend 86 the service life of those remaining timber bridges. The following discourse intends to provide an extensive 87 and 88 comprehensive review of the various deterioration 89 mechanisms, the preventive actions and possible remedial 90 options for management and maintenance of timber bridges. 91 The classified information has been summarised in a tabular 92 format and presented as a ready-reckoner taxonomy for quick reference. This taxonomy is purely a re-statement of 93 the 94 information already covered in the paper, but when presented 95 in the summary form, reference becomes highly convenient. 96 Keywords: 97 Timber bridge, deterioration mechanism, prevention, 98 remediation, taxonomy. 99 100 1. Introduction 101 It is estimated that there are currently 43,000 timber 102 bridges in use across Australia (Tingley, 2014), with most 103 being constructed before 1950 (Rashidi et al., 2017). 104 Understandably, the majority of these bridges are now 105 some of the oldest bridges in transportation networks, and 106 have become dilapidated and structurally weakened as107 a result. Though these bridges may have minimum worth 108 themselves, they have a greater value to the economy109 as they are from part of trade routes and links between 110 communities; their assessed cost under values their 111 net worth. Timber bridges in some situations are unable112 to service their community as they are no longer able113 to handle modern or increased traffic load or conditions and 114 their cost of maintenance (North, 2012). However, it115 is neither possible nor practical, economically or physically, 116 to replace all timber bridges simultaneously. Thus, they 117 must be maintained until they can be replaced or made 118 redundant (Rashidi et al., 2018b). 119 The majority of timber bridges that are currently in use 120 are not designed for current vehicles and traffic loadings, 121 as such these bridges are exceeding their antiquated 122 design capacity (RTA, 2008 ). Local government asset 123 managers must ensure that their infrastructure corridors 124 are able to satisfy the demands of their community while 125

on a limited budget, hence, bridges that are difficult to maintain are usually prioritised for replacement (Tingley, 2014). 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 (Tingley, 2014). 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” (Ingall, 2008). 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 industries have on bridges and their inability to function properly without this infrastructure (Moore et al., 2011). 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 Sri Lanka, dating back to the 13th century, and the 14th century Kapellbrücke bridge in Switzerland (Balendra et al., 2010). Even with the possibility of long services lives, timber bridges are inherently susceptible to a myriad of deterioration mechanism, to which other construction materials are resilient; such as that of natural disasters and biological hazards. The way in which these deterioration mechanisms are mitigated and managed will affect the expected 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 17.8km 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.

A Comprehensive Taxonomy for Structure and Material Deficiencies, Preventions and Remedies of Timber Bridges

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 mechanisms are forms of decay as well as insect attack. Whereas the main non-biological 1

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deterioration mechanisms come from physical decay 161 Swelling and drying due to saturation, corrosion, warping through weathering as well as forms of mechanical wear 162 and ultraviolet radiation are the main sub-categories of this (Dackermann et al., 2009). 163 mechanism.

164 165 1.1 Swelling 166 1.2 Corrosion 167 1. Weathering 168 1.3 Warping 169 1.4 Ultraviolet 170 2.1.1Termites Radiation 171 2.1 Insect Attack 2.1.2 Borers 172 173 2.1.3 Ants 2. Biological 174 2.2 Bacteria 2.3.1 Mould and 175 Stain Fungi 2.3 Fungi 176 2.3.2 Decay Fungi 177 3.1 Deck Wear 178 179 3.2 Deformation 180 3.3 Element Crushing 181 182 3. Mechanical Wear 3.4 Element Buckling 183 3.5 Delamination 184 185 3.6 Fractures 186 3.7 Loose 187 Connections 188 4.1 Knots 189 190 4. Natural Element 4.2 Checks Defects 191 4.3 Splits 192 193 Figure1. The main deterioration mechanisms of timber 194 bridges 195 Within various deterioration mechanisms, there is usually 196 an underlying cause, and the two most common 197 are moisture content and overloading. Moisture content issues 198 cause a cycle of wetting and drying which alters 199 the surface and end grains of the timber. The cross-sectional 200 movement causes the timber to warp and form splits and 201 checks. These splits and checks along with the high 202 timber moisture content, of above 20%, creates the perfect 203 environment for development of fungi and insect 204 infestation. The high moisture content also causes 205 unprotected metal components to corrode and rust. 206 Moisture content is not only a root cause of the biological 207 deterioration mechanisms, when combined with 208 overloading, which is the application of a load exceeding 209 the current load carrying capacity of either the element210 or structure, it becomes the starting point for many 211 deterioration mechanisms. Overloading causes deck 212 damage, deformations like sagging, element crushing and 213

130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 buckling, fractures and when combined with high 152 moisture content, can play a crucial role in delamination. 214 153 The main deterioration mechanisms of timber bridges 215 are 154 illustrated in Figure 1. 216 217 155 218 156 2.1 Weathering 219 157 158 This deterioration mechanism is mainly due to 220 the 221 159 environmental conditions such as moisture content and 222 160 ultraviolet radiation (Mahnert and Hundhausen, 2018).

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 microstructure to swell. The repetitious process of swelling and drying can cause leaching of heartwood toxins which preserves the timber and prevent biotic growth (Mahnert and Hundhausen, 2018). As a result of the constant moisture fluctuation, the timber can also become subject to surface checking. This defect along with overloading of the member can decrease its strength (Wilkinson, 2008). Timber deformations like grain rising, warping, cupping and checks are also results from varying moisture contents and have also been associated with a loss in strength (Sibel et al., 2011). Moisture meters can efficiently be utilised in undertaking assessments 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.

2.1.2

Corrosion

The lowest level at which corrosion of metal fastenings occur in wood is 18%, can produce loose connections. Oxidisation occurs when moisture in the timber causes metal elements (gusset plates, bolts, fasteners, 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 (Valentin, 2012). 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. Corrosion creates movement between the members and can lead to rapid wear and high maintenance costs (Wilkinson, 2008). 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 (Seavey and Larson, 2002). This takes place when iron (from fasteners) interacts with the heartwood. Figure 2 shows the corrosion of the metal support plates and bolts of a timber bridge.

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 (Mahnert and Hundhausen, 2018). Warping can cause not only aesthetic issues but can pull loose connections and fasteners which will decrease the overall structural capacity due to the 2

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ineffective transfer of loads through the defected connections. There are six major types of warp as bow, crook, twist, oval, diamond, 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 (Balendra et al., 2010).

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270 271 272 273 274 275 276 277 278 279 280 281 282 Figure 2. Corrosion of the metal support plates and bolts. 283 (Minnesota Department of Transportation, 2014) 284 285 2.1.4 Ultraviolet Degradation 286 When timber is exposed to UV radiation or sunlight, a degenerative photochemical reaction in the lignin of 287 the timber cells occurs. This reaction only directly affects 288 the aesthetics of the bridge causing the wood surface 289 to 290 become exposed and turn grey in colour. UV radiation is a 291 very slow process with an estimated rate of 63mm per 100 years. Ridout (2010), states that UV radiation affects 292 the 293 aesthetics of the timber but can also allow other 294 deterioration mechanisms to occur through minor cracks. 295 UV degradation, while not causing significant damage 296 besides surface wear, is a form of weathering and can lead to slow delamination of certain timber members such as 297 ply

Figure 3. Paint and timber deteriorated by UV radiation. (RTA, 2008)

2.2 Biological 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 (Moncmanovâa, 2007). 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 environmental conditions, 2) A source of sustenance or food, oxygen (with the exception of anaerobic organisms) and 3) Appropriate temperatures.

2.2.1

Insect Attack

2.2.1.1 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 (Wilkinson, 2008). This form of termite is more commonly found in damp tropical climates. Bakri and Mydin (2014) found that the termite damage occurs in two stages. First, there is some material lose due to the termites eating the timber and secondly, exposure to the weather and decay through the material deterioration.

and glulam timbers. 298 Ultraviolet radiation should be inspected visually. Some of 299 the most noticeable timber deterioration effects come from 300 the action of the ultraviolet portion of sunlight, which 301 chemically damages the lignin close to the surface of 302 timber (Mettem, 2011). Ultraviolet damage normally 303 causes light timber to darken and dark timber to lighten, 304 however this damage merely infiltrates a shallow distance 305 below the surface. The damaged timber is marginally 306 weaker; however the shallow depth of the damage has only 307 a small effect on its strength, except when constant removal 308 of damaged timber ultimately results in section loss. Figure 309 3 shows the deterioration of timber by UV radaiation.

Subterranean termites, as the name suggests, primarily live underground. However, as Wilkinson (2008) and Bakri and Mydin (2014) 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 (Gold, 2005). 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

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decay, the control of termites remains an important 356 At the end of the tunnel the pinhole beetle will lay her eggs consideration (VicRoads, 2018). Figure 4 illustrates 357 the and as she leaves she will often die at the entrance of the deterioration of a timber bridge affected by termite attack. 358 tunnel she has just made to protect her young eggs from 359 predators and to maintain the humid environment for the 360 fungus to germinate.

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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 permeability. 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 (Ritter et al., 2013). The galleries are free of residue and the adjacent timber is darkly blemished. Figure 5 demonstrates an example of borer deterioration.

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Figure 4. Termite deterioration (Vic Roads, 2018)

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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 (Seavey and Larson, 2002). This system is currently being implemented to detect and quantify decay, voids, and termite galleries in timber columns, beams, piles, and poles.

