NATIONAL BUREAU OF STANDARDS REPORT

9356

THERMAL- SHOCK RESISTANCE FOR

BITUMINOUS BUILT-UP ROOFING MEMBRANES -

ITS RELATION TO SERVICE LIFE

by

William C. Cullen and Thomas H. Boone

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

421.04-12-4212447 30 June 1966 9356

THERMAL- SHOCK RESISTANCE FOR

BITUMINOUS BUILT-UP ROOFING MEMBRANES -

ITS RELATION TO SERVICE LIFE

by

William C. Cullen and Thomas H„ Boone

Materials and Composites Section

Building Research Division

Institute for Applied Technology

Sponsored by

Office of the Chief of Engineers

Department of the Air Force

Bureau of Yards and Docks

IMPORTANT NOTICE NATIONAL BUREAU OF ST

for use within the Government, and review. For this reason, thi whole or in part, is not author Bureau of Standards, Washingti

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

ss accounting documents intended

subjected to additional evaluation

listing of this Report, either in

; Office of the Director, National

the Government agency for which

opies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

THERMAL- SHOCK RESISTANCE FOR

BITUMINOUS BUILT-UP ROOFING MEMBRANES -

ITS RELATION TO SERVICE LIFE

by

William C. Cullen and Thomas H. Boone

1 . INTRODUCTION

Any rupture of the waterproofing element of a roofing assembly generally leads

to failure of the total assembly, from both a weatherproofing and a heat transfer viewpoint. Moseley [1]—^ has indicated that tension splits which occur in

the built-up membrane are frequent causes of failure. Cullen [2] suggested

that the thermal movement which occurs in a bituminous built-up roofing membrane

due to rapid temperature changes is often a contributing factor in tension

splits. He proposed the utilization of a Thermal- Shock Resistance Factor [3]

to predict the ability of a built-up membrane to tolerate, without rupture, the

forces induced by such temperature changes [4]

The higher the breaking load, the lower the apparent linear thermal expansion

coefficient and the lower the modulus of elongation the more resistant the

membrane will be to rupture under thermal changes. These parameters were com- bined in a Thermal-Shock Resistance Factor given by the following equation (1)

TSRF 1=_S_ (1 ) Ma

where

TSRF^ = Thermal-Shock Resistance Factor

S = Breaking load, lb/ in^

M = Modulus of Elongation (initial tangent), lb/in^-'

a = Coefficient of linear thermal expansion

The development of the Thermal-Shock Resistance Factor has been reported, and

values for three conventional built-up membranes were given at temperatures of

-30°F (-34 .4 °C) , 0 °F (-17.8°C), 30°F (-1.1°C), and 73°F (22.8°C) [6].

1 / Figures in brackets refer to references presented at the end of this paper.

LI

The breaking loads and moduli of elongation are expressed in pounds per inch

width of membrane since a previous study [5] had shown that the effect of

the bonding bitumen on the strength characteristics of the membrane are negligible. The elongation moduli are the slopes of the initial tangent to

the load-strain curves.
2 . EXPERIMENTAL

2.1 Materials and Specimen Preparation

2.1.1 Laboratory

Four types of bituminous saturated or impregnated felts and two types of bitumen

were selected for the preparation of laboratory specimens. Table 1 identifies

the materials.

The laboratory prepared specimens consisted of two, three and four plies of

felt adhered to each other with the appropriate bitumen applied at a spreading

rate of about 25-30 lb per 100 sq. ft., which is comparable to that obtained

in good roofing practice. The laboratory prepared specimens were cut into a dumbbell- shaped according to a procedure developed at the National Bureau of

Standards [6]

.

Figure 2 shows the die, the test specimen, and a cross section

of the test specimen.

2.1.2 Field

In order to obtain samples from roofs of known history, it was necessary to

rely upon the roofing industry. Samples were obtained through the cooperation

of contractor associations, roofing manufacturers, and, in many cases, private

roofing contractors.

