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Article

Comparative Study on the Combustion Characteristics of Korean Wood with Changes in the Radiant Heat

by
Ji-Young Park
,
Seong-In Hong
,
Yun-Jeong Chio
and
Jae-Hong An
*
Department of Construction Test & Certification Center, Korea Institute of Civil Engineering and Building Technology, Ilsanseo-gu, Goyang-si 10223, Gyeonggi-do, Republic of Korea
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(18), 8360; https://doi.org/10.3390/su17188360
Submission received: 30 June 2025 / Revised: 27 August 2025 / Accepted: 15 September 2025 / Published: 18 September 2025
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Abstract

Due to the increasing demand for sustainable construction materials, wood has gained prominence owing to its renewable nature and carbon-storage capabilities. However, the flammability of wood presents critical safety concerns in its structural applications. This study investigates the combustion and charring characteristics of three Korean softwood species, namely Larix kaempferi, Pinus densiflora, and Pinus koraiensis, which were subjected to varying levels of radiant heat using cone calorimetry testing (ISO 5660-1). The fire performance was evaluated based on the peak heat release rate (HRR), total heat release (THR), surface ignition temperature, and char depth. The obtained results showed that Pinus densiflora exhibited rapid ignition and the highest THR (62.2 MJ/m2), indicating an aggressive burning behavior. Additionally, Pinus koraiensis recorded the highest peak HRR (241.6 kW/m2). In contrast, Larix kaempferi showed the most favorable fire-resistance profile, with the highest ignition temperature (~300 °C), lowest THR (31.8 MJ/m2), and shallowest char depth (7.5 mm), suggesting slower pyrolysis. These findings identify Larix kaempferi as the most fire-resilient species among those tested, making it a promising material for use in timber structures. This work offers practical insights for improving fire safety in wooden buildings and supports the strategic use of Korean wood in sustainable and fire-safe architecture.

1. Introduction

In response to the global challenges of climate change and resource depletion, there has been a growing demand for sustainable and environmentally friendly building materials in the architecture and construction sectors [1]. Among these materials, wood has attracted increasing attention due to its renewability and capacity to store carbon. As a natural carbon sink, wood continuously absorbs and sequesters carbon dioxide throughout growth and service life. Furthermore, wood is reusable and biodegradable, thereby contributing to the development of a low-carbon and resource-circulating society [2].
Advances in engineered wood technology have significantly improved the structural performance and durability of wood, expanding its application from traditional low-rise buildings to mid- and high-rise structures [3]. This evolution demonstrates that wood is now recognized not only for its environmental benefits but also for its technical viability in modern construction. In regions such as North America, Europe, and Japan, numerous studies and pilot projects have examined the sustainability, cost-effectiveness, and energy efficiency of wood-based buildings [4].
However, the combustibility of wood remains a critical concern for ensuring fire safety in wooden structures, as the charring and burning characteristics of timber directly affect its structural integrity under fire conditions. Quantitative fire performance assessments are therefore essential to the safe design and application of wood [5]. In particular, research on the fire behavior of domestic structural wood is crucial for establishing design standards and supporting the sustainable use of local wood resources [6].
Wood combustion is an oxidation process in which wood reacts with oxygen to produce heat, light, and various byproducts [7]. Its behavior can be evaluated by ignition time under flame exposure and by measuring the heat release rate (HRR) from the material surface [8]. The HRR, representing instantaneous heat generation, and the total heat release (THR), quantifying the accumulated energy output, are both key indicators of combustibility and help assess fire risk and predict flame spread [9]. Beyond these macroscopic indicators, the underlying thermal degradation process-pyrolysis-also plays a decisive role in fire performance.
Pyrolysis in low-oxygen environments generates smoke and charcoal as residue. At 200~225 °C, the process is extremely slow, producing mostly non-volatile gases. As the temperature increases to 225~275 °C, full-scale pyrolysis occurs, accompanied by ignition, with ~260 °C generally regarded as the threshold at which visible flames appear [10,11]. Below 300 °C, char formation predominates, whereas above this temperature volatile gases are released.
During pyrolysis, the char layer emits flammable gases (e.g., carbon monoxide, methane, organic acids) and non-flammable gases (e.g., carbon dioxide, water), along with solid carbon. This layer acts as a protective barrier that enhances fire resistance when heat transfer to the wood interior is insufficient. Such staged mechanisms of pyrolysis and their implications for fire behavior have been comprehensively reviewed in the recent literature [12]. As illustrated in Figure 1, the cross-sectional schematic of a wood sample highlights the formation of this char layer.
In addition, the microstructure of wood, including grain orientation, fiber density, porosity, and resin content, significantly influences combustion behavior [11]. Resin-rich woods with open cell structures (e.g., Pinus densiflora) tend to ignite rapidly and release energy intensely, whereas denser woods with compact fibers (e.g., Larix kaempferi) ignite more slowly and undergo reduced charring. Recent thermogravimetric and cone calorimetry studies have further demonstrated that density and chemical composition critically determine pyrolysis kinetics and fire performance [13].
Despite these advances, most studies have focused on foreign wood species, leaving a gap in research on domestic wood and their application to fire safety evaluations [14].
Accordingly, this study seeks to address the following research question: How do the combustion and charring behaviors of three domestically utilized structural wood—Larix kaempferi, Pinus densiflora, and Pinus koraiensis—differ under varying levels of radiant heat exposure? To answer this question, cone calorimetry experiments are conducted to analyze key fire performance parameters, including the HRR, THR, surface temperature, and char depth.
Based on the density and microstructural differences among these species, we hypothesize that Larix kaempferi, with its higher density and compact fiber arrangement, will exhibit delayed ignition, lower THR, and shallower char depth, thereby demonstrating superior fire resistance compared with Pinus densiflora and Pinus koraiensis.
The results presented herein are expected to serve as foundational data for improving the fire safety and efficiency of structural wood applications, while also supporting the development of design standards and the promotion of Korean wood utilization in sustainable building practices.