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2.2.1.2 Borers Wood borers are beetles which, at some point, during their 372 short life, use timber as a method of shelter, food or both. 373 Figure 5. Borer deterioration. (Vic Roads, 2018) In Australia there are two main type of borers that affect hardwood timber, powderpost beetle and pinhole borer. For 374 2.2.1.3 Ants the most part borers are not cause for alarm as their damage 375 Ants are insects which often create tunnels and nests in decay is usually minimal. (Creffield, 1996). Hadlington (1996) 376 cavities in timber structures. They deposit sawdust in gallery outlined the characteristics of an infestation of both 377 openings, thereby trapping moisture, the result of this is an powderpost and pinhole borers. 378 increase in the rate of decay of an element (Seavey and 379 Larson, 2002). Powderpost beetles are most commonly found in dry timber. In this method, the infestation is happened when the 380 Insect activity is usually recognised by the existence of female beetle lays its eggs in the exposed sapwood vessels 381 cavities, frass, and powder posting. For timber boring insects of hardwood timber. When the eggs hatch they begin to 382 such as ants, frass is characterised as the combination of insect feast on the starch rich sapwood for their 3-6 week life. 383 feces and hollowed out timber material from wood During their short lives the beetles mate and propagate 384 components where they are active. The presence of insects through the infected timber or structure, and thus 385 might also signify the presence of decay, as ants frequently infestations can last for generations. The powderpost 386 construct tunnels and nests in decay cavities. Ants deposit beetle’s ability of flight enables it to rapidly enlarge its area 387 sawdust in gallery openings, trapping moisture and increasing of contamination. 388 the rate of decay of a timber bridge component. The powder post beetles leave a number of small tunnels 2.2.2 Bacteria behind, filled with powderlike frass. When the larvae389 of 390 Bacteria are a single cell organisms and in wet conditions these beetles tunnel, they push frass out of the timber. This 391 can cause timber to have an increase permeability and cause frass accumulates below the attacked timber and is a 392 the timber surface to soften. Though bacterial decay is a positive indication of powder post infestation. 393 slow process and has the potential to deteriorate Pinhole borers prefer to inhabit moist timber. The way394 in preservatives and allow organisms with a reduced chemical 395 threshold to develop. (the ecology of building material). which they infest the timber is by the female beetle boring through the sapwood and, in some instances, through 396 the Softening of the timber exterior indicates that bacterial attack heartwood as well. During this process the female leaves 397 is a deterioration mechanism which is affecting one or more spores of fungus along the gallery walls which will 398 components in a timber bridge (Ritter, 1990). germinate and become food for her young when they hatch.

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2.2.3 Fungi 4

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A fungus is an organism that breaks down timber for449 a source of sustenance, which propagates through timber 450 via threadlike hyphae that grow through pits or penetrate 451 cell walls. As the fungus infests timber it secretes 452 enzymes that break down Hemicellulose (cell wall 453 constructed of short branched carbohydrates made 454 of different monosaccharides); Cellulose (insoluble 455 substance which is the main constituent of plant cell 456 walls’ constructed of long unbranched fibrils of glucose) 457 and Lignin. 458 The ways in which a fungus spreads along the structure 459 differ according to the species and method 460 of reproduction. There are three classifications of fungi viz: 461 Mould fungi, stain fungi and decay fungi all with differing 462 effects on the structure (Dahlberg, 2015). 463 The variety of fungi organisms can survive and thrive under differing environmental conditions. Soft rot is the most resilient fungi able to tolerate a wide range 464 in 465 humidity, temperature, moisture content and pH levels. 466 The likes of mould and stain fungi, and brown and white 467 rot have limited tolerances to these environmental conditions. 468 While they do not show the quantity or degree of decay, 469 fruiting bodies give an affirmative indication of fungal 470 attack.

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 (Ridout, 2001). 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 (Mahnert and Hundhausen, 2018).

2.2.3.2 Decay Fungi Ritter et al. (2013) notes that decay fungi is generally the main cause of decay in timber bridges, it has three classification’s based upon the way in which it appears and manifests itself in the timber which are: -

471 472 2.2.3.1 Mould and Stain Fungi 473 474 Mould and stain fungi damage occurs with timber with high moisture contents and the damage persists after 475 the 476 wood has dried, however this type of damage is small and 477 insignificant in terms of the timber strength. Stain fungi 478 can occur beneath coatings and eat through them causing problems when trying to seal a timber structure. If 479 the staining penetrates deep into the timber that can not 480 be 481 removed by planning. 482 Moulds can also cause patchy discoloration on the surface 483 of the timber, ranging from green to black to pink. They 484 most commonly occur in timber that has a moisture 485 content greater than the fibre saturation point which486 is

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% to 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 don’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 (Bakri and Mydin, 2014). Decay relies heavily on a combination of factors. It requires suitable temperature, appropriate moisture levels, oxygen and cellulose in timber (Richardson, 2001).

between 28%-32%. The optimum temperature range for mould growth is between 24 to 30 °C. The toughness487 of 488 the wood can be affected by moulds however have little impact on strength. A major problem with mould is that489 it 490 increases the porosity of timber members which in turn 491 opens the door to decay due to moisture deformations. 492 Figure 6 illustrates a pine timber member showing signs 493 of mould and stain fungi.

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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 (Ritter et al., 2013). 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.

499 500 501 502 503 504 505 Figure 6. Pine timber member showing signs of mould and 506 stain fungi. (Southern Pine, 2016)

- Brown Rot Brown rot is a form of decay fungi that is common in timber structures and can cause severe damage. It has an optimal growth temperature of 20º 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 5

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can cause weight losses of up to 70% in the timber 548 element. Due to the fact that brown rot removes 549 the cellulose, which provides strength to the cell, it can cause 550 strength reduction in early stages of decay. Brown 551 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. Figure 7 shows a timber element deteriorated by brown rot fungi.

surface, and single fibers are able to be peeled from the timber (Ritter et al., 2013). This gives a positive indication of white rot. Figure 8 shows timber substructure with evidence of white rot fungi.

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Figure 8. Timber substructure with evidence of white rot fungi. (Minnesota Department of Transportation, 2014)

555 556 557 558 559 560 561 562 563 564 Figure 7. Timber element deteriorated by brown rot fungi. 565 (Roads, 2005) 566 567 Brown rot fungi, as its name suggests, give decayed timber 568 a brownish appearance in colour. In progressive stages, 569 brown decomposed timber is brittle, has a dark colour and 570 has multiple cross checks alike in appearance to the surface of a cracked and severely charred timber (Ritter et 571 al., 572 2013). 573 574 - White Rot In appearance, white rot is a shade of white or tan575 in 576 colour with dark streaks present. White rot is not easily detected in the early stages of development. The way577 in 578 which it propagates is by releasing enzymes that remain

- 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: • 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 (Ritter et al., 2013). Hardwoods are more inclined to suffer from soft rot and that 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 (De Belie et al., 2000). 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.

close to the hyphae, therefore localising infestation. 579 When the rot has become advanced, it is soft in texture 580 and fibres may peel individually from the timber. White 581 rot attacks all three components of the cell wall causing extensive weight losses of up to 97% and thus582 a 583 substantial loss in strength (VicRoads, 2018) . The main 584 environmental factors causing white rot are high humidity or moisture content and appropriate temperatures 585 of 586 around 20 degrees Celsius (Singh, 1999). White rot fungi create decay which bears a resemblance587 to 588 ordinary timber in appearance, however might be whitish 589 or light tan in colour with dark streaks. In the progressive 590 stages of decay, infected timber has a particularly soft

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 (Findlay, 2013). 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 (Rashidi et al., 2016a).

2.3.1 Deck Wear 6

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Timber bridge decking comes in two common forms. 644 First, there is regular sawn decking that ranges from 645 200mm to 250mm wide and 125mm deep and second, 646 LVL or stress Laminated Timber (STL) decking. The SLT 647 system uses thin (35mm to 50mm thick) timber laminates 648 of widths from 140mm up to 290mm. These placed 649 on edge and combined together using high strength bars650 or prestressing strands. This forms a solid slab like structure. 651 (RTA, 2008). In the case of the regular sawn decking 652 the main deterioration issues are damage from abrasion and 653 friction as a result of traffic and debris. This wears away 654 the timber or any coating making the deck vulnerable to both fungal decay and insect infestation which are 655 the most prevalent forms of deck deterioration according656 to the RTA (2008). A specific issue with the SLT system,657 is that a loose tie down can cause the deck to deform, 658 causing damage, due to too much stress. All of these 659 forms also cause section loss in the elements in both 660 decking systems that has a direct impact on the strength 661 and performance of the deck (Mahnert and Hundhausen, 662 2018). Figure 9 shows some forms of deck wear. 663

664 665 666 667 668 669 670 671 672 673 Figure 9. Forms of deck wear. (Minnesota Department of 674 Transportation, 2014)

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 distribution 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 (Porteous, 2013). Any long-term sag will also increase bending in a headstock and therefore, decrease its capacity (Roads, 2005).

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 (Kumar, 2016). 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 (RTA, 2008 ). 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 (Ritter et al., 2013).

616 Timber bridge decks can be inspected by using 675 the 2.3.4 Element Buckling 617 following inspection techniques and equipment: 676 Element buckling is a deformation of the timber element. 677 A force is applied that is too high for the element to carry 618 1. Visual assessment; 678 resulting in the element distorting in a direction that is 619 2. Hammer sounding with pick hammer; 679 perpendicular to that of the force being applied (Mei et al., 620 3. Awl and level edged probes; 621 4. Moisture meters for exterior surface timber 680 with 2007). It has two forms, first Global buckling which is 681 where part or all of the length deforms longitudinally. The 622 assumed high water content; 682 second is where the cross section of the element deforms. 623 5. Stress wave timing assessment; 683 In this case the damage is localised (Bhattacharya, 2005). 624 6. Resistance microdrilling of decayed regions. 684 Buckling can be attributed to many causes depending on 625 685 the situation, they include but are not exclusive to, 626 2.3.2 Deformation 686 overloading, loose bolts or connections and scour and 687 abrasion. 627 Deformation is the altering of the shape or direction of 688 There are a few factors that can cause buckling. The first is 628 the member as a result of a load or loads being applied. 689 loading or overloading of the element, most commonly the 629 The deformation causes the movement in the entire 690 pile. The pile is unable to support the axial load and 630 structure that can result in damage to other elements 691 therefore, transfers the force in the only available direction 631 such as the more rigid surface layer (New South Wales which is lateral (Roads, 2005). Furthermore, the element 632 Government, 2008). There are two main causes 692 of 693 will buckle when rot or steel corrosion affects the pile 633 chord deformation. The first is sagging of the truss, 694 connections as these can either cause the connections to 634 results in an increase in stress on the top chord causing 695 become loose or can cause a loss of section that will reduce 635 in to warp and deform out of shape. The second cause 696 the bracing effectiveness, ultimately resulting in member 636 and perhaps the underlying cause is loading or 697 buckling. Corrosion of the pile itself, or scour, can also 637 overloading of the bridge. Ceccotti (2002) relates 698 cause a buckling and vertical failure (Cheng et al., 2014). 638 loading to long term deformation which can double or 699 Scour is a form of deterioration that is the result of the 639 even quadruple the elastic deformation. Sagging is the 700 flowing water eroding the soil, the material is carried away 640 deformation of an element within the y-axis, where the 701 from around the piers and abutments of bridges increasing 641 element sags down in the middle, and is in itself a 702 the effective length of the element and exposing footings 642 method of deformation. The two main causes of sag are 703 (Balendra et al., 2010). Contraction scour is caused by the 643 span lengths that are too long for the elements capacity 7