Two samples, a minimum of one square foot in area, were cut from the selected

roof. The samples were identified in respect to location, composition and felt

orientation, and were forwarded to NBS. A history, as well as a diagram of the

roof plan, frequently accompanied the samples. In some cases the roofs in

question were observed by a NBS representative who selected the areas from which

the samples were to be removed. In the cases of roof rupture, samples were

taken near the failure.

Information of the field samples relative to the location, age, composition,

and number of plies is given in Table 3.

The samples werfe prepared for test by removing the mineral surfacing (if present)

and any adhering insulation. Dumbbell- shaped specimens similar to the laboratory-prepared test specimens were cut. Figure 3 shows a test specimen

obtained from a roof membrane after 9 years exposure.

= 2 =
2.2 Load-Strain Properties in Tension

The load- strain properties and the breaking strengths of the specimens were measured in duplicate (triplicate when sufficient specimens were available)

employing a tensile testing machine equipped v?ith a temperature chamber to

control the specimen temperature within +5°F (+2.,7°C) during the test. The loadstrain measurements were made at 0°F (-I7.8°C). The gage length, as defined

by the distance between the jaws of the testing machine, was 4.5 inches. A

straining rate of 0.05 inches per minute (1.1% per minute) was used in each

determination.

2.3 Linear Thermal Expansion Coefficients

The linear thermal expansion measurements were made on duplicate dumbbell- shaped

specimens prepared as described in section 2.1, using a 5- inch gage length in

accordance with a procedure described by Cullen [2].

3. RESULTS

The average results of the load-strain measurements for the laboratory-prepared

specimens are reported to the nearest 10 lb. in Table 2, and those of the

samples obtained from roofs exposed under service conditions are reported in

Table 3. In each case the values for the elongation moduli are the slopes of

the initial tangent to the load-strain curves.

The apparent linear thermal expansion coefficient for the laboratory prepared

and field obtained samples were calculated from data obtained over a temperature

range of 30 to -30°F (-1.1°C to -34.4°C), and are given in Tables 2 and 3.

4. DISCUSSION OF RESULTS

4.1 Effect of Number of Plies of Reinforcing Felt on the

Strength Characteristics of a Built-Up Membrane.

Figure 4 shows that the breaking load in tension for a multiple-ply roofing

specimen increased in proportion to the number of plies of felt in the membrane.

Further, the character of the reinforcing felt had a marked effect on the values

of the breaking loads. Figure 5 shows the effect of the number of plies of

reinforcing felt in the membrane, as well as the character of the felt, on the

elongation modulus (initial tangent). Again a significant increase was noted in

this property as the number of plies was increased.

Table 2 shows that, although the coefficient of linear thermal expansion varied

with the composition of the respective sample, the number of plies of felt in

the membrane had no significant effect on the value of the coefficient of thermal

expansion.

-3,
The value for the Thermal- Shock Resistance Factor was approximate 1}/ the same for

a 2-, 3-, or 4-ply membrane, despite the increase in breaking load with number of

plies . The increased breaking load was offset by the increased elongation

modulus, while the values of the coefficient of linear thermal expansion remained

independent of the number of plies of felt*,

If the Thermal-Shock Resistance Factor is a valid criterion, it many be concluded

from the data given herein that the resistance of a membrane to thermally induced

forces is very nearly independent of the number of plies of reinforcing felt in

the membrane

.

The difference in the values of the linear thermal expansion coefficients in

respect to the orientation of the felt in a built-up membrane has been report [2],

The values of the engineering properties and those of the Thermal-Shock Resistance Factors reported here also reflected the anisotropic behavior of a built-up

roofing membrane. The difference in value of these properties between the

"machine" (longitudinal) and the "across machine" (transverse) directions of

both the organic and asbestos based felts appeared to be sufficiently large as to result in differences in performance in field service. Cullen described the

relation between felt orientation and the frequency of tension tearing of builtup roofs in field service as a splitting which generally occurred parallel to

the machine direction of the felt.

The samples prepared with glass based felts, for all practical purposes, exhibited

isotropic behavior in respect to strength properties and Thermal-Shock Resistance

Factors

.

