Chapter 19. Initial Preservative Treatment of Wood in Covered Bridges

Introduction

The chapter provides information on treatment chemicals, pretreatment specifications, and pertinent standards that help specify wood for covered bridges that will resist biodeterioration. Wherever possible, consult the original standards to confirm that the chemicals suggested are suitable for a particular application.

Historic covered bridges were masterpieces of design that allowed untreated wood to survive for many decades. However, in most covered bridges, some members eventually succumb to decay or insect attack. This chapter discusses options for preservative treatment of the replacement members, while considering factors such as appearance, odor, durability, availability, and environmental concerns.

The basic design premise employed in covered bridges is to limit exposure to water, which is one of the essential elements for biodeterioration. All organisms capable of substantial biodegradation have four basic needs: adequate temperature, oxygen, water, and a food source. The temperature ranges for most decay organisms are broad. They survive temperatures well below freezing and above 40°C (104°F) and do best between 16 and 32°C (64.4 and 89.6°F), depending on the organism. Most wood-degrading organisms are aerobic and require oxygen, but many organisms are adapted for low-oxygen environments. As a result, few strategies for preventing decay limit oxygen or temperature.

Water is essential for nearly all fungal action. It swells wood, and at higher levels, it serves as a diffusion medium for fungal enzymes and the breakdown products they produce, and is a reactant in the processes whereby the carbohydrate components of the wood are broken into smaller fragments.

Most wood-degrading agents require that free or liquid water be present in the wood before substantial attack can occur. The point where liquid water is present is called the fiber saturation point, and, for most wood species, falls between 24 and 32 percent (water-to-wood weight ratio). Covered bridges were constructed to limit the potential for the bridge deck and superstructure to become wet enough to allow fungal attack.

Wood is usually the food source for decay (although some agents, notably carpenter ants, do not consume wood as a food source). Bridge designers have poisoned this food source, depending on naturally toxic compounds present in the heartwood of those species, or, when these chemicals are absent, adding supplemental chemicals to preserve the wood.

Decay is therefore most likely to occur on decay-susceptible wood species that are exposed where they are likely to become wet above the fiber saturation point under temperature regimes favorable for growth. One can predict the rate of decay out of direct soil contact by evaluating the number of days per month with measurable rainfall and the average monthly temperature. These data are used to construct a climate index. The Scheffer Climate Index is one example of this approach and produces values from 0 to 400 for low to extremely high decay hazards, respectively. Thus, a bridge in southern Mississippi is at much greater risk for decay than a similar structure in northern Maine. These indices are reasonably predictive for above ground exposures, but are poor predictors of wood performance in direct soil contact.

Evaluating the Need for Preservative Treatment

Preservative treatment of replacement members in covered bridges may not be necessary in bridges where the wood will not become wet or where there is no risk of insect attack, but the wood in most bridge components will last longer if it receives some type of supplemental treatment. In most environments, wood must be exposed to liquid water to accumulate enough moisture to cause decay. Even in humid climates, the moisture content of wood protected from precipitation is typically below 25 percent. Covered bridge designers were aware of this moisture content relationship and effectively used it to protect the bridge components. However, even in a covered bridge, it is difficult to protect wood from liquid water continuously, and several areas of the structure are particularly susceptible to wetting. Perhaps the most important of these are supports such as sole plates members that directly contact the ground, stone, or masonry. Other areas of exposure are the weatherboarding and members near the ends of the bridges where wind-driven precipitation can reach the interior of the structure. The risk of deterioration in a covered bridge depends upon the wood species, amount of rainfall, local temperature, and ability of various design elements to shed or exclude moisture. Thus, the risk of fungal attack on interior members not exposed to wetting will be low, while large timbers exposed to wetting can experience considerable degradation.

Decay risk also can be affected by the presence of water-trapping joints between members, bolts, or connectors that channel water into the interior, and other features that encourage water ingress. While covering a bridge deck reduces the risk of biodeterioration, it cannot completely eliminate that risk.

Perhaps the most reliable indicator of whether a replacement member should be treated with preservatives is the condition of the member being replaced. If the member being replaced had suffered biological attack, and no significant changes were made in the structure to remedy the sources of moisture for that member, then preservative treatment should be strongly considered.

Types of Deterioration

Covered bridges are primarily susceptible to attack by fungi and insects, although birds such as woodpeckers and some mammals (such as voles or porcupines) can physically damage the wood as they seek food, shelter, or nesting sites.

The fungi that attack wood are divided into five broad groups, based on the type of damage they cause. All of these fungi have life cycles that begin with simple spores that land on the wood and geminate to produce hyphae that grow into the wood, where their enzymes degrade specific components of the wood. After it obtains enough energy, the fungus produces more spores, and the cycle is repeated. Fungal spores are almost always present in the air. For example, a single fungus-fruiting structure can produce nearly one billion spores in a single day. As a result, favorable conditions for fungal attack will inevitably lead to decay.

Molds and Stains

These fungi generally grow through the ray cells, which store starches and other compounds in living trees. Stain and mold generally do not attack the structural components of the wood, and their damage is primarily cosmetic. Mold fungi produce prodigious amounts of pigmented spores on the wood surface; these can be removed by brushing. Stain fungi discolor the wood green, blue, or black with brownish pigments produced in the hyphae growing within the wood. This damage penetrates deep into the sapwood and cannot be removed by brushing.

Decay Fungi

Decay fungi all affect cellulose, hemicellulose, or lignin in the wood cell and can cause substantial reductions in material properties. The three types of attack can be distinguished based on the appearance of the decayed wood.

Soft rot fungi cause a gradual degradation from the surface inward and primarily attack cellulose and hemicellulose. These fungi are more prevalent in very wet environments, in agricultural soils, or on certain preservative treatments. In covered bridges, this damage is most likely to occur wherever bridge timbers abut locations where water can collect. Soft rot attack gradually softens the outer surface of both hardwoods and softwoods. Because the attack occurs on the surface, soft rot damage can be particularly important where members are used in bending. Because of the high moisture content, the wood may appear mushy. In general, however, soft rot damage is likely to be less common than other types of damage.

