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The influence of drying method on the warp behavior of Norway spruce (Picea abies (L.) Karst.) sawn timber.

Abstract

The guarantee of a consistent wood moisture content of 15[+ or -]3 percent is an important prerequisite of timber for construction purposes, according to international standards. It was the aim of this investigation to identify and quantify the effect of two different drying methods (air-drying and kiln-drying) on the warp behavior of timber. In this study, 60 Norway spruce trees from a 111-year-old, even-aged, homogeneous stand were processed into beams of practical dimensions. After the measurement of the moisture content when green, the total group was divided into two subgroups according to the later method of drying. The beams of group A were kiln-dried using a conventional drying schedule, those of group B were air-dried and technically post-dried. For both groups, the defined average target moisture content was 15[+ or -]3 percent. To derive the distortions of each beam, the surfaces were scanned twice, before and after drying, using Freiburg's Improved Timber Scan (FRITS) measuring device. The warp variables according to international standards (E DIN 4074-1:2001-01), indicated as bow, crook, twist, as well as the angle of twist, were calculated based on mathematical algorithms. Both drying methods led to low warp with only slight differences. A t-test revealed a statistically significant difference in the twist of the sample beams dried by the two methods. Bow, crook, and the angle of twist did not significantly differ between the two groups.

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The removal of water from wood under the fiber saturation point leads to a shrinking process that tends to cause distortion. The magnitude of distortion, defined as bow, crook, twist, and angle of twist, depends on complex interactions between moisture content, wood species, and wood anatomical characteristics. These interactions have not been fully investigated. Moisture-related distortion of sawn timber has been the subject of several studies during the last decade, for example Danborg (1994), Perstorper et al. (1995a, 1995b), Cown (1996), Sandberg (1996), Ormarsson (1999), Merforth (2000), and Seeling (2001). The aim of these studies was to analyze the influence of selected wood characteristics on the warp behavior.

In further studies, different drying methods were compared exclusively in regard to the success in reducing moisture content (MC) (Schneider et al. 1979, Schneider and Wagner 1980). Welling (1996) investigated the drying quality of sawn timber and appointed a scheme for the determination and evaluation of the MC of kiln loads. He described a practical method for the assessment and visualization of the case-hardening of timber.

Teischinger (1992) investigated the effect of drying temperature on the behavior of swelling and shrinkage of Norway spruce (Picea abies (L.) Karst.) sawn timber. Studies of warp and shrinkage of conventional and high temperature dried hem-fir (Tsuga heterophylla (Raf.) Sarg.) studs were also conducted by Milota (2000). Lee and Jung (2000) compared a radio-frequency/vacuum process and conventional kiln-drying in terms of shrinkage, checking, and use of energy for Korean ash (Fraxinus rhynchophylla) squares.

[FIGURE 1 OMITTED]

Wengert and Lamb (1983) also compared conventional and new methods (predrying followed by kiln-drying, radio-frequency followed by vacuum drying, and vacuum drying only) for drying red oak. The results showed that radio-frequency and vacuum drying were at least as good as the conventional predrying-kiln-drying combination. Piao et al. (2000) designed an integrated method consisting of air-drying and kiln-drying to reduce drying stress, collapse, inner cracks, and plastic distortions of Acacia mangium timber. This combined treatment also reduced drying time, energy consumption, and drying costs.

Price and Koch (1980) investigated the impact of kiln time and temperature on warp of southern pine lumber. The boards dried for a short time at relatively higher temperatures (36 hr. at 240[degrees] F and 9 hr. at 270[degrees] F) had on the average comparatively lower crook, bow, and twist and a lower variation of crook and bow than boards dried for longer kiln times at lower temperatures (120 hr. at 180[degrees] F). Shupe et al. (1997) analyzed the impact of a conventional and a high temperature kiln schedule on spruce lumber defects. No statistical differences could be proven. However, the impact of the air-drying process was not taken into account in these investigations.