4.2 Relation Between Values of Thermal-Shock Resistance Factors and

Performance in Field Service

The effective service life of a bituminous built-up roofing system depends to a large extent on the initial strength properties of the waterproof membrane, as well as on the retention of these properties during long periods of exposure in

service. The results of this investigation not only showed that the properties

of various roofing membranes changed with exposure, but also indicated the type

of changes which may be expected upon exposure to the elements. In this con- nection, a marked decrease in values of the breaking loads of those membranes

exposed in service with some exceptions, was observed. An increase in values of

both the elongation modulus and the linear expansion coefficient was also indicated

by a comparison of the results obtained on field samples and those obtained on

the laboratory prepared specimens. When the values of the properties of the

weathered samples were substituted in equation (1), it was found that the ThermalShoek Resistance Factors of the weathered samples were considerably lower than

those obtained for the unweathered samples. It should be noted here that no data were available on the original properties of the samples obtained from the

field. However, because of the similarity of construction and composition of the

built-up field samples with those fabricated in the laboratory, the following

broad assumptions seem to be valid at this time.

- 4-
The values for the Thermal-Shock Resistance Factors of coal-tar-pitch, organic

felt membranes, although initially lox^er than those of the other samples included

in the program, exhibited the least change when exposed under service conditions.

On the other hand, the factors for the asphalt, organic felt membranes show the

greater decrease in the values followed by the asphalt, glass felt membranes. Unfortunately, samples of the asphalt, asbestos felt membrane exposed to service

conditions were not submitted under the project

If these data were applied to predict effective service life, they would indicate

that, initially, the coal-tar-pitch, organic felt built-up roof membranes are more vulnerable to thermally induced forces than the asphalt, organic or the

asphalt, glass combinations . However, as the weathering process proceeds, the

difference is susceptibility to these forces between the coal-tar-pitch and

asphaltic products in substantially reduced.

Samples were obtained from five built-up roofs each of which had exhibited tension

splits in service attributed in part to thermally induced forces, while samples were obtained from two roofs exhibiting no such failures. In each of the five cases where splitting was reported, the length of the splits were parallel to the

machine direction of the felt, without exception. This behavior was not un- expected, since previous field experiences have indicated that a relation exists

between felt orientation and splitting failures |2J. Further, when the values

for the Thermal-Shock Resistance Factors are considered in respect to felt

orientation in the membrane, the "across machine" direction appears to be the

most vulnerable to splitting failure.

In summary, the results of this study indicates that an apparent relation exists

between the incidence of splitting failure in a membrane and the values of the

Thermal-Shock Resistance Factor as given in Table 4. Further, a comparison of

results obtained in the laboratory and of observations of effective service life

of built-up roofing systems in the field, confirm our opinion that the higher the

Thermal-Shock Resistance Factor, the more resistant a membrane will be to failure

due to thermally induced forces.

5 . COMMENTS

Although all the pertinent properties of built-up membranes were not demonstrated

experimentally in this program, it is believed that the thermal shock concept

which is proposed may be useful in predicting the performance of built-up roofing

membranes in respect to properties, roof design, and the environment in which the

roofing system is expected to serve.

5

-
Splitting failures attributed in part to thermally induced forces appear to be

more prevalent in cold climates where roofing membranes were exposed to larger

and more rapid temperature changes than in areas where temperatures were more

uniform,, In order for a membrane to resist these failures due to large temperature changes at sub- freezing temperatures, it follows that a higher value for

the Thermal~Shock Resistance Factor will be required than for the same membrane

exposed to milder climates. This concept is not included in the factor given in

equation (1). Therefore, in order to reflect the temperature conditions of the

roofing system® s environment into the Thermal “Shock Resistance Factor, it is

proposed that the inclusion of another term to the right side of equation (1)

be made. Since thermally induced shrinkage failures are associated with low

and changing temperatures, the term should take into account only temperatures

below 32 °F and possible extreme changes which may occur in the climate where

the roof is exposed. Equation (1) may now be written

where: t = 32 °F

TSRF = iL_ • _I

2 Ma At

(minimum mean daily temperature, January).

