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Significance of flame-spread results
SIGNIFICANCE OF FLAME-SPREAD RESULTS
J . H . McGuire and M.V. DrSouza
This Note describes the application of the concept of flame spread o r flammability in t h e design of a building. Tt is to be emphasized t h a t although such a concept is virtually indispensible in order to achieve a reasonable measure of fire s a f e t y , no such property as
flammability or flame spread strictly exists. Some of the limitations
imposed by this contradiction a r e discussed.
Tn developing a building design aimed a t minimizing f i r e h a z a r d ,
t w o f e a t u r e s c o n t i n u e t o be o f paramount importance. The first is t h e extent to which compartmentation [into fire resistant enclosures) is planned, with a view to containing the f i r e . The second is t h e e x t e n t to which the flammability of interior finishes (ceilings, walls and f l o o r s ) , and also contents, is restricted. The flammability of every
possible contributing surface should be limited to reduce the basic
probability of t h e development of a f i r e and, j u s t as important, to
lengthen the t i m e s c a l e o f any possible development. A longer Eire- growth period enhances t h e likelihood o f escape and of successful f i r e - f i g h t i n g and g i v e s more t i m e during which d o o r s can b e c l o s e d ( p o s s i b l y automatically) to complete compartmentation and thus c o n t a i n the f i r e .
Some architects
are
dispensing with compartmentation and regarding the provision of sprinklers as an adequate substitute. This concept i snot discussed in t h i s Note, b u t it must be emphasized t h a t such a
substitution should not permit dismissal of flammability considerations.
The use of highly flammable linings and contents can give r i s e to s u c h
r a p i d f i r e development t h a t propagation proceeds beyond each area before
the associated sprinkle^ can operate.
MEASUREMENT OF FLAMMABILITY
Flammability b e i n g such an important d e s i g n factor, means of
assessing it are of equally critical importance. In North America the
most widely accepted metl~od is t h e tunnel t e s t . 1 2 p 3 Because this
apparatus is expensive and involves l a r g e specimens, another test officially known as 'The s t a n d a r d test method for surface flammability of materials using a radiant h e a t energy saurce0 4E162) has been
developed. The l a t t e r standard was i n t e n d e d f o r research and development purposes only, but is f i n d i n g other applications, for example, in t h e
regulation
of
the flammability of various components o f r a i l and r o a d vehicles operated by some city trznsit authorities.The tunnel and t h e E l 6 2 t e s t s have similar flammability rating s c a l e s
with zero representing an essentially nan-flammable m a t e r i a l with v e r y little combustible c o n t e n t to contribute to f i r e propagation. Higher
numbers represent greater f l a m a b i l i t i e s , a value of about 100 being
associated with thick heavy woods and values over 200 being regarded a s
unacceptable for applications o t h e r than f l o o r i n g .
The intended significance of such a scale is t h a t it should rank
materials (or assemblies) in
a
sequence of m e ~ i t , so t h a t if one particularmarerial is found to be acceptable (from a f l a m m a b i l i t y standpoint) then
substitution of a material with a lower ( b e t t e r ) index o r flame-spread value should also be acceptable, presenting an even lower hazard. It is usually assumed that the merit sequence is universally applicable,
regardless o f the f i r e conditions being considered. Thus T f material A has a lower flame-spread value than marerial 8 , it would be expected to give l e s s rLsk o f fire and slower development than B in any given set o f
circumstances. The r a t e o f fire propagation in a vertical s h a f t f o r b o t h
materials might be f a r faster than in the case of a l a r g e auditorium, but
f o r each of these cases, material E would be expected t o perform more
p o o r l y than material A .
It has always been known t h a t this concept is only an approximation
to the t r u t h , but
in
recent years anomalous r a t i n g s have been appearing quite f r e q u e n t l y . The following sections discuss two sets o f circumstancesi n which this can occur; a means of resolving the anomaly is given f o ~ t h e f i r s t case.
FOAMEII PLASTICS AND THERMdL INERTIA fkacl
Some years ago it w a s found t h a t many foamed plastics with tunnel
r a t i n g s of l e s s than 75
were
behaving more p o o r l y than more conventionalmaterials with r a t i n g s of several hundreds.
This
s i t u a t i o n created s u c ha potential hazard t h a t the U . S . Federal Trade Commission condemned the
use of the tunnel (and certain other tests) f o r the assessment of the
f
l a m a b i l i t y o f these materials. Following consultation with thc: S o c i e t yof the Plastics Industry, a consent order was issued recommending that
foamed plastics be cavered w i t h a substantial thickness of protecq:ive material.