Brown rot fungi also attack both cellulose and hemicellulose, but their damage is primarily internal. These fungi are a serious concern, because they tend to rapidly break down cellulose at considerable distances from where the fungus is growing. As a result, these fungi cause substantial strength losses at very early stages of attack. Brown rot fungi tend to be more prevalent on coniferous wood species. In the early stages of decay, the wood surface lacks luster and appears dull or dead, and as the decay progresses, the wood develops a brown discoloration, with cross grain checking, collapse, or crumbling, and abnormal shrinkage. The appearance is similar to wood that has been charred. Wood that has been brown rotted and subsequently dried is sometimes mistakenly referred to as dry rotted wood. If the decay has developed within a member, it may not be readily visible, but symptoms such as crushing of the wood or excessive shrinking may be apparent.

White rot fungi use all three of the structural polymers that make up wood and, at their extreme, can cause up to 97 percent weight loss. White rot attack tends to become visible as strength effects become apparent. White rot fungi are most common on hardwoods. In the early stages of decay, the wood may have a bleached appearance, and black zone lines may appear in these lighter areas. Hardwoods suffering from white rot attack do not crack across the grain or shrink excessively like brown rotted wood, unless they are severely degraded.

In general, symptoms of fungal decay include the presence of fruiting bodies (mushroom-type growths), sunken faces or localized depressions, staining or discolorations that indicate wetting, or the presence of soil, plant growth, or moss growth.

Wood Boring Insects

While a variety of insects can attack wood, three groups are responsible for most of the damage: powder post beetles, termites, and carpenter ants.

Powder post beetles are capable of attacking wood that is below the fiber saturation point, allowing them to attack timbers in the bridge superstructure that are protected from direct wetting.

Powder post beetles lay their eggs on the surface of untreated, noncoated sapwood of the desired species. The eggs hatch into larvae that tunnel into the wood, leaving little evidence of their presence inside the wood. The larvae tunnel extensively through the wood over periods extending for 1 to 7 or more years. After the larvae have obtained a sufficient amount of energy, they pupate to become adults. These adults then exit the wood, leaving round exit holes on the wood surface. This is often the first visible sign of an infestation. The inside of powder post-damaged wood tends to be crumbly and powdery. Prevention is easiest by applying paint or other sealing finishes to the wood.

Termites are social insects that live in large colonies organized in a caste system containing workers, soldiers, and reproductives. Three types of termites attack wood products. Dampwood termites are confined to the Pacific Northwest, where they attack very wet wood in a variety of environments. Damage from these termites is most easily controlled by eliminating the source of moisture, although preservative treatments are also effective. Where it is not possible to eliminate the source of moisture, preservative treatment remains the only option.

Subterranean termites are found in most regions of the United States below 50°N latitude and are distinguished by nests in soil contact. Native subterranean termites live in colonies up to one million workers. Although they require moisture for attack to occur, these termites can produce mud tunnels from the soil over nonwood obstacles and into wood that is not in direct soil contact. Workers then carry wet soil up the tubes, providing moisture that allows them to attack the otherwise dry wood. Subterranean termites cause an estimated $1 billion in damage per year, but their importance in some regions is likely to be overshadowed by an imported termite species, the Formosan termite (Coptotermis formosanus). Although the range of this species is largely confined to major ports along the Gulf Coast as well as most of the Hawaiian islands, the voracious nature of this insect has raised concerns among many wood users. Formosan termites nest in colonies as large as five million workers. These termites attack virtually all wood species and have even been known to penetrate wood with low preservative levels. The best method for preventing termite attack in covered bridges is to use wood that has been pressure treated with preservatives.

Dry wood termites, unlike other termites, can attack very dry wood above the ground. Found primarily in the desert Southwest, these insects leave little sign of their attack and are difficult to control. Fortunately, drywood termites are not often found where covered bridges are used extensively.

Carpenter ants are also social insects with castes of workers, soldiers, and reproductives, but differ in a number of important characteristics from termites. While termites are rarely seen outside the nest, carpenters must forage outside the nest for food. Carpenter ant workers are typically dark-colored and have constrictions between the body segments, while termites are cream-colored and lack constrictions on the body. Most importantly, termites use wood as a food source, while carpenter ants do not; they mine the wood to create galleries to rear their young. This sometimes makes it difficult to control carpenter ant attacks. Carpenter ants attack wood in a variety of environments, including pressure-treated wood.

Preventing Fungal and Insect Attack

Although a variety of organisms have evolved to utilize wood for food or habitat, there are numerous strategies for preventing this damage.

Ideally, wood kept below the fiber saturation point should be free of fungal attack, but can be susceptible to powder post beetles, carpenter ants, or termites. The two approaches to limiting deterioration are to use natural durable heartwood or nondurable woods that have been supplementally protected with chemicals.

The heartwood of some native species, such as redwood, western red cedar, eastern white cedar, white oak, osage orange, and black walnut has natural durability that may be sufficient to prevent decay in moderate exposures. As the sapwood of these species dies inside a living tree, it undergoes reactions that produce a series of toxic chemicals. One must stress that it is typically only the heartwood that has enhanced durability, and that the proportion of heartwood that it is practical to achieve in a member must be a consideration. For example, although heartwood of Southern Pine species may be similar in durability to that of Douglas Fir, it is very difficult to obtain large Southern Pine timbers with a high proportion of heartwood. Conversely, larger members cut from Douglas Fir trees typically contain a high proportion of heartwood. While naturally durable heartwoods resist insect and fungal attack, the degree of resistance can vary widely from tree to tree and even within the same tree. As a result, structures of naturally durable woods can exhibit variable performance, particularly in direct soil contact. In general, naturally durable woods perform best when used above ground as decking or sheathing.

