Bureau of StafTdaras

' 2 1 7967 BUILDING SCIENCE SERIES

Thermal-Shock

Resistance

for Built-Up

Membranes

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UNITED STATES DEPARTMENT OF COMMERCE

Alexander B. Trowbridge, Secretary

(J 5 , NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Thermal-Shock Resistance for

Built-Up Membranes

William C. Cullen and Thomas H. Boone

Building Research Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Building Science Series 9

Issued August 21, 1967

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li^TiOm EURcAU OF Sl^^SI^

SEP 1 2 1970

Ho.

9

LIBRARY OF CONGRESS CATALOG CARD NUMBER: 67-60063
Contents

Page

Section 1. Progress in the development of a thermal-shock resistance

factor for bituminous built-up roofing membranes

W. C. CuUen and T. H. Boone 1 Section 2. Thermal-shock resistance for bituminous built-up roofing

membranes—its relation to service hfe

W. C. CuUen and T. H. Boone 7

iii

Section 1

Progress in the Development of a Thermal-Shock Resistance Factor

for Bituminous Built-up Roofing Membranes ^

William C. CuUen and Thomas H. Boone

The resistance of bituminous built-up roofing membranes to thermally induced forces

is considered in terms of their strength properties such as breaking load in tension, modulus

of elongation and apparent linear thermal expansion coefficient. The development of a Thermal-Shock Resistance Factor is described and values are given for three bituminous

built-up membranes at temperatures of -30 °F (-34.4 °C), 0 °F (- 17.8 °C), 30 °r (- 1.1 °C)

and 73 °F (22.8 °C). The apparent relation between the values obtained in the laboratory

and the observed performance of roofing membranes in service is considered. The utilization

of the Thermal-Shock Resistance Factor in the reduction of potential failures of bituminous

built-up roofing membranes in service from thermally induced forces is also discussed. Key Words: Development, roofing membrane, strength properties, thermally induced

forces, thermal-shock resistance factor. Contents

Page

1. Introduction 1 2. Thermally and mechanically induced forces 2

3. Materials and specimen preparation 3

4. Load-strain properties in tension 3

5. Apparent linear thermal expansion coefficients 5

6. Thermal-shock resistance factor 5

7. Relation between splitting failures and the thermal- shock resistance factors 5

7.1. Climate 5

7.2. Thermal characteristics of substrate 5

7.3. Orientation of reinforcing felt 5

7.4. Composition of built-up membrane 5

8. Conclusions 6

9. References 6

1. Introduction

In 1964 the amount of built-up roofing components produced was sufficient to cover over 1.7

billion square feet of roof area at an installed cost

in excess of 450 million dollars. A large number of roofing systems constructed from these components should perform adequately for 20 or more

years. However, the small percentage which are destined to fail prematurely will present serious

and costly problems to manufacturers who pro- duce the components of the system, the architects

who design the systems, the contractors who apply

them, and the building owners and occupants who

expect good service from them.

In the design and construction of a roofing

system, the architect and roofing contractor have

little choice. They must rely either on their past

experience with the performance of a specific sys1 Also published as part of ASTM STP 409, American Society for Testing and Materials (1966).

tern under known exposure conditions or on the

material and application specifications recom- mended by manufacturers. In order to permit

greater flexibility, there is a need for design cri- teria by which the architect-engineer and materials manufacturer can select suitable materials

to assure a properly designed roofing system which

will perform adequately under specific exposure

conditions.

The performance of a roofing system cannot be

predicted or evaluated entirely on the basis of the

properties of the individual components. An understanding of the interdependence and interaction

between and among components is of paramount

importance and must be considered by the designer. This is apparent from the discussion, by

various authors, of problems such as blistering,

wrinkle cracking, and membrane shrinkage fail1
uxes [1, 2, 3, 4, 5, 6, 7],^ frequently associated with

bituminous buUt-up roofing systems. Nevertheless,

this does not mean that the properties of the in- dividual components can be ignored. Indeed, it is most important that in order to comprehend the

performance of the total system the function of each individual component must also be understood.

The object of this research was to obtain laboratory data on some engineering properties of

conventional bituminous built-up membranes and

to attempt to relate these data to the performance

desired of roofing systems operating under service

conditions. A second objective was to develop a

procediire for utilizing these data in such a manner

that would be useful as a guide to the manufacturer, the architect-engineer, and the roofing contractor in the manufactm-e, selection, and application of components of a roofing system.

2 Figures in brackets indicate literature references at the end of this paper.

2. Thermally and Mechanically Induced Forces

The authors' experience over many years with

bituminous built-up roof performance in service

has indicated th? t many failures can be attributed

to factors other than mechanically and thermally

induced forces with which this paper deals. Therefore, this paper should not be construed to imply that these other factors, such as moisture

movement, wrinkle cracking, blistering, etc., are not important considerations in many bituminous

built-up roof failures. Any rupture in the waterproofing element of the roofing system generally leads to a failiu-e of

the total roofing system from both a heat transfer

and weatherproofing viewpoint. Recent experience

has shown that splitting of built-up roofing membranes is a frequent cause of failure [7]. Cullen [6] suggested that the rather sudden

temperature changes which occur in roofing membranes are a key element in splitting failures. Thermal shock stresses therefore can be expected

to be greater in roofing materials than in other

components of a building construction. In this connection he proposed the development of a thermal-shock factor which should prove useful

for predicting the ability of a roofing membrane

to tolerate, without rupture, the movements pro- duced by the rather sudden temperature changes.