Nevertheless, different technologies for timber drying have been compared in terms of the quality of the dried timber. A systematic comparison between the very energy-efficient traditional air-drying and the most often used kiln-drying is still missing. Therefore it is the objective of the presented investigation to evaluate and quantify the influences of the two different methods of drying on the warp behavior of sawn construction timber.

Material and methods

The material for this investigation was collected in the course of different thinning treatments over a timeframe of 7 months (December 1999 until July 2000), in a homogenous, monocultural, even-aged stand, located in central Germany, with an average tree age of 111 years. In total, 60 dominant and vital Norway spruce trees (Picea abies (L.) Karst.) were selected. Diameter at breast height and tree height are shown in Table 1 to characterize the architecture of the sample trees. One sample butt log was removed from each stem from the stump to a height of 6.10 m (Fig. 1). In case of fungal decay, logs of 1 m in length from the base were removed until a 6.10-m log free of decay could be achieved. The 60 roundwood logs had an average middle diameter of nearly 37 cm (Table 2). The logs were processed into sawn timber according to the sawing pattern shown in Figure 1. This sawing pattern allowed the production of beams in practical, manageable dimensions for construction purposes. The beams were sawn in cross-sectional dimensions of 24.5 cm by 13 cm, 12 cm by 12 cm, and 12 cm by 6 cm. The middle boards, containing the pith and consequently a high percentage of juvenile wood, were excluded.

The material used in this study was very homogeneous according to wood-anatomical growing characteristics. Using a one-factorial analysis of variance, no significant differences between groups were found for a variety of wood parameters (spiral grain, ring width, maximum branch diameter, single branch quotient, proportion of compression wood, and juvenile wood). This allows us to ignore the influence of wood anatomical characteristics so we can analyze the influence of different drying methods on the warp behavior.

[FIGURE 2 OMITTED]

MC determination

The MC of the sawn timber was measured twice, before and after drying. Beginning from the former stem base, the measurements took place on both broadsides of the beams, near and far from the pith, in longitudinal direction at three points (1 m, 3 m, and 5 m) perpendicular to the fiber direction using an electronic resistance device with an active electrode.

This method was used even though the accuracy of a resistance-type moisture meter above the fiber saturation point is regarded in practice as relatively imprecise and is recommended in a range of MC between 5 and 25 percent (Du et al. 1991). For the active electrode used in this investigation, the producer specified the system for higher MCs between a value range of 40 and 200 percent. It is a considerably more exact methodology than previously used measuring devices (Bohner et al. 1983). But the accuracy of this method has not been tested systematically because even if it is not as good as the ovendry method in the presented study, the MC is only used for a direct comparison.

Timber drying

In total, 267 beams were gained for this investigation. For drying, they were solid stacked flush at both ends on seven stickers (3 by 3 cm) spaced about 1 meter apart and separated to 10 charges. Afterwards, the charges were separated into two groups (A and B) containing five packages, according to the different methods of drying.

The beams of group A were kiln-dried using a conventional drying schedule at a constant dry-bulb temperature of 55[degrees]C and a stepwise adaptation of the wet-bulb temperature (Table 3). The circulated air velocity through the loads was about 2 m/sec. After relatively short periods of kiln-drying (12 days or 22 days), all the beams of group A achieved a consistent relative target MC of 15[+ or -]3 percent.

The beams of group B were air-dried according to the recommendations of Futo (1985), i.e., protected from negative climatic influences like rain and ultraviolet light, for a total duration of 298 days. After that, the average relative MC of 21 percent measured on a random sample of 132 beams was still above the target MC of 15[+ or -]3 percent. Therefore, in a second step, an additional kiln-drying of 8 days, according to the identical drying schedule of group A (Table 3), was carried out.

Consequently, both groups had an equivalent MC. This was a prerequisite for further analysis. The groups were labeled in this paper as kiln-dried (A) and air-dried (B).

