Evaluation of Wood Species 

and Preservatives for Use in 

Wisconsin Highway Sign Posts 

Stan T. Lebow

Robert J. Ross

Samuel L. Zelinka

United States Department of Agriculture

General Technical Report

FPL–GTR–231

Forest 

Service

Forest Products

Laboratory

July

2014

In cooperation with the Wisconsin Department of Transportation.
June 2014

Lebow, Stan; Ross, Robert; Zelinka, Samuel. 2014. Evaluation of wood 

species and preservatives for use in Wisconsin highway sign posts. General 

Technical Report FPL-GTR-231. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 45 p.

A limited number of free copies of this publication are available to the 

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Abstract

The Wisconsin Department of Transportation (WisDOT) 

uses wooden posts to support many types of signs along 

state highways. WisDOT currently uses red pine or Southern 

Pine posts treated with chromated copper arsenate (CCA) 

and has generally experienced satisfactory performance and 

service life. However, there are some areas of concern, as 

well as potential opportunities for diversifying the wood 

species and preservatives used. Warp is sometimes a problem with the current pine posts, and increased use of locally 

sourced wood species for the sign posts could decrease 

transportation costs and potentially benefit Wisconsin industries. Although CCA is a highly effective preservative, it 

may not be the optimum treatment for alternative wood species. This study reviewed the characteristics of alternative 

wood species and wood preservatives and evaluated their 

potential for use in Wisconsin highway sign posts. 

The evaluation concluded that WisDOT’s current practice 

of using red pine or Southern Pine posts treated with CCA 

is logical and may be the optimum combination of wood 

species and preservatives currently available. Red pine and 

Southern Pine are readily available and relatively strong 

compared with many other softwood species. Red pine is 

also an important commercial resource for Wisconsin and 

the upper Midwest. Other Wisconsin species to consider 

include eastern white pine and soft maples. However, 

strength may be a concern with eastern white pine, whereas 

cost may be a concern with soft maples. CCA is an effective 

preservative, readily treats red pine and Southern Pine, and 

is compatible with aluminum signs. Copper naphthenate in 

oil solvent appears to be one of the most logical alternatives 

to CCA, and would be a strong candidate for treatment of 

hardwoods. Copper naphthenate is non-corrosive to aluminum and would avoid warp associated with re-drying 

after treatment with water-based preservatives. WisDOT 

may want to consider purchasing a small volume of copper 

naphthenate-treated posts to evaluate the handling characteristics of this oil-based preservative. 

Keywords: wood preservatives, wood species, sign posts, 

highway, Wisconsin

Acknowledgment

The authors acknowledge the valuable input received from 

Matt Rauch of WisDOT in refining the study problem areas, 

relating field experiences, and describing WisDOT practices 

and procedures.
Contents

Introduction..................................................................................................................................................................... 1

 Wisconsin’s Current Sign Post Standard Specifications............................................................................................. 1

 Wood Species Considerations..................................................................................................................................... 1

 Wood Preservative Considerations ............................................................................................................................. 2

Objectives ....................................................................................................................................................................... 2

Wood Preservative Background...................................................................................................................................... 2

 Pressure-Treatment Preservatives and Pressure-Treated Wood.................................................................................. 2

 AWPA vs ICC-ES and AASHTO Preservative Listings............................................................................................. 3

 Evaluating Preservative Efficacy................................................................................................................................ 3

 Role of Solvent Characteristics in Durability ............................................................................................................. 4

 Corrosion Testing........................................................................................................................................................ 4

 AWPA Use Category System ...................................................................................................................................... 5

Wood Preservative Characteristics.................................................................................................................................. 5

 Role of Copper in Wood Preservatives....................................................................................................................... 5

 Current Ground-Contact Preservatives....................................................................................................................... 6

Corrosion Aspects of Selecting Preservatives for Sign Posts........................................................................................11

 Mechanism of Corrosion in Treated Wood ................................................................................................................11

 Review of Corrosion Data from New Wood Preservatives .......................................................................................12

 Special Corrosion Considerations for Sign Posts......................................................................................................14

 Corrosion Recommendations.....................................................................................................................................14

Summary of Preservatives’ Potential for Sign Post Treatments ....................................................................................14

 Water-Based Preservatives.........................................................................................................................................15

 Oil-Based Preservatives.............................................................................................................................................16

Wood Species Options for Wisconsin DOT Signposts..................................................................................................17

 Wood Species Currently Listed in WisDOT Signpost Specifications .......................................................................17

 Hem-Fir......................................................................................................................................................................21

 Treatability and Durability of Other Wisconsin Wood Species.................................................................................23

 Ash .............................................................................................................................................................................23

 Aspen..........................................................................................................................................................................24

 Balsam Fir..................................................................................................................................................................24

 Basswood ...................................................................................................................................................................25

 Birch—Paper and Yellow...........................................................................................................................................25

 Eastern Hemlock........................................................................................................................................................26

 Elm.............................................................................................................................................................................27

 Maples, Hard..............................................................................................................................................................27

 Maples, Soft...............................................................................................................................................................28

 Spruce ........................................................................................................................................................................28

Engineering Properties of Wisconsin Wood Species.....................................................................................................29

 Common Mechanical Properties................................................................................................................................29

 Moisture Content and Shrinkage ...............................................................................................................................30

 Minimizing Warp in Sign Posts.................................................................................................................................32

Other Options: Structural Wood Composites and Naturally Durable Species...............................................................34

 Structural Wood Composites .....................................................................................................................................34

 Naturally Durable Species .........................................................................................................................................35

Summary of Potential Use of Wisconsin Species in Sign Posts....................................................................................35

Wooden Signage Post Specifications in Other States....................................................................................................36

 Wood Species Referenced by Other States................................................................................................................38

 Wood Preservatives Referenced by Other States.......................................................................................................39

Summary........................................................................................................................................................................40

 Preservatives and Wood Species................................................................................................................................40

References......................................................................................................................................................................42

Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

1

Evaluation of Wood Species and 

Preservatives for Use in Wisconsin 

Highway Sign Posts

Stan T. Lebow, Research Forest Products Technologist

Robert J. Ross, Supervisory Research General Engineer

Sameul L. Zelinka, Research Materials Engineer

Forest Products Laboratory, Madison, Wisconsin

Introduction

The Wisconsin Department of Transportation (WisDOT) 

administers approximately 11,800 miles (18,990 km) of 

state highways (WisDOT 2014). WisDOT uses preservativetreated wood posts for much of the signage along these 

highways because wood is relatively inexpensive, easy to 

install, and has the necessary strength properties to tolerate 

typical Wisconsin wind loads. Although WisDOT’s experience with wood sign posts has been generally positive, there 

are some areas of concern, as well as potential opportunities 

for diversifying the wood species and preservatives used. 

WisDOT and the USDA Forest Products Laboratory conducted a comprehensive literature review to examine those 

concerns and 

opportunities.

Wisconsin’s Current Sign Post Standard 

Specifications

Wood sign posts are covered under WisDOT Section 634 

Wood and Tubular Steel Sign Posts, but with reference to 

conformance to Section 507.2.2 Lumber and Timber (for 

grading), Section 507.2.2.6 Preservative Treatments (for 

preservative criteria), and Section 614.2.5 Wood Posts and 

Offset Blocks (for species selection and wood quality). 

However, Section 634 additionally specifies that wood sign 

posts will be either beam and stringer grade or structural 

joist and plank grade material with a minimum stress grade 

rating of 1200 fb (1200 lb/in2 fiber stress in bending) at 

19% maximum moisture content (MC). Section 634 also 

limits the choice of preservative treatments to chromated 

copper arsenate from the Section 507.2.2.6 listing that also 

includes creosote, pentachlorophenol, copper naphthenate 

(both oil and water-based), ammoniacal copper zinc arsenate 

(ACZA), ammoniacal copper quat (ACQ types A or B), and 

copper azole (types A or B). Section 507.2.2.6 specifies that 

the wood be treated to meet the penetration and retention 

requirements of American Wood Protection Association Use 

Category 4A (general use ground contact) and this specification is not amended by Section 634. Section 634 also does 

not amend the wood species listed in Section 614.2.5, thus 

allowing use of Douglas-fir, Southern Pine, ponderosa pine, 

jack pine, white pine, red pine, western hemlock, western 

larch, Hem-Fir species, and oak species. However, discussion with WisDOT personnel indicates that oaks will be 

removed from the species listed. The net effect of the WisDOT specifications is to provide sign posts with the following characteristics:

•	 Wood quality: Beam and stringer grade or structural joist 

and plank grade with minimum fiber stress in bending (fb) 

of 1,200 lb/in2.

•	 Wood species: Douglas-fir, Southern Pine, ponderosa 

pine, jack pine, white pine, red pine, western hemlock, 

western larch, and Hem-Fir (species grouping).

•	 Preservative treatment: Chromated copper arsenate to 

meet AWPA Use Category 4A (6.4 kg/m3 or 0.4 lb/ft3) 

retention.

Wood Species Considerations

Use of locally sourced wood for the sign posts decreases 

transportation costs and potentially benefits Wisconsin industries. Currently, red pine is the preferred wood species 

because it is locally grown and locally treated, but Southern 

Pine posts are also used when there is an insufficient supply of red pine. Southern Pine is easily treated with wood 

preservatives and readily available but is primarily grown in 

the southeastern United States. Possible alternative species 

include those currently allowed under WisDOT specifications (Douglas-fir, jack pine, white pine, and oak) as well 

as other softwood and hardwood species that occur commonly in Wisconsin. However, some of these species may 

not be locally available in sufficient quantity, while others 

may not grow to sufficient size, have sufficient strength 

(Kretschmann 2010), or be treatable with preservatives 

(Smith 1986; Gjovik and Schumann 1992). Some Wisconsin 

wood species have a degree of natural durability (Clausen 

2010), although it is not clear that this durability is sufficient 

for use in sign post applications. 

An acceptable wood species also must not exhibit excessive 

warp during drying or while in service. In part because of 

their length (up to 6.7 m (22 ft)), the red pine and Southern 

Pine sign posts are vulnerable to twisting or warping as they 
General Technical Report FPL–GTR–231

2

dry following treatment. Current WisDOT practice is to 

have the posts stickered after treatment to promote drying 

and to minimize the time that the posts are exposed horizontally outdoors during storage. An alternative approach is to 

close-stack the posts after treatment to minimize drying and 

warp prior to installation. However, this approach raises the 

concern that drying and warp might occur after installation 

and compromise the appearance or function of the sign. 

Wood Preservative Considerations

Most Wisconsin wood species do not have sufficient natural 

durability to be used in direct contact with soil. To provide 

the necessary durability, the wood must be pressure-treated 

with preservatives that are toxic to wood-decaying fungi. 

Currently, WisDOT is specifying that posts be treated with 

chromated copper arsenate (CCA). CCA is an effective and 

inexpensive preservative that has been widely used for treatment of wooden construction materials for decades. However, CCA is considered a Restricted Use Pesticide (RUP) 

by the U.S. Environmental Protection Agency (EPA), and 

it is possible that it will no longer be available sometime in 

the future. It is also difficult to penetrate some wood species 

with CCA during the treatment process. 

In the past decade, numerous alternatives to CCA have 

been developed, and some have gained substantial market 

share (Lebow 2010). The most readily available of these 

are water-based formulations that rely primarily on copper 

for protection against decay and termite attack. In addition, 

some of the most recent copper-based formulations would 

reportedly need to be modified to penetrate red pine or other 

Wisconsin species (McIntyre 2010). Research also suggests 

that water-based copper formulations are not as effective 

in protecting hardwoods (Lebow et al. 2010). Copper-free 

water-based formulations have also been developed but may 

not have sufficient durability to protect wood placed directly 

in contact with the ground. Oil-based preservatives such as 

copper naphthenate are another option, but oil-based preservatives can have odor and surface characteristics that are 

sometimes considered undesirable. 

Compatibility with aluminum signs is an important consideration for alternative preservative treatments. WisDOT conducted a trial with one of these copper-based formulations 

(ACQ) and found that the preservative caused corrosion of 

the aluminum sign (Wilson 2004). Manufacturers of other 

copper-based preservative formulations claim the treated 

wood can be placed in contact with aluminum, but there is 

little research available on their compatibility with aluminum signs. WisDOT currently attaches aluminum signs to 

CCA-treated posts with galvanized lag screws. When ACQtreated sign posts were evaluated, excessive corrosion of 

the aluminum occurred around the bolt-hole (Wilson 2004). 

While not specifically mentioned in the WisDOT report, the 

photographs in the report suggest that a galvanic couple was 

occurring between the galvanized lag screw and the aluminum sign. Galvanic corrosion occurs when dissimilar metals 

are electrically connected in a wet environment; the more 

anodic metal corrodes at a faster rate than it would if exposed by itself and the cathodic metal corrodes more slowly. 

Which metal acts as the anode/cathode depends on the metals and the environment. In seawater, aluminum is slightly 

cathodic to zinc. However, the photographs from the preliminary corrosion investigation (Wilson 2004) suggest that 

the aluminum sign is acting as the anode and preferentially 

corroding in the vicinity of the zinc-plated lagscrew. 

The use of alternative wood preservatives presents two 

possibilities for increased corrosion of aluminum signs: 

(1) increased general corrosion of the aluminum sign and/

or zinc-plated fastener (2) increased galvanic corrosion in 

the new environment. If the majority of the increased corrosion is caused by galvanic corrosion, placing a nylon washer 

between the lag screw and aluminum sign should greatly 

reduce the observed corrosion.

Objectives

The objectives of this project were as follows:

•	 Review properties of wood preservatives and evaluate 

their potential for use in treatment of sign posts manufactured from Wisconsin wood species.

•	 Review properties of Wisconsin wood species and evaluate their potential for use in sign post applications.

•	 Review current WisDOT post storage practices and 

recommend options for minimizing warp during storage 

and use.

Wood Preservative Background

When considered in its broadest context, a wood preservative is any substance or material that, when applied to wood, 

extends the useful service life of the wood product. In more 

practical terms, wood preservatives are generally chemicals 

that are either toxic to wood-degrading organisms or cause 

some change in wood properties that renders the wood less 

vulnerable to degradation, or both. Most wood preservatives contain pesticide ingredients, and as such must have 

registration with the U.S. Environmental Protection Agency 

(EPA). However, some preservatives such as those based on 

water repellents, work on the basis of moisture exclusion 

and do not contain pesticides. Preservatives that do contain 

pesticides are required to provide information on the type 

and concentration of pesticide on the label. Because the term 

“wood preservative” is applied to a broad range of products, 

there is often confusion or misunderstanding about the types 

of products being described, and some degree of specificity 

is needed.

Pressure-Treatment Preservatives and 

Pressure-Treated Wood

The greatest volume of wood preservatives is used in 

the pressure treatment of wood at specialized treatment 
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

3

facilities. In these treatment plants, bundles of wood products are placed into large pressure cylinders and combinations of vacuum, pressure, and sometimes heat are used to 

force the preservative deeply into the wood. Pressure-treated 

wood and the pressure-treatment preservatives differ from 

non-pressure preservatives in three important ways. 

(1) Pressure-treated wood has much deeper and more uniform preservative penetration than wood treated in other 

manners. (2) Most preservatives used in pressure treatment 

are not available for application by the public. (3) Pressuretreatment preservatives and pressure-treated wood undergo 

review by standard-setting organizations to ensure that the 

resulting product will be sufficiently durable in the intended 

end-use. Standards also apply to treatment processes and 

require specific quality control and quality assurance procedures for the treated wood product. This level of oversight 

is needed because pressure-treated wood is used in applications where it is expected to provide service for decades, 

and where premature failure could result in injury or death. 

In contrast, non-pressure preservatives may undergo relatively little review, other than the EPA evaluation of pesticide toxicity.

Preservatives have a range of properties that may make 

them more or less suitable for treatment of signposts. The 

most important of these is their efficacy in protecting the 

wood from attack by decay fungi and insects. EPA registration status and the accompanying allowable uses on the EPA 

label are also critical, but current EPA labeling puts few 

limitations on use of labeled preservatives for highway applications. Because the signage posts are typically in direct 

contact with aluminum signs, corrosiveness of the treated 

wood is also important. Other characteristics such as toxicity, odor, and surface cleanliness may also be considered, 

although these properties are less important for signage 

posts than for applications where human contact is more 

frequent. And finally, the preservative must be commercially 

available or have the likelihood of becoming available in 

the future. This last consideration is particularly relevant for 

pressure-treatment preservatives because conversion of a 

pressure-treatment facility to alternative preservatives is an 

expensive process that is unlikely to be undertaken without 

some assurance of a sustained and profitable market for the 

treated product. 

AWPA vs ICC-ES and AASHTO Preservative 

Listings

Before a wood preservative can be approved for pressure 

treatment of structural members, it must be evaluated to 

ensure that it provides the necessary durability and that it 

does not greatly reduce the strength properties of the wood. 

The EPA typically does not evaluate how well a wood preservative protects the wood. Traditionally this evaluation 

has been conducted through the standardization process of 

the American Wood Protection Association (AWPA). The 

AWPA Book of Standards lists a series of laboratory and 

field exposure tests that must be conducted when evaluating new wood preservatives. The durability of test products 

is compared with that of established durable products and 

nondurable controls. The results of those tests are then presented to the appropriate AWPA subcommittees for review. 

AWPA subcommittees are composed of representatives 

from industry, academia, and government agencies who 

have familiarity with conducting and interpreting durability evaluations. Preservative standardization by AWPA is a 

two-step process. If the performance of a new preservative 

is considered appropriate, it is first listed as a potential preservative. Secondary committee action is needed to have the 

new preservative listed for specific commodities and to set 

the required treatment level.

More recently the International Code Council–Evaluation 

Service (ICC–ES) has evolved as an additional route for 

gaining building code acceptance of new types of pressuretreated wood. In contrast to AWPA, the ICC–ES does not 

standardize preservatives. Instead, it issues evaluation reports that provide evidence that a building product complies 

with building codes. The data and other information needed 

to obtain an evaluation report are first established as acceptance criteria. AC326, which sets the performance criteria 

used by ICC–ES to evaluate proprietary wood preservatives, 

and requires submittal of documentation from accredited 

third party agencies in accordance with AWPA, ASTM, and 

EN standard test methods. The results of those tests are then 

reviewed by an evaluation committee to determine if the 

preservative has met the appropriate acceptance criteria.

The American Association of State Highway and Transportation Officials (AASHTO) also has a standard specification 

for Preservatives and Pressure Treatment Processes for Timber (AASHTO M 133). This specification is under the oversight of AASHTO Technical Section 4c–Coatings, Paints, 

Preservatives, Bonding Agents, and Traffic Markings. Unlike AWPA and ICC-ES, AASHTO does not evaluate new 

preservatives for inclusion in AASHTO M 133. Instead, 

AASHTO lists some (but not necessarily all) preservatives 

that have been either standardized by AWPA or have an 

ICC-ES evaluation report. AASHTO M 133 also refers to 

AWPA standards or ICC-ES Evaluation Reports for specifications on treatment processes and limitations.

Evaluating Preservative Efficacy 

Evaluating preservative efficacy and judging its expected 

durability for sign posts is not an exact science. Wood preservative efficacy is evaluated using a variety of laboratory 

and field exposures, and the applicability of these test results 

to real world durability depends on how well the test simulates in-service conditions. Ideally, a preservative would be 

tested under identical conditions to those in service, but this 

can become impractical for products expected to remain 

durable for many years or even many decades. In general, 

the shorter and more artificial the test conditions, the less 

confidence it provides for in-service performance. Although 
General Technical Report FPL–GTR–231

4

relatively short laboratory tests can be useful indicators in 

comparison with more established preservatives systems, 

these tests should not be viewed as absolute evidence of 

future performance. They may however, provide insight 

into a preservatives’ potential vulnerability or resistance to 

a certain type of organism. The type and severity of tests 

depends somewhat on the intended end-use of the treated 

wood. Wood intended for use in ground-contact applications 

(i.e., sign posts, piles, poles) is generally subjected to the 

most severe tests, which include multiple years of in-ground 

exposure. 

Stake and Post Field Tests

Ground-contact tests with stakes or posts are the most traditional and typically the most severe (other than marine 

exposure) method of evaluating preservatives. They are also 

the most appropriate methods for evaluating preservatives 

to be used in signage posts. In these tests, specimens ranging from thin strips to full-size posts are partially buried and 

then periodically inspected (AWPA Methods E7 and E8) 

(AWPA 2012). Inspections are typically visual ratings based 

on a scale of 10, 9, 8, 7, 6, 4, and 0 with 10 representing no 

deterioration and 0 representing failure. Prior to 1993, many 

researchers, including those at the FPL, used a 5-scale rating 

that does not exactly transfer to the ratings used since that 

time. Because of this discrepancy, the durability of FPL’s 

older “2 by 4” (38 by 89 mm) stake data will be presented 

in terms of percentage failures or average years to failure 

(Woodward et al. 2011). “Push” or “pull” tests have also 

been widely used for posts that are too large to remove from 

the ground. In a push or pull test, a set force is applied to the 

top of the post and it is given a “pass” rating if it does not 

snap at the groundline. Although “2 by 4” sized stakes were 

often used in the past, the most common specimen dimension over the last two decades has been the smaller 19- by 

19- by 457-mm (3⁄4- by 3⁄4- by 18-in.) stake. The smaller 

stakes typically fail more rapidly than larger stakes and are 

assumed to “accelerate” testing relative to specimens with 

larger dimensions or to in-service materials. The assumption of test acceleration has been used to justify shortening 

test durations in evaluating new preservatives to as little as 

3 years. However, the assumption is somewhat murky because the relationship between time to decay in “2 by 4” and 

19- by 19- by 457-mm (3⁄4- by 3⁄4- by 18-in.) stakes is highly 

variable. Deterioration is more rapid in warm moist climates 

than in cool or dry climates, and warmer climates also ensure termite activity, whereas termites may not be present in 

more northern locations. Soil properties also play a role with 

decay appearing to progress more rapidly in highly organic 

soils and more slowly in more compacted soils. The presence and extent of copper-tolerant fungi in the soil also varies within and between test sites.

Role of Solvent Characteristics in Durability

Many preservatives have been formulated for use as either 

an oil-based or water-based treatment. Differences in wood 

durability can be present depending on whether the preservative is carried in oil or water. Also, multiple types of oilbased solvents are used in wood preservative formulations, 

but for the purposes of this discussion we can group them 

into two categories: “light” and “heavy” oils. Light solvents 

(such as mineral spirits) have a lower viscosity and higher 

volatility. Heavier oils (such as diesel) have higher viscosity and lower volatility and remain in the treated product for 

many years. Preservative treatments in heavy solvents are 

usually more durable than those in light solvents because the 

solvent itself acts as a preservative. Diesel alone has been 

shown to extend the life of 2 by 4 stakes by many years 

(Woodward et al. 2011). When reviewing efficacy data between oil-based systems, it is important to consider the type 

of solvent used. Similarly, a preservative formulation applied with a heavy oil solvent will be more durable than one 

applied as a water-based formulation. There is less difference in durability between formulations applied using water 

or light solvent because the light solvent does vaporize from 

the wood. However, the light solvent treatment may have 

advantages in increased penetration and more uniform distribution of the active ingredient within the wood structure. 