2. Materials and Methods

2.1. Selection of the Target Species

The species and dimensions of the structural lumber employed in this study conform to the Korean Industrial Standards for softwood structural lumber (KS F 3020) [15] and structural glued laminated timber (KS F 3021) [16]. Table 1 presents the classification of the different softwood structural lumber species, including cross-sectional images of the Korean Larix kaempferi (Group A), Pinus densiflora (Group B), and Pinus koraiensis (Group C) specimens employed in this research. Notably, these materials are all commonly used as structural materials in wooden buildings.
To comply with the standard for structural glued laminated timber (KS F 3021), the Larix kaempferi, Pinus densiflora, and Pinus koraiensis samples were conditioned to moisture contents of ≤15%. This was achieved by monitoring the specimens with a wood moisture meter (LS Electric, ACREL, Shanghai, China) at (23 ± 2) °C and (50 ± 5)% relative humidity until they reached their respective equilibrium moisture contents (Table 2). The specimens were then cut to dimensions of (100 × 100 × 50) mm, with a moisture content of ≤15%, masses ranging from 185 to 265 g, and densities between 0.35 and 0.53 g/cm3, as detailed in Table 2.

2.2. Experimental Setup

A cone calorimeter was employed to examine the combustion characteristics (e.g., HRR and THR) of the wood samples. Originally developed in the 1980s by the United States National Institute of Standards and Technology, the cone calorimeter was designed primarily to measure the HRRs of small-scale samples [17]. Subsequently, a standardized test protocol was developed for evaluation of the combustion performance (ISO 5660-1) [18], thereby enabling the application of radiant heat with a stable heat flux. This combustion performance test is commonly used to measure fire characteristics by determining the HRR and monitoring the oxygen concentration in combustion gases. Consequently, it quantifies the oxygen consumption during uniform radiant heating from a conical heater directed horizontally toward the specimen. As shown in Figure 2, the experimental setup features a cone-shaped radiant heat source that delivers radiant heat vertically to the surface of the solid combustible material, initiating both pyrolysis and combustion reactions. The cone calorimetry experiment allows the collection of key combustion data, such as the HRR, THR, smoke production rate, and ignition time, ultimately enabling comprehensive data acquisition [19]. In addition to its practicality, cone calorimetry provides theoretical insight into fire behavior. The HRR is regarded as the most fundamental variable in fire science, since it directly governs flame spread and the potential for flashover, while the THR represents the cumulative energy release and indicates the overall fire load of a material. Moreover, ignition temperature and time are closely related to the pyrolysis mechanisms of cellulose, hemicellulose, and lignin, allowing combustion performance to be interpreted in terms of wood’s density and chemical composition [20]. In this study, the amount of radiant heat was gradually increased from 5 to 25 kW/m2 (i.e., 5, 10, 15, 20, and 25 kW/m2). However, radiant heat levels > 25 kW/m2 were avoided since they tend to cause rapid ignition and charring, thereby impeding observation of the initial heating and charring states. This limitation informed the selection of the radiant heat flux range.

