Assessing Treatments to Mitigate End-Face Cracking in Air-Dried Acacia dealbata Logs

Abstract

Acacia dealbata Link, known as Mimosa in Portugal, is an invasive hardwood species with potential for construction use, but research is limited. The available stock of small-diameter juvenile wood logs can help reduce this gap, but tangential cracking at log ends challenges fastener connections. This study evaluated different treatments to control and reduce end-face cracking in small wood logs during air drying, an economical and environmentally friendly procedure. The extreme two-thirds of sixteen Mimosa logs were subjected to two treatments: one with longitudinal kerfs 15 mm deep along the length (two and three kerfs) and the other with a hollow in the center up to half the length (16 mm and 30 mm diameters). Over 219 days of air drying and compared with the central part, kerfing treatments significantly reduced outer-wood tangential cracking (p < 0.001), with the three kerfs also reducing crack numbers (p < 0.05) but increasing significantly cracks near the pith (p < 0.01). The 30 mm central hollow significantly reduced central perforation cracking (p < 0.05). Prospectively, the results suggest that a combined treatment approach involving cross-cuts could help reduce and/or control end cracking, thereby improving the suitability of wild Mimosa logs for construction use.

1. Introduction

Acacia dealbata Link, commonly known as Mimosa in Portugal, is a hardwood species native to Australia. It is highly invasive due to its rapid growth rate and prolific seed production [1], significantly increasing fuel loads in colonized native forests [2,3]. According to the national forest inventory [4], the area covered by species of the Acacia genus, unlike other forest species, such as Pinus pinaster Link, increased by 3.7 ha between 1995 and 2005 and now occupies 8.4 thousand ha in continental Portugal. A control method involves harvesting, resulting in a residual reserve of Mimosa. Some authors indicate that using these residues helps reduce the high costs of controlling invasive species [5]. Additionally, it would support the integration of these by-products into a circular economy framework [6] and provide further environmental benefits, such as reducing fuel loads and wildfire potential [7].

Efforts to extract value from this species and offset operational costs have led researchers to explore various applications, such as fuel production [8], paper pulp [9], and textile dyeing [10]. On the other hand, the use of Mimosa in construction faces challenges due to its wild growth characteristics, including the predominance of young trees (10–20 years old) with irregular shapes and small transverse sections, which consequently yield low timber volume and limit its suitability for manufacturing boards and engineered wood products.

One promising avenue for integrating Mimosa into construction involves utilizing small-diameter juvenile wood (sjw hereafter) logs, a by-product of forest management supported by existing research [11,12,13]. Environmental, economic, and technical considerations advocate for the utilization of sjw due to its potential to diversify wood usage, prolong its lifespan beyond a mere primary by-product, and decrease energy consumption during processing, drying, and transportation, as observed by Ramage et al. [14], rationalizing the supply chain and enhancing environmental sustainability. Furthermore, Green et al. [15] and Larson et al. [16] have demonstrated that a low level of processing for sjw can be advantageous because it does not reduce the exterior resistant material (mature outer wood) and does not weaken the area surrounding the knots.

However, post-felling Mimosa $sjw$ log often develops local cracks due to environmental interactions, which compromise durability, promote water accumulation, facilitate biological colonization, and increase susceptibility to fire by expanding the exposed perimeter. Studies suggest that the formation of cracks impacts the mechanical performance of beams [17] or connectors [18]. Furthermore, a high percentage of juvenile wood increases vulnerability to brittle failure and longitudinal shrinkage [19]. Evans [20] defines cracks resulting from drying stresses as “checks” or “splits”, depending on their appearance on one or both sides of the wood piece, respectively. Other definitions, such as “collapse” or “honeycomb”, are associated with failures within the drying process [21]. Despite extensive research on cracking phenomena using small samples under controlled conditions [22,23], scale effects such as moisture content gradient (MCG) influence cracking in massive pieces like logs, primarily on the tangential plane [24].

Internal stresses from the drying process, alongside incidental stress tree growth [25] and internal factors like anisotropy or anatomical characteristics, contribute to cracking formation, further influencing susceptibility to cracking [22]. Control of environmental drying conditions, facilitated by appropriate kiln-drying protocols or pretreatments, can significantly reduce drying defects [26,27].

Due to the availability and yield limitations of Mimosa logs, a practical alternative is air drying, which can be conducted in areas near the harvest. Given their high moisture content, transporting logs from the harvest area to the drying and sawing area is one of the most expensive operations in timber harvesting [28]. From an environmental perspective, utilizing air-dried logs within nearby radii can reduce energy consumption for transportation, save fuel, and decrease kiln drying time. Additionally, drying energy constitutes a significant portion of the lifecycle-associated carbon footprint of timber materials, making energy reduction options in this stage particularly relevant [29,30].

Evans et al. [31] determined that the type of drying (air or kiln) does not significantly affect the formation of cracks in radiata and slash pine logs 125/150 mm in diameter and 700 mm in length. However, greater crack depths in air-dried logs were observed. The duration of air drying depends on the species, size (diameter and length), local environmental conditions of temperature and relative humidity during storage, and the time of year when stacking begins. In California, Simpson et al. [32] used air-dried Ponderosa pine and Douglas fir logs with diameters ranging from 102 to 203 mm and lengths of 2.4 m and achieved moisture content values close to 20% in twenty days when starting in summer (July) and in one hundred forty days when starting in autumn (October). This also conditions the use of logs in construction, as achieving conventional moisture content values for use (around 12%, for example) requires longer drying times or the assistance of artificial drying to reach service values.

Some researchers have conducted comparative studies on various “destructive” treatments to reduce or control cracking in logs [31]. One approach involves “kerfing” on the outer surface to release drying stresses. This method includes options such as creating one or more cuts along or kerfing to a certain depth depending on the log diameter [33,34] or making numerous small incisions distributed regularly over the surface [20,33]. Another strategy entails drilling a central hollow along the entire length of the log [35,36], which reduces the thickness and humidity gradient while increasing the exposed surface area. However, Lim et al. [37] found that the complete removal of material may compromise load capacity, potentially leading to failure under punctual load conditions.

Despite limited studies, Acacia gender wood exhibits noteworthy physical–mechanical properties, underscoring its potential for diversifying the timber supply chain amidst increasing demand. In Portugal, Martins [38] obtained mean values of density of 647 kg/m³, an MOE of 13,900 MPa, and a bending strength of 65 MPa for Acacia melanoxylon (Blackwood), demonstrating its applicability for Glulam. In Chile, a technical report from the Forestry Institute [39] reports mean density values of 495 kg/m³, an MOE of 11,515 MPa, and flexural strength of 65.7 MPa for Acacia dealbata, which are higher than those of the reference species for use in construction (Pinus radiata Link). Also, by using conventional kiln drying on 25 and 50 mm boards of Acacia melanoxylon and Acacia dealbata, Ananias et al. [40] achieved satisfactory results with wood free of cracks or “collapse”. These results present an opportunity to explore the suitability of Mimosa logs in truss structures, which utilize smaller, shorter (therefore, lighter) axially loaded components. However, controlling or reducing drying cracks is necessary, especially at the ends, where connections are inserted.