This may be especially true for water-based treatments that 

are dispersions or emulsions rather than true solutions.

Corrosion Testing

Corrosion testing was often an afterthought in evaluating 

wood preservatives, but has grown in importance with the 

increased use of copper-based preservatives. AWPA has 

methods for evaluating corrosiveness of preservative treatment solutions to metal treating equipment (AWPA Method 

E17) and for evaluating the corrosiveness of the treated 

wood to metal fasteners (AWPA Method E12). In the treatment solution method, metal coupons (typically 25 by 51 

by 2 mm (1 by 2 by 1/16 in.)) are weighed and suspended 

in the preservative and agitated. The test is continued for 

25 days with frequent replacement of the treatment solution. 

At the end of the test, the coupon is cleaned and re-weighed 

to determine weight loss from corrosion. Water and a known 

preservative solution are evaluated for comparison. 

To evaluate the corrosiveness of the treated wood, metal 

coupons (typically 15 by 51 mm (1 by 2 in.)) are placed 

between two blocks of treated wood, and nylon bolts joining 

the two wood blocks are tightened to ensure good contact 

between the wood and metal. These assemblies are then 

placed into a room maintained at 122 °F (50 °C) and 90% 

RH for 366 to 720 h. Extent of corrosion (mils per year) is 

calculated by dividing the weight loss by the surface area, 

metal density, and by the duration of the test. Untreated 

wood and wood treated with a known preservative are tested 

for comparison. Both methods, and particularly the treated 

wood method, have been a source of concern because of the 

poor correlation to observed in-service corrosion. 
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

5

AWPA Use Category System

The type of preservative applied is often dependent on the 

requirements of the specific application. For example, direct 

contact with soil or water is considered a severe deterioration hazard and preservatives used in these applications 

must have a high degree of leach resistance and efficacy 

against a broad spectrum of organisms. These same preservatives may also be used at lower retentions to protect wood 

exposed in lower deterioration hazards, such as above the 

ground. The exposure is less severe for wood that is partially protected from the weather and preservatives that lack 

the permanence or toxicity to withstand continued exposure 

to precipitation, but may be effective in those applications. 

Other formulations may be so readily leachable that they 

can only be used indoors. 

To guide selection of the types of preservatives and loadings appropriate to a specific end-use, the AWPA developed 

Use Category System (UCS) standards. The UCS standards 

simplify the process of finding appropriate preservatives 

and preservative retentions for specific end uses. They 

categorize treated wood applications by the severity of the 

deterioration hazard, as well as the structural significance of 

the application. The lowest category, Use Category 1 (UC1) 

is for wood that is used in interior construction and kept 

dry, while UC2 is for interior wood, completely protected 

from the weather but occasionally damp. UC3 is for exterior 

wood used above ground, while UC4 is for wood used in 

ground contact in exterior applications. UC5 includes applications that place treated wood in contact with seawater and 

marine borers. Individual commodity specifications then list 

all the preservatives that are standardized for a specific Use 

Category, as well as the appropriate preservative retention.

One of the disadvantages of the Use Category System is 

that there is no longer a separate standard specifically for 

wood used in highway construction. Wood used in highway 

construction is typically considered to fall into UC4A, B, or 

C, depending on whether the wood is used above ground or 

in ground contact, as well as its structural significance. Because signage posts are placed into contact with the ground, 

but are not considered structurally critical (in comparison 

to guardrail posts, for example), they have typically been 

considered a UC4A application. In contrast, guardrail posts 

are classified as a UC4B application. For most preservative 

systems, the only difference between UC4A and UC4B is an 

increase in the required retention (Table 1). However, a few 

preservatives standardized for UC4A are not standardized 

for UC4B. 

Wood Preservative Characteristics

Role of Copper in Wood Preservatives

Copper has been used in wood preservatives for centuries 

and remains a common component in current formulations. 

It is effective against most types of decay fungi as well as 

Table 1. AWPA or ICC-ES-ESR retentions (kg/m3) for preservatives used in ground contact. Retentions are 

for Southern Pine and red pine species; retentions for other softwoods are generally similar, but not all 

softwoods are listed for all preservatives. NL is not listed. 

Preservative 

Listed in 

AASHTO

M 133 

Type of commodity and AWPA Use Categorya

Sawn lumber/Timbers Posts Utility poles Piling 

UC4A UC4B UC4C UC4A UC4B UC4A UC4B UC4C UC4C 

Water-based 

ACC X 8.0 NL NL 8.0 NL NL NL NL NL 

ACQ-A 6.4 NL NL NL NL NL NL NL NL 

ACQ-B X 6.4 9.6 9.6 6.4 8.0 9.6 9.6 9.6 NL 

ACQ-C X 6.4 9.6 9.6 6.4 8.0 NL NL NL 12.8 

ACQ-D X 6.4 9.6 9.6 6.4 8.0 NL NL NL NL 

ACZA X 6.4 9.6 9.6 6.4 8.0 9.6 9.6 9.6 12.8 

CA-B X 3.3 5.0 5.0 3.3 4.0 5.0 5.0 5.0 6.6 

CA-C X 2.4 5.0 5.0 2.4 4.0 5.0 5.0 5.0 6.6 

CCA-C X 6.4 9.6 9.6 6.4 8.0 9.6 9.6 9.6 12.8 

CDDC 3.2 NL NL NL NL NL NL NL NL 

CuN-W (copper naph) 1.76 NL NL 1.76 NL NL NL NL NL 

KDS 7.5 NL NL 7.5 NL NL NL NL NL 

ESR-1721 uCA-B X 2.4 3.7 5.3 2.4 3.7 2.4 3.7 5.3 5.3 

ESR-1721 uCA-C X 2.4 3.7 5.3 2.4 3.7 2.4 3.7 5.3 5.3 

ESR-1980 X 5.4 9.6 NL 5.4 9.6 5.4 9.6 NL NL 

ESR-2240 X 2.4 3.7 NL 2.4 3.7 2.4 3.7 NL NL 

Oil-based 

Creosote X 160 160 192 128 160 96 120 144 192 

PCP (pentachlorophenol) X 8.0 8.0 8.0 6.4 8.0 4.8 6.1 7.2 9.6 

CuN (copper naph) X 0.96 1.2 1.2 0.88 1.1 0.96 1.28 2.1 1.6 a

ICC-ES preservatives are not specifically listed by use categories. Listings in this table are based on example applications provided in ESR 

reports. 
General Technical Report FPL–GTR–231

6

major insect pests. Unlike carbon-based preservatives, copper is not biodegraded and retains its efficacy for long periods. However, copper does have some disadvantages and 

limitations. Although it is not biodegraded, it is somewhat 

leachable and thus the reservoir of available preservative 

is slowly depleted over time. The importance of leaching 

in long-term efficacy depends on the copper concentration, 

preservative formulation, and distribution of copper within 

the treated material. Copper is also not effective against all 

types of fungi. Some mold/stain fungi can grow on coppertreated wood, and certain types of decay fungi are classified 

as “copper-tolerant.” These fungi can sporadically cause 

severe and rapid damage in wood treated with copper, 

and thus commercial copper-based preservatives typically 

include a co-biocide (i.e., arsenic, quaternary ammonium 

compounds, triazoles, naphthenic acids) to provide additional protection. A third and highly relevant limitation of 

copper-based preservatives is their potential corrosiveness 

to various metals, including (in some cases) aluminum. The 

extent of corrosion is dependent on the preservative formulation and the resulting concentration of free, water-soluble 

copper ions. Typically, water-based formulations are more 

corrosive than oil-based formulations, and water-based formulations without chromium are more corrosive than those 

with chromium. Other additives (such as borates) are also 

used to lessen corrosion. 

Historically, most copper-based wood preservatives have 

used copper that is solubilized in water, although there are 

two notable exceptions (copper naphthenate and copper8-quinolinolate). Copper metal and copper oxides and 

carbonates have relatively low water solubility, but copper 

solubility can be greatly increased with use of acidic or 

alkaline solutions. Soluble copper salts such as copper sulfate have also been used, but these formulations tend to be 

more corrosive and leachable because more copper remains 

soluble within the treated wood. As a result, current commercial formulations use either acidic solutions (chromated 

copper arsenate, acid copper chromate) or alkaline solutions that rely on either ammonia (ACQ type B, ACZA) or 

ethanolamine (i.e., ACQ types A, D, and C, copper azole, 

KDS, water-based copper naphthenate) compounds to solubilize the copper. (Note: that A, B, C, and D designations for 

ACQ refer to the order in which they were listed in AWPA 

Standards). Once the wood has been treated and allowed to 

dry, the copper becomes much less soluble because the pH 

becomes more neutral and because a portion of the copper 

undergoes ion-exchange reactions with the wood substrate. 

In the case of the ammoniacal formulations, the volatilization of the ammonia during drying also results in copper 

precipitates within the wood structure. It is important to 

note, however, that small amounts of copper remain slightly 

soluble and are gradually solubilized over time. Although 

relatively slight, this solubility allows copper to leach from 

the wood and also to potentially corrode metal in contact 

with the wood. The chromium in the acidic chromated 

copper arsenate and acid copper chromate formulations 

helps to mitigate this corrosion, but wood treated with the 

alkaline formulations remains somewhat more corrosive 

than untreated wood. Because the treatment solutions contain a much greater concentration of soluble copper ions 

than the treated wood, chromium-free copper solutions tend 

to be substantially more corrosive than the wood itself. This 

effect can be observed when fasteners are installed in pressure-treated wood before it has dried following treatment. 

In recent years, a different approach to formulating copperbased preservatives has become widely used. In these 

formulations, the copper is not solubilized. Instead, copper 

carbonate is mechanically milled to very small particles 

(generally less than 1 micron in diameter) and suspended in 

the treating solution. These particulate formulations appear 

to have several advantages, including potentially less corrosiveness because of lower concentrations of soluble copper ions in the treatment solution and treated wood. Copper 

in these formulations is also reported to be less leachable 

from the treated product, and cost savings are achieved by 

not depending on the relatively expensive ethanolamine 

used in the soluble copper preservatives. Wood treated with 

the particulate copper formulations also has less coloration 

than wood treated with soluble copper formulations, but 

this characteristic may not be of importance in treating sign 

posts. A potential disadvantage of the particulate copper 

preservatives is that they may not penetrate as deeply or uniformly into the wood substrate as the soluble copper preservatives. They are primarily used for treatment of Southern 

Pine species, which are readily penetrated with preservatives 

and have pore sizes that allow the particles to move into the 

wood. Currently, none of these particulate copper formulations have been evaluated or standardized by the AWPA. 

However, several formulations do have ICC-ES Evaluation 

Reports and have also been included in AASTHTO M 133.

Current Ground-Contact Preservatives 

A number of preservatives are currently listed for treatment of wood to be used in contact with the ground, either 

through AWPA standards or ICC-ES evaluation reports 

(Table 1). Most (but not all) of these preservatives are also 

listed in AASHTO M 133. The preservative retentions vary 

by Use Category, and type of commodity. In this section, the 

properties of the preservatives listed in Table 1 are summarized below in alphabetical order.

Acid Copper Chromate (ACC) 

ACC is an acidic water-based preservative that has been 

used in Europe and the United States since the 1920s. ACC 

contains 31.8% copper oxide and 68.2% chromium trioxide. 

The treated wood has a light greenish-brown color and little 

noticeable odor. ACC is applied by pressure treatment, but 

current use is largely limited to wood used in cooling towers. ACC is standardized by the AWPA at retentions of 

4.0 kg/m3 (0.25 lb/ft3) for above ground use and 8.0 kg/m3
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

7

(0.5 lb/ft3) in ground contact. Tests on stakes exposed to decay and termite attack indicate that wood well-impregnated 

with ACC gives acceptable service (Woodward et al. 2011) 

(Fig. 1). However, it may be susceptible to attack by some 

species of copper-tolerant fungi, and because of this its use 

is sometimes limited to above-ground applications. It may 

be difficult to obtain adequate penetration of ACC in some 

of the more difficult-to-treat wood species such as spruce or 

Douglas-fir. This is because ACC must be used at relatively 

low treating temperatures and because rapid reactions of 

chromium in the wood can hinder further penetration during longer pressure periods. The high chromium content of 

ACC, however, has the benefit of preventing much of the 

corrosion that might otherwise occur with an acidic copper 

preservative. The treatment solution does use hexavalent 

chromium, but the chromium is converted to the more benign trivalent state during treatment and subsequent storage 

of the wood. Availability of facilities pressure-treating with 

ACC is currently limited.

Ammoniacal Copper Quat (ACQ-B)

ACQ formulations combine copper and quaternary ammonium compounds (quats) to protect wood from both fungal 

and insect attack. ACQ-B (Akaline copper quat, Type B) is 

the earliest ACQ formulation standardized and commercialized. Unlike the other ACQ formulations, it relies primarily on ammonium hydroxide to solubilize the copper. The 

actives composition of ACQ-B is 66.7% CuO and 33.3% 

didecyl dimethyl ammonium chloride/carbonate (DDAC). 

ACQ-B is currently standardized by the American Wood 

Protection Association (AWPA) at retentions ranging from 

4.0 kg/m3 (0.25 lb/ft3) for above-ground use to 9.6 kg/m3

(0.6 lb/ft3) for critical ground-contact applications. ACQ-B 

treated wood has a dark greenish-brown color that fades to a 

lighter brown and may have a slight ammonia odor until the 

wood dries. It is used primarily in the western wood United 

States because the ammonia helps the preservative penetrate 

into more difficult to treat wood species such as Douglas-fir. 

Like many other soluble copper preservatives, ACQ-B solution and to some extent the treated wood can be expected 

to increase corrosion of aluminum signs and other metal 

components.

Alkaline copper quat, (ACQ Types A, D and C and 

ESR-1980)

ACQ Types A, D, and C use ethanolamine to solubilize 

the copper. Wood treated with copper ethanolamine tends 

to have less odor and a more uniform surface appearance 

than that treated with copper in ammonia, and thus is more 

widely used for easily treated species such as Southern Pine. 

ACQ-A has a higher DDAC concentration (actives ratio of 

50% DDAC, 50% CuO). ACQ-C has a different form of 

quaternary ammonium compound (alkylbenzyl dimethyl 

ammonium compound) with an actives ratio of 66.7% CuO 

and 33.3% quat. ACQ-D is the most commonly used formulation in the eastern United States. It is similar to ACQ-B, 

with an actives ratio of 66.7% CuO and 33.3% DDAC, but 

differs in the use of ethanolamine to solubilize the copper 

rather than ammonia. ACQ-A, C, and D are standardized by 

the AWPA at retentions ranging from 2.4 kg/m3 (0.15 lb/ft3) 

for above-ground use to 12.8 kg/m3 (0.8 lb/ft3) for terrestrial 

piles. Exposure data indicate that the ethanolamine formulation of ACQ-D may not be as effective as the ammoniacal 

ACQ-B formulation at low concentrations, but is similarly 

effective at higher concentrations (Fig. 2). However, compatibility with aluminum remains a concern. 

ESR-1980 is a particulate copper formulation that is produced in copper/quat ratios of 1:1 and 2:1 (analogous to 

the actives ratios in ACQ-A and ACQ-D). ESR-1980 has 

not been standardized by the AWPA, but was evaluated by 

the International Code Commission Evaluation Service 

(ICC-ES) and is included in AASHTO M 133. Its retention 

requirements range from 1.6 kg/m3 (0.10 lb/ft3 ) for certain 

above-ground applications to 9.6 kg/m3 (0.6 lb/ft3) for critical ground-contact applications. Product literature indicates 

that ESR-1980 may be less corrosive to aluminum and other 

metals than the soluble-copper formulations of ACQ. As 

with other particulate copper formulations, this formulation 

is primarily used to treat more permeable pine species. 

Ammoniacal Copper Zinc Arsenate (ACZA, 

Previously ACA)

ACZA is a refinement of the preservative ammoniacal copper arsenate (ACA) that had been in commercial use since 

the 1930s. In ACZA, a portion of the arsenic is replaced 

with zinc, yielding an actives ratio of copper oxide (50%), 

zinc oxide (25%), and arsenic pentoxide (25%). ACZA is 

standardized by the AWPA for a wide range of applications 

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Stakes remaining (%) 

Acid copper chromate retention (kg/m3)

40 years, MS

40 years, LA

60 years, MS

Figure 1. Durability of ACC-treated 38- by 89-mm (2- by 4-in. 

nominal) southern pine stakes exposed in Mississippi or 

Louisiana for 40 or 60 years.
General Technical Report FPL–GTR–231

8

at retentions from 4.0 kg/m3 (0.25 lb/ft 3) for above-ground 

use to 40 kg/m3 (2.5 lb/ft3) for marine piles. It is an effective 

preservative (Fig. 3, Table 2) and is currently used to treat 

poles, piles, timbers, and other “industrial” type products 

that need a long service life. ACZA uses ammonia to solubilize the copper, and the wood may have a slight ammonia 

odor until it is thoroughly dried after treatment. The ammonia in the treating solution, in combination with processing 

techniques such as steaming and extended pressure periods, 

allow ACZA to obtain better penetration of difficult to treat 

wood species than many other water-based wood preservatives. Treating facilities using ACZA are primarily located in 

western United States, where many of the native tree species 

are difficult to treat with other waterborne preservatives. As 

with many of the other copper-based preservatives, compatibility with aluminum is a concern. ACZA is classified as a 

RUP by the EPA. 

Chromated Copper Arsenate (CCA)

CCA is a water-based preservative that has been used for 

pressure treatment since the 1940s and was the predominant 

preservative in the United States from the 1970s through 

2003. Since 2003, its use has been limited to non-residential 

applications. Three formulations (CCA-A, CCA-B, and 

CCA-C) have been widely used in the past, but the CCA-C 

formulation has been the dominant formulation for at least 

two decades because of its combination of efficacy and resistance to leaching. CCA-C has an actives ratio of 47.5% 

chromium trioxide, 18.5% copper oxide, and 34.0% arsenic 

pentoxide. CCA-C is standardized by the AWPA at retentions ranging from 4.0 kg/m3 (0.25 lb/ft3) for above-ground 

use to 40 kg/m3 (2.5 lb/ft3) for marine piles. In addition to 

being the most common treatment for wooden sign posts, 

it is still widely used for treatment of poles, piles, and timbers. CCA has decades of proven performance in field trials 

(Woodward et al. 2011) (Fig. 4) and in-service applications. 

In accelerated testing, CCA remains the “gold standard” 

reference preservative used to evaluate the performance of 

other waterborne wood preservatives. Like ACC, CCA may 

have difficulty penetrating difficult to treat wood species 

such as Douglas-fir or larch. CCA treatment solution also 

contains hexavalent chromium, although it rapidly converts 

to trivalent chromium in the treated wood. Because of the 

chromium, CCA treating solution and treated wood is less 

corrosive than many of the other copper-based waterborne 

preservatives. CCA is classified as a RUP by the EPA. 

Coal-Tar Creosote

Coal-tar creosote is the oldest wood preservative still in 

commercial use and remains the primary preservative used 

to protect wood used in railroad construction. It is made by 

distilling the coal tar that is obtained after high-temperature 

carbonization of coal. Unlike the other oil-type preservatives, creosote is not usually dissolved in oil, but it does 

have properties that make it look and feel oily. Creosote 

contains a chemically complex mixture of organic molecules, most of which are polycyclic aromatic hydrocarbons (PAHs). The composition of creosote depends on the 

method of distillation and is somewhat variable. However, 

the small differences in composition within modern creosotes do not significantly affect its performance as a wood 

preservative. Creosote is standardized at retentions ranging 

from 128–192 kg/m3 (8–12 lb/ft3) for above ground and 

ground-contact applications. The efficacy of creosote has 

been well-established through in-service performance and 

field tests (Table 2). Creosote-treated posts installed at FPL’s 

test in in southern Mississippi had estimated 54 years to 

5

6

7

8

9

10

0 50 100 150

Average stake rating

Test (months)

ACQ-B, 0.6%

ACQ-B, 1.2%

ACQ-B, 2.4%

ACQ-D, 0.6%

ACQ-D, 1.2%

ACQ-D, 2.4%

Figure 2. Ratings of 19- by 19-mm (3⁄4- by 3⁄4-in.) Southern 

Pine stakes treated with varying solution strengths of 

ACQ-B or ACQ-D and exposed in southern Mississippi.

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16

Stakes remaining (%)

ACA or ACZA retention (kg/m3)

ACZA, 30 years

ACA, 40 years

ACA, 60 years

Figure 3. Durability of 2 by 4 (nominal) Southern Pine 

stakes treated with ACA or ACZA and exposed in Mississipi. Soild black lines on x-axis indicate range of AWPA 

standard retentions for wood to be used in ground contact.
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

9

failure despite their relatively low retentions (Freeman et al. 

2006). Although an increase in durability is not necessarily 

directly proportional to an increase in retention, the longevity of these posts would have been expected to be much 

greater if treated to current AWPA standards. The durability 

of creosote-treated posts in southern Mississippi is slightly 

below that reported for posts exposed in South Carolina. 

In that study, posts treated with creosote retentions ranging 

from 64 to 128 kg/m3 (4–8 lb/ft3) had approximately 30% 

failures after 50 years of exposure (Webb et al. 2010). Much 

greater durability was also reported for creosote-treated 

posts exposed near Corvallis, Oregon (Morrell et al. 1999) 

or Ontario, Canada (Morris and Ingram 2010). The greater 

durability reported for posts exposed in Oregon and Canada 

may reflect the lower decay hazard in cooler northern climates. The Oregon test location falls into AWPA Hazard 

Zone 3 (Intermediate), while the more northerly Ontario 

site is in Hazard Zone 2 (Moderate). In contrast, southern 

Mississippi falls into AWPA Hazard Zone 5 (Severe). Wisconsin is in AWPA Hazard Zone 2, suggesting that the durability of creosote-treated posts in Wisconsin would be more 

similar to that in Oregon or southern Canada. 

Creosote-treated wood has a dark-brown to black color and 

a noticeable odor, which some people consider unpleasant. 

Workers sometimes object to creosote-treated wood because 

it soils their clothes and photosensitizes the skin upon contact. The treated wood sometimes also has an oily surface, 

and patches of creosote sometimes accumulate, creating a 

skin contact hazard. However, creosote-treated wood has 

advantages to offset concerns with its appearance and odor. 