A close examination of the fire behaviour of foamed p l a s t i c s in t h e tunnel has led ta a means of substantially resolving the problem of
flammability assessment6 and to a p o s s i b l e explanation of the mechanisms
responsible f o r the o r i g i n a l problem.
The unsatisfactory r a t i n g s only arise f o r those foamed plastics t h a t g i v e flame fronts that i n i t i a l l y advance r a p i d l y , b u t h a l t b e f o r e r e a c h i n g t h e end of the tunnel. It has been found t h a t the hazard o f such a
material
can
be satisfactorily assessed by a~plying a rate index (now incorporated i n t h e ULC Standard] to the f l a m e - f ~ o n t behaviour during thetimes t h e main index s p e c i f i e d in t h e s t a n d a r d . For
more
conventional materials, however, discrepancies are usually minimal.The suggested explanation for the anomalous behavieur of foamed
p l a s t i c s in the t u n n e l is t h a t it is associated wEth t h e environmental
thermal i n e r t i a of t h e tunnel and the extraordfnarily low thermal
i n e r t i a and d e n s i t y of foamed p l a s t i c s . Thermal inertia ( t h e product
kpc where k = thermal conductivity and pc = volumetric heat capacity) is the thermal factor controlling the rate of heat f l o w in and onto a
specimen (and t h e su~rounding walls) to produce a particular temperature
distribution w i t h i n the specimen, A foamed plastlc with a low p r o d u c t
kpc can be warned with q u i t e small h e a t flows, whereas similar
tempexature distributions
in,
say, a dense wood would r e q u i r e much g r e a t e r flows.The implicit assumption that a flammability merit or r a n k i n g
sequence
is
independentof
the environmental thermal inertia in a givens e t of circumstances has hitherto been reasonably valid, presumably
because t h e prevailing range o f thermal i n e r t i a [and d e n s i t i e s ) of
combustible materials was l i m i t e d . With t h e advent o f foamed p l a s t i c s , however, t h i s is no longer so and t h e behaviour of some of them i n t h e
tunnel illustrates t h i s feature-
Initially, during a tunnel exposure, a foamed p l a s t i c can behave
as
if it were highly flammable. After some involvement, however, whenthe environmental thermal i n e r t i a o f t h e tunnel becomes significant, it can then behave as a relatively non-flammable material. I f t h e most a d v e ~ s e environment is to be c a t e r e d for, the flammability should be
assessed b y t h e suggested
rare
index. Such an approach is conservativc and avoids unexpected hazard, at The expense of penalising the foamed p l a s t i c in the rarer circumstance when it should be regarded as q u i t ea non-flammable material. In fact some foams have proved to be of remarkably low flammability regardless of prevailing test or f i r e conditions.
The u s e of t h e rate index can thus be regarded as a reasonably satisfactory solution to the problem associated w i t h t h e assessrnenr o f
t h e flammability of foamed plastics. NON-HOMOGENEOUS SPECIMENS
The second set of circumstances giving r i s e to anomalous merit sequences involves non-homogeneous specimens tested by a method using a
time scale n o t representative
of
t h e f i r e scenario under consideration. A f e w isolated examples o f the e f f e c t , primarily associated w i t h steel-backed specimens, have been known for many years. The advent of foil-
covered foamed plastics however has l e d to serious anomalies and it i s
now appreciated t h a t t h e mechanism responsible is similar t o t h a t
The time s c a l e of a test, or o f the development of a f i r e , will govern the depth to which t h e specimen is thermally involved, Figure 1
illustrates a test (OT fire) situation
in
which two specimens, i d e n x i c a l t o a c e r t a i n depth b u t radically d i f f e r e n t thereafter, will give t h esame performance. Figure 2 on the other hand illustrates depths o f involvement for which t h e same t w o specimens will behave markedly
d i f f e r e n t l y from each o t h e r , specTmen 2 behaving better than specimen 1 .
Figure 3 illustrates a metal-foil-covered foamed p l a s t i c and
the
depths of (thermal) involvement created by two d i f f e r e n t test methods A
and B. During the rapid test A , t h e influence of t h e metal foil (as a
heat capacity] will be substantial in view af the small depth of
combustible generating heat. The slower test B, on the other hand, will
involve m c h more combustible and hence the heat sink constituted by the metal f o i l will have much less i n f l u e n c e . Assuming that b o t h t e s t s rate most materials
in
the same sequence of merit, test A will rate t h especimen h i g h e r in t h e sequence ( i . e . , b e t t e r ) than will test B.