This study aims to control and/or reduce cracks at the ends of wild sjw Mimosa logs by combining air drying with two treatments: longitudinal kerfing and central hollowing. Additionally, it seeks to characterize this species’ specific cracking formation patterns under varying environmental conditions. It is part of a larger study to evaluate applicability in construction using low-processed air-dried logs.

2. Materials and Methods

2.1. Collection of sjw Logs

As an initial phase of a broader experimental campaign, logs of Acacia dealbata Link (hereafter referred to as Mimosa) were recollected from a large accumulation of timber waste originating from forest cleaning activities in northern Portugal. These logs were stored outdoors within the courtyard of a sawmill located in the city of Porto. The selection criteria for the logs prioritized obtaining stems with minimal sweep, devoid of visible outer surface damage, and possessing intact bark. Sixteen sjw logs of Mimosa, aged between 14 and 19 years and approximately 2000 mm long, were chosen for the study. The diameter of each log, measured by using an electronic caliper with a precision of 0.01 mm, ranged from a maximum of 121 mm to a minimum of 67 mm, all exhibiting irregular cross-sectional shapes. The bark was retained until natural detachment occurred. Within the sawmill facility, the ends were enveloped in polyethene film to mitigate moisture loss.

2.2. Sample Preparation

A reference line was drawn along the entire length of each log, and then 2 to 3 cm was trimmed from each end. Subsequently, each log was divided into three samples of 450 mm each, as depicted in Figure 1a. Three treatments were carried out. Kerfing cuts (K) 15 mm deep were carried out throughout the upper sample’s overall length, as Figure 1b shows. The central sample, shown in Figure 1c, remained untreated and served as the reference (R). In the bottom segment, a central hollow (H) was created as depicted in Figure 1d by drilling with a drill bit positioned at the geometric center of the section.

The manual kerf treatment was made along the samples with a 3 mm wide saw disc and involved the two variations depicted in Figure 1e,f. Similarly, the hollowing treatment (H) was executed with the two variations shown in Figure 1g,h. Partial drilling was conducted until 250 mm in depth, ensuring no compromise to specimen integrity.

2.3. Air Drying of the Samples

The forty-eight specimens (16 × 3) underwent air drying on the terrace of the IB-S building at University of Minho in Guimarães (41°27′11.6″ N, 8°17′26.7″ W). This location experiences a warm-summer Mediterranean climate (Csb, according to Köppen’s Climate Classification) with temperature values ranging between 0.6 °C (minimum) and 33.0 °C (maximum) and a total precipitation of 155 mm [41]. The air-drying process occurred from early February to the first week of September 2022. Periodic temperature (T) and relative humidity (RH) measurements were recorded by using a mark Govee thermo-hygrometer device. An elevated area was established approximately one meter above the ground level, shielded from rain and direct radiation, according to class 3.1 defined by standard EN 335-2: 2013 [42]. No additional treatments were applied, such as sealing or burning over the end faces of the logs. The samples were stacked with a horizontal gap of 20 mm between them and elevated by 20 mm by using dry wooden strips, as depicted in Figure 2a–c.

To measure the moisture content (MC), an electrical resistance hygrometer, the Protimeter SurveyMaster® BLD5365, was employed. The hygrometer was placed on two pairs of electrode heads inserted into each sample, located as indicated by the letter “e” in Figure 1b–d. The electrodes were positioned one-third of the distance from both ends for the untreated samples (R) and the kerfing treatment (K). For the kerfing treatment, the pairs were located between two kerfs. In the hollow treatment (H), they were positioned one-third of the distance from the massive end opposite the perforation on both sides. The electrodes were made from screws coated with insulating tape, penetrated to a depth of 20 mm, corresponding to between 15% and 25% of the log diameter, with the screw tip embedded in the sample. Each pair of electrodes was inserted perpendicular to the wood fibers into pre-drilled holes less than 20 mm deep and spaced 30 mm apart. Subsequently, the electrodes were securely screwed in, and their heads were sealed with silicone.

Sixteen disk slices were extracted from the central part of the reference sample (R) to calibrate the hygrometer readings. These disks were saturated with water and then stored in a climatic chamber maintained at 20 ± 0.5 °C and a relative humidity of 60 ± 5%. Periodic mass and moisture content measurements with the hygrometer were conducted until constant mass was achieved. The calibrated MC was determined with the dry weight of each measurement following the standard BS EN 13183-1:2002 [43] and then correlated with the MC of each hygrometer measurement, achieving a determination coefficient R² = 0.84. By using the inferred simple regression model, the MC values indicated by the hygrometer were determined.

To physically and geometrically characterize the samples in the initial air-drying stage, the following macroscopic characteristics were analyzed by using end-face pictures of all the reference samples (R) before cracks formed. Diameter (Dia.): the largest and smallest average diameter of both ends in mm; TotalArea (A_t): the area measured, excluding the bark, in mm²; Eccentricity (ecc.): the ratio of the center–pith distance to the radial distance (half diameter), measured as a percentage of the largest diameter that passes through the pith; Heartwoodpercentage (hw.): the ratio of the heartwood area to the total area (A_t), measured as a percentage; AnnualRingWidth (${\mathrm{A}}_{r_w}$): the ratio of the largest diameter to twice the number of rings counted over a radius, measured in mm per year.

Additionally, each specimen’s mass (m) was measured by using an electronic scale manufactured by Steinberg, with a maximum capacity of 50 kg and a reading precision of 1 g. The mean wood density adjusted to 12% moisture content (${\rho }_{12}$) was determined according to ISO 3131:1975 [44], using the mass/volume ratio of the 16 slices mentioned above.

2.4. Measurement of Cracking on Log End Faces

To capture the formation of cracks on the end faces of logs, photographs were taken at regular intervals (every two weeks) by using a Sony DSC-H90 digital camera with a resolution of 350 dpi. As Figure 3a indicates, the camera lens was kept 200 mm orthogonal from the log end face. The orthogonal reference axes drawn on the end face of the log shown in Figure 3b were overlaid on a transparent acrylic plate marked with axes and scale references, as illustrated in Figure 3c. Subsequently, the captured images were oriented and scaled to produce TIFF format images over a CAD file, as shown in Figure 3d.

