Effect of the knife-incising pretreatment on the surface checks occurrence of red pine heavy timber after drying

Red pine (Pinus densiflora S. et Z) wood is commonly used to build Korean traditional houses and as building material in form of posts, beams, and rafters. In particular, heavy timbers such as posts and beams should be dried properly in order to prevent deformation after construction. However, during drying of heavy timbers surface checks often occur and negatively influence the appearance of the building after construction. Consequently, various methods such as high-temperature and low-humidity pretreatment, incising (CO₂ laser), and longitudinal kerfing have been tested (Hattori et al. 1997; Lee et al. 2013; Listyanto et al. 2013). Among the incising methods, knife-incising by introducing artificial scars on the four surfaces of the timber, makes it possible to evenly distribute the stress on all surfaces. Therefore, the knife-incising treatment was assumed to be the best method to protect the surface from check occurrence by dispersing the stress during the drying. However, there have been no studies relating to the effect of the knife-incising treatment on surface checks so far. Thus, in this study, knife-incising pretreatment was applied, and after drying, the effect of knife-incising on the surface checks occurrence was evaluated by comparison with the surface checks occurred in the control and kerfed material.

Red pine heavy timber with dimensions of 20 × 20 × 360 cm³ was used in this study. Prior to drying, the heavy timbers were treated with knife-incising (Inc), longitudinal kerfing (Ker) and a combination of knife-incising and longitudinal kerfing (Inc-Ker). The knife-incising treatment was carried out on all four sides (2400 piece/m²) (Fig. 1b). After that, kerfing treatment was processed by using a circular saw (Fig. 1a). Untreated timber was used as a control (Con). Five specimens were prepared for each condition.

Drying test was carried out using a commercial dryer under conditions as follows: (1) steaming (dry-bulb 95 °C, wet-bulb 95 °C, 12 h), (2) high-temperature and low-humidity pretreatment (dry-bulb 120 °C, wet-bulb 90 °C, 48 h), (3) elevated temperature drying (dry-bulb 90 °C, wet-bulb 70 °C, 144 h). Distance between crossers and spacing between specimens were adjusted to 60 and 3 cm, respectively.

At the end of drying, number, length, and width of surface checks present on the surface of all dried specimens were measured (>2-mm check width). From these results, average number of surface checks (ANS) and average length of surface checks (ALS) were determined. Average area of surface checks (AAS) was calculated by the following formula:where, l length of each surface check, w width of each surface check, n total number of checks with width >2 mm, N number of specimens. In this formula, the denominator was set to ‘2’, because the checks have a diamond shape.

The analysis was carried out by one-way analysis of variance (ANOVA) (at ρ < 0.05). After that, Tukey–Kramer Honestly significant difference (HSD) test was used for post-mortem analysis (at ρ = 0.05).

One specimen selected from each pretreatment condition was used for the measurement of final moisture content and distribution. The moisture content (MC) was determined by the oven-dry method (at 103 ± 2 °C). The MC samples were obtained at the position of 60, 120, and 180 cm from the end surface (Fig. 1d) and cut as shown in Fig. 1c.

In this study, the following results were obtained: (1) pretreatment did not influence the final moisture content and moisture content distributions. (2) Knife-incising treatment was effective in preventing surface checks. ANS, ALS, and AAS of Inc were less than that of Con. The effect of Inc on the reduction of surface checks occurrence was similar to that of Ker. (3) Inc-Ker was most effective in preventing surface checks and produced statistically significantly fewer surface checks than Con (ρ < 0.05).