All Hands on Deck to Lessen Wildland Fire Damage

Because wildland fires pose a significant societal threat, it is important to understand how to mitigate their damage. Lives and structures are at risk, particularly in the wildland-urban interface (WUI), where homes are constructed near or among areas prone to these fires.

Testing decking materials in FPL's Fire Test Lab.

Testing decking materials in FPL’s Fire Test Lab.

The Forest Products Laboratory’s Mark Dietenberger, a research general engineer, and Laura Hasburgh, a fire protection engineer, are studying a common scenario that results in property loss due to these fires in the WUI: ignition of attached wood decks.

A recently posted Research in Progress summary titled Fire Performance of Exterior Wood Decks in Wildland-Urban Interface explains how FPL and the American Wood Council (AWC) are working together to provide mitigation strategies that will reduce wildfire threats to structures and therefore preserve the marketability of wood decks.

The objectives of the research are threefold:

  • provide the AWC with a technical assessment of the fire performance of decking when subjected to relevant fire exposure
  • assess the related fire test methodologies using state-of-the-art flammability facilities
  • identify options for policy decisions pertaining to prescriptive regulations

This project began in January 2014 and will continue for three years, with an annual report compiled each July.

Proper Soil Grading Helps Keep Decay at Bay

According to Build Green: Wood Can Last for Centuries, a common homeowner and contractor mistake following construction projects is to set the finish grade for soil or mulch above the level of wood framing. Soil contact is one of the primary culprits in wood decay.

soil_contact

Soil graded high against the exterior brick veneer will contribute to decay problems in untreated wood members below the grade line. Similar problems will occur with soil graded high against stucco and siding.

Where untreated wood is used in a structure, it should be at least eight inches above the finish grade for framing members and six inches above finish grade for siding. Composite products should never be used in contact with soil.

Preservative treatments are designated for above-ground use or in-ground contact (buried in soil or touching soil). When planning your building, it is important that you specify the right treated wood for your specific need and that you insist that the treatment be of certified quality and be labeled accordingly.

Wetted-stucco

Here’s what not to do. In this photo, the finish grade on the yard is above the level of wood framing inside the wall. To make matters worse, the lawn sprinkling system is providing constant wetting of the stucco siding on this home. (Photo provided by Steve Easley, Steve Easley & Associates, Inc.)

A vast array of treated wood is available for the homeowner. Choosing a preservative approved for ground contact, properly grading soil, and avoiding constant wetting will go a long way in protecting your outside wood structure from decay caused by the moisture in soil.

Life-Cycle Analysis of Redwood Decking

Forest Products Laboratory (FPL) scientist Rick Bergman recently led a life-cycle assessment study of redwood decking in the United States. In cooperation with the Consortium for Research on Renewable Industrial Materials (CORRIM), the School of Environmental and Forest Sciences, University of Washington, and Humboldt State University, Department of Forestry and Wildland Resources, researchers compared the use of redwood with three other decking materials.

Fig_01

Complete life cycle from regeneration of trees to disposal of wood materials

Life-cycle inventory (LCI) and life-cycle assessment (LCA) are terms we’ve been hearing around FPL in recent years with increasing attention to “green building” practices. The term life cycle connotes a fair, holistic assessment to consider all aspects of the product: raw-material production, manufacture, distribution, use, and disposal, including all intervening transportation steps.

The goal for Bergman and his fellow researchers was to conduct an LCI of California redwood (Sequoia sempervirens) decking that would quantify the critical environmental effects of decking from cradle to grave. Using LCI data, the scientists produced a life-cycle assessment for redwood decking. These results were used to compare the environmental footprint of redwood decking to similar decking materials made of plastic (cellular PVC) and wood–plastic composites.

Results of the study showed the total energy expended for redwood was substantially lower than that for the other decking products. The ranking for redwood decking was the result of the product’s ability to store carbon, originally sequestered from the atmosphere, over the life of the product.

 

 

How do Wood Extractives Affect Metal Corrosion?

Did you know that metal fasteners corrode in wood? This week we will look further into the work of Sam Zelinka on this subject. Zelinka is the FPL corrosion expert, and our post today borrows from Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives, a compilation of several papers Zelinka has written on the subject in recent years.

Wood may seem like a simple material, but the lumber you build with is actually chemically and physiologically complex. Research has shown that different wood species contain different extractives that may affect the corrosion of embedded metals. This week, we consider the chemical components of wood and the effect of tannins and pH on the corrosion process. Much of today’s post draws from a 2011 study on corrosion of steel in wood extracts.

Wood is comprised of polymers, natural and synthetic compounds of high molecular weight consisting of millions of repeated linked units. In addition to structural polymers (cellulose, hemicellulose, and lignin), wood contains a variety of additional chemical components.

Wood also consists of a variety of organic compounds that are low in molecular weight—extractives. These small molecules get their name because they can be extracted by rinsing with various solvents (including water). Generally, extractives are present in small amounts, and thousands of different extractives are present in wood.

