The charring rates of 12 different wood species originating from Europe and the tropics with densities ranging from 350 to 750kg/m3 were investigated to obtain clues on their fire resistance behavior. This was done by measuring the thickness of the charred layer after a 30-min exposure to the standard fire ISO 834-1. No correlation was observed between charring rate and density. In search of another physical property that could be used as an indicator of fire resistance, the oxygen permeabilities of the selected wood types were measured. A strong correlation between oxygen permeability perpendicular to the wood fiber direction and charring rate was found, which is quite straightforward given that oxygen is the necessary component to enable smoldering and ignition, both affecting the charring rate. It seems that oxygen permeability is potentially more suitable as a parameter to evaluate the fire resistance of char-sensitive wooden constructions, rather than density. No general preference of the tree ring orientation from 0° (tangential) to 45° and 90° (radial) was found for these measurement

Hugi, Erich

Risi, Walter

Wakili, Karim

Wuersch, Marcel

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NOTEJ Wood Sci (2007) 53:71–75 © The Japan Wood Research Society 2006DOI 10.1007/s10086-006-0816-1E. Hugi (*) · M. Wuersch · K. Ghazi WakiliBuilding Technologies, Materials Science and Technology, Empa,Duebendorf CH-8600, SwitzerlandTel. +41-44-823-4977; Fax +41-44-823-4154e-mail: erich.hugi@empa.chW. RisiWood Laboratory, Materials Science and Technology, Empa,Duebendorf CH-8600, SwitzerlandErich Hugi · Marcel Wuersch · Walter RisiKarim Ghazi WakiliCorrelation between charring rate and oxygen permeability for 12 differentwood speciesReceived: December 12, 2005 / Accepted: May 8, 2006 / Published online: July 28, 2006Abstract The charring rates of 12 different wood speciesoriginating from Europe and the tropics with densities rang-ing from 350 to 750kg/m3 were investigated to obtain clueson their fire resistance behavior. This was done by measur-ing the thickness of the charred layer after a 30-min expo-sure to the standard fire ISO 834-1. No correlation wasobserved between charring rate and density. In search ofanother physical property that could be used as an indicatorof fire resistance, the oxygen permeabilities of the selectedwood types were measured. A strong correlation betweenoxygen permeability perpendicular to the wood fiber direc-tion and charring rate was found, which is quite straightfor-ward given that oxygen is the necessary component toenable smoldering and ignition, both affecting the charringrate. It seems that oxygen permeability is potentially moresuitable as a parameter to evaluate the fire resistance ofchar-sensitive wooden constructions, rather than density.No general preference of the tree ring orientation from0° (tangential) to 45° and 90° (radial) was found for thesemeasurements.Key words Oxygen permeability · Charring rate · Woodspecies · Fiber orientation · Fire resistanceIntroductionWood is a commonly used building material appreciated forits various properties. One of these is good load-bearingcapacity under fire impact, which depends on the charringrate of the wood. Due to the thermal screening by thecharred layer, the uncharred wood remains at moderatetemperatures even in a long-lasting fire. Consequently thecharring rate is the most important property of wood withrespect to fire resistance and fire integrity. Particularly forapplications in door frames or door leaves, where a ratherthin layer of wood is installed as the room closure, thecharring rate directly influences the amount of deformationand hence the possibility of openings leading to failure infire resistance tests. The previous Swiss Standard1 for fireresistance of materials included a list of 13 different woodspecies declared as appropriate to be used as door frames ordoor leaf frames due to their density being at least 500kg/m3. This enabled the mutual substitution of the listed woodspecies without requiring the repetition of the fire resistancetest for the whole door system. This changed at the begin-ning of 2005 when the Swiss regulative board introducednew fire safety regulations, which refer to the harmonizedEuropean standards.2–4 These allow any wood type thatfulfils the fire resistance test but does not allow interchangeof the wooden parts.Numerous investigations on parameters influencing thecharring rate of wood have been carried out during the pastdecade. Some such as Mikkola5 have suggested simple mod-els including density, wood