Measuring warp

The warp of the sample beams was measured using Freiburg's Improved Timber Scan (FRITS) device, which was developed by Seeling and Merforth (2000a, 2000b) at the Institute of Forest Utilization and Work Science at the Albert-Ludwigs-University, Freiburg, Germany. The system was modified for this investigation by installing a laser scanning device. It scans the surface of the timber to determine the distances to a reference level. Based on mathematical algorithms, the variables of warp (bow, crook, and twist) can be deduced (Fig. 2). Additionally, in accordance with the prescribed method of Merforth (2000), the angle of twist [alpha] (Fig. 3) was determined. The angle of twist is less influenced by bow and crook than by twist, and varies from the twist prescribed in the European Standard E DIN 4047-1:2001-01. The warp of the sample beams was measured twice, when green and after drying to the target MC of 15[+ or -]3 percent.

[FIGURE 3 OMITTED]

Results

MC

The MC of the sawn timber in the green condition according to drying groups is summarized in Table 4. The beams were systematically located in different positions within the stem and show a characteristically different MC before drying. Separated into different cross-sectional dimensions, a one-factorial analysis of variance revealed no significant differences between the groups before drying. This means that the material is in a comparable condition in terms of the MC. After drying, all the beams showed a comparable target MC of 15[+ or -]3 percent.

Warp

To investigate the influence of different drying methods on warp (bow, crook, twist, and angle of twist), the total group was examined first. The warp variables for all sample beams were compared when green and after drying (Fig. 4).

In terms of bow, the beams of both groups (A and B) showed an almost identical mean value in the green state: 0.37 cm/2 m (group A, kiln-dried) and 0.38 cm/2 m (group B, air-dried). After the kiln-drying, the mean value of bow (0.37 cm/2 m) was unaffected and remained the same. For the air-dried beams, the mean value of bow (0.30 cm/2 m) was lower, but not statistically different. The air-dried beams have a much lower range of values.

In terms of crook, lower mean values were found before and after drying. In the green condition, the mean values for crook of both groups were similar (0.19 cm/2 m). This indicates that the beams were comparable in terms of crook before drying and the values remained almost the same after drying (0.21 cm/2 m after kiln-drying and 0.23 cm/2 m after air-drying). Using a t-test for independent samples, this small difference could not statistically be proven as significant. The low standard deviation characterizes the narrow range in both groups.

In terms of twist, the mean values of the green beams were 1.37 cm/2 m (group A) and 1.39 cm/2 m (group B). There was a larger variation for the values of twist after kiln-drying, characterized by the standard deviation of [+ or -]1.00 cm/2 m. The mean of the kiln-dried beams (1.90 cm/2 m) was significantly higher than after air-drying (1.61 cm/2 m). Therefore, it can be supposed to be a consequence of the different drying methods.

The mean angle of twist for the green beams was also very similar for group A (2.9 degrees) and group B (3.1 degrees). After air-drying, the angle of twist was 3.6 degrees and was lower than the kiln-dried beams (5.2 degrees). The standard deviation of [+ or -] 4.9 degrees characterizes the relatively larger range for group A. However, the difference could not be statistically proven.

The influence of kiln-and air-drying on the warp behavior of timber was separately investigated for the beams with different cross-sectional dimensions, which come from different positions in the stems.

In both groups, the mean bow increases with a decreasing cross-sectional dimension. For all cross-sectional dimensions, the air-dried beams show relatively lower mean values than green beams and have a marginally narrower range. Using a one-factorial analysis of variance no significant differences between the groups could be proven (Fig. 5).

The mean values for crook and twist were similar in all cross-sectional dimensions and almost totally unaffected by the different drying methods.

[FIGURE 4 OMITTED]

The highest warp values were found for twist. The kiln-dried beams of a cross-sectional dimension of 12 by 12 cm showed the highest value of 1.96 cm/2 m. In comparison, the air-dried beams in the same dimension were less twisted (1.55 cm/2 m). In both groups, the beams of all cross-sectional dimensions showed increasing values of twist after drying. However, the small differences between the groups were not significant.