While naturally durable heartwoods remain an attractive option for some applications, many of the naturally durable species have lower structural properties that limit their usefulness in highway structures and are more costly than other species. As an alternative, the use of nondurable species supplementally protected with synthetic preservatives typically is recommended.

Methods of Preservative Treatment

Attempts to protect nondurable woods from deterioration date back thousands of years, but effective wood preservation dates to the 1830s, when John Bethell patented creosote as a preservative and the Bethell, or full cell process, for delivering it into the wood. These two patents form the foundation of the current preservation industry.

Preservation seeks to render wood either toxic to the deteriorating organisms or unrecognizable as a substrate. In either case, the goal is to deliver a specific amount of chemical (retention) to a specified depth. The depth to which the treatment penetrates depends on the wood species and its intended use. For example, penetration requirements are greater in thick sapwood species such as Southern Pine than in thin sapwood species like Douglas Fir or White Oak. These differing requirements reflect the difficulty of moving liquids through heartwood as well as the knowledge that sapwood has little decay resistance. As a result, the goal of most treatments is to protect the shell of sapwood surrounding the untreated heartwood core.

There are a variety of methods for delivering chemicals into wood. Brushing is, by far, the simplest method for providing supplemental protection, but it also produces the shallowest depth of treatment. Brush treatments typically are used to provide supplemental protection when treated wood must be cut after treatment, although there is some evidence that it also improves the performance of naturally durable woods.

Dipping in preservative is used for some wood products, including windows, door frames, and pallets. In the former two instances, the shallow protection is adequate because the wood is usually coated and protected from the weather. Pallets are relatively low-value commodities that have limited service lives, making more elaborate treatments uneconomical. Dipping is also used to treat framing lumber with borates in New Zealand. Wood is dipped in a concentrated boron solution, and then stored for 30 to 45 days to allow diffusion. This process is not widely used in North America.

In general, brushing and dipping do not adequately penetrate preservative, and they are largely relegated to supplemental protection of either naturally durable or preservative-treated woods.

The most effective methods for supplemental wood protection involve using combinations of vacuum and pressure to force chemicals into the wood. The two basic approaches to pressure impregnation are the full and empty cell processes. Both are performed in pressure vessels. The goal of these treatments is to deliver a specific weight of chemical per unit area of wood (expressed on a pounds per chemical/cubic foot of wood or kg/m³ basis) to a specific depth.

The full cell process begins with a vacuum to remove as much air as possible from the wood, then adds solution under vacuum and raises pressure. As the vacuum is released, preservative is drawn into the wood. Increasing pressure then forces more chemical into the wood. Pressure is held until the desired solution is injected. The pressure is then released, and vacuum and heating periods recover excess solution and clean the wood surface. Full cell cycles produce maximum solution uptake in the treated zone. They are typically used to treat wood with creosote to marine retentions (very high loadings) or for water-based systems where water is a cheap solvent and treatment chemical concentrations can be adjusted to produce the desired retention.

The empty cell processes eliminate the initial vacuum and were designed to reduce the amount of treatment solution deposited in the wood (i.e., retention). When pressure is introduced, it traps and compresses a pocket of air. At the end of the process, the compressed air pocket expands, carrying excess preservative from the wood. This reduces the overall chemical loading in comparison with the full cell process. There are a number of variations on the empty cell process, but all are designed to reduce retention. Empty cell processes are widely used for treating wood in timber bridges.

Environmental Considerations for Treatment

For decades, wood users believed that more chemicals were better and that surface appearance was of little concern. Changing societal values and increasing environmental awareness have altered these concepts. It is now recognized that poor treatment practices can lead to surface bleeding of preservative, excessive leaching, and a far greater potential for environmental impact.

It is important to specify standards that allow the treater to produce an environmentally friendly product. The American Wood Preserver Association (AWPA) preservative retention standards (Use Category Standards (UC) 4 and 5) are sufficient for a long-lasting, durable product.^[22] Asking the treater to increase the retention beyond these guidelines increases the amount of leachable chemicals in the wood without a noticeable service gain. Similarly, retreating wood that has failed to meet AWPA standards for initial retention increases the leachable material present. Selecting material free of surface residues is another way to assist in minimizing environmental risk. Allowing time for the use of proper treatment practices is also important. A common complaint of treaters is that customers demand the treated product on very short notice. This can cause the treater to rush or delete processing steps that improve the final product. This problem is especially relevant to post-treatment conditioning of water-based preservatives.

Post-treatment conditioning, or fixation, is necessary to reduce leaching from water-type preservatives. The active ingredients of various water-based wood preservatives (copper, chromium, arsenic, and zinc) are initially water soluble in the treating solution, but become resistant to leaching when placed into the wood. This leach resistance is a result of the chemical fixation reactions that render the toxic ingredients insoluble in water. The mechanism and requirements for these fixation reactions differ depending on the type of wood preservative. Some reactions occur very rapidly during pressure treatment, while others may take days or even weeks to reach completion, depending on post-treatment storage and processing conditions. If the treated wood is placed in service before these reactions are completed, the initial release of preservative into the environment may be many times greater than for wood that has been adequately conditioned. The AWPA has recently formed several task forces to consider the development of fixation, or leaching minimization, standards for CCA-C and other wood preservatives; however, there are currently no nationally recognized standards for fixation of waterborne preservatives in the United States.

