Thesis (PhD (Forest and Wood Science))--Stellenbosch University, 2007. / ENGLISH ABSTRACT: There has been a reported reduction in the durability of creosote treated, wooden utility poles in
South 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 moisture
content (MC) after drying and drying defects were investigated. Relationships involving drying
defects and MC gradient before treatment which may help in the development of simple nondestructive
methods 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 the
durability of treated poles by avoiding unacceptable drying defects and MC values before
treatment. 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) and
a 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 depths
into the pole were taken. However, when single samples of 50 mm and 75 mm depths were
considered, the auger drill MC measurements were unreliable. It was concluded that SABS SM
983 (2000), which specifies taking single radial auger drill samples of depth 70±5 mm at pole
mid 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 and
heartwood percentage were related to final drying defects and may be used as criteria for pole
sorting before kiln drying. Also, the number of valleys per unit length (VPUL) of the
circumference at the theoretical ground line (TGL) was positively correlated with honeycomb
and closed surface checks, implying that VPUL as a parameter of the pole circumference
profile 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 resulted
into unacceptable levels of drying defects in kiln dried E. grandis poles. In addition, the drying
period 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 TGL
and not higher up since there is normally considerable MC variation in the longitudinal
direction. It is also important to consider sampling more than one position on the pole
circumference to cater for the MC variation in the tangential direction.
For poles with large sapwood depths i.e. >15mm, mild drying conditions at the
beginning of a drying run should be used, since such poles may be more susceptible to
surface checking. Poles with large heartwood percentages should also be dried with
suitable kiln schedules, i.e. with low Tdb to minimise honeycomb and collapse. In
general, Tdb lower than 80oC should be considered in order to reduce the defects to
acceptable levels.
Poles should be dried for longer than 8 days even at Tdb as high as 80oC to reduce the
MC to acceptable values and gradients.
Since only three schedules were tested, more drying schedules should be investigated to
make reliable conclusions about the effect of schedule on drying defects. In addition,
more poles per schedule should be considered in order to obtain statistically reliable
results.
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 drying
schedules on defect correlations, further studies should be done on sufficient samples to
come up with more reliable conclusions. It is also necessary to further investigate the
possible variation in correlation of VPUL and internal defects on a large sample of poles
from specific sources and drying schedules. / AFRIKAANSE OPSOMMING: Die afgelope paar jaar is waargeneem dat kreosootbehandelde transmissiepale 'n afname in
duursaamheid toon. Verskeie faktore kan hiervoor verantwoordelik wees. In hierdie studie is
huidige Eucalyptus grandis oonddrogingskedules en voggehalte- (VG) bepalingsmetodes vóór
en na droging asook drogingsdefekte ondersoek. Verwantskappe tussen drogingsdefekte en
VG-gradiënt vóór behandeling wat kan meehelp om nie-destruktiewe metodes te ontwikkel om
defekte soos oppervlaktekrake, interne barste en instorting en VG-gradiënt te bepaal, is ook
bestudeer. Die langtermyn doelwit van hierdie studie is om die duursaamheid van behandelde
pale te verbeter deur onaanvaarbare drogingsdefekte en voggehaltes vóór behandeling te vermy.
Die resultate van hierdie ondersoek word in die volgende hoofstukke bespreek:
'n inleiding waarin die doelwitte van die onderskeie ondersoeke motiveer word
(Hoofstuk 1);
'n oorsig van die literatuur relevant tot VG en drogingsdefekte in gesaagde en
rondehout (Hoofstuk 2);
persklaar manuskripte oor VG en drogingsdefekte in oondgedroogde E. grandis
pale (Hoofstukke 3 tot 7) en
'n algemene gevolgtrekking wat bevindings uit hoofstukke 3 tot 7 saamvat gevolg
deur aanbevelings (Hoofstuk 8).
Die augerboor-metode kon betroubare VG-waardes lewer indien monsters van 25mm diepteinkremente
in die paal geneem is. Augerboor-voggehaltemetings met enkel 50mm en 75mm
monsters was egter onbetroubaar. Daar is bevind dat SABS SM 983 (2000), wat 'n enkel
radiale augerboormonster tot op 'n diepte van 70±5 mm in die middel van die lengte van die
paal spesifiseer, nie 'n betroubare VG-bepalingsmetode vir oondgedroogde E. grandis pale is
nie.
Die resultate het aangetoon dat boomgroeifaktore soos spinthoutdiepte, groen voggehalte en die
persentasie kernhout aan drogingsdefekte verwant is en as basis vir sortering van pale vóór
oonddroging gebruik kan word. Ook kon die aantal valleie per eenheidslengte (e. VPUL) van
die omtrek by die teoretiese grondlyn (TGL) positief met interne barste en geslote
oppervlaktekrake korreleer word wat impliseer dat VPUL as parameter van die omtrekprofiel
van die paal gebruik kan word om onsigbare, interne defekte mee vas te stel.
Die droëbaltemperatuur (Tdb) van 80oC wat in die industrie gebruik word, was te hoog omdat
dit onaanvaarbare vlakke van drogingsdefekte in oondgedroogde E. grandis veroorsaak het.
Daarbenewens was die drogingsperiode van 8 dae by 'n Tdb van 80oC te kort om 'n aanvaarbare
VG-gradiënt te lewer.
Die volgende word derhalwe aanbeveel:
Om betroubare VG-resultate d.m.v. die augerboormetode te verkry, behoort monsters in
inkremente van 25mm geneem te word.
VG-metings behoort in die mees kritiese dele van 'n paal gedoen te word byv. by TGL
en nie hoër nie omdat daar normaalweg beduidende VG-variasie in die longitudinale
rigting is. Dis ook belangrik om op meer as een posisie op die omtrek te meet om vir die
variasie in die tangensiale rigting voorsiening te maak.
Ligte drogingstoestande behoort aan die begin van die drogingsproses by pale met groot
spinthoutdieptes, bv. > 15mm, gebruik te word omdat sulke pale meer geneigd tot
oppervlaktekrake is. Pale met hoë kernhoutpersentasies moet ook met geskikte
drogingskedules, bv. met lae Tdb, gedroog word om interne barste en ineenstorting te
verminder. Oor die algemeen behoort Tdb laer as 80oC oorweeg te word om die omvang
van defekte by aanvaarbare vlakke te hou.
Selfs by Tdb so hoog as 80oC behoort pale langer as 8 dae te droog om die VG tot by
aanvaarbare waardes en 'n gradiënt te bring.
Omdat slegs drie skedules evalueer is, behoort meer skedules ondersoek word om
betroubare gevolgtrekkings oor die effek van skedule op drogingsdefekte te verskaf.
Addisioneel behoort meer pale per skedule oorweeg word om statisties betroubare
resultate te verkry.
Die verwantskap tussen paaldeursnee en drogingsdefekte benodig ook verdere
ondersoek.
Aangesien 'n beperkte aantal pale gebruik is om die herkoms en drogingskedule met
defekte te korreleer, behoort verdere studie op genoeg pale gedoen te word om meer
betroubare gevolgtrekkings te kry. Dit is ook noodsaaklik om die moontlike variasie in
die korrelasie tussen VPUL en interne defekte op 'n groot aantal pale van spesifieke
herkoms en drogingskedules vas te stel.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/4575 |
| Date | 12 1900 |
| Creators | Mugabi, Paul |
| Contributors | Rypstra, Tim, Vermaas, H. F., Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 277 leave, preliminary pages i-viii and numbered pages 1-255. Includes bibliography, list of figures and tables. |
| Rights | Stellenbosch University |
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