An increasing angle of twist as a result of drying could be shown for all dimensions and both drying methods. The smaller beams (12 by 6 c[m.sup.2]) showed the highest values in both groups (group A: 5.4 degrees, group B: 4.7 degrees). Again, a one-factorial analysis of variance showed no significant differences between the groups.

The correlation between the shape of all beams in the green and dried conditions is shown in scatter diagrams (Figs. 6 to 9). The line in these figures (identity-line) characterizes the identical situation of warp before and after drying.

From Figure 6 it can be seen that in group A (kiln-dried) there are more beams above the identity-line than below. This means in general that the beams exhibit higher values of bow after kiln-drying. Some beams had severe bow already in the green condition, which increased during the process of kiln-drying. In group B (air-dried), there are almost the same number of beams above or below the identity-line and it is obvious that the air-dried beams have lower values of bow compared to some of the kiln-dried beams. Besides those, which had a relatively high bow in the green condition, there is a tendency in most cases to have lower values after drying. As can be seen in Figure 7, there is no difference in crook depending on the method of drying.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

In terms of twist, there are significant differences between the kiln-dried and the air-dried beams (Fig. 8). The kiln-dried beams show higher twist values than the air-dried. The variation of twist for kiln-dried is greater than air-dried timber. It is obvious that the twist is harder to predict within the kiln-drying process than for the air-drying.

Figure 9 shows that the distribution of the angle of twist values before drying lies predominantly between 0 to 4 degrees and is very low. After drying, for both methods, the values are much more widely spread, lying between 0 and 14 degrees. It is obvious for both drying methods that the beams with a low angle of twist (< 4 degrees) tend to behave very individually during the drying process. The beams with higher angles of twist tend to provide better results after drying. It can be noted that the values before and after drying are correlated very poorly. A prediction of the behavior of twist on the basis of the values in the green condition seems to be very difficult.

To evaluate the practical relevance of the measured warp (bow, crook, and twist) all dried beams were graded according to E DIN 4074-1:2001-01 (Fig. 10).

To assess the importance of the warp variables, the grading has been done separately for each one. Within the standard there are three quality classes: S13 (best), S10 (middle), and S7 (worst).

The limits for bow, crook, and twist are identical:

* S13: a maximum warp of 0.5 cm/200 cm is accepted;

* S10: a maximum warp of 0.8 cm/200 cm is accepted;

* S7: a maximum warp of 1.5 cm/200 cm is accepted.

Although the mean values for bow are very similar in both groups, it is obvious that the two different drying methods lead to different results of grading. A relatively high proportion of 84 percent of the air-dried beams could be graded in terms of bow into the best grading class S13 for which a maximum value of only 0.5 cm/200 cm is accepted. The proportions of the air-dried beams in the grading classes S10 and S7 amount to 13 and 3 percent, respectively. The proportion of kiln-dried sawn timber in the grading class S13 is comparatively lower (76%), whereas the percentages in both other grading classes show increasing tendencies (S10: 15%; S7: 9%).

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The results of grading based on crook is very similar in both groups and not affected by the different drying methods. Because of the very low values, all the beams could be graded in the classes S13 (group A: 94%; group B: 93%) and S10 (group A: 6%; group B: 7%).

It is obvious in Figure 10 that twist leads to relatively lower grades with both drying methods. The proportions in the best class S13 amount to only 5 percent for the kiln-dried and 6 percent for the air-dried beams. The proportions in class S10 are 11 percent for the air-dried beams and 8 percent for the kiln-dried beams. The proportions in the lowest grading class S7 amount to 23 percent (kiln-dried) and 29 percent (air-dried).

Clear differences between the two differently dried groups of beams arise from the higher percentages of severely twisted beams that have been graded as reject: 64 percent of the kiln-dried sawn timber could not be graded in class S7 as a result of twist. The proportion of reject of the air-dried beams is lower, but still reaches 54 percent.

