Long-Term Durability of Structural Composite Lumber in Bridge Applications

Structural composite lumber (SCL) is a family of newly engineered wood products finding increasing use in highway bridge applications. The advantages of SCL are high strength, flexibility of sizes and shapes, stiffness, and excellent treatability with preservatives. The main concerns in SCL bridge applications are the effects of moisture, exposure to ultraviolet light, and varying temperature and humidity levels on the long-term durability of the bridge members. To document these effects, monitoring of full-scale SCL T-beam bridge members was performed under ambient conditions and an accelerated aging process. A total of 16 beams were monitored under exposed weather conditions with frequent wetting and drying. Variables in the experiment were lumber type (Douglas fir and southern yellow pine), SCL type [laminated veneer lumber (LVL) and parallel strand lumber (PSL)], and preservative type [chromium copper arsenate (CCA) and pentachlorophenol (penta)]. The moisture content of the SCL, ambient temperature, humidity, and the general condition of the beams were monitored. The experiment concluded with the determination of an applicable SCL member for bridge applications. Although most of the members were found to be adequate, a Douglas fir CCA-treated LVL member was found to be the most suitable.