The thermal-shock resistance factor which was

proposed depends upon some laboratory determined properties of the built-up membrane as given in eq (1). TSF= _S_

Ma (1) where

TSF = Thermal-Shock Resistance Factor

S = Breaking load, lb/in (see footnote 3) M = Modulus of Elongation (initial tangent)

lb/in (see footnote 3). a = Apparent Linear Thermal Expansion

Coefficient, per °F.

If a restrained object is heated or cooled,

thermal stresses are induced that are proportional

to the coefficient of linear thermal expansion, the

2 The breaking loads and moduli of elongations are expressed in pounds per

inch width of membrane since a previous study (8] has shown that the overall thickness of the membrane is negligible. The elongation moduli are the

slopes of the initial tangent to the load-strain curves.

modulus of elasticity and the temperature change.

This relation has been expressed by eq (2) [9].

where

(2)

ft = Stress due to temperature change, psi

E = Modulus of elasticity, psi

e = Coefficient of linear expansion, per ° F = Change in temperature ° F.

If we rewrite eq (2) as follows

(3)

it becomes apparent that the proposed ThermalShock Resistant Factor takes on a physical

significance. Numerically it is equivalent to the i the temperature drop required to produce a

stress equal to the ultimate. This assumes

constant properties and perfect elasticity which of course is not the case here. The higher the value for the Thermal-Shock

Resistance Factor for a roofing membrane the more resistant it should be to splitting failures. Therefore, the more resistant built-up membranes

will have a high breaking load, a low modulus of

elongation, and a low coefficient of apparent linear

thermal expansion.

At this time the proposed test shoiild be!

recognized as an empirical one which may or may

not correlate with the performance of a specific

built-up roofing membrane in service. Therefore'

a high value for the factor does not necessarilj ii infer that in aU cases the membrane wiU exhibit J better performance characteristics than one dis- playing a lower value.

Initially, it was believed that roofing splits

resulted solely from shrinkage of the membrane _

due to forces induced by sudden temperature

changes. However, recent studies by Jones [8] and

Cullen [10] on engineering properties of bituminous,

built-up membranes have indicated that mechan-'

ically induced forces must also be considered.

Mechanical stresses are sometimes induced in the roofing membrane by cracks which occur in the

substrate or by differential movements between

units of the substrate. Koike [11] discusses this! problem. He derived eqs (4) and (5) below thati

2
describe the conditions for rupture of a bituminous

membrane in relation to some physical properties

of the membrane and the adhesive system used to bond the membrane to the substrate. He states that a roofing membrane will rupture if the in- crease in width (AW) of a crack in the substrate is equal to or exceeds the quantity expressed on the

right side of eq (4), provided that the shear

strength (tq) of the adhesive between the membrane and the substrate is equal to or exceeds

the quantity expressed by the right-hand side of

eq (5). On the other hand, when the shearing strength

(tq) of the adhesive is less than the value on the

right side of eq (5), a shear failure will occur

within the adhesive and, regardless of the width

of the crack produced by movement in the substrate, no ruptiu"e will occiu" in the membrane.

/- ta (4)

(G)

(EKtaKU (5)

where

AW=Increase in width of crack in the substrate,

cm

)S=Breaking load in tension, kg/cm

Modulus of elasticity (initial tangent) of membrane in tension, kg/cm^

fa =Thickness of adhesive between membrane

and substrate, cm

(?= Modulus of elasticity (secant modulus) of binding bitumen in shear, kg/cm^

fm= Thickness of membrane, cm

ra= Shearing strength of adhesive bitumen,

kg/cm^

Although Koike demonstrated the validity of the equations by laboratory experiments involving one layer of an asphalt saturated and coated felt bonded to a concrete panel with a blown petroleum

asphalt, he concluded that the equations were not

intended to predict the performance of a roofing

in service but that they may prove helpful in the

development of improved roof coverings.

It should be noted here that, to our knowledge,

data are not available on these properties for either single or multiple ply membrane currently

in use in the United States. Therefore, the use

of the equations as a guide for the performance

of built-up roof coverings in service is not pres- ently feasible.

3. Materials and Specimen Preparation

fe! Three types of bituminous saturated or impreg-

,1 nated felts and two types of bitumen, which are

ji currently being used in the construction of buUt-up

jj! roofs, were selected for the study. Table 1 identi-

' fies the materials.

Table 1. Materials used in the preparation of specimens

Materials

Bitumens:

Asphalt

Coal-tar-pitch.

Beinforcing felt: Asphalt saturated organic felt . Asphalt impregnated glass felt. Coal-tar saturated organic felt_.

ASTM Specification

D312, Mineral Sur. Flat. D450, Type A.

D226, 15 lb type.

(»). D227.

