NBSIR 86-3418

Strain Energy of Bituminous

Built-Up Membranes: An

Alternative to the Tensile Strength

Criterion

Walter J. Rossiter, Jr. Dale P. Bentz

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

September 1986

Prepared for: E.l. DuPont de Nemours and Company

NBSIR 86-3418

STRAIN ENERGY OF BITUMINOUS

BUILT-UP MEMBRANES: AN

ALTERNATIVE TO THE TENSILE STRENGTH

CRITERION

Walter J. Rossiter, Jr. Dale P. Bentz

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

September 1986

Prepared for:

E.l. DuPont de Nemours and Company

Textile Fibers Department

Old Hickory, TN 37138

U.S. DEPARTMENT OF COMMERCE. Malcolm Baldrige. Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

Page

ABSTRACT iv

LIST OF TABLES v

LIST OF FIGURES vi

1.0 INTRODUCTION 1

1.1 Background 1

1.2 Objectives 10

1.3 Scope 10

2.0 EXPERIMENTAL H

2.1 Membrane Samples 11

2.2 Tensile Test 12

2.3 Watertightness Test 13

3.0 RESULTS 14

4.0 DISCUSSION 16

4.1 Conventional Built-Up Membranes 16

4.2 Non-Conventional Bituminous Membrane Materials . . 16

5.0 REVISED PERFORMANCE CRITERION FOR TENSILE STRENGTH . . 24

5.1 Comparison of Test Samples to the Revised

Criterion 27

5.2 Comparison of Additional Samples to the

Revised Criterion 29

6.0 SUMMARY AND RECOMMENDATIONS 31

7.0 ACKNOWLEDGMENTS 34

8.0 REFERENCES 35

APPENDIX A. PERFORMANCE CRITERION FOR TENSILE STRENGTH . . A-l

APPENDIX B. PREPARATION OF BUILT-UP MEMBRANE SPECIMENS . . B-l

APPENDIX C. COEFFICIENTS OF VARIATION OF MEMBRANE SAMPLES

TESTED FOR DETERMINATION OF THE STRAIN ENERGY

CRITERION C-l

APPENDIX D. COEFFICIENTS OF VARIATION OF ADDITIONAL SAMPLES

TESTED IN COMPARISON TO THE REVISED CRITERION . D-l

iii
ABSTRACT

This study was conducted to revise the performance criterion for

tensile strength of bituminous built-up membranes. Bituminous

membrane samples, fabricated from polyester fabric, polyesterglass composite fabric, and single plies of APP- and SBS-modified

bitumen, were tested in tension to determine their load-elongation

properties and to measure their strain energy. The results of

the tensile tests of the new bituminous membranes indicated wide

variability of load and elongation among the different types of

materials

.

As an alternative to the criterion that a bituminous built-up

membrane have a tensile strength of 200 lbf/in. (35 kN/m) , it was

recommended that the strain energy should be a minimum of 3 lbf

in. /in. (13 N-m/m), when tested at 0 °F (-18 °C) in the weaker

direction. The properties of the membrane samples in the study

were compared to the suggested revised performance criterion.

Two polyester samples (without glass), having relatively low

strength and low ultimate elongation at 0 °F (-18 °C), did not

conform to the revised criterion.

Key words: bituminous roofing; built-up; low-sloped; performance

criterion; polyester; polymer-modified bitumen;

roofs; strain energy; tensile strength

iv
LIST OF TABLES

Page

Table 1. Membrane Materials Used for Test Samples for

Determination of the Strain Energy Criterion ... 38

Table 2. Membrane Materials Used for Samples Tested in

Comparison to the Strain Energy Criterion .... 39

Table 3. Tensile Strength of Membrane Samples Used in the

Determination of the Revised Criterion 40

Table 4. Elongation of Membrane Samples Used in the

Determination of the Revised Criterion 41

Table 5. Strain Energy per Unit Gage Length of Membrane

Samples Used in the Determination of the Revised

Criterion 42

Table 6. Tensile Strength, Elongation, and Strain Energy

of Typical 4-Ply Organic Built-up Membranes ... 43

Table 7. Revised Performance Criterion for Load-Elongation

Properties 44

Table 8. Comparison of the Properties of the Membrane

Samples With the Revised Performance Criterion . . 45

Table 9. Load-Elongation Properties of Additional Samples

Tested in Comparison to the Strain Energy

Criterion 46

v
LIST OF FIGURES

Page

Figure 1. Strain energy: the area under the load

elongation curve 47

Figure 2. Comparison of strain energy for materials having

variable load-elongation properties 48

Figure 3. Load-elongation curves of the membrane samples,

when tested at 0 °F (-18 °C) in the transverse

direction 49

Figure 4. Illustration showing that the load-elongation

properties of some new bituminous membranes are

comparable to those of conventional built-up

membranes, when tested at 0 °F (-18 °C) 50

Figure 5. Load-elongation curves for sample no. 5, when

tested in the transverse direction at 73, 0, and

-30 °F (23, -18, and -34 °C) 51

Figure 6. Load-elongation curves for sample no. 7, when

tested in the longitudinal direction at 73, 0,

and -30 °F (23, -18, and -34 °C) 52

Figure 7. Comparison of the load-elongation curves of

the polyester samples with that of the organic

membrane sample; tests are compared for the

weaker direction of the membrane sample at

0 ° F (-18 °C) 53

vi
1.0

INTRODUCTION

1.1 BACKGROUND

1.1.1 Bituminous Built-Up Roofing

Bituminous built-up membranes have been used for over a century

to provide the waterproofing element in low-sloped roofing

systems for industrial and commercial buildings. These membranes

are normally constructed by applying hot bitumen in consecutive,

continuous layers to adhere reinforcement sheets (felts or mats)

to each other. Until the mid-1970s, almost all low-sloped

roofing systems in the United States were the bituminous built-up

type. Since that time, built-up membrane applications have

declined as the use of elastomeric, thermoplastic, and polymermodified bituminous membranes has increased [1]. Nevertheless,

built-up roofing has retained a large share of the membrane

market. Estimates indicate, for example, that in 1985 bituminous

built-up membranes accounted for about 50 percent of those

installed [ 2 ]

.

Although over the years the majority of bituminous built-up

membranes have performed satisfactorily, premature failures have

often occurred [31. For example, the U.S. Air Force found in the

mid-1970s that the built-up roofs on its Strategic Air Bases were

lasting an average of 12 years, although a 20-year service

life was anticipated [4],

1
Since the early 1970s, a number of steps have been taken by the

industry to help assure the satisfactory performance of built-up

roofing including:

o increasing the awareness of owners, manufacturers, and

contractors of the need for proper specifications, and for

quality installation and maintenance [5-7],

o the development of preliminary performance criteria [8],

o an increase in laboratory and field research to provide the

basis for standards and solutions to problems [9], and

o changes in the types of reinforcements used in built-up

membrane fabrication [11].

The change in the types of reinforcements has been drastic, as

illustrated from survey data from the National Roofing Contractors Association (NRCA ) [10,11]. In 1977, organic felts (56%)

and asbestos felts (25%) had the major share of the reinforcement

market. At that time, fibrous glass mat reinforcements (11%)

accounted for a small share of the market and synthetic fabrics

such as polyester were essentially not used. By 1983, fibrous

glass (65%) was the most used, while the use of organic (17%) and

asbestos (2%) felts had declined significantly. At the same

time, there was also an increase in the use of polymer bituminous

felts (15%) for built-up roofing. The introduction of new types

of reinforcements for built-up roofing has resulted in a need to

reexamine and possibly modify existing performance criteria for

bituminous membranes [12,13]. This report presents the results

2
of a study to revise the performance criterion regarding tensile

strength.

1.1.2 Performance Criteria and Splitting

Performance criteria for membranes are attempts to define

performance in engineering terms for one component of the roofing

system [13]. In 1974, Mathey and Cullen reported on preliminary

performance criteria for bituminous membrane roofing [8]. They

identified 20 performance attributes for these membranes, and

suggested preliminary criteria for ten of the attributes. The

ten criteria concerned tensile strength, thermal expansion,

thermal movement, flexural strength, tensile fatigue strength,

flexural fatigue strength, punching shear strength, impact

resistance, wind resistance, and fire resistance.

The twenty attributes for bituminous built-up membranes were

selected upon consideration of the membrane characteristics

necessary to resist development of typical defects and premature

failure in service. Typical defects include splitting, blistering,

ridging, slippage, delamination, and puncturing. Splitting is

tearing of the membrane resulting from tensile stress [14] (which

exceeds the breaking strength of the membrane). Splitting is one

of the most commonly occurring defects [2,15], and as is obvious,

has disastrous consequences, since the waterproofing integrity of

the membrane is lost. Griffin [16] has recently given a summary

of the causes of splitting and its prevention. Lewis [17] has

3
applied the finite-element method (FEM) of analysis to estimate

levels of temperature-induced stress in bituminous built-up

membranes as a step towards understanding the stresses to which

membranes may be subjected in service.