It has lengthy record of satisfactory use in a wide range of 

applications and a relatively low cost. Creosote is also effective in protecting both hardwoods and softwoods, and 

is often thought to improve the dimensional stability of the 

treated wood. With the use of heated solutions and lengthy 

pressure periods, creosote can be fairly effective at penetrating even fairly difficult to treat wood species. Creosote 

treatment also does not accelerate, and may even inhibit, 

the rate of corrosion of metal fasteners relative to untreated 

wood. Three formulations of creosote are listed in AWPA 

Standards. CR is straight coal tar distillate, CR-S may be a 

mixture of coal tar and coal tar distillate, and CR-PS may 

contain up to 50% petroleum solvent. The retentions in 

Table 1 are based on straight-run creosote (CR), but in most 

cases CR-S and CR-PS are standardized at the same retentions. Creosote is a classified as a RUP by the EPA. 

Copper Azole (CA-B and CA-C, ESR-1721, 

ESR-2240)

Copper azole relies primarily on the preservative properties 

of copper but also has small amounts of azole fungicides to 

protect from attack by copper-tolerant fungi. In the “traditional” copper azole formulations, which are standardized 

by the AWPA, the copper is solubilized in ethanolamine in a 

manner similar to ACQ Types A, C, and D. CA-B has an actives ratio of 96.1% Cu and 3.9% tebuconazole, while CA-C 

has an actives ratio of 96.1% Cu, 1.95% tebuconazole, and 

1.95% propiconazole. AWPA standardized retentions for 

copper azole range from 0.96 kg/m3 (0.06 lb/ft3) for 

Table 2. Estimated years to failure for Southern Pine posts in Mississippi (25 replicates per 

treatment group)a

Preservative 

Retention

(kg/m3

)

AWPA retentionb

(%)

Failed 

(%)

Estimated years 

to failure 

Years to failure 

90% confidence limits 

Lower Upper 

Copper naph (oil) 0.48 44–55 46 65 55 78 

Coal-tar creosote 89.60 56–70 65 54 47 62 

Pentachlorophenol 5.12 64–80 29 74 60 91 

ACA (ACZA) 5.44 68–85 52 60 51 69 

Untreated 0 NA 100 2 2 3 

a

Adapted from Davidson (1977) and Freeman et al. (2006). 

b

These posts were treated to retentions well below current AWPA standards. Values in this column show the tested 

retention as a percentage of AWPA standard retention for posts (AWPA 2012).

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16

Stakes remaining (%)

CCA-A, B or C retention (kg/m3)

CCA-C, 40 years

CCA-A, 60 years

CCA-B, 61 years

Figure 4. Durability of CCA-treated 2- by 4-in. (nominal) 

Southern Pine stakes exposed for 40, 60, or 61 years in 

Mississippi. Solid black lines on x-axis indicate range of 

AWPA standard retentions for wood to be used in ground 

contact.
General Technical Report FPL–GTR–231

10

above-ground applications to 6.6 kg/m3 (0.41 lb/ft3) for terrestrial piles. As with other copper-ethanolamine formulations, compatibility with aluminum is a concern. Availability 

of wood treated with the copper-ethanolamine formulations 

has also become more limited because many treaters have 

converted to the analogous “particulate” formulations of 

copper azole. The “particulate” formulations of copper azole 

are listed by the International Code Commission under ESR1721. These formulations, which are some times referred 

to as μCA-B and μCA-C, have the same copper and azole 

compositions as the soluble copper formulations. A particulate formulation of CA-B is also listed under ESR-2240. The 

ESR retentions specified for the particulate formulations 

tend to be lower than those listed for the ethanolamine formulations in AWPA standards. The particulate copper azole 

formulations are expected to be less corrosive to aluminum 

than the ethanolamine formulations. Wood treated with particulate copper azole formulations is widely available.

Copper Naphthenate (CuN)

Copper naphthenate has been used as a wood preservative 

since the 1940s, although it is not as widely used as creosote, CCA, or pentachlorophenol. It is an organometallic 

compound formed as a reaction product of copper salts and 

petroleum-derived naphthenic acids. In recent years, it has 

been increasingly used as an alternative to pentachlorophenol. Copper naphthenate has been primarily used as an oilbased formulation, but a water-based formulation (CuN-W) 

containing ethanolamine has also been standardized by the 

AWPA for pressure treatment. The heavy solvent formulation generally provides the greatest durability, and CuN 

in heavy solvent is currently used for pressure treatment 

of poles, timbers, and glulam beams. It is standardized by 

AWPA for retentions ranging from 0.64 kg/m3 (0.04 lb/ft3) 

for above-ground use to 1.92 kg/m3 (0.12 lb/ft3) for heavy 

duty ground-contact applications (retentions expressed as 

elemental copper). Although copper naphthenate does not 

have as extensive a history of in-service durability as CCA, 

creosote, or pentachlorophenol, its efficacy has been demonstrated in field tests. Copper naphthenate posts exposed in 

southern Mississippi had expected service lives of 65 years 

despite being treated to only about half the current AWPA 

retention for posts (Freeman et al. 2006) (Table 2). The 

durability of copper-naphthenate treated posts in the Mississippi study is slightly less than that reported by Morris and 

Ingram (2010), who noted only one failure after 58 years 

for posts exposed near Ontario, Canada. Copper naphthenate is also dissolved in light solvent for pressure treatment 

of above-ground members (such as glulam beams) and for 

brush-on application to untreated wood exposed when cutting pressure-treated wood. Most commercial pressure-treating facilities are using copper naphthenate in heavy solvent. 

It is also used for non-pressure steeping (extended dipping) 

treatment of round fence posts in some western states. 

The CuN-W standardized retentions range from 1.1 kg/m3

(0.07 lb/ft3) for above-ground use to 1.76 kg/m3 (0.11 lb/ft3) 

for ground-contact applications. FPL researchers compared 

the durability of stakes treated with oil and waterborne formulations of copper naphthenate and found that the oilborne 

formulations were more durable at lower concentrations 

(Fig. 5). However, at the highest concentration evaluated 

(1.2% copper) both formulations were highly effective. 

Waterborne copper naphthenate does contain soluble copper 

ions and might be expected to contribute somewhat to corrosion of aluminum. Waterborne copper naphthenate has less 

obvious odor than the oil-borne formulation, but the odor 

is noticeable until the wood dries. There are no or limited 

pressure-treatment facilities currently using CUN-W.

KDS

KDS is another pressure-treatment preservative formulation 

that uses copper solubilized with ethanolamine along with a 

co-biocide (in this case polymeric betaine). It is produced in 

two formulations: the KDS formulation also contains boron, 

and has an actives composition of 47% copper oxide, 23% 

polymeric betaine, and 30% boric acid. KDS–B does not 

contain boron (nomenclature is non-intuitive here) and has 

an actives composition of 68% copper oxide and 32% polymeric betaine. KDS is standardized by AWPA for treatment 

of commodities used above ground and for general use in 

contact with soil at retentions ranging from 3.04–7.52 kg/m3

(0.19–0.47 lb/ft3). AWPA standards do not list KDS or KDSB for severe exposures or critical applications, but they 

are listed for these uses under ICC-ESR 2500. The soluble 

0

1

2

3

4

5

6

7

8

9

10

0 50 100 150

Average stake rating

Test (months)

CuN-W, 0.04%

CuN-W, 0.08%

CuN-W, 0.3%

CuN-W, 1.2%

CuN-O, 0.04%

CuN-O, 0.08%

CuN-O, 0.3%

CuN-O, 1.2%

Figure 5. Durability of 19- by 19-mm (3⁄4- by 3⁄4-in.) Southern 

Pine stakes treated with water or oilborne copper 

naphthenate and exposed in southern Mississippi.
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

11

corrode; the notable exception is stainless steel. The corrosion mechanism involves diffusion of the cupric ions to 

the metal surface, where they are reduced as the metal is 

oxidized. This mechanism is thermodynamically favorable 

and will occur. What is important is the rate-limiting step 

that controls the kinetics and thus how rapidly the metal will 

corrode. In theory, the rate-limiting step could be the diffusion of ions to the metal surface or the oxidation/reduction 

reaction at the metal surface. Several different experiments 

have demonstrated that the rate-limiting step is not diffusion but rather the reaction at the metal surface (Baker 1992; 

Zelinka and Rammer 2009; Zelinka and Stone 2011). This 

has large implications for corrosion as it implies that the 

corrosion rate will not decrease with time as it does in atmospheric corrosion (Zelinka et al. 2011). 

It is important to emphasize that the corrosion of metals 

in contact with treated wood is much different than atmospheric corrosion. Although the wood may be exposed to the 

atmosphere, corrosion of embedded fasteners is controlled 

by wood chemistry and moisture. This difference in environment has two important implications: (1) zinc-galvanization 

is ineffective, and (2) the corrosion rate does not decrease 

with time. Since these points seem counterintuitive, they 

will be expanded upon in the next two paragraphs.

In atmospheric corrosion, zinc oxidizes to form hydrozincite 

(Zn5(CO3)2(OH)6) and smithsonite (ZnCO3), which passivate the zinc surface; that is, these oxidized species protect 

the metal from further corrosion. Conversely, steel forms 

goethite (α-FeOOH), also called “red-rust” in atmospheric 

conditions. 

Kinetically, hydrozincite and smithosnite are better at protecting the underlying metal than goethite; that is why zinc 

corrodes more slowly than steel in atmospheric conditions 

(Zhang 2003). It is known that in certain environments, such 

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12

Stakes remaining (%)

Pentachlorophenol retention (kg/m3)

39 years, heavy oil

45 years, heavy oil

39 years, light oil

Figure 6. Durability of 2- by 4-in. (nominal) Southern Pine 

stakes treated with pentachlorophenol and exposed in 

Mississippi. Solid black lines on x-axis indicate range of 

AWPA standard retentions for wood to be used in ground 

contact.

copper used in KDS might be expected to increase the risk 

of corrosion of aluminum. Although availability of KDS is 

somewhat limited, a few treating plants are in the United 

States using this preservative.

Pentachlorophenol

Pentachlorophenol has been widely used as a pressure treatment since the 1940s. The active ingredients, chlorinated 

phenols, are crystalline solids that can be dissolved in different types of organic solvents. The performance of pentachlorophenol and the properties of the treated wood are influenced by the properties of the solvent. The heavy oil solvent 

is generally used when the treated wood is to be used in 

ground contact because wood treated with lighter solvents 

is slightly less durable in such exposures. Wood treated with 

pentachlorophenol in heavy oil typically has a brown color 

and may have a slightly oily surface that is difficult to paint. 

It also has some odor, which is associated with the solvent. 

Like creosote, it is effective in protecting both hardwoods 

and softwoods and is often thought to improve the dimensional stability of the treated wood. Pentachlorophenol in 

heavy oil has long been a popular choice for treatment of 

utility poles, bridge timbers, glulam beams, and foundation 

piling. The treated wood is quite durable, as shown in 

Table 2 (Freeman et al. 2006) and Figure 6. Lighter solvents 

are often used for treatment of wood in above-ground applications, but pentachlorophenol-light solvent treatments also 

provide substantial durability at higher retentions (Fig. 6). 

Lighter solvents provide the advantage of a less oily surface, 

lighter color, and less odor. However, one disadvantage of 

the lighter oil is that less water repellency is imparted to the 

wood. With the use of heated solutions and extended pressure periods, pentachlorophenol is fairly effective at penetrating difficult to treat species. Pentachlorophenol treatment 

does not accelerate corrosion relative to untreated wood. 

Pentachlorophenol is classified as a RUP by the EPA.

Corrosion Aspects of Selecting 

Preservatives for Sign Posts

Mechanism of Corrosion in Treated Wood

Corrosion is a redox reaction, where a metal is oxidized 

(loses electrons) and another species in the solution or 

atmosphere is reduced (gains electrons). Corrosion is nearly 

always thermodynamically favorable; what is more important is the kinetics of the reaction. Put more simply, corrosion will always happen, but it can be slowed to where it is 

not important. 

Zelinka and Stone (2011) have demonstrated that the corrosion mechanism in treated wood involves the reduction of 

free cupric ions in the wood preservative. The mechanism 

is illustrated in Figure 7. Importantly, this mechanism is 

based upon (1) copper-based wood preservatives and (2) the 

metal fastener being anodic to copper on the galvanic series. 

Most construction metals are anodic to copper and will 
General Technical Report FPL–GTR–231

12

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++ Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Cu++

Fe Cupric ions ++

Iron ions

Cu++

Fe++

Electron transfer

as immersion in saltwater (Zhang et al. 1998) or in environments with volatile acetic and formic acids (KnotkovaCermakova and Vlckova 1971), different corrosion products 

form and zinc corrodes more rapidly than steel. In addition 

to measuring the corrosion rates, Zelinka et al. (2010) examined the corrosion products on fasteners removed from steel 

and galvanized steel fasteners in wood using X-ray diffraction and did not observe smithsonite on the zinc fasteners. 

Instead, they observed hydrozincite, namuwite (Zn2(SO4)

(OH)6·4H2O), and simonkolleite (Zn5(OH)8Cl2·(H2O)), 

which are consistent with the observed relative corrosion 

rates.

During atmospheric corrosion, in many cases the corrosion 

rate decreases with time because of passivation. Empirically, 

the kinetics are frequently described by

(1)

where ΔW is the change in weight, K is a constant (the 

1-year corrosion rate), t is the time in years, and n is an 

exponent that controls the kinetics and describes passivation (Legault and Preban 1975). Theoretically, n should be 

bounded by 0.5, which represents complete diffusion control, and 1, which represents “activation control,” where the 

rate-determining step is the electron transfer reaction at the 

metal surface. In wood, n = 1 behavior has been observed by 

several researchers, which unfortunately represents a worst 

case scenario (Baker 1992; Zelinka and Rammer 2009; Zelinka and Stone 2011). 

Review of Corrosion Data from New Wood 

Preservatives

Since the 2004 change in wood preservative regulation, 

there have been several investigations of the corrosiveness 

of metals in contact with treated wood. The studies cover a 

wide range of preservatives, preservative retentions, wood 

moisture contents, and metals tested. Not surprisingly, a 

wide variation in corrosion rates have been reported (e.g., 

from 2–113 μm/y for galvanized steel in ACQ-treated 

wood). Because of the wide variations of test conditions 

used, it is best to only compare results within a single study 

or across different studies with very similar conditions.

Many of the design recommendations for materials selection in wood are based off of the recommendations of Baker 

(1992), who conducted a 17-year investigation of corrosion of metal fasteners embedded into chromated copper 

arsenate- (CCA-) and ammoniacal copper arsenate- (ACA-) 

treated wood and exposed either underground or in a room 

maintained near 100% RH. From these data, Baker concluded that at a minimum, hot-dip galvanized fasteners should 

be used in treated wood and cautioned against the use of 

aluminum fasteners. Importantly, Baker presented the corrosion data as a percentage of weight loss instead of a true 

corrosion rate because he could not calculate the surface 

area of threaded fasteners. Zelinka and Rammer reanalyzed 

Baker’s data using data in Baker’s laboratory notebook and 

an algorithm they had developed to measure the surface area 

of threaded fasteners (Baker 1992; Rammer and Zelinka 

2008; Rammer and Zelinka 2010). They found that when 

Figure 7. Mechanism of corrosion in 

treated wood illustrating the transport 

of cupric ions to the metal surface 

where they are reduced as the 

fastener is oxidized.

ΔW = Kt 

n
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

13

the corrosion rate was adjusted to a true corrosion rate, the 

corrosion rate of aluminum was in fact less than hot-dip 

galvanized steel. Baker (1992) also reported that pitting was 

observed on the aluminum fasteners. However, Zelinka and 

Rammer could not determine any evidence of pitting corrosion in similar exposure tests conducted for one year, nor 

could they find any evidence of pitting corrosion in photographs in Baker’s laboratory notebook.

Zelinka has published results of several different corrosion 

tests that were conducted at 27 °C (80 °F) and 100% RH 

(Zelinka 2007; Zelinka and Rammer 2009; Zelinka et al. 

2010; Zelinka and Stone 2011). In one study, Zelinka and 

Rammer (2009) examined the corrosion of five different 

metal fasteners (carbon steel, hot-dip galvanized steel, electroplated galvanized steel, aluminum, and stainless steel) 

embedded in wood treated with ACQ to a retention of 

4 kg/m–3. They found that the corrosion rate of hot-dip 

galvanized steel (in μm/y) was the highest (62) followed 

by carbon steel (34), aluminum (22), and electroplated galvanized steel (21). The corrosion rate of stainless steel was 

statistically indistinguishable from zero. In another study 

(Zelinka et al. 2010), steel and hot-dip galvanized steel 

fasteners were exposed to six different wood treatments: 

chromated copper arsenate (CCA), alkaline copper quaternary (ACQ-D), copper azole (CuAz-B), micronized copper 

quaternary (MCQ), didecyldimethylammonium carbonate 

(DDAC, or the “quat” in ACQ and MCQ). For galvanized 

steel, ACQ was the most corrosive (32 μm/y), followed by 

CuAz (29), MCQ (19), CCA (16), DDAC (5.5), and untreated (4.4). For carbon steel, the order was slightly different: 

ACQ (17), MCQ (13), CuAz (11), CCA (10), DDAC (1.9), 

and untreated (0.7).

Kear et al. (2009) examined the corrosion of three different metals (316 stainless steel, hot-dip galvanized steel, and 

plain carbon steel) in three different preservatives (CCA, 

ACQ, CuAz), treated to three different retention levels with 

four different test methodologies. The retention levels were 

specified in a New Zealand standard that specifies a mass 

basis (i.e., kg/kg) instead of a density basis and therefore 

cannot be directly compared with traditional U.S. retentions (in lb/ft3 or kg/m3). Differences in the corrosiveness 

of the preservatives were most apparent in a constant exposure to 90% relative humidity (RH). Corrosion rates were 

(CCA:CuAz:ACQ) 4:14:45 μm/y–1 for mild steel fasteners 

and 5:16:26 μm/y for hot-dip galvanized steel fasteners. 

Average corrosion rates as high as 113 μm/y were observed 

for galvanized steel in ACQ treated wood in a “moisture 

saturated air” condition.

Simpson Strong Tie Corporation (Pleasanton, CA) published 

a technical bulletin on the results of in-house AWPA-E12 

(AWPA 2007) corrosion tests they had conducted (Simpson 

Strong-Tie 2008). In this test method, a metal plate is sandwiched between two blocks of wood and exposed to a high 

temperature (50 °C), high humidity (90% RH) environment. 

They found that the corrosiveness of ACQ-D (carbonate) 

was roughly equivalent to that of CuAz, and both of these 

were more than twice as corrosive as CCA-C. They also 

found that ACZA was more than three times more corrosive 

than CCA and that borates were less corrosive than CCA. A 

footnote in the table mentioned that for micronized formulations like MCQ the “relative corrosiveness is somewhat 

lower than ACQ-D.”

Copper naphthenate is sometimes used as a preservative in 

timber bridges. While there are no peer-reviewed data on the 

corrosiveness of copper naphthenate, Anthony Forest Products Company (El Dorado, AK) published a technical bulletin with the results of AWPA E12 tests that compares both 

the waterborne and oilborne formulations of copper naphthenate against ACQ-D and CCA-C for mild steel, hot-dip 

galvanized steel, aluminum, and red brass (Anthony Forest 

Products Company 2013). Although no data are given on the 

variability between replicates, the data clearly suggest that 

either formulation of copper naphthenate is much less corrosive than ACQ and possibly less corrosive than CCA. For 

mild steel, the corrosion rates (in mils (0.001 in.) per year) 

were 4.9 for ACQ, 1.0 for CCA, 0.17 for waterborne copper 

naphthenate, and 0.03 for the oilborne formulation. An even 

larger difference was observed for aluminum, where the 

corrosion rates were 7.3 for ACQ, 0.3 for CCA, 0.07 for waterborne copper naphthenate, and 0 for the oilborne formulation. Although this bulletin was published by a company 

that supplies products treated with copper napthenate, the 

data suggest that copper naphthenate is much less corrosive 

than ACQ and less than or equal in corrosiveness to CCA. 

Freeman and McIntyre (2008) summarized the results of 

several unpublished corrosion tests of pressure-treated 

wood in contact with different metals which compared traditional (ACQ and CuAz) against their micronized formulations (MCQ and mCuAz). Most of the data shown was from 

AWPA E-12 tests. The results are inconclusive. In one test, 

the micronized formulation of CuAz is less corrosive than 

the standard formulation for galvanized steel but more corrosive for plain carbon steel and the micronized formulation 

is much more corrosive to aluminum. The remainder of the 

E-12 data have similar trends; in some cases the micronized 

formulations show lower corrosiveness; in other cases, the 

traditional formulations appear less corrosive. The only results that consistently show that the micronized formulations 

are less corrosive than the traditional formulations are from 

a test conducted using a protocol from the International 

Staple and Nail Tool Association (ISANTA, La Grange, 

IL). In this test, the micronized formulations appear slightly 

(approximately 30%–40%) less corrosive, with the notable 

exception of aluminum, in which case the micronized formulations are more corrosive. 
General Technical Report FPL–GTR–231

14

Special Corrosion Considerations for Sign 

Posts

The purpose of this literature review is to develop recommendations to minimize corrosion in signs attached to 

treated wood posts. WisDOT noticed severe corrosion of 

aluminum signs attached to posts treated with alkaline copper quaternary (ACQ), which is detailed in Final Report 

WI-06-04 (Wilson 2004). WisDOT currently attaches aluminum signs to CCA-treated posts with galvanized lag screws. 

Excessive corrosion was noted around the bolt hole; this 

suggests that a galvanic couple was occurring between the 

lag screw and the sign post that accelerated the corrosion of 

the aluminum sign post. Since the failure of the sign appears 

to be caused by galvanic corrosion, it is worthwhile to have 

a brief discussion of galvanic corrosion and discuss prevention strategies.

Galvanic corrosion happens when three conditions are satisfied: (1) two dissimilar metals (2) are placed in electrical 

contact (3) in the presence of an electrolyte. If any of the 

three conditions are not met (i.e., metals are not in electrical 

contact, metals are not in an electrolyte), galvanic corrosion 

cannot occur. In the case of the sign posts, the electrolyte is 

the wood, and the metals are in electrical contact because 

the galvanized bolt head is in contact with the aluminum 

sign post. When the conditions for galvanic corrosion are 

met, the more active metal (called the anode) will corrode 

more rapidly than it would otherwise corrode and the more 

noble metal (called the cathode) will corrode more slowly 

than it would otherwise corrode. Whether a metal is active 

or noble to another metal depends upon the thermodynamics 

of the oxidation reactions in that environment.

