fastenings must be cleared away to make space for the replacement pile. A 

 replacement pile can then be set in the same hole as the original and driven 

 to refusal. If it is to support a deck, the pile is driven alongside the 

 pile cap, cut off below the pile cap, and pulled over into place. The cut 

 is treated with preservatives and shims are inserted to fill the space 

 between the pile and pile cap. A driftpin is then hammered into place to 

 secure the pile. 



11. Environmental Considerations . 



a. Chemical Attacks . Chemical actions of three general types may 

 affect the strength of wood. The first causes swelling and the resultant 

 weakening of wood. This action is almost completely reversible when the 

 swelling solution is removed. The second type of action brings about perma- 

 nent changes in the wood such as hydrolysis of the cellulose by acids or 

 salts. The third action also brings about permanent change in wood and 

 involves delignification of wood and dissolving of hemicellulses by alkalies. 



(1) Saltwater and Freshwater . Saltwater and freshwater penetrate 

 the wood fibers. Between zero moisture content and the fiber saturation 

 content (about 30 percent moisture content) wood will swell. The rate of 

 swelling is proportional to the moisture content up to the fiber saturation 

 point. As wet wood dries, the outer part of the wood loses moisture faster 

 than the inner parts thus the shrinkage rate is uneven and can result in the 

 development of checks or cracks. When wood is immersed over extended peri- 

 ods, water can soften the fibers. 



Water and particularly saltwater carries dissolved oxygen and marine 

 biota that can severely impact wood or wood fastenings. As a bearer of 

 oxygen, water enhances corrosion of iron and steel fastenings. When wood is 

 periodically wetted and dried in the presence of oxygen it becomes suscept- 

 ible to fungus which causes dry rot. 



(2) Strong Acids . Strong acids (such as nitric and hydrochloric) 

 and highly acidic salts (such as zinc chloride) tend to hydrolyze wood and 

 cause serious strength loss if they are present in sufficiently high con- 

 centrations. When the pH of aqueous solutions of weak acids is above 2, the 

 rate of hydrolysis of wood is small and is dependent on the temperature. 



(3) Wood Oxidation . Wood oxydation by air in dry locations is slow 

 and attacks the spring wood first to produce a rough or weathered looking 

 surface. Very dry wood can resist hundreds of years of normal exposure to 

 oxidation. Wood can be dissolved by strong acids but basically wood is 

 considered to be somewhat resistant to the action of acids and basic hydro- 

 xides. Wood is also resistant to most commercial solvents. 



b. Pollutant Attacks . Pollution in both the air and water environments 

 may have the effect of prolonging the useful life of wood by reducing the 

 oxygen supply that oxidizes the wood and supports the biota that attack 

 wood. 



c. Sunlight Exposure Effects . Wood in sunlight will expand because of 

 the increase in temperature. In most structures the wood's increase in 

 length for normal rise in temperature is negligible and as a result secondary 



273 