Oil-type preservatives offer a unique difficulty, in that such preservatives sometimes bleed to the surface of the wood. This problem may be apparent immediately after treatment, but in some cases the problem does not become obvious until after the product is placed in service in a location where it is exposed to direct sunlight. The volume of preservative that bleeds out of the wood and may drip into the environment is typically quite small, but it may appear much larger if it spreads on the surface of standing water. It should be made clear that pieces with oily surfaces (or bleeders) will not be accepted when specifying wood treated with oil-type preservatives. However, bleeding is not always apparent initially, and may occur later when the wood is placed in service. This problem is best addressed by controlling treatment processes. The processes that reduce bleeding vary somewhat, depending on the type of preservative. The treaters also typically use a combination of these processes to obtain a clean surface. Some of the key steps include maintaining clean facilities and working solutions, avoiding over-treatment, and using post-treatment conditioning techniques such as final vacuum, steaming, and expansion baths.

Construction practices can also make a difference in preservative released into the environment. Treated material shipped to the job site should always be stored free from standing water and wet soil, and protected from precipitation. Field fabrication of treated wood that cannot be handled before applying treatment should be done carefully. Construction debris like sawdust has a disproportionately high surface-area-to-volume ratio, leaching more chemicals into water. Taking practical steps to collect such debris can minimize the threat.

In some regions of the country, the industry has responded to environmental concerns by developing a series of best management practices (BMPs) designed to fix or immobilize preservative, produce retentions that are reasonably close to the target, and reduce the risk of bleeding from the finished product. While BMPs are not mandatory, their use is highly recommended for any treated wood used over or in water. Where applicable, descriptions of appropriate BMPs are included for the preservatives discussed in this chapter.

Treatment Specifications

The primary standards for pressure treatment of wood in the United States are promulgated by AWPA, which are widely referenced in the AASHTO standards. Founded in 1904, the AWPA is a consensus standards-writing body consisting of wood treaters, chemical suppliers, and general interest members. The primary objective of the AWPA standards is to protect the consumer or user of treated wood products.

The AWPA standards specify treatments according to the risk of decay and the commodity. Thus, for the same decay hazard, specifications will be far more rigorous for a bridge timber than for a fence post. In addition, the standards recognize regional differences in wood species employed. Covered bridge designers have always experienced difficulties in using the AWPA standards because of requirements to incise some difficult-to-treat species and the lack of nonpigmented preservatives for many applications.

While it may occasionally be necessary to refer beyond the AWPA standards, the goal of these standards is to assist designers in specifying properly protected wood without compromising the aesthetics of the finished product.

AWPA standards are considered to be minimums for chemical loading and penetration. Specifications may be made more rigorous; however, it should be clear that the intended change and the associated costs truly improve reliability. In addition, the AWPA standards are results-oriented; within specific process limitations, the method for achieving the desired retention and penetration is largely left to the treater.

Methods for Improving Performance of Treated Wood

The performance of treated wood is largely a function of the initial treatment quality. The primary goal of treatment is to produce an envelope of protection around the untreated core. Any damage to this treated shell opens the interior to possible fungal or insect attack. These openings can be created naturally through the development of seasoning checks that extend beyond the original depth of treatment, or they can originate from damage due to cutting, notching, or drilling a member after treatment.

To limit the risk of these problems, the wood should be seasoned before treatment. This helps ensure that checks have already formed and creates a better treatment envelope. However, seasoning to in-service conditions is generally not practical. Most large timbers are treated while their core moisture contents are above 30 percent, a practice that guarantees the development of some post-treatment checking. Wherever possible, simple practices such as placing the member so that the largest check faces down in a structure can reduce the risk of wetting through checks.

It is nearly impossible to completely eliminate the need for onsite fabrication; however, a majority of cutting and drilling can be done before treatment. In cases where holes must be redrilled because of twisting or slight misalignments, application of preservative to the freshly exposed wood is recommended.

In addition to drying and precutting/boring, preservative penetration can be improved in many species by incising, a process that drives metal teeth 15.2 to 19 mm (0.6 to 0.75 inches) into the wood. Incising opens the wood to flow and improves treatment to the depth of the incisions. Incising is required to effective treat many thin sapwood conifers as well as many hardwoods. While it reduces strength (up to 15-20 percent on 2 by 4s), the effect is reduced as timber size increases, and any losses are more than offset by the improved performance of the treated wood.

In some instances, even for historic structures, it may be more advantageous to substitute engineered wood products or smaller sizes of timber that are either less likely to check or be treated evenly. A large timber is aesthetically pleasing, but it is far more prone to checking and has a proportionally smaller treated area than several smaller timbers bolted together. The smaller timbers will be better treated and are less likely to develop deep checks. Similarly, substituting glue-laminated beams or laminated veneer lumber can produce products that are less likely to check in-service because they are thoroughly dried before treatment. Clearly, these products must be used carefully to retain the historic character of the bridge, but they offer considerable advantages with regard to treatment characteristics.

Preservative Chemicals

Wood preservatives are generally classified or grouped by the type of application or exposure environment in which they are expected to provide long-term protection. Some preservatives have sufficient leach resistance and broad spectrum efficacy to protect wood that is exposed directly to soil and water. These preservatives also will protect wood exposed above ground, and may be used in those applications at lower retentions (concentrations in the wood). Other preservatives have intermediate toxicity or leach resistance that allows them to protect wood fully exposed to the weather, but not in contact with the ground. Still other preservatives lack the permanence or toxicity to withstand continued exposure to precipitation, but may be effective with occasional wetting. Finally, there are formulations that are readily leachable to the extent that they can only withstand very occasional, superficial wetting. It is not possible to evaluate a preservative's long-term efficacy in all types of exposure environments, and there is no set formula for predicting exactly how long a wood preservative will perform in a specific application. This is especially true for above-ground applications, because preservatives are tested most extensively in ground contact. To compensate for this uncertainty, the tendency is to be conservative in selecting a preservative for a particular application.

Preservatives are also classified by their solubility in either water- or oil-type solvents. This classification is not always absolute, because some preservatives can be formulated to be used with either type of solvent. Water-based preservatives often include some type of cosolvent, such as amine or ammonia, to keep one or more of the active ingredients in solution. Each solvent has advantages and disadvantages depending on the application.