Cash [15] has described characteristic condtions under which

splits frequently appear in service:

o northern climates,

o winter months,

o well-insulated roofs,

o roofing systems with poor adhesion between the deck and

insulation, and

o roofing systems with weak membranes.

The first three of these factors are unavoidable, since buildings

(and thus well-insulated roofs) are constructed in northern

climates. The last two factors can be avoided through proper

design, material selection, and system construction.

In reporting on the factors that influence the splitting of

asbestos membranes, McCorkle et al . [18] stated that membrane

strength is generally overrated as a factor in splitting. They

indicated that the factors affecting asbestos membrane splitting

were :

o deck type and deflection characteristics,

o insulation anchorage technique.

4
o number of felt plies in the membrane, and

o type of mopping asphalt used in the membrane

Of the preliminary performance criteria suggested by Mathey

and Cullen, the tensile strength criterion is considered important

to the ability of the built-up membrane to resist stresses

imposed in service [13]. This criterion has been the most widely

referenced of the twenty criteria suggested. For example, the

National Roofing Contractors Association ( NRCA ) has used it as

the basis for tensile testing of bituminous built-up membranes

cited in the NRCA "Roofing Materials Guide."

The suggested preliminary performance criterion [8] for tensile

strength is that the membrane should have a minimum strength of

200 lbf/in. (35 kN/m) , when tested at 0 °F (-18 °C) in the

weakest direction of the membrane (Appendix A). The development

of this criterion was based on the load-elongation properties of

bituminous built-up membranes available at the time and their

performance in service. For example, when tested as described

in the tensile strength criterion, many traditional built-up

membranes had tensile strengths greater than 200 lbf/in. (35

kN/m) and elongations at break in the range of 1-3 percent.

However, some of the new (or non-con ventional ) membranes fabricated

from synthetic reinforcements such as polyester fabrics or from

polymer-modified bitumens have tensile strengths less than 200

lbf/in. (35 kN/m) and elongations at break of 20 percent or

5
more. For the limited time in service, the performance of many

of these non-conventional materials has been satisfactory [19—

21]. Concerns that they may be prone to splitting have not been

reported, suggesting that the performance criterion of 200

lbf/in. (35 kN/m) is not a requisite for acceptable performance

of bituminous built-up membranes in cases where they are relatively

extensible (e.g., having an extensibility greater than 1-3 percent).

1.1.3 Strain Energy

A preliminary study [13] was conducted at the National Bureau of

Standards (NBS) to propose possible alternative approaches which

might be taken to revise the tensile strength criterion. The

study suggested that a strain energy approach be used. Strain

energy is the energy that a material absorbs as a result of its

deformation [22]. An important advantage to the strain energy

approach was that it was compatible with the original tensile

strength criterion in that tensile strength and strain energy are

properties measured in the same tensile test. As a consequence,

previous data, developed as the basis of the original tensile

strength criterion, could again be used in the development of a strain energy criterion.

The use of strain energy as an approach for revision of the

tensile strength criterion can be summarized as follows. When a load (force) is applied to a material, work is done as the load

moves through certain distances as the material undergoes

6
deformation, and energy is absorbed. In a uniaxial tensile test,

the total work (W) done on the material is by definition [22]:

The total work (W) done in deforming the material to some value,

e, is equal to the area under the load-deformation curve (Figure

1 ) . It represents the total energy absorbed by the material

during strain and is, thus, called the strain energy [23].

The area under the entire load-deformation curve is a measure of

the strain energy required to rupture a material [22] and is

related to the toughness of the material. Toughness represents

a material's ability to resist energy loads before rupture [22].

As is evident from Figure 2, a high strength material having low

capacity to deform may have less toughness than a low strength

material having high capacity to deform.

Traditional built-up membranes may be considered as having

relatively high strength and low deformation, while some of the

non-conventional membrane materials may have relatively low

strength and high deformation (Figure 2). When in place on a given building, a non-conventional membrane material would be

exposed to the same environmental conditions (which can produce

splitting forces) as experienced by a traditional bituminous

built-up membrane. Thus, as a measure of splitting resistance

which includes both strength and extensibility of the membrane.

u where P is the load, and e is the deformation.

w

7
it is considered that the non-conventional membrane materials

should have strain energies ^at least comparable to those of the

traditional bituminous built-up membranes that have historically

provided acceptable long-term performance.

The use of strain energy as a criterion must also include a consideration of the watertightness of the membrane upon

elongation. Some of the non-conventional bituminous built-up

membranes, having greater elongation at break than the traditional ones [19], may not remain watertight when elongated at low

temperatures to values approaching ultimate. The bitumen in the

composite built-up membrane is brittle at low temperatures (for

example, less than 40 °F or 4 °C). During low-temperature

elongation of a relatively extensible membrane, the bitumen may

crack before the membrane ruptures. The result would be loss of

watertightness, since the bitumen provides the waterproofing for

the built-up membrane. Thus, in applying the strain energy

approach to the performance of bituminous built-up membranes, the

membrane should remain watertight after deformation to the

minimum strain energy suggested as a performance criterion.

It is noted that the strain energy of a material, determined as

the area under the load-elongation curve, is dependent upon the

size of the specimen (or, gage length over which strain is

measured, if the specimen size is held constant) [22]. To

eliminate the effect of specimen size on the determination of

8
strain energy as a material property, the strain energy per unit

volume (sometimes called strain energy density) is often considered.

Strain energy density is determined by dividing the strain energy

by the initial volume of the specimen. However, for bituminous

built-up membranes, the use of strain energy density is not

practicable. The thickness of a built-up membrane is normally

not homogeneous for the specimen, and consequently, in conducting

load-deformation tests, the load is reported in force per unit

specimen width [24]. For the same reason, it is thus considered

that the strain energy of bituminous built-up membranes, based on

unit gage length, should be reported for specimens of constant

width and not strain energy density.

The application of strain energy in the specification of bituminous built-up membranes having varying load-deformation

properties is not a new concept [13]. The Canadian General

Standards Board (CGSB) Standard [25] and the Midwest Roofing

Contractors Association (MRCA ) Performance Criteria [26] for

polymer modified bitumens both have requirements for minumum

strain energy for these materials. However, these two documents

refer to strain energy at break and do not consider the watertightness of the membrane after some limited elongation which

does not reach the break point.

9
1.2 OBJECTIVES

The objectives of this study are:

1) to determine the load-deformation properties including

strain energy of some non-conventional bituminous

membrane materials, and

2) to suggest a strain energy criterion as an alternative

for the tensile strength criterion given in NBS

Building Science Series 55 [8].

1.3 SCOPE

This study concerned the load-deformation properties including

strain energy of bituminous built-up membrane specimens fabricated

in the laboratory. The primary type of built-up membrane

investigated was reinforced with polyester fabric, although

specimens having organic felt and fibrous glass mat reinforcements

were included. Load-deformation properties were also measured

for two polymer-modified bitumens. In addition, after a strain

energy criterion was suggested based on the initial test data,

additional polyester-based membrane specimens were tested to

compare their strain energy with that of the revised criterion.

10
2.0

EXPERIMENTAL

2.1 MEMBRANE SAMPLES

2.1.1 Samples Used in Determination of the Strain Energy Criterion

The bituminous membrane samples (nos. 1-9) used for the determination

of the strain energy criterion are described in Table 1. All

materials were commercially available for low-sloped roofing.

The built-up membrane samples were fabricated using either hot

asphalt or an asphaltic emulsion (Table 1) put on in quantities

normally applied in practice. The procedure is given in Appendix

B. The temperature of the asphalt during application was a

minimum of 425 °F (218 °C). No asphalt was applied to the outer

surfaces of the samples. After fabrication, the samples were

kept at ambient laboratory temperature for at least 72 hours

before testing. The orientations of the test specimens, relative

to the direction of the manufactured roll of reinforcing fabric,

were longitudinal (machine-direction), transverse (cross-machine

direction), and diagonal (45° to the machine direction).

2.1.2 Samples Used for Comparison with the Suggested Strain

Energy Criterion

Four other polyester-based built-up membrane samples (nos. 10-13)

were prepared using asphaltic emulsion and tested in tension for

comparison of their strain energy with that suggested in the

revised criterion. These samples are described in Table 2.

For these four samples, a coating of the emulsion was applied on

either one or both outer surfaces.

11
2.2 TENSILE TEST

All samples were tested in tension using a universal testing

machine at a rate of 0.08 in. /min (2 mm/min). The machine was

equipped with a microcomputer for data acquisition, reduction,

and storage, as well as for calculating the strain energy of the

specimens. Tests were carried out at 73, 0, and -30 °F (23, -18,

and -34 °C). The test-specimen configuration was as described in

ASTM D 2523 [24]. Five specimens of each membrane sample were

tested at each temperature for each orientation.