Discussion

As an effect of the age and large dimension of the sample trees, the sawn timber was of high quality. So-called wood defects that influence the warp behavior (pith and juvenile wood) were eliminated due to the sawing pattern, the other wood characteristics (spiral grain, ring width, maximum branch diameter, single branch quotient, proportion of reaction wood, and juvenile wood) were similar for the beams of both groups. Therefore it is possible to evaluate separately the influence of the different drying procedures.

The low occurrence of warp confirms the results of Johansson (2002), who found values for bow between 0.27 and 0.45 cm/2 m during her investigation of moisture-induced distortion in Norway spruce timber; the mean values of crook varied between 0.19 and 0.27 cm/2 m and those for twist between 2.07 and 5.16 degrees. Schumacher (1993) compared Norway spruce sawn timber from Germany and Sweden and found that they both have relatively low values of crook (0.2 to 0.3 cm/2m). Sandberg (1996) investigated kiln-dried and rewetted Norway spruce sawn timber and found similar results. The beams, dried to an MC of 12 percent, showed bow values of 0.26 to 0.55 cm/2 m and crook of 0.16 to 0.3 cm/2 m. A study by Merforth (2000) showed sporadically lower values of warp after drying than before drying.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

The methodology of this investigation ensured that the MC before and after drying was identical for both groups. To measure wood MC, an electronic resistance based moisture meter was used. According to specifications of the producer, the appointed active electrode is especially manufactured for measurements of high wood MCs. Even if the absolute values are less precise than measured by the ovendry method, it can be assumed that it is possible to use the data for the internal comparison. It must be noted that the air-dried timber (after 298 days) did not reach the target MC of 15[+ or -]3 percent and therefore required an additional 8 days of kiln-drying.

With regard to warp, the beams behaved in a similar way and were only slightly affected by the different methods of drying. An analysis of variance revealed that there was no significant difference in bow, crook, and angle of twist between the groups. Twist was the only variable of warp that was found to be different between the two groups. This result confirms the study from Perstorper et al. (1995a) who stated that twist was considerably more affected by moisture cycling than bow and crook.

It is obvious that a single comparison of the mean values of warp does not give a complete picture of the quality of drying. In fact, the mean values of the two different treatments may be close to one another but for the individual grading of the beams the warp behavior of every single case is relevant. So the warp of the dried sawn timber, even if only slightly influenced by the drying method, is decisive for the output of straight timber used for construction purposes. The results of the grading in this investigation show that this is especially true for bow and twist.

This confirms the results found in the investigation of Shelly and Simpson (2000). The average twist in their sample material produced of suppressed Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees was greater than that accepted in the higher lumber grades, whereas other variances of warp (cup, bow, and crook) were not so high that they had any influence on the use of the material.

It is assumed that long lasting and gentle air-drying processes may be more appropriate to achieve low warp, especially with sawn timber for construction purposes, than the kiln-drying schedule used in this study. Investigations by Glos et al. (1989) showed similar results. In their study, they showed that the air-drying process for construction timber largely relieved stresses and had a positive influence on warp behavior. This perception also confirms the results of Arganbright et al. (1978). In their studies, high temperature drying was not as effective as conventional kiln-drying with restraint or air-drying in reducing warp. In contrast, Trnka and Ladomersky (1985) reported on the higher dimensional stability of kiln-dried spruce timber compared to air-dried timber. A reason for the contradictory results of this study could be the different wood species that were investigated and simultaneously the different wood characteristics of the sample material. The varying moisture cycles and the different techniques of drying and drying schedules in these investigations could possibly be another explanation for the contradictions.

This investigation was an analysis of the influence of different drying methods (kiln-drying and air-drying) on the warp behavior (measured as bow, crook, twist, and angle of twist) of sawn timber. The investigation showed that the more time-consuming but energy-saving air-drying process followed by a short phase of kiln-drying leads to a generally lower range of values compared to those of the exclusively kiln-dried beams.

