Effects of age and moisture content on mechanical properties and twisting of Finnish round and sawn pine (Pinus sylvestris) and spruce (Picea abies)

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1 Effects of age and moisture content on mechanical properties and twisting of Finnish round and sawn pine (Pinus sylvestris) and spruce (Picea abies) Boren, Hannu 1 ABSTRACT The primary aim of the study was to find out the effect of age on mechanical properties and twisting of pith enclosed round and sawn pine (Pinus sylvestris) and spruce (Picea abies) produced from small-size thinning wood. Effects of defects, moisture content, physical properties, form and size of the specimen, wood processing, origin of timber and tree species are studied, also. There was found an interaction between age and moisture. The younger the age the greater the effect of moisture on twisting and mechanical properties. A possible reasons for that are: There is more tracheids and cell gaps per volume unit and tracheids comprise more lignin and hemicellulose and have a greater microfibril angle in the juvenile wood than the mature wood. Previous studies have shown that the microfibril angle and chemical composition of timber affects mechanical properties, and the twisting is affected by microfibril angle, in particular. The great twisting of pith enclosed timber produced from small-size thinning wood might cause problems in prefabricated components exposed to humidity variations. INTRODUCTION The studied material was collected in the EU-Project Small Diameter Round Timber for Construction (FAIR CT95-91). More information about the project can be found papers presented by Ranta-Maunus (1999) and (). In this paper the effects of age and moisture content on mechanical properties and twisting of pith enclosed Finnish round and sawn pine (Pinus sylvestris) and spruce (Picea abies) are studied and discussed. Twisting, bending and compression strength and stiffness are presented and discussed in order to determine possible obstacles to utilise the pith enclosed Finnish pine and spruce from thinning in constructions. The paper is based on the author s doctor thesis Factors affecting properties of round and sawn Finnish pine (Pinus sylvestris) and spruce (Picea abies). (Boren ). The hypotheses of the paper are listed below. They are based on a theoretical background of wood properties and features, and are verified by measurements of a properties and statistical methods. 1. Because of the large extent of juvenile wood, the age and moisture content have a great effect on the twisting and the mechanical properties of pith enclosed timber. 2. The pith enclosed timber has not negative size effect on the bending strength. A high proportion of juvenile wood in smaller sections suggest that strength of pith enclosed timber should increase with growing height i.e. age, which is the opposite to the EN 3 standard size adjustment for sawn timber. MATERIALS AND METHODS The quality of the tested timber was characterized by the measurements and the data was analyzed by a multiple regression analysis. The following variables and notations for them are used in this paper: - a = age (years), measured at or close the failure point. The measurement method is shown in Figure 1. - b = deviation on timber surface (mm/m) caused by twisting - c = circumference of the specimen (mm), measured at or close the failure point - d = diameter of the specimen (mm), measured at or close the failure point - h = heigth of the specimen (mm), measured at or close the failure point - f i = forest from 1 to, where the specimen was taken from. - G = gross grain (mm/m) - KAR = ks/c = knot sum per circumference (%), measured at or close the failure point 1 Senior researcher, Finnish Forest Research Institute, Joensuu Research Station, Box, FIN-1, Joensuu, Finland

2 - KAR m (%) = knot sum of machine processed surface per total circumference (manually debarked pine: KAR m = ). - l = length (m) - ρ = density at % moisture content (kg/m 3 ) at or close the failure point - r = ring width (mm), measured at or close the failure point - S = tree species, pine = and spruce =1 - ss = form of tension zone in bending. Sawn (plane) surface = 1 and round =. - t = average taper (mm/m) of timber - T = twisting ( /m) of timber. The measurement method is shown in figure 2. - u = moisture content, mass of water per dry mass (%), measured at or close the failure point - E c, = modulus of elasticity in compression parallel to the grain (kn/mm 2 ) - E m = modulus of elasticity in bending (kn/mm 2 ) - f c, = compression strength parallel to the grain (N/mm 2 ) - f m = bending strength (N/mm 2 ) Both the bending and compression testing method followed EN3 standard as closely as practical. The principle to measure twisting ( o /m) is presented in Figure 2. Used age in Models 1, 2, 5 and for round form. Bending direction Used age in Models 3 and for round form. Bending direction Used age in Models 1, 2, 5 and for partly sawn form. Used age in Models 3 and for partly sawn form. Used age in Models 1, 2, 5 and for squared sawn form. Bending direction Used age in Models 3 and for squared sawn form. Figure 1. Used age for different forms of timber in the models. Used age is a mean value of ages calculated by directions of arrows. b = deviation on timber surface (mm/m) Initial positions of twisting lines Position of twisting lines after seasoning and conditioning T=twisting angle ( o /m) Test specimen Figure 2. Method used to measure twisting of timber.