» "Perma Ply No. 11," as manufactured by the Owens-Corning Fiberglas

'j| Corporation. The commercial material is identified in this paper in order to

specify the experimental procedure adequately. In no case does such identifi- cation imply recommendation or endorsement by the National Bureau of Standards nor does it imply that the material identified is necessarily the

best available for the purpose. Each specimen consisted of four plies of the felt adhered to each other with the selected bitumen

[j applied at a spreading rate of about 25 lb per 100

sq ft, which is comparable to normal roofing

J practice.

y The four-ply built-up specimens were prepared

I by heating the bituminous cement to 212 to 266 °F

(100 to 130 °C). The heated bitumen was poured

on an 8- X 4-in section of the saturated felt placed on a larger piece of imlacquered cellophane

and a second section of felt was placed on the

bitumen to form a sandwich. This in turn was

covered with imlacquered cellophane and placed

in an unheated laboratory press. Sufficient pres- sure was applied so that the platens of the press were separated from each other by two 0.125-in

diam spacers, placed on either side and adjacent

to the specimen. When cool, the two-ply sandwich was taken from the press, the top piece of cello- phane removed and two additional plies were

applied (one at a time) by repeating the above

process using spacers having diameters of 0.218 in

and 0.312 in for the third and fourth plies,

respectively.

The specimens were chilled to about 32 °F(0 °C)

and dumbbell-shaped specimens were stamped out

using a suitable die on the laboratory press. Figure 1 shows the die, the test specimen, and a

cross section of the test specimen.

The felts in any single specimen were oriented

and identified in respect to longitudinal (with

machine) and transverse (across machine) directions since they exhibit anisotropic behavior.

4. Load- Strain Properties in Tension

The load-strain properties of three specimens in each direction were measured at —30 °F ( —34.4

°C), 0 °F (-17.8 °C). 30 °F (-1.1 °C), and

73 °F (22.8 °C) employing a tensile testing machine equipped with temperature chamber to control the specimen temperature mthin ±5 °F

J
(±2.7 °C) during the test. The temperatures

selected were believed to cover a realistic temperature range for roofs exposed in many areas of the

United States where failures have occurred. The

gage length as defined by the distance between the

jaws of the testing machine was 4.5 in and a straining rate of 0.05 in per minute (1.1% per minute) was used in each determination. We recognized

that this rate of loading which was at the slowest

rate permitted by the testing apparatus, was

significantly higher than that likely to be experienced at subfreezing temperatures in service. In

our opinion the rate of loading will influence both

the values obtained for the Thermal-Shock Resistant Factors and the performance of built-up

roofing membranes in service.

Table 2. Properties of bituminous built-up membranes at —SO °F

The averaged results of the load-strain measure- ments are reported to the nearest 10 lb in tables 2,

3, 4, and 5. The values for the elongation moduli

are the slopes of the initial tangent to the loadstrain curves. The standard deviation among

replicates for breaking loads is 20 lb/in; thus the

20

standard error for the values in the tables is -7=^ 12

V3

lb/in; or an uncertainty of ±25 lb/in. For elongation modulus, the standard deviation among

replicates is a function of the temperature and has

the following values:

Temperature (°F) —30 0 30 73 Standard deviation (lb/

in) 1200 1600 2100 2300

Apparent

Breaking Elongation linear Thermal- Membrane Direction • loadCS) b modulus thermal shock

(M) b exp. coeffi- factor cient (a) "

lb/in lb/in (° F)-^ Asphalt, asphalt-sat. organic felt L 480 4.3X10< nxio-6 1000 T 310 3. OXW 21X10-' 500

Coal-tar-pitch, coal-tar-sat. organic felt L 380 4. 0X10* 19X10-8 500 T 210 2. 6X10< 29X10-' 300

Asphalt, asphalt-sat. glass felt L 230 2. 0X104 18X10-' 650 T 170 1. 7X104 26X10-' 400

' L—Longitudinal (with machine). T—Transverse (across machine).

' Average of three specimens.

• Temperature Range +30 to —30 °F. Source: NBS Mono. No. 89 (1965). Table 3. Properties of bituminous built-up membranes at 0 °F

Membrane Direction » Breaking

load(S) b Elongation modulus

(M) b

Apparent

linear thermal

exp. coeffi- cient (a) 0

Thermalshock

factor

Ibjin lb/in Asphalt, asphalt-sat. organic felt L 400 2.8X104 11X10-' 1300 T 300 1. 9X104 21X10-' 750

Coal-tar-pitch, coal-tar-sat. organic felt L 400 4.9X104 ' <io-« 450 T 180 2. 9X104 2aX10-' 200

Asphalt, asphalt-sat. glass felt L 200 1.7X104 18X10-' 650 T 200 1. r-X10» 26X10-' 500

" L—Longitudinal (with machine). T—Transverse (across machine),

b Average of three specimens.

0 Temperature Range +30 to —30 °r. Source: NBS Mono. No. 89 (1965). Table 4. Properties of bituminous built-up membranes at SO °F

Apparent

Breaking Elongation linear Thermal- Membrane Direction » load(S) b modulus thenral shock

(M) b exp. coeffi- factor cient (a) 0

lb/in lb/in C f)-

Asphalt, asphalt-sat. organic felt L 270 2.1X104 11X10-9 1200 T 180 1.3X104 21X10-' 700

Coal-tar-pitch, coal-tar-sat. organic felt L 260 3. 0X104 19X10-' 500 T 120 1. 2X104 29X10-' 350

Asphalt, asphalt-sat. glass felt L 180 1.3X104 18X10-' 800 T 150 1.1X104 26X10-' 500

» L—Longitudinal (with machine). T—Transverse (across machine),

b Average of three specimens.

