BRIDGE REPAIR/REHABILITATION FEASIBILITY STUDY

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MITC-FUME Levels in Southern Pine 0.3 m Below Treatment MITC-FUME Levels in Douglas-fir 0.3 m Below Treatment Figure 2-Residual levels of MITC in Douglas-fir and southern pine poles one to seven years after application of MITC-Fume. 450
of reinvasion (Forsyth and Morrell, 1993). One other advantage of this chemical is its existing registration for application to non-food crops, making it far easier to register for wood application. Ultimately, strategies utilizing solid fumigants which can decompose slowly over a several year period can provide a safer method for preventing internal decay using volatile chemicals. Alternative to Fumigants While fumigants have proven to be highly effective, their handling properties have encouraged a search for less toxic decay control strategies. One alternative to fumigants are water diffusible fungicides including boron and fluoride. These compounds do not volatilize like fumigants, but they are able to diffuse from areas of high to low concentrations whenever free water is present in the wood. Both boron and fluoride have been used for many years for protecting a variety of products from decay, but their use for internal decay control in large timbers in North America is a relatively recent development (Becker, 1976). Boron is highly effective against most decay fungi and insects, although the levels required for control can vary quite widely. Typically, a target boron retention between 0.25 and 0.5% by weight is required for wood protection. Levels required for preventing wood attack where the Formosan termite is present are many times higher. Fluoride is generally only used for controlling decay fungi. In a number of studies, boron and fluoride have moved well through moist wood, but move very little when the moisture content falls below 30% (Smith and Williams, 1969). Proponents of these systems have pointed out that substantial fungal decay does not occur when the moisture content falls below 30%, therefore, it should not matter if the diffusible compound does not move in dry wood since no decay can occur under these conditions. However, this approach ignores the fact that wood moisture contents can vary widely along the length of large timbers. As a result, the boron or fluoride may be applied to a dry zone, where no movement will occur, while an adjacent wet area contained actively growing decay fungi. Judicious application can help overcome some of this limitation, but there remains the risk that the improperly placed chemical will not diffuse to the points where it is needed. Two formulations of boron and fluoride are labeled for wood use in the U.S. Fused boron rods are produced by heating boron to high temperatures and pouring this molten material into molds. The boron cools and hardens into a glass-like rod which is applied to the same steep angled holes used for fumigant treatment. Boron diffuses from the rods in the presence of moisture (Morrell et al., 1990) and moves well through a variety of North American wood species (Morrell et al., 1992b). Sodium fluoride is available in rod form and has a long history of use in railroad ties, but has only recently been labeled for other wood uses. Field trials are currently underway to evaluate the performance of these materials in larger timbers. An additional formulation which is not currently labeled in the U.S. is composed of both fluoride and boron in a rod form (Preschem Ltd., Cheltenham, Australia). Field trials with this formulation suggest that the rate of chemical movement from the rods remains slower than that found with fumigants (Table 3). Field trials of boron in fused boron rods have shown that boron diffusion away from the application point in Douglas-fir poles takes up to 3 years to achieve chemical levels which can provide effective fungal control (Table 4)(Morrell and Schneider, 1995). Since decay continues while this diffusion occurs, the user takes a risk that the timber will deteriorate to an unsafe condition before boron levels are sufficient to effect fungal control. Trials with southern pine poles have proven more successful, perhaps reflecting the more permeable nature of this wood species (Zahora et al., 1996). Trials with a fluoride/boron rod have shown that boron has moved more rapidly than the fluoride over a 2 year period. These results are interesting since a prior trial of groundline preservative pastes containing fluoride and boron showed the opposite effect in Douglas-fir posts (Morrell et al., 1994). A final diffusible preservative system available for timber in bridges is a water soluble copper naphthenate/boron paste. Limited field trials with this formulation indicate that the boron moves well from the point of application, while the copper naphthenate moves to only a limited extent (Forsyth and Morrell, 1992). As a result, this treatment might be useful for treating the inner surface of large voids, where the copper naphthenate would coat the surface of the void, while the boron would diffuse further into the wood. This treatment, however, would be unlikely to completely eliminate established decay fungi. Selecting Remedial Treatments Bridge maintenance specialists have a variety of options for arresting internal decay in their bridges. Each chemical has certain pros and cons which may make it especially attractive for specific applications. For example, where decay is actively occurring in a bridge located away from inhabited structures, chloropicrin 451
