1. Introduction
Since motor gasoline is readily available in gas stations, it is often used as a combustion accelerant in arson cases. Gasoline hazards are characterized by "high ignitability," "high combustibility," and "high volatility." Based on past arson cases involving gasoline (e.g., the Kyoto Animation Arson Murder Case in 2019 and the Osaka Kita-Shinchi Building Arson Murder Case in 2021), when arson with gasoline spills occurs inside a building, the high combustibility of gasoline causes rapid fire spread in the building and extensive damage.
In criminal investigations in Japan, arson crime against a building (Articles 108 and 109 of the Japanese Penal Code) is considered to have been committed when a part of the building is burned irrespective of the arson medium. Because this concept of burn damage is based on the assumption of an old-style wooden house, it is difficult for a modern flame-retardant building to cause obvious burn damage, even in the case of arson with liquid fuel. In other words, there is a mismatch between dangerous criminal acts with gasoline and burn damages. This can lead to cases where arson crime against buildings cannot be applied to arson cases with liquid fuels, albeit considerable danger generated by producing hot smoke and toxic gases.
Shinohara[1] conducted FT-IR, XRD, and SEM-EDX analyses on wood boards exposed to gas burners or flames of ignition medium to clarify differences in the degree of carbonization depending on the burning temperature to identify the presence or absence of carbonization of building materials from a forensic viewpoint. Yang et al.[2] proposed a model for predicting the charring rate of wood exposed to time-increasing and constant heat fluxes based on the assumption that the char layer can be obtained at 300°C. DiDomizio et al.[3] established a method for producing well-controlled, time-varying exposures using the conical radiant heater of a cone calorimeter. They accurately predicted the non-piloted ignition of wood exposed to time-varying radiant heating in the cone calorimeter using a numerical model of the thermal response of the material. Babrauskas[4] offered insights into experimental studies on wood to interpret carbonization patterns in fire investigations. Hakkarainen[5] measured the charring of laminated heavy-timber members in a full-scale room fire fueled by wood cribs. Puttori[6] showed that less than 1 mm liquid depth is generally achieved with a pour of liquid fuel on a level floor and that the fire burns out in roughly 1 min. Okamoto et al.[7,8] conducted experiments assuming an arson case in which liquid fuel was spilled on the floor and burned. They clarified the liquid fuel burning rate, flame height, and thermal effects on the surroundings. In these previous studies, experiments were conducted under open conditions in which oxygen was fully supplied during combustion. However, when liquid fuels are used in buildings, the oxygen in the room is rapidly consumed and ventilation is limited, exposing the building materials to heat fluxes different from those under open conditions. As a result, neither the danger to those within the building nor the possibility of a fire spreading to other buildings for arson via liquid fuels is fully understood.
In this study, we conducted fire experiments using liquid fuel in a full-scale building, assuming arson with liquid fuel, and observed the burning behavior of the liquid fuel and burned damage to the building. We focused on the burned damage of floorboards, which are exposed wooden combustible materials without fire-retardant treatment, even in fire-resistant buildings, for proof of arson crime. X-ray computed tomography (X-ray CT) can be used to reconstruct the internal structure of wood non-destructively[9,10]. We employed the technology to study the progress of burn damage to floorboards. Particularly, we performed X-ray CT imaging of burned floorboards to observe burn damage and established a method to identify the presence or absence of burn damage to floorboards from the images for the purpose of proving continuous burning of floorboards independent of liquid fuel flame.
2. Experimental Method
2.1 Ignition and burning experiments of floorboards using a cone calorimeter
Wooden floorboard panels (depth: 12 mm, width: 300 mm, length: 1,800 mm) were cut into a square of 100 mm to prepare test pieces for ignition and burning experiments using a cone calorimeter. Ignition and burning experiments were conducted using a Cone Calorimeter C4 manufactured by Toyo Seiki Seisaku-sho, Ltd. In ignition experiments, the test pieces were exposed to heat fluxes (50, 40, 30, 25, 20, and 15 kW/m2), and the piloted ignition times were measured. Experiments were conducted three times for each heat flux exposure condition. In burning experiments, weight loss was measured and the heat release rate was obtained using an oxygen consumption method at a heat flux of 50 kW/m2. Further, test pieces were burned after the piloted ignition at a heat flux of 50 kW/m2 to prepare samples for X-ray CT observation. Burning test pieces were extinguished by cooling under a CO2 atmosphere after reaching a predetermined weight loss (10, 20, 30, 40, and 50 wt%). The heating times and weight losses of the prepared samples are listed in Table 1.
