United States

Department of

Agriculture

Forest Service

Forest Products

Laboratory

Research

Paper

FPL-RP-501

Thermal Degradation

of Fire-RetardantTreated Plywood

Development and Evaluation

of a Test Protocol

Jerrold E. Winandy

Susan L. LeVan

Robert J. Ross

Scott P. Hoffman

Craig R. McIntyre
Abstract

Although untreated plywood has given satisfactory

performance as roof sheathing for more than 50 years,

some fire-retardant-treated plywood products have not

performed satisfactorily in recent years. Thermally

induced in-service failures have occurred with some

fire-retardant-treated plywood roof sheathing. This

paper describes the development and evaluation

of a new test protocol for screening potential fire

retardant treatments for plywood that is continuously

or periodically exposed to elevated temperatures.

In the protocol, untreated and monoammoniumphosphate-treated Southern Pine plywood specimens

were exposed to various exposure temperatures and

durations under steady-state environments of 130°F

(54°C)-73 percent relative humidity (RH), 150°F

(65°C)-76 percent RH, 170°F (77°C)-79 percent RH,

or 170°F (77°C)-50 percent RH. All specimens were

mechanically tested in either bending or tension.

June 1991

Winandy, Jerrold E.; LeVan, Susan L.; Ross, Robert J.;

Hoffman, Scott P.; McIntyre, Craig R. 1991. Thermal

degradation of fire-retardant-treated plywood: Development

and evaluation of test protocol. Res. Pap. FPL-RP-501.

Madison, WI: U.S. Department of Agriculture, Forest

Service, Forest Products Laboratory. 21 p.

A limited number of free copies of this publication

are available to the public from the Forest Products

Laboratory, One Gifford Pinchot Drive, Madison, WI

53705-2398. Laboratory publications are sent to more than

1,000 libraries in the United States and elsewhere.

The Forest. Products Laboratory is maintained in cooperation with the University of Wisconsin.

Monoammonium-phosphate-treated plywood had

lower bending and tension strength than did untreated

plywood at all temperatures. The strength degradation

rate of untreated and treated plywood increased as

exposure temperature increased and appeared constant

for any treatment-temperature combination (that

is, linear over time). The strength degradation rate

was greater at 170°F (77°C)-79 percent RH than

at 170°F (77°C)-50 percent RH for both untreated

and treated plywood. Within the RH limits studied,

the magnitude of the RH effect did not appear to be

as influential as the temperature effect. The results

indicate the protocol provides an effective screening

method for comparing the effects of extended exposure

to elevated temperature on strength of untreated

plywood and plywood treated with commercial fireretardant formulations.

Keywords: Plywood, roof sheathing, mechanical

properties, strength effects, temperature, thermal

effects, treatment, fire retardants
Thermal Degradation of Fire-RetardantTreated Plywood

Development and Evaluation of Test Protocol

Jerrold E. Winandy, Research Wood Scientist

Susan L. LeVan, Supervisory Chemical Engineer

Robert J. Ross, Supervisory General Engineer

Forest Products Laboratory, Madison, Wisconsin

Scott P. Hoffman, Senior Laboratory Associate

Craig R. McIntyre, Manager, Research and Development

Hickson Technical Center, Hickson Corporation, Conley, Georgia

Introduction

For more than 50 years, untreated and fire-retardant-

(FR-) treated lumber and plywood have been successfully used in structures exposed at or near room temperature. We know of no practical in-service strength

reductions in that exposure. Untreated plywood also

has a long record of adequate performance when used

as roof sheathing. Since the early 1960s, FR-treated

plywood and lumber have been used in the roof structures of commercial buildings on a limited scale. However, 10 years ago, two major model building codes allowed the use of FR-treated plywood roof sheathing as

a replacement for noncombustible deck and parapetwall systems in some multifamily structures. In addition, at the time the codes were changed, several FR

formulators changed their chemical formulation. In the

past decade, a number of roof failures have occurred

in structures having some type of FR-treated plywood

used as roof sheathing (APA 1989a).

Roof sheathing is periodically exposed to temperatures

as high as 175°F (80°C) (Heyer 1963, APA 1989b).

Temperature has been shown to be a primary factor

in strength loss of FR-treated wood, be it initial

strength loss (Gerhards 1970, Winandy and others

1988) or in-service strength loss (LeVan and others

This paper is dedicated to the memory of

Dr. John J. “Jack” Kozak, our friend and coworker.

1990). The FR-treated roof sheathing that undergoes

in-service thermal degradation usually exhibits several

classic visual characteristics. Degraded plywood roof

sheathing darkens in color, exhibits a dry-rotted-like

appearance, crumbles easily when abraded, and often

exhibits excessive cross-grain checking. However, the

sheathing does not necessarily exhibit each of these

characteristics.

Objective

This report describes the development and evaluation

of a test protocol for assessing thermally induced

strength loss of FR-treated plywood. The protocol was

developed to screen potential commercial formulations

that are intended for FR treatments of structural

plywood used in environments continuously or

periodically exposed to elevated temperatures. The

protocol was developed because no standardized

comparative procedure exists. This protocol provides

a laboratory method for comparing treated plywood

products to untreated plywood, which has been proven

to be satisfactory for structural roof sheathing. Many

hypothesis were evaluated during the 2-year evolution

of the final protocol. In this report, the background

information and test results used to develop the

final protocol will be discussed. That test protocol is

currently being considered as an Emergency Standard

by Subcommittee D07.06 of the American Society for

Testing and Materials (ASTM).
Background

Fire-retardant treatments can affect the strength of

wood both at the time of processing and while the

wood is in service. Initial processing effects are related

to treatment and redrying conditions. Secondary effects

are associated with in-service exposure of the treated

wood to elevated temperatures.

Initial Processing Effects

The work of many investigators has indicated that

FR treatments initially reduce strength of lumber and

plywood 10 to 25 percent. The magnitude of strength

reduction depends on the FR chemical, the severity

of treatment and processing conditions, and the wood

property under consideration. Taken as a whole, the

literature has shown that the temperature used in kiln

drying FR-treated lumber and plywood after treatment

is one of the factors responsible for the magnitude of

initial strength reduction. Accordingly, American Wood

Preservers’ Association Standards C-20 (for FR-treated

lumber) and C-27 (for FR-treated plywood) both

require that the kiln temperature after treatment be

limited to ≤160°F (≤71°C) until the average moisture

content of the FR-treated material is below 25 percent

(AWPA 1990a,b). In general, these initial processing

effects can be and are accommodated within existing

design procedures.

Secondary In-Service Effects

The FR treatments generally lower the thermal

degradation temperature of most wood-based materials

(LeVan 1984). When some FR formulations are

exposed to elevated temperatures, such as when treated

wood is used as roof sheathing, thermally induced

strength reductions can occur in service. Recent

reports have described strength reduction of FR-treated

plywood roof sheathing (LeVan and Collet 1989),

thermochemical factors (LeVan and Winandy 1990),

and design guidelines (Winandy 1990).

In a recent study of six generic FR formulations in

small clear wood specimens, the long-term effects of

steady-state exposure for 6 months at 130°F (54°C)-

73 percent relative humidity (RH) were found to

be negligible (LeVan and others 1990). However,

the bending strength of all treated specimens and

the untreated control was reduced after 6 months

of exposure at 180°F (82°C)-50 percent RH. After

thermally induced strength loss had been initiated, the

rate of strength loss over time of exposure was similar

between the FR-treated and untreated materials.

Thus, the real differences between the performance

of FR chemicals studied were the magnitude of the

initial strength effect and the duration of elevated

2

temperature exposure required to initiate secondary

strength effects.

Rationale

The final protocol represents the cooperative effort

of many persons (Table 1). The protocol evolved

over a 2-year technical assessment that addressed key

questions about the scope, design, and accuracy of the

proposed test method. These questions identified

1. wood species, plywood quality, and specimen size,

2. mechanical properties,

3. simulation of field conditions in the laboratory,

4. exposure temperature, humidity and duration, and

5. experimental design considerations.

We will discuss the rationale underlying many of the

experimental design decisions and the adaptation and

refinement of the initial experimental techniques for the

final test protocol.

Species, Plywood Quality, and Specimen Size

Southern Pine is probably the most popular plywood

species grouping for use with fire retardants because

of its low cost and superior treatability. It is also the

most readily available wood for the Eastern Seaboard

market where building codes allow the use of FRtreated plywood. Accordingly, FR-treated Southern

Pine plywood is also the material that has undergone

the most problems with in-service strength loss. For

these reasons, Southern Pine plywood was selected as

the material most appropriate for the test protocol.

