Physical and mechanical properties of laminated strand lumber treated with fire retardant

This study investigated the effects of fire retardant chemical treatment on the physical and mechanical properties of commercial laminated strand lumber (LSL). Thirty-five nominal 2- by 4- by 96-inch LSL studs were nondestructively tested to determine MOE and sorted into two stiffness matched groups: 17 each for untreated and fire-retardant (FR) treated 3.8- by 8.9- by 194-cm (1.5- by 3.5- by 76.5-in.) bending specimens. The 50-cm end from each stud was cut into three matched pieces to be used for untreated, FR-treated, and water-treated 3.8- by 6.35- by 15.2-cm (1.5- by 2.5- by 6-in.) compression specimens. Treatment was via full-cell vacuum/pressure impregnation. The treated specimens were kiln-dried after treating, and all specimens were equilibrated at 23oC, 65 percent relative humidity before testing. Experimental results for untreated LSL showed a high correlation between density, MOE, and MOR, with quite low coefficients of variation. FR treatment reduced LSL MOE and MOR values by 17 and 20 percent, respectively. With compression parallel to grain specimens, both water and FR treatment decreased average maximum crushing strength, water by 13 percent and FR treatment by a somewhat larger, but statistically insignificant, 16 percent. This indicates that property reductions are due primarily to the water absorption, and not the FR chemical. Both FR and water treatment showed slightly higher equilibrium moisture content values than the untreated control specimens. Also, FR-treated specimens had moderately higher swelling values than water-treated specimens in both width and thickness directions.

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