Structural Flakeboardsand Composites 2907 



To impart resistance to fluctuations in humidity and temperature, and to rain 

 wetting, structural flakeboards are bonded with exterior resins — chiefly phenol- 

 formaldehyde formulations. (See sec. 24-5.) 



All of the commercially manufactured structural flakeboards are formed in 

 three layers, i.e. , with a core layer and two face layers. Flakes in the face layers 

 generally are thinner and may also be longer than those in the core layer. In most 

 waferboards, flakes within all three layers are randomly placed with regard to 

 grain direction (fig. 24-2 top). 



In oriented-strand boards (fig. 24-2 bottom), flakes in the two face layers 

 are aligned with grain parallel to the major panel axis, e.g. , parallel to the 8-foot 

 edges of 4- by 8-foot panels. Core flakes may be randomly disposed, but more 

 commonly are aligned at right angles to those in the face layers. 



24-2 STANDARDS 



Tentative standards and specifications for structural flakeboard were adopted 

 in 1980 and 1981 and continue to evolve. In general terms, the industry seeks a 

 product which can replace the construction exterior grade (CDX) plywood 

 marketed in the United States. Vajda (1978a) summarized the significant proper- 

 ties of CDX plywood as follows: 



Property Value 



Modulus of rupture in bending 



Along the grain 8,000 psi 



Across the grain 4,000 psi 



Modulus of elasticity 



Along the grain 1,200,000 psi 



Across the grain 500,000 psi 



Linear expansion (30 to 90 percent relative 

 humidity) 



Along the grain 0. 10 percent 



Across the grain .14 percent 



CDX plywood weighs about 36 pounds per cubic foot (dry basis) and retains 60 

 to 70 percent of its original strength under severe long-term exposure to weather 

 or under tests which simulate such exposure (Vajda 1978a). 



BUILDING CODES 



Jorgensen (1978) described the history of requirements, codes, and U.S. 

 Forest Service goals for properties of structural flakeboard. He noted that in late 

 1972 the New Jersey State Building Code approved the use of Canadian wafer- 

 board in thickness dimensions equivalent to those in use in Canada and equiv- 