Because our objective was to develop a comparative procedure rather than to establish design values, N-grade plywood was chosen to eliminate the

uncontrollable influence of knots and voids. The 4-

by 8-ft (1.22- by 2.44-m) plywood panels used in developing the test protocol were special lay-ups of

all N-grade veneers (U.S. Department of Commerce

1983). The N-grade veneer is the highest quality

specified in PS-1-83, the Plywood Products Standard. This grade allows no knots or voids, allows no

splits >0.06 in. (>1.6 mm), and limits nonoverlapping repairs (e.g., patches) to <0.98 in. (<25 mm)

wide and <3.5 in. (<89 mm) long. The decision

to use N-grade plywood presumed that comparative estimates of relative thermal effects based on

high-quality material are representative of field effects with commercial panels. Subsequent work at

Forest Products Laboratory (FPL) is examining that

hypothesis.
Table 1—Task group responsible for developing

test protocol

Name Affiliation

Craig R. McIntyre,

Chairman

Kendall H. Bassett

Thomas R. Flint

Scott P. Hoffman

Barry W. Holden

Edward G. King, Jr.

John J. Kozaka

Joseph J. Kusarb

Alan L. Lambuth

Susan L. LeVan

William S. McNamera

Fred L. Omundson

Kenneth R. Peterson

Nicholas V. Poletika

Alan F. Preston

David M. Roberts

Robert J. Ross

Gerald E. Sherwood

Glenn A. Wilson

Jerrold E. Winandy

Steven C. Zylkowski

Hickson Corporation

Weyerhaeuser Company

American Plywood Association

Hickson Corporation

Hoover Treated Wood

Products, Inc.

Wood Construction

Technologies, Inc.

Osmose Company

Tolleson Lumber Company

Boise-Cascade, Inc.

Forest Products Laboratory’

Osmose Company

CSI, Inc.

Georgia-Pacific, Inc.

J.H. Baxter, Inc.

CSI, Inc.

CSI, Inc.

Forest Products Laboratoryc

National Forest Products

Association

Hoover Treated Wood

Products, Inc.

Forest Products Laboratoryc

American Plywood Association

aDeceased.

b

Initial Task Group Chairman, from December 1987

to October 1989.

cUSDA Forest Service, Madison, Wisconsin.

Early in the development of the protocol, the task

group considered the size of test specimens. Capacity

of available environmental chambers, economic

considerations, and intended use of the protocol

as a comparative procedure (rather than a designsetting procedure) were all considerations. These

considerations dictated the use of small specimens

rather than full-size panels. Mechanical property

estimates based on results from small, defect-free

specimens cut from full-size panels usually do not

precisely predict the properties of full-size panels

because of knots and voids within the panels. McNatt

(1984) studied the accuracy of property estimates for

large plywood panels based on small specimen results.

He found that modulus of elasticity (MOE) values of

small specimens were usually 10 to 20 percent lower

than MOE values of full-sized panels; modulus of

rupture (MOR) values of small specimens were usually

6 to 30 percent greater than MOR values of full-sized

panels. McNatt and others (1990) also presented a

series of predictive equations that allow prediction of

full-sized panel properties from small specimen data.

Based on this information, the task group decided that

3-in.-wide by 24-in.-long (75-mm-wide by 610-mm-long)

(face veneer parallel to long axis) bending specimens

provided repeatable estimates of material properties, as

well as a workable specimen size.

Mechanical Properties

Bending properties were evaluated because bending

loads were considered critical for plywood roof

sheathing. The effects of extended exposure at elevated

temperature on the tensile properties of FR-treated

plywood were also studied because we anticipated

a differential effect between bending and tensile

properties. A cutting procedure was chosen that

provided an equal number of bending and tensile

specimens. This cutting format provided 20 bending

and 20 tensile specimens per exposure combination

(treatment-temperature-humidity-exposure duration).

Laboratory Simulation of Field Conditions

Plywood roof sheathing is exposed to a constantly

changing environment. Solar radiation causes roof

sheathing to heat up and cool on a daily cycle. Roof

sheathing moisture content follows an opposite cycle.

Temperature increases during the day tend to dry

roof sheathing, and cooler evening temperatures can

cause moisture adsorption. Seasonal cycles occur as

well. Changes in moisture content are predominantly

controlled by seasonal rather than daily conditions

because wood has a relatively low moisture diffusion

coefficient. Ideally, an optimum laboratory exposure

technique would mimic these cyclic patterns of

temperature and relative humidity. However, such

a simulation would tend to last many years and

to be prohibitively expensive. Accelerated cyclic

environments were considered to speed the process, but

the time required was still prohibitive.

The final laboratory exposure technique considered

was steady-state, elevated temperature exposure. This

exposure is fast and it indicates whether particular

chemicals are activated at the tested temperature.

Steady-state exposure at elevated laboratory conditions

was chosen as the most reasonable compromise

between time, cost, and the need for reliable data on

comparative performance.

The task group recognized that any laboratory

exposure scenario has problems because of a lack of

constitutive relationships between laboratory and field

(in-service) conditions. The relationships between

3
cumulative thermal-moisture exposure and strength

loss over time for both laboratory and in-service

exposures will need to be developed to obtain reliable

in-service predictive capability from laboratory tests.

Future research will address these needs.

Exposure Conditions

The protocol was not intended to predict roof sheathing performance exactly. Rather, the intention was to

produce comparative data of the response of treated

and untreated plywood to elevated thermal conditions.

The task group decided that data would be needed for

three critical temperatures: 130°F, 150°F, and 170°F

(54°C, 65°C, and 77°C). These temperatures respectively represent a daily temperature commonly obtained in plywood roof sheathing, a critical temperature

limit for long-term exposure of wood products (NFPA

1986), and a periodically obtained daily maximum temperature (Heyer 1963, APA 1989b).

Because acid-catalyzed dehydration reactions are

believed to be the primary mechanism of strength

loss (LeVan and Winandy 1990), plywood moisture

content exaggerates the thermal degradation process.

Thus, the protocol needed to take into account the

effect of moisture. The task group collectively thought

that plywood roof sheathing would vary on a seasonal

basis between 6 percent and 12 percent equilibrium

moisture content assuming the presence of an attic with

near code-specified ventilation and a well-constructed

roofing membrane. Therefore, two matched 170°F

(77°C) exposures were performed, one at 79 percent

RH and the other at 50 percent RH. The first exposure

produces 12 percent equilibrium moisture content in

untreated wood and the second 6 percent equilibrium

moisture content in untreated wood (Forest Products

Laboratory 1987).

Duration of exposure was another important variable.

The task group recognized that 170°F (77°C) would

induce a given strength reduction in a shorter period

than would 130°F (54°C). However, the question

of a thermal threshold, below which little practical

thermally induced strength reductions would occur,

needed to be addressed. Thus, lower temperature

exposures were run for a progressively longer time.

The 170°F (77°C) exposures ran for 63 days, the 150°F

(65°C) exposures for 112 days, and the 130°F (54°C)

exposures for 140 days.

Experimental Design

Wood is a variable material. The mechanical properties

of specimens from a single plywood panel generally

tend to have less variability among themselves than do

4

specimens cut from two different panels. Distinguishing

within-panel variability from between-panel variability

can greatly increase the sensitivity of statistical

analysis. Statistical designs that use the technique

of segregating independent variabilities are known

as blocked designs (Snedecor and Cochran 1967).

However, it is sometimes more efficient to increase the

number of specimens used in an experiment than to

control unspecific variability. To determine the actual

benefits of blocking, we compared two methods of

assigning specimens to experimental groups. In the

first method, specimens were allotted to the groups by

way of a random-shuffle technique. The second method

used a blocked experimental design. In this method,

the specimens were assigned to experimental groups in

a predesignated manner so that each group was allotted

only two specimens from any single panel of plywood.

Each 0.625-in.- (15.9-mm-) thick Southern Pine panel

(4 by 8 ft (1.22 by 2.44 m)) was cut into four 4- by 2-ft

(1.22- by 0.61-m) sheets. For each panel, two sheets

were arbitrarily kept as untreated controls and two

sheets were pressure treated in a laboratory cylinder

at the Hickson Corporation Research and Development

Center. An 8.3-percent solution of technical grade

monoammonium phosphate (MAP) in water was used

to achieve a final retention of 3.15 lb/ft3 (50.4 kg/m3)

MAP. Experience has shown that this retention

provides plywood with a Class A flame-spread rating.

All treated specimens were kiln dried at dry-bulb and

wet-bulb temperatures of 160°F (71°C) and 150°F

(65°C) for 114 h to final average moisture content of

15 percent. Ten bending and 10 tension specimens

were cut from each sheet. This procedure provided 20

untreated and 20 treated specimens from each panel

for both bending and tension tests. Bending specimens

were 3 in. wide by 24 in. long (75 mm wide by 610 mm

long). Tensile specimens were 1 in. wide by 24 in. long

(25 mm wide by 610 mm long) and necked to 0.5 in.

(12.5 mm) over the middle 2.5 in. (6.4 mm). Twenty

specimens were allotted to each exposure combination.

Exposure Conditions

Prior to elevated temperature exposure, all specimens

were conditioned to constant weight at 74°F (23°C)-

65 percent RH, which represents approximately

12 percent equilibrium moisture content conditions for

untreated wood (Forest Products Laboratory 1987).

