# ASTM D6841-16

Designation: D6841 − 16Standard Practice forCalculating Design Value Treatment Adjustment Factors forFire-Retardant-Treated Lumber1This standard is issued under the fixed designation D6841; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers procedures for calculating treat-ment adjustment factors to be applied to design values forfire-retardant-treated lumber used at ambient temperatures[service temperatures up to 100°F (38°C)] and as framing inroof systems.1.2 These design value treatment adjustment factors for theproperties of extreme fiber in bending, tension parallel to grain,compression parallel to grain, horizontal shear, and modulus ofelasticity are based on the results of strength tests of matchedtreated and untreated small clear wood specimens after condi-tioning at nominal room temperatures [72°F (22°C)] and ofother similar specimens after exposure at 150°F (66°C). Thetest data are developed in accordance with Test Method D5664.Guidelines are provided for establishing adjustment factors forthe property of compression perpendicular to grain and forconnection design values.1.3 Treatment adjustment factors for roof framing applica-tions are based on computer generated thermal load profiles fornormal wood roof construction used in a variety of climates asdefined by weather tapes of the American Society of Heating,Refrigerating and Air-Conditioning Engineers, Inc.(ASHRAE).2The solar loads, moisture conditions, ventilationrates, and other parameters used in the computer model wereselected to represent typical sloped roof designs. The thermalloads in this practice are applicable to roof slopes of 3 in 12 orsteeper, to roofs designed with vent areas and vent locationsconforming to national standards of practice and to designs inwhich the bottom side of the roof sheathing is exposed toventilation air. For designs that do not have one or more ofthese base-line features, the applicability of this practice needsto be documented by the user.1.4 The procedures of this practice parallel those given inPractice D6305. General references and commentary in Prac-tice D6305 are also applicable to this practice.1.5 The values stated in inch-pound units are to be regardedas standard. The SI units listed in parentheses are provided forinformation only and are not considered standard.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3D9 Terminology Relating to Wood and Wood-Based Prod-uctsD5664 Test Method for Evaluating the Effects of Fire-Retardant Treatments and Elevated Temperatures onStrength Properties of Fire-Retardant Treated LumberD6305 Practice for Calculating Bending Strength DesignAdjustment Factors for Fire-Retardant-Treated PlywoodRoof Sheathing3. Terminology3.1 Definitions:3.1.1 Definitions used in this practice are in accordance withTerminology D9.3.2 Definitions of Terms Specific to This Standard:3.2.1 bin mean temperature—10°F (5.5°C) temperatureranges having mean temperatures of 105 (41), 115 (46), 125(52), 135 (57), 145 (63), 155 (68), 165 (74), 175 (79), and185°F (85°C).3.2.2 thermal load profile—the cumulative time per year ineach 10°F (5.5°C) temperature bin.1This practice is under the jurisdiction of ASTM Committee D07 on Wood andis the direct responsibility of Subcommittee D07.07 on Fire Performance of Wood.Current edition approved March 1, 2016. Published April 2016. Originallyapproved in 2002. Last previous edition approved in 2015 as D6841 – 15. DOI:10.1520/D6841-16.2Available from American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (ASHRAE), 1791 Tullie Circle, NE, Atlanta, GA30329, http://www.ashrae.org.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14. Summary of Practice4.1 Test results developed in accordance with Test MethodD5664 are used in conjunction with computer generatedthermal load profiles to calculate treatment factors that areapplied to published design values for untreated lumber. Thesetreatment adjustment factors account for the combined effect offire-retardant-treatment and service temperatures.5. Significance and Use5.1 Fire-retardant-treatments are used to reduce the flame-spread characteristics of wood. Chemicals and redrying condi-tions employed in treatments are known to modify the strengthproperties of the wood product being treated. This practicegives procedures for fire-retardant chemical manufacturers touse to calculate the effects of their treatment on lumber used innormal and elevated temperature service conditions.5.2 The effect of fire-retardant treatments on the strength oflumber used in roof framing applications is time related. In thispractice, the cumulative effect on strength of annual thermalloads from all temperature bins is increased 