The type and amount of extractives vary widely among wood species. In some naturally durable species (such as locust or white oak), extractives can protect the wood from decay.

Although a single piece of wood can contain over 700 different extractives, only three types have been thought to affect the corrosion of metals in contact with wood or the black liquors of wood pulp: small organic acids, tannins, and phenols.

According to Zelinka, “many researchers have found correlations between the acidity of wood and its corrosiveness, and pH is largely controlled by formation of acetic [essentially vinegar] and formic acid. However pH cannot be the only variable that affects corrosion.”

Zelinka tells us, “Although the pH of wood, a solid material, is not well defined, the water extracts of nearly all wood species are acidic. The reason for this acidity is that in the presence of water, acetyl and formyl groups in the hemicelluloses are hydrolyzed [decompose by reacting with water] to form different kinds of acid. Many researchers have found correlations between the acidity of wood and its corrosiveness, and pH is largely controlled by formation of acetic and formic acid.”

Research has shown that this process is chemical, rather than biological. Previous research on sawblade corrosion suggested that wood tannins accelerated the corrosion of sawblades; however, in general, tannins are known as a corrosion inhibitor. In the 2011 study, Zelinka and Stone showed that tannins in solid wood act as a corrosion inhibitor to the embedded fasteners. In addition to the corrosion rate data, Zelinka and Stone observed a blue-black patina forming on the steel, indicative of the formation of iron-tannate, a stable blue/black corrosion product.

Blue-black precipitate forming on the surface of the steel plug
exposed to synthetic oak extract.

Tannins and other extractives are often mentioned in the literature as compounds that affect corrosion in wood. The different behavior of tannins most likely depends on what application is being studied. For instance, Zelinka and Stone discovered that tannins act as a corrosion inhibitor in wood extracts, which contradicts the earlier sawblade corrosion findings. The difference is most likely due to the friction and heat produced during sawing.

By combining kinetic models in the literature, Zelinka and Stone created an isocorrosion map for wood extracts as a function of pH and tannins. An isocorrosion map is a kind of tool used to recognize high-corrosion situations during the design process of equipment.

“This map,” says Zelinka, “was based on limited data and it does not explain why synthetic extracts behave differently; nevertheless, in the future with additional data such maps may be able to assess the relative effects of these chemicals when developing a new, non-metallic preservative system.”

After all this work and explanation, Zelinka tells us that “the effect of tannins on the corrosion of metals in wood remains unclear.” As Zelinka and his colleagues continue to study corrosion, perhaps this question will have more answers.

 

Why and How Do Metals Corrode in Treated Wood? Chemistry and Design Mysteries Revealed

Scientist Sam Zelinka is the FPL expert on corrosion of fasteners in treated wood. This week we continue our discussion of this subject from Zelinka’s publication Guide for Materials Selection and Design for Metals Used in Contact with Copper-Treated Wood and a related publication, Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives.

In these publications, Zelinka discusses the chemistry of waterborne wood preservatives. These preservatives contain cupric ions (copper molecules) that are thermodynamically unstable in the presence of steel or zinc galvanized fasteners. A chemical reaction, such as the one that will be discussed here, requires a lot of energy to be formed. To be thermodynamically unstable is to be unfavorable and means that the reaction is not spontaneous. It requires energy.

Let’s talk about the mechanism of corrosion in treated wood. This process, according to Zelinka, involves “the transport of cupric ions through the wood to the fastener surface, where the cupric ions are reduced and the fastener (zinc or iron) is oxidized.”

What does that mean?  In chemistry, oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. Reduction, on the other hand, is the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.

How does that work? The following graphic illustrates this point by showing the mechanism of corrosion in treated wood.  We can see how cupric ions migrate through the wood to the metal surface where they are reduced as the fastener is oxidized.

fig_04

Zelinka tells us, “For carbon steel and zinc-galvanized fasteners, the reduction of cupric ions is thermodynamically favorable and will occur. The corrosion of embedded metals is strongly dependent upon moisture content.” This, according to Zelinka, “is a war that Nature will eventually win.”

However, Zelinka also gives the homeowner numerous tips for prolonging the life of their outdoor projects through careful materials selection and design. By understanding the corrosion mechanism, “it is possible to develop strategies for maximizing the life of embedded fasteners.”

One strategy starts with understanding that “When wood is dry, embedded metals do not corrode.” Although your deck will obviously be exposed to rain and snow, Zelinka assures us that sound design principles can help, such as keeping rainwater from seeping in through the end grain and designing roofs and overhangs so they do not drain onto lower structures.

Another strategy Zelinka suggests for making sure that the fasteners holding that new deck might have a long life is using a metal noble to copper.  Noble metals are metallic chemical elements that have outstanding resistance to oxidation, the most common (and affordable) being stainless steel. This means that the nail is hot-dipped in a noble metal, and if these nails or other fasteners are not damaged in construction, the coating can go a long way in protecting against corrosion.

With this knowledge at hand, your outdoor structures can (and will) last a long time.