( 2 )

The relative position of a bituminous built-up membrane in a roofing system

will also influence its effective service life. It has been demonstrated ex- perimentally that bituminous built-up membranes placed over insulation were

subject to greater temperature changes than similar membranes placed over more

dense substrates [4]

.

It has also been observed that tension splits attributed

in part to thermally induced forces occur more frequently in membranes placed

over substrates having high thermal insulating values. Therefore, it appears

that the ability of components of the roof system to conduct heat to and away

from the roofing membrane will, by regulating the amount and rate of temperature

change of the membrane, affect the service life of the membrane. The thermal

diffusivity of the substrate would appear to be a relevant factor in the ThermalShock Resistance Factor and may be Included by rewriting equation (2)

where

TRSF

3

_S

Ma

K = thermal conductivity of the substrate

p = specific heat of the substrate

d = density of the substrate.

Based on the foregoing comments the authors suggest the above equation (3) as

a possible useful revision of the Thermal-Shock Resistance Factor and feel that

it should be tested by further investigations of the engineering behavior of

the whole roof assembly and of the interrelationship of its components under

different thermal exposures.

"6
6 . ACKNOWLEDGMENT

The authors acknowledge with thanks the assistance of A. J» Turner in the preparation of the specimens and in making the measurements reported in this paper.

The authors also acknowledge the advice and encouragement given to them by

W. W. Walton and J. R. Wright.

The authors appreciate the cooperation of the Midwest Roofing Contractors

Association, Inc., in making available, through their membership, exposed roof

cut-outs and first-hand knowledge of their performance.

- 7-
REFERENCES

[ 1] G. N. Moseley, "Built-Up Roofing Tension Splits," The Journal of the

Building Research Institute, Vol. 1, No. 6, 31 (Nov. -Dec. 1964).

[2] W. C. Cullen, "Effects of Thermal Shrinkage on Built-Up Roofing,"

NBS Monograph No. 89 (1965).

[ 3j J. Martin, Mechanical Behavior of Engineering Materials (PrenticeHall, Inc.), 338 (1962).

[4] W. C. Cullen, "Solar Heating, Radiative Cooling and Thermal Movement - Their Effects on Built-Up Roofing," NBS Technical Note 231

(December 1963).

[5] P. M. Jones, "Recent Research on Bituminous Materials," ASTM STP-347

(1963).

[6] W. C. Cullen and T. H. Boone, "Progress in the Development of a Thermal-Shock Resistance Factor for Bituminous Built-Up Roofing

Membranes, "ASTM Symposium, Paper 169, June 1966.

8-
Table 1. Materials Used in the Preparation

of Laboratory Specimens

Materials

Bitumens

Asphalt

Coal- tar-pitch

Reinforcing Felts

Asphalt Saturated Asbestos Felt

Asphalt Saturated Organic Felt

Asphalt Impregnated Glass Felt

Coal-tar Saturated Organic Felt

ASTM Specification

D312, Mineral Sur. Flat

D450, Type A

D250, 15 lb type

D226, 15 lb type

1 /

D227

—1 / "Perma Ply No. 11," as manufactured by the Owens-Corning

Fiberglas Corporation. The commercial material is identified

in this paper in order to specify the experimental procedure

adequately. In no case does such identification imply re- commendation or endorsement by the National Bureau of Standards

nor does it imply that the material identified is necessarily

the best available for the purpose.
Table 2 . Properties of Laboratory Prepared

Built-Up Membranes at 0*F

Felt No. Direct-— Break Elong

.