Oil-type systems are among our oldest and most effective preservatives. These systems are usually used in medium to heavy oils that leave the wood surface a dark brown color, although some lighter solvents can minimize color changes. Oil-based systems are widely believed to experience reduced checking and splitting, although this is often difficult to document. Oil-based systems have provided excellent protection in a number of highway applications and are compatible with most wood species. These systems can be very difficult to paint or stain because the initial oil preservative can migrate through or discolor the coating. The use of more volatile solvents can reduce this problem.

Water-based preservatives are often used when cleanliness and paintability of the treated wood are required. Wood treated with water-based preservatives also typically has less odor than wood treated with some types of oil-based preservatives. However, unless supplemented with a water repellent, the water-based systems do not confer dimensional stability to the treated wood. In addition, water-based preservatives that use copper as a fungicide may not adequately protect hardwood species under conditions that favor soft rot attack. (As mentioned above, the high moisture levels needed to promote soft rot attack are unlikely to occur in covered bridge components.) Some water-based preservatives may increase the rate of corrosion of mild steel fasteners. The most widely used waterborne treatment is a mixture of chromic acid, cupric oxide, and arsenic pentaoxide, called chromated copper arsenate (CCA). Developed in India in the 1930s, CCA is used to impregnate 60 percent of the wood treated in the United States. CCA is strongly fixed to the wood and has provided excellent protection in a variety of environments. The primary drawbacks to CCA are those associated with any heavy metal preservatives. While CCA is fixed, small amounts of copper and arsenic are continually solubilized, and those losses have raised environmental concerns. Over the past 10 years, a variety of alternative chromium and/or arsenic-free systems have been developed, but none has had the cost and performance advantages of CCA. As a result, alternatives such as ammoniacal copper quat, ammoniacal copper zinc arsenate, and copper azole have relatively small U.S. markets. It is also notable that all of the recently standardized CCA alternatives contain, and appear to leach, much higher levels of copper than does CCA. Thus, it is probably advisable to use available aquatic models to assess the risk of copper leaching where invertebrate fish may be sensitive to copper concentrations in water and sediment. However, these alternative treatments are available commercially and may be appropriate where CCA is not considered to be suitable.

Each preservative also has many unique characteristics that might affect its suitability for a particular application. These include factors such as appearance, odor, toxicity, wood species compatibility, and availability. All of the preservatives mentioned in the sections below are registered with the U.S. Environmental Protection Agency (EPA) for use as wood preservatives.

In some instances, the preservative may be classified as a restricted-use pesticide, meaning that the applicator must be licensed by the appropriate State agency to apply chemicals. The use of preservative treated wood, however, does not require a license, even if the treatment chemical is classified as a restricted-use pesticide.

Preservatives That Are Effective in High-Decay Hazard Applications

These preservatives exhibit sufficient toxicity and leach resistance to protect wood in contact with the ground or in other high-moisture, high-deterioration hazard applications. In bridges, these types of applications may include sole plates resting on piers or wooden members that directly contact footings in abutments or approaches. Preservatives listed in this section are also effective in preventing decay in other, less severe exposures.

Water-Based Preservatives

Acid Copper Chromate (ACC)-Acid copper chromate (ACC) has been used as a wood preservative in Europe and the United States since the 1920s. ACC contains 31.8 percent copper oxide and 68.2 percent chromium trioxide. The treated wood has a light greenish-brown color, and little noticeable odor. Tests on stakes and posts exposed to decay and termite attack indicate that wood impregnated with ACC gives acceptable service, although it may be susceptible to attack by some species of copper tolerant fungi. It is listed in AWPA standards for a wide range of softwood and hardwood species, with a minimum retention of 4.0 or 8.0 kg/m ³ (0.25 lbs/ft³ or 0.5 lbs/ft³) for wood used above ground or in ground contact, respectively. However, in critical structural applications such as highway construction, its AWPA listings are limited to sign posts, handrails, guardrails, and glulam beams used above ground. It may be difficult to obtain adequate penetration of ACC in some of the more refractory wood species such as White Oak or Douglas Fir. This is because ACC must be used at relatively low treating temperatures (38°C to 66°C (100°F to 150°F)), 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. Only a limited number of treatment facilities use ACC, primarily for treating wood used in cooling towers.

ACC does not contain arsenic and thus may offer environmental and handling advantages in some applications. The treatment solution does utilize hexavalent chromium, but the chromium is converted to the more benign trivalent state during treatment and subsequent storage of the wood. This process of chromium reduction is the basis for fixation in ACC, and is dependent on time, temperature, and moisture. Fixation standards or BMPs have not yet been developed for ACC, because of its relatively low usage. As a general guide, the fixation considerations discussed for CCA (see below) can be applied to ACC.

Ammoniacal Copper Citrate (CC)-Ammoniacal copper citrate (CC) is a wood preservative that uses copper oxide (62 percent) as the fungicide and insecticide, and citric acid (38 percent) to help distribute copper within the wood structure. The color of the treated wood varies from light green to dark brown. The wood may have a slight ammonia odor until it is thoroughly dried after treatment. Exposure tests with stakes and posts placed in ground contact indicate that the treated wood resists attack by both fungi and insects. However, it is possible that the lack of a cobiocide may render the wood vulnerable to attack by certain species of copper-tolerant fungi. CC is listed in AWPA standards for treating a range of softwood species and wood products. The minimum CC retention is 4.0 and 8.0 kg/m³ (0.25 and 0.4 lb/ft³) for wood used above ground or in ground contact, respectively. CC is not listed in AWPA standards for use in highway construction or other structurally critical applications (the AWPA standards typically employ higher preservative loadings for highway construction in consideration of the need for a higher level of reliability than might be required for fence). As with other preservatives containing ammonia, CC has an increased ability to penetrate into difficult-to-treat wood species such as Douglas Fir. The CC treatment can accelerate corrosion in comparison to untreated wood, necessitating the use of hot-dipped galvanized or stainless steel fasteners. Few treating plants currently use CC, and wood treated with this product may not be readily available in most areas.