<= Temperature Range +30 to —30 °F. Source: NBS Mono. No. 89 (1965). 4
Table 5. Properties of bituminous built-up membranes at 73 °F

ATptti Hro nft Direction Breaking Elongation

moQuliis (M) t>

Apparent

linear

t Vi Of )' ct I 6Xp. C06fl^~ cient(a) °

Thermalfsctor

Iblin Iblin (° F)'i Asphalt, asphalt-sat. organic felt L 190 1. 9X10< 3.2X10-' 3000 T 130 9 4X10^ 5 3X10"' 2600

Coal-tar-pitch, coal-tar-sat. organic felt L 160 1.3X10'' 2. 0X10-' 6150 T 90 5. 2X103 3.2X10-3 5400

Asphalt, asphalt-sat. glass felt. , L 180 1.4X10* 2.6X10-3 5150 T 170 1.2X10« 2. 5X10-3 5650

» L—Longitudinal (with machine)

.

T—Transverse (across machine).

•> Average of three specimens.

" Temperature Range 100 to -60 °F. Source: NBS Tech. Note 231 (De;. 1963).

5. Apparent Linear Thermal Expansion Coefficients

The apparent linear thermal expansion coefficient for four-ply built-up membranes over a temperature range of -f-SO to —30 °F (—1.1 to —34.4 °C) were taken from reference [10] and are given in tables 2, 3, and 4. Coefficients for similar

membranes over a temperature range of 100 °F

(37.8 °C) to 60 °F (15.6 °C) were calculated from

the curves given in reference [6] and are given in

table 5.

6. Thermal- Shock Resistance Factor

In order to establish a numerical scale for thermal shock resistance, a Thermal-Shock Resistance

Factor was calculated for each of the composite

membranes included in the program. The values

7. Relation Between Splitting Failures and the Thermal- Shock Resistance Factors

were obtained by substituting the data in eq (1)

(see sec. 2). The factors were rounded off to the

nearest 50 and are given in tables 2, 3, 4, and 5.

Investigations by Cullen [10] of splitting failures

in bituminous built-up membranes during recent

years on roofs exposed to low and changing

temperatures have revealed some factors which

frequently contribute to such failures. These

include, among others, climate, orientation of the

felt in the membrane and the thermal characteristics of the substrate.

It is interesting to speculate on a relationship

between the frequency of splitting failures of

membranes exposed to actual weathering conditions and the value of the Thermal-Shock Resistance Factors reported herein.

7.1. Climate

The observed incidence of splitting failures

which were attributed in part to thermal shrinkages was greater in the colder climates and failures were frequently reported following periods of extremely cold weather with little or no snow

cover. The results of this study show that with

few exceptions the Thermal-Shock Resistance

Factor is lower in value at low temperature.

7.2. Thermal Characteristics of Substrate

Splitting failiu"es were observed more frequently

in bituminous built-up membranes applied to substrates having high thermal insulating values

than in those placed directly on more dense

substrates.

Built-up roof membranes placed on insulation

are subjected to both higher temperatures when

exposed to solar heating and to lower temperatures during the nighttime due to radiative cooling

than are their non-insulated counterparts [12]. Therefore, the lower temperatures experienced by

a membrane placed over insulation will result in a lower value for the Thermal-Shock Resistance

Factor than for a similar membrane placed over another substrate. The lower values for the

Thermal-Shock Resistance Factor in the Laboratory were obtained at the lower temperatures

indicating consistency with field observations.

7.3. Orientation of Reinforcing Felt

Splitting failures in built-up membranes re- sulting in part from thermal shrinkage were observed almost exclusively perpendicular to the

transverse direction of the reinforcing felt. This

indicated the anisotropic behavior of the bitumi- nous built-up membrane. The Thermal-Shock

Resistance Factor data given in the tables also

show this anisotropic behavior probably because

the strength is appreciably lower and the apparent

linear thermal expansion coefficient is considerably

higher in the transverse direction as compared to the longitudinal direction.

7.4. Composition of Built-Up Membrane

The significant differences in the values of the

Thermal-Shock Resistance Factors among the

membranes would imply differences in perform-

^ 259-508 0-67-2
ance among these membranes under service conditions regarding splitting failures. However, to our knowledge, this has not been established. In

fact, splitting failures under service conditions

have been observed and reported for each type of bituminous built-up membrane included in this

investigation. Further, there are factors other than

thermal movement, such as moisture or structural

movements, which are known to contribute to a

lesser or greater extent in the observed failures

and they must also be considered. There is a need

for further investigation in this area as stated in section 2.

8. Conclusions

We recognize the preliminary nature of both the

test methods and the test resiilts used to determine

the values of the Thermal-Shock Resistance Factors reported herein. Nevertheless we believe that

a knowledge of this factor prior to installation wUl

prove useful in predicting the ability of a built-up

membrane to withstand forces induced by thermal

shock. Further, the value of the factor assigned to a roofing memljrane will depend to a large extent on the climate to which exposed, whether the

membrane is placed over insulation or not, the

longitudinal or transverse orientation of the felt in the membrane and to the area of the roofing

unbroken by expansion joints. For example, a

higher Thermal-Shock Resistance Factor will be

required for a membrane exposed in an area ex- periencing an average January temperature of 20 °F than that required for another membrane

to be exposed where the average January temperature is 30 °F. Similarly, a higher factor will be

necessary for a membrane placed over a substrate

having a high thermal insulating value than that

needed for a membrane placed directly on a concrete deck. We would also expect that a higher

Thermal-Shock Resistance Factor would be desirable for membranes on roofs where large areas are involved as opposed to those divided into smaller

areas by expansion joints.