2.3. Experimental Method

After raising the temperature to the set point using the temperature control device, the emitted heat flux value at the target temperature was measured using a radiant heat sensor. Once the heat flux value stabilized, the prepared wood specimen was secured in the mold. The specimen was then placed in the frame at a distance of 25.4 mm (1 inch) from the bottom of the cone heater. The ignition device was inserted, and the thermal barrier was removed, allowing sample heating for 600 s. All data were recorded using data-collection equipment to measure the surface temperature of the specimen. The time to ignition (transient flaming time) was also recorded, along with the moment when continuous combustion was established; at this point, the ignition device was immediately removed. Upon completion of the combustion experiment, data collection was stopped, and a thermal insulation device was installed to protect the equipment. The samples were carefully examined after removing the attached thermocouples and fixtures. Figure 3 shows representative photographic images of the charring patterns (side views) for the Larix kaempferi, Pinus densiflora, and Pinus koraiensis specimens after the cone calorimetry combustion process performed at 25 kW/m2. From a theoretical perspective, the evaluation of post-test char depth provides valuable insight into the pyrolysis and combustion behavior of wood. Previous studies on biomass pyrolysis have shown that char is widely recognized not merely as a byproduct but as a protective barrier that reduces oxygen diffusion, mitigates mass loss, and slows heat transfer into the underlying material [21]. The amount and structure of char are strongly influenced by the chemical composition and density of the biomass, with higher lignin content generally leading to the formation of more stable char residues. Accordingly, the measurement of char depth after cone calorimetry provides a reliable indicator of the thermal degradation characteristics and inherent fire resistance of the specimens. This interpretation is consistent with the findings of experimental studies on char production from the pyrolysis of biomass, which highlight the significant role of char in enhancing fire resistance and long-term thermal stability [22].

