Minimizing Corrosive Action in Timber Bridges Treated with Waterborne Preservatives: Fid-Bau Portal

Minimizing Corrosive Action in Timber Bridges Treated with Waterborne Preservatives

Zelinka, Samuel L. / Rammer, Douglas R. / Wacker, James P.

Wood preservatives are required by AASHTO specifications in bridge applications to extend the service life of the wood members. The only exception is for naturally durable species, which are significantly more expensive. While the majority of wood used in bridge applications is treated with oil-type preservatives, wood treated with waterborne preservatives has been used in timber bridge applications. Many design engineers are choosing waterborne preservatives due to oil-type exudation concerns. As of January, 2004, the most common waterborne preservative of the past thirty years, Chromated Copper Arsenate (CCA), was voluntarily withdrawn from use in residential applications. While this withdrawal does not directly affect bridge applications, some designers are choosing to use alternative treatments to CCA, such as Alkaline Copper Quaternary (ACQ), Copper Azole (CuAz). It is believed that these preservatives are more corrosive towards metal than CCA because CCA contains hexavalent chromium and arsenic, both of which typically act as corrosion inhibitors, and are not contained in the CCA alternatives. In addition, these preservatives contain more copper than CCA, and the mechanism of corrosion of metals in contact with treated wood is believed to be related to the free copper within the wood. Unfortunately, there has been very little published research on the corrosiveness of wood treated with these preservatives because of the difficulties of measuring the corrosion rate in these environments. This work will briefly review published literature and current research activities on the corrosion of metals in contact with wood treated with waterborne alternatives to CCA. In addition, recommendations to minimize these corrosive effects in timber bridges will be discussed.