moisture content, air oxygenconcentration, and heat flux as parameters influencing thecharring rate and used the cone calorimeter method formodel validation. A linearly decreasing correlation be-tween the charring rate and the density was reported byNjankouo et al.6 A closer analysis of the tabulated valuesshowed a weak correlation as the coefficient of determina-tion was only r2 = 0.4 where 0 ≤ r2 ≤ 1. Others such as White,7Lingens et al.,8 and Frangi and Fontana9 found no correla-tion between density and the charring in the range of 350–750kg/m3 as well as no correlation between charring rateand wood moisture content in the range of 8%–15% (typi-cal for wood in buildings). In a recent review article,Babrauskas10 reported that high density timber productswithout any gaps or joints, when under postflashover roomfires, will char at similar rates found in fire resistance tests,namely at 0.5–0.8mm/min. Out of these considerations theSwiss Association of Door Manufacturers (VST) becameinterested in investigating parameters that are strongly cor-72related to the fire resistance of a large variety of woodspecies of both central European and tropical origin. Thisled to the present study, which was carried out at Emparesulting in a diploma thesis at the School of Architecture,Civil, and Wood Engineering in Biel/Switzerland byWuersch.11 In the research work, different parameters ofwood with an assumed impact on its fire resistance wereexamined. They were partitioned in primary parameters(density, oxygen permeability, and fiber orientation) andsecondary parameters resulting from fire tests (charringrate, deflection, temperature increase) using specially de-veloped equipment. The purpose was to determine if anycorrelation existed among the above parameters, whichcould be used as an indicator for future classification withrespect to the fire resistance of wooden door systems.ExperimentalTwo different experiments were carried out for each of the12 selected wood species (Table 1). First, the charring ratemeasurements were carried out in the Empa small horizon-tal furnace. Figure 1 gives a view on the opening of thefurnace and its ventilation hood. For each of the 12 woodspecies, three specimens of 1000 × 90 × 45mm were cutdiffering in their tree ring orientation. The visible tree ringorientation made an angle of 0° (tangential cut), 45°, and90° (radial cut), respectively, with the surface of the samplefacing the furnace. All samples were conditioned for 3weeks in a climate chamber at 20°C and 60% relative hu-midity. These special sizes were used because of the mini-mum required dimensions of door or door leaf frames whena fire resistance time of 30min has to be achieved. Theexposure to unilateral fire load took place according tothe standardized temperature–time curve ISO 834-1.12 Thesamples were arranged in a special sample holder andequipped with displacement transducers on their unex-posed side to investigate deflections primarily induced byloss of moisture, which is not discussed here and will be thesubject of a future publication. After a fire exposure of30min, the charring rate was measured by removing thesamples, cooling them with water, cutting five slices of5-mm thickness from each specimen, abrading the charTable 1. Summary of the measured values for density, charring rate, oxygen permeability index(OPI), and moisture content of the 12 investigated wood speciesWood species Tree ring Density Charring Oxygen Moistureorientation (kg/m3) rate permeability content(mm/min) index (% w/w)Abachi 0° 435.6 0.726 9.85 9.0Triplochiton 45° 478.0 0.750 9.26 9.5scleroxylon 90° 446.6 0.757 9.58 9.6Abura 0° 575.2 0.791 9.57 5.8Hallea ciliata 45° 555.7 0.818 9.73 5.890° 539.6 0.752 9.63 5.3Maple 0° 601.5 0.690 10.11 9.0Acer pseudoplatanus 45° 600.3 0.639 9.64 9.390° 615.0 0.623 9.76 10.1Beech 0° 744.9 0.564 10.30 10.0Fagus sylvatica 45° 664.0 0.703 10.08 10.290° 685.2 0.700 10.15 11.4Oak 0° 659.5 0.557 10.77 11.3Quercus robur 45° 646.1 0.571 10.78 8.090° 572.7 0.544 10.75 10.3Ash 0° 713.9 0.613 9.97 12.0Fraxinus excelsior 45° 566.3 0.681 10.36 7.390° 560.8 0.640 10.26 7.3Spruce 0° 419.8 0.582 10.15 9.1Picea abies 45° 422.0 0.573 10.15 8.890° 420.1 0.568 10.26 8.8Koto 0° 598.3 0.651 9.79 6.0Pterygota 45° 640.4 0.636 9.70 6.0macrocarpa 90° 615.3 0.629 9.88 4.0Meranti dark red 0° 655.4 0.515 10.72 15.4Shorea ssp. 45° 625.5 0.519 10.72 16.190° 644.8 0.528 10.53 16.9Meranti light red 0° 620.4 0.570 10.60 16.7Shorea ssp. 45° 592.4 0.533 10.60 15.690° 586.9 0.580 10.43 16.7Sipo 0° 677.0 0.499 10.84 9.0Entandophragma 45° 722.7 0.511 11.12 9.0utile 90° 716.8 0.492 10.72 9.0Fir 0° 364.7 0.796 9.52 7.5Abies alba 45° 389.3 0.624 9.91 7.590° 388.0 0.709 9.83 8.073layer, and comparing the resulting thickness (Fig. 2) withthat of the initial sample. In other words, the remaining(uncharred) thickness drem was determined by averagingover 15 sites for each specimen.  