3 RESULTS Twisting Regression models for twisting was done separately for the pine and the spruce. For the pine was found that the age, gross grain, moisture content and density have a statistically significant effect on the twisting (Model 1). For the spruce was found the interaction between the age, gross grain and moisture content (Model 2). The diameter of specimen had also the effect on the twisting in both tree species, but the better indicator of the juvenile wood was the age. Model 1 for pine is shown below. The multiple coefficient of determination for the model (adjusted R 2 ) is.2 and the standard error (s).17. T =.92-.lg a +.1lg (G+1) +.77ρ.57 lg u 1 Model 2 for spruce is shown below. The multiple coefficient of determination for the model (adjusted R 2 ) is. and the standard error (s).175. T = * (G+1) * (2 u) / a - 1 All multipliers of the Models 1 and 2 are significant and the basic statistical assumptions are met. The length of twisting specimens were four meters. The low multiple coefficient of determination (adjusted R 2 ) of the Models 1 and 2 indicates a great variability and that there are several other factors affecting on the twisting of structural size timber. Figure 3 shows the means and standard errors for the twisting by the age classes for the pine and the spruce. In addition, the mean moisture content of the samples are presented. Within the presented samples, the moisture content is constant along the age distribution. In addition, the effect of age at moisture content % and % on the twisting of the pine and the spruce by the Models 1 and 2, respectively, are presented in Figure 3. The younger the age the greater the twisting. In addition, there can be seen that the moisture content have remarkable effect on the twisting. In addition, there can be seen the interaction between the moisture content and the age: The higher the age the lower the effect of moisture content. Twisting (o/m) 3,5 3, 2,5 2, 1,5 1, Twisting (o/m) 3,5 3, 2,5 2, 1,5 1, Twisting (o/m), 3,5 3, 2,5 2, 1,5 1,, , u =, % 51-, u =,3 % 1-5, u =,5 % -1, u =, % , u =,3 % 17-, u = 19,3 % 1-5, u =,5 %, , u =, % 1-1, u = 19, % , u =,5 %, Age (year), G = mm/m, p = kg/m3 and u = %, G = mm/m, p = kg/m3 and u = %, G = mm/m and u = %, G = mm/m and u = % Figure 3. Means and standard errors for twisting by age class (left: pine, middle: spruce) and the effects of age on twisting by Models 1 and 2 for pine and spruce, respectively (right). Mechanical properties An interesting phenomena found in the statistical analysis of mechanical properties were an interaction between the moisture content and the age. This interaction is shown by scatter-plots in Figures and 5.

4 2 2 2 Em (kn/mm2) Em (kn/mm2) Em (kn/mm 2 ),7,,9 1, 1,1 1,2 1,3 1, 1,5 a (year) u (%) LOG u / LOG a fm (N/mm 2 ) fm (N/mm2) fm (N/mm2) 5 15 a (year) u (%) u / LOG a Figure. Scatter-plots of the bending stiffness and strength versus age, moisture and age divided by moisture. The stiffness and the strength are at test moisture content. Ec, (kn/mm 2 ) Ec, (kn/mm2) Ec, (kn/mm2) ,7 -, -,5 -, -,3 -,2 -,1,,1,2,3, a (year) u (%) LOG (u/a) fc, (N/mm2) fc, (N/mm2) fc, (N/mm2) , -, -, -,2,,2, a (year) u (%) LOG (u/a) Figure 5. Scatter-plots of the compression stiffness and strength versus age, moisture and age divided by moisture. The stiffness and the strength are at test moisture content.