2.2 Burning experiments with liquid fuels in a full-scale building
A full-scale building for burning experiments is shown in Figure 1. The building is a wooden frame structure (length and width: 2,600 mm, height: 2,400 mm) with wooden floorboards. The pillars are completely covered with walls and ceiling materials so that they cannot be seen from the inside. The walls and ceiling materials are 12.5- and 9.5-mm-thick gypsum boards with white wallpapers applied, respectively. The floorboards used are 12-mm-thick wooden floorboards (the same product used in Section 2.1). A window (length: 1,840 mm, width: 1,670 mm) was installed in the front, and an opening was set on the right rear side. The interior of the experimental building is shown in Figure 2. Burning experiments were conducted in a fire laboratory (width: 15 m, depth: 15 m, height: 15 m), and an electrostatic precipitator (treatment capacity: 700 m3/min) was operated during the burning experiments. No fixtures were installed, only ceiling lights were installed for room observation to eliminate the effects of combustibles, except for the building and liquid fuel. Three burning experiments were conducted using 2 L of gasoline (Experiment A), 4 L of gasoline (Experiment B), and 2 L of kerosene (Experiment C). After the experiments, the interior (floor, walls, and ceiling) was completely restored using the same type of building materials. The experimental conditions are presented in Table 2. A circular bank with a diameter of 1.6 m was created on the floor center of the experimental building, and liquid fuel was spilled inside the bank. The burning experiments were initiated by directly igniting gasoline with an ignition rod in Experiments A and B. In Experiment C, a rag was used as the medium to ignite kerosene because kerosene has low flammability. After the burning experiments, floorboard pieces of approximately 100 mm square were cut from the burned floor.
2.3 Observation of burned floorboards by X-ray CT system
Three-dimensional X-ray CT imaging was performed on burned floorboard samples cut from a liquid fuel burning experiment in a full-scale building (prepared in Section 2.1) exposed to a heat flux of 50 kW/m2 by a cone calorimeter (prepared in Section 2.1) using an inspeXio SMX-225CT FPD HR microfocus X-ray CT system manufactured by SHIMADZU Corporation. The X-ray tube target was tungsten. At a tube voltage of 115 kV, tube current of 70 µA, and two integrations, 2,400 projection data of 2048 × 2048 pixels (16-bit tif format, uncompressed) were acquired per measurement in a continuous scan. The distance between the X-ray source and sample (source- to-rotation center distance) was 306.1 mm, and the voxel size of the obtained cross-sectional images was 0.075 mm/voxel. VGSTUDIO MAX 3.2.4 (Volume Graphics) was used to analyze the measurement data, and CT images inside the burned floor samples were obtained.
3. Results and Discussion
3.1 Ignition experiments of floorboards using a cone calorimeter
There is a linear relationship between the -0.5 power of the ignition time and heat flux when a sample is exposed to a constant heat flux[11]. The -0.5 power of the ignition time measured using a cone calorimeter is plotted against the heat flux in Figure 3. The ignition-limiting heat flux of the floorboards was obtained as 8.54 kW/m2 from the x-intercept of the regression line. The ignition times predicted from the regression line are listed in Table 3.
3.2 Ignition experiments of floorboards using a cone calorimeter
The time variation of heat release rate per unit area, Q ˙ '' ω ˙ t
Assuming that the samples are of uniform materials and they burn evenly in the horizontal direction, the burn progressive rate in the depth direction, d ˙
where ρ denotes the sample density and S denotes the surface area of a sample, which are 543 kg/m3 and 0.01 m2, respectively, in this study.
Immediately after a sample was ignited, the burn progressive rate exceeded 2 mm/min; however, during the steady-state burning period, the burn progressive rate remained at approximately 1 mm/min.