It is important to point out a couple of common misunderstandings of galvanic corrosion at this point. For instance, 

commonly it is assumed that only the anode (“sacrificial 

anode”) corrodes in a galvanic couple. This is not true; both 

metals corrode, but the cathode may corrode slowly enough 

that it does not fail in a normal service life. The second 

common misperception is that there is a single “galvanic 

series” that one can use to tell if a given metal will be active 

in a given environment. While galvanic series have been 

tabulated for seawater and other electrolytes (Matsukawa et 

al. 2011), the positions of metals can change dramatically 

depending on the environment. 

From the WisDOT final report (Wilson 2004), it is clear 

that galvanic corrosion is occurring and that in ACQ-treated 

wood, zinc is cathodic (more noble) to aluminum, and the 

aluminum signs failed because of galvanic corrosion near 

the fastener. If the galvanic corrosion could be stopped, it is 

possible that the signs would last much longer. As it is not 

possible to change the metals used or the electrolyte, the 

only way to prevent galvanic corrosion in this situation is 

to electrically isolate the bolt from the aluminum sign. 

This should be possible by using a nonconductive washer 

between the head of the lag screw and the wooden post 

(Fig. 8).

Corrosion Recommendations

•	 The corrosion of metals in contact with wood is different 

from atmospheric corrosion.

•	 The corrosion is extremely sensitive to the wood moisture content. Preventing the wood from getting wet, if 

possible, is the easiest way to prevent corrosion. 

•	 Existing corrosion data are somewhat dependent on how 

the tests were conducted. Despite this, it appears that the 

relative corrosiveness of different preservatives appears 

to be as follows:

 oil CuN ≤ water CuN ˂˂ CCA < MCQ, mCuAz ≈ ACQ, 

 CuAz ˂˂ ACZA

•	 There appears to be a galvanic couple between the zinc 

galvanized lag screw and the aluminum sign when they 

are in contact with treated wood. The zinc and aluminum should be separated by a dielectric to increase the 

service life. 

Summary of Preservatives’ Potential 

for Sign Post Treatments

In this section, the properties of the preservatives are summarized, and they are given a relative ranking regarding the 

suitability for sign posts. Only those preservatives judged to 

be effective in protecting wood used in ground contact are 

discussed. This review indicates that CCA, the preservative 

currently being used for sign post treatment, remains one 

of the best options. Another strong candidate is oil-based 

copper naphthenate, but the possibility of worker concerns 

Figure 8. Strategies to mitigate galvanic corrosion. A 

non-conductive washer between the head of the lag screw 

and the sign should eliminate electrical contact between 

the bolt and the sign (Better). If there is a tight clearance 

between the hole and the bolt, it might be necessary to 

coat the edge of the holes to prevent electrical contact 

(Best).
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

15

associated with the odor and the potentially oily surface may 

need to be assessed. 

Water-Based Preservatives

Water-based preservatives have the general advantage of 

leaving posts with relatively low odor and clean surface. As 

a group, they also tend to not be as effective in protecting 

hardwood species as the oil-based preservatives, although 

this limitation can be overcome through the use of higher retentions. Water-based preservatives may increase the likelihood of signpost warping as the wood dries after treatment.

CCA

CCA, the preservative currently used by the Wisconsin DOT 

and by other states, remains a strong candidate for treatment 

of sign posts. It has decades of proven efficacy, is readily 

available, and is compatible with aluminum. On the negative side, CCA does not penetrate refractory wood species as 

well as some of the other preservatives (such as ACZA and 

ACQ-B) that use ammonia and can be heated during treatment. CCA is also a RUP that contains arsenic. However, 

CCA was recently re-registered by the EPA, and current regulatory concerns appear to be focused on improving worker 

safety at pressure-treatment facilities. 

•	 Suitability for use with current pine species: High

•	 Suitability for use with less treatable softwoods: 

Medium-high

•	 Suitability for use with hardwoods: Medium

•	 Overall potential: High

Particulate Copper Preservatives (Micronized Copper 

Quat, Micronized Copper Azole)

These formulations have become widely available and have 

been used to some extent by Wisconsin DOT for treatment 

of sign posts. They have not been in use long enough to 

document their ability to provide long-term durability, but 

short-term stake testing indicates that potential. These formulations do not contain arsenic, chromium, or other RUPs, 

and thus may be less likely to be restricted by regulatory 

agencies. The use of the particulate rather than soluble copper decreases, but does not eliminate, concerns about corrosiveness toward aluminum, and additional steps may be 

needed to minimize corrosion. These formulations may have 

difficulty penetrating refractory wood species and there are 

little data on their ability to protect hardwoods. 

•	 Suitability for use with current pine species: Mediumhigh (corrosion concerns)

•	 Suitability for use with less treatable softwoods: Low 

(penetration problems)

•	 Suitability for use with hardwoods: Medium (little data, 

but probably similar to other water-based preservatives)

•	 Overall potential: Medium-high

Water-Based Copper Naphthenate

Water-based copper naphthenate has seen little commercial 

pressure-treatment use, but stake testing indicates that it is 

an effective preservative. There is relatively little information on its ability to penetrate refractory wood species. Water-based copper naphthenate solution is available for consumer purchase and application, and thus restriction from 

use as a pressure-treatment preservative does not appear 

likely. The compatibility of water-based copper naphthenate 

with aluminum has not been well documented, but corrosion 

is expected to be relatively low. A major disadvantage of 

water-based copper naphthenate is its lack of availability as 

a pressure-treatment preservative. 

•	 Suitability for use with current pine species: Medium 

(availability)

•	 Suitability for use with less treatable softwoods: Medium 

(availability)

•	 Suitability for use with hardwoods: Medium-low (efficacy, availability) 

•	 Overall potential: Medium 

ACZA

ACZA is an effective preservative with a long history of 

commercial use. Because of the ammonia used in the formulation, it is better able to penetrate refractory wood species 

than some other water-based preservatives and has been 

widely used for treatment of Douglas-fir. It has also been 

standardized by AWPA for treatment of hardwood railroad 

ties. However, like CCA it contains arsenic and is a RUP. 

It is less available than CCA, although it is being used by 

a treatment facility in Minnesota. A potential disadvantage 

of this preservative is corrosiveness to aluminum. Although 

several states do include ACZA in their sign post specifications, manufacturers do not recommend that aluminum 

be placed in direct contact with ACZA-treated wood. It is 

possible that states with ACZA in their sign-post specifications are either not actually using it or are using it with signs 

made with other materials.

•	 Suitability for use with current pine species: Mediumlow (corrosion concerns)

•	 Suitability for use with less treatable softwoods: 

Medium-low (corrosion concerns) 

•	 Suitability for use with hardwoods: Medium-low (corrosion concerns)

•	 Overall potential: Medium

Soluble Copper Preservatives (ACQ, Copper 

Azole, KDS, Copper-HDO)

ACQ has been used commercially for nearly two decades 

and has established an accompanying level of confidence 

in its ability to protect wood from decay. Copper azole has 
General Technical Report FPL–GTR–231

16

a slightly shorter history of use, but it and the other copperbased systems are likely to provide adequate protection at 

equivalent copper concentrations. These formulations do 

not contain arsenic and chromium, and may be less likely to 

face regulatory scrutiny than those preservatives classified 

as restricted-use pesticides. They are also relatively capable 

of penetrating a range of wood species, especially if ammonia is used in the formulation. Some of these preservatives 

(particularly ACQ) are commercially available. However, 

these preservatives can be damaging to aluminum signs, as 

demonstrated by prior WisDOT observations (Wilson 2004).

•	 Suitability for use with current pine species: Mediumlow (corrosion concerns)

•	 Suitability for use with less treatable softwoods: 

Medium-low (corrosion concerns)

•	 Suitability for use with hardwoods: Medium-low 

(efficacy, corrosion concerns)

•	 Overall potential: Medium

Acid Copper Chromate (ACC)

ACC has been a commercial wood preservative for decades, 

although not used as widely as other wood preservatives. 

Although wood treated with ACC is generally durable, studies indicate that it is vulnerable to the copper-tolerant fungi 

that occur in some locations. The practical impact of this 

vulnerability is unclear because the distribution and prevalence of these types of fungi is unknown. Although ACC 

does not contain arsenic, the treatment solution does contain 

hexavalent chromium, and the EPA has restricted its use to 

applications similar to CCA. Like CCA, ACC is expected 

to cause little corrosion of aluminum. A major limitation of 

ACC is its lack of availability. 

•	 Suitability for use with current pine species: Mediumlow (availability, efficacy)

•	 Suitability for use with less treatable softwoods: 

Medium-low (availability, efficacy)

•	 Suitability for use with hardwoods: Low (availability, 

efficacy)

•	 Overall potential: Medium 

Oil-Based Preservatives

Oil-based preservatives are less likely to cause warping of 

sign posts, and tend to be more compatible with hardwoods 

than the water-based preservatives. Their major disadvantages are odor and the potential for having an oily surface.

Copper Naphthenate (Oil-Based)

Although not as widely used as preservatives such as 

creosote, pentachlorophenol and CCA), oil-based copper 

naphthenate has a long history of commercial use (Fig. 9) 

and its efficacy has been demonstrated in long-term post 

tests. Although not as widely available as some other preservatives, there are to be pressure-treatment facilities in 

Michigan and South Dakota using oil-based copper naphthenate. Copper naphthenate solution is available for consumer 

application, and is unlikely to be restricted from pressuretreatment application. It is also compatible with aluminum. 

The primary disadvantages of copper naphthenate are its 

odor and the possibility of the treated wood having an oily 

surface.

•	 Suitability for use with current pine species: High

•	 Suitability for use with less treatable softwoods: 

Medium-high (lack of data on treatability)

•	 Suitability for use with hardwoods: High 

•	 Overall potential: Medium-high (overall rating reduced 

because of the potential for worker concerns with odor 

and oily surface)

Pentachlorophenol

Pentachlorophenol has a long history of use and proven 

efficacy as a wood preservative. Although not as widely 

available as some other preservatives, there are pressuretreatment facilities in Minnesota, Missouri, and Iowa using 

pentachlorophenol. Pentachlorophenol is classified as a RUP 

by EPA and has the associated safety and handling concerns. 

It is compatible with aluminum, but the treated wood has an 

odor and the potential for an oily surface.

•	 Suitability for use with current pine species: Mediumhigh (possible worker concerns)

•	 Suitability for use with less treatable softwoods: 

Medium-high (possible worker concerns)

•	 Suitability for use with hardwoods: Medium-high (possible worker concerns)

•	 Overall potential: Medium-high

Figure 9. Oil-based copper naphthenate is sometimes used 

for treatment of salt storage sheds.
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

17

Creosote

Creosote has a long history of use and is an effective preservative. It has been used more widely and with a broader 

range of wood species than any other preservative. There 

are currently fewer treating plants using creosote than other 

types of preservatives, but there is a facility producing creosote-treated wood in Wisconsin. Like the other oil-based 

preservatives, creosote is compatible with aluminum. Creosote is a restricted-use pesticide and has a strong odor. It is 

also a skin sensitizer and may elicit concerns from workers 

handling the treated wood.

•	 Suitability for use with current pine species: Medium 

(possible worker concerns)

•	 Suitability for use with less treatable softwoods: Medium 

(possible worker concerns)

•	 Suitability for use with hardwoods: Medium (possible 

worker concerns)

•	 Overall potential: Medium-high

Wood Species Options for 

Wisconsin DOT Signposts

Wood Species Currently Listed in WisDOT 

Signpost Specifications

Sign posts are specified in “Section 634, Wood and Tubular Steel Sign Posts.” That specification refers to “Section 

614.2.5, Wood Posts and Offset Blocks,” for a listing of 

wood species. The species listed are Southern Pine, red pine, 

ponderosa pine, jack pine, white pine, Hem-Fir, oak, Douglas-fir, western hemlock, and western larch. In this section 

we discuss the characteristics of these wood species (or species groups) as they relate to use for Wisconsin sign posts. 

Southern Pine

Southern Pine is currently used by the Wisconsin DOT for 

sign posts. Southern Pine is a grouping of wood species 

grown in the southeastern United States and includes loblolly (Pinus taeda), longleaf (P. paulstris), shortleaf (P. echinata), and slash pine (P. elliottii). Southern Pine wood species 

are commonly used for preservative treatment because they 

are relatively strong (among softwoods) and have a large, 

easily treated sapwood zone. Southern Pine is the most readily available treated wood species east of the Rocky Mountains and is a reasonable choice for use in Wisconsin sign 

posts. It is also worth noting that the bulk of wood preservative standards and associated research is based on treatment 

of Southern Pine species. Thus, the treatability of Southern 

Pine and its durability when adequately treated are better 

understood than for other wood species. However, Southern 

Pine does have moderately high shrinkage that can lead to 

problems with warping, especially in longer sign posts. It 

is also somewhat of a changing resource, with trees now 

grown more rapidly than in the past. These faster growing 

trees have wider growth rings and may have a greater proportion of juvenile wood, which further contributes to warping. In many structures, this warping in treated Southern 

Pine can be minimized by securely fastening (i.e., screwing 

or bolting) the members in place before the wood dries. 

However, this is a less viable option for sign posts. 

Red Pine

Red pine (P. resinosa) is a Wisconsin wood species that is 

currently used by for sign posts by WisDOT. Although red 

pine is a minor species from a national perspective, it is an 

important component of the forest resource in Wisconsin 

(Table 3), Minnesota, Michigan, and northeastern states. 

It has been widely planted in plantations, which allows for 

relatively economical harvesting (Fig. 10). Red pine is considered a “treatable” wood species and is often grouped with 

Southern Pine and ponderosa pine in treatment standards. 

However, it does appear to be somewhat less easily treated 

than Southern Pine, or at least less consistently treatable. 

Gjovik and Schumann (1992) evaluated the treatability of 

several northeastern wood species and noted that preservative penetration in red pine was generally less than that in 

eastern white pine (Fig. 11). Variability in the treatability 

of red pine appears to be associated with geographic source 

and with the sapwood to heartwood transitional growth rings 

that visually appear to be sapwood but have permeability 

more similar to heartwood (Lebow et al. 2006). However, 

that study also noted that more consistent penetration could 

be achieved by modifying the treatment conditions. When 

treated with ground-contact preservatives, red pine is highly 

durable. Red pine posts thermally treated (a non-pressure 

process) with creosote to have had no failures after 71 years 

of exposure at a test site in Ontario, Canada (Morris and 

Ingram 2010). Posts pressure-treated with pentachlorophenol in heavy solvent have had no failures after 41 years at 

that test site, which is at approximately the same latitude as 

Hayward, Wisconsin. Red pine has lower strength properties 

than Southern Pine species, but higher strength than other 

pine species such as eastern white and ponderosa pine. Like 

Southern Pine species, red pine has moderately high shrinkage, which can lead to warping, especially in longer sign 

posts.

Ponderosa Pine

Ponderosa pine (P. ponderosa) is not a Wisconsin species, 

but is a major wood species in western states and as far 

east as the Black Hills of South Dakota. Ponderosa pine is 

considered a treatable species and is typically grouped with 

Southern Pine and red pine in treatment standards. However, 

ponderosa pine is softer and weaker than Southern Pine or 

red pine, and its use as a treated structural product has been 

somewhat less common. Ponderosa pine also has a more 

uniform grain and less shrinkage than southern or red pines, 

which creates profitable markets in other applications. There 

is relatively little data on the long-term durability of treated 
General Technical Report FPL–GTR–231

18

ponderosa pine, although one study did report no failures 

after 30 years for posts pressure-treated with pentachlorophenol in heavy oil and exposed in South Dakota (Markstrom and Gjovik 1992). An earlier report also noted no 

failures after 27 years for ponderosa pine posts treated with 

a creosote–oil solution and exposed in South Dakota (Kulp 

1966). 

Jack Pine

Jack pine (P. banksiana) is a native Wisconsin wood species, but is not currently used for sign posts. The volume 

of jack pine in Wisconsin is only about one-fifth that of red 

pine or eastern white pine, and it has suffered declines in 

growth volume over the last several decades. The diameter 

of jack pine stems is also typically smaller than that of red 

pine, eastern white pine, or Southern Pine, somewhat limiting their value in lumber production. In addition, jack pine 

stems tend to have a greater proportion of less treatable 

heartwood than do the pine species more commonly used 

for preservative treatment. When sawn to produce posts, 

(such as sign posts) there is a higher likelihood that jack 

pine will have at least two faces with little sapwood remaining. Jack pine is listed as a post species in AWPA standards, 

but has lesser penetration requirements than red, white, 

or Southern Pine because of its narrower sapwood band. 

AWPA standards also specify that sawn jack pine posts be 

incised (run through rollers that cut many small slits in the 

wood) prior to treatment to improve the depth and uniformity of preservative penetration (Fig. 12). Incising is not 

required for round posts on the assumption that they will 

have an intact outer band of treatable sapwood. 

The long-term durability of preservative-treated jack pine 

posts has not been the subject of extensive research. FPL 

researchers did expose posts pressure-treated with 5% tetrachlorophenol (precursor to pentachlorophenol) near Madison, Wisconsin, and reported no failures after 29 years. Jack 

pine is an important component of the Canadian species 

mix, and several post durability tests have been conducted at 

a test site in Ontario, Canada. Only 1 of 20 jack pine posts 

treated with creosote by a thermal (non-pressure process) 

have failed after 71 years of exposure and only 1 of 14 posts 

pressure treated with copper naphthenate have failed after 

58 years (Morris and Ingram 2010). Posts pressure treated 

Table 3. Growth volume and extent of current utilization of major Wisconsin wood speciesa

Wood species or species groupinga

Existing

volume

(×106

 ft3)

Annual growth

(×106 ft3)

Annual

removals

(×106 ft3)

“Unused” 

growth

(×106 ft3)

b

Eastern white pine 1,667 68.7 9.4 59.3 

Soft maple (red, silver) 2,494 75.0 31.6 43.4 

 Red pine 1,646 71.4 34.5 36.9 

Hard maple (sugar, black) 2,367 61.2 26.5 34.7 

Ash (white, black, green) 1,386 42.0 10.8 31.2 

Red oaks (red, black, pin) 2,680 58.4 39.6 18.8 

Basswood 1,204 26.9 10.7 16.2 

White oaks (white, burr, swamp) 1,042 17.6 10.0 7.6 

Spruce (white, black) 460 11.2 4.5 6.7 

Eastern hemlock 459 8.0 1.7 6.3 

Elm (American, slippery, rock) 339 3.3 1.0 2.3 

Balsam fir 410 7.0 6.1 0.9 

Yellow birch 263 3.0 2.6 0.4 

Jack pine 246 6.0 12.7 –6.7 

Aspen (quaking, bigtooth) 2,324 60.5 71.0 –10.5 

Paper birch 540 –6.8 12.6 –19.4 a

Adapted from Forest Resources Annual Report (Wisconsin DNR 2012). 

http://dnr.wi.gov/topic/ForestBusinesses/documents/WisconsinForestResources.pdf b

Annual growth – Annual removals. For relative ranking purposes only. This column was not included in the Wisconsin DNR 

report. 

Figure 10. Red pine has been widely planted in Wisconsin, 

Minnesota, and Michigan.
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

19

with CCA formulations have also been very durable, with 

no failures after 57 years for posts treated to 5.1 kg/m3

(0.32 lb/ft3), no failures after 48 years for posts treated to 

7.8 kg/m3 (0.49 lb/ft3), and one failure (out of 29 posts) after 

48 years for posts treated to 3.7 kg/m3 (0.23 lb/ft3). However, it should be noted that all of the post studies mentioned 

here appear to have been conducted with round posts which 

have a treatable outer band of sapwood. Jack pine has moderately low strength and moderately low shrinkage.

White Pine (Eastern)

Eastern white pine (P. strobus) is an important timber species in Wisconsin but is not currently used for sign posts. Its 

primary current uses are pulpwood and lumber, and it has 

moderate value relative to other Wisconsin species (Fig. 13). 

Wisconsin’s white pine appears to be a somewhat “underutilized” resource as annual growth is currently almost four 

times annual removals (Table 3). Eastern white pine wood 

0

100

200

300

400

500

600

700

800

Balsam

fir

Eastern

hemlock

Eastern

spruce

E. white

pine

Red

pine

S

olution uptake (kg/m3

)

0

20

40

60

80

100

120

Balsam

fir

Eastern

hemlock

Eastern

spruce

E. white

pine

Red

pine

Preservative penetration (%)

Wood species

CCA-incised

CCA-unincised

ACA-incised

ACA-unincised

Figure 11. Preservative uptake and penetration for 

CCA- or ACA-treated incised or unincised northeastern 

softwoods. Note: Eastern white and red pine specimens 

were predominately composed of less treatable heartwood. 

Adapted from Gjovik and Schumann (1992). 

Figure 12. Incising is used on less treatable wood 

species to improve depth and uniformity of 

treatment.

0

50

100

150

200

250

300

350

400

Dollars per ×103 b

oard feet 

Wood species or species grouping

State of Wisconsin estimate

Industry survey

Balsam fir

E. hemlock

Aspen

Jack pine

Red pine

Elm

E. white pine

Basswood

Ash

Paper birch

Spruce

Soft maple

Yellow birch

White oaks

Red oaks

Hard maple

Figure 13. Stumpage value of saw timber as estimated 

by the State of Wisconsin or an industry survey. Source: 

Adapted from Wisconsin DNR (2012). The State of 

Wisconsin estimate is based on the total value of timber 

sales and products produced; the industry survey is a 

survey of prices reported by timber buyers, sellers, and 

their agents.
General Technical Report FPL–GTR–231

20

has relatively low in strength, and it has never been widely 

used for preservative treatment. Perhaps because of the 

lack of use it has a somewhat unusual classification within 

AWPA standards. Despite its apparent treatability, incising 

is required for treatment with all preservatives except CCA. 

The exception was made for CCA when a treating company 

in the Northeastern United States provided AWPA with data 

demonstrating eastern white pine’s treatability with that 

preservative. Because CCA is not exceptional in its ability 

to penetrate during treatment, it appears likely that incising 

would not be needed for at least some other preservatives as 

well, but those changes have not been made to the standards. 

Smith (1986) also found eastern white pine to be readily 

treated and questioned the need for incising in AWPA standards. Treatability evaluations at FPL indicate that although 

penetration in eastern white pine is not equivalent to that in 

Southern Pine, it is at least as great as that in red pine and 

greater than that in other Wisconsin species (Figs. 11, 14). 

Durability data for 2 by 4 (nominal) stakes exposed at FPL’s 

test site in southern Mississippi also indicates that treated 

eastern white pine is similar in durability to treated red pine, 

with or without incising (Table 4). Canadian researchers 

also report the durability of white pine, with no failures in 

20 posts thermally treated with creosote after 71 years of 

exposure in Ontario, Canada (Morris and Ingram 2010). An 

advantage of eastern white pine is its relatively low shrinkage and high dimensional stability. Given its combination of 

growth volume in Wisconsin, treatability with preservatives, 

and good dimensional stability, eastern white pine appears 

to be a potential candidate species for use in sign posts. 

