Ridging, shrinkage and splitting of built-up roofing membranes

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Ridging, shrinkage and splitting of built-up roofing membranes

R I D G I N G , SHRINKAGE AND

SPLITTING

OF BUILT-UP ROOFING

MEMBRANES

by

K . R . Solvason and G.O. Handegord

As the o u t e r component of t h e roofing system, a b u i l t - u p r o o f i n g [BUR) membrane is s u b j e c t e d t o all t h e elements of the o u t s i d e environ-

ment, w i t h only p a r t i a l protection provided by t h e top cover o f gravel or o t h e r c o a t i n g . I t s performance w i l l t h u s depend on its response to

t h i s environment which can b e s t b e explained by c o n s i d e r i n g i t s _physical

p r o p e r t i e s i n r e l a t i o n to the environmental factors to which it is exposed.

The p r i n c i p a l environmental f a c t o r s likely to a f f e c t membrane

performance a r e moisture, a i r temperature and s o l a r r a d i a t i o n . Bitumens are r e l a t i v e l y unaffected by m i s t u r e due p a r t l y to the non-wetting

character o f t h e i r surface, but mre particularly because of t h e i r non-

porous structure. S o l a r r a d i a t i o n and c y c l i c a l temperatures, however, can r e s u l t in a hardening o f t h e surface and thermal c o n t r a c t i o n l e a d i n g to microcracking o f t h e exposed surface.

Shielding the surface from solar radiation w i t h gravel or w i t h o t h e r surface c o a t i n g s provides some radiation p r o t e c t i o n and the "self

heal ingl' c h a r a c t e r i s t i c of bitumen may act to maintain its i n i t i a l water- p r o o f i n g qualities. This accepted "self healing" f e a t u r e of bitumen

recognizes that the material C T ~ C ~ S at times and t h e n reseals due to

p l a s t i c flow a t h i g h e r temperatures. T h i s c h a r a c t e ~ i s t i c is i n d i c a t i v e

of the change in t h e physical properties o f bitumen w i t h temperature.

PROPERTIES O F BUR MEMBRANES

Only at very low temperatures or under a small, rapidly a p p l i e d l o a d do bitumens e x h i b i t t h e properties of an e l a s t i c solid t h a t deforms

under load but r e t u r n s to its o r i g i n a l dimensions when t h e l o a d i s

released [represented by line "A" in Figure 1 ) . A t temperatures such a s

those experienced in p r a c t i c e , o r w i t h a slower r a t e of application, the

same l e a d w i l l r e s u l t in a greater deformation (as i n curve "Bt'). I f the l o a d is released quickly the bitumen will p a r t i a l l y r e t u r n to iss o r i g i n a l

dimension (as in curve "C"), but if t h e l o a d simply maintains a f i x e d

deformation or s t r a i n , p l a s t i c flow o r creep w i l l allow the material to

a d j u s t t o i t s

new

dimension as t h e load gradually decreases t o _{zeyo [as} in line ' v D f f ) . This last-mentioned situation i s r e p r e s e n t a t i v e of the

loading induced kn a menibrane

in

practice by thermal expansion and

contraction: the membrane i s r e s t r a i n e d to a f i x e d dimension and t h e

load results from i t s tendency to expand

or

contract.

Some o f

the

properties o f bitumens axe imparted to t h e f e l t s used

i n b u i l t - u p

r o o f i n g through the factory saturation and coating process as

we1 1 as _{i n the f i e l d application o f the composite membrane. Both wood} f i b r e and asbestos f e l t s are made by a _{manufacturing process which}

produces a f i b r e o r i e n t a t i o n in the Pong dimension of t h e sheet and

r e s u l t s in a d i r e c t i o n a l variation in properties, These d i r e c t i o n a l

p r o p e r t i e s are f u ~ t h e r exaggerated in the process of applying t h e f e l t s .

The Fibre ~ e i n f o r c e m e n t i s essentially unbroken in the f e l t d i r e c t i o n but

across t h e f e l t d i r e c t i o n , t h e 36-in.-wide f e l t s are simply overlapped

and the f i b r e s a r e discontinuous. Any l a t e r a l forces applied outside t h e

dimensional limits of 36 in, are t h u s resisted by the bonding bitumen

between t h e f e l t s i n shear.

Although s l i g h t variations e x i s t between d i f f e r e n t felt-bitumen combinations, t h r e e general characteristics are most likely to i n f l u e n c e t h e i r performance in practice:

1. bkmbranes tend to change i n dimension w i t h changes in

temperature, the change b e i n g g r e a t e r across t h e

f e l t: d i r e c t i o n

.

