ASTM D6841-21

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Designation: D6841 − 16 D6841 − 21
Standard Practice for
Calculating Design Value Treatment Adjustment Factors for
Fire-Retardant-Treated Lumber
This standard is issued under the fixed designation D6841; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This practice covers procedures for calculating treatment adjustment factors adjustment factors that account for the effects of
fire-retardant treatment on design properties of lumber. The adjustment factors calculated in accordance with this practice are to
be applied to design values for untreated lumber in order to determine design values for fire-retardant-treated lumber used at
ambient temperatures [service temperatures up to 100°F (38°C)] 100 °F (38 °C)] and as framing in roof systems.
NOTE 1—This analysis focuses on the relative performance of treated and untreated materials tested after equilibrating to ambient conditions following
a controlled exposure to specified conditions of high temperature and humidity. Elevated temperature, moisture, load duration, and other factors typically
accounted for in the design of untreated lumber must also be considered in the design of fire-retardant-treated lumber, but are outside the scope of the
treatment adjustments developed under this practice.
1.2 These design value treatment adjustment factors for the properties of extreme fiber design properties in bending, tension
parallel to grain, compression parallel to grain, horizontal shear, and modulus of elasticity are based on the results of strength tests
of matched treated and untreated small clear wood specimens after conditioning at nominal room temperatures [72°F (22°C)] [72
°F (22 °C)] and of other similar specimens after exposure at 150°F (66°C). 150 °F (66 °C). The test data are developed in
accordance with Test Method D5664. Guidelines are provided for establishing adjustment factors for the property of compression
perpendicular to grain and for connection design values.
1.3 Treatment adjustment factors for roof framing applications are based on computer generated thermal load profiles for normal
wood roof construction used in a variety of climates as defined by weather tapes of the American Society of Heating, Refrigerating
and Air-Conditioning Engineers, Inc. (ASHRAE). The solar loads, moisture conditions, ventilation rates, and other parameters
used in the computer model were selected to represent typical sloped roof designs. The thermal loads in this practice are applicable
to roof slopes of 3 in 12 or steeper, to roofs designed with vent areas and vent locations conforming to national standards of practice
and to designs in which the bottom side of the roof sheathing is exposed to ventilation air. For designs that do not have one or
more of these base-line features, the applicability of this practice needs to be documented by the user.
1.4 The procedures of this practice parallel those given in Practice D6305. General references and commentary in Practice D6305
are also applicable to this practice.
1.5 The values stated in inch-pound units are to be regarded as standard. The SI units listed in parentheses are provided for
information only and are not considered standard.
This practice is under the jurisdiction of ASTM Committee D07 on Wood and is the direct responsibility of Subcommittee D07.07 on Fire Performance of Wood.
Current edition approved March 1, 2016June 1, 2021. Published April 2016July 2021. Originally approved in 2002. Last previous edition approved in 20152016 as
D6841 – 15.D6841 – 16. DOI: 10.1520/D6841-16.10.1520/D6841-21.
Available from American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (ASHRAE), 1791 Tullie Circle, NE, Atlanta, GA 30329,
http://www.ashrae.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6841 − 21
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D9 Terminology Relating to Wood and Wood-Based Products
D5664 Test Method for Evaluating the Effects of Fire-Retardant Treatments and Elevated Temperatures on Strength Properties
of Fire-Retardant Treated Lumber
D6305 Practice for Calculating Bending Strength Design Adjustment Factors for Fire-Retardant-Treated Plywood Roof
Sheathing
3. Terminology
3.1 Definitions:
3.1.1 Definitions used in this practice are in accordance with Terminology D9.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 bin mean temperature—10°F (5.5°C) 10 °F (5.5 °C) temperature ranges having mean temperatures of 105 (41), 115 (46),
125 (52), 135 (57), 145 (63), 155 (68), 165 (74), 175 (79), and 185°F (85°C).185 °F (85 °C).
3.2.2 thermal load profile—the cumulative time per year in each 10°F (5.5°C) temperature bin.
4. Summary of Practice
4.1 Test results developed in accordance with Test Method D5664 are used in conjunction with computer generated thermal load
profiles to calculate treatment factors that are applied to published design values for untreated lumber. These treatment adjustment
factors account for the combined effect of fire-retardant-treatment and service temperatures.
5. Significance and Use
5.1 Fire-retardant-treatments are used to reduce the flame-spread characteristics of wood. Chemicals and redrying conditions
employed in treatments are known to modify the strength properties of the wood product being treated. This practice gives
procedures for fire-retardant chemical manufacturers to use to calculate the effects of their treatment on lumber used in normal and
elevated temperature service conditions.establishes the procedures for determining adjustment factors that account for the isolated
effects of fire-retardant treatment on design properties of lumber. These effects are established relative to performance of untreated
lumber.
5.2 The effect of fire-retardant treatments on the strength of lumber used in roof framing applications is time related. In this
practice, the cumulative effect on strength of annual thermal loads from all temperature bins is increased 50 times to establish
treatment adjustment factors for fire-retardant treated lumber roof framing.