3. Results

3.1. Determination of the HRR

The HRR and THR values of the wood samples, along with their peak HRR times, were determined to analyze the effects of the radiant heat on their combustion characteristics, and the obtained values are presented in Table 3.
For the Larix kaempferi samples, the maximum peak HRR of 194.9 kW/m2 was achieved for the LA-20 specimen, while a minimum HRR of 2.0 kW/m2 was observed for the LA-15 specimen. In terms of the time to reach peak the fastest (185 s). These results indicate that the HRR of the LA-20 specimen increased sharply before reaching its maximum, whereas in the LA-25 specimen the peak HRR was significantly reduced. The highest average HRR value for Larix kaempferi was 113.2 kW/m2 (LA-20), while the LA-25 specimen showed a markedly lower value (72.1 kW/m2). This reduction was attributed to the formation of a protective char layer, which enhanced the fire resistance of the LA-25 specimen after an initially rapid rise in HRR.
For the Pinus densiflora samples, the highest peak HRR was observed in the PI-25 specimen, reaching 198.7 kW/m2 after 75 s, while the PI-5 specimen exhibited the lowest value (11.1 kW/m2). An intermediate value of 147.3 kW/m2 was recorded for the PI-10 specimen, suggesting that Pinus densiflora releases a larger amount of instantaneous heat compared with the other species, thereby potentially contributing more significantly to fire spread. Furthermore, the average HRR was 115.8 kW/m2 for the PI-10 specimen and 112.1 kW/m2 for the PI-25 specimen, indicating that despite a substantial increase in applied radiant heat (from 10 to 25 kW/m2), Pinus densiflora maintains a relatively high and sustained HRR. This behavior is consistent with previous studies [23], which reported that the crown fuels of pine trees exhibit high flammability and sustain combustion under dry-season conditions, thereby accelerating fire spread and prolonging burning duration. Such consistency suggests that the resinous and volatile compounds in Pinus densiflora play a critical role in sustaining elevated HRR values, highlighting the inherently fire-prone characteristics of this species.
For the Pinus koraiensis samples, the highest peak HRR (241.6 kW/m2) was recorded for the NU-20 specimen, whereas the NU-5 specimen showed the lowest value (9.1 kW/m2). The time to reach peak HRR was the longest for the NU-15 specimen (430 s) and the shortest for the NU-10 specimen (70 s). For the NU-15, NU-20, and NU-25 specimens, the HRR tended to rise sharply and then stabilize, indicating that combustion was activated after a certain delay, followed by either a steady or reduced HRR. The highest average HRR was observed for the NU-15 specimen (116.6 kW/m2), while the NU-5 and NU-10 specimens recorded very low values of −0.4 and −0.6 kW/m2, respectively, due to the near absence of combustion reactions. In contrast, the NU-15 specimen exhibited prolonged burning, as heat release continued even after the formation of a char layer. These findings are consistent with previous research [24], which showed that the combustion behavior of Pinus koraiensis is strongly influenced by the development of protective char layers and inherent fire-retardant properties, leading to reduced peak HRR under low-intensity heat fluxes and extended burning at higher intensities. This suggests that the thermal degradation and charring mechanisms of Pinus koraiensis play a decisive role in sustaining combustion once ignition has occurred.

3.2. Determination of the THR

The THR refers to the total amount of heat generated during a fire and is one of the key factors in predicting flashover, i.e., a phenomenon wherein an entire space is engulfed in flames almost instantaneously during an indoor fire [14]. The THR analysis results presented in Table 4 demonstrate that Pinus densiflora wood (PI-25) produced the highest value of 62.2 MJ/m2, followed by Pinus koraiensis (NU-25, 45.5 MJ/m2) and Larix kaempferi (LA-25, 31.8 MJ/m2). These differences were attributed to the relatively high average density of Pinus densiflora and its heat accumulation characteristics as the combustion duration increased.

3.3. Evaluation of the Surface Temperature

Wood is a natural material whose physical properties vary depending on the surrounding environment and the region in which it grows. Therefore, when exposed to elevated temperatures, charring characteristics can differ depending on the growth environment. Accordingly, the surface temperature for each species was analyzed over time, with the corresponding curves presented in Figure 4.
Figure 4 shows the surface temperatures at the point of ignition for the various specimens. Specifically, higher surface ignition temperatures were recorded for the larger specimens, with the LA-20 and LA-25 specimens exhibiting ignition temperatures of ~355 and 300 °C, respectively. In contrast, for the PI-20 and PI-25 Pinus densiflora samples, relatively low ignition temperatures of ~190 and 210 °C, respectively, were obtained. Intermediate values were obtained for the Pinus koraiensis samples, with the NU-20 and NU-25 specimens exhibiting ignition temperatures of ~260 and 240 °C, respectively. These results indicate that the surface temperature depends on the vegetative environment, which ultimately influences the thermal properties of the wood. Generally, the surface temperature represents the temperature at which rapid thermal decomposition begins, producing flammable gases and subsequently forming a char layer.