CC does not contain arsenic or chromium, and thus may offer environmental or safety advantages in some situations. The copper is solubilized by the ammonia in the treating solution, and becomes insoluble (fixed) as the ammonia volatilizes from the wood. Wood treated with CC should not be exposed to precipitation or other sources of environmental moisture until this fixation process is complete. This time period varies with temperature. If the wood arrives at the job site appearing heavy, wet, and with a strong ammonia odor, it may not be thoroughly fixed. Fixation can be ensured by placing sticks between courses of the wood (stickering) and placing it in an area with good air flow to facilitate drying. Although the wood should be covered to protect it from precipitation, it should not be covered in a way that impedes air circulation until after it has dried and the ammonia odor has dissipated. There currently are no fixation standards or BMPs to ensure fixation in CC-treated wood. However, the BMPs developed by the Western Wood Preservers Institute (WWPI) for ammoniacal copper zinc arsenate (ACZA)-treated wood are applicable (see below).^[25]

Alkaline Copper Quat (ACQ)-Alkaline copper quat (ACQ) is one of several wood preservatives that has been developed in recent years because of environmental or safety concerns with CCA. The fungicides and insecticides in ACQ are copper oxide (67 percent) and a quaternary ammonium compound (quat). Multiple variations of ACQ have been standardized or are in the process of standardization. ACQ type B (ACQ-B) is an ammoniacal copper formulation, ACQ type D (ACQ-D) is an amine copper formulation, and ACQ type C (ACQ-C) is a combined ammoniacal-amine formulation with a slightly different quat compound. 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. Wood treated with ACQ-D has a light brown color and little noticeable odor, while wood treated with ACQ-C varies in appearance between that of ACQ-B and ACQ-D, depending on the formulation. Stakes treated with these three formulations have demonstrated efficacy against decay fungi and insects when exposed in ground contact. The ACQ formulations are listed in AWPA standards for a range of applications and many softwood species, although the ACQ-C listings are limited because it is the most recently standardized. Minimum retentions of 4.0, 6.4, or 9.6 kg/m³ (0.25, 0.4 or 0.6 lbs/ft³) are specified for wood used above ground, in ground contact, or for highway construction, respectively. The multiple formulations of ACQ allow some flexibility in achieving compatibility with a specific wood species and application. When ammonia is used as the carrier, ACQ has improved ability to penetrate into difficult-to-treat wood species. However, if the wood species is readily treated, such as Southern Pine, an amine carrier can be used to provide a more uniform surface appearance. All of the ACQ treatments accelerate corrosion of metal fasteners relative to untreated wood, and hot-dipped galvanized or stainless steel fasteners are a necessity. The number of pressure treatment facilities using ACQ is increasing. In the western United States, the ACQ-B formulation is used because it allows better penetration in difficult-to-treat western species. Treating plants in the rest of the country generally use the ACQ-D formulation. Researchers are currently evaluating the performance of a secondary highway bridge constructed from ACQ-D-treated Southern Pine lumber.^[26] Use of the more recently standardized ACQ-C formulation is expected to increase in both parts of the country.

ACQ-treated wood does not contain arsenic or chromium, and thus may offer handling and environmental advantages in some applications. ACQ-treated wood contains copper, which can be of concern in aquatic environments. The release and environmental impact of copper from ACQ-B-treated wood was recently evaluated in a wetland boardwalk study.^[23] Soil and sediment samples removed from within a few feet of the boardwalk did contain elevated levels of copper, but there was no measurable impact on either the number or diversity of aquatic invertebrates adjacent to the structure. Specific standards or BMPs for fixation of ACQ are not yet available, although it is likely that the BMPs developed for ACZA (see below) could be applied to wood treated with ACQ-B.

Ammoniacal Copper Zinc Arsenate (ACZA)-Ammoniacal copper zinc arsenate (ACZA) contains copper oxide (50 percent), zinc oxide (25 percent), and arsenic pentoxide (25 percent). ACZA is a refinement of an earlier formulation, ACA, that is no longer available in the United States. The color of the treated wood varies from olive to bluish green. The wood may have a slight ammonia odor until it is thoroughly dried after treatment. ACZA is an established preservative that protects wood from decay and insect attack in a wide range of exposures and applications. It has also protected stakes and posts placed in ground contact in exposure tests. ACZA is listed in AWPA standards for treating a range of softwood and hardwood species and wood products. The minimum ACZA retention is 4.0 or 6.4 kg/m³ (0.25 and 0.4 lb/ft³) for wood used above ground or in ground contact, respectively. A slightly higher retention (9.6 kg/m³ or 0.6 lb/ft³) is required for wood used in highway construction and for critical structural components used in high decay hazard exposures. 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. ACZA is frequently used in the western United States for treating the Douglas Fir lumber and timbers used in construction of secondary highway bridges, trail bridges, and boardwalks. The ACZA treatment can accelerate corrosion in comparison to untreated wood, requiring the use of hot-dipped galvanized or stainless steel fasteners. Treatment facilities using ACZA are currently located in western States, where many of the native tree species are difficult to treat with CCA.