The heat of combustion per unit weight of the samples, ΔH [kJ/g], was obtained by dividing the heat release rate by the weight loss rate, Q ˙
where Q ˙
The heat of combustion per unit weight remained at 12-14 kJ/g from ignition to approximately 7 min 30 s, except for the peak at ignition, but increased afterward when the weight loss exceeded 65 wt%, reaching ≥30 kJ/g. The weight loss rate began to decrease after approximately 7 min 30 s and finally dropped to approximately 1/5 of the steady-state combustion. Because the carbide was observed to burn red-hot without flame (flameless combustion), we considered that the moisture and resin components contained in the samples were lost by combustion and that the remaining carbide was decomposed and burned. During flameless combustion, water and other noncombustible materials were removed and only the combustible carbide remained, which is considered to have approximately doubled the heat of combustion per unit weight. The average heat of combustion per unit weight was 14.09 kJ/g from ignition to 80 wt%.
3.3 Burning experiments of liquid fuel in a full-scale building
3.3.1 Experiment A
Immediately after gasoline spilled in the circular bank was ignited, combustion expanded to the entire liquid surface, flames covered the entire room and ejected from the openings, and the fire burned violently. At 5 s, the fire waned due to the lack of oxygen in the room caused by the combustion; however, at 28 s, the window was broken by the heat exposure, allowing airflow into the room, and the combustion regained intensity and peaked at 45 s. The combustion intensity was reduced by fuel burnout after 1 min, and it reached a state of spontaneous extinguishment at 3 min. Because burning up was not observed again, this experiment was terminated after 5 min.
Photographs of burned floorboards in the burning experiments are shown in Figure 8. The burning of the liquid fuel and wallpaper caused extensive burn damage not only to the area where the liquid fuel was spilled but also to a wide area of the entire floor surface. Some parts of the floorboard surface burned out locally; overall, the burn damage was weak, with the floorboard surface soot.
3.3.2 Experiment B
In Experiment B, as in Experiment A, immediately after ignition, the flame quickly expanded to cover the entire room and ejected from the openings. However, the fire waned in 7 s due to lack of acidity; by 20 s, the flame was invisible due to the black smoke that filled the room. At 24 s, a flame plume was observed as unburned combustible gas mixed with outside air at the openings. At 46 s, the window was damaged by heat exposure, and the combustion peaked immediately. When the combustion weakened and the room became visible at 1 min 8 s, ignition of the floorboard on the far side of the room was observed; however, the flames were almost extinguished at 3 min. Smoldering of the floor in contact with the right wall was observed at 8 min; thus, the observation was continued. This experiment was terminated after confirming that the smoldering had transitioned to flaming combustion at 68 min.
The burned area covered the entire floor, and the degree of burn damage was stronger than that in Experiment A. Burning of the floorboard surface layer was observed in areas where burn damage was strong due to the air flowing in through the openings or where there was a transition to flaming combustion (Figure 8).
3.3.3 Experiment C
After igniting a rag used as the ignition medium for kerosene, the flame gradually expanded on the kerosene liquid surface starting from the rag. At 58 s, the flame spread over the entire kerosene. At 1 min 7 s, a flame plume was observed in the openings; at 1 min 16 s, the window was broken and the flame peaked; at 1 min 40 s, the combustion weakened; and at 4 min, the fire was extinguished spontaneously. This experiment was terminated because there was no possibility of combustion again.
Although the burn damage extended over the entire floor, the damage was limited to the surface only. At the walls, the floorboard coating material was also unburned, and the burn damage in Experiment C was the weakest among the three experiments (Figure 8).
3.4 X-ray CT imaging of burned floorboard
The relatively high and low burn damage areas were visually determined for the floorboards after the experiments, and the samples were cut to approximately 100 mm square for X-ray CT in each experiment. The locations where the samples were collected for X-ray CT are shown in Figure 8 and summarized in Table 4. The burn damages of the samples were observed by X-ray CT.
3.4.1 Experiment A
Surface images and X-ray CT images of the burned floorboard samples collected in Experiment A are shown in Figure 9 and Figure 10, respectively. The X-ray CT images show that the floorboard used in this study comprised six layers of wooden materials. In A-1, which was collected at a location with strong burn damage inside the gasoline spill area, swelling and deformation were observed in the second layer and microcracks were observed in the third layer. In A-2, which was collected at a point of weak burn damage inside the gasoline spill area, microcracks were observed only in the first layer and no burn damage was observed in the second layer or below.