Individual groups of specimens were then exposed

for designated durations at the controlled, steadystate, elevated temperature-humidity conditions

shown in Table 2. After the allotted exposure duration

and before mechanical testing, the specimens were

again conditioned to constant weight at 74°F (23°C)-

65 percent RH.
Table 2—Exposure combinations for test protocola

Temperature RH

(°F (°C)) (%)

Exposure Test

Design b

(days) mode

80 (27)c 30 Random 0 Bending

and and

blocked tension

130 (54) 73 Random 14, 28, 56, Bending

84, 116, 140 and

tension

150 (65) 76 Blocked 7, 14, 28, Bending

56, 84, 112

170 (77) 79 Random 7, 14, 21, Bending

35, 49, 63 and

tension

170 (77) 79 Blocked 7, 14, 21, Bending

35, 49, 63 and

tension

170 (77) 50 Random 7, 14, 21, Bending

35, 49, 63

a Same exposure combination were used for untreated

and treated specimens. Treatment with monoammonium phosphate (MAP), 3.5 lb/ft3

 (56 kg/m3).

bSpecimens allotted to blocked design were tested by

Hickson Corporation. All other specimens were

allotted to random design and tested at the FPL.

c Control.

Mechanical Testing

Mechanical testing in bending followed procedures

outlined in ASTM D-3043 Method A (ASTM 1990b).

Specimens were center-point loaded using rotational

end-supports over a span of 22 in. (560 mm). Rate

of loading in bending was 0.2 in/min (5 mm/min).

Tension tests followed procedures outlined in ASTM D3500 Method A, Specimen type B (ASTM 1990c). Rate

of loading in tension was 0.04 in/min (0.9 mm/min).

Evaluation

Experimental results are given in Appendix A. Mean

trends are shown in Figures 1 to 9.

Experimental Design

Differences between the blocked experimental design

and the random design can be evaluated by comparing

the two sets of data for the 170°F (77°C)-79 percent

RH exposure (Figs. 1, 2). The trend shown by the

mean results is similar for MAP-treated and untreated

material in tension (Fig. 2) and MAP-treated materials

in bending (Fig. 1), but not for untreated materials

in bending (Fig. 1). This difference in mean trends

between the two experimental designs in bending is

probably related not to the experimental design but

rather to a difference between the manner in which

the two testing laboratories performed the mechanical

tests. The bending specimens from the blocked design

that were exposed for ≤35 days were loaded until the

first major drop in load (crack); the load was then

removed. Specimens from the blocked design that

were exposed for >35 days and specimens from the

random-design were tested in bending until they would

no longer support load. Untreated plywood can be

extremely tough and durable; with continued loading,

the maximum load is sometimes obtained after the first

major drop in load. Conversely, treated material in

bending and treated and untreated material in tension

tend to be more brittle, and the first drop in load is

nearly always the maximum sustainable load. Thus,

treated material in bending would be less sensitive

than untreated material to premature termination of

the test. This appears to be the case. Otherwise, mean

trends for MOR and UTS are quite similar (Figs. 1, 2).

The efficiency in analysis of wood mechanical property

data from blocked experimental designs is usually

greater than that from random designs if sufficient

degrees of freedom exist in the overall experimental

design. Recall that overall standard error is represented

by the standard deviations listed (Appendix A,

Tables A1 to A9). Also recall that for data from the

blocked design, a portion of that overall standard

error can be specifically attributed to panel-to-panel

variability and the remainder attributed to withinpanel variability. To evaluate this, we performed an

analysis of variance on the blocked experimental results

assuming a (1) random and (2) blocked design. We

found that the specific partitioning of standard error

in the blocked design effectively minimized the least

significant difference calculated from the blocked design

by approximately 6 to 8 percent when compared to the

random design. Based on this information, we decided

that a blocked experimental design was the preferable

methodology.

Exposure Conditions

Bending and tensile strength degradation of treated

plywood was related to exposure temperature, duration

of exposure, and relative humidity (Tables A1 and A2).

We also examined the effect of wood specific gravity

and moisture content on wood degradation.

5
Figure 1—Effect of 170°F (77°C)-79 percent

RH exposure on modulus of rupture (MOR).

1 lb/in2 = 6.89 kPa.

Figure 2—Effect of 170°F (77°C)-79 percent

RH exposure on ultimate tensile stress (UTS).

1 lb/in2 = 6.89 kPa.

Temperature

As exposure temperature and duration increased,

mechanical properties generally decreased. Examples

of these relationships are shown for MOR and ultimate

tensile stress (UTS) of untreated plywood exposed at

130°F (54°C)-73 percent RH, 150°F (65°C)-76 percent

RH, and 170° (77°C)-79 percent RH (Figs. 3, 5) and

for MOR and UTS of MAP-treated plywood (Figs. 4,

6) (Tables A3 to A6). These data show four trends:

1. Strength of MAP-treated plywood was initially

reduced compared to that of untreated plywood.

2. Effect of temperature on MOR of untreated and

MAP-treated plywood was generally similar to that

on UTS.

3. Rate of strength degradation (as shown by general

slope in strength over time relationship) was directly

related to exposure temperature.

4. Rate of degradation over duration of exposure

in both bending and tensile strength generally

appeared constant for any exposure combination.

6

Figure 3—Effect of exposure to various

temperatures on modulus of rupture (MOR) of

untreated plywood. 1 lb/in2 = 6.89 kPa.

Figure 4—Effect of exposure to various

temperatures on modulus of rupture (MOR) of

MAP-treated plywood. 1 lb/in2 = 6.89 kPa.

Figure 5—Effect of exposure to various

temperatures on ultimate tensile stress (UTS)

of untreated plywood. 1 lb/in2 = 6.89 kPa.

Although linear (first-order), second-order, and several

independent-variable transformations were considered,

none seemed to substantially fit the data better

than the linear model. Thus, a first-order model was
Figure 6—Effect of exposure to various

temperatures on ultimate tensile stress (UTS)

of MAP-treated plywood. 1 lb/in2 = 6.89 kPa.

selected because of its simplicity. A linear or first-order

relationship was noted for a parallel study conducted

simultaneously using untreated specimens, MAPtreated specimens, and specimens treated with several

other FR formulations (LeVan and others 1990).

The rate of strength degradation over duration of

exposure generally appears to be a function of exposure

temperature (Figs. 3 to 7). Specifically, the data for

treated material seem to refute the existence of a

temperature threshold, below which thermally induced

degradation does not occur and above which permanent

thermally induced degradation does occur. This would

imply that the performance of a treated product after

an extended period at some elevated temperature can

be used to predict laboratory performance at higher or

lower temperatures. Thus, we believe that running the

thermal exposures at 170°F (77°C)-67 percent RH over

an extended period of at least 175 days is sufficient to

indicate products or treatments that are susceptible to

thermally induced degradation.

Relative Humidity

Increasing the RH increased the moisture content of

untreated or MAP-treated plywood. The increase

in moisture content, in turn, increased the rate of

thermally induced strength loss over the duration of

exposure at 170°F (77°C) (Fig. 7). A comparison

of untreated and MAP-treated plywood exposed

at 79 percent RH (12 percent equilibrium moisture

content) to plywood exposed at 50 percent RH

(6 percent equilibrium moisture content) shows that the

rate of thermally induced strength loss was increased

at the higher wood equilibrium moisture content.

However, the reader should note that the magnitude of

the increased effect resulting from increased equilibrium

moisture content (Fig. 7) was not as great as the effect

of increased temperature (Figs. 3 to 6).

Figure 7—Effect of exposure to 170°F (77°C)

at different relative humidities on modulus of

rupture (MOR) of untreated and MAP-treated

plywood. 1 lb/in2 = 6.89 kPa.

Our practical experience in running the protocol was

that RH above 70 percent at 170°F (77°C) promoted

excessive corrosion in the equipment. We also noted

that although temperature within an environmental

chamber was relatively easy to monitor and control,

accurate monitoring and control of RH at the elevated

temperatures was more difficult. Thus, the desired

RH condition was not as controllable as it should be

for a standard test method. These findings, together

with the discrepancy in strength results related to RH

(Fig. 7), indicate the need for an empirical procedure

to account for the effect of different RH. The following

procedure uses the relative vapor pressures of water at

various RH to adjust data obtained at one temperature

and RH condition to another RH condition. This

procedure and examples of its use are described in

the new ASTM Standard (ASTM 1991). Using this

procedure, we can calculate an experimentally derived

rate of thermal degrade (k 1 = d(MOR)/dt) at the

experimental temperature and RH condition through

linear regression:

MOR = b0

 + k1t (1)

where

t is time (days),

b0

 regression constant (at T1 and RH1),

k1 regression slope (at T1 and RH1),

T1 temperature (experimental), and

RH1 relative humidity (experimental).