Appar .Lin

(bitumen) Plies ion Load . (S)Mod . . (M)Therm. Exp

Coef f

.

lb/ in lb / in (°F)-1

4

Organic 2 L 180 1.3x10 10x10°

(asphalt) T 130 .9x10 17x10

6

„ 4 — 6

3 L 280 i.9xio;

q. 10x10 c T 180 1.2x10 24x10

l\ 4 L 400 2.8x107 iixi°_;

T 300 1.9x10* 21xl0-b

4 -6

Asbestos 2 L 170 2.0x10)/ 10x10

(aspha It) T 80 .9x10 26x10"

4 — 6

3 L 270 2.8x10); 8x 10 _;

T 120 1.3xl0 4 30xl0

-b

4

4 L 310 3.3x10); 11x10 ° T 130 1.6x10* 32xlO _b

4 -6

Glass 2 L 110 .9x10); 20x10°

(asphalt) T 80 .7x10* 20xl0“ b

4

3 L 160 1.3x101 22x10 ° T 140 1.1x10* 35x10"

4

4 L 210 1.6x10* 2°x 10 ° T 180 1.3x10* 22xl0" b

4 Organic 2 L 200 2.5x10)7 22x10 °

— n (coal tar) T 100 1.7x10 33x10

4 -6

3 L 330 3.9x107 25x10 I T 150 3.3x10* 42x10

4 -6

4 L 450 4.8x107

q

27x10

T 220 3.9x10 42x10

1/ L - Longitudinal (with machine)

T - Transverse (across machine)

2/ Average of three specimens

3/ Temperature Range +30°F to -30°F

Thermal

Shock

Factor

1350

850

1500

650

1300

750

850

300

1200

300

850

250

620

600

540

360

660

610

370

190

330

110

350

140
Table 3. Properties of Built-Up Roofing

Field Samples at 0°F

Felt No. Age Source Dir.—^Break Elong„ Appar.Lin. Thermal

(bit.) Plies Yrs . Load^ Mod.^^ Therm. Exp^ Shock

(S) CM) “ Coef f . (a )— Factor

lb/ in lb/ in (“F)" 1

Organic 3 6 RockfordL 240 4.5x10^ 16x10"^ 750

(asphalt) 111. T 140 4.7x10 41x10 500

Organic 3 22 K„ C. , L 220 2.3x10^ 15x1 o”f 620

(asphalt) Mo. T 130 1.6x10 30x10 260

Organic 4 8 Waterloo

L 230 1.4x10* 25x10“^ 650

(asphalt) Iowa T 80 l.lxlO4 23x10 350

Glass

(asphalt)

3 9 Omaha

Nebr

L

T

90

80

1.8xlo£

1.2x10

20xl0~?

19x10“

450

350

Organic 3 12 Rockford L 210 3.8x10* 20x10“^ 300

(coal-tar) 111. T 70 2.0xl04 31x10 100

Organic 4 20 Waterloo L 270 5.1x10* 550

(coal-tar) Iowa T 200 2.5x10 16x10 500

Organic 4 25 K. C. s L 490 5.1x10* 13x10~£ 560

(coal-tar) Mo. T 260 4.9x10 23x10 230

1/ L - Longitudinal (with machine ) T - Transverse (across machine)

2 / Average of three specimens

3 / Temperature Range +30 °F to -30 °F
Table 4. Relation Between Values of Thermal Shock-Resistance and

Splitting Failures Observed Under Service Conditions

Thermal Shock-Resistance Factor

(Transverse Direction)

Felt

(bitumen)

No.

Plies

Age

Yrs

.

Source Membrane Exhibiting

Tension Splits

No Evidence of

Tension Splits

Organic

(asphalt)

3 6 Rockford

,

111.

500

3 22 Kansas City,

Mo.

260

4 8 Waterloo

,

Iowa

350

Glass

(asphalt)

3 9 Omaha

,

Nebr

.

350

Organic

(coal-tar)

3 12 Rockford

,

111.

100

4 20 Waterloo

,

Iowa

500

4 25 Kansas City,

Mo.

230
CAPTIONS FOR FIGURES

Figure 1. Roof sample taken from field. Note membrane split.

Figure 2. Cutting die, four-ply built-up roofing specimen and cross

section of membrane.

Figure 3. A prepared test specimen of a membrane taken from the field

after nine years exposure.

Figure 4. Breaking load of built-up roofing membranes at 0°F.

Figure 5. Elongation modulus (initial tangent) of built-up roofing

membranes at 0°F.



BREAKING

LOAD,

LB/

IN

PLIES OF FELT IN MEMBRANE
PLIES OF FELT IN MEMBRANE