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BRIDGE REPAIR/REHABILITATION FEASIB
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1.0 EXECUTIVE SUMMARY The existing
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such there is a risk that the proje
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3.0 BRIDGE DESCRIPTION Bridge Numbe
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Photo 2 - Present Mitchell River Br
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4.0 ELEMENT CONDITION & REHABILITAT
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Based on the 1997 load rating analy
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promotes growth in the number and s
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4.2.2 Deck NBIS Condition Rating: 5
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Rehabilitation Scope: In order to m
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4.2.5 Railings NBIS Condition Ratin
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Functionality and Safety: Eliminati
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Although in-place preservative trea
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The bascule span timber stringers c
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Repair Scope: Although the timber m
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Use of an all steel counterweight w
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additional force effects, the speci
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typically loose. The steel guardrai
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Functionality and Safety: The cap b
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The surface decay and deterioration
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the interior of the piles where the
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Load Capacity: The current loss in
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Visual Impacts: Replacement of the
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Condition Description: There are cu
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5.0 OTHER CONSIDERATIONS 5.1 Naviga
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SUMMARY OF TIMBER ELEMENT SERVICE L
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5.3 Funding The project is currentl
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navigation opening. The estimated c
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• Continued replacement, repair a
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APPENDICES A. Existing Bridge Plans
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APPENDIX B Structures Field Inspect
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PAGE 3 OF 22 CITY/TOWN B.I.N. BR. D
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CITY/TOWN B.I.N. BR. DEPT. NO. 8.-S
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PAGE 2 OF 7 CITY/TOWN B.I.N. BR. DE
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APPENDIX C Load Rating Summary (199
Page 237 and 238: MAINTENANCE AND REHABILITATION OF T
Page 239 and 240: Figure 14-1. - Many covered bridges
Page 241 and 242: (discussed later in this chapter),
Page 243 and 244: Figure 14-5. - Liquid wood preserva
Page 245 and 246: wood, and other openings to the atm
Page 247 and 248: Table 14-1. - Number and size of ho
Page 249 and 250: member noting treatment information
Page 251 and 252: Figure 14-11. - Splicing and scabbi
Page 253 and 254: STRESS LAMINATING Stress laminating
Page 255 and 256: ond between separated sections, inc
Page 257 and 258: onding the old and new pile section
Page 259 and 260: GENERAL PROCEDURES FOR EPOXY REPAIR
Page 261 and 262: openings will allow epoxy to escape
Page 263 and 264: Another quality control problem is
Page 265 and 266: 6. Avent, R.R. 1985. Factors affect
Page 267 and 268: Integrated Remedial Protection of W
Page 269 and 270: Figure l-Effect of fumigant applica
Page 271: Table 1- Residual MITC content in D
Page 275 and 276: might represent the best option, wh
Page 277 and 278: In: Ritter, M.A.; Duwadi, S.R.; Lee
Page 279 and 280: Present and Potential Commercial Ti
Page 281 and 282: 2 U.S. DEPT. OF AGRICULTURE HANDBOO
Page 289 and 290: 10 U.S. DEPT. OF AGRICULTURE HANDBO
Page 305 and 306: APPENDIX F FRP Pile Jackets
Page 307 and 308: about its hazard to human health. T
Page 309 and 310: Fig. 5. Cross section of wood pile
Page 311 and 312: (Lopez-Anido et al. 2004c). For exa
Page 313 and 314: Table 1. Cost Items for FRP Composi
Page 315 and 316: Hardcore Composites. (2000). Hardsh
Page 317 and 318: CALCULATION COVER SHEET Project Nam
Page 319 and 320: URS Corp. 260 Franklin Street Bosto
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URS Corp. 260 Franklin Street Bosto
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APPENDIX I Correspondence
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BRIDGE REPAIR/REHABILITATION FEASIBILITY STUDY

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