We then adjust that experimentally derived rate

constant (k 1) to a standard rate of thermal degradation

(k2) at some predefined consensus RH condition:

7
(2)

where RH2 is currently defined as 67 percent RH

(ASTM 1991). Using this approach, the effects of

temperature on strength at one RH condition were

found to reasonably predict the effects on strength at

another RH condition.

Specific Gravity

Extended thermal exposure apparently exerts no

measurable effect on the specific gravity of untreated

or MAP-treated plywood (Table A7). This lack

of measurable effect eliminates the possibility of

monitoring specific gravity (that is, density) to

nondestructively evaluate the possible extent of

thermally induced strength loss.

Moisture Content

A definite moisture content hysteresis is apparent in the

results shown in Table A7 and Figures 8 and 9. Untreated specimens exposed at 130°F (54°C)-73 percent RH and 170°F (and 77°C)-50 percent RH approached equilibrium under adsorbing conditions and

equilibrated at approximately 10 percent moisture content. Untreated specimens exposed at 150°F (65°C)-

76 percent RH and 170°F (77°C)-79 percent RH approached equilibrium under desorbing conditions and

equilibrated at approximately 12 percent moisture content. The MAP-treated specimens also followed this

pattern of hysteresis, but they tended to equilibrate

at about 1 to 2 percent higher equilibrium moisture

content than their untreated counterparts. In the future, this protocol should be run in a way that assures

that all specimens approach moisture equilibrium under

either all adsorbing or all desorbing conditions. This

would facilitate comparison of mechanical property results.

Note that regardless of whether the specimens approached moisture equilibrium under adsorbing or

desorbing conditions, the eventual equilibrium moisture content of both treated and untreated specimens

tended to be slowly reduced after extended exposure

to elevated temperature (Figs. 8, 9). The exceptions

were the untreated 130°F and 150°F (54°C and 65°C)

exposure groups and the untreated and MAP-treated

groups from FPL exposed at 170°F (77°C)-79 percent

RH. Note that treated specimens tended to exhibit decreased hygroscopicity more than did untreated specimens. This permanent, reduction in plywood hygroscopicity as witnessed by reduced equilibrium moisture

content after extended exposure to elevated temperature is similar in nature and in magnitude to the trend

noted earlier in our work with small clear specimens of

Southern Pine (LeVan and others 1990). In that report,

8

Figure 8—Effect of exposure to various

temperatures on equilibrium moisture content

of untreated plywood.

Figure 9—Effect of exposure to various

temperatures on equilibrium moisture content

of MAP-treated plywood.

we showed that the permanent reduction in equilibrium

moisture content resulted from the hydrolysis of certain

hemicelluloses that provide secondary bonding sites for

bound water.

Mechanical Testing

Extended exposure to elevated temperatures induced similar strength property reductions in bending

(Figs. 1, 3, 4) and tensile (Figs. 2, 5, 6) properties. Accordingly, we decided to discontinue the tension tests.

In addition, we decided to mandate the calculation and

reporting of work to maximum load (WML) because

a load that induces a bending stress was considered to

be a more likely scenario with plywood roof sheathing

than a load that induces a pure tensile stress. Our results confirm that WML is the most seriously affected

‘mechanical property (Tables A8, A9). Three other advantages of reporting WML for roof sheathing panels

are as follows:
1. WML may be a legitimate predictor of a panel’s

ability to deform to accommodate normal irregularities in truss height,1

2. changes in WML tend to be of a greater magnitude

than changes in MOR and thus WML changes are

easier to identify, and

3. changes in WML seem to act as a precursor of

future trends in MOR.

Cutting Pattern

The ramifications of treatment gradients were considered during the evolution of the protocol. To eliminate

the undesirable influence of a treatment gradient associated with differential chemical absorption along the

edges of a treated plywood sheet, all material within

2 in. (50 mm) of the long edges of the original 4- by

8-ft (1.22- by 2.44-m) plywood panels should be eliminated.

Distributional Effects

Wood properties exhibit variability about some average

tendency. Thermal exposure could affect both the

average and the distributional form of those properties.

Thus, although the experiments were primarily

designed to address mean effects, the distributional

characteristics of the mechanical properties studied

were also considered and evaluated.

Mean MOR and MOE values over the duration of

exposure were compared to fifth percentile estimates

of the MOR and MOE distributions. Distributions

were fitted for normal or two-parameter Weibull

distributions. Nonparametric fits were also obtained by

using a rank-order technique for the 20 specimens. As

might be expected when generating lower tail estimates

based on sample sizes of 20 specimens, the variability of

the fifth percentile estimates for any given mechanical

property was great. After careful examination, no

single fit or estimation technique could be considered

as describing the central tendency (mean) or lower tails

any better than any other technique. Most importantly,

no apparent differences were discernible between the

trends in the lower tails and mean trends. Thus, all

further evaluations were based on the more stable

estimates of mean.

Increasing the number of specimens in each bending

group from 20 to 30 specimens might be beneficial.

1

 Zylkowski, S.C. 1990. Personal communication

(December 7, 1990), with Steven C. Zylkowski,

Manager, Research and Development, American

Plywood Association, Tacoma, WA.

Although this larger sample size would not greatly

increase the accuracy of estimates of mean trends,

it would allow investigators to place a 75-percent

confidence limit on the nonparametric estimate of the

fifth percentile (ASTM 1990a). Specifically, this change

would provide an ability to monitor distributional

effects on bending properties.

Predictability

Strength loss can be predicted from exposure time

at elevated temperature, based on earlier work by

Millett and coworkers (Mitchell and others 1953, 1967,

1972). Accordingly, the strength data were evaluated

using kinetic theory by deriving an Arrhenius-type

relationship to predict thermally induced strength

loss. Previous work by Stamm (1964) also supports

this approach. All data generated from the protocol

suggest a linear relationship between mechanical

properties and duration of exposure to elevated

temperatures. Prediction of a first-order chemical

or thermal effect requires an estimate of the rate of

change. For cellulosic materials, this rate constant

can represent a. change in concentration of a chemical

constituent or a change in a measured property, such

as strength (Millett and Gerhards 1972). The rate

constant is a function of temperature. Changes in the

adjusted standard rate constant k2

 can be related to

changes in temperature by an Arrhenius equation:

(3)

where

A

Ea

R

T

is adjusted standard rate constant at 67 percent

RH,

pre-exponential factor,

activation energy,

gas constant, and

temperature (K).

When the Arrhenius equation is expressed in terms of

natural logrithims,

(4)

The activation energy Ea and pre-exponential factor

A can then be determined graphically from a plot

of the logarithm of adjusted standard rate constant

as a function of reciprocal temperature or by linear

regression. Once these two parameters are known, the

rate constant at other temperatures can be predicted.

9
Figure 10—Arrhenius plot of reaction rate

(In(-k2 ) versus 1/T). The value k2 is the

standard rate of thermal degrade derived in

Equation (2) at each temperature (TK ).

The application of an Arrhenius-kinetic approach to

modeling chemical rate constants for wood and woodbased materials is more completely discussed by Millett

and Gerhards (1972) and Stamm (1964).

Using the initial data generated at three temperatures

in the test protocol, we applied Arrhenius theory to

predict the rate constants associated with the change

in some mechanical property to other temperatures.

Examples of this relationship are shown for MOR in

Figure 10. Note that both the untreated and MAPtreated materials could generally be considered as

undergoing a linear rate of change. Also note that

when using this first-order (linear) analysis, the rates

of change between treated and untreated materials

are somewhat similar. These results tend to support

a linear cumulative damage model. Variations of such

models have been theorized (Stamm 1964, Gerhards

1979, Caulfield 1985). These results also substantiate

the work of Millett and Gerhards (1972) in that

kinetic-based models can successfully predict thermally

induced mechanical property degradation for wood

materials. Although our current ability to predict

thermally induced degradation is elementary, the

long-range goal of developing an ability to predict

serviceability may be obtainable.

Further Considerations

The processing conditions employed in this study

were closely matched to the conditions imposed

in commercial treating and redrying schedules.

Nevertheless, the magnitude of strength loss in our

experiments was less than that of some field experience.

This might have been due to the quality of the

chemicals used or to processing conditions. Our recent

10

work with small clear specimens of Southern Pine

pointed to the importance of chemical formulation

and post-treatment redrying in accelerating the

subsequent susceptibility of the wood to thermally

induced strength loss (LeVan and others 1990).

Based on the unacceptable performance of some

FR treatments, FR-treated materials should not

be considered interchangeable commodity items.

Architects and engineers must be aware of the fact

that different FR treatments may behave differently

under thermal exposure. An approved third-party

inspection agency should certify compliance with all

existing standards, processing methods, and testing

procedures. Such procedures include compliance with

AWPA C20/C27 and ASTM E-84, with chemical

formulation and post-treatment drying temperatures

employed in developing the original certification,

and with performance data from the new ASTM test

method (1991) for assessing ongoing thermally induced

strength degradation.