704 narrowing of the waterway as it approaches the structure 705 and the accelerated flow that it creates. Local scour is a 706 result of the interference of the piers and abutments to the 707 flow. In all cases, due to the increase in the flow speed 708 and volume of water, the incident of flooding greatly 709 increases the severity of the scour as well as reducing the 710 time in which said scour will occur. 711 All of these factors that attribute to the development of 712 element buckling have been found to timber pile load 713 carrying capacity (Roads, 2005). In severe cases, if 756 the 714 buckling causes total failure of the element 757 the Figure 10. Delamination of plywood. (BRANZ, 2013) 715 deterioration can cause the entire structure to fail. 716 758 The following items are a concise list of the areas for the 759 essential visual assessment of SLT bridges (New South 717 2.3.5 Delamination 760 Wales Government, 2008): 718 Delamination is the process of separation and 761 -The SLT deck must be assessed under traffic loading for 719 deterioration of the layers of certain timber products such 762 excessive deformation or movements; 720 as glue laminated timber, plywood and laminated veneer 763 -Special consideration must be given to potential slip 721 lumber or LVL. It occurs when glued-laminated layers 764 amongst the laminates under substantial loads; 722 separate as the adhesive that bonds the layers fails 765 -Drainage systems must be assessed for obstructions or 723 (Valentin, 2012). It can transpire locally in the case of end 766 debris; 724 grains but can also be a gradual process where layer 767 by -The wearing surface must be assessed for cracks and 725 layer the timber is deteriorated, each time revealing new 768 deterioration; 726 undamaged material that is then subject to deterioration.769 -The waterproofing system, including the edge flashing, 727 The main causes of delamination occurring involve either 770 must be assessed for deterioration and seepage; 728 weathering and overloading. Transit New Zealand (2001) 771 -All regions of exterior surface timber decking must be 729 attribute the process to movement and shrinkage of 772 the assessed for fractures, deterioration and indications of 730 timber. The movement can be a result of the warping from 773 moisture and staining; 731 moisture or the deflection from overloading. The 774 -The wood directly beneath the prestressing arrangement 732 Queensland Government Department of Main Roads 775 must be visually assessed, giving particular attention to: 733 (2004) believes that the timber bridge location and 776 -Deterioration of the anchorage protection system; 734 environment are keys to the deterioration, where tropical 777 -Excessive deformation of the anchorage system; 735 areas and frequent submergence of timber elements are 778 -The deck tie down bolts and deck joint bolts must be 736 often the cause. They also outline weathering and UV assessed for tightness and a minimum of 5%, but greater 737 degradation as factors influencing delamination which779 is 780 than 12, of the tie down bolts must be physically inspected 738 supported by Ridout (2000) who also attributes UV 781 for tightness. 739 degradation to the deterioration mechanism, stating that it 782 740 does not cause significant damage besides surface wear 783 2.3.6 Fractures 741 but can lead to slow delamination of certain timber 742 members such as ply and glulam timbers. 784 Fractures are cracks in timber as a result of beams being 743 The effect that delamination has on the timber element under flexural loading. The fractures are influenced by 744 and structure as a whole was put forward in a paper 785 by 786 various mechanical properties and loading conditions of 745 Valentin (2012), where he says that delamination provides 787 the timber element such as knots present within the 746 openings for decay to begin as moisture can penetrate and 788 element as well as the grain of the timber and the loading 747 be trapped between layers creating a humid environment, 789 in relation to that grain, whether it be parallel or 748 perfect for fungi and insects. He also attributes a reduction 790 perpendicular (Alam, 2009). Elements around shear plates 749 in strength of the element as the loads cannot be 791 and keys are subject to high amounts of bearing stress and 750 effectively transferred through the damaged element. 792 shear forces when loads are applied, which can result in 751 Feeler gages and awls should be utilised to measure the 793 fractures in the timber around the plates and keys (New 752 degree of delamination (Seavey and Larson, 2002). Figure 794 South Wales Government, 2008). Loading is not however 753 10 demonstrates and shows an example of delamination of 795 the only cause of fractures. Pipinato (2015) attributes 754 plywood. 796 moisture to producing fractures. The constant changes in 755 797 volume throughout the entire member, as a result of 798 799 800 801 802 803 804 805 806

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. (Alam, 2009). Figure 11 shows some longitudinal fractures on a timber bridge. 8

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Figure 11. Longitudinal fractures (Minnesota Department 861 of Transportation, 2014)

810 2.3.7 Loose Connections 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857

photo) and severe through split (right photo) in some timber pieces.

862 863

Connections are an area of bridges that are subject864 to 865 many forms of deterioration as a result of cut ends, fixture material removal and moisture retention. These 866 all contribute to the weakening of the connections. Valentin 867 (2012) states vehicle traffic loads crossing the bridge 868 along with weathering crush the wood around 869 the fasteners due to the repetitive impact. The loading wears 870 on the connection (fasteners and their holes) causing them to loosen. Loading, vibration and weathering are the main 871 factors causing the loosening. The effect this has on 872 the structure is a reduction in the bridges load carrying 873 capacity while also severely reducing the structural 874 stability. From Yu et al. (2014), a direct relationship 875 between ineffective fasteners and connections and 876 an increase in the peak displacement of the timber member is 877 evident, which could then result in partial or complete 878 failure of the element or structure. 879 A good indication of loose connections is when primary 880 timber bridge members (for example primary deck or slab 881 members) are out of alignment and are not operating as 882 intended (Zealand, 2001).

Figure 12. Left: Small end checks. Right: Severe through split. (Minnesota Department of Transportation, 2014) 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 (Seavey and Larson, 2002). Feeler gages and awls should be implemented to measure the extent of splits.

2

Inspection, Condition Assessment and Remediation Planning

The majority of state bridge authorities use three levels of bridge inspection procedures. These are: Level 1 – Routine maintenance inspection; 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 tables 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 inspectors 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 elements (either tension or compression), shear forces and bending moments at points

883 884 Not all timber deterioration mechanisms are the result of a 885 third party, some are naturally occurring defects that come 886 about through the growing of the tree. Knots, checks and 887 splits are the main three natural element defects. 888 Knots are defects that arise when a piece of branch889 or limb that was growing on the tree has been incorporated 890 into the timber element that has been milled, they are891 a natural product of growth. Balendra et al. (2010) attributes 892 a reduction in strength and load carrying capacity to 893 the presence of knots while Tingley (2014) says that894 a reduction in mechanical properties results from knots 895 reducing the effective cross section while causing 896 localised sloping of grain. Splits and checks are similar897 in both their cause and effect on the structure. Mettem 898 (2011), outlines checks are a separation of wood occurring 899 perpendicular to the cross sectional grain or growth rings 900 and splits are a separation of wood from one surface901 to another, usually parallel to the grain. Both of which are a 902 results of the differential shrinkage during drying 903 or seasoning. The outcome of the two deterioration 904 mechanisms is a reduction in strength and load carrying 905 capacity as forces cannot effectively be transferred 906 through the members and structure while also opening 907 the timber to further weathering and deterioration (Valentin, 908

2.4 Natural Element Defects

2012). Figure 12 shows some small end checks (left

9

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927 928 929 930 931 932 933 934 935

along the bridge. 936 937 Inspecting for deterioration mechanisms is an important 938 aspect regarding the lifecycle management of timber 939 bridges. Whether it is visual inspection or inspecting bridge components with equipment or apparatus, it940 is essential to know what type of deterioration mechanism941 is affecting the bridge in order to know which kind 942 of preventative or remedial methods should be utilised. 943 944 Visual inspection of bridge elements is usually undertaken 945 when deterioration mechanisms are affecting the exterior 946 surface of the bridge. This commonly can be the first step 947 of inspection, followed by the use of equipment and measuring devices to measure the extent of 948 the 949 deterioration. While visual inspection can be much less 950 accurate with relation to indicating the correct deterioration mechanism, it is an adequate inspection 951 procedure for a large number of mechanisms. 952 953 Table1. Element Ratings (RTA, 2008 ) 954 Condition Description 955 State The timber is in good condition with no 956 evidence of decay. There may be 957 cracks, splits and checks having no 958 1 959 effect on strength or serviceability. All connections are in good condition 960 961 and bolts are tight. Minor decay, insect infestation, 962 splitting, cracking, checking or 963 crushing may exist but none is 964 2 sufficiently advanced to affect 965 966 serviceability. Joint connections may be slightly loose 967 968 but does not affect the serviceability. Medium decay, insect infestation, 969 970 splitting, cracking or crushing has 971 produced loss of 3 972 strength of the element but not of a 973 sufficient magnitude to affect the 974 serviceability of the bridge. 975 Joint connections may be slightly loose 976 but the serviceability of the bridge is 977 not 978 significantly affected. 979 Advanced deterioration. Heavy decay, 980 insect infestation, splits, cracks or 981 crushing has produced loss of strength 982 4 that affects the serviceability of the 983 bridge. 984 Connections are very loose causing 985 large movements, bolts are corroded and ineffective or missing and the serviceability of the bridge is affected. Inspection equipment is used to detect, measure, assess and quantify deterioration mechanisms in timber bridges. Equipment has been found 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 986 timber bridges need remediation, condition ratings 987 are

implemented. Timber bridge elements are given a condition state from one to four, as shown in Error! Reference source not found. 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, squared meters 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 (Rashidi and Gibson, 2012) 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 deteriorated beyond repair and has become structurally incompetent or outdated. It is often done to increase the load carrying capacity to cope with the demands of modern traffic conditions. Rashidi et al. (2018b) 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 struggling 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. Figure 13 demonstrates a decision tree including the major remedial strategies.