It is interesting to note that the data reported

by Koike [11] and that reported herein support the

validity of some suggestions made by Cullen [10] regarding the alteration of some conventional

application techniques to reduce the incidence of

splitting failures resulting from mechanically and

thermally induced forces, as:

(a) The placement of the insulation boards with

their long dimension parallel to the short dimension

of the roof.

(b) The orientation of the roofing felt parallel

to the long dimension of the roof.

(c) The use of an adhesive of optimum strength

to secure the membrane to the substrate.

The authors acknowledge with thanks the assist- ance of W. H. Appleton and 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 the members

of the Tri-Service Committee.

The investigation was conducted under the TriService Engineering Investigations of Building

Construction and Equipment, NBS, sponsored by

the Bureau of Yards and Docks, U.S. Navy; the

Office of the Chief of Engineers, U.S. Army; and

the Directorate of Civil Engineering, U.S. Air Force

9. References

[1] C. E. Lund and R. M. Granum, Univ. of Minnesota

Eng. Exp. Stn. Bulletin No. 34 (May 1952).

[2] W. B. Warden, Building Research Institute Monograph No. 1 (I960).

[3] D. E. Brotherson, Research Report 61-2, Small

Homes Council-Building Research Council, University of Illinois (1961).

[4] G. O. Handegord, Technical Paper No. 182 of the Division of Building Research, National Research

Council, Canada (June 1964).

[5] F. A. Joy, Better Building Report No. 5, College of Engineering, The Pennsylvania State University

(Sept. 1963).

[6] W. C. Cullen, NBS Technical Note No. 231 (Dec.

1963).

[7] G. N. Moseley, An Explanation of Built- Up Roofing

Tension Splits, Building Research Institute 1964

Fall Conference (November 1964).

[8] P. M. Jones, Recent Research on Bituminous Materials, ASTM STP-347 (1963).

[9] Perry, Robert H. and John H., (ed) Engineering

Manual (1959).

[10] W. C. Cullen, NBS Mono. No. 89 (1965).

[11] Michio Koike, BRI Occasional Report No. 15, Building Research Institute, Japan (Dec. 1963).

[12] W. C. Cullen, W. H. Appleton, The Construction

Specifier 16, 5, 35 (October 1963).

Figure 1 . Cutting die, four-ply built-up roofing specimen and

cross section of membrane.

6
Section 2

Thermal-Shock Resistance for Bituminous Built-up Roofing

Membranes—Its Relation to Service Life

William C. GuUen and Thomas H. Boone

The assignment of a service life to a bituminous built-up roofing system is frequently

difficult because of the many variables involved. A knowledge of these variables, and of their effect on the performance of the total building system, will greatly assist in the selec- tion of a roofing assembly and the assignment of a service life to such an assembly.

Some of the factors such as breaking load in tension, modulus of elongation, and apparent linear thermal expansion coefficient of roofing membranes of different composition are given for both laboratory-prepared and field-obtained samples. Membranes of 2, 3, and

4-plies of felt are included. The relations of some engineering properties of a roofing membrane

to performance in service as expressed by a Thermal-Shock Resistance Factor are also given.

Ways and means to reduce potential failures of bituminous built-up roofing membranes

resiilting from thermally induced forces are discussed. Key Words: Bituminous-built-up roofing, roofing membrane, service life, thermal- shock resistance factor, tension splitting.

Contents

Page

' 1. Introduction 7

ij 2. Experimental detail 8

2.1. Materials and specimen preparation 8

i( a. Laboratory 8

ji b. Field 8

2.2. Load-strain properties in tension 9

I 2.3. Linear thermal expansion 9

j 3. Results 9

i 4. Discussion of results 9

4.1. Effect of number of plies of reinforcing felt on

the strength characteristics of a built-up

membrane 9

! 4.2. Relation between values of thermal-shock

I resistance factors and performance in field

i service 11

' 5. Comments 12

i 6. References 13

1. Introduction

Any ruptiire of the waterproofing element of a

I roofing assembly generally leads to failure of the

I total assembly, from both a weatherproofing and

jj a heat transfer viewpoint. Moseley [1] ' has indiI cated that tension splits which occur in the builtI up membrane are frequent causes of failure. Cullen

[2] suggested that the thermal movement which

i occurs in a bituminous built-up roofing membrane

, due to rapid temperature changes is often a conj tributing factor in tension splits. He proposed the

I utilization of a Thermal-Shock Resistance Factor

1 to predict the ability of a built-up membrane to

I tolerate, without rupture, the forces, induced by

II such temperature changes [3].

i 1 Figures in brackets indicate the literature references at the end of this

t paper.