Because of the possibility that high moisture contents

may interact with FR-treated material exposed to

elevated temperatures, AWPA Standards C27 for

FR-treated plywood and C20 for FR-treated lumber

recognize that all FR-treated material must be dried

after treatment to ≤15 percent moisture content

for plywood and ≤19 percent moisture content for

lumber. These moisture content levels are required

for proper structural performance, and especially for

avoiding fastener corrosion. Thus, adequate precautions

must also be used during shipping and at the job site

to prevent moisture from rewetting the FR-treated

material. If FR-treated material is exposed to rain, it

should be allowed to dry thoroughly before it is covered

with roofing material.

Conclusions

Using the new test protocol (ASTM 1991), thermally

induced strength losses were evident in laboratory

simulations within a reasonably short period. The

environmental conditions used in the laboratory

activated chemical reactions that affected mechanical

properties in a manner similar to that experienced

in the field. The results of this test protocol seem to

present the engineering community with a reasonable

comparative procedure for assessing the potential

of commercial FR treatments to cause thermally

induced in-service strength loss. Thus, results from

this protocol can be used to begin the process of

substantiating acceptable field performance for new

or existing FR-treatments before they are used in

service conditions with periodic or sustained exposure

to elevated temperatures.
References

APA. 1989a. Fire-retardant-treated plywood roof

sheathing: Field failures. American Plywood Association,

Tacoma, WA.

APA. 1989b. Fire-retardant-treated plywood roof

sheathing: General information. American Plywood

Association, Tacoma, WA.

ASTM. 1990a. ASTM D2915-90: Standard practice for

evaluating allowable properties for grades of structural

lumber. Annual Book of ASTM Standards V4.09.

American Society for Testing and Materials, Philadelphia,

PA.

ASTM. 1990b. ASTM D3043-90: Standard methods

for testing structural panels in flexure. Annual Book of

ASTM Standards V4.09. American Society for Testing and

Materials, Philadelphia, PA.

ASTM. 1990c. ASTM D3500-90: Standard methods

for testing plywood in tension. Annual Book of ASTM

Standards V4.09. American Society for Testing and

Materials, Philadelphia, PA.

ASTM. 1991. ASTM ES-XX-91: Standard test method

for evaluating the mechanical properties of fire retardant treated softwood plywood exposed to elevated temperatures. American Society for Testing and Materials,

Philadelphia, PA.

AWPA. 1990a. C20: Fire-retardant treated lumber.

AWPA Book of Standards. American Wood Preservers’

Association, Stevensville, MD.

AWPA. 1990b. C27: Fire-retardant treated plywood.

AWPA Book of Standards. American Wood Preservers’

Association, Stevensville, MD.

Caulfield, D.F. 1985. A chemical kinetics approach to

the duration-of-load problem in wood. Wood and Fiber

Science. 17(4): 504-521.

Forest Products Laboratory. 1987. Wood handbook:

Wood as an engineering material. Agric. Handb. 72.

(Rev.) Washington, DC: U.S. Department of Agriculture.

466 p.

Gerhards, C.C. 1970. Effect of fire-retardant treatment

on bending strength of wood. Res. Pap. FPL-145.

Madison WI: U.S. Department of Agriculture, Forest

Service, Forest Products Laboratory.

Gerhards, C.C. 1979. Time-related effects of loading on

wood strength: A linear cumulative damage theory. Wood

Science. 11(3): 139-144.

Heyer, O.C. 1963. Study of temperature in wood parts

of houses throughout the United States. Res. Note. FPL012. Madison WI: U.S. Department of Agriculture, Forest

Service, Forest Products Laboratory.

LeVan, S.L. 1984. Chemistry of fire retardancy. In:

Rowell, R.M., ed. Chemistry of solid wood. Advances

in chemistry series 207. Washington, DC: American

Chemistry Society: 531-574. Chapter 14.

LeVan, S.L.; Collet, M. 1989. Choosing and applying

fire retardant-treated plywood and lumber for roof designs.

Gen. Tech. Rep. GTR-62. Madison WI: U.S. Department

of Agriculture, Forest Service, Forest Products Laboratory.

LeVan, S.L.; Ross, R-J.; Winandy, J.E. 1990. Effect of

fire-retardant chemicals on the bending properties of wood

at elevated temperature. Res. Pap. FPL-498. Madison

WI: U.S. Department of Agriculture, Forest Service, Forest

Products Laboratory.

LeVan, S.L.; Winandy, J.E. 1990. Effects of fire

retardant treatments on wood strength: A review. Wood

and Fiber Science. 22(1): 113-131.

McNatt, J.D. 1984. Static bending properties of structural wood-based panels: Large-panel versus small-specimen

test. Forest Products Journal.

34(4): 50-54.

McNatt, J.D.; Wellwood, R.W.; Bach, L. 1990.

Relationships between small-specimen and large panel

bending tests on structural wood-based panels. Forest

Products Journal. 40(9): 10-16.

Millett, M.A.; Gerhards, C.C. 1972. Accelerated aging:

Residual weight and flexural properties of wood heated in

air at 115 to 175°C. Wood Science. 4(4): 193-201.

Millett, M.A.; Western, L.J.; and Booth, J.J. 1967.

Accelerated aging of cellulosic material: Design and

application of a heating chamber. Tappi Journal. 50(11):

74-80.

Mitchell, R.L.; Seborg, R.M.; Millett, M.A. 1953.

Effect of heat on the properties and chemical composition

of Douglas-fir wood and its major components. Forest

Products Journal. 3(11): 38-42, 72-73.

NFPA. 1986. National Design Specification for Wood

Construction. National Forest Products Association,

Washington, DC.

Snedecor, G.W.; Cochran, W.G. 1967. Statistical

Methods. 6th ed. Ames, IA: Iowa State University Press.

Stamm, A.J. 1964. Wood and Cellulose Science. New

York: Ronald Press: 308-311.

U.S. Department of Commerce. 1983. U.S. Product

Standard PS 1-83 for construction & industrial plywood

with typical APA grade-trademarks. Reprinted by

American Plywood Association, Tacoma, WA.

Winandy, J.E. 1990. Fire-retardant treated wood: Effects

of elevated temperatures and guidelines for design. Wood

Design Focus. 1(2): 8-10.

Winandy, J.E.; LeVan, S.L.; Schaffer, E.L.; Lee,

P.W. 1988. Effect of fire-retardant treatment and

redrying on the mechanical properties of Douglas-fir and

aspen plywood. Res. Pap. FPL-485. Madison WI: U.S.

Department of Agriculture, Forest Service, Forest Products

Laboratory.

Appendix A. Results of

Experimental Tests

The tables in the Appendix (Tables A1-A6, A8-A9)

show the effects of extended exposure at elevated temperature on mechanical properties of untreated and

fire-retardant-treated plywood. Table A7 shows moisture content and specific gravity values of untreated

and treated plywood for each exposure combination

(treatment-temperature-humidity-exposure duration).

11
Table A1—Modulus of elasticity and stiffness of untreated material exposed to elevated temperatures

Modulus of elasticity Stiffness

(×106 lb/in2 (GPa))

Temper-

(×103 lb·in2 (N·m2))