Douglas-Fir

Douglas-fir (Pseudotsuga menziesii) is not native to, but can 

be grown in, Wisconsin. Preservative-treated Douglas-fir is 

widely used for structural elements, particularly in western 

states. Douglas-fir’s large size, straight growth, and relatively high strength (for a softwood) make it particularly useful 

for large timbers and poles. From a treatment perspective, 

Douglas-fir is characterized by a relatively narrow sapwood 

band and a high proportion of difficult to treat heartwood. 

Because of this treatment difficulty, incising is required for 

Douglas-fir, and some preservatives (such CCA) are not recommended. Preservatives commonly used to treat Douglasfir include the ammoniacal formulations (i.e., ACZA and 

ACQ-B) as well as the oil-type formulations. Treatment 

schedules for Douglas-fir also tend to be substantially longer than those for pine species. Douglas-fir grown in drier 

climates and higher elevations tends to be less treatable than 

that grown along the west coast, and some specifications 

limit the use of Douglas-fir to that from coastal areas. The 

durability of adequately treated Douglas-fir has been well 

established through years of in-service performance as well 

as with some test data. However, much of the in-service 

durability of Douglas-fir has been documented with round 

utility poles, which have an intact outer band of treatable 

sapwood. There is less documentation of the durability of 

sawn Douglas-fir, although research at Oregon State University indicates that durability is excellent when incised and 

treated with effective preservatives (Morrell et al. 1999). 

Square posts incised and treated with a creosote–oil mixture 

have had no failures after 57 years. Unincised square posts 

treated with precursors of CCA or ACZA had average lives 

of 36 and 33 years, respectively. 

Western Larch

Western larch (Larix occidentalis) is not native to Wisconsin, and primarily grows in eastern Oregon and Washington, 

western Montana, and northern Idaho. Although not as 

common or as widely used, western larch has similarities to 

Douglas-fir and the two species are sometimes grouped in 

the Douglas-fir–Larch species group. The wood is moderately stiff and strong, but does have moderately high shrinkage. 

0

5

10

15

20

25

30

35

40

45

SYP

Prservative penetration (m

m)

Wood species without incisions

ACQ

Copper azole

Copper citrate

CCA

0

5

10

15

20

25

30

35

Balsam

fir

East.

hemlock

Eastern

spruce

Red

maple

White

pine

Preservative penetration (m

m)

Wood species incised

Balsam

fir

East.

hemlock

Eastern

spruce

Red

maple

Figure 14. Preservative penetration in unincised or incised 

northeastern wood species treated one of four types of 

preservatives. Adapted from Lebow et al. 2005a.
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

21

Like Douglas-fir, western larch has a large, difficult to treat 

heartwood, and in it is often considered even more difficult 

to treat than Douglas-fir. With the notable exception of utility pole cross-arms, western larch is not listed in the AWPA 

sawn lumber and timbers standards. However, it is listed for 

use in round posts, poles, and piles that maintain an outer 

layer of treatable sapwood. In general, there has been relatively little research on either the treatability or durability of 

western larch. Field exposure data for western larch tends to 

reflect older preservatives and treatment methods, making it 

difficult to determine if the lack of treatability or the preservative efficacy controlled durability. However, it is notable 

that posts pressure-treated with creosote had an average life 

of only 20 years when exposed in Mississippi, while those 

pressure-treated with zinc chloride lasted an average of 

only 15 years in Wisconsin (Gjovik and Davidson 1975). 

Post treated by steeping with either mercuric chloride or 

sodium fluoride were more durable than the pressure-treated 

posts, with average lives of 39 or 28 years (respectively) 

when exposed in Wisconsin. All of these posts were round, 

however, and thus should have had an outer layer of treatable sapwood.

Hem-Fir

Hem-Fir is a species grouping that includes western 

hemlock (Tsuga heterophylla), red fir (Abies magnifica), 

Table 4. Condition of ACA- or CCA-treated 38- by 89-mm (2- by 4-in. nominal) stakes after 24 years in southern 

Mississippi. The AWPA specified ground-contact retention for ACZA and CCA is 6.4 kg/m3

 (0.4 lb/ft3) for 

normal use and 9.6 kg/m3

 (0.6 lb/ft3) for critical structures. 

ACA-treated stakes CCA-treated stakes 

Wood species Incised? 

Retention

(kg/m3)

 Some decay 

(%)

Failed

(%)

Retention

(kg/m3)

a

Some decay

(%)

Failed

(%)

Eastern white pine No 4.16 100 100 4.96 30 0 

Eastern white pine No 6.72 100 44 7.04 0 0 

Eastern white pine No 11.52 10 10 10.4 0 0 

 

Eastern white pine Yes 4.16 100 50 5.28 22 0 

Eastern white pine Yes 6.72 89 22 8.32 0 0 

Eastern white pine Yes 11.68 0 0 11.84 0 0 

 

Red pine No 4.64 100 100 3.68 90 40 

Red pine No 4.8 100 67 5.12 44 22 

Red pine No 7.04 11 0 7.36 11 0 

 

Red pine Yes 4.96 90 70 4 60 0 

Red pine Yes 6.4 67 11 6.4 0 0 

Red pine Yes 10.24 22 0 8.8 0 0 

 

Eastern spruce No 2.56 100 100 2.24 100 80 

Eastern spruce No 4 100 100 3.2 70 50 

Eastern spruce No 6.24 100 50 11.36 0 0 

 

Eastern spruce Yes 3.84 90 80 

Eastern spruce Yes 4.48 100 70 Not tested 

Eastern spruce Yes 7.68 20 0 

 

Balsam fir No 4.16 100 100 

Balsam fir No 5.12 100 67 Not tested 

Balsam fir No 9.44 0 0 

 

Balsam fir Yes 4.64 100 33 

Balsam fir Yes 6.4 40 20 Not tested 

Balsam fir Yes 11.68 0 0 

 

Eastern hemlock No 3.2 100 100 

Eastern hemlock No 5.12 100 60 Not tested 

Eastern hemlock No 5.92 30 0 

 

Eastern hemlock Yes 5.44 100 62 

Eastern hemlock Yes 6.08 10 0 Not tested 

Eastern hemlock Yes 10.88 0 0 
General Technical Report FPL–GTR–231

22

grand fir (A. grandis), noble fir (A. procera), white fir 

(A. concolor) and Pacific silver fir (A. Amabilis). The native range of these species is the Pacific coast from northern 

California up into Alaska. The species are characterized by 

light-colored, light weight wood with little obvious color 

differentiation between sapwood and heartwood. The group 

has moderate strength and shrinkage that varies from low to 

moderately high. The largest component of the grouping is 

typically western hemlock. As with other species groupings, 

the Hem-Fir grouping appears to have developed somewhat 

out of convenience because the species, sometimes grown in 

mixed stands, are difficult to differentiate once milled, and 

have somewhat similar properties. However, there can be 

a substantial difference in mechanical properties and treatability between the species. Although species in the HemFir grouping tend to be somewhat more treatable than the 

Douglas-fir grown in the same region, they are less treatable 

than pine species and incising of all species in the grouping 

is required by AWPA standards. There is relatively little durability data on many of the true firs in the Hem-Fir grouping, although some data are available for western hemlock 

(see western hemlock heading in this section). In general, 

care should be taken in the use of this species grouping for 

sign posts as the properties of the posts may vary depending 

on the species mix. 

Western Hemlock

Western hemlock (Tsuga heterophylla) is often included 

within the Hem-Fir species grouping and typically represents the largest proportion of that grouping. The natural 

range of western hemlock is along the Pacific coast from 

Oregon north to Alaska, although some does occur in northern Idaho. It is a relatively large and common tree, making 

it second only to Douglas-fir in economic importance in 

that region. Western hemlock has moderate strength and 

moderately high shrinkage. It typically has a narrow sapwood band, but the differentiation between sapwood and 

heartwood is not always visible once the wood has dried. 

Although more treatable than Douglas-fir, treatability of 

western hemlock is variable between trees and within a 

single board, and incising is required by AWPA standards. 

In many cases, western hemlock can be treated adequately 

enough to be durable without incising; researchers at Oregon State University report that unincised sawn western 

hemlock posts treated with precursors of CCA or ACZA had 

average lives of 49 and 32 years, respectively (Morrell et al. 

1999). However, incising should increase average durability 

as well as decreasing the likelihood of early failure.

Oaks (Red and White)

The current WisDOT specification does not differentiate 

among oak species. There are numerous oak species in 

Wisconsin and additional species in other states, but for the 

purposes of this report we will group those species under the 

broad categories of red and white oak. Red oaks are generally somewhat treatable with preservatives but have relatively low natural durability. In contrast, white oaks are very 

resistant to preservative treatment but have moderately high 

natural durability. In Wisconsin, red, black, and pin oak are 

examples of red oaks, whereas white, burr, and swamp oak 

are examples of white oaks. Both red and white oaks occur 

in substantial volume in Wisconsin, although there is a substantially greater volume of red than white oaks (Table 3).

Red Oaks

Red oaks (Quercus spp.) are an important resource in Wisconsin with both substantial growth volume and substantial 

removals for commercial use. Red oaks are among the most 

valuable of Wisconsin species (Fig. 13), with much of the 

higher quality red oak used for saw logs and veneer, where 

the monetary values obtained are likely to greatly exceed 

that obtained for sign post products. A large volume is also 

used for firewood, but it is likely that much of this volume 

is derived from small stems and branches, or from lower 

value large trees. The potential value of red oak for other 

applications and its growth form may make it challenging to obtain the dimensions needed for sign posts at costs 

similar to those for currently paid for softwood species. Red 

oak is stronger than other species listed in Wisconsin DOT 

signpost specifications, but does exhibit fairly high shrinkage. Red oak species are listed in AWPA standards for a 

variety of uses, including sawn posts. The most common 

use for treated red oak is railroad ties, but treated timbers 

are also sometimes used for bridge construction. Research 

indicates that red oak is moderately treatable (Crawford et 

al. 2000; Laks et al. 1996); it is less treatable maple or pine 

but more treatable than white oaks. Once adequately treated, 

red oak is durable, although, like other hardwoods, it tends 

to be somewhat less durable than softwoods when treated 

with water-based preservatives (Fig. 15). Hardwoods are 

more commonly treated with oil-type preservatives such as 

creosote, pentachlorophenol, and copper naphthenate. The 

durability of red oak is similar to that of Southern Pine when 

treated with creosote (Fig. 16). 

White Oaks

Although white oaks (Quercus spp.) do not occur in as 

great abundance as red oaks, they remain an important species mix in Wisconsin. Like red oak, the larger and higher 

quality logs are valuable when used for lumber and veneer 

production (Fig. 13), but much of the volume is used for 

firewood. Also like red oaks, white oak wood is strong but 

undergoes moderately high shrinkage during drying. In the 

context of preservative treatment and durability, the major 

difference between white and red oak is the presence of 

tyloses in the vessels of white oak. The tyloses hinder fluid 

flow within the wood, making white oak difficult to treat but 

also causing it to be much more durable than red oak. White 

oak is listed in AWPA standards, but its use is generally limited to railroad ties. In deference to poor treatability, it is 

the only species group in which AWPA standards still 

allow “treatment to refusal” as a measure of treatment 
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

23

quality rather than specifying minimum penetration and 

retention requirements. However, a recent proposal to 

AWPA by the manufacturers of the preservative ACZA included data showing that some penetration in white oak is 

possible, albeit to a lesser extent than red oak. Their proposal, which was subsequently adopted by the AWPA, does call 

for minimum penetration and retention specifications when 

white oak is treated with ACZA. There is relatively little 

information on the durability of pressure-treated white oak. 

However, one study did find that white oak posts thermally 

treated (a non-pressure method) with creosote had an average life of 37 years when exposed in Maryland. Other durability evaluations with non-pressure treatments of white oak 

posts produced less impressive results, with average lives 

ranging from 13 to 29 years (Gjovik and Davidson 1975). 

Treatability and Durability of Other Wisconsin 

Wood Species

One of the objectives of this report is to review the potential for increased use of Wisconsin wood species for sign 

posts. Use of locally sourced wood for sign posts decreases 

transportation costs and may also provide benefit Wisconsin 

land-owners and industries. Currently red pine is the only 

Wisconsin wood species frequently used for sign posts, 

although white pine and oak are also allowed under the 

specification. In this section, the treatability and durability 

of Wisconsin wood species not currently included in the 

sign post specification are reviewed for their possible use as 

sign posts. Species were selected for review based on their 

classification as a major Wisconsin wood species by the 

Wisconsin Department of Natural Resources (Table 3). 

Ash

Wisconsin has a significant ash resource, with white (Fraxinus Americana), black (F. nigra) and green (F. pennsylvanica) ash all present in the state. The volume of ash growth 

each year is also approximately four times greater than that 

harvested (Table 3), indicating that ash could be available 

for sign posts without conflicting with existing types of 

utilization. As of 2012, ash had moderate commercial value 

(Fig. 13), but those data may not reflect the recent increase 

in ash removal in anticipation of the potential for infestation 

by the invasive emerald ash borer. This devastating insect 

has been associated with high ash mortality in states where 

it has become established. Currently emerald ash borers 

have been detected in several Wisconsin counties, and widespread infestation appears inevitable. In the near term, ash 

availability is expected to increase as local governments begin to remove and replace ash trees in anticipation of future 

infestation. There is great interest in finding ways to utilize 

these trees (Brashaw et al. 2012) and the City of Madison, 

Wisconsin, recently expressed interest in renting a portable 

sawmill (WSJ 2012) so that the ash trees removed could 

be utilized for wood products other than chips. In the much 

longer term, however, availability of ash wood could decline 

substantially if the emerald ash borer becomes widely 

established. 

Ash has not traditionally been used for durable applications 

and has relatively little natural durability. Pressure-treated 

ash is occasionally used for railroad ties as part of a mixed 

species grouping, but there is little use of preservative-treated ash in other applications. However, the little information 

that is available does indicate that ash is somewhat treatable. 

Early researchers at FPL conducted creosote treatments of 

25 hardwood species and grouped them into three categories 

based on extent of creosote penetration (Teesdale and 

MacLean 1918). Both green and white ash were placed 

into the most treatable category. Subsequently, Tesoro et al. 

(1966) compared the extent of creosote penetration and retention in nine hardwoods and six softwoods, and found that 

ash was more treatable than many species, although greater 

penetration and retention was observed for maple. Koch 

(1985) reviewed data on uptake of creosote or pentachlorophenol solution by a range of hardwood species with 

non-pressure treatments and found that ash was among the 

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3 4 5 6

Average condition rating

Preservative solution strength (%)

CCA, pine

CCA, red oak

ACQ-B, pine

ACQ-B, red oak

Figure 15. Comparison of the condition or Southern Pine 

and red oak stakes after treatment with water-based preservatives and 11 years of exposure in southern Mississippi.

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70

A

verage condition rating

Creosote solution strength (%)

Red maple

Red oak

Southern pine

Figure 16. Comparison of the condition of red maple, red 

oak, and Southern Pine stakes after treatment with creosote 

and 9 years of exposure in southern Mississippi.
General Technical Report FPL–GTR–231

24

most treatable species. The exception to this relatively positive portrayal of ash treatability was as an evaluation of the 

pressure treatment of a 22 hardwood species with a pentachlorophenol solution (Koch 1985). In that study, green ash 

fell in the middle in terms of treatability, while white ash 

ranked in the lower third. 

There are a little data on the durability of preservative-treated ash. The average life of treated green ash fence posts has 

been reported to range from 12–30 years, while two trials 

with white ash noted average lives of 11 and 21 years (Blew 

and Kulp 1964). Durability varied depending on preservative, method of application, and severity of exposure site; 

none of the posts used in these tests were pressure-treated. 

Aspen

Wisconsin has two native species of aspen: quaking (Populus tremuloides) and bigtooth (P. grandidentata) aspen. 

Aspen has one of the highest growth volumes in Wisconsin, 

but is currently being utilized at a higher rate than it is being 

replenished (Table 3). Aspen traditionally has not been used 

for durable wood products and information on its treatability and subsequent durability is limited. Like other North 

American hardwoods, it theoretically would be allowed for 

use in railroad ties under AWPA’s “Mixed Hardwoods” category, but such use would be unusual. Aspen has little natural durability and must be preservative treated for outdoor 

applications (Blew and Kulp 1964). Research that has been 

conducted with quaking aspen indicates that its treatability can vary substantially both within and between pieces 

(Cooper 1976; Kaufert 1948; Mackes and Lynch 2001). The 

sapwood is reported to be treatable (Smith 1986) and it has 

been recommended that treatments focus on small diameter younger trees that have greater proportion of sapwood 

(Wengert 1985). Cooper (1976) reported that heartwood in 

some pieces was also treatable, and that heartwood treatability appeared to be a function of geographic source. It should 

be noted that Cooper (1976) used relatively rigorous treatment conditions (heated solution and 3- to 20-h pressure 

periods) that could be considered onerous by treating plants 

more accustomed to treating pine species. It has also been 

reported that small dimensions are more treatable than large 

dimensions, and that sawn lumber treats more consistently 

than round stock (Kaufert 1948). Kaufert attributed the latter observation to the exposure of cell lumens on the wood 

surface during milling, but it is probable that this effect is 

attributable to more uniform drying of smaller dimension 

material. Aspen wood can contain “wet pockets” that have 

much higher moisture content than the surrounding wood 

and resist drying (Mackes and Lynch 2001; Wengert 1985). 

Some of the variability in aspen treatability is likely a result 

of these wet pockets interfering with preservative flow. The 

literature indicates that effective treatment of aspen may be 

possible after thorough drying, although a rigorous quality 

control process would be necessary to account for variability in treatment. 

Much of the data on durability of preservative-treated quaking aspen are based on posts treated by non-pressure methods or with preservatives that are no longer in use or both. 

Blew and Kulp (1964) report average lives for treated aspen 

posts ranging from as little as 6 years to as much as 37 years 

in Wisconsin. The latter was achieved with a creosote retention of 150 kg/m3 (9.4 lb/ft3), which is similar to that currently specified for pressure treatment of posts. Posts treated 

with water-based zinc chloride to a retention of 21 kg/m3 

(1.32 lb/ft3) were nearly as durable, with an average life of 

33 years (zinc chloride is no longer used as a wood preservative). Blew and Kulp (1964) also report on one test with 

bigtooth aspen posts, noting that they had an average life of 

15 years after cold-soaking in a pentachlorophenol solution. 

In general, the post data do indicate that aspen posts can be 

quite durable if adequately treated. 

Balsam Fir

Balsam fir (Abies balsamea) is a relatively minor species 

in Wisconsin, and its utilization volume is currently nearly 

equivalent to its growth volume (Table 3). When judged 

solely on availability, balsam fir does not appear to be an 

ideal candidate for production of large volumes of sign 

posts. Research also indicates that balsam fir is somewhat 

resistant to preservative penetration. Gjovik and Schumann 

(1992) found that incising was required to obtain adequate 

penetration of CCA in balsam fir, although penetration was 

substantially better with ACA (Fig. 11). As a note of explanation, Gjovik and Schumann intentionally selected red pine 

and white pine specimens with substantial heartwood, and 

thus penetration in these pine species was less than might be 

expected for sapwood. Lebow et al. (2005b) also reported 

poor penetration of CCA and copper from another waterbased preservative (CuBor) in balsam-fir (Fig. 17). Penetration of boron was much greater, but the boron in current formulations is leachable and would not provide long-term protection for sign posts. Poor treatability with CCA was again 

confirmed in another study of the treatability of northeastern 

species (Lebow et al. 2005a), but this study also indicated 

that better penetration was possible with ACQ (Fig. 14). Improved treatment was achieved with incising, but still lagged 

well below that of Southern Pine.

As part of their evaluation of northeastern species, Gjvoik 

and Schumann (1992) installed sets of 38 by 89 by 

457 mm (2 by 4 by 18 in. nominal) treated stakes in FPL’s 

test site within Harrison Experimental Forest in southern 

Mississippi (Table 4). For balsam fir, only ACA-treated 

stakes were included in the tests. These stakes have been 

evaluated for their extent of decay and insect attack for 

24 years. The ACA-treated balsam fir stakes have been durable when treated to above the AWPA UC4A retention 

(6.4 kg/m3) for ACA (ACZA) general-use posts, but two 

stakes treated to 6.4 kg/m3 have failed. None of the stakes 

treated to higher retentions suffered significant attack, even 

when treated without first incising the wood. It is worth 
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

25

noting that for eastern spruce, which was treated with both 

ACA and CCA in this study, stakes treated with CCA appeared to be more durable than those treated with ACA 

(Table 4) even though preservative penetration was greater 

with ACA (Fig. 11). This may indicate that balsam fir stakes 

would also have been more durable if treated with CCA. 

Basswood

There is an intermediate volume of basswood (Tilia americana) in Wisconsin, and currently the volume of growth 

is approximately twice that utilized (Table 3). Like aspen, 

basswood has very little natural durability. Also like aspen, 

basswood is technically allowed for use in railroad ties under AWPA’s “Mixed Hardwoods” category, but such use is 

not typical. There appears to have been little interest in the 

preservative-treatment of basswood, with the exception of 

on-site treatment of fence posts in the first half of the 20th 

century. The research that is available generally indicates 

that basswood is treatable, at least when pressure-treated. 

Teesdale and MacLean (1918) included basswood among 

the most treatable of three groups of hardwoods when 

pressure-treated with creosote, and a high retention was 

achieved when round basswood fence posts were pressuretreated with creosote (Gjovik and Davidson 1975). Tesoro et 

al. (1966) also found that creosote penetration in basswood 

heartwood was among the highest of the mixed hardwood 

and sapwood species he evaluated. Research on treatability 

of basswood by non-pressure means is less conclusive. MacDonald (1915) reported that creosote uptake by basswood 

posts during thermal treatment was good, but that transverse 

(across the grain) penetration was only about 8 mm (1/3 in.). 

Blew (1961) also noted that penetration of basswood during 

cold-soaking was primarily through the end-grain, and that 

satisfactory treatment could only be obtained on relatively 

short pieces. Canadian researchers reported substantial uptake with thermal treatment, although penetration data was 

not provided (Kulp 1966). Gjovik and Davidson (1975) 

reported intermediate non-pressure preservative uptake 

in basswood posts when compared with other species 

evaluated. 

When adequately treated, basswood can be quite durable. 

Posts pressure-treated to a high retention with creosotesolvent solution had no failures after 56 years in test in 

Wisconsin, while posts treated by non-pressure means (and 

with lower retentions) had average lives ranging from 5.4 to 

32 years (Gjovik and Davidson 1975). The lower durability 

appears to be for posts that were only treated on one end. 

MacDonald (1915) estimated that basswood posts soaked in 

creosote would last an average of 25–30 years, although this 

estimate appears to be based on an exposure period of less 

than 10 years. 