A pattern of circular concentric rings centered on the pith was superimposed onto the CAD file to study the formation of tangential cracks on the end face. Image analysis was performed by using the freely available software ImageJ 1.53k, which has been used in other reference studies [45,46]. As shown in Figure 3d, separated each 10 mm, 3 or 4 rings (depending on size) were overlaid: ring 1, named r1; ring 2, named r2; ring 3, named r3; ring 4, named r4; and as the last ring, the perimeter without bark, named rp. For each ring, intersections between visible cracks and the rings were counted, and the width of each intersection was measured with a tolerance of ±0.5 mm. Non-perpendicular crack axes were measured at their minimal width. Based on these data, the following three parameters were defined in lowercase letters.

The relative cracking value $c_r$, measured as a percentage (%), calculated by using the formula

$$c_r=\frac{\mathrm{\Sigma }{w}_c}{P_r}\times 100$$

(1)

where $\mathrm{\Sigma }{w}_c$ is the sum of all widths of tangential cracks and $P_r$ is the perimeter of the corresponding circular ring; for rp, the latter is the perimeter of log without bark. For instance, in Figure 3d, $c_r$ of ring 4 is the sum of widths $w_1$ + $w_2$ + …, divided by the perimeter of ring 4, $P_{r4}$, expressed as a percentage.

Likewise, to compare the different treatments concerning their reference samples, a dimensionless reduction indicator, $i_{red}$, is defined as the ratio

$$i_{red}=\frac{c_{r\left(R\right)}}{c_{r(K,H)}}$$

(2)

where $c_{r_{\left(R\right)}}$ is the relative cracking for the reference samples (R) and $c_{r_{(K,H)}}$ corresponds to the mean $c_r$ value for their respective treated samples, either kerfing (K) or hollowing (H). Values close to unity indicate no differences between untreated and treated samples.

Finally, the relative number of cracks parameter $n_r^{\circ }$, corresponding to the number of cracks per decimeter ($n^{\circ }dm$), defined as

$$n_r^{\circ }=\frac{n_c^{\circ }}{P_r}\times dm$$

(3)

where $n_c^{\circ }$ is the total number of cracks and $P_r$ is the perimeter of the corresponding circular ring; for rp, the latter is the perimeter of log without bark. For example, in Figure 3d, $n_r^{\circ }$ of ring 4 is the total number of cracks, $n_1$ + $n_2$ + …, divided by the perimeter of ring 4, $P_{r4}$, expressed as the number of cracks per decimeter.

Some adaptations were made for each specific treatment. In the kerfing technique (K), the total sum of crack widths was recorded and analyzed separately to evaluate the specific effect of the kerfs. For this analysis, the width of the saw blades used was subtracted from the total crack width: for 2 kerfs (2k), 6 mm was subtracted, and for 3 kerfs (3k), 9 mm was subtracted. In the case of the central hollow technique (H), the center for concentric rings was not the pith but rather the center of the perforation. Additionally, for the H-30mm variant, ring 1 (r1) was excluded from the measurements.

2.5. Experimental Design and Statistical Analysis

A descriptive statistical analysis was performed to analyze response variables across all end faces studied. Mean values were compared by using t-tests. The assumptions of normality were verified using the Shapiro–Wilk test, and the equality of variance was assessed by using the Levene test. All tests were conducted at a 95% confidence level and were executed by using Excel’s Real Statistics Release 8.8.1 tool. To compare the $c_r$ and $n_r^{\circ }$ response variables on each ring, analysis of variance (ANOVA) was employed, followed by post hoc group comparisons using Tukey’s Honestly Significant Difference (HSD) test at a 95% confidence level. These analyses were conducted by using RStudio 2023.12.0 [47].

Table 1. A description of the sample sizes for this study. The pairs of ends in the second row refer to pairs formed by an untreated face and a treated face, and the third row indicates their specific variations.

	Reference (R)	Kerfing (K)		Hollowing (H)
Samples	16	16		16
End-face pairs	-	16		16
Variations	-	8 (K-2k)	8 (K-3k)	8 (H-16 mm)	8 (H-30 mm)

summarizes the experimental design. The first row indicates the total number of samples. The second row specifies the number of untreated/treated end-face pairs. The last row shows the end-face variations studied for each treatment.

Due to errors in sample preparation, the number of end-face pair variations for the central hollowing treatment (H) was adjusted, resulting in 10 treatment variations of H-16 mm and 6 of H-30 mm.

3. Results

3.1. Physical Features of Samples

Table 2. Physical features of Reference samples.

	Dia.	At	ecc.	hw.	${\mathbf{A}}_{\mathit{rw}}^{*}$	${\mathit{\rho }}_{12}$
	(mm)	(103 mm2)	(%radio)	(%)	mm/year	kg/m3
Mean	88.91	6.35	10.77	31.05	2.94	573.33
SD	12.03	1.68	6.72	8.63	0.75	31.76
Median	85.90	5.71	8.92	29.35	2.77	571.84
Maximum	117.09	10.05	26.86	49.51	4.87	640.12
Minimum	85.84	3.70	2.48	11.39	2.20	532.70

describes the physical features of the reference samples (R), measured on both end faces of logs at the beginning of air drying. Due to their irregularity, all samples exhibited differences in size and shape. A high dispersion in eccentricity (ecc) with a coefficient of variation of 62.40% implies the presence of reaction wood in some specimens [25].

In some hardwood species, it is difficult to distinguish growth rings to determine the age of the log with the naked eye. However, immediately after we oven-dried the 16 slices used to calibrate the hygrometer, differences in surface porosity between earlywood and latewood became more distinguishable. Consequently, the annual ring width (${\mathrm{A}}_{rw}$) was determined for each specimen by using a handheld binocular loupe (10×). The density value adjusted to 12% humidity, ${\rho }_{12}$, was 573.3 kg/m³ (SD = 31.76). Pinilla et al. [39] reported a lower value, 495 kg/m³ (SD = 72.10), in Acacia dealbata logs with a diameter at breast height (dbh) of 360 mm, around 15 years old, from the southern part of Chile. Although higher, another reference value is that indicated by Machado et al. [48], 654.0 kg/m³ for Acacia melanoxylon Link logs between 33 and 51 years old, 400 mm dbh, from four sites in Portugal.