Do Nothing Downgrade Remediation Strategies

Preventative Maintenance

Minor Rehabilitation

Rehabilitation Major Rehabilitation Replacement

Figure 13. Remediation decision tree (Rashidi et al., 2016b) 10

988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050

1051 1052 1053 Generally the service life of a bridge can be sub-divided into 1054 four different phases (ARRB 2000; Rashidi and Gibson 2012): 1055 Phase A – design and construction; 1056 Phase B –deterioration has not yet started but initiation 1057 processes are underway; 1058 Phase C – damage propagation has just begun; 1059 Phase D – extensive damage is occurring. 1060 According to the Law of Fives, 1 dollar spent in Phase1061 A equals 5 dollars spent in Phase B; 25 dollars in Phase 1062 C and 125 dollars in Phase D, implying this law is the 1063 corner stone of any asset management decision-making. 1064 From this rule it can be deduced that implementing 1065 preventive maintenance must become of paramount 1066 importance to avoid further deterioration and achieve 1067 structural longevity and long term economic benefit. 1068 1069 3.1 Asphalt / Bitumen 1070 Asphalt is a dark viscous liquid which is created as a by1071 product of distilling petroleum; though it can also be a 1072 naturally occurring product as the prelude to petroleum. 1073 This primordial sludge is able to preserve fossils for 1074 palaeontologist and the decayed remanence of prehistoric 1075 organisms can act as a wonderful hydrophobic membrane 1076 for timber. Coating elements of the bridge in asphalt, 1077 namely the deck and penetrations, provide a physical 1078 barrier between the timber elements and wear form traffic 1079 or the elements (Roads, 2005). Furthermore, this physical 1080 barrier insulates timber from changing moisture content 1081 and subsequent conditions which are conducive to 1082 biological decay such as cycling dimensional loading 1083 which can cause splits, checks and warping in elements. 1084 1085 3.2 Heat Treated Timber 1086 Heat treating timber is by no means a new method of 1087 wood modification, though it is also far from being 1088 antiquated. In 1920 heat treated timber was shown to be 1089 effective for dimensional stabile of timber and subsequent 1090 research in the prevailing decades has also furthered this 1091 observation. Though the specifics of heat treatment for 1092 heat treatment differs from manufacture to manufacture, 1093 the method and principles employed are the same (Esteves 1094 and Pereira, 2009). Thermowood, retailer of heat treated 1095 timber in Europe, has the following methodology: 1096 1) The lumber must be placed in a humid atmosphere 1097 for 2-10 hours at temperatures exceeding 150ºC to 1098 obtain a mass loss of 3%. 1099 o 2) A vapour with a treatment at 100 C is then applied 1100 and the oven temperature is slowly increased to 1101 o 130 C with almost no humidity. 1102 3) The temperature is then raised again to 185-230oC 1103 for two to three hours to complete the treatment. 1104 The above process has profound effects on timber viz. cell 1105 and molecular changes, increased durability, dimension 1106 stability, mass loss, and altered mechanical properties. 1107 The main effect of heat treatment is the reduction of1108 the moisture equilibrium with a subsequent stability in 1109 shrinking and swelling. The degree to which 1110 the equilibrium is adjusted depends on several factors such as, 1111 species of timber, temperature of treatment and duration. 1112 The main reason for decrease in the moisture equilibrium 1113

3. Prevention Strategies

11

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 treatment 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 (Yildiz et al., 2006). 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 microfibrils are unable to expand with absorbed water. Heat treated timber also exhibits resistance to rot fungi 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 (Esteves and Pereira, 2009). 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 (Yildiz et al., 2006).

3.3 Flashings A flashing according to the Penguin Civil Engineering Dictionary, is “a strip used to seal a junction between two surfaces to exclude rainwater”. 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 the element, water will become stagnant and soaking into the element creating suitable conditions 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 (Mettem, 2011).

3.4 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 precludes incurrence or further development, of splits and checks (Pipinato, 2016). If paint

1114 1115 1116 1117 1118

is improperly applied or is in need of maintenance it1175 can be detrimental to the structure as it allows a method 1176 of ingress for insects and moisture, the painted surface 1177 then provides shelter from the sun and will decay from1178 the inside out (New South Wales Government, 2008). 1179

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3.5 Design

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3.5.1 Timber Species and Section Size

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 remanence of the nurturance transport system of the once living tree stays on. Hence, end grain should be one of the first items to be considered to prevent moisture intrusion into timber elements; this is often carried out with the 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 (Mettem, 2011). Figure 14 presents an eight years old footbridge detailed for durability.

1180 1181 The design of a timber bridge will extend the service1182 life of a timber bridge just as much as paints, treated timber 1183 and regular maintenance. A properly designed and 1184 well maintained timber bridge will able to last a minimum of 1185 70 years. There are several considerations which must be 1186 taken into account when a bridge is being designed or 1187 upgraded, which are: 1188 -Timber species and section size; 1189 -Design detailing. 1190 The most important part of timber design is to direct 1191 water and moisture away from the structure, thus much1192 of the design detailing is to about ensuring there are minimal moisture traps.

The durability of timber varies with the species of timber, as a general 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. The standard gives a 1193 guide to the expected life of untreated timber to in ground 1194 and above ground use and a resistance rating against 1195 hazards of termites and lyctid attack. Hence it can be determined that if more durable timber is 1196 implemented throughout the structure, it will fare better 1197 against deterioration. AS5604 is only able to provide a guide to the expected durability of timber, as timber1198 is a 1199 biotic material there are variations from tree to tree caused 1200 by conditions during growth. Further, the environment in which the timber is situated will affect the rate1201 or 1202 sustainabilyty of decay; such an example is that of marine 1203 borer attack, in AS5604 durability is based on samples in 1204 southern waters which are not as hazardous to attack as 1205 northern waters. The natural durability of timber is owed to extractives formed when sapwood metamorphoses1206 into 1207 heartwood (AS5604). When this transformation occurs tannins and other substances are contained within1208 the parenchyma cells, which often become toxic and 1209 have reduced porosity (De Belie et al., 2000). Incidentally,1210 the reduced porosity increases the timbers resistance1211 to shinkring and swelling, and thus decay relating1212 to 1213 dimensional stability. These extracts are also responsible 1214 for the darker hue that can be observed in durable timber, 1215 though this is not always the case. Hence, it can be noted 1216 that heartwood is responsible for the primary durability of 1217 each timber species, and should be used during 1218 construction as much as possible. 1219 A timber bridge can be designed with inevitable deterioration in mind. If the timber members 1220 are oversized, it will increase the initial load capacity of1221 the bridge. However, as the members start to deteriorate 1222 they 1223 will approach the designed maximum load limit.

Figure 14. An eight years old footbridge detailed for durability (RTA, 2008)

3.6 Acetylated Timber Acetylation is a timber modification process whereby the sub-straight of the timber is altered to provide the desired durable and 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, hemicellulose 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. Additionally, these acetyl groups are considerably larger and heavier than their acetic anhydride hydroxyl counterparts (Yu et al., 2014). The enlargement of molecules within the timber polymer causes the treated timber to be in permanently swollen state, thus increasing its dimensions, and increased mass; the degree to which mass gain is measured is 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

1224 1225 3.5.2 Design Detailing 1226 Design detailing is mainly about drainage which keeps the 1227 timber dry and prevents a myriad of deterioration.

12

1228 1229 1230 1231 1232

subject to internal stress that occur from swelling 1289 and shrinking that cause splits, checks, cracking and warping, 1290 nor does it convey stresses onto external coatings, such as 1291 paint, causing them to crack, therefore imposing the need 1292 of resurfacing (Balendra et al., 2010).

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environment and must be implemented in accordance with 1299 AS3660. 1300 Stainless steel meshing and finely divided granite barriers are the two ubiquitous physical barriers which 1301 are employed to prevent the manifestation of termites1302 in 1303 timber structures. The ideology behind both methods differs, however, both create a barrier which termites 1304 find 1305 difficult to traverse. Stainless steel meshing is simply a 1306 mesh which is so fine that termites are unable to penetrate 1307 (Hadlington, 1996); in Australia the maximum aperture sized used is 0.66 x 0.45mm (with the exception1308 of 1309 northern Australia where 0.4x0.4 mm is used). The mesh 1310 must be made out of stainless steel to prevent corrosion from the soil and environmental conditions in the soil.1311 For the most part the mesh is usually placed around1312 the 1313 footing of the structure. 1314 A granite barrier is a layer of graded basalt upon which the footing rests. The theory behind this barrier is that1315 the particles are larger and heavier than the termites are 1316 able to move; therefore, preventing them from entering1317 the structure. Like many inert barriers they are able to1318 be 1319 circumvented and the inspections must be paramount 1320 (Gerozisis et al., 2008).

1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288

they all intend to achieve the following: • 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. In general, soft wood timbers absorb treatments better than hardwoods.

1293 1294 3.7 Physical Barriers With the exception of treated timber, there are usually1295 two 1296 other ways in which termites are prevented, physical barriers and chemical treatment of foundations. However, 1297 the latter has the potential to have adverse effects on1298 the

3.9.1 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 (De Belie et al., 2000), and contains over 300 different substances, unfortunately, 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 (De Belie et al., 2000). 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 .