For those samples that underwent relatively little extension

(< 5 percent) during testing, percent elongation was determined

using a 2-in. (50-mm) gage-length extensometer attached at

specimen mid-length. The samples for which the extensometer

was used were:

o sample nos. 1 and 2 (transverse direction) at 0 °F (-18 °C),

o sample nos. 3 and 4 (longitudinal and transverse directions)

at 0 and -30 °F (-18 and -34 °C),

o sample no. 8 (all directions) at all temperatures, and

o sample no. 9 (longitudinal and transverse directions) at

-30 ° F (-34 °C).

In the majority of the cases using the extensometer, specimen

rupture occurred within the extensometer gage length. When

specimen rupture took place outside the extensometer gage length,

the measured extension was considered to be indicative of the

extension occurring in the area of the break.

12
Percent elongation of the other samples (i.e., those not listed

above) at the remaining conditions of test was determined by

measuring the amount of separation which occurred between the

specimen grips during testing. Observation of the specimens

during testing did not indicate noticeable slippage in the

grips. Any slippage in the grips was considered to be negligible

in relation to the total elongation of the specimen.

2 . 3 WATERTIGHTNESS TEST

Watertightness tests were conducted according to the procedure

given in the Canadian General Standards Board (CGSB) Standard for

polymer-modified bitumens [25]. Test specimens had an area of

2 2

4 in. (2500 mm ), and the diameter of the column holding the

water was 1.8 in. (44 mm). Before conducting the watertightness

tests, the specimens were elongated at 0 °F (-18 °C) at a rate

of 0.08 in. /min (2 mm/min) to a percent corresponding to the

minimum strain energy suggested in the revised performance

criterion (see section 4.2.3). Upon removal from the environmental

chamber, the ends of the elongated, cold specimens were immediately

nailed to a plywood substrate for conducting the watertightness

test

.

13
3.0 RESULTS

When tested in tension, the 9 membrane samples described in Table

1 showed, as expected, varying load-elongation properties. This

is illustrated in Figure 3, which presents the results of the

tests at 0 °F (-18 °C) for specimens tested in the transverse

direction. Several samples (Nos. 1-4, and 8) exhibited

relatively high peak loads that occurred at break with relatively

little elongation. Peak load is defined as the maximum load

recorded during the tension test. One sample (No. 9) gave a peak

load during the initial portion of the test (due to break of the

reinforcement), and had a relatively high elongation at break.

For another sample (No. 7), the peak load occurred essentially at

break with relatively high elongation. Two samples (Nos. 5 and

6) exhibited low load and intermediate elongation.

Tables 3, 4, and 5 present summaries of the average tensile

strength, percent elongation, and calculated strain energy for

the tension tests. Tensile strength is given as the peak load

experienced during testing. Depending on the sample, the peak

load occurred at the initial stages of elongation or at the

ultimate elongation. For those cases where the peak load

occurred during initial specimen elongation, the percent elongation

and strain energy are given for peak loads and also for the break

point of the specimens (referred to in the tables as ultimate

elongation and total strain energy, respectively). When the peak

load occurred at specimen break, only values of ultimate elongation

14
and total strain energy are reported (Tables 4 and 5). In some

tests, a rapid, large loss of load occurred just prior to

specimen break (for example, see Figure 3, specimen no. 7). In

these cases also, only the ultimate elongation and total strain

energy are reported.

Appendix C presents a summary of the coefficients of variation

for all tests of samples listed in Table 1. The coefficient of

variation of average tensile strength of all specimens was

generally 15 percent or less. With the exception of sample

nos. 4 and 5 at 0 and -30 °F (-18 and -34 °C), the coefficients

of variation of average percent elongation and strain energy were

generally 25 percent or less. In the case of the exceptions, the

values of the coefficient of variation for several of the test

conditions were in the range of 50 to 90 percent. The degree of

bonding (or interlocking) of the fibers in the fabrics has a

significant effect on the reproducibilty of the elongation and

strain energy data.

15
4.0

DISCUSSION

4.1 CONVENTIONAL BUILT-UP MEMBRANES

Conventional 4-ply bituminous built-up membranes using organic

and glass felts (sample nos. 1 and 2, respectively) were included

in the study to compare their load-elongation properties at 0 °F

(-18 °C) with those of the non-conventional membrane materials.

It was particularly of interest to determine the strain energy of

the conventional samples at 0 °F (-18 °C), since this property

had not been reported previously by Mathey and Cullen L 8 ] during

the development of the preliminary performance criteria.

The strain energies at 0 °F (-18 °C) for the 4-ply organic and

glass built-up membranes were 3.2 and 4.0 lbf-in./in. (14.0

and 17.6 N*m/m), respectively. The greater value of strain

energy for the glass felt membrane was attributed, for the

most part, to its greater tensile strength (Table 3). The

ultimate elongations for the two conventional membrane samples

were found to be similar (Table 4). The values of tensile

strength of both the organic and glass felt membrane samples

(Table 3) were similar to the those reported by Mathey and Cullen

[8] for these types of materials.

4.2 NON-CONVENTIONAL BITUMINOUS MEMBRANE MATERIALS

Little data on the load-elongation properties of polyester

reinforced bituminous built-up membranes have been reported for

different test temperatures and orientations of fabric in the

16
membrane specimen [19]. Thus, for purposes of characterization,

3-ply specimens of polyester-based bituminous built-up membranes

(sample nos. 3-7), oriented in the longitudinal, transverse,

and diagonal directions of the fabric, were tested in tension at

73, 0, and -30 °F (23, -18, and -34 °C).

Splitting in bituminous built-up membranes usually occurs

perpendicular to the weaker orientation of the reinforcement

[15]. For this reason, tensile properties of built-up membranes

are normally measured for the longitudinal and transverse

directions of the felt to determine the stronger orientation.

The membrane can be oriented on the roof to have maximum resistance

to areas of stress concentration. For example, Griffin [27] has

indicated that the direction of reinforcing felts applied on

insulation substrates should be perpendicular to long, continuous

joints between the adjacent insulation boards.

Tensile properties for the diagonal direction of conventional

membrane materials are normally not measured. In this study, the

tensile properties of the non-conventional membrane materials

were determined for the diagonal direction, since some of these

materials contained reinforcement which could contribute to

strength of the fabric in the longitudinal and transverse

directions, but might have little influence on strength in the

diagonal direction. For such materials, the diagonal direction

of the fabric would then be the weakest orientation.

17
The results of the study showed that the polyester fabrics having

the bonded glass net (sample nos. 3 and 4) were weakest in the

diagonal direction (Table 3). However, this finding should have

little impact on the use of the glass-reinforced polyester

fabrics for built-up membranes, since in normal practice, the

fabric orientation during application would be perpendicular to

the long joints between insulation boards. Thus, further

discussion of the data regarding the orientation of the fabric in

the test specimens will be limited to the longitudinal and

transverse directions.

The APP (atactic polypropylene)- and SBS ( styrene-butadienestyrene ) -modif ied bitumens were included in the test program to

compare the load-elongation properties of typical polymermodified bitumens to those of bituminous built-up membrane

materials. In developing the scope of the study, it was intended

to explore whether the revised performance criterion for tensile

strength would have applicability to typical polymer-modified

bitumens .

4.2.1 Tensile Strength

With one exception, the membrane samples were weaker in the

transverse direction than the longitudinal direction at each of

the test temperatures (Table 3). One polyester sample (no. 7)

was weaker in the longitudinal direction. The method of manufacture

for this polyester fabric was different than that used for the

18
other two polyesters (sample nos. 5 and 6). The finding that the

weaker orientation of a polyester fabric may be either the

longitudinal or transverse direction has significance for roofing

applications. The load-elongation properties of polyesters in

both directions should be known before application so that proper

orientation of the weaker direction with respect to insulation

boards may be achieved.

For a given orientation (e.g., transverse), all membrane samples

were weakest at 73 °F (23 °C) (Table 3). At the lower test

temperatures, the strength of the samples increased. Sample

nos. 3, 4, and 9 had greatest tensile strengths at -30 °F (-34

°C). The polyester samples (nos. 4-6) were strongest at 0 °F

(-18 °C), although their strengths at -30 °F (-34 °C) were not

much different than those at 0 °F (-34 °C). Sample no. 8 (APPmodified bitumen) had a much greater tensile strength at 0 °F

(-18 °C) than at -30 °F (-34 °C).

The load-elongation behavior of three samples (nos. 3, 4, and 8)

for the transverse direction at 0 °F (-18 °C) was similar to that

of the conventional built-up membranes (nos. 1 and 2), as

illustrated in figure 4. The tensile strength of the three

samples was greater than 200 lbf/in. (35 kN/m) , indicating

conformance to the preliminary performance criterion for bituminous

built-up membranes [8].