ACZA does contain inorganic arsenic, and is classified as a restricted-use pesticide by the EPA. Producers of treated wood, in cooperation with the EPA, have created Consumer Information Sheets (CIS) that give guidance on appropriate handling and use site precautions for wood treated with inorganic arsenic. These CIS should be made available to all personnel involved in handling ACZA-treated wood. The CIS limitations on the uses of ACZA-treated wood do not affect covered bridges. The environmental impact of ACZA-treated wood in sensitive environments has been evaluated. One study found that soil and sediment samples removed from directly under a large ACZA-treated elevated walkway had elevated levels of copper, zinc, and arsenic.^[23] However, samples removed at greater distances were not elevated, and there was no detectable impact on either the number or diversity of aquatic invertebrates adjacent to the structure. Another study found only occasional elevated samples, even immediately under the dripline of a wetland boardwalk.^[26]

As with all water-based preservatives, the safety and environmental compatibility of the treated wood depends on completion of fixation reactions to reduce the solubility of preservative components. In ACZA, the copper and zinc are solubilized by the ammonia in the treating solution, and become insoluble as the ammonia volatilizes from the wood. As the copper and zinc precipitate, they form insoluble zinc arsenates or copper arsenates within the wood. Wood treated with ACZA should not be exposed to precipitation or other sources of environmental moisture until this fixation process is complete. The WWPI has developed BMP processing guidelines to help ensure that ACZA-treated wood is fixed before it leaves the treating plant. These BMPs specify that the treated wood either be air dried for three weeks at a temperature above 16°C (60°F) or kiln dried to below 30 percent moisture content. The air drying time may be shortened if an in-cylinder ammonia removal step is incorporated into the treatment cycle. If the wood arrives at the job site appearing heavy, wet, and with a strong ammonia odor, it may not be thoroughly fixed. Fixation can be ensured by stickering the material and placing it in an area with good air flow. Although the wood should be covered to protect it from precipitation, it should not be covered in a way that impedes air circulation until after it has dried and the ammonia odor has dissipated.

Chromated Copper Arsenate (CCA)-Chromated copper arsenate (CCA) is the most commonly used of all wood preservatives, and until very recently represented more than 90 percent of the sales of waterborne wood preservatives. CCA-treated wood is commonly sold at retail lumber yards as green-treated wood, although most residential applications were withdrawn from the market at the end of 2003. However, it will still be allowed in nonresidential applications such as highway construction. CCA-treated wood has no odor, and as suggested by its common name, a light green color. However, it is often sold with commercially applied dyes or stains to give the wood more of a brown appearance. There are three standardized formulations of CCA: CCA Type A, CCA Type B, and CCA Type C. However, CCA Type C (CCA-C) is the formulation used by nearly all treating facilities because of its leach resistance and demonstrated efficacy. CCA-C is comprised of 47.5 percent chromium trioxide, 18.5 percent copper oxide, and 34.0 percent arsenic pentoxide dissolved in water. CCA-C has decades of proven performance in field trials and in-service applications. In accelerated testing, it is the reference preservative used to evaluate the performance of other waterborne wood preservatives. Because it has been so widely used for many years, CCA-C is listed in AWPA standards for a wide range of wood products and applications. The minimum retention of CCA-C in the wood is 4.0, 6.4, or 9.6 kg/m³ (0.25, 0.40, or 0.60 lbs/ft³) for above ground, ground contact, and critical structural applications, respectively. As with ACC, it may be difficult to obtain adequate penetration of CCA in some difficult-to-treat wood species. Temperature limitations during treatment and rapid reaction of chromium within the wood structure can hinder penetration during longer pressure periods. However, the chromium does serve as a corrosion inhibitor, and corrosion of fasteners in CCA-treated wood is not as much a concern as it can be with some of the chromium-free alternative preservatives. Treating facilities that use CCA are widespread within the United States, making it the most readily available of all preservatives.

CCA does contain inorganic arsenic, and is classified as a restricted-use pesticide by the EPA. Producers of treated wood, in cooperation with the EPA, have created CIS that give guidance on appropriate handling and use site precautions for wood treated with inorganic arsenic. These CIS should be made available to all personnel involved in handling CCA-treated wood. The CIS limitations placed on the uses of CCA-treated wood do not impact covered bridges. The environmental impact of CCA-treated wood has been evaluated in two timber bridges in Florida.^[25] In some cases, samples removed from under the bridge did have evaluated levels of copper or chromium or arsenic, but there was no detectable effect on aquatic invertebrate populations. Similar results were found in an evaluation of the environmental impact of CCA-treated wood used in construction of a wetland boardwalk.^[23] CCA may offer an advantage over the newer alternative preservatives for use in aquatic environments, because the treated wood contains and leaches less copper. Although arsenic raises concerns as a toxin for mammals, copper may be more of a concern for fish and aquatic invertebrates.^[23]

As with other water-based preservatives, the risk of chemical exposure from CCA-treated wood is minimized after chemical fixation reactions to lock the chemical in the wood. The CCA treating solution contains hexavalent chromium, but the chromium reduces to the less toxic trivalent state within the wood. This process of chromium reduction is also critical in fixing the arsenic and copper in the wood, and CCA-treated wood should not be exposed to precipitation or other sources of environmental moisture until this fixation process is complete or nearly complete. The rate of fixation of CCA is highly temperature-dependent, requiring only a few hours at 66°C (150°F), but weeks or even months at temperatures below 16°C (60°F). Because of this temperature relationship, some treating facilities use kilns, steam, or hot water baths to accelerate fixation. An AWPA test method, the chromotropic acid test, can be used to determine if fixation in CCA-treated wood is complete.^[20] There is some concern that the chromotropic acid test may be overly conservative because it requires more than 99.5 percent of the chromium to be reduced to the trivalent form. However, it is currently the only standardized test available, and is specified in the BMPs developed by the WWPI.^[21]