3.9.2 Fixed and Non- Fixed Water Soluble Preservatives

1321 1322 Treated soils are intended to either deter or adversely 1323 affect the termites that pass through the impregnated area, 1324 depending on which chemical is utilised. Traditionally 1325 chlorinated hydrocarbon insecticides have been used to 1326 treat soils for termites, which have an efficacy of 30-40 1327 years (in undisturbed conditions i.e under slabs). 1328 However, during the 1980s many of these were banned as 1329 they had considerable impacts on the environment, and 1330 were replaced with other commercially available 1331 chemicals; with services lives of 5-10 years. 1332 For treated soils to be more effective they must be treated 1333 before construction (Hadlington, 1996) to ensure that the 1334 perimeter of the in ground element has been properly 1335 covered. Chemical barriers are prone to failure as 1336 conditions around the element change and cause leaching 1337 soil redistribution and allow termite impregnation 1338 (Gerozisis et al., 2008). 1339 1340 3.9 Chemical Preservatives There are two methods of application for chemical 1341 preservatives, these are pressurised and non-pressurised; 1342 both methods, however, require the timber to be seasoned 1343 so that the majority of water is removed from the timber 1344 cells. With a variety of chemical preservatives available 1345 for a myriad of potential maladies, there are three 1346 categories into which they can be classified: Oil Based, 1347 Water Soluble, Organic Solvents. Though the method of 1348 application and makeup of the preservative may change, 1349 3.8 Treated Soils

There are two forms of water soluble preservatives, fixed and non-fixed (De Belie et al., 2000). Fixed soluble preservatives are those which usually contain arsenic, copper and chromium salts, while the most common nonfixed water soluble preservatives are boron compounds while boric acid, sodium 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 treatment to prevent fungus and decay. The ratio of the components is generally as follows (Beder, 2003): -Copper 23-25%; -Chrome 38-45%; -Arsenic 30-37%. Copper cations preserve the cellulose while chromium anions preserve 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 effective and durable method of preserving timber it has been found to leach out over time (Mettem, 2011). The rate at which leaching occurs is dependent on many factors 13

1350 such as timber age, acidity of rain and or soil, original 1411 1351 amount of CCA applied (Beder, 2003). 1412 A 1352 recommendation from the review by the Australian 1413 1353 Pesticides & Veterinary Medicines Authority (APVMA) 1414 1354 in 2005 said that CCA treated timber should not be 1415 used 1355 for handrails or other areas which are in common contact 1416 1356 with humans. 1417 1418 1357 3.9.3 Organic Solvents 1419 1358 Organic solvents consist of active chemicals, generally 1420 1359 less than 10%, which have been dissolved in an organic 1421 1360 solvent like that of petroleum distillate. The common 1361 active preserving agents in organic solvents 1422 are: 1423 1362 Pentachlorophenol, Lindane, Dieldrin, Tributyl tin oxide 1424 1363 (TBTO), Copper 8-quinolinolate and Copper napthenate. 1364 Each of these preservatives have varying effects in 1425 their 1426 1365 preservative ability, they have been known to leach out of 1427 1366 the timber over time. Usually being highly viscous, these 1428 1367 preservative treatments are applied using through 1429 1368 brushing, spraying or immersion. 1430 1369 3.10 Load Testing 1431 1370 For older timber bridges overloading is a highly probable 1432 1371 and serious deterioration mechanism. Before overloading 1433 1372 causes structural failure, it can invite a wide variety of 1434 1373 other decay agents to manifest themselves. The simplest 1435 1374 way to ensure that timber bridges are not being 1436 1375 overloaded is to perform a level three engineering 1437 1376 inspection to determine the safe loading capacity of1438 the 1377 bridge. It is recommended that Dynamic Frequency 1439 1378 Analysis (DFA) or hammer testing is utilised rather 1440 than 1379 the traditional load test. This is due to the fact 1441 that 1380 ultrasonic testing is able to determine load capacities 1442 1381 without further stressing overloaded members 1443 and 1382 connections (Rashidi et al., 2017). 1444 1383 4. Remedial Options 1445 1384 The main goal of any remediation strategy is to provide a 1385 sufficient level of reliability with a bridge network at1446 the 1447 1386 lowest cost to life-cycle maintenance. The different 1387 remediation work can not only extend the life span of1448 the 1449 1388 bridge but can also improve the quality and reliability of 1389 the bridge as it ages and increases the safety of1450 the 1451 1390 structures for the public (Rashidi et al., 2010). 1452 1391 4.1 Tightening of Bolts and Other Fixings 1453 1392 When it comes to loose ineffective connections 1454 and 1455 1393 fasteners, the most appropriate remedial strategy is to 1394 simply retighten the bolts back to the specified torque. 1456 1395 (Ryall, 2001). This has the benefit of not only stopping 1457 1396 overstressing on the connection but also minimises water 1458 1397 penetration into the timber member (New South Wales 1459 1398 Government, 2008). Transit New Zealand (2001) not 1460 only 1399 supports tightening but also the sealing of the bolts1461 that 1400 provides an increase in waterproofing. This is vital1462 as 1401 many of the holes used for the connections extend through 1463 1402 the entire length of the timber element and the moisture 1464 1403 ingress could cause internal corrosion which is difficult to 1465 1404 detect. 1466 1467 1405 4.2 Removal, Repair and Resealing 1468 1406 The method of removal of the deteriorated section of an 1469 1407 element varies depending upon the element and1470 the 1408 deterioration mechanism. For metal fasteners that are1471 not 1409 so severely corroded as to be needing replacement, 1472 1410 removal of rust should be carried out using a wire brush

and rust remover if needed, then painted to prevent further corrosion (Mettem, 2011). A similar process should be applied for mould and stain fungi removal. First spot cleaning and scrapping is needed to remove the defect before the application 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 60cm of surrounding timber in the direction of the grain (Clark, 1977). To repair the element once the affected area is removed, an epoxy polymer should be applied to fill any holes left from debris removal (Dahlberg, 2015). 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 deteriorating the element (Roads, 2005). 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 (New South Wales Government, 2008). 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 the creation of a physical barrier from the weathering mechanisms. Both an undercoat and top coats can 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 (Mettem, 2011).

4.3 Replacement of Deteriorated Elements The 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 components, Mettem (2011) states that metallic elements should be replaced with materials which are resistant to corrosion like galvanised or stainless steel. Ryall (2001), the Queensland Government Department of Main Roads (2005) and Transit New Zealand (2001) 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. (Singh, 1999). The criteria for replacement of the top or bottom chord is when the chord is outside half of its own width from center line (New South Wales Government, 2008). 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 (New South Wales Government, 2008). Timber used as the replacement should be either a preservative treated softwood or a naturally durable hardwood to ensure that deterioration mechanisms do not affect the new element (Zealand, 2001). There are many reasons why replacement might be the most appropriate remedial 14

1473 1474 1475 1476

strategy. However they all have the same goal, which 1534 is to either restore the structure to its original load carrying 1535 capacity or to upgrade the structure to support 1536 new demands. 1537

1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492

4.4 Insect Remediation

1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533

deterioration mechanisms from overloading, like delamination, deformation and localised crushing (Roads, 2005). 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 (Dahlberg, 2015). 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 longitudinal 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 (Roads, 2005). 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 (Seavey and Larson, 2002). Finally, metal shims, which are effective packers or spacers, are used to elevate the decking in line where headstock sag is less than 50mm (Roads, 2005).

1538 1539 The main procedure to deal with termite attack on1540 the surface or in the pipe comes with two scenarios. The 1541 first is if the affected area of the timber member is more 1542 than 35%, the timber should be removed and replaced with a 1543 new treated piece. The second scenario is when1544 the termite attack has damaged less than 35%, in this case,1545 the remediation method is to treat the area and surrounding 1546 members with a termicide to eradicate the termite 1547 infestation. 1548 Borer eradication, revolves around marine borers and it 1549 outlines its main strategy in the eradication of them to 1550 reduce the oxygen content of the water surrounding1551 the piles. This relies on the fact that the damage done has1552 not sufficiently affect the strength of the timber piles or piers 1553 (Balendra et al., 2010). 1554 1555 4.5 Bracing and Sister Members 1556 Sometimes it isn’t feasible to replace deteriorated 1557 elements due to their position in the structure. This is 1558 when the use of bracing or sister members is used to reinforce the structure (Zealand, 2001). Bracing assists in 1559 remedying many deterioration mechanisms, often 1560 associated with overloading, one of which is deformation. 1561 Strengthening of the element can be achieved by bracing 1562 the member along the length, while bracing can also be 1563 used to assist in relieving permanent forces on cross 1564 members, chords and piles. Seavey and Larson (2002) 1565 explain how bracing can be used to remedy buckling 1566 as it reduces the chance of further deterioration while 1567 increasing the strength and capacity closer to its original 1568 state. 1569 Another remedial method for strengthening deteriorated 1570 structures is the use of sister members. A sister member is 1571 a member that is able to support the loans that were 1572 applied to the previous member and is placed next to1573 the existing deteriorated element. The sister member can 1574 also increase the capacity to remedy deterioration such as sag 1575 in an element. Queensland Government Department of 1576 Main Roads (2005) also points out that the supplementary 1577 member can be used to remedy strength loss from overloading and the splitting that it causes (Seavey1578 and 1579 Larson, 2002).

4.7 Fumigants Fumigants, like vapam and chloropicrin, are a type of chemical preservative that come in the form of either a liquid or a solid. They are placed into predrilled holes in order to stop any internal decay and insects (Zealand, 2001). 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 meters, from the point of origin, in the direction of the grain and can remain effective from between 10 to 15 years. Fungicides 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 (Zealand, 2001).