The effect of t h e metal facing, as j u s t described, is the opposite
of the metal backing, described e a r l i e r . All o t h e r aspects of t h e i r behaviaur also d i f f e r . For example t h e metal facing produces t h e
greatest e f f e c t with a s h o r t time-scale test, and a n e g l i g i b l e effect
with a long time-scale t e s t . Metal backing, on the o t h e r hand, has l i t t l e influence fn s h a ~ t time-scale tests but some e f f e c t w i t h slower
t e s t s . Usually t h e magnitude o f t h e effects is also less marked. The discussion on non-homogeneous materials has beem confined to
composites involving combustibles and metals, the density and thermal d i f f u s i v i t y of t h e l a t t e r being so markedly d i f f e r e n t from t h o s e of combustibles t h a t t h e e f f e c t described is maximum. Ifiere the component
materials are
more
similar, t h e effect may still occur, but will be of smaller magnitude. This i s illustrated by f i ~ e and test experience withthin plywoad veneers. The f i r e behavlour of 118-in. ( 3 - 2 mm) plywood
veneer backed by glass-fibre insulation h a s not been inconsistent w i t h
tunnel r e s u l t s for t h e same t e s t assembly, In constructing mobile homes,
however, designers have used flame-spread results, intended for an
i n s u l a t e d assembly, that have been derived from specimens o f the 1 / 8 - i n .
plywood backed by asbestos cement. These have proved anomalous and f i r e
performance h a s been much poorer than i n d i c a t e d by the spurious test r e s u l t . When t h e e f f e c t o f I-in. glass-fibre backing w a s investigated,
a test result of 197 was obtained, whereas t e s t s
on
the same plywoodd i r e c t l y backed by t h e asbeszos cement of the tunnel l i d gave an average of 118.
From the foregoing discussion t h e following conclusions can be dram.
(1) It s h o u l d be assumed t h a t t h e function o f a flame-spread (or flammability) test is to list materials in a sequence
o f
m e This assumption is seasonably valid for conventional
homogeneous materials and most composites o t h e r t h a n t h o s e t h a t
aTe metal -foil faced,
( 2 ) The test assembly must correspond to t h e assembly for which information i s required, to the depth o f themal involvement in
t h e t e s t . T h i s will usually not exceed an i n c h . An example o f a n
invalid t e s t i s t h e tunnel t e s t i n g o f plywood veneer backed by t h e
asbestos cement of t h e tunnel lid, when the assembly under discussion involves a lightweight insulation behind the veneer.
( 3 ) \fiere the material involved has a very low thermal i n e r t i a [product kpc) as with a foamed plastic, the assumption of a universally applicable merit sequence may n o t b e a satisfactory approximation to t h e truth. I n a conventional kpc environment t h e material may n o t propagate f i r e as far as some standard reference
material, whereas in a low LPC environment it may propagate it faster and f u r t h e r . A merit sequence applicable to adverse conditions f o r t h e lightweight material may be derived from a
tunnel test by using a ate index.
( 4 ) Where the assembly involves laminates of sharply d i f f e r e n t thermal
p r o p e r t i e s , a test method may rate the assembly satisfactorfly o n l y f o r fire scenarios involving similar time-scales. The effect will probably be substantial o n l y where metals are involved and then
o n l y when t h e metal is a facing or is very n e a r to the surface o f t h e assembly.
REFERENCES
1 "Standard Method of T e s t f o r Surface Burning Characteristics of
Building Materials,'VUnerrwriters' Laboratories of Canada,
ULC-S102-1977.
2 '3Standard Method of Test f o r Surface Burning Characteristics of Flooring, Floor Covering and Miscellaneous Materials," Underwritcrsr Laboratories of Canada, ULC-5102.2-1978.
3 "Standard Test Method for Surface Burning Characteristics of Building Materials,"AAST/ASTM E84-76a-
4 "Standard T e s t Method f o r Surface Flammability of Materials Using a
Radiant Heat Energy Source,'qASTM E162-76.
5 "Cellular P l a s t i c s Products," U.S- Federal R e g i s t e r , Vol. 40,
No.
1 4 2 ,July 23, 1975, p . FR 30842.
6 D%oauza,
M.V.
and J.H. McGuire. "ASTM E-84 and the Flammability of Foamed Thermosetting P l a s t i c s , ' q i r e Technology, Vo9, 13, No. 2, May 1977, pp. 85-94.7 McGuire, J . H . "Flammability Merit Sequence and Specimen H o m ~ g e n e i t y , ~ ~
S P E C I M E N 1 S P E C I M E N 2 E X P O S E D S U R F A C