Copper Azole (CBA and CA-B)-Copper azole is another recently developed preservative formulation that relies primarily on amine copper, but with cobiocides, to protect wood from decay and insect attack. The first copper azole formulation developed was copper azole-Type A (CBA-A), which contains 49 percent copper, 49 percent boric acid, and 2 percent tebuconazole. More recently, the copper azole-Type B (CA-B) formulation was standardized. CA-B does not contain boric acid, and is comprised of 96 percent copper and 4 percent Tebuconazole. Wood treated with either copper azole formulation has a greenish-brown color and little or no odor. The copper azole formulations have been evaluated with in-ground stake tests and found to protect wood against attack by decay fungi and insects. The formulations are listed in AWPA standards for treatment of a range of softwood species. Minimum retentions of CBA-A in the wood are 5.28, 6.56, or 9.76 kg/m³ (3.3, 0.41, or 0.61 lb/ft³) for wood used above ground, in ground contact, or in critical structural components, respectively. Minimum retentions of CA-B in the wood are 1.6, 3.2, or 4.96 kg/m³ (0.10, 0. 21, or 0.31 lb/ft³) for wood used above ground, in ground contact, or in critical structural components, respectively. Although listed as an amine formulation, copper azole may also be formulated with an amine-ammonia formulation. The ammonia may be included when the copper azole formulations are used to treat refractory species, and the ability of such a formulation to adequately treat Douglas Fir has been demonstrated. The inclusion of the ammonia, however, is likely to have slight effects on the surface appearance and initial odor of the treated wood. The copper azole treatments do increase the rate of corrosion of metal fasteners relative to untreated wood, and hot-dipped galvanized or stainless steel fasteners are recommended. Because copper azole has been developed only recently, relatively few treating facilities are currently using this preservative.

The copper azole formulations do not contain arsenic or chromium, and thus may offer environmental or safety advantages in some applications. Copper azole-treated wood does contain copper, which can be of concern in aquatic environments. Proper handling and post-treatment conditioning of copper azole-treated wood is important to ensure that leaching and potential environmental impacts are minimized. The copper is solubilized in the treating solution by the presence of the amine (and in some cases ammonia). The mechanism of copper fixation in the wood is not completely understood, but appears to be strongly influenced by time, temperature, and retention. Copper fixation in the CBA-A formulation is extremely rapid (within 24 hours) at the lowest retention 3.36 kg/m³ (0.21 lb/ft³), but slows considerably at the higher retentions unless heat is used to accelerate the fixation. As with other waterborne preservative formulations, the treated wood should not be exposed to precipitation or other sources of water until these fixation reactions are complete. Specific standards or BMPs for fixation of copper azole are not yet available.

Oil-Type Preservatives

Coal-Tar Creosote-Coal-tar creosote is the oldest wood preservative still in commercial use in the United States. 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 affect its performance as a wood preservative. Creosote-treated wood has a dark brown to black color and a noticeable odor, which some people consider unpleasant. It is very difficult to paint creosote- treated wood. Workers sometimes object to creosote-treated wood because it soils their clothes and photosensitizes the skin on contact. The treated wood sometimes also has an oily surface, and patches of creosote sometimes accumulate, creating a skin contact hazard. Because of these concerns, creosote-treated wood is often not the first choice for applications such as handrails, where there is a high probability of human contact. This is a serious consideration for members in covered bridges that are readily accessible to the public.

However, creosote-treated wood has advantages to offset concerns about its appearance and odor. It has a 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. Creosote is listed in AWPA standards for a wide range of wood products and wood species. Minimum creosote retentions are in the range of 80 to 128 kg/m³ (5 to 8 lb/ft³) for above-ground applications, 160 kg/m³ (10 lb/ft³) for wood used in ground contact, and 192 kg/m³ (12 lb/ft³) for wood used in critical structural applications such as highway construction. With the use of heated solutions and lengthy pressure periods, creosote can be fairly effective at penetrating even fairly difficult-to-treat wood species. Like other oil-type systems, creosote is suitable for treatment of glue-laminated members. Creosote treatment also does not accelerate, and may even inhibit, the rate of corrosion of metal fasteners relative to untreated wood. Treating facilities using creosote are widely distributed in the United States, making it one of the more readily available preservative treatments.

Creosote is classified as a restricted-use pesticide by the EPA. Producers of treated wood, in cooperation with the EPA, have created CIS that give guidance on appropriate handling and use site precautions for wood treated with creosote. These CIS should be made available to all personnel involved in handling creosote-treated wood. People are sometimes concerned that creosote-treated wood used in aquatic applications, such as bridges, may harm the environment. This concern recently was evaluated in a study of the environmental impact of two creosote-treated wooden bridges in Indiana.^[27] In each case, elevated levels of PAH were detected in sediments 1.8 to 3 m (6 to 10 ft) downstream from the bridges, and these levels approached levels of concern for one bridge. However, no significant effect on aquatic invertebrate populations was noted for that bridge. There did appear to be a reduction in the population and diversity of aquatic insects within 6 m (20 ft) downstream for the other bridge, but the author postulated that this trend was caused by the deposition of maple leaves in this area.

As with other preservatives, the environmental impact of creosote-treated wood is a function of treatment practices. Creosote in treated wood may sometimes bleed or ooze to the surface and drip to surfaces or the water below. This problem may be apparent immediately after treatment, and such members should not be used in bridges or other aquatic applications. However, in other cases, the problem does not become obvious until after the product is placed in service in a location where it is exposed to direct sunlight. This problem is best addressed through the control of treatment processes and BMPs. The WWPI's BMPs for creosote-treated wood call for the use of either an expansion bath or final steaming at the end of the pressure period:

• Expansion Bath: Following the pressure period, the creosote should be heated -12.2 to -6.6°C ( 10-20°F) above press temperatures for a minimum of 1 hour. Pump creosote back to storage and apply a minimum vacuum of 609 mm (24 inches) for a minimum of 2 hours.
• Steaming: Following the pressure period and after the creosote has been pumped back to the storage tank, a vacuum shall be applied for a minimum of 2 hours at not less than 559 mm (22 inches) of vacuum to recover excess preservative. Release vacuum back to atmospheric pressure and steam for a 2-hour period for lumber and timbers, and 3 hours for piling. Maximum temperature during this process shall not exceed 115.5°C (240°F). Apply a second vacuum for a minimum of 4 hours at 559 mm (22 inches) of vacuum.