b = = −Dtd dtinit rem (1)where D is the charred thickness (mm), dinit is theinitial thickness (mm), drem is the remaining (uncharred)thickness (mm), t is time (min), and b is the charring rate(mm/min).The permeability to oxygen perpendicular to the woodfiber direction was measured next at room temperature bymeans of a test method that was originally developed byBeushausen et al.13 for concrete specimens and was adaptedby the authors for wood specimens. The test method in-volved measurement of the pressure decay of oxygen pass-ing through a 25-mm-thick slice of a 68-mm-diameter coreof wood (Fig. 3). The sample holder included a hose clampto guarantee tightness of the whole assembly as shown inFig. 4. In the test, the samples were initially subjected tooxygen at a pressure of 100kPa and the pressure decay withtime was monitored. The coefficient of permeability wasdetermined from the slope of the line produced when thenatural logarithm of the ratio of the initial pressure to theinstantaneous pressure was plotted against time. The oxy-gen permeability index (OPI) is defined as the negativelogarithm of the coefficient of permeability.  kV g dR A tPP t=⋅ ⋅ ⋅⋅ ⋅ ⋅× ( )wqln 0 (2)  OPI = − ( )log10 k (3)Fig. 1. The small horizontal furnace of Empa with an exposed area of1.0 × 0.8 m where samples of 1.25 × 1.05 m size can be subjected to thestandard fire ISO 834-1166 166 166 166 166 1664522.5 22.590DdremFig. 2. Five slices of 5 mm in thickness were cut out of each sampleafter fire exposure. One of them with the 3 sites of charring depthmeasurement is shown enlarged (sizes in mm)Plastic pipe section and PVC spacer Silicone rubber collar Gas outlet valve Gas inlet valve Base plate Pressure gauge or transducer Perforated cover plate          Sample             Rubber O-ring           in 5 mm groove Fig. 3. Apparatus for measuring the oxygen permeability index (OPI)Fig. 4. Special sample holder for OPI measurement74where OPI is the oxygen permeability index, k is the coeffi-cient of permeability of the test sample (m/s), w is the molarmass of gaseous oxygen (32g/mol), V is the volume ofthe pressure cylinder (m3), g is gravitational acceleration(9.81m/s2), R is the universal gas constant (8.313Nm/Kmol), A is the sample cross section (m2), d is the samplethickness (m), q is absolute temperature (K), t is time (s), P0is the pressure at the start of the test (kPa), and P(t) is thepressure at time t (kPa).For each of the 12 wood species, two series of threespecimens of the aforementioned sizes but different treering orientations of 0°, 45°, and 90° with respect to thespecimen surface were cut out of the bulk (Fig. 5). Allspecimens were cut out of a bulk of 2.5 × 0.09 × 0.045mof the corresponding wood type and kept 3 weeks understandard conditions for wood, i.e., 20°C and 60% relativehumidity, prior to testing.ResultsThe measured charring rates of all wood samples are re-presented as a function of measured density and tree ringorientation (Fig. 6) and as a function of measured oxygenpermeability index and tree ring orientation (Fig. 7). Inboth figures, the orientation of the indicated, and thesamples of the same wood type are connected by dottedlines. A summary of the measured values for density, char-ring rate, and oxygen permeability index (OPI) is given inTable 1.DiscussionAccording to Fig. 6, there is no correlation between thecharring rate and the density for the investigated range of350–750kg/m3. Looking in the x- and y-directions of Fig. 6,the influence of tree ring orientation on density and char-ring rate, respectively, seems to be unique for each woodtype. For some, the dispersion is only in density, while forothers is in the charring rate so that no general tendency isdetectable. Figure 7 on the contrary, shows a strong correla-tion between the charring rate and the oxygen permeabilityindex (OPI). Looking in the x-direction of Fig. 7, no corre-lation between the tree ring orientation of the woodsamples and their oxygen permeability index is observed.The coefficient of determination resulting from an appliedlinear regression gives a quite promising value (r2 = 0.70). Apossible explanation of this behavior is that during fire ex-posure a gradient of oxygen concentration will emerge fromthe char front (oxygen sink) toward the fire