ature RH

(°F (°C)) (%) Designa

Exposure Standard Standard Sample

(days) Mean deviation Mean deviation size

80 (27) 65 Random

Blocked

130 (54) 73 Random

150 (65) 76 Blocked

170 (77) 79 Random

170 (77) 79 Blocked

170 (77) 50 Random

0

0

14

28

56

84

116

140

7

14

28

56

84

112

7

14

21

35

49

63

7

14

21

35

49

63

7

14

21

35

49

63

1.557 (10.74) 0.280 (1.93) 65.66 (188.43) 13.51 (38.77) 19

1.381 ( 9.52) 0.308 (2.12) 67.31 (193.17) 15.32 (43.96) 21

1.496 (10.31) 0.207 (1.43) 66.31 (190.30) 16.16 (46.38) 20

1.575 (10.86) 0.242 (1.67) 67.46 (193.60) 10.31 (29.59) 20

1.626 (11.21) 0.351 (2.42) 69.82 (200.37) 15.20 (43.62) 19

1.540 (10.62) 0.317 (2.19) 65.98 (189.35) 14.55 (41.76) 20

1.692 (11.67) 0.396 (2.73) 67.08 (192.51) 15.88 (45.57) 20

1.781 (12.28) 0.268 (1.85) 69.58 (199.68) 12.25 (35.16) 20

1.401 ( 9.66) 0.220 (1.52) 63.73 (182.89) 10.85 (31.14) 21

1.243 ( 8.57) 0.263 (1.81) 56.72 (162.78) 13.40 (38.46) 21

1.270 ( 8.76) 0.292 (2.01) 59.02 (169.38) 13.89 (39.86) 20

1.340 ( 9.24) 0.239 (1.65) 59.18 (169.84) 9.36 (26.86) 21

1.298 ( 8.95) 0.269 (1.85) 58.88 (168.97) 13.36 (38.34) 21

1.396 ( 9.63) 0.276 (1.90) 63.06 (180.97) 11.78 (33.81) 21

1.449 ( 9.99) 0.286 (1.97) 65.10 (186.82) 12.87 (36.93) 20

1.515 (10.45) 0.261 (1.80) 65.74 (188.66) 12.05 (34.58) 20

1.516 (10.45) 0.311 (2.14) 66.85 (191.85) 13.34 (38.28) 19

1.458 (10.05) 0.154 (1.06) 64.82 (186.02) 7.34 (21.06) 20

1.280 ( 8.83) 0.242 (1.67) 59.47 (170.67) 11.34 (32.54) 20

1.268 ( 8.74) 0.281 (1.94) 58.95 (169.18) 13.50 (38.74) 20

1.353 ( 9.33) 0.434 (2.99) 62.21 (178.53) 18.00 (51.66) 21

1.322 ( 9.12) 0.217 (1.50) 62.04 (178.04) 10.61 (30.45) 21

1.234 ( 8.51) 0.227 (1.57) 60.82 (174.54) 11.08 (31.80) 21

1.206 ( 8.32) 0.287 (1.98) 57.31 (164.47) 13.37 (38.37) 21

1.282 ( 8.84) 0.242 (1.67) 58.99 (169.29) 10.64 (30.53) 21

1.380 ( 9.52) 0.279 (1.92) 62.38 (179.02) 12.93 (37.11) 21

1.558 (10.74) 0.301 (2.08) 65.08 (186.77) 12.01 (34.47) 20

1.662 (11.46) 0.265 (1.83) 64.94 (186.36) 11.08 (31.80) 20

1.583 (10.91) 0.217 (1.50) 66.83 (191.79) 10.06 (28.87) 20

1.502 (10.36) 0.317 (2.19) 62.55 (179.51) 13.09 (37.56) 20

1.430 ( 9.86) 0.348 (2.40) 58.01 (166.48) 13.30 (38.17) 20

1.625 (11.20) 0.314 (2.17) 67.89 (194.83) 12.98 (37.25) 20

a

Specimens allotted to blocked design were tested by Hickson Corporation.

All other specimens were allotted to random design and tested at the FPL

12
Table AP—Modulus of elasticity and stiffness of treated material exposed to elevated temperatures

Modulus of elasticity Stiffness

(×106 lb/in2 (GPa)) (×103

 lb·in2 (N·m2))

Temperature RH

(°F (°C)) (%) Designa

Exposure Standard Standard Sample

(days) Mean deviation Mean deviation size

80 (27)

130 (54)

65 Random

Blocked

73 Random

150 (65) 76 Blocked

170 (77)

170 (77)

170 (77)

79 Random

79 Blocked

50 Random

0

0

14

28

56

84

116

140

7

14

28

56

84

112

1.468 (10.12) 0.268 (1.85) 67.53 (193.80)

1.208 ( 8.33) 0.271 (1.87) 63.01 (180.83)

1.348 ( 9.29) 0.229 (1.58) 62.93 (180.60)

1.382 ( 9.53) 0.256 (1.77) 64.46 (184.99)

1.498 (10.33) 0.272 (1.88) 71.71 (205.79)

1.560 (10.76) 0.192 (1.32) 74.06 (212.54)

1.698 (11.71) 0.247 (1.70) 76.02 (218.16)

1.617 (11.15) 0.336 (2.32) 69.80 (200.31)

1.152 ( 7.94) 0.212 (1.46) 56.82 (163.06)

1.204 ( 8.30) 0.231 (1.59) 60.19 (172.73)

1.266 ( 8.73) 0.264 (1.82) 61.47 (176.41)

1.236 ( 8.52) 0.228 (1.57) 59.48 (170.70)

1.222 ( 8.43) 0.232 (1.60) 60.15 (172.62)

1.231 ( 8.49) 0.253 (1.74) 60.14 (172.59)

7

14

21

35

49

63

1.326 ( 9.14) 0.258 (1.78) 63.19 (181.34)

1.258 ( 8.67) 0.229 (1.58) 58.13 (166.82)

9.11) 0.238 (1.64) 62.90 (180.51)

9.01) 0.265 (1.83) 62.50 (179.36)

8.60) 0.224 (1.54) 58.91 (169.06)

9.07) 0.255 (1.76) 62.58 (179.59)

1.321 (

1.307 (

1.247 (

1.315 (

7 1.242 ( 8.56) 0.244 (1.68) 62.80 (180.22)

14 1.317 ( 9.08) 0.203 (1.40) 65.40 (187.68)

21 1.273 ( 8.78) 0.263 (1.81) 65.16 (187.00)

35 1.126 ( 7.76) 0.249 (1.72) 56.52 (162.20)

49 1.195 ( 8.24) 0.274 (1.89) 59.49 (170.72)

63 1.192 ( 8.22) 0.227 (1.57) 56.58 (162.37)

7

14

21

35

49

63

9.90) 0.338 (2.33) 65.24 (187.23)

10.04) 0.302 (2.08) 63.89 (183.35)

10.09) 0.275 (1.90) 66.27 (190.18)

10.15) 0.254 (1.75) 66.33 (190.35)

1.489 (10.27) 0.367 (2.53) 66.26 (190.15)

1.546 (10.66) 0.236 (1.63) 70.72 (202.95)

1.436 (

1.456 (

1.464 (

1.472 (

12.09 (34.70) 19

15.20 (43.62) 21

10.55 (30.28) 20

11.41 (32.74) 20

12.25 (35.16) 20

9.29 (26.66) 20

10.87 (31.19) 19

14.05 (40.32) 20

10.17 (29.19) 21

11.57 (33.20) 21

13.39 (38.43) 21

11.00 (31.57) 21

12.45 (35.73) 21

13.24 (38.00) 21

11.98 (34.38) 19

9.86 (28.30) 20

11.23 (32.23) 20

13.16 (37.77) 20

10.71 (30.74) 20

12.10 (34.72) 20

11.93 (34.24) 21

9.07 (26.03) 21

13.99 (40.15) 21

11.60 (33.29) 21

14.98 (42.99) 21

11.54 (33.12) 21

16.08 (46.15) 20

13.98 (40.12) 20

12.87 (36.93) 20

12.29 (35.27) 20

15.82 (45.40) 20

11.99 (34.41) 20

a

Specimens allotted to blocked design were tested by Hickson Corporation.

All other specimens were allotted to random design and tested at the FPL.

13
Table A3—Modulus of rupture and maximum bending moment of untreated material

exposed to elevated temperatures

Modulus of rupture Maximum bending moment

(×103 lb/in2 (MPa)) (× 103

 lb·in (N·m))

Temperature RH

(°F (°C)) (%) Designa

Exposure Standard Standard

(days) Mean deviation Mean deviation

80 (27) 65 Random

Blocked

130 (54) 73 Random

150 (65) 76 Blocked

170 (77) 79 Random

170 (77) 79 Blocked

170 (77) 50 Random

0

0

14

28

56

84

116

140

7

14

28

56

84

112

7

14

21

35

49

63

7

14

21

35

49

63

7

14

21

35

49

63

10.81 (74.5) 2.14 (14.8) 1.65 (186.4) 0.34 (38.4)

8.50 (58.6) 1.30 ( 9.0) 1.42 (160.4) 0.20 (22.6)

10.35 (71.4) 1.34 ( 9.2) 1.58 (178.5) 0.20 (22.6)

10.39 (71.6) 1.94 (13.4) 1.60 (180.8) 0.30 (33.9)

9.88 (68.1) 1.81 (12.5) 1.53 (172.9) 0.28 (31.6)

9.75 (67.2) 1.63 (11.2) 1.51 (170.6) 0.27 (30.5)

10.47 (72.2) 2.21 (15.2) 1.53 (172.9) 0.32 (36.2)

10.52 (72.5) 1.58 (10.9) 1.53 (172.9) 0.26 (29.4)

9.52 (65.6) 1.20 ( 8.3) 1.53 (172.9) 0.23 (26.0)

9.32 (64.3) 1.74 (12.0) 1.50 (169.5) 0.31 (35.0)

9.08 (62.6) 1.49 (10.3) 1.48 (167.2) 0.24 (27.1)

9.66 (66.6) 1.66 (11.4) 1.52 (171.7) 0.25 (28.2)

8.86 (61.1) 1.53 (10.5) 1.42 (160.4) 0.26 (29.4)

9.64 (66.5) 1.66 (11.4) 1.54 (174.0) 0.26 (29.4)

9.40 (64.8) 1.74 (12.0) 1.50 (169.5) 0.27 (30.5)

9.86 (68.0) 1.46 (10.1) 1.53 (172.9) 0.23 (26.0)

9.06 (62.5) 1.45 (10.0) 1.43 (161.6) 0.22 (24.9)

8.81 (60.7) 1.14 ( 7.9) 1.40 (158.2) 0.20 (22.6)

7.86 (54.2) 1.12 ( 7.7) 1.28 (144.6) 0.19 (21.5)