50 times toestablish treatment adjustment factors for fire-retardant treatedlumber roof framing.5.3 The procedures of Test Method D5664 employ anelevated temperature intended to produce strength losses in ashort period of time. Although the exposure is much moresevere than that which occurs in an actual roof system, thechemical reactions that occur in the laboratory test are consid-ered to be the same as those occurring over long periods oftime in the field.5.4 Treatment adjustment factors developed under this prac-tice apply to lumber installed in accordance with constructionpractices recommended by the fire-retardant chemical manu-facturer which include avoidance of direct wetting, precipita-tion or frequent condensation. Application of this practice islimited to roof applications with design consistent with 1.3.6. Test Data6.1 Test Method D5664 describes the procedures used toobtain the data needed to calculate the ratios of average treatedand average untreated values for the strength properties.6.1.1 Procedure 1 of Test Method D5664 provides data forcomparing the initial effects of fire-retardant treatments tountreated controls for bending, tension parallel, compressionparallel, and horizontal shear properties. The procedure usessmall clear specimens.6.1.2 Procedure 2 of Test Method D5664 provides data forassessing the differential trends between treated and untreatedspecimens on bending and tension parallel properties over thecourse of a prolonged exposure to elevated temperature. Theprocedure uses small clear specimens.6.1.3 Procedure 3 of Test Method D5664 is an optionalprocedure to provide additional information on size effects.The results are used to modify the test results for the smallclear specimens of Procedure 1 and 2.6.2 In Test Method D5664, specimens subjected to pro-longed exposure to elevated temperature are exposed in acontrolled environment of 150 6 4°F (66 6 2°C) and ≥50 %relative humidity (RH). Durations of exposure are 36, 72, and108 days.NOTE 1—Exposure durations may vary –0%, +5% to allow convenientscheduling for the laboratory conducting the test.7. Calculation of Strength Loss Rates7.1 For each species and property evaluated, calculate theratio of the average treated value to the average untreated valuefor the specimens conditioned at room temperature only(unexposed specimens) and for specimens exposed for thesame period of time at elevated temperature.7.1.1 The treated and untreated specimen averages used tocalculate each ratio shall include the same number of speci-mens and each treated specimen value shall be matched to anuntreated specimen value obtained from the same source pieceof lumber.NOTE 2—Test data show that the ratio of average treated and averageuntreated values is a more conservative measure of treatment effect thanthe median or the average of the individual matched specimen ratios.7.2 The ratio of the average property value for unexposedtreated specimens to the average value for unexposed untreatedspecimens shall be designated the initial treatment ratio, Ro.7.3 Using the ratios of average treated to untreated speci-mens exposed to elevated temperature for the same period oftime, Rti, determine by least squares the linear regression.Rti5 a1kt~D! (1)where:Rti= ratio of average treated to untreated values,D = number of days specimens exposed at elevatedtemperature,a = intercept, andkt= slope, strength loss rate.7.3.1 The ratio, Ro, for unexposed specimens (conditionedat room temperature only) shall be included in the regressionanalysis.7.3.2 A property for which the strength loss rate, kt,isnotnegative is assumed to be unaffected by the elevated tempera-ture exposure.7.3.3 The strength loss rate, kt, shall be adjusted to a 50 %RH basis by the equation:k505 kt~50/RHi! (2)where:k50= strength loss rate at 50 % RH, andRHi= elevated temperature test RH.7.4 Calculate strength loss per day rates for bin meantemperatures of 105 (41), 115 (46), 125 (52), 135 (57), 145(63), 155 (68), 165 (74), 175 (79), and 185°F (85°C) using theArrhenius equation:ln~k50/k2! 5 @Ea~T12 T2!