Effect of incipient decay on compressive strength and stiffness of timber piles.
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Effect of incipient decay on compressive strength and stiffness of timber piles.

  • 1997-12-01

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  • English

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      The goal of this study was to develop guidelines for bench marking strength and stiffness properties of deteriorated in-service timber piles as a function of the age of the pile and the in-service environment. A parallel research project undertaken to evaluate the load capacity of hollowed timber piles showed a significant reduction in the strength and stiffness of the undamaged portions of the piles. While it is well known that preservative treatment adversely affects the mechanical properties of timber piles, the level of reduction in these properties in the undamaged portion of the piles was significantly higher than expected. There are two key factors that control strength properties of the seemingly undamaged sections of a given pile in service. They are: (a) the amount of cellulose material in the wood; and (b) the quality of the resin bond between the wood fibers. Piles which appear to be undamaged can exhibit a significant loss in strength due to incipient damage occurring in the piles. This incipient damage is generally due to fungus infestation which is not obvious to the naked eye. If the infestation is widespread the pile could suffer a loss of the cellulose material which can explain the decrease in the strength properties. Also, the leaching of the creosote into the interior of the piles can contribute to some loss in strength. Small coupons were taken from most of the piles to determine the basic physical and mechanical properties. Several chemical analysis tests were conducted on specimens taken from pile cross-section to determine the level of creosote, lignin, holocellulose, and the degradation of the holocellulose. These tests were conducted in accordance with specified ASTM standards. The mechanical properties obtained from the coupon tests were compared with those obtained from the full-scale pile tests and formula for estimating the pile strength and stiffness from the coupon properties were developed. The investigation has led to the following conclusions: (1) The pile strength and stiffness are correlated to the strength and stiffness, respectively, of the coupons; (2) The mechanical properties of the solid coupons are independent of their location in the pile cross-section; (3) The compressive strength of the coupons was independent of the specific gravity of the coupons; (4) The creosote level in the pile decreased towards the center of the pile; (5) The lignin level remained unchanged across the cross section of the pile indicating that this level is not influenced by either the decay or the service life of the pile; (6) The holocellulose content indicates that degradation occurred in the outer portions of the pile which are most exposed to the environment; and (7) The decay level was unrelated to the compressive strength of the coupons.
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