Annals of Warsaw University of Life Sciences - SGGW

The scope of work included determination of the most significant properties having an
impact, first of all, on durability and quality of wood in elements used outdoors, exposed to
variable weather conditions. The effect of a gradual degradation of wood caused by variable
thermal and moisture conditions was determined on the basis of tests of wood density
(according to Polish Standard PN-77/D-04101) and wood compressive strength along the
fibres (according to Polish Standard PN-79/D-04102). The use of slightly smaller dimensions
of samples, i.e. 15 x 15 x 22 mm, was a departure from the above-mentioned norms. The
deviation from demands of the Polish Standard concerning the sample size occurred in view
of the fact that no relationship existed between the compressive strength along the fibres and
the size of samples when they geometrically similar and when section of the samples
contained at least a couple of annual increments. Testers of wood of every kind were being
acquired from one board, getting so-called „identical samples”. Thanks to that a density
moved close and a structure were kept for so that happening changes in the ageing process are
a main factor deciding on examined properties.
RESULTS
The samples of the tested wood species developed a colour changes as a result of
accelerated ageing. At first the colour was becoming darker, and then wood was assuming the
colour grey-ashen. As a result of the ageing process on the surface of wood cracks appeared.
As first running cracks were turning up at radial direction, and then in tangential direction.
Coming into existence of cracks of surface layers of wood, leading to the degradation
of this material, is a result of uneven drying outside and of inner layers generating strong
desorption stresses.
The ageing has caused a decrease in density of the tested wood species. The higher the
number of ageing cycles, the bigger the difference in density between the unaged wood and
the wood treated by the process. Artificial ageing caused wood fall in the density in case of
red oak wood from the initial average value of 804 kg/m 3 up to 771 kg/m 3 what constitutes c.
12,7 %. It is possible to suppose this fall is triggered above all with scouring the row of nonstructural
substances from cell walls and to a lesser degree with partial hydrolysis
hemicelluloses and celluloses in layers surface of samples. In order to illustrate the impact of
accelerated ageing on density of the tested wood species, an approximate percentage decrease
corresponding to one cycle of the process has been determined (fig. 1). The first artificial
wood ageing cycles had the biggest impact on density. Non-structural substances were being
washed out from the wood cell walls in the most intensive way. The decrease in density
related to one ageing cycle tended to be smaller with a growing number of artificial ageing
cycles completed and was nearing a constant value (asymptotically to zero).
With the increase number of cycles of ageing, at the testing compressive strength
along the fibres an image is changing scrap of samples – from pressing for oblong cracks. The
change of character of damage is attesting to the fall in the strength of wood triggered process
artificial ageing. Analyzing the average values of wood compressive strength along the fibres,
it has been observed that artificial ageing caused a decrease in strength of red oak wood from
initial value 64,3 MPa to 57,9 MPa (fall about c. 7,1 %). The decrease in wood compressive
strength is caused by changes occurring in the wood structure. Cyclic changes in thermal and
moisture conditions resulted in strong stresses exceeding internal cohesion forces of the
wood. It caused this occurrence of fractures in cell walls. Loss of mass also took place to the
effect of washing the row of nonstructural substances out of cell walls and relaxing the
skeleton lignin- cellulose. In order to illustrate the impact of ageing on compressive strength
along the fibres, an approximate percentage decrease of this feature corresponding to one
cycle was determined (fig. 2). The nature of the changes is the same as in the case of density.
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