19
The strength of sample nos. 3 and 4 at 0 °F (-18 °C) was attributed to the glass net bonded to the polyester fabric. Peak load

was achieved when the glass net failed. The polyester fabric did

not break at this point. However, delamination of the three

plies of polyester fabric in the specimens occurred simultaneously

with the glass-net rupture. Because of the delamination, the

membrane specimens were considered failed, and the tension test

was terminated.

4.2.2 Percent Elongation

For all orientations at 73 °F (23 °C), the non-conventional

bituminous membrane materials had, with one exception, ultimate

elongations ranging from approximately 15 to 150 percent (Table

4 ) . These values were substantially greater than the ultimate

elongations of conventional built-up membranes, which may range

from about 1 to 4 percent'*' at ambient temperatures. The ultimate

elongation of sample no. 8 was only slightly greater than 1

percent at 73 °F (23 °C).

As the temperature was decreased below 73 °F (23 °C), the

ultimate elongations of all samples except no. 8 decreased.

In general, sample no. 8 exhibited little change in percent

elongation over the range of test temperatures. On the average,

it had its greatest percent elongation (2.6 percent) at 0 °F (-18

°C). Reasons for this finding were not investigated.

1. Unpublished NBS data.

20
Three of the polyester-based samples (nos. 3, 4, and 7) had

comparable elongations at 0 and -30 °F (-18 and -34 °C) for a

given orientation. Polyester sample no. 5 had its lowest

percent elongation (for all orientations) at 0 °F (-18 °C). The

percent elongation was twice as great at -30 °F (-34 °C) as at

0 ° F (-18 °C)

,

as shown in figure 5, which gives load-elongation

curves in the transverse direction at the three test temperatures

Polyester sample no. 6 showed similar behavior for the transverse

direction, having about twice the extensibility at -30 °F (-34 °C

than at 0 °F (-18 °C).

The SBS-modified bitumen (sample no. 9) was the only material

which was significantly more extensible at 0 °F (-18 °C) than at

-30 ° F (-34 °C). Complete embrittlement of the membrane material

had apparently not occurred at 0 °F (-18 °C), because of the

modification with the SBS polymer.

4.2.3 Strain Energy

The total strain energies of the non-conventional membrane

materials varied considerably between some samples, which

reflected the differences in their peak loads and ultimate

elongations (Table 5). For the longitudinal and transverse

directions, four polyester-based samples (nos. 3-6) were found

to have their greatest total strain energy at 73 °F (23 °C),

which was attributed to their relatively high elongations at this

temperature. For these four samples, as the test temperature

21
decreased below ambient, the total strain energy decreased.

For sample nos. 3 and 4 f the total strain energy was comparable

at 0 °F (-18 °C) and -30 °F (-34 °C). In the cases of sample

no. 5 in the longitudinal and transverse directions and sample

no. 6 in the transverse direction, the strain energy at -30 °F (-

34 °C) was about twice as great as that at 0 °F (-18 °C). This

was primarily due to the lower percent elongation at 0 °F (-18

°C), as indicated previously (section 4.2.2). From figure 5, it

is evident that, for three test temperatures, the total strain

energy of sample no. 5 was least at 0 °F (-18 °C).

For a given orientation, the total strain energies of polyester

sample no. 7 were comparable at the three test temperatures. In

this case, relative increases in load and decreases in ultimate

elongation, as the temperature was reduced below 73 °F (23 °C),

produced little effect on the strain energy (figure 6). In the

case of both polymer-modified bitumens (sample nos. 8 and 9),

the relative changes which occurred in load and ultimate elongation,

as the test temperature was decreased, resulted in the samples

having their greatest total strain energy at 0 °F (-18 °C) (Table

5 )

.

At 0 ° F (-18 °C) for the weaker membrane direction, two of the

five polyester-based samples (nos. 3 and 7) had a total strain

energy greater than that of the 4-ply organic membrane sample

(no. 1) (Table 5). In contrast, for the same conditions, the

22
other three polyester-based samples (nos. 4, 5, and 6) had a

total strain energy less than that of the organic membrane

sample. Figure 7 compares typical load-elongation curves for the

polyester samples (nos. 5, 6, and 7) with a curve for the organic

membrane sample (no. 1). It is evident that the area under the

curve for sample no. 7 is much greater than that for the organic

membrane sample. It is not evident that the areas under the

curves for sample nos. 5 and 6 are less than that of the organic

membrane sample. It was found by calculation that the total

strain energy of polyester sample nos. 5 and 6 was le-ss than that

of the organic membrane sample.

23
5.0 REVISED PERFORMANCE CRITERION FOR TENSILE STRENGTH

The original tensile strength performance criterion of 200

lbf/in. (35 kN/m) for bituminous built-up membranes (having

relatively high strength and low extensibility) has provided a technical basis for the U.S. roofing industry in the selection of

membrane materials that provide satisfactory long-term performance.

The original criterion should be continued to be used. However,

the original tensile strength criterion should have an alternative

approach to provide applicability to the non-conventional

bituminous membrane materials that have relatively low strength

and high elongation, and that have shown satisfactory

in-service performance regarding splitting resistance.

The approach taken for revising the tensile strength performance

criterion is to use the strain energy of the membrane as an

alternative to tensile strength. A major difficulty in suggesting

the use of strain energy is the selection of a minimum value

which the membrane material should possess under the given

test conditions. As a first step to resolve this difficulty, it

is considered that the minimum value of strain energy should

be based on the strain energy of built-up membrane materials

which have provided acceptable performance. In this regard,

four-ply organic built-up membranes, properly applied and

maintained, have, in general, performed satisfactorily for many

years [8, 29]. Until recent times, membranes having organic

felts were the most commonly used, and provided an industry

24
benchmark against which the performance of other types of

mambrane materials was compared. For this reason, it is suggested

that a minimum value of strain energy (for use as a performance

criterion) be comparable to that of typical 4-ply organic

bituminous built-up membranes. It is noted that 4-ply organic

membranes generally have shown conformance to the tensile

strength criterion of 200 lbf/in. (35 kN/m)

.

Although 4-ply organic built-up membranes having different brand

names are generically the same type of material, they do not all

have equivalent strain energies. This is due, in part, to

differences in load-elongation properties imparted to the felts

during the manufacturing process. Table 6 compares the tensile

strength, ultimate elongation, and strain energy of some typical

4-ply organic membranes. With the exception of the sample tested

in the present study, the strain energy of the other samples was

estimated from load-elongation data given in the referenced

papers [8, 28, and 30], Strain-energy data were not presented in

these references. Table 6 indicates that the strain energy of

the samples was in general estimated to be about 3 lbf*in./in.

(13 N*m/m) or greater. In one case of a coal tar pitch membrane,

the estimated strain energy was 1.7 lbf*in./in (7.6 N.m/m), which

was attributed to the low elongation (0.9 percent) of the sample.

Based on the limited data given in Table 5, it is suggested

that a preliminary minimum value of strain energy of

25
3 lbf»in./in. (13 N*m/m) be used in the revision of the performance

criterion for tensile strength. Additional data from loadelongation tests of 4-ply organic built-up membranes may result

in a value of strain energy superseding the presently suggested

value

.

The revised performance criterion for tensile strength is

presented in Table 7. As previously discussed, a watertightness

test must be conducted on membrane materials which conform to the

strain energy portion of the criterion. The watertightness test

should be conducted on the specimen after it is elongated at 0 °F

(-18 °C) to a percent at which the strain energy is equivalent to

the criterion value of 3 lbf in. /in. (13 N m/m). If the specimen

is not watertight after that elongation, it is considered to be

non-functional, and would not meet the criterion.

The watertightness test in the CGSB Standard for polymer-modified

bitumens [25] was used in the present study to demonstrate

watertightness of the elongated membrane specimens. The test was

selected because it was available in a national standard developed

in North America. The description of the procedure in the

standard indicated that the test should be easily conducted

in the laboratory. However, difficulty was encountered in

sealing membrane test specimens to the vertical tubes containing

the columns of water that provided pressure for the tightness

test. Hot wax was found to provide the best seals, but sealing

26
was still difficult to accomplish. The test was considered

adequate for purposes of demonstrating watertightness in this

study. Because of the difficulties encountered in the present

study, it is suggested that, for future application of the

tensile strength criterion, another watertightness test should be

2 used or an improved test procedure be developed.

5.1 COMPARISON OF TEST SAMPLES TO THE REVISED CRITERION

Table 8 compares the bituminous membrane materials included in

the study with the revised performance criterion for tensile

strength. Five samples (nos. 1-4, and 8) showed conformance to

the original criterion of having a tensile strength of 200

lbf/in. (35 kN/m) . Watertightness tests were not conducted on

these samples, since loss of watertightness of built-up membranes

(which have relatively low extensibilty ) during elongation to

break has never been of concern. These materials provide

waterproofing capability as long as they do not split, puncture,

or incur similar damage.