4.8 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 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 (Balendra et al., 2010). 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 (3668%), 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 (Buell and Saadatmanesh, 2005). As for fiber reinforced polymer rods, they are pre-made spikes of FRP and are inserted into the affected member

1580 4.6 Member Augmentation and Mechanical 1581 Repair 1582 The process of member augmentation and mechanical 1583 repair is centered around the use of extra material, such as 1584 timber, steel, concrete and metal fasteners, in order to 1585 reinforce and strengthen the deteriorated elements which 1586 all aim to increase the effective section and therefore 1587 load capacity. 1588 The use of metal or timber plates as reinforcement is 1589 known as splicing or scabbing (Mettem, 2011). The plates 1590 are placed on either side of the deteriorated area 1591 and bolted or screwed together increasing the effective 1592 area around the localised damage (New South Wales 1593 Government, 2008). This method is also known1594 as clamping and stitching and is used in the remediation of 1595 15

1596 1597 1598 1599 1600 1601 1602

longitudinally throughout, they are then covered 1658 with epoxy to seal the hole from moisture and other 1659 deterioration mechanisms (Rashidi, 2014). The FRP 1660 rods increase the stiffness, strain and ultimate flexural strength 1661 of the element and are used to prevent any increase 1662 of fractures and splits and to restore the load capacity of1663 the effected element (Burgers et al., 2005). 1664

1603 1604 1605 1606 1607 1608 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622 1623 1624 1625 1626 1627 1628 1629 1630 1631 1632 1633 1634 1635 1636 1637 1638 1639 1640 1641 1642 1643 1644 1645 1646 1647 1648 1649 1650 1651 1652 1653 1654 1655 1656 1657

4.9 Composite Structures

1665 1666 Composite timber structures describe any structure 1667 that uses timber in combination with another material, whether 1668 that be concrete, steel or modified wood products 1669 (Zealand, 2001). An example of this is hybrid composite1670 timber that is created using laminated veneer lumber 1671 (LVL) layers with a reinforced core material added between layers giving this form the characteristics1672 of 1673 exceptional hardwood (Balendra et al., 2010). The most 1674 common, and effective, form of composite timber 1675 structure is the composite concrete-timber. This is where a reinforced concrete slab is poured over the top of1676 an existing timber bridge. It is bonded using non-slip joints 1677 and gluing steel connectors (shear spikes) into the timber 1678 (Balendra et al., 2010, Ceccotti, 2002). The combination 1679 of concrete and engineered wood products can improve 1680 the load capacity by 3 fold. (Makippuro et al. 1996). 1681 This increase makes timber composite structures a great 1682 remedial option for deterioration from overloading like 1683 fractures, buckling, crushing and other deformations. The 1684 only thing to consider is the additional dead load that the 1685 concrete will incur (Rashidi et al., 2018a). 1686 5. Taxonomy 1687 All the information that has been collected throughout the 1688 research and presented above, has been collated into a taxonomy table which can be seen in Appendix A. 1689 The 1690 taxonomy has been classified based on the main 1691 deterioration mechanisms as below: 1692 -Weathering; 1693 -Biological; 1694 -Mechanical Wear; 1695 -Natural Defects. 1696 Contained within each of these categories is an inventory 1697 of deterioration mechanisms. Each of the deterioration 1698 mechanisms is accompanied with a description which 1699 details the method in which the mechanism deteriorates the structure. From this description, the type1700 of deterioration can be identified, and the reason as to 1701 why deterioration has occurred. Once the root cause of1702 the deterioration mechanism has been identified, 1703 an appropriate preventive measure can be taken. For 1704 each deterioration mechanism, there are corresponding 1705 remediation and preventative measures which 1706 are specialised to particular forms of deterioration. Thus from 1707 the taxonomy, not only the method of deterioration and 1708 root cause can be determined, a series of rectification and 1709 preventive measures can also be identified. 1710 6. Conclusion 1711 Timber bridges play a critical role specially in rural 1712 transportation networks, and any disruption in their 1713 function may lead to huge economic losses. Globally, 1714 aging bridges are becoming troublesome to maintain, particularly for timber bridges that still form part1715 of 1716 transportation networks. These remaining timber bridges 1717 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.

References ARRB. 2000. Local Roads Bridge Manual. Melbourne: Transport Research Ltd. AUSTRALIAN STANDARDS. (2005). AS5604 Timber Natural Durability Rating. Standard. Australia. ALAM, P. A. 2009. Mechanical repair of timber beams fractured in flexure using bonded-in reinforcements Composites Part B: Engineering, 40, 95-106. BAKRI, N. N. O. & MYDIN, M. A. O. 2014. General Building Defects: Causes, Symptoms and Remedial Work. European Journal of Technology and Design, 3, 4-17. BALENDRA, T., WILSON, J. L. & GAD, E. F. 2010. Review of Condition Assessment and Retrofitting Techniques for Timber Bridge Assets in Australia. Advances in Structural Engineering, 13, 171-180. BEDER, S. 2003. Timber Leachates Prompt Preservative Review. Engineers Australia, 75, 32-34. BHATTACHARYA, S. C. 2005. Frontiers in offshore geotechnics : ISFOG 2005 / [edited by] Susan Gourvenec & Mark Cassidy. , London : Taylor & Francis, c2005. , International Symposium on Frontiers in Offshore Geotechnics (1st : 2005 : University of Western Australia) BRANZ. (2013). Plywood cladding - delamination. Retrieved from BRANZ Maintaining My Home: http://www.maintainingmyhome.org.nz/issues-andrepairs/issue/plywoodcladding-delamination/ BUELL, T. & SAADATMANESH, H. 2005. Strengthening Timber Bridge Beams Using Carbon Fiber. Journal of Structural Engineering (New York, N.Y.), 131, 173-187. BURGERS, T., GUTKOWSKI, R., RADFORD, D. & BALOGH, J. 2005. Composite Repair of FullScale Timber Bridge Chord Members through the Process of Shear Spiking. CECCOTTI, A. 2002. Composite concrete-timber structures. Progress in Structural Engineering and Materials, 4, 264-275. CHENG, L., JIE, H., BENNETT, C. & PARSONS, R. L. 2014. Behavior of laterally loaded piles under scour conditions considering the stress history of undrained soft clay.(Report)(Author abstract). Journal of Geotechnical and Geoenvironmental Engineering, 140. 16

1718 1719 1720 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 1742 1743 1744 1745 1746 1747 1748 1749 1750 1751 1752 1753 1754 1755 1756 1757 1758 1759 1760 1761 1762 1763 1764 1765 1766 1767 1768 1769 1770 1771 1772 1773 1774 1775 1776

CLARK, J. W. 1977. Decay in Wood Bridges: 1777 Inspection and Preventative and Remedial1778 Maintenance, Madison: : U.S. Department1779 of Agriculture, Forest Products Laboratory 1780 CREFFIELD, J. W. 1996. Wood destroying insects1781 : wood borers and termites, East Melbourne, 1782 Vic., East Melbourne, Vic. : CSIRO. 1783 DACKERMANN, U., LI, J. & SAMALI, B. 2009.1784 Damage Identification in Timber Bridges 1785 Utilising the Damage Index Method and Neural 1786 Network Ensembles. Australian Journal of1787 Structural Engineering, 9, 181-194. 1788 DAHLBERG, J., PHARES, B, & KLAIBER, W 2015. 1789 Development of Cost-Effective Timber Bridge 1790 Repair 1791 Techniques for Minnesota. Bridge Engineering Center 1792 and National Center for Wood 1793 Transportation Structures, Iowa State University 1794 DE BELIE, N., RICHARDSON, M., BRAAM, C.1795 R., SVENNERSTEDT, B., LENEHAN, J. J. & 1796 SONCK, B. 2000. Durability of Building 1797 Materials and Components in the Agricultural 1798 Environment: Part I, The agricultural 1799 environment and timber structures. Journal of 1800 Agricultural Engineering Research, 75, 2251801 241. 1802 ESTEVES, B. & PEREIRA, H. 2009. Wood 1803 modification by heat treatment: A review 1804 BioResources, 4, 370-404. 1805 FINDLAY, W. P. K. 2013. Timber Pests and Diseases: 1806 Pergamon Series of Monographs on Furniture 1807 and Timber, Pergamon. 1808 GEROZISIS, J., HADLINGTON, P. & STAUNTON, 1809I. 2008. Urban pest management in Australia, 1810 Sydney, N.S.W. : University of New South1811 Wales Press Ltd, 2008. 1812 GOLD, R. E. 2005. Subterranean termites. Texas 1813 FARMER Collection, 1-10. 1814 HADLINGTON, P. W. 1996. Australian termites :1815 and other common timber pests, Kensington, 1816 N.S.W., Kensington, N.S.W. : UNSW Press. 1817 INGALL, J. 2008. Unexpected bridge collapse. 1818 Australian 1819 Broadcasting Corporation, September 5, 2016. 1820 KUMAR, P. 2016. Use of Timber as a Construction 1821 Material. Academia. 1822 MAHNERT, K.-C. & HUNDHAUSEN, U. 2018. 1823 A review on the protection of timber bridges.1824 Wood Material Science & Engineering, 13, 1825 152-158. 1826 MEI, H., HUANG, R., CHUNG, J. Y., STAFFORD, C. 1827 M. & YU, H.-H. 2007. Buckling modes of1828 elastic thin films on elastic substrates. Appl. 1829 Phys. Lett., 90. 1830 METTEM, C. J. 2011. Timber Bridges, Abingdon,1831 OX: Spon Press. 1832 MINNESOTA DEPARTMENT OF 1833 TRANSPORTATION, 2014. Advanced Timber 1834 Bridge Inspection Field Manual for Inspection 1835