2 - Except at: very low temperatures or under r a p i d loading, bituminous membranes do not behave e l a s r i c a l l y but

t e n d to creep or f l o w , r e s u l t i n g in a permanent defor- mation w i t h T i m e .

3. Membranes incorporating mais tare- s e n s i ti ve f e l t s tend

to expand and contract w i t h i n c r e a s e s o r decreases

i n

moisture content, t h e changes b e i n g greater across t h e

f e l t d i r e c t i o n .

E NVP RONMENTAL CONDITIONS

In practice the membrane in a _{conventional r o o f i n g system i s} expased

almost d i r e c t l y t o t h e outside a i r temperature, The membrane surface temperature tends to follow t h i s a i r temperature v a r i a t i o n , b u t will also

h e s i g n i f i c a n t l y a f f e c t e d by the s o l a r r a d i a t i o n conditions t h a t occur on

a d a i l y b a s i s , as i n d i c a t e d in Figure 2.

Under sunny conditions the surface temperature of a roof membrane

rises lo a p e a k well above a i r temperature. During the n i g h t , surface

temperatures will dmp t o a i r temperature or lower because

of

r a d i a t i o n t o the clear n i g h t s k y . More r a p i d changes in temperature occur when

s o l a r r a d i a t i o n is i n t e r c e p t e d by p a s s i n g clouds, such effects b e i n g

Although t h e maximum range o f temperature t h a t the membrane

experiences will b e t h a t between t h e coldest c l e a r n i g h t in winter and the

sunniest

day in summer, it will undergo a s i g n i f i c a n t and more r a p i d

change in temperature on a r e p e t i t i v e , daily b a s i s , particularly

i n

summer. I n view o f i t s sensitivity to temperature i t s p o s s i b l e response

to these changes should be examined.

FEMBWNE MOVEMENT

-

R I D G I N G AND

SHRINKAGE

I f t h e membrane i s r e s t r a i n e d to a f i x e d dimension and subjected to t h e d a i l y temperature cycle it tends to expand on h e a t i n g and compressive

stresses will develop. P l a s t i c flow o r creep will also b e g i n to occur at a rate t h a t w i l l increase w i t h i n c r e a s i n g temperature. Because of t h i s

c h a r a c t e r i s t i c p r o p e r t y , a l l the stresses in the membrane can relax in time. When t h e membrane b e g i n s cooling, due to a reduction in solar

r a d i a t i o n and a i r temperature, it tends to s h r i n k and t e n s i l e stress will

develop because it I s r e s t r a i n e d . In time, p l a s t i c flow or creep

in

t h e

membrane could again allow the stresses t o r e l a x .

A bituminous roofing membrane w ill thus experience both compressive s t r e s s e s and tensile stresses under a typical d a i l y temperature c y c l e .

Increasing temperatures w i l l generally induce compressive stress;

d e c r e a s i n g temperatures will induce tensile s t r e s s . There will a l s o be

periods during which t i m e and temperature will permit p l a s t i c flow to occur and relax the stresses ( t h e r a t e o f and tendency f o r p l a s t i c f l o w

b e i n g greater a t higher temperatures).

There are several ways

in

which stresses in membranes might b e relaxed. IVhen under compression, p l a s t i c flow could allow t h e rnernbrane

t o increase in t h i c k n e s s . Plastic flow in s h e a r between the l a y e r s could allow membrane movement and if the membrane is not h e l d clown at a

p a r t i c u l a r l o c a t i o n , b u c k l i n g could occur. When the membrane i s subse- quently placed i n t e n s i o n , p l a s t i c flow in t h e bitumen l a y e r between t h e

membrane and insulation may a l l o w t h e buckle o r ridge to f l a t t e n . I f

r i d g e s had n o t formed, t e n s i l e forces could cause t h e membrane to decrease in t h i c k n e s s or c o u l d r e s u l t in slippage berween the p l j e s , a t the membrane i n s u l a t i o n i n t e r f a c e or between t h e i n s u l a t i o n and t h e deck.