3.2. Cracking Formation During Air-Drying

3.2.1. Evolution of Environmental Conditions and Moisture Content (MC)

Figure 4a outlines the temporal evolution of environmental parameters, and Figure 4b shows the average variation in moisture content (MC). The calibrated equilibrium moisture content (EMC) was achieved when a constant value of approximately 11.94% (standard deviation, SD = 2.18) was reached for the reference samples and a value 11.45% (SD = 2.10) for the treated samples. This value was attained after more than 6 months of drying under conditions of shielding from direct sunlight and rain from spring to summer. During this period, the variation in MC proceeded schematically in three stages (Figure 4b), corresponding to changes in the environmental conditions of temperature and relative humidity. The first stage started with the samples being saturated (over 60% MC, corresponding to >FSP in Figure 4b) until the fiber saturation point (≈FSP), near 28%, was reached. The intermediate stage progressed until a value close to the equilibrium moisture content (≈EMC) was reached. The final stage was when the equilibrium moisture content (EMC) was reached.

In the initial stage (from day 0 to day 67 in Figure 4b), the saturated samples evaporated surface free water at a rate of between 4% and 6% every 10 days, reducing their moisture content (MC) as they approached the fiber saturation point (FSP) at approximately 28%. This corresponds to the spring months, when the average temperature was 15.52 °C (SD = 2.80 °C), with maximum and minimum temperatures of 18.75 °C and 12.35 °C, respectively. The average relative humidity was 66.51% (SD = 14.31%), with maximum and minimum values of 87.50% and 51.85%, respectively.

Subsequently, the MC curves entered stage 2 (from day 67 to day 140 in Figure 4b), coinciding with a change in environmental conditions from spring to summer, resulting in increased temperature and decreased relative humidity. This is a transition stage, at the cellular tissue level, marking the end of free water loss and the beginning of bound water loss. The drying process slowed to between 2% and 3% every 10 days until the MC approached 15%, closer to the equilibrium moisture content (≈EMC). During this stage, the average temperature rose to 23.01 °C (SD = 6.81 °C), with maximum and minimum temperatures of 29.11 °C and 12.73 °C, respectively. The average relative humidity was 49.45% (SD = 20.54%), with maximum and minimum values of 74.42% and 34.3%, respectively.

Finally, the last stage (from day 140 to day 218 in Figure 4b) corresponds to the summer months, during which the MC reduced and stabilized at a value close to 12%. Both treated and untreated samples exhibited a temporary increase in MC, coinciding with an exceptional rise in temperature and a decrease in relative humidity. This phenomenon is counter-intuitive and lacks a clear explanation. Hypothetically, it can be associated with a humidity gradient: the increase in temperature (over 30 °C) may have triggered a mobilization effect of the available vapor in the environment, which was registered by electrodes inserted at a depth of 20 mm. This behavior could be primarily due to the climatic conditions typical of the transition from spring to summer. After this specific event, the MC continued to decrease until reaching the EMC. Consistent with the summer season, the average temperature increased to 29.67 °C (SD = 5.00 °C), with maximum and minimum temperatures of 35.83 °C and 23.59 °C, respectively. The average relative humidity was lower, at 47.99% (SD = 10.90%), with maximum and minimum values of 63.60% and 38.20%, respectively.

3.2.2. Evolution of Cracking over End Faces

Macroscopic cracks appeared and progressed during the monitoring phase, becoming visible in the regularly captured images. Figure 5 illustrates the evolution of the average relative cracking variable ($c_r$) for each ring on the 30 end faces of fifteen reference samples (R). Log number 8 was excluded from all $c_r$ analyses due to an exceptional type of splitting crack, likely associated with the release of growth stresses. Measurements of ring 4 ($r4$) were restricted to the larger-diameter logs, comprising half of the samples, totaling 8 logs and 16 end faces. Additionally, Figure 5 shows the curves beginning on day 0, with an average initial cracking greater than zero for each ring despite the samples’ ends being protected before the first record.

Figure 5 shows a cracking pattern related to temperature fluctuations and inversely related to relative humidity levels. Peaks in the average relative cracking variable ($c_r$) coincide with the maximum temperatures and minimum relative humidity levels shown in Figure 4a. During these episodes, environmental conditions cause the evaporation of surface water from within the log, emerging in the exposed fibers at the log ends. The moisture differential between the exterior and interior causes the outer fibers to shrink, forming cracks. Subsequent temperature decreases and humidity increases reverse this process, causing the fibers and cracks to swell and close. Ultimately, these alternating cycles of tangential contraction and swelling reflect environmental fluctuations. Some cracks do not close and become permanent on the log’s end face. The average curve in Figure 5 shows a general upward trend until the fiber saturation point (FSP) is reached, after which the cycle stabilizes with more or less constant development until the end of the process, when EMC is reached. This behavior suggests that the drying environment characteristics and the hygroscopic properties of the wood determine the air-drying rate, as some authors have demonstrated [32].

To describe the relationships between environmental conditions (temperature and relative humidity) and moisture content MC variation and cracking formation, the graphs in Figure 6a–c show trend lines associated with each ring for each variable. Additionally,

Table 3. Correlation coefficients between relative cracking $c_r$ for each ring ($r1$, $r2$, $r3$, $r4$, and $rp$) and temperature (T), relative humidity (RH), and moisture content (MC).

	${\mathit{c}}_{\mathit{r}}$ (%)
	r1	r2	r3	r4	rp
Temperature	0.82	0.79	0.81	0.81	0.81
Relative humidity	−0.74	−0.73	−0.74	−0.65	−0.59
Moisture content	−0.71	−0.73	−0.73	−0.85	−0.89

presents the correlation coefficients between these factors and each ring’s relative cracking ($c_r$).

Consolidating the previous observations, strong correlations were determined between the analyzed factors and cracking formation with clear trends (Figure 6). An increase in temperature is strongly correlated with greater relative cracking in all rings, and conversely, an increase in relative humidity correlates with a reduction in cracking. At the MC level, an increase in moisture within the log is directly related to increased cracking across all rings.

In the first stage (from day 0 to day 67 in Figure 4b), the end cells exposed to environmental conditions underwent shrinkage, and with higher moisture inside, a stress differential triggered the formation of cracks. In this initial drying stage with water-saturated samples, cracks were more widespread on the exposed face. In the transition stage (from day 67 to day 140 in Figure 4b), the cells tended to shrink more with the rise in temperatures and the decrease in relative humidity. Some cracks closed, while others, typically two or three, increased in width and differentiated in size. In the final drying stage (from day 140 to day 218 in Figure 4b), with significant increases in the temperature and relative humidity of the air and less moisture within the log, drying stresses were absorbed by the now consolidated differentiated cracks (checking or splitting).

There is a clear seasonal effect on cracking formation, explained by the observed values of relative humidity and temperature being the main environmental drivers of moisture content variation in air-dried logs, creating internal stress differentials that trigger cracking formation. Supporting this idea, Filbakk et al. [49] found that the air-drying rates of whole broadleaf trees at three sites in Norway were primarily explained by air temperature and environmental humidity caused by precipitation.