3.4.2 Experiment B
Surface images and X-ray CT images of the burnt floorboard collected in Experiment B are shown in Figure 11 and Figure 12. In Experiment B, the floorboards outside the gasoline spill area ignited, and the floorboards continued to burn even after the gasoline burned out. B-1 was collected from floorboards that burned independently of gasoline outside the gasoline spill area. In B-1, partial burnout, deformation, and cracking were observed in the first and second layers and microcracks were observed in the third layer. In B-2, which was collected at a point of weak burn damage inside the gasoline spill area, cracks were observed in the first layer and no burn damage extended to the second layer or below.
3.4.3 Experiment C
Surface images and X-ray CT images of the burnt floorboard collected in Experiment C are shown in Figure 13 and Figure 14, respectively. In C-1, which was collected at a location with strong burn damage inside the kerosene spill area, cracking in the first layer, swelling due to cracking and deformation in the second layer, and slight deformation in the third layer were observed. In C-2, which was collected at a location with weak burn damage under the rag for ignition medium, only the first layer was slightly burned and no burn damage extended below the second layer.
4. Distinguishing between Burned and Unburned Floorboard Samples Using an X-ray CT System
To establish a method for evaluating burn damage to floorboards in situations where arson crimes against buildings using liquid fuel are to be proven, burned floorboard samples (11.5, 22.2, 30.6, 41.2, and 50.7 wt%) prepared using a cone calorimeter were observed by X-ray CT, and the relationship between the weight loss and the progression of burn damage to the depth was quantitatively analyzed. X-ray CT images of the burned floorboard samples are shown in Figure 15. For the burned floorboard samples with 11.5 and 22.2 wt%, we observed that burn damage did not progress to the second layer. Given that the weight loss rate at peak floorboard combustion is 0.18 g/s, it is estimated that it will take at least 74 seconds for the weight burn loss percent to progress to 22.2 wt%, assuming the initial weight of the floorboard sample is 60 g. According to the burning experiments, the duration of intense burning of liquid fuel in buildings is less than 1 minute. Therefore, it is presumed that the burnt damage of the floorboard due to exposure to liquid fuel combustion should be limited to the surface layer only, and burnt damage should not progress to a layer deeper than the second layer. Further, the samples collected from the location where the floorboard was not independently burned by the liquid fuel flame showed that the burn damage did not progress to the second layer or below. Therefore, the possibility of distinguishing whether the floorboard has burned was found by observing the progression of burn damage to the second layer and below by X-ray CT.
5. Conclusion
In this study, we focused on the floorboard as a building material that can be damaged by fire in an arson crime against a building with liquid fuel. We conducted burning experiments using liquid fuels in a full-scale building of 7.5 m2 and observed the burning characteristics of the liquid fuels and the burn damage to floorboards. Data on the ignition characteristics and burning behavior of the floorboards were collected using a cone calorimeter. In addition, X-ray CT imaging was applied to observe the burnout of the floorboards and a method to distinguish between burned and unburned floorboards using the X-ray CT images was established.
From the results of burning experiments in which gasoline and kerosene were spilled on the floor of a building and ignited, we confirmed that when there are few combustibles, such as fixtures in the room, the liquid fuel burns out immediately and the burning time is extremely short, leaving only minor traces of soot on the floorboard surface layer. In Experiment B, in which 4-L gasoline was spilled on the floor, we confirmed that the floorboards were ignited by the heat from the liquid fuel flame and continued to burn even after the liquid fuel had burned out, resulting in a transition to flaming combustion.
X-ray CT imaging of the burned floorboards obtained from the experiments and cone calorimeter test revealed the internal condition of the floorboards, comprising six layers, and allowed us to determine the number of layers that suffered burnout, i.e., the depth of burnout. We could clarify the occurrence of surface layer burnout and internal microcracks, which are difficult to visually observe. In addition, we found that in arson cases in a building using liquid fuel, if the floorboard does not ignite, the fire damage does not progress below the second layer. Further, we established a novel method to distinguish between burned and unburned floorboards using X-ray CT imaging. The results of this study indicate that X-ray CT is effective in evaluating burn damage to floorboards in arson cases.
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