The resistance o f f e r e d by the bitumen layers in shear w i l l b e l e s s

at h i g h e r than at lower temperatures,

\+%en

s u b j e c t e d t o a d a i l y c y c l e ,

slippage o f one f e l t over another i s more likely t o t a k e place on h e a t i n g

t h a n when t h e membrane goes i n t o t e n s i o n on c o a l i n g . Thus, if t h e mem- brane is n o t s e c u r e d a t its perimeter, such cyclical h e a t i n g and cooling

could r e s u l t in an incremental, progressive shrinkage away f ~ o m t h e edges o f t h e roof until the n e t r e s u l t becomes obvious. Such o v e r - a l l

s h r i n k a g e will b e greater acmss the f e l t s s i n c e the c o e f f i c i e n t of

thermal e x p a n s i o n is g r e a t e r and in t h i s d i r e c t i o n nonreversible slippage

between overlapping p l i e s could a c c u r by shear

in

the bimnen layers. En the fe1 t direction the stresses will be more uniformly d i s t r i b u t e d

EFFECT

OF

MINUNIFOW HEATING AND COOLING

Temperatures are n o t l i k e l y to be uniform on

an

actual r o o f . Shaded

areas tend to

cool

to a i r temperature, there

will

be d i f f e r e n c e s

in

sur- face colour and heat absorption, and the air flow p a t t e r n over

the

roof will usually b e such as t o d i s s i p a t e h e a t more r a p i d l y

in some

areas t h a n

in o t h e r s .

In

Figure 3, for example, hot a i r r i s i n g above the roof surface will

d r a w in air E m m around the perimeter and set up convection c u r r e n t s

causing t h e central area to h e a t up faster than the surrounding area. As the c e n t r a l area heats

up

it will expand and go into compression; this

expansion will be r e s i s t e d by she surrounding area as it gradually

increases in temperature and goes

i n t o

compression. Eventually the whale

membrane will b e in compression but t h e central area will b e at a h i g h e r temperature and creep or p l a s t i c f l o w will allow t h e stresses to r e l a x .

If the stresses in the central area are relaxed while the surround- ings are still in compression, there will be a tendency f o r the surround-

i n g E e l t s _{to push inward towards t h e central area. These inward forces} may promote slippage of one f e l t over anether, a l o c a l i z e d increase o f

the thickness of f e l t , or they could r e s u l t in buckling at

a

p o i n t where t h e membrane has t h e l e a s t adhesion to the substrate. As the temperature

of t h e whole membrane continues to rise, or if s u f f i c i e n t time e l a p s e s ,

p l a s t i c flow or creep could allow the whole mHnb~anC? to relax and if

s u f f i c i e n t material has been pushed

i n t o

t h e c e n t r a l area,

the

ridges in the central area may remain.

When the cooling cycle begins, a l l areas o f the membrane w i l l

develop t e n s i l e s t r e s s e s but t h o s e developed in the central area wif 1 be h i g h e r since it cools through a greater range o f temperatures. Under

these circumstances the central area w i l l tend t o p u l l i n material f r o m

its perimeter as i n d i c a t e d

in

Figure 4 . As t h e whole membrane relaxes, in t i m e , an increased amount of material may remain

in

this l o c a l area.

T h i s mechanism w i l l a l s o lead to the migration of the f e l t s towards ~e

centre r e s u l t i n g in even greater shrinkage at t h e edges

of

the

roof

if

t h e membrane is not restrained at the perimeter. If t h e membrane i s

r e s t r a i n e d at the perimeter, slippage between overlapping f e l t s could

take place w i t h o u t serious consequences and w i t h no evidence of shrinkage

at the

roof

edges.

'EHE ROLE

OF

INSULATION

JOINTS AND M3ISTUE

Faen

a p o r t i o n of t h e membrane

is

under compressive stress, and particularly when material i s being pushed into an area, any buckling of t h e membrane is m s t l i k e l y to occur at a location where the membrane

is n o t held down to the s u b s t r a t e . A joint in the insulation may well

provide such a l o c a t i o n .

I f

the i n s u l a t i o n boards aTe separated by a

crack, a narrow unbonded s r r i p occurs.

If

the

u p e r

surfaces o f adjacent

will e x i s t . Thermal expansion o f the t o p p o r t i o n o f t h e i n s u l a t i o n at

this p o i n t could add a n o t h e r a s s i s t i n g f o ~ c e .

If

m o i s t u r e had concen-

t r a t e d a t the j o i n t p e v i o u s l y , some deterioration i n t h e bond between membrane and t h e i n s u l a t i o n could have occurred. Any l a t e r a l p e n e t r a t i o n

of moisture i n t o the f e l t at t h i s l o c a t i o n would increase s t i l l f u r t h e r the width of t h e unbonded area.