3.2.3. Cracking Formation Pattern

To illustrate the formation of cracks over time, Figure 7 highlights three relevant measurement points, considering only reference samples (R). These three measurement points are shown in Figure 5 as the “x”, “y”, and “z” axes. The initial point, “x”, corresponds to ten days after the first measurement. The next point, “y”, aligns with the peak of relative cracking (maximum $c_r$), which occurred ninety-eight days after the initial measurement. The point “z” represents the final measurement taken two hundred nineteen days after the initial measurement.

These points are illustrated with images in Figure 7, which presents two representative specimens. To indicate the predominance of certain crack widths, the bar charts in Figure 7c,f,i display the average number of relative cracks $n_r^{\circ }$. The black bars describe each ring’s total number of cracks (Total). To distinguish different crack widths, the hatched bars separate the total into three groups: less than 0.5 mm (i), between 0.5 and 1.5 mm (ii), and greater than 1.5 mm (iii).

At the first point described by Figure 7a–c, the exposed ends of the samples, still confined by intact and firm bark, begin to crack extensively. Due to the bark’s protection, the moisture content at the surface is significantly lower than inside, creating a longitudinal moisture gradient. The MC of 57.48% indicates that the logs still contain free water. However, the exposed fibers at the ends have likely already reached 30% (FSP) MC or less, triggering a dimensional change process in the fibers. The interior–exterior moisture difference restricts this, generating drying stresses that result in widespread surface cracks. Another relevant interval corresponds to values near the FSP, which was reached after about 60 days and maintained for more than 30 days; at this point, the highest total relative cracking $\left(c_r\right)$ is reached (see Figure 5). In the distribution by groups at this first point, cracks of less than 0.5 mm predominate (between 75% and 85%). The intermediate group ranges between 17% and 25%, and cracks greater than 1.5 mm are almost non-existent (around 1% of the total).

The second point in Figure 7d–f corresponds to the maximum cracks across all rings, with an average MC of 29.97% (Figure 5). It was observed that the specimens had less bonding bark, potentially reducing the confinement effect. The mean number of cracks $n_r^{\circ }$ followed the previous pattern; however, each group’s distribution showed a trend change. The percentage of smaller cracks decreased to 52% and 72% of the total; the intermediate group remained relatively constant except for the central ring, $r1$, which increased, and the larger group increased by 11% to 21%. Some of the small cracks were nearly imperceptible. In contrast, one or two of the more significant cracks began to stand out, with some deep cracks causing severe damage to the integrity of the piece, as seen in Figure 7d,e.

The third point in Figure 7g–i shows specimens with over 75 days of bark loss and an MC of 12.60%. The total number of visible cracks decreased, affecting the total relative average number of cracks $n_r^{\circ }$ and the group distribution. While cracks in the smallest group had a high percentage of the total, between 47% and 60%, their absolute number significantly decreased. This was especially true in $r2$, $r3$, and $r4$ (from 4.1 to 2.2, from 4.7 to 1.9, and from 3.5 to 1.2 $n_r^{\circ }$), respectively because some previously visible cracks were no longer perceptible. In contrast, the larger cracks consolidated, particularly the most significant ones (Figure 7g,h).

At the beginning of the air-drying process, when the moisture content exceeds the FSP, the exposed faces serve as the main interfaces for moisture exchange due to the confinement effect of the bark. Consequently, a moisture gradient initiates the formation of widespread radial cracks from the pith, as shown in Figure 7a,b. As air drying progresses and the adhering bark area gradually decreases, some cracks differentiate, forming checking or splitting, as evidenced in Figure 7d,e. While some cracks become imperceptible, others become more pronounced. With the loss of bark, this process consolidates, and larger cracks capture a significant percentage of the total number of cracks, especially the smaller ones, as inferred from Figure 7g,h. This model is consistent with some of the cracking formation patterns proposed by Hu et al. [50] on real logs. In the final drying stage, with logs approaching the equilibrium moisture content (EMC), drying stresses tend to reduce due to the lower moisture gradient than that in the stages near the FSP. As these stresses decrease, the cell wall experiences less mechanical stress, resulting in a lower proportion of cracking [51].

3.2.4. Treatment Performance

The curves in Figure 8a,b illustrate the evolution of the reduction indicator ($i_{red}$) variable for the kerfing treatment, indicating that in the perimeter ($rp$)—excluding the effects of the kerfs—the treated pieces exhibited consistently lower relative cracking throughout the process. Additionally, for r3, there was a reduction in cracking between 1.0% and 2.0% compared with the reference for K-3k and less than 1.5% for K-2k. Differences in cracking were minimal for the inner rings ($r1$ and $r2$).

Figure 8 also reveals a similar temporal variation pattern for the treated and reference samples. Both curves began with an initial segment of approximately 50 days, during which the relative cracking in the reference samples was about 2.0 times higher for K-2k and 3.0 times higher for K-3k. During this period, the saturated logs, with a moisture content (MC) well above the FSP, lost moisture while approaching an MC of 30%. The kerfs act as additional interfaces for water exchange alongside the end faces, facilitating the evaporation of free water and potentially reducing surface stress at the log end faces.

Following this initial stage, the relative cracking in the reference samples increased to nearly three times that of K-2k and over five times that of K-3k compared with the treated logs. This subsequent stage lasted approximately 30 days, with the MC ranging between 20% and 30%, corresponding to the transition from above to below the FSP. This significant difference is attributed to the additional checks and splitting that appeared and consolidated during this period, contributing to the increase in the relative cracking value, $c_r$.

In the kerfing-treated pieces, this difference was absorbed by the kerfs in the superficial rings. The striped curves for $rp+k-effects$ in Figure 8a,b show the reduction indicator ($i_{red}$) for $rp$, including the contribution of the kerfs’ total width after subtracting the saw blades’ thickness. It was observed that general cracking was reduced, and additional cracks generated from the kerfs resulted in values below unity (approximately 0.6). This pattern actively counteracted the formation of cracks, as indicated by the consistent line formed in Figure 8a,b. This demonstrates that the kerfing treatment effectively mitigates cracking by redistributing stress and providing controlled release points, thus lowering the overall cracking observed.

Figure 9a,b show the reduction curves for the two hollowing treatment variants, H-16 mm and H-30 mm. The general trend is the opposite of the kerfing treatment. Toward the interior, in the rings closest to the central perforation, a reduction in relative cracking was observed, as indicated by the curves for $r1$ in Figure 9a and $r2$ in Figure 9b. In contrast, the reduction was minimal in the outer rings, which were very close to unity. The reduction in the H-16 mm variant was very small, never exceeding 1.5 times the reference. However, the difference was more pronounced with H-30 mm. In $r2$, after the PSF, the cracking was reduced by two and almost three times compared with the reference.