As a conclusion, it should be noted that twist was the only warp variable that differed significantly between the two investigated drying methods.
Table 1.--Descriptive statistics for the stem architecture (diameter at
breast height [DBH] and stem height) of the sample trees.

Variable No. Mean Median [X.sub.min] [X.sub.max] SD COV

 (%)
DBH (cm) 60 54.86 53.00 40.00 77.00 6.44 0.12
Height (m) 60 32.13 33.00 21.37 45.09 3.84 0.12

(a) [X.sub.min] = minimum; [X.sub.max] = maximum; SD = standard
deviation; COV = coefficient of variation.

Table 2.--Descriptive statistics for the dimension of the logs at tree
base. (a)

Variable No. Mean Median [X.sub.min] [X.sub.max] SD COV

 (%)
Middle
 diameter
 (cm) 60 36.83 37.00 23.00 57.00 6.33 0.17
Length Consistently 6.10 m

(a) [X.sub.min] = minimum; [X.sub.max] = maximum; SD = standard
deviation; COV = coefficient of variation.

Table 3.--Kiln-drying schedule.

 Dry-bulb Wet-bulb
Drying time temperature Step no. temperature
(days) ([degrees]C) ([degrees]C)

 13 51
 12 50
 11 49
 10 48
22 9 47
(24.5 cm by 12 cm) 8 46
10 55[degrees]C 7 45
(12 cm by 12 cm) 6 44
(12 cm by 6 cm) 5 43
 4 42
 3 41
 2 40
 1 39

 Dry-bulb Wet-bulb Equilibrium
Drying time temperature depression MC
(days) ([degrees]C) ([degrees]C) (%)

 4
 5
 6
 7
22 8
(24.5 cm by 12 cm) 9
10 55[degrees]C 10
(12 cm by 12 cm) 11 20
(12 cm by 6 cm) 12 19
 13 18
 14 17
 15 16
 16 15

Table 4.--Moisture content of the beams (%) in green condition (before
drying) separated into groups and cross-sectional dimensions.

 Collective Cross-sectional No. Mean Median [X.sub.min]
 dimension

 (%)
 24.5 cm by 13 cm 29 49 43 34.37
 12 cm by 12 cm 59 50 44 35.68
Kiln-dried (A) 12 cm by 6 cm 41 63 50 35.48
 Total 129 54 46 34.37
 24.5 cm by 13 cm 29 47 42 33.55
 12 cm by 12 cm 58 51 44 35.30
Air-dried (B) 12 cm by 6 cm 42 62 53 34.70
 Total 129 54 46 33.55

 Collective [X.sub.max] SD COV

 %
 84.82 14.15 0.29
 97.58 14.97 0.30
Kiln-dried (A) 129.40 26.79 0.42
 129.40 20.22 0.37
 75.32 11.80 0.25
 97.67 14.95 0.29
Air-dried (B) 118.95 22.77 0.37
 118.95 18.17 0.34

(a) [X.sub.min] = minimum; [X.sub.max] = maximum; SD = standard
deviation; COV = coefficient of variation.


[c]Forest Products Society 2004.

Forest Prod. J. 54(4):79-87.

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Volker Klaiber

Ute Seeling

The authors are, respectively, PhD candidate, and Professor, Institute of Forest Utilization and Work Sci., Albert-Ludwigs-University Freiburg, Werderring 6, 79085, Freiburg, Germany. This investigation was supported with funding and technical support of the Forest Administration of North-Rhine-Westphalia, represented by Forstamt Schmallenberg, Germany, who also provided the sample material, and with cooperation and technical support from the sawmill H. Hegener-Hachmann, Hanxleden, 57392 Schmallenberg, Germany, where the sawn timber could be produced and differently dried. This paper was received for publication in August 2002. Article No. 9529.
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Author:Klaiber, Volker; Seeling, Ute
Publication:Forest Products Journal
Geographic Code:4EUGE
Date:Apr 1, 2004
Words:5431
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