The r o l e o f moisture i n reducing t h e bond between p l i e s or between

menibrane and s u b s t r a t e is suggested in t h i s analysis as a c o n t r i b u t i n g factor t o ridge formation, Arguments based on t h e moisture expansion of

f e l t s as t h e prime cause of r i d g i n g have a l s o been advanced o v e r t h e

years.

Felts will expand due to an i n c r e a s e i n moisture content b u t in many

actual r i d g i n g cases the increase in the dimension of t h e membrane

r e q u i r e d to produce t h e ridge appears t~ be many times t h e increase t h a t could b e achieved by moisture expansion alone. Most f i e l d observations

i n d i c a t e that a movement o f material to t h e l o c a t i o n is necessary to

provide t h e amount required to farm the ridge and it i s suggested that c y c l i c a l h e a t i n g and cooling can p r o v i d e t h i s type of progressive

movement.

Joints i n t h e i n s u l a t i o n are locations where a c o n c e n t r a t i o n

of

m o i s t u r e due to condensarion w i l l occur on the underside of the menibsane.

P e n e t r a t i o n o f t h i s moisture i n t o the f e l t s w i l l b e i n h i b i t e d b u t n o t

necessarily s togped by any bitumen c o a t i n g such as t h a t on a coated base s h e e t . The p r o t e c t i v e e f f e c t o f the a s p h a l t c o a t i n g will slow down t h e

rate a t which moisture i s accumulated, reducing b o t h the amount of l o c a l i z e d expansion and t h e d e t e r i o r a t i o n o f t h e bond a t this l o c a t i o n .

If t h e felt is unprotected by bitumen at t h e joint, this lateral migra- tion o f m i s t u r e w i l l be a c c e l e r a t e d and a reduction of bond at this

c r i t i c a l location could r e s u l t

in

less time.

MEMBRdNE SPLITTING

Unequal r a t e s of cooling of various areas of the _{r o o f i n g membrane}

surface could create l o c a l i z e d areas of membrane shrinkage and hence

strain in t e n s i o n . ? a e n such an area i s rapidly cooled, t h e temperature

of t h e upper surface of the membrane will d m p f a s t e r than that of the

u n d e r l y i n g p l i e s . Any t e n s i l e stresses e x i s t i n g i n the sursounding

membrane could produce a t e n s i l e farce t h a t i s resisted primarily by the upper p l y s i n c e t h e lower p l i e s will b e warmer and more s u b j e c t to

p l a s t i c flaw. With a11 o f t h e t e m i l e force produced by the adjacent area a c t i n g on the upper p l y i t w i l l experience a t e n s i l e stress much

A f u r t h e r concenrration o f stress in t h e u p p e r p l y could r e s u l t from a r e d u c t i o n in thickness due to microcracking of the upper surface. Such

fissures may also have permitted moisture t o penetrate into the f e l t ,

f u r t h e r reducing i t s s t r e n g t h at t h i s c r i t i c a l l o c a t i o n . Piny

tension

f a i l u r e of t h e upper p l y could lead t o the progressive

failure

o f t h e u n d e r l y i n g p l i e s until a s p l i t develops completely t h r o u g h t h e membrane.

CONCLUSION

The daily cycle of heating and cooling of bituminous membranes can result in the development of t e n s i o n and compression forces which may a c t to produce membrane movement. This cyclical movement and the l o c a l i zation of s t r a i n s in t h e membrane will be augmented by the uneven temperature patterns on the r o o f occasioned by shading, colour variation and air movement. Consideration of these factors in r e l a t i o n to membrane proper-

ties can serve to explain t h e observed performance of b u i l t - u p roof

membranes in regard to ridging, shrinkage and splitting, Improvements

in

performance could b e o b t a i n e d by r e s t r a i n i n g The membrane at the r o o f p e r i m e t e r and by reducing the temperature c y c l e to which it is exposed,

through the use of r e f l e c t i v e coatings, or the a d d i t i o n of thermal insula-

D E F O R M A T I O N F I G U R E 1 LOAD-DEFORMATI ON CHARACTER I S T I C S OF B I T U M I N O U S M A T E R I A L S 1 2 TIME. h F I G U R E 2 D A I L Y C Y C L E OF MEMBRANE T E M P E R A T U R E I M SUMMER

M E M B R A N E M I G R b T l O N B Y AON-UlYlF.ORM H E A T I N G

MEMBRANE M I G R A T I O N B Y N O N - U N I F O R M C O O L I N G