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Timber Preservation
Treatments for Highway
Applications
tech transfer summary
October 2007 Wood preservatives are evaluated for their performance effectiveness
in building new timber bridges in Iowa.
RESEARCH PROJECT TITLE Objective
Field Evaluation of Timber Preservation
Treatments for Highway Applications The primary objective of this research was to evaluate the performance of
different wood preservatives in the fi eld and to review current specifi ca-
SPONSORS tions and testing procedures to determine whether they provide the level
Iowa Highway Research Board (TR-552) of timber treatment required for acceptable performance.
Iowa Department of Transportation
(CTRE Project 06-252) Problem Statement
PRINCIPAL INVESTIGATOR Timber can be a cost-effective building material for new bridge construc-
Terry J. Wipf tion. The single most limiting factor for increased use of timber bridges
Professor, Civil, Construction, continues to be concerns with durability. The durability of timber
and Environmental Engineering bridges is largely a product of the initial preservative treatment used to
Iowa State University
515-294-6979 protect the wood, although construction practices and maintenance also
tjwipf@iastate.edu play an important role.
CO-PRINCIPAL INVESTIGATOR Proper preservative treatment creates an excellent barrier against fungi
F. Wayne Klaiber and insects, which can destroy the wood; however, the preservative
Professor, Civil, Construction, barrier can be compromised during on-site installation or as a result of
and Environmental Engineering checks and cracks from normal weathering and moisture changes. Any
Iowa State University
515-294-7622 break in the treatment barrier may expose untreated wood to fungal or
klaiber@iastate.edu insect attack.
AUTHORS The Iowa State University Bridge Engineering Center (BEC), in conjunc-
Jake J. Bigelow, Carol A. Clausen, tion with the United States Department of Agriculture Forest Products
Stan T. Lebow, and Lowell Greimann Laboratory (FPL), evaluated the various types of wood preservatives
BEC used in Iowa and outlined recommendations for their use in new bridge
Iowa State University construction.
2711 S. Loop Drive, Suite 4700
Ames, IA 50010-8664 60
515-294-8103
www.bec.iastate.edu
50
The Bridge Engineering Center (BEC) is part 43 Yes No
of the Center for Transportation Research 42
and Education (CTRE) at Iowa State Univer- 40 38
sity. The mission of the BEC is to conduct 33
research on bridge technologies to help 30
bridge designers/owners design, build, and 25
maintain long-lasting bridges. Number of Counties
21
The sponsors of this research are not respon- 20 17
sible for the accuracy of the information 14
presented herein. The conclusions expressed
in this publication are not necessarily those 10
of the sponsors.
0
Backwalls & Wingwalls Pilings & Substructure Superstructure Guardrail & Sign Post
New Bridge Components
Use of timber for construction of new bridges by Iowa counties (results
from 63 respondents)
Research Description
The research team conducted on-site visual inspections of 47 Longitudinal
bridges in eight Iowa counties using different preservative splitting and
types. The goal of the inspections was to evaluate the perfor- cracking
mance of current preservatives used in Iowa. The following
preservative types were evaluated:
• creosote Vegetation growth—
• pentachlorophenol indication of decay
• copper naphthenate
• ammoniacal copper zinc arsenate (ACZA)
• chromated copper arsenate (CCA)
• alkaline copper quaternary (ACQ) Decay infi ltration
at cracks
When conducting the inspections, all available piles, cap
beams, backwalls, stringers, decking, and guard railing were
inspected for decay, physical defects, and damage—signs that
the preservative treatment is not performing effectively or may
have been compromised for future protection.
Preservative Evaluation
Highway applications of timber material in Iowa vary greatly Common visual signs of interior decay on bridge piles located
and include bridge pilings, abutment backwalls, guardrail in stream channels
posts, bridge deck planking and many others. Currently, vari- Plant-Applied Preservative Treatments
ous in-plant preservative treatments are being used in Iowa to
extend the service life of structures. Creosote has been the in- Plant-applied wood preservatives can be broadly classifi ed
plant preservative of choice for many years; however, due to as either oilborne or waterborne, based on the chemical
environmental concerns and handling issues, a movement is composition of the preservative and the solvent/carrier used
being made away from creosote to other preservative alterna- during the treating process. Generally, oilborne preservatives
tives. Remedial, or in-place, preservative treatments have seen are used with petroleum based solvents ranging from heavy
minimal usage in the state of Iowa. As Iowa’s timber bridges oils to liquefi ed gases. Waterborne preservatives are applied
become older, the implementation of in-place treatments will using water-based solutions, such as water and ammonia.
be necessary to reduce future costly repair and replacement. There are advantages and disadvantages associated with us-
ing each type that depend upon the application.