of Minnesota Timber Bridges MONCMANOVÂA, A. 2007. Environmental deterioration of materials, Southampton, Southampton : WIT Press. MOORE, J. C, GLENNCROSS-GRANT, R, MAHINI, S, and PATTERSON, R. 2011. Towards Predictability of Bridge Health. Sustaining Our Regions: The Engineering Challenge: Proceedings of the 2011 Regional Convention, Newcastle Division, Engineers AustraliaHeld at University of New England, Armidale, NSW 16th -18th September 2011: 103-110. NEW SOUTH WALES GOVERNMENT, T. N., ROADS AND MARITIME SERVICES (RMS) 2008. Timber Bridge Manual, Section Eight, Preservative and Protective Treatments. Timber Bridge Manual. 1 ed.: Roads and Maritime Services. NORTH, M. 2012. Spreading the Load: The Management of Heritage Timber Truss Bridges in the NSW Road Network. Australian Journal of Multi-disciplinary Engineering, 9, 79-85. PIPINATO, A. A. 2016. Innovative bridge design handbook : construction, rehabilitation and maintenance, Waltham, MA : Elsevier, 2016. PORTEOUS, J. A. 2013. Structural Timber Design to Eurocode 5. Oxford. Rashidi, M. (2014), 'Finite element modeling of FRP wrapped high strength concrete reinforced with axial and helical reinforcement', International Journal of Emerging Technology and Advanced Engineering, vol 4, no 9 , pp 728 - 735. RASHIDI, M., ANCICH, E., GHODRAT, M. & BUCKLEY, P. S. Review of the most common repair techniques for reinforced concrete structures in coastal areas. IABSE Conference: Engineering the Developing World, 2018a Kuala Lumpur, Malaysia. 370-377. RASHIDI, M. & GIBSON, P. 2012. A Methodology for Bridge Condition Evaluation. Journal of Civil Engineering and Architecture, 6, 1149–1157. RASHIDI, M., LEMASS, B. & GIBSON, P. A Decision Support System for Concrete Bridge Maintenance 2nd International Symposium on Computational Mechanics and the 12th International Conference on the Enhancement and Promotion of Computational Methods in Engineering and Science, 2010 Hong Kong‐ Macau (China) American Institute of Physics (AIP), 1372-1377. RASHIDI, M., SAMALI, B. & SHARAFI, P. 2016a. A new model for bridge management: Part A: condition assessment and priority ranking of bridges. Australian Journal of Civil Engineering, 14, 35-45. RASHIDI, M., SAMALI, B. & SHARAFI, P. 2016b. A new model for bridge management: Part B: Decision Support System for Remediation Planning. Australian Journal of Civil

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1836 1837 1838 1839 1840 1841 1842 1843 1844 1845 1846 1847 1848 1849 1850 1851 1852 1853 1854 1855 1856 1857 1858 1859 1860 1861 1862 1863 1864 1865 1866 1867 1868 1869 1870 1871 1872 1873 1874 1875 1876 1877 1878 1879 1880 1881 1882 1883 1884 1885 1886 1887 1888 1889 1890 1891 1892 1893 1894

Engineering, 14, 46-53. 1895 Retrofitting of Timber Girders. Queensland: RASHIDI, M., SAMALI, B., ZHU, X., AZAD, A.1896 & Queensland University of Technology. GHODRAT, M. 2017 Case study : structural 1897 YILDIZ, S., GEZER, E. D. & YILDIZ, U. C. 2006. health monitoring of timber bridges using 1898 Mechanical and chemical behavior of spruce dynamic frequency analysis (DFA). Structural 1899 wood modified by heat. Building and Health Monitoring Of Intelligent Infrastructure 1900 Environment, 41, 1762-1766. (Shmii-8), Brisbane. Queensland University of YU, L., WANG, J. & HUANG, T. 2014. Mechanical 1901 Technology. 1902 Properties of Wood and Timber Bridge RASHIDI, M., ZHANG, C., GHODRAT, M., 1903 Evaluation. Zurich. 1904 KEMPTON, S., SAMALI, B., AKBARNEZHAD, A. & ZHU, L. 2018b. 1905 Bridge Abutment Movement and Approach Settlement — A Case Study and Scenario Analysis. International Journal of Structural Stability and Dynamics, 18, 1840011. RICHARDSON, B. A. 2001. Defect and Deterioration in Building (Vol. 2). RIDOUT, B. 2001. Timber Decay in Buildings: The Conservation Approach to Treatment. APT Bulletin, 32, 58-60. RITTER, E., DE ROSA, M., FALK, A., CHRISTENSEN, P. & LOKKE, S. 2013. Wood as construction material: a "common" choice for carbon management? Environ Sci Technol, 47, 11930-1. ROADS, Q. G. D. O. M. 2005. Timber Bridge Maintenance Manual Parts 1-4 Timber Bridge Maintenance Manual. Queensland Government. RTA. 2008 Timber Bridge Manual - Section 1-8. Roads & Maritime Services RYALL, M. J. 2001. Bridge management, Woburn, MA Butterworth-Heinemann. SEAVEY, R. & LARSON, T. 2002. Inspection of Timber Bridges - Minnesota Local Road Research Board. SIBEL, Y., UMIT CAFER, Y. & EYLEM DIZMAN TOMAK, M. 2011. THE EFFECTS OF NATURAL WEATHERING ON THE PROPERTIES OF HEAT-TREATED ALDER WOOD. BioResources, 6, 2504-2521. SINGH, J. 1999. Review: dry rot and other wooddestroying fungi: their occurence, bioloogy, pathology and contro Indoor and built environment, 1, 3-20. SOUTHERN PINE. 2016. http://www.southernpine.com/using-southernpine/blue-stain-faqs/ TINGLEY, D. 2014. 2014. Extending the Life of Hardwood Timber Bridges. Brisbane: IPWEA QLD. VALENTIN, H. 2012. Defects in concrete and wooden bridges. Analele Universitatii DinOradea 18, 156-170. VICROADS 2018. Road Structures Inspection Manual Part 1-3. WILKINSON, K. 2008. Capacity Evaluation and 18

906

Appendix A-Taxonomy Table

907 Number

Deterioration Mechanism

Description of Deterioration Mechanism and Main Cause

Effect on Structure

1 1.1

Swelling and drying due to Saturation

Remediation Strategy Weathering

-Warping 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.

Prevention

-Element Replacement

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.

-Tightening of all bolts and connections

Heat Treated Timber

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 conducive to biological growth

Flashings

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.

Sealing of Penetrations

Penetrations through timber members should be sealed with an appropriate preservative decreasing the risk of moisture ingress through connection holes.

Paint

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

-Cupping -Checks -Loss in strength

-Improper / degenerated painting

Main Cause: Rain, submerged timber, environmental conditions, poorly installed flashings, removal of protective coating.

Ventilation

Use of acetylated 19

Prevention Description

Number

1.2

Deterioration Mechanism

Corrosion

Description of Deterioration Mechanism and Main Cause

Description: 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.

Effect on Structure

-Reduction in connection strength

Remediation Strategy

Prevention

Prevention Description

timber

in the cell into acetic acid. The conversion causes the wood to swell. The acrytylation process causes the timber to become more dimensionally stable

Material choice

The non-corrosive metals like galvanised or stainless steel fixings and non-corrosive metal components.

Replacement of corroded fixings

Paint

Microporous water repellent paint creates a physical barrier between the metallic surface and oxygen subsequently preventing oxidisation.

Bracing warped members

Paint, stain, seal

The protective coatings along with ventilation reduce the wetting and drying effects from the environment, which cause the warping of the timber.

Cleaning and painting of the metal components

-Decay of surrounding timber

Main cause: Salt in timber, moisture in timber, abundance of excessive water

1.3

Warping

Description: Timber deforming from its original geometry is known as ‘warping.’ The classification of warping depends on the plane in which the timber has deformed i.e., cupping, around the minor axis, bowing, the major axis.

Decreased structural capacity

Member augmentation

Main cause: Sporadic moisture content within the timber, growth stresses

Element replacement

20

Ventilate members

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.

Number

1.4

Deterioration Mechanism

Ultra Violet Radiation (UV)

Description of Deterioration Mechanism and Main Cause

Description: 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 63mm per 100 years. Main cause: Exposure to sunlight

Effect on Structure

-Affects the aesthetics of the timber -Can allow other deterioration mechanisms to occur through minor cracks caused by UV radiation

2

2.1 2.1.1

Remediation Strategy

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.

Paint, stain or seal

Microporous water replant paint or pigment stain (with fungicide recommended) maintain and provide a physical barrier between incoming UV radiation.

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 acetylation process allows the timber to become more dimensionally stable.

Cladding

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.

-Replacement

-Termite guards

-Construction detailing

-Termicide

• 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

-Paint, stain or seal effected members

-Replacement

Biological

Insect attack Termite

Description: Well-established termite attack normally damages wood rapidly, however it is uncommon for termite attack to take place in durable hardwoods usually utilized in bridge assembly without some pre-existing fungal decay. The decay accelerates when the termites extend their galleries

Reduced strength and structural capacity

-Site clearance -Fumigants

21

Number

Deterioration Mechanism

Description of Deterioration Mechanism and Main Cause throughout the bridge, moving fungal spores and moisture around with their bodies. Therefore, while the 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.

Effect on Structure

Main cause: -Timber bridges being situated in humid regions. -Pre-existing fungal decay in timber bridges. -Moist conditions provided by improperly installed flashings

Remediation Strategy

Prevention 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.

Treat soil

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.

Placement of stainless steel mesh around footings

Strip shielding

Timber selection

Use of acetylated timber

22

Prevention Description

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.

Number 2.1.2

Deterioration Mechanism Borer

Description of Deterioration Mechanism and Main Cause Description: In general, wood borers are beetle which at some point, during their short life, use timber as a method of shelter, food or both.

Effect on Structure -Reduced strength and structural capacity

Remediation Strategy -Replacement

Prevention Timber selection

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.

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. Copper (23-25%), chrome (38-45%) and arsenic (3037%) (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.

-Construction detailing

Metallic Salts -Fumigants

Use of acetylated timber

23

Prevention Description

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

Number 2.1.3

Deterioration Mechanism Ants

Description of Deterioration Mechanism and Main Cause Description: Ants are insects that frequently build passageways and nests in decay cavities in timber structures.

Effect on Structure Increases the rate of decay of a timber component.

Main cause: Ants deposit sawdust in gallery openings, thus trapping moisture

Remediation Strategy Fumigants

Prevention

Prevention Description

Metallic salts

Copper (23-25%), chrome (38-45%) and arsenic (3037%) (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. 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.

Construction detailing

Detailing

2.2

Bacteria

Description: 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.

It can cause timber to have an increase permeability and cause the timber surface to soften.

24

Heat treated timber

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

Construction detailing

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.