Sample nos. 7 and 9 did not conform to the tensile strength

criterion, but did meet the strain energy criterion of 3 lbf*

in. /in. (13 N»m/m). These samples were also watertight when

2

. During the course of the study, conversations were held

with Dr. R. Booth, present chairman of the CGSB task group having

responsibility for the standard. He indicated that the standard

is being reviewed for possible revision. One concern being

addressed is the significance of the current test procedure

for demonstrating watertightness of a damaged membrane specimen.

27
tested according to the procedure in the CGSB Standard. Prior to

the watertightness test, sample no. 7 was elongated 5 percent at

0 °F (-18 °C). Sample no. 9 was elongated to 3.5 percent. At

these elongations, the strain energy of the specimens was about 3

lbf-in./in. (13 N»m/m).

Two polyester samples (nos. 4 and 5) did not show conformance to

either the tensile strength or strain energy criterion. As a consequence, there was no need to conduct the watertightness test

on these samples.

The two polymer-modified membrane materials met the revised

performance criterion. The APP-modified bitumen (no. 8) showed

conformance to both the tensile strength and strain energy

portions of the criterion. The SBS-modified bitumen (no. 9) was

in conformance with the strain energy section. Additional

research should be conducted on other polymer-modified bituminous

membranes to obtain further evidence that the revised criterion

would have applicability to the many polymer-modified bitumens

that are presently being marketed.

Finally, it is noted that the tests of the bituminous built-up

membranes were conducted only on new samples prepared in the

laboratory. The test specimens were not subjected to any

exposure conditions, either in the laboratory or outdoors, which

might be considered deleterious to the membranes. The minimum

28
strain energy of 3 lbf«in./in. (13 N.m/m) should be maintained by

a membrane as it ages. Changes in membrane properties that occur

due to weathering and diurnal cycling should not result in a

strain energy less than the suggested minimum. Further research

is needed to investigate whether the non-conventional bituminous

built-up membranes maintain an acceptable minimum value of strain

energy after performance in service.

5.2 Comparison of Additional Samples to the Revised Criterion

Table 9 presents the tensile strength, ultimate elongation, and

total strain energy for the additional polyester-based membrane

samples (nos. 10-13). As previously indicated, these samples

were tested for comparison of their strain energy with that

suggested for the revised performance criterion. Three replicate

specimens of each sample were tested at 0 °F (-18 °C) in the

transverse direction of the fabric. Coefficients of variation

for these tests are given in Appendix D. As is evident in Table

9, the total strain energies of the four specimens were found to

be greater than the suggested criterion value of 3 lbf«in./in.

(13 N*m/m) . Watertightness tests were not conducted on the four

specimens. To determine conformance to the revised criterion, a

watertightness test of the samples would need to be conducted.

In conducting the additional tests, it was of interest to

investigate whether a surface coating of asphaltic emulsion would

influence the sample tensile strength. Sample nos. 10 and 11

29
were prepared from the same fabric and were identical except that

one outer surface of no. 10 was coated, whereas the two surfaces

of no. 11 were coated (Table 9). Sample nos. 12 and 13 also

formed an identical pair except for surface coating. Strict

comparison of the effect of the surface coating is difficult to

make because of the limited number of replicate specimens

tested. As given in Table 9, the former pair (nos. 10 and 11),

had comparable tensile strengths, whereas the latter pair

(nos. 12 and 13) had tensile strengths differing by about 25

percent. Visual examination of the test specimens seemed to

indicate that the asphaltic emulsion penetrated the polyester

fabric used for sample nos. 12 and 13 more than it penetrated the

fabric used to prepare sample nos. 10 and 11. The penetration of

the asphaltic emulsion into the fabric could provide increased

fabric strength.

30
6.0 SUMMARY AND RECOMMENDATIONS

This study was conducted to revise the performance criterion for

tensile strength of bituminous built-up membranes. This criterion

is that the tensile strength of the membrane should be a minimum

of 200 lbf/in. (35 kN/m) , when tested in the weaker direction at

0 °F (-18 °C). The development of this criterion was based on

the load-elongation properties of traditional built-up membranes

available in the early 1970s and their performance in service.

Alternative bituminous membranes, based on synthetic reinforcements

and polymer-modified bitumens are now available. The, nonconventional materials have generally performed satisfactorily,

particularly with regard to splitting resistance. However, many

of them do not have tensile strengths conforming to the minimum

value suggested in the original criterion. They have relatively

low strength (< 200 lbf/in. or 35 kN/m) and high elongation in

relation to the traditional bituminous built-up membranes, which

may be considered as relatively high strength and low elongation

materials

.

Bituminous membrane samples, fabricated from 3 plies of polyester

fabric, 3 plies of polyester-glass composite fabric, and 1 ply of

APP- and SBS-modified bitumen, were tested in tension to determine

their load-elongation properties and to measure their strain

energy. The tests were conducted in the longitudinal, transverse,

and diagonal directions at 73, 0, and -30 °F (23, -18, and -34

°C). The single-ply materials were included to investigate the

31
applicabilty of the revised criterion to these types of membranes.

Limited tensile tests of conventional 4-ply organic and glass

built-up membrane specimens were also carried out to compare the

load-elongation properties with those of the non-conventional

membrane materials.

The results of the tensile tests of the non-conventional bituminous

membranes indicated wide variability of load and elongation among

the different types of materials. The polyester and SBS-modified

materials generally exhibited relatively low strength and high

elongation; whereas, particularly at the lower temperatures, the

polyester-glass composite materials and APP-modified bitumen

showed load-elongation properties similar to those of conventional

built-up membranes.

A strain energy approach was taken to revise the tensile strength

criterion. A strain energy criterion considers both the strength

and extensibility of the membrane, and is related to the toughness

of the material to withstand energy loads before rupture. As an

alternative to the criterion that a bituminous built-up membrane

have a tensile strength of 200 lbf/in. (35 kN/m) , it was recommended

that the strain energy should be a minimum of 3 lbf*in./in. (13 N»

m/m), when tested at 0 °F (-18 °C) in the weaker direction. This

value of strain energy was selected based on a summary of limited

load-elongation data for conventional 4-ply organic bituminous

built-up membranes.

32
When strain energy is used as a criterion, it is necessary to

conduct a watertightness test to assure that limited strain of

the membrane at low temperatures does not occur to an extent that

cracks the bitumen, resulting in loss of waterproofing integrity.

It was recommended that a watertightness test be conducted on

specimens after partial elongation to a percent corresponding to

the minimum strain energy of 3 lbf«in./in. (13 N»m/m).

The properties of the membrane samples in the study were compared

to the suggested revised performance criterion. Two polyester

samples (without glass), having relatively low strength and low

ultimate elongation at 0 °F (-18 °C), did not conform to the

revised criterion. The other samples showed conformance due to

minimum tensile strength, minimum strain energy, or both. Two

samples (a 3-ply polyester without glass and the SBS-modified

bitumen), which conformed due to minimum strain energy, were

shown to remain watertight after less-than-ultimate elongation

(< 5 percent) at 0 °F (-18 °C).

The data on which the strain energy criterion was developed were

obtained from tests of new laboratory-prepared specimens. A

membrane should maintain a strain energy above the suggested

minimum throughout its service life. Further research is needed

to investigate whether the non-conventional bituminous built-up

membranes maintain an acceptable minimum value of strain energy

after long-term in-service performance.

33
7 . 0 ACKNOWLEDGMENTS

This study was sponsored by E.I. du Pont de Nemours & Co.,

Textile Fibers Department. The authors express their thanks and

appreciation to M.S. Herring, V. Kumar, and S.K. Rudys , Du Pont

Project Managers, for their valuable assistance and encouragement

during the course of the study.

The authors also acknowledge with thanks the contributions and

assistance of their NBS colleagues. J.C. Hairston, E.J. Embree,

and N. Nicastro provided specimen preparation and tes-ting.

R.J. Fields provided assistance and equipment for the measurement

of specimen strain. D. Herbert provided typing and editorial

assistance.

Special thanks are given to W.C. Cullen, L.W. Masters, and

R.G. Mathey for their valuable comments and suggestions provided

during their reviews of a draft of this report.

34
8.0 REFERENCES

1. Rossiter, Walter J. , Jr., "Single-Ply Roofing — A Decade of

Change," Standardization News (September 1985), pp. 32-35.

2. Toland , Richard M. , "Single-Ply Tops the Market," Contractors

Guide (March 1986), pp. 54-55.

3. Griffin, C.W. , "Manual of Built-Up Roof Systems," 2nd

Edition, McGraw-Hill, New York (1982), p. 1.

