Mechanical analysis of wood-fiber cement sheets under constant and repeated loading

Inorganic-bonded panels have been successfully utilized for many years around the world. Cellulose materials are extensively used for cement-bonded particleboard (CBP) and for fiber-reinforced cement (FRC) composites worldwide. Particularly in Europe, this family of composites is used, among other applications, for building construction. Use of wood-fiber cement (WFC) composites in North America has been steadily increasing over the last 10 years. Problems encountered with resin-bonded wood products used in exterior environments have resulted in litigation and search for viable products. WFC sheets are currently filling this need and gaining market share by virtue of their own superior properties. This study was designed to provide basic information currently lacking in literature and important to the wise application of WFC sheets. Experimental autoclaved WFC flat sheets made with kraft Douglas fir fiber and with recycled old corrugated containers (OCC) fiber were manufactured and the results compared with an available commercial product. This experimental program was subdivided into three manuscripts. The first manuscript evaluates whether the actual mechanical properties of WFC sheets can be predicted using nondestructive parameters of the material by applying stress wave time techniques. The second manuscript deals with characterization of the WFC sheets. Physical and mechanical properties were evaluated and results discussed with the use of a scanning electronic microscopic (SEM) analysis. Manuscript three examines the viscoelastic behavior of the material at constant and repeated loading conditions. The nondestructive evaluation (NDE) of the material showed good correlation between dynamic and static modulus of elasticity (MOE). A multivariate linear regression analysis provided the strongest correlation (R = 0.828) for static MOE as a function of wave speed, density, and dynamic MOE. Results from Manuscript 2 revealed that WFC sheets manufactured with Douglas fir fiber presented superior mechanical and physical properties as compared to the commercial sheets. Sheets made with recycled OCC fiber presented properties comparable to the commercial product, except for modulus of rupture. However, defects detected after the autoclave process make OCC fibers unsuitable as fiber reinforcement for the sheets are to be cured in autoclave. Finally, results from Manuscript 3 showed that WFC sheets were sensitive to high moisture environment but not to high temperature. Creep deflection was higher under high humidity conditions. Even though moisture played a major role on the creep behavior of the products tested, values of deformation and relative creep were much lower when compared to resin-bonded wood composites. Furthermore, a 5-element model developed to predict creep deflection of the material studied produced coefficient of determination (R{dollar}sp2{dollar}) over 0.977. Results obtained in the current study showed that WFC flat sheets tested are potential candidates for use in structural applications.