Estimated service life of treated round fence post in southern
Treatment Mississippi
barrier 90% confi dence
Average Estimated limits for service
Preservative retention service life (yrs)
3
Seasoning (lb/ft ) life (yrs) Lower Upper
cracks Copper
naphthenate 0.03 65 55 78
Penetration at Creosote 5.6 54 47 62
incising marks Pentachloro-
phenol 0.32 74 60 91
ACA 0.34 60 51 69
Pentachlorophenol treated railing post with fi eld-cut end grain Untreated 0 2.4 2.1 2.7
and no in-place treatment which increased the amount of
physical defects
Properties and uses of plant-applied preservatives for timber bridges
Standardized Preservative Solvent Surface Color Odor Fastener
Uses Characteristics Characteristics Corrosion
All uses Creosote Oil-type Oily, not for frequent Dark brown Strong, No worse than
human contact lasting untreated wood
All uses Ammoniacal copper Water Dry, but contains Brown, possible Mild, short Worse than
zinc arsenate arsenic blue areas term untreated wood
All uses Chromated copper Water Dry, but use is Greenish brown, None Similar to
arsenate restricted by EPA weathers to gray untreated wood
All uses (except Pentachlorophenol No. 2 fuel oil Oily, not for frequent Dark brown Strong, No worse than
in seawater) Type A (heavy oil) human contact lasting untreated wood
All uses (except Copper No. 2 fuel oil Oily, not for frequent Green, weathers Strong, No worse than
in seawater) naphthenate human contact to brownish gray lasting untreated wood
All uses (except Alkaline copper Water Dry, okay for human Greenish brown, Mild, short Worse than
in seawater) quat contact weathers to gray term untreated wood
All uses (except Copper azole Water Dry, okay for human Greenish brown, Mild, short Worse than
in seawater) contact weathers to gray term untreated wood
Above ground, Pentachlrophenol Mineral spirits Dry, okay for human Light brown, Mild, short No worse than
fully exposed Type C (light oil) contact if coated weathers to gray term untreated wood
Above ground, Oxine copper Mineral spirits Dry, okay for human Greenish brown, Mild, short No worse than
fully exposed contact weathers to gray term untreated wood
Above ground, Copper HDO Water Dry, okay for human Greenish brown, Mild, short Worse than
fully exposed contact weathers to gray term untreated wood
The most common oilborne preservatives are creosote, In-Place Preservative Treatments
pentachlorophenol, and copper naphthenate. Conventional
oilborne preservatives, such as creosote and pentachlo- For best performance, as much fabrication should be com-
rophenol, have been confi ned largely to uses that do not pleted prior to pressure treatment to allow all exposed
involve frequent human contact. The exception is copper surfaces to be protected. On-site fabrication of timber bridge
naphthenate, a preservative that has become available more components typically results in breaks in the protective bar-
recently but has been used less widely. rier. Pile tops, which are typically cut to length after installa-
tion, need reapplication of the preservative to the cut ends.
Oilborne preservatives may be visually oily, oily to the Likewise, the exposed end-grain in joints and the immediate
touch, and sometimes have a noticeable odor. However, the area around all fasteners, including drill holes, require supple-
oil or solvent that is used as a carrier makes the wood less mental on-site treatment.
susceptible to cracks and checking and provides a barrier
against moisture, making them the preferred preservative for Periodic inspections should seek to identify cracks, splits,
bridge structural elements. and checks that result from normal seasoning as well as areas
of high moisture or exposed end grain in joint areas. These
Waterborne preservatives are formulations of inorganic areas require periodic reapplication of supplemental preserva-
arsenical compounds that react with or precipitate in treated tive. Supplemental in-place treatments are available in several
wood. The reaction takes place when members are treated, forms: surface-applied chemicals, pastes, diffusible chemicals,
“fi xing” the precipitants (e.g., copper, chromium, and/or ar- and fumigants.
