The available wood supply in Uruguay comprises trees that grow so fast in intensively managed plantations that they reach saw timber size in 25 years or fewer. Trees harvested at this age contain high proportions of juvenile wood that may lead to lumber low in stiffness and strength. A project was conducted to characterize fast-growing wood, determine engineering properties, and assign visual structural grades of lumber. The present study evaluated properties of 15-and 25-year-old loblolly (Pinus taeda L.) and slash (Pinus elliottii Eng.) pine to better understand the current available locally produced wood material. A total of 175 stump bolts from trees from two commercial plantations provided inner and outer small clear specimens for property evaluation. Specific gravity, bending, compression parallel to grain and perpendicular to grain, and shear tests were conducted. Most properties significantly increased radially away from the pith. The outer wood appears to be denser, stiffer, and stronger than the inner wood in both plantations. Wood from 15-year-old San Jose ´trees showed significantly lower properties than 25-year-old Paysandu ´trees, and had considerably inferior properties compared with values listed in the Wood Handbook (US Department of Agriculture 1999). Our results on 25-year-old Paysandu ´small clear specimens showed properties similar to those of previous studies on small clear and structural size pieces. Therefore, it can be expected that lumber from 25-year-old Paysandu ´trees will eventually comply with required properties for structural use. A second on-going phase of this study addressing structural size specimens will help to establish more definite conclusions.
RESUMENLa disponibilidad de madera aserrada producida en Uruguay ha tenido un incremento significativo en los últimos treinta años debido a la política gubernamental para promover plantaciones de rápido crecimiento. Para aplicaciones estructurales, la madera debe estar clasificada según su resistencia mecánica, rigidez y densidad, y sus propiedades disponibles para profesionales y técnicos. Este trabajo estuvo motivado por la falta de madera clasificada y de un sistema de clases resistentes en Uruguay. El objetivo fue caracterizar mecánicamente madera aserrada de Pinus elliottii y P. taeda y asociar sus propiedades a grados estructurales. El estudio evaluó una muestra representativa del material que se produce y comercializa en Uruguay, que incluyó madera de 25 años del litoral y de 15 años del suroeste. Aproximadamente 900 piezas de tamaño estructural previamente clasificadas en tres grados cualitativos fueron ensayadas en flexión, compresión y tracción-paralela a la fibra. Cada grado fue asociado a una familia de tensiones y comparado con los valores de EN 338 y NCh 1198. El grupo superior con piezas de 25 años del litoral pudo ser asignado a la clase C14, mientras que ninguno a las exigencias de NCh 1198. Ningún grupo de 15 años del suroeste cumplió con los requisitos estructurales internacionales.
Palabras clave: Grados estructurales, madera estructural, Pinus elliottii, Pinus taeda.
ABSTRACTThe availability of local produced lumber has significantly increased in the last thirty years in Uruguay due to a governmental policy to promote fast-grown plantations. For structural applications lumber must be strength graded and the properties should be available for professionals and customers. This research was motivated by the lack of strength-graded lumber and a strength class system in Uruguay. The objective was to characterize Pinus elliottii and Pinus taeda timber and to assign strength grades. The study evaluated a representative sample of currently produced material in Uruguay, including lumber from a 25-year-old west and 15-year-old southern-west plantations. Approximately 900 structural pieces were graded in three qualitative grades, and then tested in bending, compression-and tensile-parallel to grain. For each grade, a group of strength properties was associated. Comparison with EN 338 strength classes and NCh 1198 indicated that the superior group comprised of 25-year-old timber could be assigned to C14 class, while none complied with NCh 1198 requirements. None of the 15 year-old groups attained the international structural requirements.
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