3.4. Evaluation of the Char Depth

The char layer formed on wood is critical in the context of fire safety, as it serves as a natural protective barrier that imparts the material with fire-resistance characteristics [17]. To identify the species-specific char layers formed during the thermal decomposition of wood, the char depth was measured at four points on each sample, as illustrated in Figure 5.
The char depth was measured using the cross-section of each specimen following the combustion test. For this purpose, each specimen was divided into four distinct zones to capture the effects of varying thermal exposure across different areas. In each zone, the central axis served as the reference line, with five measurement points being designated to assess whether differences in heat exposure were adequately represented. The measurement process followed the Detailed Operational Guidelines from the Quality Certification and Management Standards for Building Materials (Notice by the Ministry of Land, Infrastructure and Transport, Korea). As shown in Figure 5, the char depth was measured at four of the five designated points, and the average of these four measurements was taken as the representative value. This approach was adopted to account for localized variations in heat exposure across the specimen, thereby enhancing the reliability and consistency of the data.
As detailed in Table 5, the char depth tended to increase for all species as the radiant heat intensity increased. This was attributed to the accelerated pyrolysis of the wood surface under higher radiant heat conditions, which promotes the formation of a char layer. The resulting charring patterns for each species, corresponding to the varying levels of applied radiant heat, are illustrated in Table 5.
As outlined in Table 6, upon exposure to 25 kW/m2 radiant heat, the average char depths were 13.5, 11.0, and 7.5 mm for the Pinus koraiensis, Pinus densiflora, and Larix kaempferi specimens, respectively. At a slightly lower radiant heat of 20 kW/m2, the corresponding depths were 11.5, 8.0, and 5.5 mm, indicating that Pinus koraiensis underwent the most extensive carbonization. Pinus densiflora ignited later than Larix kaempferi and Pinus koraiensis; however, once combustion began, its char depth increased rapidly with rising heat flux, implying that this species responds more sensitively to thermal exposure. These findings are consistent with previous studies [25], which reported that charring rates of softwood species depend strongly on heat flux, density, and resin content. In particular, Pinus species, owing to their high levels of volatile and resinous compounds, undergo accelerated pyrolysis and rapid char formation under elevated thermal conditions, whereas denser species such as Larix kaempferi tend to form comparatively thinner char layers. Taken together, these results underscore the decisive role of chemical composition and structural density in determining species-specific fire resistance characteristics.

3.5. Discussion of the Combustion Characteristics

Given that the most intense combustion and pyrolysis occurred under a radiant heat flux of 25 kW/m2 (Figure 6), the subsequent analysis focused on specimens tested at this condition. The peak HRR values for the three species were 189.3 kW/m2 for Larix kaempferi, 198.7 kW/m2 for Pinus densiflora, and 221.9 kW/m2 for Pinus koraiensis. The times to reach peak HRR were 185 s for Larix kaempferi, 75 s for Pinus densiflora, and 85 s for Pinus koraiensis, indicating that Pinus densiflora exhibited rapid and intense combustion immediately after ignition. The THR values were 31.8, 62.2, and 45.5 MJ/m2 for Larix kaempferi, Pinus densiflora, and Pinus koraiensis, respectively, which can be attributed to the relatively high density and prolonged burning of Pinus densiflora. These findings are consistent with previous studies [26], which reported that charring rate and char depth are strongly correlated with wood density and mass loss under cone calorimeter tests. In particular, higher-density species tend to char more slowly but sustain combustion for longer durations, providing a theoretical explanation for the elevated THR observed in Pinus densiflora compared with Larix kaempferi and Pinus koraiensis.

3.6. Evaluation of the Charring Characteristics

The charring characteristics of the three wood species were subsequently analyzed, as shown in Figure 7. More specifically, the surface ignition temperatures were determined to be ~300, 210, and 240 °C for the Larix kaempferi, Pinus densiflora, and Pinus koraiensis samples, respectively, indicating that the Larix kaempferi sample underwent a more gradual pyrolysis process than the other species. The corresponding char depths were 13.5 mm for Pinus koraiensis, 11.0 mm for Pinus densiflora, and 7.5 mm for Larix kaempferi, with Pinus koraiensis forming the thickest char layer. Although Larix kaempferi ignited later than the other species, once combustion began it developed char relatively quickly. These results are consistent with previous research [27], which emphasized that the combustion behavior of wood species is strongly influenced by heat flux. Specifically, higher heat flux has been shown to reduce ignition time and accelerate pyrolysis, while species-specific density and chemical composition determine the extent of char formation. Together, these findings provide a theoretical explanation for the lower ignition temperature of Pinus densiflora and the thicker char layer observed in Pinus koraiensis under the same heat flux conditions.

