Moisture content and drying defects in kiln-dried Eucalyptus grandis poles

[摘要] ENGLISH ABSTRACT: There has been a reported reduction in the durability of creosote treated, wooden utility poles inSouth Africa in recent years. Several factors could have been responsible for this. In this study,Eucalyptus grandis pole drying schedules currently used, methods of measurement of moisturecontent (MC) after drying and drying defects were investigated. Relationships involving dryingdefects and MC gradient before treatment which may help in the development of simple nondestructivemethods of assessing defects such as surface checking, honeycomb and collapse,and MC gradient were also explored. The long term goal of this study was to increase thedurability of treated poles by avoiding unacceptable drying defects and MC values beforetreatment. The results of this investigation are presented in the following chapters:an introduction motivating the aims of the investigations (Chapter 1);a review of literature relevant to MC and drying defects in sawn or round wood(Chapter 2);ready for press manuscripts on MC and drying defects in kiln-dried E. grandis poles(Chapters 3 to 7) anda general conclusion that links up chapters 3 to 7, and recommendations (Chapter 8).The auger drill method gave reliable MC values when samples in increments of 25 mm depthsinto the pole were taken. However, when single samples of 50 mm and 75 mm depths wereconsidered, the auger drill MC measurements were unreliable. It was concluded that SABS SM983 (2000), which specifies taking single radial auger drill samples of depth 70±5 mm at polemid length, is not a reliable method of measuring MC in a kiln-dried E. grandis pole.Correlation results indicated that tree growth factors such as sapwood depth, green MC andheartwood percentage were related to final drying defects and may be used as criteria for polesorting before kiln drying. Also, the number of valleys per unit length (VPUL) of thecircumference at the theoretical ground line (TGL) was positively correlated with honeycomband closed surface checks, implying that VPUL as a parameter of the pole circumferenceprofile can be used to assess invisible, internal defects.The dry bulb temperature (Tdb) of 80oC, used to dry poles in industry, was too high and resultedinto unacceptable levels of drying defects in kiln dried E. grandis poles. In addition, the dryingperiod of 8 days was too short to attain an acceptable MC gradient in poles.It is, therefore, recommended that:To use the auger drill method to reliably determine the moisture content of a pole,samples in increments of 25 mm should be taken.MC measurements should be made at the most critical zones of a pole such as the TGLand not higher up since there is normally considerable MC variation in the longitudinaldirection. It is also important to consider sampling more than one position on the polecircumference to cater for the MC variation in the tangential direction.For poles with large sapwood depths i.e. >15mm, mild drying conditions at thebeginning of a drying run should be used, since such poles may be more susceptible tosurface checking. Poles with large heartwood percentages should also be dried withsuitable kiln schedules, i.e. with low Tdb to minimise honeycomb and collapse. Ingeneral, Tdb lower than 80oC should be considered in order to reduce the defects toacceptable levels.Poles should be dried for longer than 8 days even at Tdb as high as 80oC to reduce theMC to acceptable values and gradients.Since only three schedules were tested, more drying schedules should be investigated tomake reliable conclusions about the effect of schedule on drying defects. In addition,more poles per schedule should be considered in order to obtain statistically reliableresults.The relationship of pole diameter and drying defects also requires further investigation.Since a limited sample was used to test for the effect of source of poles and dryingschedules on defect correlations, further studies should be done on sufficient samples tocome up with more reliable conclusions. It is also necessary to further investigate thepossible variation in correlation of VPUL and internal defects on a large sample of polesfrom specific sources and drying schedules.

[发布机构] Stellenbosch University