#/RT1T2(3)where:k2= strength loss rate at bin mean temperature,D6841 − 162Ea= 21 810 cal/mol, (1)4,5(91 253 J/mol),R = 1.987 cal/mol-K (8.314 J/mol-K), gas constant,T1= test temperature, K, andT2= bin mean temperature, K.7.4.1 Where the treatment effect was evaluated at more thanone elevated temperature [for example 130°F (54°C) and150°F (66°C)], the strength loss rates associated with the binmean temperatures shall be calculated for each temperatureseparately and the rates averaged for determination of capacityloss values associated with thermal load profiles.NOTE 3—This practice constructs an Arrhenius plot using classicalchemical kinetics techniques, which is the simplest modeling approach.Other more sophisticated modeling techniques are available but require adifferent procedure for calculating strength loss rate (2, 3).68. Calculating Capacity Loss for Roof FramingApplications8.1 Thermal load profiles applicable to roof framing aregiven in Table 1. The loads represent the cumulative days peryear framing temperatures fall within 10°F (5.5°C) of the binmean temperatures of 105 (41), 115 (46), 125 (52), 135 (57),145 (63), 155 (68), 165 (74), 175 (79), and 185°F (85°C).Tabulated values are based on a verified attic temperature andmoisture content model developed by the USDA, ForestService Forest Products Laboratory (4) and reference yearweather tapes. Input parameters used in the model werea3in12 roof slope, south exposure, roofing absorptive factor of 0.65and ventilation rate of 8 air changes per hour (ach).NOTE 4—Additional information on the computer model and the inputparameters used is given in Practice D6305.8.1.1 Two thermal load profiles are given in Table 1. Thefirst profile shall be used with all properties except tensionparallel to grain. This profile represents a weighted average ofbin temperature days for the bottom of the roof sheathing andfor the attic air with weights of 0.25 and 0.75 respectively. Thesecond load profile shall be used for tension parallel to grainand is based on bin temperature days for the attic air.NOTE 5—Field temperatures for upper and lower chords of roof rafters(truss) for two locations have been studied (5)(Fig. 1). This data indicatesthat the upper chord temperature tracks closely with the attic airtemperature. The use of a weighted average of bottom sheathing and atticair temperatures for properties other than tension parallel to grainrepresent a conservative approach for locations where field data is notavailable.NOTE 6—Thermal load profiles in Practice D6305 represent the binningof the average of the hour by hour temperatures at the top and bottom ofthe roof sheathing.NOTE 7—Thermal loads in Table 1 have been indexed to a 50 % RHbasis by multiplying model generated loads by the ratio of the timeweighted average attic RH for all temperatures of 80°F and above and50 %. The adjustment is based on the use of a linear adjustment of teststrength loss rates for RH and the use of a linear regression model tocharacterize strength loss over time.8.2 Calculate capacity loss for each property as the negativevalue of the rates (k2) as determined in 7.4 for each bintemperature by the cumulative days per year for that bin for theapplicable zone and property from Table 1. The summation ofthe capacity loss values for each temperature bin shall bedesignated as the total annual capacity loss (CLT) for thatproperty and zone.9. Treatment Adjustment Factors9.1 For each property and zone, a treatment adjustmentfactor for design values shall be calculated as:TF 5 @1 2 IT 2 n~CF!~CLT!# (4)where:TF = treatment adjustment factor = (1 − IT),IT = initial treatment effect=1−Ro,n = number of iterations = 50,CF = cyclic loading factor = 0.6, andCLT = total annual capacity loss.9.1.1 Where a property has been evaluated at more than oneelevated temperature, IT in Eq 4 shall be taken as the averageof the Roratio for each temperature data set.9.2 Where the properties of compression parallel to grainand horizontal shear have not been evaluated at elevatedtemperatures for a species, the CLT determined for bending andfor tension parallel to grain, whichever is greater, shall be usedin Eq 4 to determine treatment adjustment factors for theseproperties.9.3 Where a property shows no strength loss when exposedat elevated temperature (CLT = 0), the property treatmentadjustment factor, TF, for all thermal load zones shall be equalto (1 − IT), or Ro.9.4 Atreatment adjustment factor for applications involvingservice temperatures up to 100°F (38°C) shall be (1 − IT), orRo, for all properties.9.5 Compression perpendicular to the grain design valuesare based on a deformation limit which is related to specificgravity. Although reductions in specific gravity are generallynot observed at 150°F (66°C) temperature exposure, a TF of4The boldface numbers in parentheses refer to a list of references at the end ofthis standard.5Pasek and McIntyre have shown that the Arrhenius parameter, Ea, forphosphate-based retardants for wood averages 21 810 cal/mol (91 253 J/mol). Othervalues are appropriate for fire retardants that are not phosphate based.6A description of other models is available in Refs (2, 3).TABLE 1 Reference Thermal Load ProfilesBin MeanTemperature,°F (°C)Cumulative days per yearBottom of roof sheathing/attic air Attic airZone 1AAZone 1BAZone 2AZone 1AAZo