4. Discussion

The present study investigated the combustion and charring characteristics of three Korean structural wood—Larix kaempferi, Pinus densiflora, and Pinus koraiensis—under radiant heat fluxes of 5–25 kW/m2 using cone calorimetry (ISO 5660-1). The results demonstrated clear differences among the species in terms of ignition behavior, heat release, and char depth.
In terms of ignition, Larix kaempferi exhibited the highest ignition temperature (~300 °C), significantly higher than Pinus densiflora (~210 °C) and Pinus koraiensis (~240 °C). This indicates that Larix kaempferi undergoes a slower pyrolysis process, thereby delaying combustion onset. Regarding heat release, Pinus densiflora showed the greatest total heat release (62.2 MJ/m2), almost double that of Larix kaempferi (31.8 MJ/m2), with Pinus koraiensis in between (45.5 MJ/m2). The high THR of Pinus densiflora suggests a strong potential to accelerate fire spread, which is consistent with previous studies highlighting the flammability of resin-rich pine species. In contrast, the low THR of Larix kaempferi supports its superior performance in limiting overall fire energy.
The analysis of peak HRR further confirmed species-specific differences. Pinus koraiensis reached the highest peak HRR of 241.6 kW/m2 (NU-20), followed by Pinus densiflora (198.7 kW/m2, PI-25) and Larix kaempferi (189.3 kW/m2, LA-20). However, Pinus densiflora reached its peak within 75 s, compared to 185 s for Larix kaempferi, indicating rapid and intense combustion after ignition. This instantaneous behavior reflects the presence of volatile compounds in Pinus densiflora, whereas Larix kaempferi ignited later and released heat more gradually.
Char depth measurements also highlighted distinct resistance characteristics. At 25 kW/m2, Pinus koraiensis developed the thickest char layer (13.5 mm), followed by Pinus densiflora (11.0 mm) and Larix kaempferi (7.5 mm). The rapid char growth in Pinus densiflora suggests accelerated pyrolysis under high flux, while the relatively shallow char in Larix kaempferi indicates reduced mass loss and improved fire resistance. These findings align with prior studies, which reported that charring behavior is governed by both species density and chemical composition.
Taken together, the experimental results confirm that wood density, resin content, and fiber structure play decisive roles in combustion dynamics. The superior fire resistance of Larix kaempferi can be attributed to its higher density and compact fiber structure, which delay ignition and limit heat release. Conversely, Pinus densiflora exhibited rapid ignition and high heat release due to its resinous nature, while Pinus koraiensis demonstrated delayed but intense combustion, with protective char formation that sustained burning at higher fluxes. It is also worth noting that previous studies have indicated that preservative treatments such as impregnation do not always enhance fire performance and in some cases may even deteriorate thermal stability, which underscores the importance of considering chemical modifications in assessing wood fire safety [28,29].
Beyond the experimental findings, these results have important implications for sustainability. From an academic perspective, this study provides new quantitative evidence on the fire performance of domestic Korean wood, addressing a gap in existing literature dominated by foreign species. From a practical perspective, identifying Larix kaempferi as the most fire-resistant species supports its application in eco-friendly structural wood construction. Promoting the use of domestic wood reduces reliance on imports, lowers the carbon footprint of building materials, and contributes to sustainable building design.
Nevertheless, it should be noted that cone calorimetry provides results under controlled laboratory conditions. Future research should include large-scale fire resistance testing and environmental variability to validate the findings. Such studies will further enhance the applicability of domestic wood resources in sustainable and fire-safe construction practices.
In this context, conducting full-scale fire resistance tests that meet building regulations will be an essential step. To this end, future research will employ a stepwise approach, beginning with standardized small-scale fire tests and progressing to intermediate- and large-scale evaluations in collaboration with specialized research facilities. This systematic approach will ensure both scientific rigor and regulatory compliance for the application of Korean wood as a sustainable structural material.

