Performance of Laminated Preservative-Treated Railroad Bridge Stringers

Twenty-eight laminated bridge stringers were prepared from southern yellow pine lumber treated with Wolman salts to a dry salt retention of 0.3 lb. per cu. ft. or with creosote to 8 lb. per cu. ft. Four additional stringers were laminated from untreated material and treated after gluing. Six different resorcinol glues were used. Dry shear block tests and accelerated soaking-drying delamination tests were made on a 9-inch sample from each stringer. Shear strength and the percentage of wood failures obtained with the untreated material (treated after gluing) appeared satisfactory, which indicated adequate control of the gluing process. In general, the material that was Wolmanized before gluing also had high strength values and a high percentage of wood failures. For specimens from 12 stringers made of Wolmanized material, the average shear strength was 1,835 psi, and the percentage of wood failure was 80 percent, as compared to 1,864 psi, and 86 percent for specimens from 4 stringers made of untreated material. The lowest average percentage of wood failure for a single section made of Wolmanized material was 60 percent. The specimens from the stringers made of creosoted material did not produce the uniformly high strength values that were found with the Wolmanized or untreated material. The average strength for specimens from 12 creosoted stringers was 1,582 psi, as compared to 1,835 psi for the Wolmanized and 1,864 psi for the untreated material. The wood failures were high for the creosoted material except in two stringers. The results of the accelerated soaking-drying delamination test indicate adequate glue bonds, except in three stringers. The maximum delamination developed in any section, however, was 12.6 percent, which is only slightly more than the delamination permitted in current Navy specifications for laminated material. The appearance of the sections made of Wolmanized material indicates adequate glue bonds, even for the section that developed the most delamination. Of the creosoted sections two specimens developed slightly more delamination than is usually regarded acceptable for material intended for exterior service. The remainder of the specimens made of creosoted material showed high resistance to delamination. This indicates that adequate glue bonds can be obtained with resorcinol glues on creosoted yellow pine, where the treatment is not sufficiently heavy to cause bleeding in subsequent surfacing and gluing operations. The data also indicate that southern yellow pine treated with Wolman salts can be glued satisfactorily with resorcinol resin glues, under adequate control of the gluing process. The stringers were examined after 3 years service in a bridge and found to be in excellent condition.

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