Birch—Paper and Yellow

Although there is a substantial volume of birch in Wisconsin, the growth volume is low in comparison to the volume 

currently used (Table 3). This is particularly true for paper 

birch (Betula papyrifera), for which annual removal currently greatly exceeds annual growth. Neither paper nor yellow 

birch (B. alleghaniensis) is naturally durable, and historically there has been little interest in preservative treatment 

of birch, and particularly paper birch, because of its value in 

other applications. Teesdale and MacLean (1918) did evaluate the treatability of yellow birch with creosote and placed 

it into the most treatable group of the species they evaluated. More recently, FPL researchers pressure-treated yellow 

birch heartwood stakes (19 by 19 by 457 mm) with either 

CCA or ACQ as part of a larger durability evaluation. Uptake by the yellow birch stakes was similar to that of Southern Pine (Fig. 18), but caution is warranted in interpreting 

these results because of the small specimen dimensions. 

Paper birch posts have been subjected to preservative treatment by non-pressure methods. Gjovik and Davidson (1975) 

report that paper birch posts immersed in a pentachlorophenol/diesel solution obtained uptakes ranging from 

11.2 kg/m3 (1.7 lb/ft3) to 107 kg/m3 (6.7 lb/ft3) for 

0

100

200

300

400

500

600

700

Red spruce Balsam fir White spruce

S

olution uptake (kg/m3

)

CCA CuBor DOT

0

5

10

15

20

25

30

35

40

45

Red spruce Balsam fir White spruce

Preservative penetration (m

m)

Wood species

CCA-copper

CuBor-copper CuBor-boron

DOT-boron

Figure 17. Preservative uptake and penetration in unincised 

spruce or balsam-fir. Adapted from Lebow et al. 2005b.
General Technical Report FPL–GTR–231

26

immersion periods ranging from 8 to 168 h. Blew (1961) 

conducted cold-soak treatments with a range of species and 

concluded that paper birch was characterized by adequate 

end-grain penetration but minimal penetration across the 

grain. 

Much of the long-term durability data available for birch is 

for fence posts that were treated without pressure. Yellow 

birch posts soaked for 48 h in a creosote–oil solution had 

average lives in Mississippi of 8 years without incising but 

20 years if the lower portion was incised before treatment, 

presumably because the incisions increased preservative 

penetration and uptake (Gjovik and Davidson 1975). White 

birch posts soaked in a 5% pentachlorophenol–oil solution 

for 8 to 168 h had average lives ranging from 4 to 16 years 

when exposed in southern Mississippi, while those soaked 

for 48 h lasted an average of 20 years in Wisconsin. In a 

separate study, sets of white birch posts soaked in a 5% pentachlorophenol–oil solution had averages lives ranging from 

16 to 29 years when installed in Minnesota (Kulp 1966). 

Longer service (over 32 years) was reported for white birch 

posts thermally treated with creosote and place in test in Ontario, Canada (Kulp 1966). More recently, FPL researchers 

evaluated the durability of 19- by 19-mm (3⁄4- by 3⁄4-in.) yellow birch stakes treated with a range of preservatives as part 

of a larger durability evaluation (Fig. 19). After 11 years 

exposure in southern Mississippi, it appears that the yellow 

birch stakes are less durable than Southern Pine stakes when 

compared at equivalent solution concentrations. However, 

durability equivalent to Southern Pine was observed for the 

highest concentration (2.4%) of ACQ-B. 

Eastern Hemlock

Both the existing and growth volume of eastern hemlock 

(Tsuga canadensis) in Wisconsin is less than that of many 

other major species. However, it is also the least utilized 

of major Wisconsin species (Table 3). The size and form 

of older eastern hemlock trees would allow milling of relatively long posts. Eastern hemlock is moderately low in 

strength, but also moderately low in shrinkage. Eastern hemlock has not been favored for lumber production because the 

wood is coarse and uneven in texture, and tends to splinter 

0

100

200

300

400

500

600

700

800

Hard maple

heart

Hard maple

sap

Red

maple

Red

oak

Silver

maple

S

outhern

pine

Yello

w

birch

S

olution uptake (kg/m3

)

Wood species

ACQ

CCA-C

Figure 18. Uptake of preservative by 19- by 19- by 457-mm 

(0.75- by 0.75- by 18-in.) stakes during pressure treatment. 

Note that the small dimension of these stakes tends to 

minimize species differences.

0

1

2

3

4

5

6

7

8

9

10

ACQ-B 1.2%

ACQ-B, 2.4%

ACQ-D, 1.2%

ACQ-D, 2.4%

CCA, 1.2%

CCA, 2.4%

CuN H2O, 1.2%

0

1

2

3

4

5

6

7

8

9

10

A

verage condition rating

Wood species and type

CuN oil, 1.2%

OxCu, 1.2%

OxCu, 2.4%

OxCu, 4.8%

Hard maple

heart

Hard maple

S

sapwood

outhern

Pine

Yello

w

birch heart

Hard maple

heart

Hard maple

S

sapwood

outhern

Pine

Yello

w

birch heart

Figure 19. Average condition of 19- by 19- by 457-mm (0.75- 

by 0.75- by 18-in.) treated stakes after 11 years exposure in 

southern Mississippi. Upper graph and lower graphs show 

water-based and oil-based treatments, respectively. Some 

bars are absent because not all combinations were tested.
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

27

when cut or drilled. However, a greater disadvantage relative to use in sign posts is the tendency of eastern hemlock 

trees to suffer from “ring shake.” Ring shake is a condition 

where the wood separates longitudinally parallel to the annual growth rings. Strength can be substantially impacted 

when this occurs. Eastern hemlock is included in AWPA 

standards for treated sawn products, but incising is required. 

Studies on the treatability of eastern hemlock have produced 

somewhat varying results. Gjovik and Schumann (1992) 

found that treatability of eastern hemlock was less than that 

of other northeastern species unless incised (Fig. 11), whereas Smith (1986) found eastern hemlock to be fairly treatable 

even without incising. Lebow et al. (2005a) reported poor 

penetration in eastern hemlock without incising, but much 

better penetration after incising (Fig. 14). 

Relatively little long-term data exist on the durability of 

treated eastern hemlock. However, exposure tests of 2 by 4 

(nominal) stakes in southern Mississippi indicate that eastern hemlock stakes treated with ACA (precursor to ACZA) 

are at least as durable as those of other northeastern species 

when treated to similar retentions (Table 4). Incised stakes 

had higher ACA retentions and were more durable. 

Elm

Although common in southern Wisconsin, American (Ulmus 

americana), slippery (U. rubra), and rock (U. thomasii) elm 

species represent a relatively minor proportion of Wisconsin’s major timber species. In Wisconsin, elm is currently 

used primarily for firewood and pulpwood, with relatively 

little lumber production. Elm has value in veneer production (for furniture), but the prevalence of Dutch elm disease 

prevents most trees from reaching merchantable size. Elm 

has moderately high strength and shrinkage. Elm has little 

natural durability, and there is little evidence of its use with 

preservative treatment. However, like other hardwoods it 

may be a component of the “mixed hardwoods” grouping 

allowed for treatment of railroad ties (AWPA 2013). The 

limited research available on preservative treatment of elm 

species indicates that they are at least moderately treatable. 

Teesdale and Maclean (1918) grouped American and slippery elm with the most treatable species for pressure treatment with creosote, and classified rock elm as moderately 

treatable. American elm was also among the most treatable 

hardwood species for pressure or thermal treatment with 

pentachlorophenol, but only moderately treatable by coldsoaking with pentachlorophenol or water-based solutions 

(Koch 1985). The durability data for elm species is inconclusive. Non-pressure treatments with pentachlorophenol 

formulations provided only moderate or even low protection 

of round posts, but the retentions were relatively low. Pressure and thermal treatments with creosote yielded higher 

retentions and no failures after 18 (Mississippi) or 12 (Wisconsin) years, but inspections on these posts were discontinued (Gjovik and Davidson 1975). However, split posts 

thermally treated with creosote to only 48 kg/m3 (3.0 lb/ft3) 

lasted an average of 32 years in Minnesota.

Maples, Hard

Sugar (Acer saccharum) and black (A. nigrum) hard maples 

are among the most abundant of Wisconsin timber species, 

although not as abundant as the soft maples discussed below. Despite their value in pulp, furniture, and flooring production, as well as high stumpage prices, the annual growth 

of hard maples in Wisconsin greatly exceeds the annual 

harvest. Hard maples are high in strength, moderately high 

in shrinkage, and have no natural durability. Hard maples 

have limited use with preservative treatments, but the maple 

genus (Acer spp.) is listed in AWPA Commodity Specification A (Sawn Products) for use above ground as well as for 

general-use ground-contact applications (including sign 

posts). However, creosote is the only preservative currently 

standardized for treatment of the maple species.

There is relatively little treatability information that is specific to hard maples. Teesdale and Maclean (1918) evaluated 

the treatability of sugar maple with creosote and placed it in 

the “intermediate” category. FPL researchers treated 19- by 

19-mm (3⁄4- by 3⁄4-in.) hard (sugar) maple specimens with 

ACQ and CCA as part of a durability evaluation and found 

that while uptake in the sapwood stakes was similar to other 

species, uptake in hard maple heartwood was diminished 

(Fig. 18). Kamdem and Chow (1999) treated similar size 

specimens of sugar maple, red maple, red oak, and beech 

with copper naphthenate and CCA. They found no statistical difference in treatability between sugar maple and red 

maple, but did note that in some cases the maples had significantly greater retention than red oak or beech. 

Note, however, that the small dimensions of specimens in 

both studies may have minimized treatment differences between species. Because hard maple species tend to have a 

relatively thick sapwood band, it is likely that sawn products 

from these species will have a substantial proportion of the 

more treatable sapwood. 

There is also relatively little information on the durability of 

hard maple species following pressure treatment with preservatives. The data that are available appear to be limited 

to relatively small specimens. FPL researchers compared the 

durability of hard maple sapwood and heartwood to that of 

Southern Pine and yellow birch heartwood following treatment with a range of water or oil-based preservatives 

(Fig. 19). The results indicate that treated hard maple sapwood is more durable than treated hard maple heartwood, 

possibly because of the lower solution uptake by the latter 

during treatment. It is also apparent that treated hard maple 

is less durable than treated Southern Pine when compared at 

lower treatment solution concentrations (Fig. 19). A possible 

exception to this trend is the relatively equivalent durability of hard maple sapwood and Southern Pine when treated 

with oil-borne copper naphthenate. 
General Technical Report FPL–GTR–231

28

Maples, Soft

The Wisconsin soft maple wood species are primarily red 

(A. rubrum) and silver (A. saccharinum), with red maple 

comprising over 85% of the growth volume. This species 

grouping has the greatest growth volume as well as the 

highest proportion of “unused” growth volume of any species grouping in Wisconsin (Table 3). The wood of the soft 

maples is not as strong as the hard maples, but still stronger 

than Wisconsin’s softwood species. The soft maples have 

moderate shrinkage (less than that of the hard maples). Like 

the hard maples, soft maples have little natural durability. 

Soft maples are primarily used for pulpwood, although some 

are used for veneer production and firewood. Their sawlog 

value is not as great as the hard maples, but well above that 

of most softwood species (Fig. 13). There has been some 

interest in the preservative treatment and use of red maple 

following an initiative in the 1990s to find value-added application for underutilized wood species in the northeastern 

United States. As part of this effort, red maple was added 

to AWPA standards as a species for use in the production of 

glulam timbers. Currently only creosote formulations are 

standardized for treatment of red maple glulams. 

Studies of the treatability of soft maple species have produced conflicting results. Teesdale and MacLean (1918) 

placed silver maple sapwood into the most treatable grouping and silver maple heartwood into the moderately treatable 

category following pressure treatment with creosote. Koch 

(1985) reports that studies found red maple to be relatively 

well-treated with pentachlorophenol by either pressure or 

non-pressure means. A study of the feasibility of using red 

maple and yellow poplar lumber for glulam beams noted 

excellent penetration of creosote into the red maple lumber. 

Penetration exceeded 2 in., and the uptake of creosote solution in red maple was approximately two times greater than 

that in red oak (Baileys et al. 1994). Kamdem and Chow 

(1999) reported red maple to be as least as treatable, and 

sometimes more treatable, than red oak when treated with 

copper naphthenate or CCA, and FPL researchers found that 

in small stakes, uptake of ACQ and CCA in silver maple 

and red maple was similar to that of Southern Pine 

(Fig. 18). However, studies using slightly larger specimens have indicated that red maple does not treat as well 

as Southern Pine, eastern white pine, or eastern hemlock 

(Crawford et al. 2000; Lebow et al. 2005a) (Fig. 14).

The relative durability of treated soft maple appears to differ 

with type of preservative. FPL researchers found red maple 

to be as durable as Southern Pine or red oak when treated 

with creosote (Fig. 16), but less durable than Southern Pine 

when treated with water-based preservatives (Fig. 20). This 

is similar to the findings noted for hard maple and to those 

reported by Slahor et al. 2001 and Lebow et al. 2010. These 

findings help to explain why maple species are currently 

only standardized by AWPA for treatment with creosote. 

These studies do indicate, however, that red maple can be 

as durable as Southern Pine if treated with higher concentrations of water-based preservatives. 

Spruce

White (Picea glauca) and black (P. mariana) spruce are 

relatively minor commercial timber species in Wisconsin 

and grow primarily in the northern third of the state. For 

commercial use, black and white spruce are often not distinguished from the red spruce (P. rubens) found in the northeastern United States, and the three combined species are 

called eastern spruce. Spruce is primarily used for pulpwood 

in Wisconsin, although it does have some value for saw 

logs. The estimated value of spruce logs differs substantially 

whether determined by the State of Wisconsin methods or 

industry survey (Fig. 13). Spruce wood has moderately low 

strength, moderate shrinkage, and little natural durability. 

White and black spruce are included in the Spruce–Pine–Fir 

species mix within AWPA standards and thus are standardized for use above ground and when treated with ACQ, 

ACZA, or borates. This listing does not include ground-contact applications such as sign posts. However, black spruce 

is also included in the Spruce–Pine–Fir (West) species 

group, which does allow use in ground contact (including 

sign posts) when treated with ACQ, ACZA, or CCA. With 

one exception, AWPA standards require that spruce species 

be incised prior to treatment to achieve sufficient penetration. The exception applies to borate treatment of wood to 

be used indoors.

Both white and black spruce are considered relatively resistant to preservative treatment. As with balsam fir and 

eastern hemlock, the sapwood and heartwood of the spruces 

species are difficult to distinguish once the wood has dried. 

0

1

2

3

4

5

6

7

8

9

10

C

C

A-0.6

C

C

A-1.2

A

C

Q-0.3

A

C

Q-0.6

A

C

Q-1.2

A

C

Q-2.4

A

verage condition rating

Preservative and concentration (%)

Red oak

Red maple

Silver maple

Southern Pine

Figure 20. Durability of 19- by 19- by 457-mm (0.75- by 

0.75- by 18-in.) stakes of several species after treatment 

with ACQ or CCA and 11 years of exposure in Mississippi.
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

29

One laboratory evaluation of white spruce heartwood found 

that an average of 25% to 42% (depending on the moisture 

content) of the cross section was penetrated following pressure treatment with CCA (Cech et al. 1974). In their study of 

the treatability and durability of refractory species, Richards 

and Inwards (1989) reported that white spruce was among 

the most difficult to treat species, although trees obtained 

from one geographic area in Canada were more treatable. A 

subsequent laboratory evaluation of lumber sections found 

that both white and red spruce were very resistant to preservative treatment with CCA (Fig. 17, Lebow et al. 2005b). A 

larger scale study of black spruce reported that it could 

be treated to AWPA standards if incised at a density of 

9,500 incisions/m2 (883 incisions/ft2) and pressed for over 

4 h with a heated treatment solution (Hosli and Zahora 

1996). Some evaluations of spruce treatability simply refer 

to “eastern spruce” and do not distinguish the individual 

species. One such study indicates that the treatability of 

eastern spruce is similar to that of eastern hemlock or balsam fir, but less than that of eastern white pine (Fig. 11) 

(Gjovik and Schumann 1992). However, a more recent 

study found that eastern spruce was less treatable than balsam fir and eastern hemlock (Lebow et al. 2005b). In general, both white and black spruce should be considered difficult to treat, and adequate treatment will require incising. 

The difficulty of treating spruce also appears to affect its 

durability. In their evaluation of 38- by 89-mm (2- by 4-in. 

nominal) ACA- or CCA-treated stakes in Mississippi, Gjovik and Schumann (1975) observed numerous failures in 

spruce stakes but many of these stakes were treated to lower 

retentions than other species. The treated spruce stakes appeared nearly as durable as other species at comparable 

retentions, and the CCA-treated stakes appeared to be more 

durable than the ACA-treated stakes. A subsequent study 

with smaller stakes also found eastern spruce to be one of 

the least durable species, but again this effect appeared to 

at least partly relate to retention differences (Lebow et al. 

2010). Richards and McNamara (1997) noted little deterioration in 38- by 140-mm (2- by 6-in. nominal) white spruce 

stakes treated with CCA and exposed for 8 years in Florida. 

Some of those stakes were treated to relatively low overall 

retentions, but probably had much higher retentions near 

the surface because of the poor penetration. Stake tests can 

sometimes underestimate the risks associated with poor 

penetration because there are fewer breaks in the treated 

shell than would typically occur with wood in service. In 

general, these studies indicate that spruce can be durable if 

adequately treated. 

Engineering Properties of 

Wisconsin Wood Species

Common Mechanical Properties

Common mechanical properties, at 12% moisture content, 

for various woods are summarized in Table 5 (Kretschmann 

2010). The following is a brief description of these properties. The values reported were obtained from small, clear 

specimens of wood taken from the corresponding tree species. They do not represent structural design values for a 

specific grade of timber; hence, they should not be used 

directly for design. However, they can be used for comparing the potential performance of products from one species 

of wood with another. Further information on the design 

specifications for wooden signposts can be found in Section 

9 of the Standard Specifications for Structural Supports for 

Highway Signs, Luminaires, and Traffic Signals (AASHTO 

2013).

Note that wood density significantly influences mechanical 

properties of wood products. In general, denser woods have 

higher mechanical properties (higher density woods are 

generally stronger than lower density woods). Examination 

of Table 5 reveals that species having densities similar to 

or greater than those currently used for signposts have mechanical properties that are comparable. 

Modulus of Elasticity (MOE)

Elasticity implies that deformations produced by low stress 

are completely recoverable after loads are removed. When 

loaded to higher stress levels, plastic deformation or failure 

occurs. The three moduli of elasticity are the elastic moduli 

along the longitudinal, radial, and tangential axes of wood. 

These moduli are usually obtained from compression tests. 

The modulus of elasticity determined from bending may 

be the only modulus of elasticity available for a species. 

Average MOE values obtained from bending tests are given 

in the attached table. As tabulated, MOE values include an 

effect of shear deflection; MOE values from bending can be 

increased by 10% to remove this effect approximately. 

Modulus of Rupture (MOR)

Reflects the maximum load-carrying capacity of a member 

in bending and is proportional to maximum moment borne 

by the specimen. Modulus of rupture is an accepted criterion of strength, although it is not a true stress because the 

formula by which it is computed is valid only to the elastic 

limit.

Impact Bending (Impact)

In the impact bending test, a hammer of given weight is 

dropped upon a beam from successively increased heights 

until rupture occurs or the beam deflects 152 mm (6 in.) or 

more. The height of the maximum drop, or the drop that 

causes failure, is a comparative value that represents the 

ability of wood to absorb shocks that cause stresses beyond 

the proportional limit.

Work to Maximum Load in Bending (WML)

Work to maximum load in bending represents the ability to 

absorb shock with some permanent deformation and more 

or less injury to a specimen. Work to maximum load is a 
General Technical Report FPL–GTR–231

30

measure of the combined strength and toughness of wood 

under bending stresses.

Lag Screw Withdrawal Resistance

Lag screws are commonly used because of their convenience, particularly where it would be difficult to fasten a 

bolt or where a nut on the surface would be objectionable 

(Rammer 2010). Commonly available lag screws range from 

about 5.1 to 25.4 mm (0.2 to 1 in.) in diameter and from 

25.4 to 406 mm (1 to 16 in.) in length. The length of the 

threaded part varies with the length of the screw and ranges 

from 19.0 mm (0.75 in.) with the 25.4- and 31.8-mm (1- and 

1.25-in.) screws to half the length for all lengths greater than 

254 mm (10 in.). Lag screws have a hexagonal-shaped head 

and are tightened by a wrench (as opposed to wood screws, 

which have a slotted head and are tightened by a screw driver). The following equations for withdrawal loads are based 

on lag screws having a base metal average tensile yield 

strength of about 310.3 MPa (45,000 lb/in2) and an average 

ultimate tensile strength of 530.9 MPa (77,000 lb/in2).

The results of withdrawal tests have shown that the maximum direct withdrawal load of lag screws from the side 

grain of seasoned wood may be computed as follows:

p = 8,100G3/2 D3/4 L (inch–pound) 

where p is maximum withdrawal load (lb), D shank diameter (in.), G specific gravity of the wood based on oven dry 

weight and volume at 12% MC, and L length (in.) of penetration of the threaded part. (1 lb = 4.45 N; 1 in. = 25.4 mm.)

Assuming equivalent bolt diameter and penetration, the relative withdrawal resistance of species under consideration for 

sign post use can be compared on the basis of specific gravity (Table 5). Oaks, black locust, and hard maple species 

are likely to have the greatest maximum withdrawal loads, 

whereas white pine and eastern hemlock have the least. 

Southern Pine and red pine, the species currently utilized, 

have intermediate specific gravity. 

Moisture Content and Shrinkage

The following section briefly describes several important 

definitions and characteristics of wood that relate its relationship with moisture (Glass and Zelinka 2010). Green 

moisture content values and shrinkage characteristics for 

clear wood from several species are shown in Table 6. 

Green Moisture Content

Green wood is often defined as freshly sawn wood in which 

the cell walls are completely saturated with water and additional water may reside in the cell lumens. The moisture 

Table 5. Mechanical properties of Wisconsin species in comparison to Southern Pine and Douglas-fir

Clear wood propertiesa

Specific 

gravity

Modulus of 

elasticity 

(×106

 lb/in2)

Modulus of 

rupture

(lb/in2)

Impact 

(in.) 