Author Contributions

Conceptualization, J.-Y.P. and S.-I.H.; methodology, S.-I.H.; software, J.-Y.P.; validation, Y.-J.C., J.-Y.P. and J.-H.A.; formal analysis, Y.-J.C. and J.-Y.P.; investigation, S.-I.H.; resources, J.-H.A.; data curation, Y.-J.C.; writing—original draft preparation, J.-Y.P. and S.-I.H.; writing—review and editing, J.-Y.P. and Y.-J.C.; visualization, S.-I.H.; supervision, J.-H.A.; project administration, Y.-J.C.; funding acquisition, J.-H.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Korea Forest Service (Korea Forestry Promotion Institute), under project number RS-2023-KF002506.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are openly available at Korean Standards and Certification. KS F 3020:2023 (Softwood structural lumber). Available online: https://www.standard.go.kr/KSCI/standardIntro/getStandardSearchList.do (accessed on 30 June 2025) [15]. KS F 3021:2022 (Structural glued laminated timber). Available online: https://www.standard.go.kr/KSCI/standardIntro/getStandardSearchList.do (accessed on 30 June 2025) [16].

Conflicts of Interest

The authors declare no conflicts of interest. The sponsors had no role in the design, execution, interpretation, or writing of the study.

Abbreviations

The following abbreviations are used in this manuscript:
CLTCross-laminated timber
GLTGlued laminated timber
HRRHeat release rate
THRTotal heat release