7.45 (51.4) 1.11 ( 7.7) 1.21 (136.7) 0.17 (19.2)

8.22 (56.7) 1.58 (10.9) 1.34 (151.4) 0.28 (31.6)

8.10 (55.8) 1.68 (11.6) 1.33 (150.3) 0.29 (32.8)

7.71 (53.2) 1.35 ( 9.3) 1.31 (148.0) 0.23 (26.0)

7.69 (53.0) 1.80 (12.4) 1.27 (143.5) 0.30 (33.9)

8.24 (56.8) 1.35 ( 9.3) 1.34 0.21

8.98 (61.9) 1.70 (11.7)

(151.4) (23.7)

1.44 (162.7) 0.29 (32.8)

10.19 (70.3) 2.19 (15.1) 1.55 (175.1) 0.32 (36.2)

10.69 (73.7) 1.81 (12.5) 1.55 (175.1) 0.27 (30.5)

10.14 (69.9) 12.8 ( 8.8) 1.55 (175.1) 0.20 (22.6)

10.22 (70.5) 1.83 (12.6) 1.55 (175.1) 0.27 (30.5)

9.10 (62.7) 2.31 (15.9) 1.36 (153.7) 0.34 (38.4)

9.98 (68.8) 2.13 (14.7) 1.51 (170.6) 0.32 (36.2)

a

Specimens allotted to blocked design were tested by Hickson Corporation.

All other specimens were allotted to random design and tested at the FPL.

Sample size was the same as that for experiments on MOE and stiffness (see Table Al).

14
Table A4—Modulus of rupture and maximum bending moment of treated material

exposed to elevated temperatures

Modulus of rupture Maximum bending moment

(×103 lb/in2 (MPa)) (lb·in (N·m))

Temperature RH

(°F (°C)) (%) Designa

Exposure Standard Standard

(days) Mean deviation Mean deviation

80 (27)

130 (54)

150 (65)

170 (77)

170 (77)

170 (77)

65 Random

Blocked

73 Random

76 Blocked

79 Random

79 Blocked

50 Random

0

0

14

28

56

84

116

140

7

14

28

56

84

112

7

14

21

35

49

63

7

14

21

35

49

63

7

14

21

35

49

63

8.68 (59.8) 1.65 (11.4) 1.41 (159.3) 0.26 (29.4)

7.14 (49.2) 1.09 ( 7.5) 1.25 (141.2) 0.20 (22.6)

8.32 (57.4) 1.45 (10.0) 1.36 (153.7) 0.24 (27.1)

7.99 (55.1) 1.29 ( 8.9) 1.31 (148.0) 0.21 (23.7)

8.33 (57.4) 1.32 ( 9.1) 1.39 (157.1) 0.21 (23.7)

7.64 (52.7) 1.23 ( 8.5) 1.27 (143.5) 0.21 (23.7)

7.94 (54.7) 1.37 ( 9.4) 1.27 (143.5) 0.24 (27.1)

7.73 (53.3) 1.81 (12.5) 1.20 (135.6) 0.29 (32.8)

7.48 (51.6) 0.90 ( 6.2) 1.27 (143.5) 0.15 (16.9)

7.29 (50.3) 1.04 ( 7.2) 1.25 (141.2) 0.18 (20.3)

7.28 (50.2) 1.48 (10.2) 1.22 (137.8) 0.25 (28.2)

6.10 (42.1) 1.13 ( 7.8) 1.02 (115.2) 0.19 (21.5)

5.69 (39.2) 1.08 ( 7.4) 0.96 (108.5) 0.19 (21.5)

4.94 (34.1) 1.10 ( 7.6) 0.83 ( 93.8) 0.20 (22.6)

6.79 (46.8) 1.36 ( 9.4) 1.13 (127.7) 0.24 (27.1)

5.78 (39.9) 1.04 ( 7.2) 0.94 (106.2) 0.16 (18.1)

5.91 (40.7) 1.54 (10.6) 0.98 (110.7) 0.25 (28.2)

5.48 (37.8) 1.36 ( 9.4) 0.91 (102.8) 0.23 (26.0)

4.23 (29.2) 0.92 ( 6.3) 0.70 ( 79.1) 0.15 (16.9)

4.54 (31.3) 1.12 ( 7.7) 0.75 ( 84.7) 0.19 (21.5)

6.60 (45.5) 0.86 ( 5.9) 1.14 (128.8) 0.14 (15.8)

5.90 (40.7) 1.37 ( 9.4) 1.00 (113.0) 0.22 (24.9)

5.43 (37.4) 1.13 ( 7.8) 0.94 (106.2) 0.20 (22.6)

4.46 (30.8) 0.76 ( 5.2) 0.77 ( 87.0) 0.13 (14.7)

3.92 (27.0) 1.29 ( 8.9) 0.66 ( 74.6) 0.22 (24.9)

3.50 (24.1) 0.99 ( 6.8) 0.58 ( 65.5) 0.16 (18.1)

8.06 (55.6) 1.43 ( 9.9) 1.29 (145.8) 0.24 (27.1)

6.57 (45.3) 1.54 (10.6) 1.03 (116.4) 0.25 (28.2)

6.20 (42.7) 1.37 ( 9.4) 0.99 (111.9) 0.20 (22.6)

6.04 (41.6) 1.17 ( 8.1) 0.97 (109.6) 0.20 (22.6)

5.85 (40.3) 1.74 (12.0) 0.93 (105.1) 0.27 (30.5)

5.45 (37.6) 1.21 ( 8.3) 0.87 ( 98.3) 0.18 (20.3)

“Specimens allotted to blocked design were tested by Hickson Corporation.

All other specimens were allotted to random design and tested at the FPL.

Sample size was the same as that for experiments on MOE and stiffness (see Table A2).

15
Table A5—Ultimate tensile stress of untreated material exposed

to elevated temperatures

Ultimate tensile stress

(× 103 lb/in2 (MPa))

Temperature RH

(°F(°C)) (%) Designa

Exposure Standard Sample

(days) Mean deviation size

80 (27) 65 Random

Blocked

130 (54) 73 Random

150 (65) 76 Blocked

170 (77) 79 Random

170 (77) 79 Blocked

0

0

14

28

56

84

116

140

7

14

28

56

84

112

7

14

21

35

49

63

7

14

21

35

49

63

6.67 (46.0) 1.41 ( 9.7) 20

5.69 (39.2) 1.04 ( 7.2) 20

5.66 (39.0) 1.49 (10.3) 20

5.76 (39.7) 1.06 ( 7.3) 20

5.80 (40.0) 1.37 ( 9.4) 20

6.51 (44.9) 1.19 ( 8.2) 20

5.93 (40.9) 1.03 ( 7.1) 20

6.13 (42.3) 1.96 (13.5) 20

5.46 (37.6) 1.15 ( 7.9) 20

6.12 (42.2) 1.24 ( 8.5) 20

5.61 (38.7) 1.14 ( 7.9) 20

5.67 (39.1) 1.03 ( 7.1) 20

5.70 (39.3) 1.12 ( 7.7) 20

5.67 (39.1) 1.03 ( 7.1) 20

5.78 (39.9) 1.17 ( 8.1) 20

5.96 (41.1) 1.20 ( 8.3) 20

5.83 (40.2) 1.85 (12.8) 20

5.52 (38.1) 1.25 ( 8.6) 20

5.63 (38.8) 1.23 ( 8.5) 20

4.88 (33.6) 0.84 ( 5.8) 20

5.31 (36.6) 1.01 ( 7.0) 20

5.39 (37.2) 1.29 ( 8.9) 20

5.13 (35.4) 0.98 ( 6.8) 20

4.90 (33.8) 1.20 ( 8.3) 20

5.15 (35.5) 1.07 ( 7.4) 20

4.80 (33.1) 1.36 ( 9.4) 20

“Specimens allotted to blocked design were tested by Hickson Corporation.

All other specimens were allotted to random design and tested at the FPL.