3.3. Log End-Face Final Cracks

3.3.1. Characteristics of Cracking

Table 4. Descriptive statistics of end-face crack results: relative cracks ($c_r$) and number of cracks ($n_r^{\circ }$). Includes mean, standard error (S.Error), standard deviation (SD), maximum (Max) and minimum (Min) values, and count (Count).

	${\mathit{c}}_{\mathit{r}}$ (%)					${\mathit{n}}_{\mathit{r}}^{°}$ (${\mathit{n}}^{°}$ dm)
	r1	r2	r3	r4	rp	r1	r2	r3	r4	rp
Reference
Mean	5.36	3.75	3.68	3.79	3.36	6.86	4.18	3.68	2.93	2.37
S.Error	0.47	0.27	0.24	0.20	0.18	0.32	0.26	0.21	0.19	0.14
SD	2.55	1.48	1.34	0.75	0.96	1.83	1.49	1.21	0.73	0.79
Max	10.76	6.82	6.94	4.82	5.73	11.14	7.16	6.90	3.98	3.81
Min	1.03	0.84	1.13	2.63	1.01	4.77	1.59	1.59	1.99	0.77
Count (uts.)	30	30	30	14	30	32	32	32	14	32
K-2k
Mean	4.61	3.31	1.99	1.24	0.74	6.57	5.17	4.05	2.29	1.39
S.Error	1.50	0.75	0.64	0.33	0.18	0.87	0.54	0.55	0.59	0.20
SD	3.97	1.98	1.70	0.67	0.49	2.47	1.53	1.55	1.19	0.57
Max	11.55	6.54	5.48	1.84	1.52	9.55	7.16	6.90	3.98	2.44
Min	0.64	1.25	0.45	0.28	0.28	1.59	3.18	2.65	1.19	0.52
Count (uts.)	7	7	7	4	7	8	8	8	4	8
K-3k
Mean	4.28	3.54	2.15	1.08	0.70	6.17	5.17	3.65	2.92	1.16
S.Error	1.00	0.60	0.62	0.22	0.38	0.47	0.50	0.80	0.13	0.40
SD	2.82	1.70	1.76	0.38	1.07	1.33	1.41	2.28	0.23	1.14
Max	9.23	6.25	5.13	1.37	3.20	7.96	7.16	6.90	3.18	3.06
Min	0.89	1.45	0.33	0.66	0.00	4.77	3.18	0.53	2.79	0.00
Count (uts.)	8	8	8	3	8	8	8	8	3	8
H-16 mm
Mean	3.78	3.31	3.22	3.33	3.03	6.68	5.25	4.24	4.24	2.36
S.Error	0.89	0.63	0.49	0.56	0.42	0.62	0.38	0.36	0.87	0.13
SD	2.68	1.88	1.48	0.97	1.26	1.96	1.20	1.15	1.51	0.41
Max	8.57	6.37	6.10	4.43	5.53	9.55	7.16	5.84	5.97	3.12
Min	0.65	1.08	1.71	2.62	1.75	4.77	3.98	2.65	3.18	1.66
Count (uts.)	9	9	9	3	9	10	10	10	3	10
H-30 mm
Mean	-	2.55	2.94	3.11	3.11	-	4.51	4.16	3.28	2.61
S.Error	-	0.52	0.36	0.43	0.25	-	0.34	0.59	0.25	0.38
SD	-	1.28	0.87	0.85	0.61	-	0.82	1.44	0.50	0.93
Max	-	4.47	4.15	4.21	4.02	-	5.57	6.90	3.98	4.21
Min	-	0.92	2.03	2.20	2.38	-	3.18	2.65	2.79	1.79
Count (uts.)	-	6	6	4	6	-	6	6	4	6

provides descriptive statistical data for the relative cracking ($c_r$) and the relative number of cracks ($n_r^{°}$) of the end faces of all samples, treated and untreated. The results are presented for each ring (r1, r2, r3, r4, and rp), considering the last measurement, that is, for the equilibrium moisture content (EMC). Differences were observed between rings, and to evaluate them, ANOVA was applied to the faces of the reference samples (R).

Table 5. Analysis of variance and Tukey’s mean comparison test to evaluate differences between the different radial positions for the variables relative cracking ($c_r$) and number of cracks ($n_r^{\circ }$).

	${\mathit{c}}_{\mathit{r}}$					${\mathit{n}}_{\mathit{r}}^{\circ }$
	r1	r2	r3	r4	rp	r1	r2	r3	r4	rp
r1	-	-	-	-	-	-	-	-	-	-
r2	0.002 *	-	-	-	-	<0.001 *	-	-	-	-
r3	<0.001 *	0.999 ns	-	-	-	<0.001 *	0.569 ns	-	-	-
r4	0.026 *	0.999 ns	0.999 ns	-	-	<0.001 *	0.032 *	0.039 *	-	-
rp	<0.001 *	0.884 ns	0.941 ns	0.927 ns	-	<0.001 *	<0.001 *	<0.001 *	0.694 ns	-
Value-F	6.96					51.29
Value-p	<0.001 **					<0.001 **

* Significative at significance level of 5% using Tukey’s HSD test. ^ns No significantly different, at significance level 5% using Tukey’s HSD test. ** Significant at 5% significance, using the F-test.

presents the results. Significant differences were observed between rings for both variables, with p < 0.0001 and F values of 6.969 and 51.29, respectively.

Tukey’s test on the relative cracking ($c_r$) variable (Table 5) revealed differences between $r1$ and the remaining rings. Comparisons with $r3$ and $rp$ showed a significantly higher level (p < 0.001), while the significance decreased for $r2$ (p < 0.01) and r4 (p < 0.05). No significant differences were observed among the remaining rings, $r2$, $r3$, $r4$, and $rp$, with all p-values above 0.05.

Regarding the number of cracks ($n_r^{\circ }$), very significant differences were observed in $r1$ vs. all rings, with p-values < 0.001. There were no significant differences between rings $r2$ and $r3$. These two rings differed very significantly from $rp$ (p < 0.001) and at a lower level of significance from $r4$ (p < 0.05).

3.3.2. Effectiveness of Treatments

Relative Cracking, $c_r$

Table 4 corroborates the findings from the Treatment Performance section (Section 3.2.4). The mean values of the kerfing treatments (K) are effective compared with the reference (R), reducing cracking outside the areas adjacent to the kerfs and showing a gradual decrease in effectiveness towards the interior. Significant variability in the data was observed, especially in K-3k, due to differences between specimens with exceptional cracks (checking/splitting) and those with no cracks outside the kerfs when the relative cracking ($c_r$) approached zero. For instance, the maximum value for K-3k was 3.20%, while the minimum was 0.00%.