4. Marien, Harry R. , "The Finishing Touch," Engineering &

Services Quarterly (Summer 1982), pp. 10-11.

5. "A Guide to Preparing Built-Up Roofing Specifications,"

Asphalt Roofing Manufacturers Association, Rockville, MD.

6. "The NRCA Roofing & Waterproofing Manual," 2nd Edition,

National Roofing Contractors Association, Chicago, IL

(1985 ) .

7. "Built-Up Roof Management Program," Air Force Manual AFM 91-

36, Department of the Air Force (3 September 1980).

8. Mathey, Robert G. and Cullen, William C., "Preliminary

Performance Criteria for Bituminous Membrane Roofing,"

National Bureau of Standards (U.S.), Building Science Series

55 (November 1974), 12 pages.

9. Busching, Herbert W. and Porcher, Joel P. , Jr., "Roofing

Industry Perspective and Research Capability," ORNL/SUB/82-

22293/2, Oak Ridge National Laboratory (March 1985), 85

pages

.

10. "1977 Final Project Pinpoint Returns," Bulletin #3,

National Roofing Contractors Association, Chicago, IL.

11. Cullen, William C., "Survey Compares Practices to Problems,"

Roofing Spec (October 1984), pp. 37-40.

12. Cullen, William C., Rossiter, Walter J. , Jr., Mathey, Robert

G., and Clifton, James R. , "Low-Sloped Roofing Research

Plan," National Bureau of Standards (U.S.), Special Publication

659 (July 1983), 40 pages.

13. Busching, Herbert W. , Rossiter, Walter J. , Jr., and Mathey,

Robert G., "Alternatives for Revision of Preliminary

Performance Criteria for Tensile and Fatigue Strength

Tests of Bituminous Membrane Roofing," National Bureau of

Standards (U.S.), NBSIR 86-3347 (April 1986), 59 pages.

35
14 . "Standard Definition of Terms Relating to Roofing, Waterproofing,

and Bituminous Materials," ASTM D 1079, Volume 04.04,

American Society for Testing and Materials (1984), pp. 133-139.

15. Cash, C.G., "Thermal Warp — A Hypothesis for Built-Up

Roofing Splitting Failures," in Roofing Systems, ASTM STP

603, American Society for Testing and Materials (1976),

pp. 114-135.

16. Griffin, C.W. , op. cit., pp. 302-323.

17. Lewis, J.E., "Preliminary Stress Evaluation of the Effects

of Gaps Between Roof Insulation Panels," Journal of Thermal

Insulation, Vol . 4 (July 1980), pp. 3-26.

18. McCorkle, J.E., Dupuis, R.M. , and LaCosse, R.A., "Membrane

Integrity Depends on Characteristics of Entire Roof

Assembly," Paper No. 7, Proceedings, 6th Conference on

Roofing Technology, National Roofing Contractors- Association

,

Chicago, IL (1981), pp. 34-41.

19. Sands, Seymour, "New BUR Technology Using Spunbonded

Polyester Reinforcing Fabrics," Paper No. 10, Proceedings,

7th Conference on Roofing Technology, National Roofing

Contractors Association, Chicago, IL (1983), pp. 45-48.

20. Dupuis, Rene, "Use of One Plies Expected to Continue to Grow

in '85," Roof Design, 1985 Handbook of Commercial Roofing

Systems (January/February 1985), pp. 17-19.

21. "RSI Survey: Contractors More Flexible," RSI (February

1986), p. 58.

22. Byars, Edward F. and Snyder, Robert D. , "Engineering

Mechanics of Deformable Bodies," 3rd Edition, Intext

Educational Publishers, New York (1969), pp. 92-94.

23. Timoshenko, S. and MacCullough, Gleason H. , "Elements of

Strength of Materials, 3rd Edition, Van Nostrand, New York

(1949), pp. 320-322.

24. "Standard Practice for Testing Load-Strain Properties of

Roofing Membranes," ASTM D 2523, Annual Book of ASTM

Standards, Volume 04.04, American Society for Testing and

Materials (1984), pp. 257-259.

25. "Standard for Membrane, Modified, Bituminous, Prefabricated,

and Reinforced for Roofing," Canadian General Standards

Board, 37-GP-56M (July 1980).

36
26 . "Recommended Performance Criteria for Modified Bitumen Roof

Membrane Systems," Technical Document MB-30, Midwest Roofing

Contractors Association, Kansas City, MO (1983).

27. Griffin, C.W. , op. cit., p.89.

28. "Phase Two of Coal Tar Study Finds Mechanical Application

EVT," NRCA Task Group Report, The Roofing Spec (February

1986 ) , pp. 41-49

.

29. Griffin, C.W ., op. cit., pp. 138-140.

30. Rissmiller, Ed, "Development of Laboratory Performance Tests

of Built-Up Roofing," Paper No. 12, Proceedings, 6th

Conference on Roofing Technology, National Roofing Contractors

Association, Chicago, IL (1983), pp. 63-68.

37
Table 1. Membrane Materials Used for Test Specimens for

Determination of the Strain Energy Criterion

Sample

No.

Plies

No.

£ Membrane Reinforcement Interply Bitumen

1 4 Organic Felt Asphalt

2 4 Glass Felt Asphalt

3 3 c d Polyester/Glass Fabric ' (2 ounce/square yard)Asphalt

4 3 Polyester/Glass Fabric6

(2 ounce/square yard)

Asphalt

5 3 Polyester Fabric

(2 ounce/square yard)

Asphaltic Emulsion

6 3 Polyester Fabric

(2 ounce/square yard)

Asphaltic Emulsion

7 3 Polyester Fabric

(3 ounce/square yard)

Asphaltic Emulsion

8 1 APP-Modified Bitumen^ None

9 1 SBS-Modified Bitumen^ None

a . The organic felt was ASTM D 226, Type I; the glass felt was ASTM

D 2178, Type IV; there were no ASTM standards available for the

polyester or polymer modified bitumen materials.

b. The asphalt was ASTM D 312, Type III? the asphaltic emulsion did

not conform to an ASTM standard.

c. This material was a composite of a filament glass net (5 strands

to the inch) bonded between two layers of polyester fabric.

d. The parenthetical expression is a fabric descriptor. The textile

industry describes fabrics by mass per unit area, normally ounces

per square yard or grams per square meter.

e. This material was a composite of a filament glass net (6 strands

to the inch) bonded on one surface of a polyester fabric.

f. APP indicates that the modifier was atactic polypropylene.

g. SBS indicates that the modifier was styrene-butadiene-styrene.

38
Table 2. Membrane Materials Used for Samples Tested in

Comparison to the Strain Energy Criterion

Sample

No.

Plies

No.

Membrane

Reinforcement3 Interply

Bitumen13 Surface

Coating0

10 3 Polyester Fabric ^

(3 ounces/sq yard;

Asphaltic

Emulsion

1 side

11 3 Polyester Fabric

(3 ounces/sq yard)

Asphaltic

Emulsion

2 sides

12 3 Polyester Fabric

(3 ounces/sq yard)

Asphaltic

Emulsion

1 side

13 3 Polyester Fabric

(3 ounces/sq yard)

Asphaltic

Emulsion

2 sides

a. There were no ASTM standards available for the polyester

fabric materials.

b. The asphaltic emulsion did not conform to an ASTM standard.

c. Emulsion was applied to one or both outer surfaces of the test

specimens in addition to the interply application.

d. The parenthetical expression is a fabric descriptor. The textile

industry describes fabrics by mass per unit area, normally ounces

per square yard or grams per square meter.

39
Table

3.

Tensile

Strength

of

Membrane

Samples

Used

in

the

Determination

of

the

Revised

Criterion,

Ibf/in.

(kN/m)

40

« x> u -o V

Unles

otherwise

indicated,

average

of

five

tests

Longitudinal

direction.

Transverse

direction.

Diagonal

direction.

Average

of

four

tests.
Table

4.

Elongation

of

Membrane

Samples

Used

in

the

Determination

of

the

Revised

Criterion,

percent 0) c

4.1

O

COa -u •H CO

4-4 bo

-h

a

/'“N z o

CJ O 4-1 fa 00 < o

fa ocn

r-.