5.3 The procedures of Test Method D5664 employ an elevated temperature intended to produce strength losses in a short period
of time. Although the exposure is much more severe than that which occurs in an actual roof system, the chemical reactions that
occur in the laboratory test are considered to be the same as those occurring over long periods of time in the field.
5.4 Treatment adjustment factors developed under this practice apply to lumber installed in accordance with construction practices
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
D6841 − 21
recommended by the fire-retardant chemical manufacturer which include avoidance of direct wetting, precipitation or frequent
condensation. Application of this practice is limited to roof applications with design consistent with 1.3.
6. Test Data
6.1 Test Method D5664 describes the procedures used to obtain the data needed to calculate the ratios of average treated and
average untreated values for the strength properties.
6.1.1 Procedure 1 of Test Method D5664 provides data for comparing the initial effects of fire-retardant treatments to untreated
controls for bending, tension parallel, compression parallel, and horizontal shear properties. The procedure uses small clear
specimens.
6.1.2 Procedure 2 of Test Method D5664 provides data for assessing the differential trends between treated and untreated
specimens on bending and tension parallel properties over the course of a prolonged exposure to elevated temperature. The
procedure uses small clear specimens.
6.1.3 Procedure 3 of Test Method D5664 is an optional procedure to provide additional information on size effects. The results
are used to modify the test results for the small clear specimens of Procedure 1 and 2.
6.2 In Test Method D5664, specimens subjected to prolonged exposure to elevated temperature are exposed in a controlled
environment of 150 6 4°F (66 6 2°C) and ≥50 % relative humidity (RH). Durations of exposure are 36, 72, and 108 days.
NOTE 2—Exposure durations may vary –0%, +5% to allow convenient scheduling for the laboratory conducting the test.
7. Calculation of Strength Loss Rates
7.1 For each species and property evaluated, calculate the ratio of the average treated value to the average untreated value for the
specimens conditioned at room temperature only (unexposed specimens) and for specimens exposed for the same period of time
at elevated temperature.
7.1.1 The treated and untreated specimen averages used to calculate each ratio shall include the same number of specimens and
each treated specimen value shall be matched to an untreated specimen value obtained from the same source piece of lumber.
NOTE 3—Test data show that the ratio of average treated and average untreated values is a more conservative measure of treatment effect than the median
or the average of the individual matched specimen ratios.
7.2 The ratio of the average property value for unexposed treated specimens to the average value for unexposed untreated
specimens shall be designated the initial treatment ratio, R .
o
7.3 Using the ratios of average property values of treated to untreated specimens exposed to elevated temperature for the same
period of time, R , determine by least squares the linear regression.regression in the form of Eq 1.
ti
R 5 a1k ~D! (1)
ti t
where:
R = ratio of average treated to untreated values,
ti
R = ratio of average property value of treated specimens to average property value of untreated specimens,
ti
D = number of days specimens exposed at elevated temperature,
D = number of days of elevated temperature exposure,
a = intercept, and
k = slope, strength loss rate.
t
7.3.1 The ratio, R , for unexposed specimens (conditioned at room temperature only) shall be included in the regression analysis.
o
7.3.2 A property for which the strength loss rate, k , is not negative is assumed to be unaffected by the elevated temperature
t
exposure.
D6841 − 21
7.3.3 The strength loss rate, k , shall be adjusted to a 50 % RH basis by the equation:
t
k 5 k 50/RH (2)
~ !
50 t i
where:
k = strength loss rate at 50 % RH, and
RH = elevated temperature test RH.
i
RH = relative humidity in elevated temperature test.
i
7.4 Calculate strength loss per day rates for bin mean temperatures of 105 (41), 115 (46), 125 (52), 135 (57), 145 (63), 155 (68),
165 (74), 175 (79), and 185°F (85°C) 185 °F (85 °C) using the Arrhenius equation:
ln k /k 5 E T 2 T /RT T (3)
~ ! @ ~ !#
50 2 a 1 2 1 2
k E T 2 T
~ !
50,i a 1 2
ln 5 (3)
k RT T
2 1 2
where:
k = strength loss rate at bin mean temperature,
4,5
E = 21 810 cal/mol, (1) (91 253 J/mol),
a
R = 1.987 cal/mol-K (8.314 J/mol-K), gas constant,
T = test temperature, K, and
T = bin mean temperature, K.
7.4.1 Where the treatment effect was evaluated at more than one elevated temperature [for example 130°F (54°C) and 150°F
(66°C)], the strength loss rates associated with the bin mean temperatures shall be calculated for each temperature separately and
the rates averaged for determination of capacity loss values associated with thermal load profiles.
NOTE 4—This practice constructs an Arrhenius plot using classical chemical kinetics techniques, which is the simplest modeling approach. Other more
sophisticated modeling techniques are available but require a different procedure for calculating strength loss rate (2, 3).
8. Calculating Capacity Loss for Roof Framing Applications
8.1 Thermal load profiles applicable to roof framing are given in Table 1. The loads represent the cumulative days per year framing
temperatures fall within 10°F (5.5°C) of the bin mean temperatures of 105 (41), 115 (46), 125 (52), 135 (57), 145 (63), 155 (68),
TABLE 1 Reference Thermal Load Profiles
Cumulative days per year
Bin Mean
Temperature, Bottom of roof she
...