Table 4 shows that both K variants, 2k and 3k, reduced relative cracking ($c_r$) to less than 25% of the reference in the perimeter ($rp$). However, these results are less pronounced towards the interior, with values similar to the reference but with higher variability. This is how the $c_r$ values of the K-2k and K-3k variants for ring $r1$ correspond to 86% and 80% of the $cr$ of the reference sample.

For the central hollowing treatment (H), the relative cracking ($c_r$) at the perimeter ($rp$) was scarcely distinguishable from the reference (R). It is observed in Table 4 that the $c_r$ values of the H-16 mm and H-30 mm variants for ring $rp$ correspond to 90% and 93% of the $c_r$ of the reference sample, respectively. Towards the interior, this trend decreases, as shown in Table 4, where the $c_r$ of the rings adjacent to the perforations is lower. Thus, the $c_r$ values of the H-16 mm and H-30 mm variants correspond to 70% in both cases of the $c_r$ of the reference sample for ring $r1$ and ring $r2$, respectively. This reduction may be due to the perforation itself, which reduced the proportion of less permeable heartwood and exposed the more permeable sapwood, facilitating smoother water circulation and causing fewer tensions on the hollow inner surface.

Figure 10 presents boxplots comparing the reference (R) with their respective treated faces: K-2k, K-3k, H-16 mm, and H-30 mm. The analysis focuses on rings where trends were more evident ($r2$ and $rp$), discarding r1 due to different performance observed with ANOVA and non-compliance with the normality criterion. A t-test for related samples was also performed to compare the significance levels between means.

Regarding $c_r$, Figure 10a confirms the effectiveness of both kerfing treatments (K), K-2k and K-3k, in significantly reducing the relative total crack on the perimeter compared with the reference, with a high level of significance (p < 0.00). Figure 10b shows these treatments were ineffective in the inner rings.

For the central hollowing treatment (H), the H-30 mm variant effectively reduced relative cracking ($c_r$) with a significance level of p = 0.06, while the H-16 mm variant was ineffective. The 30 mm perforation reduced the total size of cracks around the perforation ($r2$), as corroborated graphically in Figure 9b. The t-test also indicated its effectiveness on the perimeter ($rp$). This observation suggests that a larger perforation exposes more sapwood, facilitating log drying.

Relative Number of Cracks, $n_r^{\circ }$

Regarding the relative number of cracks $n_r^{\circ }$, Table 4 shows similar trends but weaker ones, with a reduction in the mean values of cracks in the outer rings but similar values towards the interior. Like $c_r$, extreme values also affected the variability in the data; for example, the maximum value for K-3k was 3.06 n°/dm, while the minimum was 0.00 n°/dm. Table 4 indicates that the number of cracks ($n_r^{\circ }$) values at the treated samples’ perimeter level ($rp$) correspond to 60% and 50% of the reference samples for K-2k and K-3k, respectively. In contrast, the percentages of $n_r^{\circ }$ for the specified variants for ring $r1$ correspond to values close to 100% of the $n_r^{\circ }$ of the respective reference samples in both cases.

No trend differences were observed for either variation in the hollowing treatment (H). Indeed, the H-30 mm variant exhibited a greater number of cracks than its reference. The boxplots in Figure 11 compare untreated and treated pairs of end faces. After reviewing assumptions, a t-test for related samples was performed to compare the significance levels between mean values.

Similarly, it was confirmed that the kerfing treatment (K) reduced the relative number of cracks on the perimeter. Still, this reduction was significant only for K-3k, as shown in Figure 11a. However, the t-test also revealed a negative secondary effect of the K-3k technique, visible in Figure 11b. In the inner ring ($r2$), the number of cracks increased significantly (p < 0.01). On the other hand, the central hollowing treatment (H) did not affect the number of cracks in either ring; in fact, it significantly increased with the H-16 mm variant, as seen in Figure 11b.

3.3.3. Kerfing for Crack Control

Another pertinent aspect to consider is whether the treatments contributed to controlling or partially predicting the occurrence of cracking. Regarding the kerfing treatment, it is noteworthy that the zone adjacent to the kerfs captured a higher percentage of total cracking, potentially promoting its occurrence. Figure 12a compares the final relative cracking ($c_r$) over the perimetral ring ($rp$) of the reference samples (in red) with the treated samples for the two variants, 2k and 3k (black bars). The central gray columns also distinguish between the area within the kerfs (light gray) and outside the kerfs (dark gray).

Notably, a significant reduction in cracking was observed outside the kerfing zone, with less than a third of the reference for 2k and less than a quarter for 3k. The kerfs absorbed the difference to the reference (red line transparent bars), resulting in a 60% increase in the total $c_r$ of the treated pieces (black bars). After a specific count, it was observed that most cracks originated at the kerfs, with 75% and 67% in the 2k and 3k treatments, respectively. Moreover, the kerfs acted as driving elements in crack formation, determining the formation of “quadrants” between kerfs, as shown in Figure 12b.

However, as noted earlier, this can be a negative side effect. While external cracks may be reduced towards the interior of the ends of the logs, the number of cracks is not significantly reduced due to the additional cracks “promoted” by the kerfs.

4. Discussion

For the application in construction of wildwood logs with minimal processing, research has been developed based on non-destructive methods for the mechanical classification of wood logs, demonstrating consistent performance [52,53]. However, other aspects associated with wood technology, such as drying and its consequences on mechanical properties, are still under investigation. Mergny et al. [17], using an analytical and numerical approach, demonstrated that in solid oak wood beams, the presence of cracks reduces stiffness by 10%, although depending on the severity, this value can reach 25%.

Many wood structure typologies are formed by connected linear elements, so the connections between these are critical from the perspective of structural safety. The early research work by Eckelman et al. [18] suggests that the tendency to develop splitting at the ends of the pieces must be considered to determine safety factors in truss structures. Based on this, it suggests developing drying practices to force the development of cracks in predetermined locations to avoid weakening the connections, especially in fastener-type connections. Another type of connection, such as the use of elements inserted parallel to the grain, like glued-in rods, can also be affected by crack formation, as suggested by Dias et al. [54], who used logs of Portuguese Maritime pine ($Pinuspinaster$, Ait). However, in a more contemporary approach, Chahade et al. [55], using glued-in threaded rods inserted parallelly at the center of the ends of Douglas fir ($Pseudotsugamenziesii$) logs, demonstrates that crack formation does not affect the load-carrying capacity.

Air-dried logs’ moisture content varies according to environmental conditions [32]. Due to the element’s size and the logs’ anatomical characteristics, an internal moisture gradient forms, creating an uneven distribution of internal stresses. Exacerbated by anisotropic shrinkage, these stresses affect the weaker different tangential planes, leading to tangential cracking [24].