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Figure 1. Cross-sectional schematic of charred wood [11].
Figure 1. Cross-sectional schematic of charred wood [11].
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Figure 2. Schematic representation of the cone calorimetry experimental setup and a photograph of the combustion process.
Figure 2. Schematic representation of the cone calorimetry experimental setup and a photograph of the combustion process.
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Figure 3. Charring patterns of the (a) Larix kaempferi, (b) Pinus densiflora, and (c) Pinus koraiensis specimens (side view) following combustion under a radiant heat of 25 kW/m2.
Figure 3. Charring patterns of the (a) Larix kaempferi, (b) Pinus densiflora, and (c) Pinus koraiensis specimens (side view) following combustion under a radiant heat of 25 kW/m2.
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Figure 4. Variation in sample surface temperatures over time during heating under different radiant heat conditions: (a) Larix kaempferi; (b) Pinus densiflora; and (c) Pinus koraiensis.
Figure 4. Variation in sample surface temperatures over time during heating under different radiant heat conditions: (a) Larix kaempferi; (b) Pinus densiflora; and (c) Pinus koraiensis.
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Figure 5. Measurement of char depth. The left-hand panel shows a schematic representation of the measurement points, while the right-hand panel shows a corresponding photographic image.
Figure 5. Measurement of char depth. The left-hand panel shows a schematic representation of the measurement points, while the right-hand panel shows a corresponding photographic image.
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Figure 6. (a) HRR and (b) THR values of the three wood species upon the application of a radiant heat intensity of 25 kW/m2.
Figure 6. (a) HRR and (b) THR values of the three wood species upon the application of a radiant heat intensity of 25 kW/m2.
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Figure 7. (a) Surface ignition temperatures and (b) char depths of the three wood species upon the application of a radiant heat intensity of 25 kW/m2.
Figure 7. (a) Surface ignition temperatures and (b) char depths of the three wood species upon the application of a radiant heat intensity of 25 kW/m2.
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Table 1. Classification of the softwood structural lumber species employed in the present study (KS F 3020).
Table 1. Classification of the softwood structural lumber species employed in the present study (KS F 3020).
GroupGroup AGroup BGroup C
Representative species Larix kaempferi Pinus densiflora Pinus koraiensis
Cross-sectional imageSustainability 17 08360 i001Sustainability 17 08360 i002Sustainability 17 08360 i003
Table 2. Details regarding the Larix kaempferi, Pinus densiflora, and Pinus koraiensis specimens.
Table 2. Details regarding the Larix kaempferi, Pinus densiflora, and Pinus koraiensis specimens.
SpeciesSpecies IDSample Specifications
(Length × Width × Height) (mm)		Water
Content
(%)		Mass
(g)		Density
(g/cm3)
Larix kaempferi LA-5 (5 kW/m2)100 × 100 × 508.9252.90.51
LA-10 (10 kW/m2)9.1254.20.51
LA-15 (15 kW/m2)8.8250.80.50
LA-20 (20 kW/m2)9.3255.10.51
LA-25 (25 kW/m2)9.0250.00.50
Pinus
densiflora 		PI-5 (5 kW/m2)7.8264.80.53
PI-10 (10 kW/m2)9.0256.00.51
PI-15 (15 kW/m2)8.0254.30.50
PI-20 (20 kW/m2)8.1253.70.51
PI-25 (25 kW/m2)7.4262.80.53
Pinus koraiensis NU-5 (5 kW/m2)8.8192.20.38
NU-10 (10 kW/m2)8.5187.60.38
NU-15 (15 kW/m2)7.5213.80.43
NU-20 (20 kW/m2)7.9187.10.37
NU-25 (25 kW/m2)7.9194.40.39
Table 3. HRR values for the various specimens.
Table 3. HRR values for the various specimens.
SpeciesSpecimenHRRmean (kW/m2)HRRpeak (kW/m2)HRRpeak_time (s)
Larix
kaempferi 		LA-51.815.9340
LA-10−0.83.4315
LA-15−0.12.0425
LA-20113.2194.9450
LA-2572.1189.3185
Pinus
densiflora 		PI-5−0.011.165
PI-10115.8147.3550
PI-1585.5160.9210
PI-2093.9180.9115
PI-25112.1198.775
Pinus
koraiensis 		NU-5−0.49.1145
NU-10−0.69.370
NU-15116.6206.8430
NU-2087.2241.6180
NU-2584.5221.985
Table 4. THR values (MJ/m2) for the various specimens under different radiant heat conditions (heating time: 0–600 s).
Table 4. THR values (MJ/m2) for the various specimens under different radiant heat conditions (heating time: 0–600 s).
Species5 kW/m210 kW/m215 kW/m220 kW/m225 kW/m2
Larix kaempferi 1.30.10.219.731.8
Pinus densiflora 0.315.640.749.462.2
Pinus koraiensis 0.40.222.839.045.5
Table 5. Charring patterns produced by the different wood species according to different radiant heat values.
Table 5. Charring patterns produced by the different wood species according to different radiant heat values.
Species5 kW/m210 kW/m215 kW/m220 kW/m225 kW/m2
Larix kaempferi Sustainability 17 08360 i004Sustainability 17 08360 i005Sustainability 17 08360 i006Sustainability 17 08360 i007Sustainability 17 08360 i008
Pinus densiflora Sustainability 17 08360 i009Sustainability 17 08360 i010Sustainability 17 08360 i011Sustainability 17 08360 i012Sustainability 17 08360 i013
Pinus koraiensis Sustainability 17 08360 i014Sustainability 17 08360 i015Sustainability 17 08360 i016Sustainability 17 08360 i017Sustainability 17 08360 i018
Table 6. Average char depths by species according to the quantity of radiant heat applied.
Table 6. Average char depths by species according to the quantity of radiant heat applied.
Species5 kW/m210 kW/m215 kW/m220 kW/m225 kW/m2
Larix kaempferi ---5.5 mm7.5 mm
Pinus densiflora -3.5 mm5.0 mm8.0 mm11.0 mm
Pinus koraiensis --7.5 mm11.5 mm13.5 mm
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Park, J.-Y.; Hong, S.-I.; Chio, Y.-J.; An, J.-H. Comparative Study on the Combustion Characteristics of Korean Wood with Changes in the Radiant Heat. Sustainability 2025, 17, 8360. https://doi.org/10.3390/su17188360

AMA Style

Park J-Y, Hong S-I, Chio Y-J, An J-H. Comparative Study on the Combustion Characteristics of Korean Wood with Changes in the Radiant Heat. Sustainability. 2025; 17(18):8360. https://doi.org/10.3390/su17188360

Chicago/Turabian Style

Park, Ji-Young, Seong-In Hong, Yun-Jeong Chio, and Jae-Hong An. 2025. "Comparative Study on the Combustion Characteristics of Korean Wood with Changes in the Radiant Heat" Sustainability 17, no. 18: 8360. https://doi.org/10.3390/su17188360

APA Style

Park, J.-Y., Hong, S.-I., Chio, Y.-J., & An, J.-H. (2025). Comparative Study on the Combustion Characteristics of Korean Wood with Changes in the Radiant Heat. Sustainability, 17(18), 8360. https://doi.org/10.3390/su17188360

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