senic) within the cells of the wood to help prevent leaching
and migration. Specifi cations and Guidelines
Waterborne preservatives, however, are used due to their State of Iowa specifi cations pertaining to the handling and
preferred handling properties, clean surfaces, and low preservative treatment of timber used for bridges can be
leaching levels. Waterborne preservative treatments have found in the Iowa Department of Transportation Standard
been found to reduce the mechanical properties of wood Specifi cations, which can be found online at http://www.erl.
under some conditions. Energy-related properties are often dot.state.ia.us/. The American Wood-Preservers’ Association
reduced slightly; however, strength and elasticity properties (AWPA) is the primary standard-setting body for preservative
are generally not affected when correct treatment levels are treatment in the United States. Their timber standards can be
used. purchased online at http://www.awpa.com/standards/ucs.asp.
Properties and uses of in-place preservatives for timber bridges
In-place Active Solvent Internal vs. Leeching or Bridge location Handling and
preservative type ingredient type external diffusing other
Surface treatment Copper naphthe- External Bolt holes, exposed
liquid nate Oil sprayed or Insoluble in water end grain, checks Non-RUP
brushed and splits
Surface treatment External Leech away by pre- Bolt holes, exposed
liquid or powder Borate solutions Water sprayed or cipitation end grain, checks Non-RUP
brushed and splits
CuNap, sodium External Boron & fl uoride Ground line area of
Surface treatment fl uoride, borates Water and covered move into wood, terrestrial piles & Non-RUP
paste Cu-Hydrooxide with wrap Copper stays at under pile caps
surface
Diffusible Boron, fl uoride, Internal Needs moisture to Pile & deep Non-RUP, low
chemical liquid copper Water through diffuse into wood timbers with drill toxicity & ease
drilled holes accessibility of handling
Internal Volatizes into gas & Pile & deep
Fumigant liquid Chloropicrin NA through moves into wood timbers with drill RUP
drilled holes accessibility
Internal Volatizes into gas & Pile & deep
Fumigant solid Solid-melt MITC NA through moves into wood timbers with drill RUP
drilled holes accessibility
Methan Sodium Internal Volatizes into gas & Pile & deep
Fumigant liquid (Vapam) NA through moves into wood timbers with drill RUP
drilled holes accessibility
Granular Internal Volatizes into gas & Pile & deep
Fumigant solid dazomet NA through moves into wood timbers with drill RUP
drilled holes accessibility
Findings and Recommendations bridges constructed with state or federal funding in
The results of this study led to the following conclusions the state of Iowa. If the bridges are being constructed
and recommendations: without state or federal funding, the Iowa DOT specifi -
cations and plant certifi cations are still recommended.
1. Copper naphthenate is recommended as the plant-ap- 4. Treated Southern Pine piles are recommended to have
plied preservative treatment for timber bridge elements. penetration of 3.0 in., or 90% of sapwood penetration.
Copper naphthenate has been tested extensively by The penetration is in accordance with AWPA standards
the FPL in past years and has been shown to have and is currently stricter than Iowa DOT specifi cations.
comparable, if not better, performance to other com- 5. Timber bridge maintenance programs need to be devel-
monly used preservatives, such as creosote. Additional oped and implemented. A maintenance program that
reasons for recommending copper naphthenate include utilizes combinations of inspection tools and various
good handling characteristics, clean surfaces, compara- in-place treatments can easily extend a bridge’s service
ble availability to other preservatives, and the potential life. Future work could entail development of a timber
for lower environmental impact. bridge maintenance program for bridge owners. An
2. During the construction of timber bridges, the Best effective maintenance program contains many compo-
Management Practices should be followed to minimize nents that need to be developed, including 1) person-
environmental impacts to the surrounding ecosystem nel training and education, 2) inspection procedures,
and ensure quality treatment of both plant-applied and 3) evaluation of structure and restoration, 4) in-place
in-place preservatives. In addition to the best manage- treatment procedures, and 5)records and data manage-
ment practices, bridge owners need to insure that pile ment.
tops and cap beams are protected from moisture by use 6. Future workshops and/or short courses presenting
of metal covers and that all fi eld cuts are treated with biodeterioration and preservative concepts to timber
in-place treatments. bridge owners, designers, and inspectors are recom-
3. The AWPA standards are the basis for the Iowa DOT mended in order to implement the information and
specifi cations, which are the regulating standards for procedures presented in this study.
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