In this study, the moisture content removal rate pattern matches the descriptions in the literature: above the fiber saturation point (FSP), free water is rapidly eliminated as the moisture content (MC) approaches 30%, after which the bound water in the tissue is removed at a slower rate [32,56].

The cracking formation profile overlaid on the described pattern demonstrates that crack formation is directly related to the environmental conditions. By observing the reduction curves, we can confirm that the treatments partially mitigate crack formation, thereby reducing the impact of environmental factors. The treatments contributed to reducing cracking, although their effectiveness varied depending on the specific treatment and the location observed. Regarding total cracks, both kerfing treatments were significantly effective in the outer rings (Figure 13). In contrast, the larger perforation (H-30 mm) treatment was moderately effective in the inner rings adjacent to the perforation (Figure 13). About the number of cracks, no differences were observed compared to the references, except for K-3k, which reduced the number at the external level of the log but simultaneously significantly increased the number of cracks in an inner ring, which can be associated with the “promoter” effect of the kerfs: the number of naturally occurring cracks is augmented by those artificially triggered by the kerfs, leading to an overall increase in the total number of cracks.

When examining performance over time, before reaching the FSP, the K-3k variant shows greater water loss than the reference, achieving MC values near 30% approximately 15 days earlier than the other samples. In line with Lee et al. [57], this suggests that incisions or cuts affect moisture distribution within the log, influencing the formation of the moisture gradient. The three kerfs reduce the distance between the core and the outer wood, diminishing the moisture differential, facilitating water transport, and increasing the moisture exchange area along the entire piece. This results in greater effectiveness on the log’s exterior, as Figure 13 shows for the perimetral ring, rp.

The results also indicate that throughout the air-drying process, the kerfs themselves not only absorb the tangential dimensional variations of the untreated areas but also actively capture more cracks; at the beginning of the process, an “instant” crack is activated adjacent to the notches, which affects almost 4% of the perimeter (Figure 13). Then, this percentage increases slowly but gradually and continues until it stabilizes after passing through the transition zone between above and below the FSP.

Evans et al. [31] studied differences in the number and depth of kerfs and observed that two kerfs instead of one and deeper kerfs (20% of the diameter each) reduce the mean depth, width, and length of checks. In this study, due to variations in the logs’ shape and diameter, the kerf depth was 15 mm, ranging from 10% to 20% of the diameter per kerf. However, although the total perimeter cracks were significantly reduced, the K-2k variant did not show a difference in water loss compared to the reference. This suggests that the number of kerfs to consider for MC reduction in future research might be a variable.

In the drying process of the H-30 variant of the hollowing treatment, starting from approximately 30 days from day 0, the moisture loss was slower than the reference, R. At the end of the drying process, a significant reduction in relative cracking $c_r$ was observed in the ring adjacent to the perforation (Figure 13). One reason is the reduction in heartwood due to the larger perforation diameter. In all studied logs, heartwood accounted for 31.05% (SD = 8.63) of the total area in the reference pieces. According to Moore et al. [19] and others, heartwood has distinct characteristics from sapwood that influence its performance. It contains a high content of juvenile wood with lower density, modulus of elasticity, and rupture compared with the mature outer wood, consistent with the larger size and number of relative cracks around ring 1, $r1$, near the pith.

Both treatments shortened the moisture gradient distance and, therefore, its steepness, locally reducing the stresses that trigger cracking. In untreated pieces, these stresses were abruptly released at the ends as checking and splitting. As described, these effects emerge, develop, and consolidate during drying, compromising the piece’s integrity. However, from a drying perspective, these cracks serve as moisture exchange interfaces, accelerating the drying process [28] and hypothetically “capturing” the tangential shrinkage/swelling variation at the log end.

In kiln drying, environmental variables (temperature and relative humidity) are controlled to prevent tangential stresses from exceeding the threshold that causes wood tissue failure [56]. In air drying, while some general aspects (using well-ventilated enclosures or protecting from rain and direct sunlight) can be controlled, this is not feasible to the same extent.

Another strategy to evaluate is manipulating the drying effect, i.e., cracking. As observed with the kerfing treatment, the kerfs guided crack formation in more than two-thirds of the specimens, creating quadrants. In some cases, no visible cracks formed within these quadrants, while in others, cracks did appear—some minor and others severe, like checking.

A proposed solution, hypothetically capturing tangential stresses in the kerfs, involves artificial cross-splitting, as Figure 14 exposes. These cross-cuttings extend throughout the extreme section to a defined depth, combining the advantages of both treatments studied. The two diameters would capture naturally formed checks or splits, and the area exposed by the cut depth would accelerate log drying.

The four formed quadrants might house a connection. Dias et al. [54] demonstrated that using four glued bars arranged around the perimeter of round timber logs subjected to tension can achieve high structural performance. Fastener connections can also be evaluated, provided that critical distances from loaded edges are measured from the end of the executed kerf. Additionally, the early work by Huybers [58] suggests a practical approach to controlling log end expansion using a wire lacing tool.

5. Conclusions

The results demonstrate that using air drying—a method that is easy to implement, economical, and environmentally friendly—along with destructive treatments such as kerfing and central hollowing can significantly reduce the formation of cracking at the ends of small logs of juvenile Acacia dealbata Link (named Mimosa in Portugal) wood with minimal processing.

More than two kerfs, each 15 mm deep along the entire log length, significantly reduce the total sum and number of surface cracks. During drying, the kerfs actively capture surface cracking triggered by environmental variations. However, the results at the center of the log ends do not show reductions different from the untreated samples and reveal a significant increase in the number of cracks, some of which may be large.

Although less effective than the kerfing treatment, a 250 mm long perforation in the center of the log (in this case, 30 mm), which exposes the more permeable sapwood, facilitates moisture transport. This results in less surface tension around the perforation during air drying and thus fewer total cracks. However, the results indicate that it does not significantly reduce the number of interior cracks.

This experimental design lacked greater differentiation between treatment variations, which could have provided clearer and more comparable results—for example, including four kerfs along the log and a complete hollowing. Additionally, a larger number of specimens with greater statistical weight could have improved the quality of the results.

This work is part of a broader study to evaluate the applicability of wild Acacia dealbata Link small-diameter logs as construction material in log structures as a by-product of control activities of an invasive species. It complements other studies that include, among other things, mechanical properties and the development of connection solutions. In this sense, advancing in studying the reduction in and/or control of cracking, especially at the ends of the logs, is a step towards the material’s applicability for these uses. Consequently, the next stage is based on these findings. It involves studying the formation of cracks at the ends of the logs by proposing two perpendicular cuts at the ends of the logs, as previously discussed. The cuts function as an artificial splitting, increasing the exposed area at the end of the log and theoretically capturing the tangential variation due to air drying.