Work to maximum load 

(in-lbf/in3)

Southern Pine Loblolly 0.51 1.79 12,800 30 10.4 

Slash 0.59 1.98 16,300 na 13.2 

Longleaf 0.59 1.98 14,500 34 11.8 

Shortleaf 0.51 1.75 13,100 33 11.0 

Douglas-fir Inland 0.46–0.50 1.49–1.83 11,900–13,100 20–32 9.0–10.6 

Coast 0.48 1.95 12,400 31 9.9 

Wisconsin hardwoods

Oak White 0.68 1.78 15,200 37 14.8 

Northern red 0.63 1.82 14,300 43 14.5 

Aspen Bigtooth 0.39 1.43 9,100 na 7.7 

Quaking 0.38 1.18 8,400 21 7.6 

Maple Red 0.54 1.64 13,400 32 12.5 

Sugar 0.63 1.83 15,800 39 16.5 

Black locust 0.69 2.05 19,400 57 18.4 

Basswood 0.37 1.46 8,700 16 7.2 

Ash Black 0.49 1.6 12,600 35 14.9 

Green 0.56 1.66 14,100 32 13.4 

White 0.6 1.74 15,000 43 16.6 

Wisconsin softwoods

Eastern white pine 0.35 1.24 8,600 18 6.8 

Balsam fir 0.35 1.45 9,200 20 5.1 

Eastern hemlock 0.4 1.2 8,900 21 6.8 

Red pine 0.46 1.63 11,000 26 9.9 

Black spruce 0.42 1.61 10,800 23 10.5 

White spruce 0.36 1.43 9,400 20 7.7 

Jack pine 0.43 1.35 9,900 27 8.3 

 a1 lb/in2 = 47.880 Pa; 1 in. = 25.4 mm; 1 in-lbf/in3 = 6.89 kJ/m3
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

31

content of green wood can range from about 30% to more 

than 200%. In green softwoods, the moisture content of 

sapwood is usually greater than that of heartwood. In green 

hardwoods, the difference in moisture content between 

heartwood and sapwood depends on the species. The average moisture content of green heartwood and sapwood of 

several species is given in Table 6. These values are considered typical, but variation within and between trees is considerable. Variability of green moisture content exists even 

within individual boards cut from the same tree. 

Shrinkage Values

Wood is dimensionally stable when moisture content is 

greater than the fiber saturation point. Below the fiber saturation point, wood changes dimension as it gains moisture 

(swells) or loses moisture (shrinks), because volume of 

the cell wall depends on the amount of bound water. This 

shrinking and swelling can result in warping, checking, and 

splitting of the wood, which in turn can lead to decreased 

utility of wood products, such as loosening of tool handles, 

gaps in flooring, or other performance problems. Therefore, 

it is important that the dimensional stability be understood 

and considered when a wood product will be exposed to 

large moisture fluctuations in service.

With respect to dimensional stability, wood is an anisotropic material. It shrinks (and swells) most in the direction of 

the annual growth rings (tangentially), about half as much 

across the rings (radially), and only slightly along the grain 

(longitudinally). The combined effects of radial and tangential shrinkage can distort the shape of wood pieces because 

of the difference in shrinkage and the curvature of annual 

rings. The major types of distortion as a result of these effects are illustrated in Figure 21.

Transverse and Volumetric Shrinkage

Shrinkage values, expressed as a percentage of the green 

dimension, are listed in Table 6. The shrinkage of wood is 

affected by a number of variables. In general, greater shrinkage is associated with greater density. The size and shape 

of a piece of wood can affect shrinkage, and the rate of drying can affect shrinkage for some species. Transverse and 

volumetric shrinkage variability can be expressed by a coefficient of variation of approximately 15%.

Longitudinal Shrinkage

Longitudinal shrinkage of wood (shrinkage parallel to the 

grain) is generally quite small. Average values for shrinkage 

from green to oven dry are between 0.1% and 0.2% for most 

Table 6. Average moisture content and shrinkage of Wisconsin species in comparison with Southern Pine 

and Douglas-fir

Wood species

Average moisture content of green 

wood (%)

Average shrinkage

(%)

Heartwood Sapwood Radial Tangential Volumetric

Southern Pine Loblolly 33 110 4.8 7.4 12.3

Slash NAa NA 5.4 7.6 12.1

Longleaf 31 106 5.1 7.5 12.2

Shortleaf 32 122 4.6 7.7 12.3

Douglas-fir Inland NA NA 3.8–4.8 6.9–7.5 10.7–11.8

Coast 37 115 4.8 7.6 12.4

Wisconsin hardwoods

Oak White 64 78 5.6 10.5 16.3

Northern red 80 69 4 8.6 13.7

Aspen Bigtooth NA NA 3.3 7.9 11.8

Quaking 95 113 3.5 6.7 11.5

Maple Red NA NA 4 8.2 12.6

Sugar 65 72 4.8 9.9 14.7

Black locust NA NA 4.6 7.2 10.2

Basswood 81 133 6.6 9.3 15.8

Ash Black 95 NA 5 7.8 15.2

Green NA 58 4.6 7.1 12.5

White 46 44 4.9 7.8 13.3

Wisconsin softwoods

Eastern white pine NA NA 2.1 6.1 8.2

Balsam fir 88 173 2.9 6.9 11.2

Eastern hemlock 97 119 3 6.8 9.7

Red pine 32 134 3.8 7.2 11.3

Black spruce 52 113 4.1 6.8 11.3

White spruce NA NA NA NA NA

Jack pine NA NA 3.7 6.6 10.3 a

NA is not applicable.
General Technical Report FPL–GTR–231

32

species of wood. However, certain types of wood exhibit excessive longitudinal shrinkage, and these should be avoided 

in uses where longitudinal stability is important. Reaction 

wood, whether compression wood in softwoods or tension 

wood in hardwoods, tends to shrink excessively parallel 

to the grain. Wood from near the center of trees (juvenile 

wood) of some species also shrinks excessively lengthwise. 

Reaction wood and juvenile wood can shrink 2% from 

green to oven dry. Wood with cross grain exhibits increased 

shrinkage along the longitudinal axis of the piece. Reaction 

wood exhibiting excessive longitudinal shrinkage can occur in the same board with normal wood. The presence of 

this type of wood, as well as cross grain, can cause serious 

warping, such as bow, crook, or twist, and cross breaks can 

develop in the zones of high shrinkage.

Minimizing Warp in Sign Posts

Warp in wood is caused by differential shrinkage as wood 

dries. Warp is typically grouped into five categories: bow, 

crook, twist, diamonding, and cup (Fig. 22) (Simpson 1991). 

Bow is a deviation from a straight line drawn from end to 

end on the widest face of a piece of lumber or timber. Crook 

is similar to bow except that the deviation is edgewise rather 

than flatwise. Both crook and bow are associated with differential longitudinal shrinkage on opposite faces. Twist 

is the turning of the four corners of any face of a post so 

that they are no longer in the same plane. It occurs in wood 

containing spiral, wavy, diagonal, distorted, or interlocked 

grain (Simpson 1991). Bow, crook, and twist are the likely 

the most frequent types of warp in sign posts (Fig. 23). Diamonding is a form of warp found in squares or thick lumber 

such as sign posts. In a square, the cross section assumes 

a diamond shape during drying. Diamonding is caused by 

the difference between radial and tangential shrinkage in 

squares in which the growth rings run diagonally (Simpson 

1991). Although diamonding can occur in the dimensions 

used for sign posts, it is not commonly observed in pressuretreated pine. Cup is a deviation across the width of a board 

causing the edges of the board to be higher than the middle. 

Although sometimes a problem in treated decking, it is typically not a concern with the dimensions of lumber used in 

sign posts. 

Green sign posts are initially dried (usually in a dry-kiln) 

under controlled conditions prior to pressure treatment. 

Some warp may be observed after this initial drying but 

ideally those pieces would be rejected prior to pressure 

treatment. Pressure treatment with water-based preservative 

then re-wets the wood. Moisture contents after treatment 

with water-based preservatives vary, but are consistently 

above 30% moisture content. Because nearly all shrinkage 

occurs as the wood dries below 30% MC, pressure treatment with water-based preservatives does cause subsequent 

shrinkage and creates and opportunity for warp to occur. 

Pressure treatment with oil-type preservatives such as creosote, pentachlorophenol, and copper naphthenate does not 

reintroduce water to the wood, and thus shrinkage and warp 

is limited to that which occurs when drying the green posts 

prior to treatment. 

Although the extent of shrinkage that occurs does vary by 

species (Table 6), these differences are relatively small and 

Figure 21. Characteristic shrinkage and distortion of flat, 

square, and round pieces as affected by direction of growth 

rings. Tangential shrinkage is about twice as great as 

tangential. Source: Glass and Zelinka (2010).

Figure 22. Common types of warp that develop during 

drying. Source: Simpson (1991).
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

33

not always a good indicator of the potential for warp. Instead, warp is usually associated with wood quality issues 

within individual pieces. Although the cause of warp is not 

always obvious, it is often associated with the presences of 

juvenile wood or reaction wood. When juvenile wood or 

reaction wood is present on one side of the post but not on 

the opposite face, the differential in longitudinal shrinkage 

can cause warp. Growing conditions that create these wood 

characteristics can be the greatest contributor to warp. In 

some cases, stands of timber may have an unusually high 

proportion of reaction wood or juvenile wood. If these trees 

are sent to single mill, sawed, dried, and then pressure-treated by a single facility, it can create a run of material from 

that treater with a greater than normal volume of problem 

pieces. In part because of their length (up to 22 ft (6.7 m)) 

sign posts are vulnerable to warping as they dry following 

pressure treatment. 

Because wood characteristics are the primary contributor 

to excessive warp, there is no easy solution to lessening the 

extent of warp in treated sign posts. Current WisDOT practice is to have the posts stickered after treatment to promote 

drying and to minimize the time that the posts are exposed 

horizontally outdoors during storage. Casual observation of 

in-service WisDOT posts indicates that relatively few have 

obvious warp, but this does not account for the number of 

posts that may have been rejected by WisDOT personnel 

prior to installation. 

A supplier has suggested that WisDOT might experience 

less warping if the posts were close-stacked after treatment 

to minimize drying prior to installation. It is unclear if less 

warp will occur in sign posts if drying and shrinkage occurs 

after installation, but this type of approach is effective in 

other applications. Treated deck boards, for example, can 

be prevented from warping if securely screwed into place 

prior to drying. However, sign posts are not as thoroughly 

restrained as deck boards. In two-post signs, some restraint 

is provided by the ground contact on the bottom end and 

the sign on the top end. The extent of restraint in this case 

would depend on the firmness of the post embedment, the 

length of the posts, and the dimensions of the sign. In single 

post signs, there is no restraint on the upper end of the post, 

allowing warp to occur in the length of the post that is above 

ground (Fig. 24). However, there are at least two other possible mechanisms that could lead to reduced warp in sign 

posts installed while wet. First, the vertical exposure of a 

sign post may allow more uniform drying than the horizontal exposure of stacked posts. The outer layers of stacked 

posts are subjected to differential drying because the outer 

surfaces are exposed to more airflow than the inner surfaces. 

Bow

Twist

Figure 23. Examples of bow and twist in sign posts. 

Source: Matt Rauch, WisDOT. 

Figure 24. Warp appears to be more common, 

or at least more visible, in single-post than 

two-post signs.
General Technical Report FPL–GTR–231

34

In addition, the upper surface of the top layer of posts may 

be subjected to direct sunlight while the bottom surface of 

those posts remains relatively cool. In contrast, in-service 

sign posts have roughly equivalent air flow to all sides (except the area contacting the sign) and although the sun is 

likely to hit one or two sides of a post more than others, the 

angle of the sun is much less extreme. It is also possible that 

shrinkage would be lessened in installed posts because they 

would not dry to a moisture content as low. Moisture from 

the ground is likely to prevent drying in portions of the post 

near the groundline, while precipitation will keep the upper 

end of the post moist for a substantial portion of the time. 

Thus, the length of the post subjected to drying stresses 

might be substantially reduced. It is important to note, however, that these potential mechanisms of reduced warp are 

speculative and have not been evaluated.

The most common approach to minimizing warp during 

drying is the application of physical restraints (Simpson 

1991) and reduction of warp by physical restraint during kiln-drying appears to be fairly permanent (Shmulsky 

2006). The concept is similar to screwing a deck board 

securely in place before it dries. In kiln drying, this may be 

accomplished by placing concrete weights (exerting at least 

244 kg/m2 (50 lb/ft2)) on top of the stack in the kiln. This 

approach is fairly effective at preventing bow or twist, but 

less effective in preventing crook because the pieces can 

slide along the stickers. Research indicates that, in combination with top-loading, crook can also be reduced if serrated 

(toothed) aluminum stickers are used to prevent pieces from 

sliding on the stickers (Koch 1974). However the use of serrated stickers does not appear to have gained commercial 

acceptance. 

There has been relatively less research on minimizing warp 

during drying after preservative treatment, but physical 

restraints may be used in a manner similar to initial kilndrying. One study noted that warp in CCA-treated Southern 

Pine 4 by 4s (nominal 3.5 by 3.5 in. (89 by 89 mm) could be 

significantly reduced by applying pressure with pneumatic 

cylinders during kiln-drying (Shumlsky 2006). There is also 

little research on the potential use of physical restraint during air-drying to minimize warp in treated wood, although 

it is likely to provide a similar benefit. The state of Kansas 

sign post specification includes the direction to “... tightly 

band with spaced layers to permit air flow between each 

layer and minimize warping. Banding consists of 1 band 

for each 4 feet of bundle length, with a maximum spacing of 

four feet between bands, end bands being not more than 1 

foot from the end of the bundles. Place spacers (stickers), a 

minimum of 1/8 inch in thickness, between each horizontal 

layer of posts at each banding location.” (KDOT 2007). 

It is likely that the intent of the banding requirement is to 

restrain the posts from warping during drying, but it is unclear how effective this approach might be. Once the wood 

begins to shrink the bands will no longer restrain the wood, 

and thus benefit is probably limited to prevention of more 

extreme cases. Application of a top load might provide additional benefit. 

Other Options: Structural Wood 

Composites and Naturally Durable 

Species

Structural Wood Composites

Structural wood composites are sometimes used as alternatives to solid-sawn wood products. Structural wood composites are lumber, veneers, or strands glued together to 

form larger, longer members. The most common example 

is the glulam beams used for both interior and exterior 

construction. Less common, but still well established in the 

marketplace, are parallel strand lumber (PSL) and laminated 

veneer lumber (LVL). All of these structural composites 

allow the production of wood products that are tailored to 

dimensions and mechanical properties needed for a specific 

application. The exterior adhesives allow the composites to 

be used outdoors, and even in ground contact. Although still 

a small percentage of the pole market, glulam utility poles 

are sometimes used in areas where predictable, engineered 

strength properties are particularly important. The lumber 

used in glulam posts, poles, and beams can potentially be 

pressure-treated either before or after gluing. If treated before gluing, water-based preservatives are typically used to 

minimize interference with adhesive bonding. If treated after 

gluing, oil-type preservatives are generally used to minimize 

the drying stresses created by the swelling and shrinking 

associated with water-based preservatives. PSL is typically 

treated after gluing and can be treated with water or oil-type 

preservatives. LVL is only standardized for treatment with 

creosote after gluing. Southern Pine and Douglas-fir are the 

woods primarily used in structural composites, although 

western hemlock, Hem-Fir, red oak, and red maple are also 

standardized, and in theory, a range of other wood species 

could also be used. 

Structural composites potentially offer several advantages 

and at least two disadvantages for use in sign posts. In general, the properties of the structural composite posts would 

be less variable and more predictable than solid sawn posts. 

They could be engineered to more closely meet strength 

requirements and it may even be possible to design posts so 

that drilling of holes to ensure break-away on impact would 

not be necessary. They would also be less likely to warp 

than solid sawn posts. It is also possible that a greater range 

of Wisconsin wood species could be used because it would 

no longer necessary to obtain 5.5–6.7 m (18–22 ft) long, 

straight pieces. The primary disadvantage of structural composite posts would be additional cost. A secondary disadvantage would be fewer potential suppliers and more limited 

availability. 
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

35

Naturally Durable Species

Wisconsin has several wood species with some natural durability. The most common of these are black locust (not 

native and often considered invasive), eastern red cedar 

(native but sometimes considered invasive), northern white 

cedar, and white oak. Black locust was widely planted for 

erosion control by the Civilian Conservation Corps and has 

since become naturalized. It reproduces vigorously by root 

suckering and stump sprouting and forms a dense canopy 

that crowds out native species. Black locust appears to be 

among the most durable of trees growing in Wisconsin and 

often grows to sufficient size to obtain the dimensions used 

for sign posts. However, the trees suffer from attack by a 

stem borer that can cause deformed growth and reduce the 

value of the wood. The wood is also difficult to dry without 

warping. Eastern red cedar and northern white cedar do not 

appear to be quite as durable black locust, and suffer from 

variability in their durability. It is uncertain that either of 

these species would consistently provide a 20+ year service 

life. In addition, trees of both species are relatively small, 

making it difficult to obtain the long dimensions needed for 

posts. Larger trees of both cedar species are also likely to 

have greater value in other applications. White oak also does 

not appear to be consistently durable enough to provide the 

expected service life of sign posts and like cedar, the higher 

quality wood has greater value in other products. Overall, 

none of the natural durable species in Wisconsin appear 

to be ideally suited for use in sign posts. Black locust may 

have the most potential because of its durability and size, 

and because it does not as great of value for other applications. However, the supply of suitable black locust posts is 

likely to be relatively low. 

Summary of Potential Use of 

Wisconsin Species in Sign Posts

Wisconsin DOT’s current use of red pine is a logical choice. 

Red pine is available in the necessary dimensions, is readily 

treated with preservatives, has relatively low cost, and has 

adequate strength properties. Choices among other Wisconsin wood species are less obvious. Although they are not 

currently being used, Wisconsin DOT’s current specification 

lists jack pine, eastern white pine, and oaks as acceptable 

species. Jack pine does not appear to be an ideal candidate 

for use in sign posts. The supply of jack pine in Wisconsin is 

relatively limited, and it is currently being utilized at a rate 

that exceeds is growth. Although jack pine is pressure-treated for various applications, it has a relatively thin sapwood 

band and requires incising to obtain adequate treatment. In 

contrast, eastern white pine appears to have some potential 

for use. There is a moderate supply of white pine in Wisconsin, and its rate of growth currently greatly exceeds its utilization. It is also a large tree that should allow mills to obtain 

posts of the necessary dimensions (Fig. 25). Eastern white 

pine also appears to be treatable with preservatives, and may 

be less prone to warping than red or Southern Pine. The concern with eastern white pine is its relatively low strength. 

Testing may be needed to determine if eastern white pine 

is has sufficient strength, especially for longer posts. In 

contrast, oak species have more than sufficient strength, but 

have other drawbacks in terms of signpost usage. One of 

these is cost; oaks have substantial value for other applications and oak posts are likely to be more expensive than 

pine posts. This is likely to be especially true for the higher 

quality oak wood. Red oak is moderately treatable, but higher retentions of water-based preservatives may be needed to 

impart durability. An oil-based preservative such as copper 

naphthenate may be more appropriate for red oak. White 

oak is extremely difficult to treat with preservatives, and 

although it does have some natural durability it is unlikely 

that it would be consistently durable in ground contact. 

Other species of potential interest are the soft maples and 

ash species. Soft maples are abundant in Wisconsin and their 

rate of growth greatly exceeds that of their utilization. Soft 

maple saw timber has moderately high value; less than the 

oaks but more than that of softwood species. Although not 

as strong as many other hardwoods, soft maple’s strength 

Figure 25. Eastern white pine's height and form 

should allow milling to obtain long posts.
General Technical Report FPL–GTR–231

36

easily exceeds that needed for sign posts. It also appears to 

be fairly treatable, although as with other hardwood species 

it appears to be most durable when treated with oil-based 

preservatives or higher retentions of water-based preservatives. A concern with the soft maples is whether their growth 

habit will make it practical to obtain the long-straight dimensions needed for sign posts. 

Potential use of ash is interesting because of concerns associated with the emerald ash borer and the increased harvesting of ash by various municipalities. Finding value-added 

applications for these trees may make their removal more 

palatable. Ash saw timber currently has moderate value but 

that may decline as the number of trees harvested to prevent 

disease increases. There has been relatively little research 

on the preservative treatment of ash, or its durability once 

treated. Treatability evaluations may be warranted before 

pursuing the use of ash sign posts. 

Of the species in Wisconsin with natural durability, black locust may have the greatest potential for sign post use. Black 

locust has sufficient strength and fairly high durability. It 

is also considered a weedy, potentially invasive species for 

which removal is often encouraged. However, black locust 

does have disadvantages for use in sign posts. It is susceptible to insect damage and heartwood decay (in the living 

tree) and it will be difficult to obtain large volumes of long, 

straight, defect-free black locust posts. 

Structural composite posts suffer little or no warp and could 

be manufactured to a wide range of dimensions and tailored 

closely to a targeted end use. They allow use of smaller trees 

and could potentially expand the number of useful Wisconsin wood species. However, composites posts are likely to 

be substantially more expensive than solid-sawn posts, and 

availability of posts constructed from Wisconsin wood species may be limited.

Wooden Signage Post 

Specifications in Other States

One approach to evaluating viable wood species/wood preservative combinations for Wisconsin wooden sign posts is 

to review the specifications of other states that have some 

similarities in forest resources. Not surprisingly, neighboring Michigan and Minnesota share the greatest similarity 

in forest types with Wisconsin (Fig. 26). However, there is 

some commonality in other Midwestern and northeastern 

states, and these states were included to expand the survey. 

Nearby states with relatively little native forest were also 

included to determine if they were utilizing species from the 

Great Lakes area. Several eastern Rocky Mountains states 

were also included to evaluate their possible use of aspen or 

small-diameter pine species (Fig. 26). 

 Of the 27 states surveyed, 18 include some form of wooden 

signposts in their state highway specifications (Table 7). 

However, one of those states (Ohio) only has specifications 

for posts constructed from laminated box beams. In addition, no evidence of permanent wooden sign post usage 

was seen in a Google Street View sampling of 7 of the 

18 states that do include wood in their specifications 

(Table 7). In Colorado, wooden posts were only observed 

Figure 26. Major forest types of the lower 48 states. Outlined states were selected for 

review of highway specifications for wooden signposts. Adapted from Forest Cover 

Types, National Atlas of the United States of America. U.S. Forest Service, Forest 

Inventory and Analysis, National Office, Arlington, VA. http://www.fia.fs.fed.us/library/

maps/docs/forestcover.pdf
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

37

Table 7. Wood species and preservatives referenced by states with specifications that include wooden sign posts

State Wood species Preservative treatment In use?a

Colorado Douglas-fir (South) or Douglas-fir–Larchb No pressure treatment specified. “Underground portions 

of timber sign posts, plus at least 6 inches above 

groundline, shall be painted with one of the preservatives 

listed in AASHTO M 13 3 (CDOT 2011).” (Currently 

copper naphthenate is the only AASHTO M 133 

preservative available for brush-on applications.) 