A roofing membrane will rupture when the

thermal contraction equals or exceeds the ultimate

elongation of the membrane. Since the force-extension curve is approximately linear below 30 °F

(—1.1 °C), the ratio of tensile strength (S) to modulus of elongation (M) may be used to estimate

the ultimate elongation. Thermal contraction is proportional to the coefficient of linear thermal

expansion (a). Hence the merit of a membrane

may be expressed by a Thermal-Shock Resistance

Factor (TSRF) given by the following eq (1)

^^^^'^Wc (1)7
where

T<S'i?Fi =Thermal-Shock Resistance Factor

<S'=Breaking load, lb/in (see footnote 2) M=Modulus of elongation (initial tangent), lb/in (footnote 2)

Q:=Linear thermal expansion, per °F. From this equation, it is seen that the higher

the breaking load, the lower the coefficient of linear

thermal expansion, and the lower the modulus of elongation, the more resistant to rupture is the

membrane under thermal changes.

Cullen and Boone reported values for TSRF of three convential membranes at temperatures of -30 °F (-34.4 °C), 0 °F (-17.8 °C), 30 °F

(-1.1 °C), and 73 °F (22.8 °C) in section 1 of

this publication.

This paper presents values for Thermal-Shock

Resistance Factors at 0 °F (—17.8 °C) for four

built-up membranes constructed in the laboratory,

consisting of 2, 3, and 4 plies of felt, respectively.

In addition values for samples of bituminous

built-up roofs with up to 25 years exposure in

service are also given. In some cases the roof

membranes from which the samples were obtained had tension splits which were attributed in

part to thermally induced forces as Ulustrated by

the sample shown in figiu'e 1. In others no

evidence of this type of faUiu'e was observed.

2 The breaking loads and moduli of elongation are expressed in poimds

per inch width of membrane since a previous study [4] had shown that the

effect of the asphalt-cement on the strength characterisitcs of the membrane

are negligible. The elongation moduli are the slopes of the initial tangent to the load-strain curves.

>

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

2. Experimental Details

2.1 Materials and Specimen Preparation

a. 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.

Table 1. Materials used in the preparation of laboratory

specimens

Materials ASTM specification

Bitumens:

Asphalt _.. . D312, Mineral Sur. Flat. D450, Type A.

D250, 15 lb type. D226, 15 lb type. w.

D227.

Coal-tar-pitch ___

Reinforcing felts: Asphalt saturated asbestos felt.-. Asphalt saturated organic felt Asphalt impregnated glass felt... Coal-tar saturated organic felt ...

a "Penna Ply No. 11," as manufactured by the Owens-Commg 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 recommendation or endorsement by the National Bureau of Standards nor does it imply that the material identified is necessarily the best

available for the purpose. 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 to 30 lb per 100

sq ft, which is comparable to that obtained in

good roofing practice. The laboratory samples . were prepared, and dumbbell-shaped test speci- mens were cut by a procedure developed at the

National Bureau of Standards as described in

section 1 of this publication.

b. 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 i cooperation of contractor associations, roofing 'I manufacturers, and, in many cases, private ? roofing contractors. i Two samples, a minimum of one square foot I in area, were cut from the selected roof. The t samples were identified in respect to location, ' composition and felt orientation, and were for- ( warded to NBS. A history, as weU as a diagram ' of the roof plan, frequently accompanied the ) samples. In some cases the roofs in question i were observed by a NBS representative who \ selected the areas from which the samples were

8
to be removed. In the cases where tension splits were apparent, samples were taken at or near

the failure. Information on the field samples relative to the location, age, composition, and number of

plies is given in table 3. The samples were prepared for test by removing

the mineral surfacing (if present) and any adhering insulation. Dumbbell-shaped specimens

similar to the laboratory-prepared test speci- mens were cut. Figure 2 shows a test specimen

obtained from a roof membrane after 9 years

exposm-e.

2.2. Load-Strain Properties in Tension

was significantly higher than that likely to be

experienced in service. No doubt the rate of loading

will influence both the values obtained for the

Thermal-Shock Resistant Factors and more important the performance of a built-up roofing

membrane in service.

2.3. Linear Thermal Expansion

The linear thermal expansion measurements

were made on duplicate dumbbell-shaped speci- mens prepared as described in section 2.1, using a

5-in gage length in accordance with a procedure

previously described by CuUen [21.

The load-strain properties and the breaking

strength of the specimens were measured in dupli-

! cate (triplicate when sufficient specimens were

available) employing a tensUe testing machine

equipped with a temperature chamber to control

the specimen temperature within ±5 °F (±2.7 °C)

during the test. The load-strain measurements

I were made at 0 °F (-17.8 °C). The gage length,

as defined by the distance between the jaws of the

testing machine, was 4.5 in. A straining rate of

0.05 in per minute (1.1% per minute) was used in

each determination.

We recognized that this rate of loading which

I was the slowest permitted by the testing apparatus.

Figure 2. A prepared test specimen of a membrane taken from the field after nine years exposure.

3. Results

The average results of the load-strain measure- ments 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 standard deviation among replicates for the

breaking loads is 20 lb/in; thus the standard error

for values in the tables is 12 lb/in; or an uncertainty of ±25 lbs/in. For the elongation

modulus, the standard deviation among replicates

is about 1600 lb/in at 0 °F. The values of the apparent linear thermal

expansion of the laboratory-prepared and field- obtained samples were calculated from data

obtained over a temperature range of 30 to —30 °F

(—1.1 to —34.4 °C), and are given in tables 2 and 3.