16
Table A6—Ultimate tensile stress of treated material exposed to

elevated temperatures

Ultimate tensile stress

(×103 lb/in2 (MPa))

Temperature RH Exposure Standard Sample

(°F(°C)) (%) Designa

(days) Mean deviation size

80 (27) 65 Random

Blocked

130 (54) 73 Random

150 (65) 76 Blocked

170 (77) 79 Random

170 (77) 79 Blocked

0

0

14

28

56

84

116

140

7

14

28

56

84

112

7

14

21

35

49

63

7

14

21

35

49

63

4.85 (33.4) 0.74 (5.1)

5.13 (35.4) 1.39 (9.6)

5.39 (37.2) 0.89 (6.1)

4.85 (33.4) 0.90 (6.2)

4.82 (33.2) 0.80 (5.5)

4.35 (30.0) 1.09 (7.5)

4.32 (29.8) 1.01 (7.0)

4.15 (28.6) 0.63 (4.3)

4.35 (30.0) 1.04 (7.2)

4.67 (32.2) 0.97 (6.7)

4.24 (29.2) 1.20 (8.3)

3.36 (23.2) 0.96 (6.6)

2.83 (19.5) 0.55 (3.8)

2.74 (18.9) 0.63 (4.3)

3.77 (26.0) 0.95 (6.6)

3.60 (24.8) 0.63 (4.3)

2.90 (20.0) 0.66 (4.6)

2.32 (16.0) 0.55 (3.8)

2.01 (13.9) 0.47 (3.2)

1.89 (13.0) 0.38 (2.6)

4.54 (31.3) 1.10 (7.6)

3.81 (26.3) 0.68 (4.7)

2.82 (19.4) 0.87 (6.0)

2.09 (14.4) 0.55 (3.8)

1.94 (13.4) 0.54 (3.7)

1.45 (10.0) 0.48 (3.3)

20

20

20

20

20

20

20

20

20

20

20

20

20

20

18

20

20

20

19

20

20

20

20

20

20

20

a

Specimens allotted to blocked design were tested by Hickson Corporation.

All other specimens were allotted to random design and tested at the FPL.

17
Table A7—Moisture content and specific gravity of plywood

exposed to elevated temperatures

Groupa

Moisture content

(%) Specific gravity

Temperature RH Time Standard Standard Test

(°F (°C)) (%) (days) Mean deviation Mean deviation siteb

Control

Untreated

Treated

Untreated

Treated 130 (54)

Untreated 150 (65)

Treated 150 (65)

Untreated 170 (77)

80 (27)

80 (27)

130 (54)

65

65

73

73

75

75

79

0

0

9.7 0.5 0.55

11.3 0.9 0.58

12.0 0.2 0.57

13.1 0.2 0.60

14 9.8 0.3 0.52

28 9.7 0.3 0.55

56 9.7 0.2 0.54

84 9.6 0.3 0.54

116 9.7 0.2 0.54

140 9.7 0.2 0.54

14 11.7 0.4 0.55

28 11.7 0.1 0.56

56 11.2 0.1 0.57

84 11.0 0.2 0.57

116 11.2 0.1 0.58

140 10.9 0.1 0.57

7 12.8 0.4 0.57

14 12.9 0.3 0.56

28 12.6 0.6 0.55

56 11.9 0.3 0.57

84 12.0 0.4 0.55

112 11.5 0.4 0.57

7 14.5 0.4 0.59

14 14.8 0.4 0.58

28 14.3 0.3 0.59

56 13.5 0.6 0.58

84 13.5 0.3 0.57

112 13.1 0.5 0.56

7 12.6 0.2 0.55

14 12.2 0.3 0.54

21 12.8 0.3 0.53

35 13.1 0.5 0.53

49 13.4 0.3 0.54

63 13.9 0.3 0.51

0.03

0.04

0.02

0.02

0.02

0.02

0.03

0.03

0.02

0.03

0.03

0.03

0.02

0.02

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.02

0.02

0.02

0.03

0.02

0.03

0.03

0.02

0.03

0.03

0.03

0.02

F

H

F

H

F

F

F

F

F

F

F

F

F

F

F

F

H

H

H

H

H

H

H

H

H

H

H

H

F

F

F

F

F

F

18
Table A7—Moisture content and specific gravity of plywood

exposed to elevated temperatures-concluded

Groupa

Moisture content

(%) Specific gravaity

Temperature RH Time Standard Standard Test

(°F (°C)) (%) (days) Mean deviation Mean deviation siteb

Treated 170 (77)

Untreated 170 (77)

Treated 170 (77)

Untreated 170 (77)

Treated 170 (77)

79 7 13.8 0.3 0.56

14 13.4 0.3 0.56

21 13.8 0.3 0.56

35 14.3 0.2 0.55

49 14.7 0.3 0.54

63 14.5 0.3 0.54

79 7 14.2 1.1 0.56

14 13.9 0.4 0.55

21 13.6 0.5 0.54

35 13.4 0.4 0.54

49 12.1 0.2 0.55

63 11.9 0.5 0.56

79 7 15.6 0.3 0.57

14 15.5 0.5 0.58

21 15.5 0.9 0.57

35 15.5 0.6 0.56

49 13.8 0.4 0.57

63 14.0 1.0 0.56

50 7 9.6

14 9.4

21 9.3

35 9.3

49 9.5

63 9.5

0.3

0.2

0.2

0.2

0.2

0.1

0.2

0.2

0.3

0.2

0.2

0.2

0.53

0.53

0.56

0.55

0.54

0.58

50 7 10.5

14 10.1

21 10.1

35 9.8

49 10.0

63 9.1

0.58

0.57

0.58

0.59

0.58

0.55

0.04

0.02

0.02

0.03

0.03

0.03

0.02

0.02

0.03

0.02

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.02

0.02

0.03

0.03

0.03

0.02

0.03

0.03

0.03

0.03

F

F

F

F

F

F

H

H

H

H

H

H

H

H

H

H

H

H

F

F

F

F

F

F

F

F

F

F

F

F

aTreated with monoammonium phosphate, 3.5 lb/ft3

 (56 kg/m3).

b

Specimens allotted to blocked design were tested by Hickson Corporation (H).

All other specimens were allotted to random design and tested at the FPL (F)

19
Table A8—Work to maximum load of untreated material exposed

to elevated temperatures

Work to maximum load

(in·lb/in3

(kJ/m3))

Temperature RH Exposure Standard

(°F(°C)) (%) Designa

(days) Mean deviation

80 (27) 65 Random

Blocked

130(54) 73 Random

150 (65) 76 Blocked

170 (77) 79 Random

170 (77) 79 Blocked

170 (77) 50 Random

0

0

6.66 (45.9)

—

2.90 (20.0)

—

14 6.91 (47.6) 1.93 (13.3)

28 5.97 (41.2) 2.58 (17.8)

56 5.07 (35.0) 2.23 (15.4)

84 6.20 (42.7) 2.32 (16.0)

116 4.26 (29.4) 2.03 (14.0)

140 4.56 (31.4) 1.20 ( 8.3)

7

14

28

56

84

112

—

—

—

—

—

—

—

—

—

—

—

—

7 6.43 (44.3) 2.59 (17.9)

14 5.96 (41.1) 1.85 (12.8)

21 5.30 (36.5) 2.01 (13.9)

35 5.13 (35.4) 1.79 (12.3)

49 5.53 (38.1) 2.17 (15.0)

63 4.71 (32.5) 1.72 (11.9)

7

14

21

35

49

63

— —

— —

— —

—

—

—

—

—

—

7 6.50 (44.8) 3.17 (21.9)

14 6.58 (45.4) 2.65 (18.3)

21 5.51 (38.0) 2.18 (15.0)

35 5.80 (40.0) 2.00 (13.8)

49 5.03 (34.7) 2.71 (18.7)

63 5.48 (37.8) 3.24 (22.3)

a

Specimens allotted to blocked design were tested by

Hickson Corporation.

All other specimens were allotted to random design and

tested at the FPL.

20
Table A9—Work to maximum load of treated material exposed

to elevated temperatures

Work to maximum load

(in·lb/in3 (kJ/m3))

Temperature RH Exposure Standard

(°F(°C)) (%) Designa

(days) Mean deviation

80(27)

130 (54)

65 Random

Blocked

73 Random

150 (65) 76 Blocked

170 (77)

170 (77)

170 (77)

79 Random

79 Blocked

50 Random

0

0

4.66 (32.1) 1.61 (11.1)

— —

14

28

56

84

116

140

7

14

28

56

84

112

4.99 (34.4) 2.32 (16.0)

4.27 (29.4) 1.75 (12.1)

3.83 (26.4) 1.49 (10.3)

3.37 (23.2) 1.51 (10.4)

3.09 (21.3) 1.55 (10.7)

2.89 (19.9) 1.44 ( 9.9)

— —

— —

— —

— —

— —

— —

7

14

21

35

49

63

7

14

21

35

49

63

3.33 (23.0) 1.74 (12.0)

2.17 (15.0) 1.30 ( 9.0)

2.06 (14.2) 1.11 ( 7.7)

1.75 (12.1) 0.72 ( 5.0)

1.05 ( 7.2) 0.49 ( 3.4)

1.26 ( 8.7) 0.68 ( 4.7)

— —

—

—

—

— —

—

—

—

—

—

7 3.40 (23.4) 1.37 ( 9.4)

14 2.42 (16.7) 1.35 ( 9.3)

21 1.81 (12.5) 0.68 ( 4.7)

35 1.65 (11.4) 0.61 ( 4.2)

49 1.62 (11.2) 0.87 ( 6.0)

63 1.37 ( 9.4) 0.66 ( 4.6)

a

Specimens allotted to blocked design were tested by

Hickson Corporation.

All other specimens were allotted to random design and

tested at the FPL.

21

U.S. GOVERNMENT PRINTING OFFICE: 1991/543-045/40001