5 The tested pine and spruce material was combined in the statistical analysis of mechanical properties. The multiple regression models 3,, 5 and for E c,, E m, f c, and f m, respectively, are shown below. The pine and the spruce deviated for each other in every case. This deviation was caused by the density, so that in the similar density the spruce was stronger. Result indicated that in the bending the variable ss, sawn surface in tension zone, has the negative effect on the bending strength. The manually debarked round timber were stronger than the machine processed, also. Furthermore, in the bending the machine processing interacted with the knots, i.e. the wood surface containing the knots increase the negative effect of machine processing. Differences between the origin of timber were found in the compression. Model 3 for E m is shown below. The multiple coefficient of determination for the model 3 (adjusted R 2 ) is. and the standard error (s) The new variable used is: - lg u / lg a = logarithm from moisture content (%) per logarithm from age (a) E m =.2.1KAR m +.19ρ +.132ρ S.1t 5.lg u / lg a Model for f m is shown below. The multiple coefficient of determination for the model (adjusted R 2 ) is.7 and the standard error (s).51. The new variable used is: - u/lg a = moisture content (%) per logarithm from age (a) f m = 1..7h.22KARm +.ρ +.519ρS.32r.23ss.37u/lg a Model 5 for E c, is shown below. The multiple coefficient of determination for the model 5 (adjusted R 2 ) is. and the standard error (s) The new variable used is: - lg(u/a) = logarithm from moisture content (%) per age (a) E c, = f.33KAR +.159ρ +.7ρ S 2.2LOG r 2.27lg(u/a) Model for f c, is shown below. The multiple coefficient of determination for the model (adjusted R 2 ) is.3 and the standard error (s) f c, =..57d.5f.9KAR +.27ρ +.ρ S 21.13lg(u/a) All multipliers included in Models 3,, 5 and are significant and the basic statistical assumptions behind the regression models are met. The age and the ring width determine the size of timber i.e. the diameter, height or circumference. In addition, according to the statistical analysis the age and the moisture content have an interaction. The effects of age, moisture content, ring width and size of timber on the mechanical properties are presented in Figures, 7 and by Models 3,, 5 and for the machine round pine at the Grade A and B limits. A synopsis of visual grading parameters for the machine round Finnish and British pine and Finnish spruce for the Grades A and B are given in Table 1. (Ranta-Maunus 1999). Table 1. Values for strength grading parameters of machine round Finnish and British pine and Finnish spruce for the Grades A and B. Ranta-Maunus (1999). Visual grading parameter Grade A Grade B Knot sum per diameter ks/d [%] 75 Max knot per diameter mk/d [%] Ring width r [mm] 3 5 In Figure is presented the effect of size on the bending strength of the machine round pine at the Grades A and B limits. At the Grade A limit the age range is from to year and at the Grade B limit the age range is from 5 to year. The height is ranging from 75 to 1 mm, respectively. The EN 3 size adjustment is presented from height 75 mm to the reference point 1 mm (dash line). The Model for the bending strength shows the negative size effect in the first look, but the Model takes into account also the age and the ring width. Therefore the Model gives both a negative and a positive size effect, although the EN 3 standard give a negative size effect. For the other mechanical properties the analysis shows a clear positive size effect.

6 55,, fm (N/mm2) 5,,,,, Heigth (mm) Grade A limits and u = % Grade A limits and u = % Grade B limits and u = % Grade B limits and u = % EN 3 size adj. from 75mm to ref. point 1 mm Figure.. Effect of height and moisture content on the bending strength at the Grade A and B limits for the machine round pine by model. The EN 3 size adjustment is calculated for the bending strength from height 75 mm to the reference point 1 mm (dash line). For both the grades is used the density kg/m 3. The effect of moisture content by models 3,, 5 and are presented and compared to the moisture content adjustment of EN 3 standard in Figure 7. Because adjustments of the EN 3 standard are suggested to use 5-percentile values of the sample, the comparison is made at the grading limits of the Grade A and B. The moisture content has a remarkable effect on the bending strength f m although the EN 3 does not give adjustment at all. The Model for the compression strength f c, shows similar adjustment than the EN 3 for the Grade A, but for the Grade B, the EN 3 adjustment is one unit lower. For the modulus of elasticity E Models 3 and 5 give a significantly lower adjustment for the Grade A than the EN 3, but for the Grade B adjustments are similar.

7 Stiffness (kn/mm2) 11,5 11,,5, 9,5 9,,5, 7,5 7,,5 Moisture content (% ) Em (grade A limits) Em (grade B limits) Ec, (grade A limits) Ec, (grade B limits) EN 3 moisture adjustment Strength (N/mm2) 7,5 2,5 37,5 32,5 27,5,5 17,5,5 M oisture content (% ) fm (grade A lim its) fm (grade B lim its) fc, (grade A lim its) fc, (grade B lim its) EN 3 moisture adjustment Figure. 7. The effect of moisture content by models 3,, 5 and in comparison to the moisture content adjustment of the EN 3 standard (dash line) at the Grade A and B limits for the machine round pine. For both the grades is used the density kg/m 3 and the diameter 5 mm. The effect of age by Models 3,, 5 and are presented for the machine round pine at the Grade A and B limits in Figure. The diameter is ranged mm and for both grades is used the density kg/m 3. Figures indicates that in the cases E c, o, E m and f c, the age has a clear positive effect. The positive effect of age, however, is diminishing after years. The effect of age on the bending strength f m is connected to height of timber. Therefore Figure shows both a positive and a negative age effect for the bending strength f m. Figure shows that the effect of moisture content is stronger for the younger specimens than the older.