(V c

4->

O « -H a « H «

4-1 bO H C 3 O

iH

4-> fa <

0)

4-*

C

(0

O

a th

•H 4J

4-1 CQ -H bO a co

4J r-( < w

<Da§

on

Os 14~\ cn CM CM sO • • • • • • • a m m sO •3

CM CM

© Os O' SO cn

H • • • • • • • CM CM cn 00 00 CM CM CM o CM CM CM sO SO

hJ • • • • • • • Cl CM sO m CM

CM CM

cnm 1 1 1 1 1 a • • 1 l 1 1 1 CM CM oz m

0) 0)

m >3

m cn >3 • • • • • • • 00 m Os sO m CM cn

CM CM V

00 sO <3 o SO sO • • • • • • • CM sD CM o CM 00 cn

*4-1 cn 00 >3 3 sO • • • • • • • CM Os m CM >3

CM

H «

cn

m cn 00 1 1 • • • • 1 • sr *3 cn o sO

*3 CM 1 1 o i 1 • • 1 1 • l 1 cn cn m Os | 1 • 1 1 1 • 1 <3 1 1 1 CM m 00 00 cn SO • • • • • • • o m cn

*3- cn Os

m 3-

0) sO OS sO CM 00 • • • 00 • • • m cn 3 >3 Os sO CM H -3 cn

a) O' -3 o 3 O 3 sO • • • • • • • CM Os o 00 CM cn

CM cn >3 cn

CM 00 • • i 1 1 l | 3 o i 1 1 i 1 cn cn

<3- Os i 1 1 i 1 • • i 1 1 i | CM CM

a> cn cn i 1 1 1 • • • i 1 1 • m

CObOCo

0)

a;

4-4 «a

0) -C

CO>s

4-4 C

0)

CDCO

0) *o

0)IJu3ooo

•o

COo

COua

a> .c«

-C

CM on

O

D CJ "O <D U-I

41

Longitudinal

direction

Transverse

direction.

Diagonal

direction.
Table

5.

Strain

Energy

per

Unit

Gage

Length

of

the

Membrane

Samples

Used

in

the

Determination

of

the

Revised

Criterion,

lbf*in./in.

(N*m/m) 9QUo

T3

tooM

*0a

to X« G O O • ^

«HC "O

u

O • 6 u c V *-> o U U *4 • u v

tj »-•

u c

•rl i -H

H t lH « ** C « *0 TJ

•H « $

T3 LH £ 3 n t «

4J > C « H O ^

tJC c bo C <C * V 3

<0 X3

U T3

O

42

Average

of

four

tests.

Average

of

three

tests.
Table 6. Tensile Strength, Elongation, and Strain Energy

of Typical 4-Ply Organic Built-up Membranes

Interply

Bitumen

Tensile

Strength

Ultimate

Elongat

.

Strain

Energy

Comment

lbf/in

.

(kN/m)

% lbf in. /in.

(N m/m)

Asphalt 287

(50.3)

1.5 3.2

(14.0)

Strain energy determined in

the present study.

Asphalt 267

(46.8)

2.1 3.9

(17.3)

Strain energy estimated from

Mathey and Cullen data [8].

Asphalt 208

(36.4)

1.9 2.8

(12.5)

Strain energy estimated from

Rissmiller data [30].

CT Pitch 1 265

(46.4)

0.9 1.7

(7.6)

Strain energy estimated from

Mathey and Cullen data [8].

CT Pitch 181

(31.7)

3.8 4.9

(21.8)

Strain energy estimated from

NRCA data [28].

CT Pitch 305

(53.4)

1.6 3.5

(15.6)

Strain energy estimated from

Rissmiller data [30].

1. Coal Tar Pitch

43
Table 7. Revised Performance Criterion for Load-Elongation Properties

Requirement The roof membrane shall withstand, without rupture,

the normal stresses imposed from internal or

external causes.

Criterion The tensile strength shall not be less than 200

lbf/in. (35 kN/m) in the weaker direction (longitudinal or transverse) of the membrane when tested at

0 °F (-18 °C).

OR

The strain energy shall not be less than

3 lbf in. /in. (13 N m/m) in the weaker direction

(longitudinal or transverse) of the membrane when

tested at 0 °F (-18 °C); in addition, the membrane

shall remain watertight after elongation at 0 °F

(-18 °C) to a percent at which the strain energy is

equivalent to the criterion strain energy of

3 lbf in. /in. (13 N m/m).

Test ASTM D 2523, Testing Load-Strain Properties of Roof

Membranes

.

Commentary Certain membranes exhibit anisotropic behavior.

Therefore, the results of tests in the weaker

direction should apply. For conventional bituminous

built-up membranes, the transverse direction is

usually weaker; whereas for the new membrane

materials, the weaker direction may be either

longitudinal or transverse.

Excessive elongation of the membrane may cause cracking

of the interply bitumen and loss of watertightness.

Thus, a watertightness test is conducted at a percent

elongation corresponding to the minimum strain energy.

The watertightness test used in the present study was

based on that given in the CSGB Standard for polymermodified bitumens. Because of difficulties encountered

in conducting this test, it is suggested that, for

future application of the proposed criterion, another

test be used or an improved watertightness test be

de ve loped.

44
Table 8 . Comparison of the Properties

With the Revised Performance

of the Membrane

Criterion

Samples

Sample Weaker Conformance to Conformance to

No. Direction Tensile Strength Strain Energy Water tightness

1. T

1 Yes Yes Not 3 needed

2. T Yes Yes Not needed^

3. T Yes Yes Not 3 needed

4. T Yes No Not 3 needed

5. T No No Not 4 needed

6. T No No Not needed^

7. L

2 No Yes Yes

8. T Yes Yes Not needed^

9. T No Yes Yes

1. Transverse direction.

2. Longitudinal direction.

3. The watertigthness test is not needed since the membrane specimen

conforms to the criterion for tensile strength.

4. The watertigthness is not needed since the membrane specimen does

not conform to either the tensile strength or strain energy

criterion

.

45
Table 9. Load-Elongation Properties of Additional Samples Tested

in Comparison to the Strain Energy Criterion3

Sample

No.

Tensile Strength

Peak Load

Elongation

Ultimate

Strain Energy

Total

Ibf /in

.

( kN/m)

percent lbf • in . /in

.

( N • m/m

)

10 72.5 19.6 12.6

(12.7) (55.8)

11 77.6 20.9 14.9

(13.6) (66.4)

12 141 11.4 11.8

(24.7) (52.5)

13 190 12.9 17.6

(33.3) (78.3)

a. Average of three tests. Tests were conducted at 0 °F (-18 °C) in

the transverse direction of the sample.

46
STRAIN

ENERGY <Ozo

Ul

47

Figure

1.

Strain

energy:

the

area

under

the

load-elongation

curve.
High

strength

o

I-

<

o

z

o

UJ

avon

48

Figure

2.

Comparison

of

strain

energy

for

materials

having

variable

load-elongation

properties.
Note:

Numbers

with

curves

represent

membrane

sample

numbers

w/n>i ‘avon

o o o o o O o o

o lO o LO o LO o LO

CO CO CM CM

*U!/*q| ‘avon

~o

<u

CO

a)>

ooc

03

t-

+->

o

+-> TO a

E

<uc

+>

oo>

i-o

a

c

o

4-> 03ooco

<u

I •o

TOO

CO

<u

ELONGATION,

percent
400 h

Note:

Numbers

with

curves

represent

w/N>l ‘QVOl oT

CO

k-0

-QEDC

0)aE

CO

CO0c

CO

L_ nE0E o O

O

O

O o co in oo cm -i- o o o o o o o o

to o lO o in o m

CO CO C\J CM t-

•ui/jqi

Q<O-1

a> -O

*T3

03CLEou

ai

s_03

to

a>c

03

s_ -OE

cuE

030c13

-Q

O0 5 00

03 i I C |

a>E Li- es o

oi oO 03

to XJ

03 03

•r- 4_>

+-» to

1- 03 03 4-3 a.O C

4- 03 CL _C5 co •> •r- tO

4-3 03 03 C

03 03 C t_ O JO — E

03 03 -i E

03 CL O 3

4-3 03 — JC -r- 4-> 3

JO

4-3 03 «—

JC 03

4-3

Co

03 -r- c 4-3 C 2 03 O >

-C c

to ou cO 4_

•f- o

4-3 03 03

k- to

4-3

O

to JC

Z3 4_» »— o

03U3

03

50
o

w/n>i ‘avcn

O 0.0 o o o o

cd lo "tt co eg -

Uj/Wl ‘avon

c

<DOu.<Da

O

I-

<

0

LU

51

Figure

5.

Load-elongation

curves

for

sample

no.

5,

when

tested

in

the

transverse

direction

at

73,

0,

and

-30°F

(23,

-18,

and

-34°C).
52

ELONGATION,

percent
300

-

i

Note:

Numbers

with

curves

represent

/

membrane

sample

numbers

o

10

T

w/n>i ‘avoi

o o o o

Tt 00 CVJ T- o

“1 1 1 1 lO

CNJ

53

Figure

7.

Comparison

of

the

load-elongation

curves

of

the

polyester

samples

with

that

of

the

organic

membrane

sample;

tests

are

compared

for

the

weaker

direction

of

the

membrane

sample

at

0°F (

-

18°C )

APPENDIX A. PERFORMANCE CRITERION FOR TENSILE STRENGTH

This appendix presents the performance criterion for tensile strength

for bituminous built-up membranes as suggested in 1974 by Mathey and

Cullen [8]. The format considered in the performance approach had

four key elements:

1. the requirement — This was a qualitative statement which

describes what the membrane was to accomplish.