4. Discussion of Results

4.L Effect of Number of Plies of Reinforcing

Felt on the Strength Characteristics of a Built-up Membrane

As expected, figure 3 illustrates that the breaking

load in tension for a multiple-ply built-up roofing

specimen increased in proportion to the number

of plies of felt in the membrane. Further, the composition of the bituminous-satiu-ated roofing felt had a marked effect on the values of the breaking

loads. Figure 4 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 linear thermal

expansion varied with the composition of the re- spective sample, the number of plies of felt in the

membrane had no significant effect on the value

of the coefficient of thermal expansion.

The value for the Thermal-Shock Resistance

Factor was approximately 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 may be concluded from the data

9
Table 2. Properties of laboratory prepared built-up membranes at 0 °F

Felt (bitumen) No. Plies Direction " Break

load (S)b Elong. Mod.(M) b Appar. lin. therm, exp

coeff. (a) "

ThermalShock

factor

lb/in Ibjin Organic (asphalt) 2 L 180 1. SXIO* 10X10-' 1, 350 T 130 .9X10< 17X10-6 '850

3 L 280 1.9X10^ 10X10"' 1 600 T 180 1.2X10^ 24X10-6 '650

4 L 400 2.8X10" 11X10-6 1, 300 T 300 1.9X10< 21X10-6 '750

Asbestos (asphalt) 2 L 170 2.0X10* 10X10-6 850 T 80 0. 9X10« 26X10-6 300

3 L 270 2.8X10' 8X10-6 1, 200 T 120 1.3X10* 30X10-6 '300

4 L 310 3.3X10'' 11X10-6 850 T 130 1. 6X10« 32X10-6 260

Glass (asphalt) 2 L 110 0. 9X10* 20X10-6 620 T 80 . 7X101 20X10-6 600

3 L 160 1. 3X10* 22X10-6 540 T 140 1.1X10* 35X10-6 360

4 L 210 1. 6X10* 20X10-6 660 T 180 1.3X10* 22X10-6 610 Organic (coal tar) 2 L 200 2. 5X10* 22X10-6 370 T 100 1. 7X10* 33X10-6 190

3 L 330 3.9X10* 25X10-6 330

1 150 42X10"^ 110

4 L 450 4.8X10* 27X10-6 350 T 220 3.9X10* 42X10-6 140

" L—Longitudinal (with machine). T—Transverse (across machine)

Average of thiree specimens.

« Temperature Range 4-30 to —30 ° F. Table 3. Properties of built-up roofing field samples at 0 °F

Dir.' Breakload (S) b Elong. mod. (M) b Appar. lin. therm, exp.

coefE. (a) 0

Thermalshock

factor

Iblin Ibjin L 240 4. 5X10* 16X10-6 750 T 140 4. 7X10* 41X10-6 500 L 220 2.3X10* 15X10-6 620 T 130 1.6X10* 30X10-6 260 L 230 1.4X10* 25X10-6 650 T 80 1.1X10* 23X10-6 350 L 90 1.8X10* 20X10-6 450 T 80 1. 2X10* 19X10-6 350 L 210 3.8X10* 20X10-6 300 T 70 2. 0X10* 31X10-6 100 L 270 5. 1X10* 9X10-8 550 T 200 2.5X10* 16X10-6 500 L 490 5. 1X10* 13X10-6 560 T 260 4. 9X10* 23X10-6 230

Felt (bit.) No.

plies Age yr Source

Organic (asphalt)

Organic (asphalt)

Organic (asphalt)

Glass (asphalt).. Organic (coal-tar)

Organic (coal-tar)

Organic (coal-tar)

22

12

20

25

Rockford, ni... K. C, Mo

Waterloo, Iowa. Omaha, Nebr.

Rockford, HI...

Waterloo, Iowa

K. C, Mo

» L—Longitudinal (with machine). T—Transverse (across machine)

b Average of three specimens.

« Temperature range 4-30 to —30 °F.

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 property in respect to the orientation

of the felt in a buUt-up membrane has previously

been reported [2]. The values of the engineering

properties and those of the Thermal-Shock Resist- ance Factors reported here also reflected the anisotropic behavior of a built-up roofing membrane

constructed from organic and asbestos base felts. The difference in value of these properties between

the "machine" (longitudinal) and the "across machine" (transverse) directions of both the or- ganic and asbestos based felts appeared to be

sufficiently large as to result in differences in per- formance in field service. CuUen described the re- lation between felt orientation and the frequency

of tension tearing of built-up roofs in field service

as a splitting which generally occurred parallel to the machine direction of the felt [2]. The samples prepared with glass based felts, for

all practical purposes, exhibited isotropic behavior

in respect to strength properties and ThermalShock Resistance Factors.

10
-ASPHALT IMPREGNATED GLASS FIBER / ASPHALT A- COAL TAR SATURATED ORGANIC / PITCH

PLIES OF FELT IN MEMBRANE

Figure 3. Breaking load of built-up roofing membranes at 0

°F.

• - ASPHALT SATURATED ASBESTOS /ASPHALT O - ASPHALT IMPREGNATED GLASS FIBER /ASPHALT A- COAL TAR SATURATED ORGANIC / PITCH

PLIES OF FELT IN MEMBRANE

Figure 4. Elongation modulus {initial tangent) oj built-up

roofing membranes at 0 °F.