8 , 11 11,, Ec, (kn/mm2) 9 7 Em (kn/mm2) 9,, 7,, 5 5,, , 5 15 Grade A limits and u = % Grade A limits and u = % Grade B limits and u = % Grade B limits and u = % Grade A limits and u = % Grade A limits and u = % Grade B limits and u = % Grade B limits and u = % fc, (N/mm2) 2 fm (N/mm2) Grade A limits and u = % Grade A limits and u = % Grade B limits and u = % Grade B limits and u = % 5 15 Grade A limits and u = % Grade A limits and u = % Grade B limits and u = % Grade B limits and u = % Fig.. The effect of age on the strength and stiffness at the Grade A and B limits for the machine round pine by the models 3,, 5 and.the diameter is ranged mm and for the both grades is used the density kg/m 3. DISCUSSION AND CONCLUSIONS The results of this study can be generalised for the following restrictions: The results concern on the pith enclosed pine and spruce timber produced from butt logs up to four meters stem height and harvested from the thinnings. The age of timber is ranging from to years. But the generalisation has to be made conservatively, because the studied material originated only from the six forest. The results concerning on the factors affecting twisting and mechanical properties are in consistence of the results of previous studies, however. According to the this study, a pith enclosed timber from thinning wood, younger than years, is prone to twist, in particular. For example, Perstorper (199) has concluded similarly. In his study the ratio of grain angle to pith distance was the best parameter for twist prediction. In addition, the studies have shown that the microfibril angle and the chemical composition of timber affects the mechanical properties and the twisting of timber. The juvenile wood comprises more tracheids and cell gaps per volume unit than the mature wood. And the tracheids of juvenile wood comprise more lignin and hemicellulose and have a greater microfibril angle than those of the mature wood. (McMlillin 1973, Erickson & Arima 197, Olesen 1977, Hillis 19, Bendtsen & Senft 19, Zopel & van Buijtenen 199). The result of this study shows that

9 the age of wood has effect on the twisting and the mechanical properties. In addition, there exists interaction between the age and moisture. This due to the fact that the composition of juvenile wood differs the composition of mature wood. The conclusions drawn from this study can be summarised as follows: 1. The twisting of pith enclosed pine and spruce timber is great, about 1.5 o /m at moisture content % and age of years. 2. The twisting of pith enclosed timber could cause problems for prefabricated components, which have to assembly subsequently and are stored in varied humidity conditions. 3. For this material the moisture content influences the E c,, f c,, E m and f m. An average, the effect of moisture is similar than given in EN 3 standard for E c,, f c, and E m, but unlike prescribed the bending strength should also be adjusted.. The pith enclosed timber has not clear size effect on the bending strength. Both a negative and a positive size effect was observed. For the other mechanical properties the size effect is clearly positive. 5. The lower the age the higher the effect of moisture content on twisting and mechanical properties. The effect of age and moisture content is the greatest for the specimens younger years. The specimens older than years twisted two times less the younger specimens. And the modulus of elasticity and compression strength were about % greater the specimens older than years than the younger. ACKNOWLEDGEMENT The project "Round small-diameter timber for construction" (FAIR-CT95-91) was part of the EC s th framework programme. The contribution of EU and several national organisations is gratefully acknowledged. REFERENCES Bendtsen, B. A. & Senft, J. 19. Mechanical and anatomical properties in individual growth rings of plantation-grown Eastern cottonwood and Loblolly pine. Wood and Fiber Science (1), pp Boren, H.. Factors affecting properties of round and sawn Finnish pine (Pinus sylvestris) and spruce (Picea abies). Manuscript for doctoral thesis, University of Joensuu, Finland. Erickson, H. D. & Arima, T Douglas-fir wood quality studies, Part II: Effects of age and stimulated growth on fibril angle and chemical constituents. Wood Science and Technology :5-. Hillis, W.E. 19. High temperature and chemical effects on wood stability Part 1: General consideration. Wood Science and Technology : McMlillin, C. W Fibril angle of loblolly pine wood as related to specific gravity, growth rate and distance from the pith. Wood science and technology 7(): 1-5. Olesen, P. O The variation of the basic density level and tracheid width within the juvenile wood of Norway spruce. For. Tree Impr. : 21 pp. Perstorper, R. M. (199). Quality of structural timber - end-user requirements and performance control. Chalmers University of Technology, Division of Steel and Timber Structures, Göteborg, Sweden. ISSN Ranta-Maunus, A Round Small Diameter Timber for Construction. Final report of Project FAIR CT VTT Technical Research Centre of Finland, Building Technology. VTT Publications 33: 1-2. ISBN X. ISSN -21. Ranta-Maunus, A.. Bending and compression properties of small diameter round timber. WCTE. Zopel, B & van Buijtenen, J. P Wood variation, its causes and control. Springer-Verlag, Berlin Heidelberg, German. 33 p. ISBN X. ISSN X.