Replacement

Metallic salts

Copper (23-25%), chrome (38-45%) and arsenic (3037%) (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.

Number

2.3

Deterioration Mechanism

Fungi

Description of Deterioration Mechanism and Main Cause

Description: 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 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

Description: 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.

Effect on Structure

-Decreased structural capacity

Remediation Strategy

Prevention Description

Use of acetylated timber

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.

Replacement

Ventilation

Fumigants

Timber selection Use of acetylated timber

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. 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.

-Potential for other biological detrition -Aesthetic appearance affected Risk of multiple elements being effected by organisms

Under suitable conditions timber degrade causing reduced toughness and -Increased 25

Prevention

Brushing and scrapping

Prevention of excessive timber moisture content or stagnant water on structure Ventilation

Paint, stain or seal

Please see section 2.1 for more information.

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

Number

Deterioration Mechanism

Description of Deterioration Mechanism and Main Cause Main cause:

Effect on Structure permeability.

High moisture content

-Can be the precursor to other more detrimental organisms

Remediation Strategy

Prevention

content and the likelihood of fungal infestation.

Epoxy resin

Heat treated timber

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.

Timber selection Use of acetylated timber

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.

Prevention of excessive timber moisture content or stagnant water on structure Metallic salts

26

Prevention Description

See moisture content for more information.

Copper (23-25%), chrome (38-45%) and arsenic (3037%) (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

Number

Deterioration Mechanism

Description of Deterioration Mechanism and Main Cause

Effect on Structure

Remediation Strategy

Prevention

Prevention Description are outlawing the use of such treatments.

2.3.2

Decay fungi

Description: 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

Checks and splits can grow to a substantial depth in the internal untreated wood.

Removal of effected area

Fumigants

Main cause:

Paint stains & seal

Please see section 2.3 for more iformation.

End caps

Please see section 2.3 for more iformation.

Timber selection Ventilation

Please see section 2.3 for more iformation.

Heat treated timber

Please see section 2.3 for more iformation.

Use of acetylated timber

Please see section 2.3 for more iformation.

Metallic salts

Please see section 2.3 for more iformation.

Paint, stain, seal

Please see section 2.3 for more iformation.

Please see section 2.3 for more iformation.

Environmental conditions

2.3.2a

Brown Rot

Description: 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

-High reduction in wright loss and strength -Can affect multiple section of the structure once the decay process has commenced 27

Fumigants

End caps

Replacement

Use of acetylated

Please see section 2.3 for more iformation.

Please see section 2.3 for more iformation.

Number

Deterioration Mechanism

Description of Deterioration Mechanism and Main Cause 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.

Effect on Structure with little or no sign of decay

Remediation Strategy

Prevention timber Ventilation

Prevention Description

Please see section 2.3 for more iformation.

Heat treated timber

Please see section 2.3 for more iformation.

Metallic salts

Please see section 2.3 for more iformation.

Paint, stain, seal

Please see section 2.3 for more iformation.

Ventilation

Please see section 2.3 for more iformation.

Heat treated timber

Please see section 2.3 for more iformation.

Use of acetylated timber

Please see section 2.3 for more iformation.

Main cause: Environmental conditions 2.3.2b

White Rot

Description: 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. When the rot has become advanced it is soft in texture and fibres may peel individually from the timber.

-Extensive reduction in wright loss and strength -Can affect multiple section of the structure once the decay

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.

28

Replacement

Number

Deterioration Mechanism

Description of Deterioration Mechanism and Main Cause

Effect on Structure

Remediation Strategy Fumigants

Prevention

Prevention Description

Metallic salts

Please see section 2.3 for more iformation.

Paint, stain, seal

Please see section 2.3 for more iformation.

End Caps

Please see section 2.3 for more iformation.

Ventilation

Please see section 2.3 for more iformation.

Heat treated timber Use of acetylated timber

Please see section 2.3 for more iformation.

Metallic salts

Please see section 2.3 for more iformation.

Timber selection

Please see section 2.3 for more iformation.

Enzymes released by the rot remain close to the hyphae therefore localising infestation. Main cause: Environmental conditions 2.3.2c

Soft Rot

Description: 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 moisture content of timber

Structural deterioration

Replacement

Fumigants

changing

Presents of additional sustenance for fungi

3.

Mechanical Wear

29

Please see section 2.3 for more iformation.

Number 3.1

Deterioration Mechanism Deck Damage

Description of Deterioration Mechanism and Main Cause Description: Narrowly spaced sawn timbers up to 125mm 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.

3.1b

LVL & stress Laminated Timber (STL) decking damage

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.

3.2

Effect on Structure 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. 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.

Deformation

30

Remediation Strategy -SAPA Decking Asphalt

-Pressure treatment

-Cross Laminated Timber

Rehabilitate or replace damaged wooden laminates.

Prevention

Prevention Description

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".

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.

Preservative treatment.

SLT decking protection consists of the preservative treatment of softwood timbers and the sapwood in some hardwood.

Protection against direct moisture ingress.

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 comprise of galvanised metal or a rubber style material which can be situated beneath the anchorages.

Number 3.2a

Deterioration Mechanism Chord Deformation

Description of Deterioration Mechanism and Main Cause Description: 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.

Effect on Structure Deformation can cause subsequent damage to the surface layer resulting in further deterioration.

Remediation Strategy -Splicing

Sagging

Description: 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)

3.3

Element Crushing

Description: 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

FRP Rods

-Strengthening the bracing

Prevent moisture content changes

Any longterm sag will increase bending in a headstock and therefore decrease its capacity.

Prevention Description 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.

-Replacement

Main cause: Sagging of the truss, deterioration of deck components, overloading 3.2b

Prevention

-Metal shims

-Sag rods

-Reseating

-Replacement

-Relieving arch

Please see section 2.1 for more information.

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.

-Sister member

Causes loss of strength and affects serviceability of timber components.

31

Replacement

Bolts

Bolts are fixed through the ends of vulnerable components with the aim of preventing crushing.

Number

Deterioration Mechanism

Description of Deterioration Mechanism and Main Cause 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.

Effect on Structure -Decay

Remediation Strategy -Strengthening

-Crushing will deteriorate wooden piles, normally at or close to the waterline.

Main cause: Over tightened connections, bridge loading

Prevention

Prevention Description

Anti-crush plate

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.

FRP wrapping

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

Bracing of the pile can greatly reduce the chance of the element buckling.

Tightening of bolts

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

-Supplementary member

Timber girders are specifically vulnerable to crushing when substantial loss of section has taken place. 3.4

Element buckling

Description: 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.

Decreases wooden pile capacity.

-Strapping

-Replacing

-Concrete Jacket

Main cause: Buckling is caused by wooden piles being unable to support axial load. If bolted connections come to be loose because of decay in wooden piles or if there is a substantial loss of section

32

Number

Deterioration Mechanism

Description of Deterioration Mechanism and Main Cause because of steel corrosion, the efficiency and function of bracing components will be lost, causing the component to buckle.

Effect on Structure

Remediation Strategy

Prevention Gabions

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.

Vertical failure of a bridge can also be caused by corrosion of the pile or scour, due to moving water. 3.5

Delamination

Description: Delamination is the separation of layers in timber from plywood to glulam. It occurs when moisture penetrates the ply or when gluedlaminated layers separate as the adhesive that bonds the layers fails

They provide openings for decay to begin and may cause a reduction in strength

-Clamping and stitching -Composite concrete timber structure

Prevent moisture content changes

Use sealant, for example bitumen, to the unprotected exterior surface of plywood decking, to prevent delamination of laminates.

-Element replacement

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. 3.6

Fractures

Description: Beams under flexural loading can exhibit factures which are influenced by various mechanical properties and loading conditions of the timber element.

Reduced Strength

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

-Steel banding

-FRP rods

33

Prevent moisture content changes

Please see section 2.1 for more information.

Number

Deterioration Mechanism

Description of Deterioration Mechanism and Main Cause course of the member. This volume change combined with low strength normal to the timber grain cause cracks to develop.

Effect on Structure

Remediation Strategy

Prevention FRP rods

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.

FRP wrapping

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 of fixings

Routine tightening is the scheduled activity of examining and maintaining the current level of service of a bridges connection.

-Replacement

Loose connection

3.7

Description: 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.

Loose connections can reduce the bridge’s loadcarrying capacity

-Replace damaged fixings

Prevention Description

-Tighten fixings

Main cause: Traffic loading, vibration and weathering

4.0 4.1

Natural Element Defects Knots

Description: Knots are a piece of branch or limb that has been incorporated into the timber member

Reduce strength and load carrying capacity

Main cause: Natural product of tree growth 34

-Wrap member in carbon fibre fabric

Prevent moisture content changes

Please see section 2.1 for more information

Number 4.2

Deterioration Mechanism Checks

Description of Deterioration Mechanism and Main Cause Description: Checks are a separation of wood occurring perpendicular to the cross sectional grain or growth rings Main cause: Seasoning, weathering

4.3

Split

Description: Splits are a separation of wood from one surface to another, usually parallel to the grain Main cause: Seasoning, weathering

Effect on Structure Reduce strength and load carrying capacity Opens the timber to further weathering and deterioration Reduce strength and load carrying capacity Opens the timber to further weathering and deterioration

35

Remediation Strategy -Epoxy fill

Prevention

Prevention Description

Prevent moisture content changes

Please see section 2.1 for more information

Prevent moisture content changes

Please see section 2.1 for more information

-Element replacement

-Split resistant bolts

-Steel banding

-Element replacement

1908

36

A Comprehensive Taxonomy for Structure and Material Deficiencies, Preventions and Remedies of Timber Bridges Highlights: • Timber bridges are critical assets hence their maintenance is highly important. • Weathering, biological, mechanical wear and natural defects are the main deterioration mechanisms. • The deterioration mechanisms, preventive actions and remedial options are summarised in a taxonomy. • The provided taxonomy table can assist with the optimal asset management of timber bridges.

Declaration of interests ☒ 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. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Dr. Maria Rashidi