4.2. Relation Between Values of ThermalShock Resistance Factors and Performance

in Field Service

The effective service life of a bituminous builtup roofing system depends to a large extent on

the initial properties of the waterproof membrane,

as well as on the retention of these properties

diiring long periods (20 years or more) 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 connection, 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 thermal expansion was also in- dicated by a comparison of the results obtained on field samples and those obtained on the laboratory prepared specimens. When the values

obtained during the testing procedure on the

exposed specimens were substituted in eq (1),

it was found that the Thermal-Shock Resistance

Factors of the weathered samples were considerably lower than those obtained for the

unweathered samples. It is extremely important

to note here that no data were available on the

original properties of the samples obtained from

the field. Nevertheless, because of the similarity

of construction and composition of the built-up

field samples vnth those prepared in the laboratory, the foUomng broad assumptions seem

to be valid at this time.

The values for the Thermal-Shock Resistance

Factors of coal-tar-pitch, organic felt membranes,

although initially lower 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 made available under

the project.

If these data were applied to evaluate effective

service life, they would indicate that, initially,

the coal-tar-pitch, organic felt buUt-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 in susceptibility to these forces between the coal-tar-pitch and aspaltic products is 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 unexpected, since previous field experiences

have indicated that a relation exists between felt orientation and splitting faUiu-es [2]. Further, when

the values for the Thermal-Shock Resistance

Factors are considered in respect to felt orientation

11
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 in- cidence 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 buUt-up roofing systems in

the field, confirm our opinion that the higher the

Thermal-Shock Resistance Factor, the more re- sistant a membrane will be to failure due to thermally induced forces.

Table 4. Relation between values of thermal shock-resistance and splitting failures observed under service conditions

Felt (bitumen) No. plies Age yr.

Thermal shock-resistance factor (transverse direction)

Source Membrane

exhibiting tension splits

No evidence

of tension

splits

Organic (asphalt)

Glass (asphalt) Organic (coal-tar) _

622

1220

25

Rockford, ni Kansas City, Mo.

Waterloo, Iowa. .

Omaha, Nebr

Rockford, El

Waterloo, Iowa.__ Kansas City, Mo.

260

350

350

100

500

230

500

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 buUt-up roofing

membranes in respect to properties, roof design,

and the environment in which the roofing system

is expected to serve. The authors' experience over many years with

the performance of built-up roofings in service

had indicated that many failures can be attributed

to factors other than thermally induced forces

with which this paper deals. Therefore, this paper

should not be construed to imply that other

factors as mechanically induced stresses, moisture

migration, wrinkle cracking, flashing failures, and

chemical degradation are not important considerations in many bituminous built-up roofing failures. Splitting failures attributed in part to thermally

induced forces appeared to be more prevalent in

climates where roofing membranes were exposed

to larger and more rapid temperature changes

especially at subfreezing temperature than in areas where temperatures were mUder and more uniform.

In order for a membrane to resist forces due to large temperature changes at subfreezing 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 parameter is not reflected in the

factor given in eq (1). Therefore, in order to take

into account the temperature conditions of the

roofing system's ambient environment into the

Thermal-Shock Resistance Factor, it is proposed

that addition of another factor to the right side of

eq (1) be made. Since thermally induced shrinkage

failures are associated with low (subfreezing) and

changing temperatures, the factor should take into

account the temperatures experienced by a built-up

roofing membrane in the area where the roof is exposed. Equation (1) may now be written:

TSRFo _S ]_

~Ma Atm (2)

where: A^m=32 °F minus (temperature of the

membrane).

The membrane temperature at any one time will be influenced by a number of factors. These include

the thermal characteristics of the roofing system,

the inside temperature of the structure, the

ambient temperature, the wind conditions, and

the presence of moisture, water or snow. Equation (2) approximates the ratio of the

ultimate elongation of the membrane to the

thermal contraction that occurs under the particular environmental conditions to which the

membrane is subjected. As mentioned previously,

the ratio S/M approximates the ultimate elongation. The thermal contraction equals the average

coefficient of linear thermal expansion multiplied

by the actual change in temperature of the

membrane.

Based on the foregoing comments the authors

feel there is a need for investigations of the

engineering behavior of the whole roof assembly

and the interrelationship of its components under

different thermal exposures.

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.

12
The authors appreciate the assistance of Edwin

Carlson, Jim Lenzner, Tom Manson, Paul Morris,

and Milton Olsen of the Midwest Roofing Contractors Association, Inc., in making available

exposed roof cutouts and first-hand knowledge of their performance.

6. References

[1] G. N. Moseley, built-up roofing tension splits, The

Journal of the Building Research Institute 1, No.

6, 31 (Nov.-Dec. 1964).

[2] W. C. CuUen, Effects of thermal shrinkage on built-up

roofing, NBS Mono. No. 89 (1965).

[3] W. C. Cullen, Solar heating, radiative cooling and

thermal movement—Their effects on built-up roofing, NBS tech. Note 231 (Dec. 1963).

[4] P. M. Jones, Recent research on bituminous materials,

ASTM STP-347 Am. Soc. Testing Mats. (1963).

13

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