No

Illinois Southern Pine, Douglas-fir AASHTO M 133, Excluding ACQ types B and C and 

copper azole CA-B and CBA-A 

 Yes 

Indiana Southern Pine, coastal Douglas-fir, catalpa, 

northern white cedar, eastern and southern red 

cedar, black locust, yellow locust, mulberry, 

red, black, and white oak group, osage orange, 

redwood, sassafras 

 In accordance with AWPA Standards C14 and C2. 

Use heavy oil with oil-type preservatives, but post must 

be paintable 

 No 

Iowa Southern Pine, Douglas-fir ACZA, CCA, creosote, pentachlorophenol (heavy oil), 

copper naphthenate (heavy oil) 

 Yes 

Kansas Southern Pine, Douglas-fir ACA (precursor of ACZA) and CCAc Yes 

Maine Southern Pine (treated) or spruce, hemlock, 

cedar or Douglas-fir 

 CCA for treatment of southern pine if treated posts are 

specified. Other species are apparently used untreated in 

some situations. 

 Yes 

Maryland AASHTO M 168 AASHTO M 133 Yes 

Michigan Southern Pine, Douglas-fir, balsam-fir, eastern 

hemlock, eastern white pine, tamarack (eastern 

larch) 

 ACZA, CCA, pentachlorophenol. ACQ and copper azole 

were recently withdrawn from the signpost specification 

but are still allowed for guardrail posts and blocks. 

 Yes 

Missouri AWPA Standards (general reference)d

 AASHTO M 133, and also “other preservatives and wood 

species in accordance with AWPA Standards (MODOT 

2011).”

 Yes 

Montana Not specifiede

 ACA (precursor of ACZA), CCA and pentachlorophenol Yes 

Nebraska Coastal Douglas-fir ACA (precursor of ACZA), ACZA, CCA, 

pentachlorophenol, or copper naphthenate. Requires redrying to maximum of 30% moisture content. 

 Yes 

New Jersey Southern Pine, Douglas-fir CCA or pentachlorophenol (southern pine); ACZA 

(Douglas-fir) 

 No 

Ohio Laminated veneer box beams Pentachlorophenol No 

Pennsylvania Southern Pine, Douglas-fir–Larchb ACA (precursor of ACZA), CCA Yes 

South Dakota Coastal Douglas-fir, western larch, western 

redcedar 

 ACZA, CCA, pentachlorophenol, copper naphthenate. 

None required for western redcedar

 No 

Vermont Oak, cedar, spruce, western fir, or other 

approved wood 

 ACQ, CCA, creosote, pentachlorophenol (either light or 

heavy oil) 

 No 

Virginia Southern Pine CCA Yes 

West 

Virginia 

 AASHTO M 168 and structural softwoods 

meeting (SPIB rules) or hardwoods meeting 

NELMA structural gradesf

 For softwoods, creosote or water-based preservative listed 

in AWPA C14 and AWPA P5. Treat hardwoods with 

water-based preservative according to AWPA C2, 

excluding CCA (this would include ACC, ACA, ACZA, 

ACQ, CA-B, and CBA-A) 

 No 

Wyoming Southern Pine, Douglas-fir, lodgepole pine, 

ponderosa pine 

 Preservatives listed in AWPA Standard C14 and approved 

for ground contact. 

 Yes 

a

Permanent wooden signposts visible in a Google Street View sampling of state and interstate highways. b”Douglas-fir South” refers to a Western Wood Products Association (WWPA) species group originating from Douglas Fir growing in Arizona, Colorado, 

Nevada, New Mexico, and Utah. Douglas-fir–Larch refers to a mixture of Douglas-fir and western larch species. In the intermountain west these species are 

sometimes combined in dimension lumber products because they have similar physical properties. c

Also includes this drying instruction: “Stack and tightly band with spaced layers to permit air flow between each layer and minimize warping. Banding 

consists of 1 band for each 4 feet of bundle length, with a maximum spacing of four feet between bands, end bands being not more than 1 foot from the end of 

the bundles. Place spacers (stickers), a minimum of 1/8 inch in thickness, between each horizontal layer of posts at each banding location.” (KDOT. 2007) d

For general posts, AWPA Standards list Douglas-fir, western larch, western hemlock, and six species of pine. Hardwoods and additional softwoods are 

included under “sawn products.”

e

Other commodities: Allow Douglas-fir, hemlock, ponderosa pine, spruce, larch, or lodgepole pine for guardrail and fence posts, but only Douglas-fir, western 

larch or southern yellow pine for structural support piles.

f

Limiting softwoods to Southern Pine Inspection Bureau rules would limit species to Southern Pine group. NELMA stands for Northeastern Lumber 

Manufacturers Association.
General Technical Report FPL–GTR–231

38

as supports for temporary road construction signage. Although this brief sampling does not demonstrate that no 

wood posts are used in those states, it does indicate that they 

are used infrequently. No mention of the use of wooden sign 

posts was found in the specifications of the other nine states 

surveyed. However, none of these states explicitly prohibit 

the use of wooden sign posts, and all do allow wood for 

other structural members in ground contact such as guardrail 

posts or piles (Table 8). 

Wood Species Referenced by Other States

Many states do not appear to be focusing on the use of local wood species for sign posts. In states that do list specific 

wood species, Southern Pine and Douglas-fir are the most 

widely listed even though in most cases those species are 

not native to the state. It is likely that Southern Pine and 

Douglas-fir are specified because of familiarity and these 

species long history of use in structural members. However, 

some states do include other local species in their specifications. Perhaps the most relevant of these is Michigan, which 

lists balsam fir, eastern hemlock, eastern white pine, and 

tamarack (eastern larch) in addition to Southern Pine and 

Douglas-fir. Colorado does not specify Southern Pine and 

limits the wood species to Douglas-fir and western larch 

species groups that are predominately found in the Rocky 

Mountain states. Indiana’s specification includes a range of 

local species in addition to Southern Pine, Douglas-fir, and 

redwood. Although the state of Maine specification requires 

Southern Pine for treated posts, it also appears to allow use 

of untreated spruce, hemlock, cedar, and Douglas-fir for 

some applications. The neighboring state of Vermont does 

not specify either Southern Pine or Douglas-fir, but instead 

lists oak, cedar, spruce, and western fir. It is possible that 

western fir is intended to specify Douglas-fir, although there 

are several fir species in the western states. South Dakota 

specifies western larch and western redcedar in addition to 

coastal Douglas-fir (all non-native species) but somewhat 

surprisingly does not list the ponderosa pine that is plentiful 

in the Black Hills region. The state of Wyoming does incorporate native species by listing lodgepole pine and ponderosa pine along with Southern Pine and Douglas-fir. Three 

states (Maryland, Missouri, and West Virginia) cite either 

AASHTO M 168 or AWPA standards for allowable species 

rather than listing individual species. 

States that do not specify wood for sign posts also most 

commonly specify Southern Pine and Douglas-fir for 

other treated wood commodities (Table 8). A notable 

Table 8. Examples of other preservative-treated wood products specified by states that do not specify wooden 

sign posts 

State Commodities Wood species Preservatives

Connecticut Guard rail posts Southern Pine or 

Douglas-fir–Larcha

ACZA

Light poles AWPA C4b CCA, creosote, pentachlorophenol

Delaware Piles, guardrail posts Southern Pine or 

Douglas-fir

CCA (creosote was recently withdrawn)

Kentucky Guardrail posts AWPA C2 (includes 

many species)

AWPA C14, except creosote only for hardwoods 

Massachusetts Guardrail posts Unclear: implies 

AWPA standards

ACA (precursor to ACZA) or CCA

Piles Southern Pine or 

Douglas-fir

AASHTO M 133

Minnesota Guardrail posts Jack pine, red pine, 

lodgepole pine, 

ponderosa pine, or 

Southern Pine

ACA (precursor to ACZA), CCA, creosote, 

pentachlorophenol (heavy oil) 

New Hampshire Guardrail posts AASHTO M 168 Pentachlorophenol or water-based meeting preservative 

meeting AASHTO M 133

New York Guardrail posts Treated: Douglas- fir, 

pine, oak, birch, apple, 

beech 

Untreated: redwood, red 

cedar, cypress or

black locust

ACQ, CA-B or other preservative conforming to AWPA 

standards (documentation required). CCA is expressly 

prohibited based on state law.

North Dakota Piles, guardrail posts, 

piles

Only specify design 

properties

AASHTO M 133

Rhode Island Guardrail posts Spruce or fir AASHTO M 133

Piles Southern Pine or 

Douglas-fir

AASHTO M 133

a

Douglas-fir–Larch refers to a mixture of Douglas-fir and western larch species. In the intermountain west, these species are sometimes combined in 

dimension lumber products because they have similar physical properties.

b

AWPA Standard C4 (Poles) lists Southern Pine, coastal Douglas-fir, jack pine, red pine, lodgepole pine, ponderosa pine, western larch, western 

redcedar, and Alaska yellow cedar.
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

39

exception is Minnesota, which does include red pine, jack 

pine, lodgepole pine, and ponderosa pine in its specification 

for guardrail posts. New York state also has an expanded list 

of species for guardrail posts, including pine (no specifics on 

species), oak, birch, beech, and even apple. They also allow 

redwood, red cedar, cypress, and black locust as untreated 

guardrail posts. Rhode Island also allows use of spruce and 

fir in guardrail posts, but limits pile species to Southern Pine 

and Douglas-fir. Connecticut cites AWPA Standard C4 for 

wood species used in light poles, which would appear to 

allow jack pine, red pine, lodgepole pine, ponderosa pine, 

western larch, western redcedar, and Alaska yellow cedar. 

The highway specifications from other states (Table 7) 

should be interpreted carefully for their applicability to using alternative Wisconsin wood species for sign posts. Relevant alternative species listed in other states’ specifications 

include balsam fir, eastern hemlock, eastern white pine, oak, 

spruce, and tamarack (eastern larch), as well as the naturally 

durable species eastern red cedar and black locust. However, 

all states except Vermont also include Southern Pine or 

Douglas-fir in their specification, and it is likely that these 

two species account for the bulk of their wood sign posts. In 

other cases, a state may list wooden sign posts in their specification but not actively use them. Communication with staff 

at Vermont’s DOT revealed that they typically do not use 

wooden sign posts of any species, a finding that agrees with 

the lack of wooden posts observed in Google Street View. 

Indiana lists many species but some do not appear to be suitable for production of large volumes of longer sign posts 

(although perhaps they could be occasionally used for shorter posts), and wooden posts were not observed in a Google 

Street View sampling of that state’s highways. Thus, inclusion of alternatives species in a state’s specification does not 

necessarily indicate that these species (or wood sign posts in 

general) are successfully utilized on a consistent basis. Similarly, states that do use wooden sign posts may not use them 

in the same lengths or for as broad a range of applications as 

does Wisconsin. 

The most relevant specification for comparison appears to 

that of the state of Michigan. Michigan’s specification for 

wooden signposts is detailed and has been kept current with 

AWPA Standards. Michigan also includes three wood species (balsam fir, eastern hemlock, and eastern white pine) 

which are major wood species in Wisconsin (Table 7). However, in response to an inquiry, the Michigan Department of 

Transportation indicated that their suppliers are generally 

providing Southern Pine posts, and that their experience 

with other species is minimal.

Although an examination of California’s sign post specification was not included in this review, it is interesting to 

note that California uses wooden signposts extensively, and 

for larger signs than either Wisconsin or Michigan (Fig. 

27). California uses post dimensions as large as 6 by 10 in. 

(nominal) or round poles to support larger signs.

Wood Preservatives Referenced by Other 

States

A fairly broad range of sign post preservative treatments 

are allowed by state’s highway specifications, with ACZA 

(ACA) and CCA most frequently specified (Table 7). However, several states also list oil-type treatments, while others 

refer to AASHTO M 133 or AWPA Standard C14 (Highway 

Construction) rather than specifying individual preservatives. As shown in Table 9, AASHTO M 133 and AWPA 

Standard C14 list most of the major ground-contact oil and 

water-type ground-contact wood preservatives. An important difference between the two standards is that AWPA 

Standard C14 does not list the most recent version of copper 

azole (CA-C) or the newer particulate (micronized) copper 

preservatives that have received evaluation reports from the 

ICC-ES. Because AWPA Standard C14 was replaced as part 

of the conversion to the Use Category System, it has not 

been updated since 2004 and will not be updated in the future. Michigan’s specification cites the more current AWPA 

standard, which does include CA-C, but does not include the 

micronized copper preservatives because data packets for 

these preservatives have not been submitted to AWPA for 

review. 

Those states that specify all preservatives in AASHTO M 

133 or AWPA Standards (or individually specifying ACA 

or ACZA) appear to be allowing use of the high-copper 

preservatives (ACQ, copper azole, ACZA) that are potentially corrosive in contact with aluminum signs. However, 

because CCA is also allowed under those specifications it 

is likely that most of their post suppliers have continued to 

use CCA. It is also possible that these states are taking other 

steps to minimize corrosion, although such measures are not 

readily evident in their specifications. The state of Illinois 

Figure 27. California uses round poles or large sawn posts 

to support larger highway signs.
General Technical Report FPL–GTR–231

40

is an exception in that it does specify AASHTO M 133, but 

specifically excludes ACQ and copper azole formulations, 

presumably because of corrosion concerns. Similarly, Michigan specifies use of ACZA, CCA, or pentachlorophenol 

but specifically excludes use of ACQ and copper azole for 

sign posts even though those preservatives are allowed for 

guardrail posts. Michigan also adds a footnote: “Non-Metallic washers or spacers are required for timber and lumber 

treated with ACQ or CA placed in direct contact with aluminum (MDOT. 2012).” Currently, the suppliers to Michigan 

DOT are primarily providing sign posts treated with CCA 

(personal communication on February 19, 2013, with Steven 

Kahl, Supervising Engineer, Michigan Department of Transportation, Lansing, Michigan.)

There appears to be little attempt to minimize use of the 

conventional “heavy duty” preservatives ACZA, CCA, 

creosote, and pentachlorophenol preservatives in state sign 

post specifications. This may in part reflect that some of the 

specifications have not been updated recently (as evidenced 

by specifying ACA and AWPA Standard C14) but even the 

more current specifications include the heavy duty preservatives. The extent of human contact is low for highway sign 

posts in comparison to some construction applications, and 

this may have lessened perceived health concerns associated 

with the older preservatives.

Summary 

Preservatives and Wood Species

WisDOT’s current practice of using red pine or Southern 

Pine posts treated with CCA is logical and may be the 

optimum combination of wood species and preservatives 

currently available. Red pine and Southern Pine are readily 

available wood species with relatively large and treatable 

sapwood zones. They are also relatively strong compared 

with many other softwood species. Pressure-treated Southern Pine is widely used for structural purposes, including 

for sign posts by other states. Use of red pine is more geographically limited, but it is an important local resource and 

a logical choice for use in Wisconsin. The use of CCA wood 

preservative is also a logical choice. CCA is an effective 

preservative with a long track record and is compatible with 

aluminum signs. Although CCA does contain arsenic, and 

is a RUP, it is still commonly used for treatment of utility 

poles, marine piles, and bridge timbers. It also appears to be 

the preservative most widely used by other states for treatment of sign posts. A potential disadvantage of CCA is that 

it does not appear to protect hardwoods species as well as 

it does softwood species. If WisDOT begins utilizing hardwood species for sign posts, it may be necessary to either 

increase the CCA retention requirement or specify that the 

hardwoods be treated with an oil-type preservative.

Table 9. Sign post applicable preservatives listed in either AASHTO M 133 or AWPA 

Standard C14a

Listed byb

Pressure-treatment preservative 

AASHTO

M 133 

AWPA 

Standard C14 

Water-type preservatives 

Acid copper chromate (ACC) Yes Yes 

Alkaline copper quat (ACQ) Type B Yes Yes 

Alkaline copper quat (ACQ) Type C Yes Yes 

Alkaline copper quat (ACQ) Type D Yes No 

Ammoniacal copper arsenate (ACA) No Yes 

Ammoniacal copper zinc arsenate (ACZA) Yes Yes 

Chromated copper arsenate (CCA) Yes Yes 

Copper azole Type A (CBA-A) Yes Yes 

Copper azole Type B (CA-B) Yes Yes 

Copper azole Type C (CA-C) Yes No 

Micronized/dispersed) copper azole (MCA or μCA-C) Yes No 

Micronized copper quat (MCQ) Yes No 

Oil-type preservatives 

Copper naphthenate Yes Yes 

Creosote and creosote solutions Yes Yes 

Pentachlorophenol (heavy or light oil) Yes Yes a

States that do not specify wood for sign posts do specify similar preservatives for other treated wood commodities 

(Table 8). Most commonly these states reference AASHTO M 133, but ACZA, CCA, creosote, and pentachlorophenol 

are also individually specified. However, two of these states expressly exclude either creosote (Delaware) or CCA 

(New York). The prohibition on use of CCA in New York appears to be based on general state policy rather than 

concerns about the risks associated with use of CCA-treated wood in highway construction. ACQ and copper azole are 

suggested alternatives, but it should be noted that New York does not use wooden sign posts and thus contact with 

aluminum is less of a concern. In Delaware, removal of creosote from the specification appears to derive from concerns 

about the potential environmental impact of treated wood placed into aquatic environments (i.e., bridge piles). b

From the 2010 version of AASHTO M 133 and the 2004 (last) version of AWPA Standard C14. AWPA Standard C14 

is no longer being updated because AWPA converted to the Use Category System. 
Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

41

Considerations for Alternative Preservatives

Potential alternatives to CCA fall into two categories: 

water-based preservatives and oil-based preservatives. The 

water-based preservatives that are suitable for sign posts 

rely primarily on copper for efficacy and appear to present 

an increased risk of corrosion of the aluminum signs. The 

particulate copper systems that have become widely used in 

residential decking are marketed as having increased compatibility with aluminum, but may still be more corrosive 

than CCA. The particulate copper systems are also less able 

to penetrate difficult to treat wood species than water-soluble preservatives. Thus, there are no obvious alternatives to 

CCA among the current water-based preservatives. Because 

compatibility with aluminum is the greatest obstacle to using copper-based wood preservatives, it may be desirable to 

evaluate additional approaches to isolating the sign from the 

post. 

All of the common oil-type preservatives (creosote, copper naphthenate, and pentachlorophenol) would effectively 

protect sign posts from decay, and all have the additional 

advantage of compatibility with aluminum. In addition, they 

are effective in protecting both softwood and hardwood 

species, and are likely to diminish the occurrence of warp 

in the sign posts. However, creosote and pentachlorophenol 

are RUPs, and thus would not overcome potential toxicity 

concerns associated with CCA. Copper naphthenate is not a 

RUP and may be a reasonable alternative to CCA. It does, 

however, have a noticeable odor and wood may initially 

have an oily surface. 

Considerations for Alternative (Wisconsin) Wood 

Species

In addition to red pine, WisDOT’s current specification 

lists jack pine, eastern white pine, and oaks as acceptable 

Wisconsin species. The supply of jack pine in Wisconsin is 

relatively limited, and it has a relatively thin sapwood band 

and requires incising to obtain adequate treatment. Eastern 

white pine is available in larger volumes, appears to be 

treatable with preservatives, and may be less prone to warping than red or Southern Pine. However, white pine may not 

have sufficient strength for sign posts. In contrast, oak species have more than sufficient strength, but the long, straight 

dimensions needed for sign posts are likely to have much 

greater value in other applications. In addition, white oak is 

difficult to treat with preservatives. 

Additional species of interest not currently listed in WisDOT specifications are the soft maples and ash. Soft maples 

are abundant in Wisconsin and their rate of growth greatly 

exceeds that of their utilization. 

Soft maple also appears to be fairly treatable, although as 

with other hardwood species they appear to be most durable 

when treated with oil-based preservatives or higher retentions of water-based preservatives. A concern with the soft 

maples is whether their growth habit will make it practical 

to obtain the long-straight dimensions needed for sign posts. 

Ash is currently being harvested by various local governments in anticipation of emerald ash borer infestation and 

there is interest in finding value-added applications for these 

trees. However, there is relatively little experience with ash 

treatability and durability, or its susceptibility to warp when 

used in long dimensions. In general, the supply of sign posts 

from alternative Wisconsin wood species might be expected 

to be more limited and less consistent than that of red pine. 

However, it may be possible to increase use of alternative 

species by focusing their use on shorter posts or posts with 

lesser strength requirements. 

WisDOT’s current use of red pine or Southern Pine posts 

treated with CCA may be the optimum combination of wood 

species–preservative currently available. Continued use of 

this combination is recommended unless/until CCA and/

or these wood species become unavailable and WisDOT 

choses to purchase posts of one or more hardwood species 

(in which case an oil-type preservative would be more appropriate).

Copper naphthenate in oil solvent appears to be one of the 

most logical alternative wood preservatives to use instead 

of CCA. However, wood treated with copper naphthenate 

has some odor and may initially have a somewhat of oily 

surface. WisDOT may want to consider purchasing a small 

volume of copper naphthenate-treated posts to evaluate their 

handling characteristics. Copper naphthenate is less widely 

used than CCA, and there are currently no pressure-treatment facilities in Wisconsin using copper naphthenate. 

Of Wisconsin tree species, red pine appears to be best suited 

for use in sign posts, and its continued use is recommended. 

Other Wisconsin species are likely to be either less readily 

available in necessary dimensions, less treatable with preservatives, lacking in needed strength properties, or more 

costly. If use of other Wisconsin species becomes a priority, 

species to consider include the following:

•	 Eastern white pine, which is already allowed under WisDOT specifications. Strength may be a concern with this 

species.

•	 Soft (red and silver) maple, which are not currently in 

WisDOT specifications but which grow in relatively 

large volumes in Wisconsin and are relatively underutilized. Cost, availability in needed dimensions, and warp 

may be considerations with these species. 

•	 No clear-cut solution was identified for reducing the 

incidence of warp in sign posts, although it is likely that 

warp could be reduced by use of an oil-type preservative. The most effective approach to minimizing warp 

during drying appears to be physical restraint of the 

stickered stack of posts during drying. Such physical 

restraint may be impractical to employ on a routine basis, but WisDOT may want to consider placing weights 

(such as concrete traffic barriers) on some stacks of 
General Technical Report FPL–GTR–231

42

drying posts to evaluate this option. WisDOT may also 

want to consider installing some posts while still wet as 

suggested by a supplier. These posts would need to be 

identified and monitored for future evaluation of warp 

development. Ideally they would be compared with posts 

from the same supplier shipment that had been dried 

before installation. When posts are stacked outdoors to 

dry, direct contact of sunlight on the outer surface of the 

upper layer of posts could cause more rapid drying on 

that face and contribute to increased drying stresses and 

warp. The practice of drying posts indoors should help to 

minimize this problem and should be continued. If posts 

are dried outdoors, it may be worthwhile to cover the upper surface to protect it from direct sunlight and rainfall. 

For example, sheets of plywood could be placed on stack 

of posts (with additional stickers between the posts and 

the plywood). 

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