2. the criterion — This was a quantitative express of the

level of performance which the membrane should have to

perform acceptably.

3. the test — This was the test method which was used to

determine that the membrane conforms to the sbated

criterion

.

4. commentary — This was to provide comment concerning

an explanation of the reason for, or intent of, the stated

criteria

.

The tensile strength criterion is as follows:

Requirement The roof membrane shall withstand, without rupture,

the normal stresses imposed from internal or

external causes.

Criterion The tensile strength shall not be less than 200

lbf/in. (35 kN/m) in the weakest direction of the

membrane when tested at 0 °F (-18 °C).

Test ASTM D 2523, Testing Load-Strain Properties of Roof

Membranes

.

Commentary This criterion is based on performance in service.

Certain membranes exhibit anisotropic behavior.

Therefore, the results of tests in the weakest

direction (usually transverse or "cross machine"

direction) should apply.

A-l
'
APPENDIX B. PREPARATION OF BUILT-UP MEMBRANE SPECIMENS

The built-up membrane test specimens were prepared from the materials

described in Tables 1 and 2. Sheets of reinforcement, having dimensions

of 8 x 10 in. (200 x 250 mm), were cut from the rolls of felt or

fabric. The 10-in. (25-mm) dimension corresponded to the orientation

of the felt or fabric (longitudinal, transverse, and diagonal) in the

test specimen. One sheet of the reinforcement was coated with the

between-ply bitumen (hot asphalt or asphaltic emulsion), and a second

sheet was immediately applied on the bitumen. The resulting 2-ply

membrane sandwich specimen was placed in a laboratory press. Two

spacing rods, having diameters approximately equal to the desired

thickness of the resultant 2-ply membrane sandwich specimen, were set

at the two edges of the lower platen of the press. A minimum force of

500 Ibf (4.90 kN) was applied to the membrane specimen and spacing

rods. In this manner, the between-ply bitumen layer was pressed to

the desired thickness, and excess bitumen was forced to flow out of

the edges of the membrane specimen. Additional plies of membrane were

in turn built-up from the 2-ply specimen in a similar way using

spacing rods having appropriately selected, larger diameters. Tensile

test specimens were cut from the membrane sandwiches using the die

described in ASTM D 2523 [24].

B-l

APPENDIX C. COEFFICIENTS OF VARIATION OF MEMBRANE SAMPLES TESTED

FOR DETERMINATION OF THE STRAIN ENERGY CRITERION

Appendix C gives a summary of the coefficients of variation for the

average values of tensile strength, percent elongation, and strain

energy for the tests (sample nos. 1-9) conducted to revise the tensile

strength criterion. Unless otherwise noted in the tables, five

membrane specimens were tested for each condition.

C-l
Table

Cl.

Coefficients

of

Variation

for

Tensile

Strength

of

Membrane

Samples

Used

in

the

Determination

of

the

Revised

Criterion

C-2

Longitudinal

direction

Transverse

direction.

Diagonal

direction.
Table

C2.

Coefficients

of

Variation

for

the

Percent

Elongation

of

Membrane

Samples

Used

In

Determination

of

the

Revised

Criterion

C-3

M-locV

x:c

0)

co

CD

0) T3

0)

J-i Duuo

T3

COO

Q)

<D<X

0)

<0 x:

io x>u x) a) ^

Longitudinal

direction

Transverse

direction.

Diagonal

direction.
Table

C3.

Coefficients

of

Variation

for

Strain

Energy

of

Membrane

Samples

Used

in

the

Determination

of

the

Revised

Criterion

O fa 00 00

Q CO ON 00 r>« co co o co CM CM CM

fa

csj u

b

T

m o ON fa CM

m -J o <3- m CO fa o CM CO CM CM CM m m CO

CO

a CJ fa o co o o ON 00 "H »-h s3 •CM CO

si/ o oo 1 1 1 1 tt( I 0 O ON o O G CNI fa

CO1 T3

COo

fa fa | 1 1 1 1 1* H 1 1 CO

<D fau<

co | 1 1 1 1 1 1 fa

01 <D 00 O CM m ON fa

u

a

a 'Om fa fa -3" rs» CM CM CO CM

a> fa CM fa CM O fa CO CO

cO fa • • • • a

aa

u

H fa CO co 00

m r>*

OH

H »-h CO CM CO CO CM

4-i CM <—* CM ON m CM

CO

a CJ fa o CM ON o -o CM o <NI —4

co

00

(1) fa 00 r>* fa

fa

a

a i 1 a

O

O 00 fa *0 *0 I 1 ON o a —H

fa

m S3 -a

cooJM •a o m i 1 cO fa 1 1 1 a i 1 a

<DH i 1 1 1 ON o ON

fa 00 in u<

c0 Hx

m

fa 1 | a i i 1 a 1 1 i i 1 r>» co

CO in ON fa in ON

O

G CM ON

m co CM OO r>.

(D ON CM ON fa r^.

c0 fa • a

a

aa • u H 00 00 —H CO fa r** <3 O H CM •—> —-4 CO H

01

to „ o fa 00 00 CO

/~\ faaa

a

a

U

m 00 CO

m S3 ON 0 CM CO

CM

N-*'

U

O fa a

a | 1 i 1 ! 0 -o co ON i 1 i 1 1 CO CO r-s S

fa CO CO 1 | i 1 , CO H • a 1 1 i 1 1 0) ON m

fa CM CM

fa

V < fa fa <T

CO • 1 1 I I fa rg 1 i i i 00 CO CM

0) fa

fa • a a O CM co NT in fa r** oo ON

c0 z

C/3

M

<0V

Q.

a o O • *H

«HC

V

u o . c U -H O -H « «-» o U o <*4 00

II

u ^

*o

u

u

a

H T3 ^ H 2 * *o

•H • D

T3 lH £ 3 0* * • u > d * h a o i3 DO C 00 d « <* d OH « U O <1 *M

C-4

Average

of

four

tests.

Average

of

three

tests.
APPENDIX D. COEFFICIENTS OF VARIATION OF ADDITIONAL SAMPLES

TESTED IN COMPARISON TO THE REVISED CRITERION

Appendix D gives the coefficients of variation for the average values

of tensile strength, percent elongation, and strain energy for the

tests on the additional membrane samples. (Table 2). Three replicates

were tested for each sample at 0 °F (-18 °C) in the transverse

direction of the specimen.

Table Dl. Coefficients of Variation for Tensile Strength, Elongation,

and Strain Energy of the Additional Membrane Samples, percent

Sample

No.

Tensile Strength

Peak Load

Elongation

Ultimate

Strain Energy

Total

10 5.3 49.2 50.8

11 5.8 37.7 43.3

12 11.8 14.0 21.7

13 12.8 13.5 22.9

D-l
-
NBS-114A (REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3.

NBSIR 86-3418

Publication Date

SEPTEMBER 1986

Strain Energy of Bituminous Built-Up Membranes:

An Alternative to the Tensile Strength Criterion

5. AUTHOR(S)

Walter J. Rossiter Jr. and Dale P. Bentz

6. PERFORMING ORGANIZATION (If joint or other than NBS. see in struction s) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

E. I. DuPont de Nemours and Company

Textile Fibers Department

Old Hickory, TN 37138

10.

SUPPLEMENTARY NOTES

1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi b I i o graph y or literature survey, mention it here)

This study was conducted to revise the performance criterion for tensile strength of

bituminous built-up membranes. Bituminous membrane samples, fabricated from

polyester fabric, polyester-glass composite fabric, and single plies of APP- and

SBS-modified bitumen, were tested in tension to determine their load-elongation

properties and to measure their strain energy. The results of the tensile tests

of the new bituminous membranes indicated wide variability of load and elongation

among the different types of materials.

As an alternative to the criterion that a bituminous built-up membrane have a tensile strength of 200 lbf/in. (35 kN/m), it was recommended that the strain

energy should be a minimum of 3 lbf*in./in. (13 N*m/m), when tested at 0 °F (-18 °C)

in the weaker direction. The properties of the membrane samples were compared to

the suggested revised performance criterion. Two polyester samples (without glass),

having relatively low strength and low ultimate elongation at 0 °F (-18 °C), did

not conform to the revised criterion.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semi colon s) bituminous roofing; built-up; low-sloped; performance criterion; polyester;

polymer-modified bitumen; roofs; strain energy; tensile strength

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

LX] Unlimited

| 1 For Official Distribution. Do Not Release to NTIS 67

1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. 15. Price

[Xj Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $11.95

USCOMM-DC 6 04 3 -P 8