as well as theroom side. As the char layer itself consumes oxygen for90°45° 0°Fig. 5. The small test specimens for the measurement of OPI in thethree fiber directions0.450.500.550.600.650.700.750.800.85350 400 450 500 550 600 650 700 750Density [kg/m3]Charring rate [mm/min]90°45°0°Fig. 6. Charring rate as a function of density and tree ring orientation.Samples of the same wood species are connected by dotted lines0.450.500.550.600.650.700.750.800.859.00 9.50 10.00 10.50 11.00 11.50Oxygen Permeability Index (OPI)Charring rate [mm/min] 90°45°0°y = -0.1654x + 2.3102r 2 = 0.7Fig. 7. Charring rate as a function of OPI and fiber orientation.Samples of the same wood species are connected by dotted lines. Alinear regression is also shown75completing pyrolysis, the oxygen supply originates mainlyfrom the room side. Deviations in Fig. 7 from the maintendency cannot be due to moisture content (Table 1) be-cause for oak, which has a high moisture content, the char-ring rate was higher than expected, and for koto, which hasa lower water content, the charring rate was lower thanexpected (Table 1). With the charring rates of the samewood type with different fiber orientations being located onboth sides of the regression line, a tendency depending onsoftwood or hardwood as stated by Harada14 cannot beconfirmed.To obtain more information on the influence of oxygenpermeability on the fire resistance behavior of differentwood species, further investigations on sealed samples (theroom side only) and on fire-induced mechanical deforma-tions are planned. This will also be helpful in the validationof wood combustion modeling at different levels of sophis-tication as proposed by Bruch et al.15Acknowlesdgments The authors express their gratitude to the entirestaff of the Fire Laboratory, Empa, for their support with the experi-mental work, W. Trindler for providing access to the OPI apparatus,D. Heer of the Wood Laboratory, Empa, for sample preparation, andK. Richter and E. Pöhler for helpful discussions. This study receivedfinancial support from the Swiss Association of Door Manufacturers(VST).References1. SIA/Lignum (1997) Brandschutz im Hochbau. SIA Dokumenta-tion 83, Zurich2. European Standard EN 1363-1 (1999) Fire resistance tests part 1:general requirements. CEN Central Secretariat, Brussels3. European Standard EN 1634-1 (2000) Fire resistance tests fordoors and shutter assemblies part 1: fire doors and shutters. CENCentral Secretariat, Brussels4. European Standard EN 13501-2 (2002) Fire classification of con-struction products and building elements part 2: classification usingdata from fire resistance tests, excluding ventilation services. CENCentral Secretariat, Brussels5. Mikkola E (1990) Charring of wood (in Finnish). VTT ResearchReport 689, Espoo6. Njankouo JM, Dotreppe JC, Franssen JM (2004) Experimentalstudy of the charring rate of tropical hardwoods. Fire Mater 28:15–247. White RH (2000) Charring rate of composite timber products. In:Proceedings of wood and fire safety (part one), 4th internationalscientific conference, May 14–19, 2000. Zvolen, Slovak Republic,pp 353–3638. Lingens A, Windeisen E, Wegener G (2005) Investigating the com-bustion behaviour of various wood species via their fire gases.Wood Sci Technol 39:49–619. Frangi A, Fontana M (2003) Charring rates and temperatureprofiles of wood sections. Fire Mater 27:91–10210. Babrauskas V (2005) Charring rate of wood as a tool for fireinvestigation. Fire Safety J 40:528–55411. Wuersch M (2005) Klassierungssystemzum Brandverhaltenverschiedener Holzarten. Diploma Thesis N r. F/4/375/05/0, Schoolof Architecture, Civil and Wood Engineering, Biel, Switzerland12. International Standard ISO 834-1 (1999) Fire resistance tests –elements of building construction part 1: general requirements.Geneva13. Beushausen HD, Alexander MG, Mackechnie J (2003) Concretedurability specifications in an international context. BFT ConcretePlant Precast Technol 7:22–3214. Harada T (1996) Charring of wood with thermal radiation. 2. Char-ring rate calculated from mass loss rate. Mokuzai Gakkaishi42:194–20115. Bruch C, Peters B, Nussbaumer T (2003) Modelling wood combus-tion under fixed bed conditions. Fuel 82:729–738The publication of this article was made possible by an Emachu Re-search Fund. The authors are grateful for the fund.

Correlation between charring rate and oxygen permeability for 12 different wood species

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Correlation between charring rate and oxygen permeability for 12 different wood species

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