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Correlation between the severities of the ASTM E119 and ISO 834 fire
exposures
Ser
TH1
National Research Conseil nationalN21d
*I
Council Canada de recherchar Canadano.
1 5 5 5
Institute forc. 2 Research in lnstitut de recherche en
BLDG Construction construction
Correlation Between the Severities of the
ASTM
E l 19
and IS0
834
Fire Exposures
by T.Z. Harmathy and M.A. Sultan
Reprinted from Fire Safety Journal Vol. 13, No. 2 & 3, 1988
p. 163-168
(IRC Paper No. 1555)
ANALYZED
NRCC 2931 8i
L I B R A R Y
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1 CNRC-
ICIST1
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On a utilis6 l'augmentation de tempdramre dans une brique rkfractaire coul6e dont les propri6ds thermiques dtaient bien connues pour etablir une co&lation entre les degrds d'exposition au feu selon ASTM El 19 et selon IS0 834. On a constat6 que I'exposition selon 1'ISO est lkg&ment moins forte que celle selon I'ASTEA, mais le gain de rksistance au feu procur6 par la dabsation de l'essai d'aprks la norme de 1'ISO est habituellement de cinq minutes ou moins.
Fire S a f e t y Journal, 13 (1988) 163
-
168Correlation between the Severities of the
ASTM
E l 1 9 and IS0
834
Fire
Exposures
T. Z. HARMATHY and M. A . SULTAN
Institute for Research in Construction, National Research Council, O t t a w a , O n t . K I A O R 6 (Canada)
(Received August 5 , 1987; in final form December 2 , 1987)
SUMMARY
The temperature rise in a castable refrac- tory brick specimen o f well-defined thermal properties was used to develop a correlation between the severities o f the ASTM E l 19 and the I S 0 834 fire tests. It was found that the I S 0 fire test is slightly less severe than the ASTM test, but the gain in fire endurance on account o f conducting the test according to the I S 0 standard is usually five minutes or less.
When the rewriting and updating of the ASTM E l 1 9 fire endurance test method was undertaken, it was generally agreed that one of the aims of the rewrite would be to bring the ASTM test procedure closer to the international procedure, by adopting certain desirable features of the I S 0 834 test standard. One such feature of the I S 0 standard is that the so-called "furnace temperature" is measured by bare thermo- couples, and controlled to follow a tempera- t u r e t i m e curve defined by analytical ex- pression.
As pointed out earlier [ I ] , the "furnace temperature" is a nominal value with no definite meaning. If measured with bare thermocouples, it will lie somewhere between the temperature of the furnace gases and the average surface temperature of the furnace chamber (part of which is the test specimen itself). If measured in the ASTM way (with the thermocouples enclosed in protective tubes), it may fall below any temperature prevailing in or along the boundaries of the furnace chamber, especially during the first 1 5 min of the test when the rise of tempera- ture in the furnace is very steep.
Although the advantage of switching over t o the I S 0 way of measuring and controlling the temperature conditions in the test furnace was generally recognized, there has been some uneasiness about the significance of the switch. To explore the effect of adopting the I S 0 ways on the test results, the researchers of the National Research Council Canada conducted an experimental study in which the severity of the specimen exposures in the ASTM and I S 0 tests was compared [2]. The study showed that the IS0 test was slightly less severe than the ASTM test but, with tests of up to 1.5 h duration, the dif- ference in the fire endurance values yielded by the two tests would not amount to more than 3
-
6 min.Since this conclusion was drawn from fire tests of 45 and 90 min duration, it seemed advisable t o follow them up with further tests suitable for developing a general cor- relation between the severities of the ASTM and I S 0 test exposures. These latter tests and the analysis of their results are discussed in this paper.
EXPERIMENTAL PROCEDURE
Two tests, each of two hours, were con- ducted in the National Research Council's floor furnace. The experimental set-up was the same as that shown in Fig. 1 of ref. 2. The furnace dimensions were 4570 mm X 3660 mm, and 2440 mm deep. The walls and the floor of the furnace were covered with 25-mm-thick fibrous ceramic blanket. The test specimen was made from castable refractory brick, marketed as KS-4. It was composed of twenty rectangular slabs, 1220 mm X 788 mm, and 150 mm thick, suspended on steel beams. The slabs were Elsevier SequoiaIPrinted in The Netherlands
tightly butted (with ceramic fibre sheets along their perimeters) to form a test specimen of 4880 mm X 3940 mm. (The overall dimensions of the test specimen were slightly larger than the furnace opening, to allow the specimen to sit on top of the furnace walls.)
The furnace temperature was measured in two ways:
(1) by nine Chromel-Alumel thermocouples in Inconel protective tubes, arranged to comply with the ASTM E l 1 9 specification; the time constant of the thermocouple assembly was 5 min when measured according to ref. 3 ;
(2) by twelve bare, 18-gauge, Chromel- Alumel thermocouples, their junctions set 100 mm from the exposed surface of the test specimen.
Five 36-gauge, Chromel-Alumel thermo- couples were installed inside the test speci- men. They were placed in five slabs in such a way as to have their junctions positioned approximately at the centre of the specimen and at the centres of its quarter sections. As in the tests described in ref. 2, the thermo- couples were located 40 mm from the surface exposed to fire. The lead wires were conducted at the same depth to the edges of the slabs, and then through the test specimen t o the data acquisition system.
The thermal properties of the castable refractory brick were determined at various temperatures up t o 600 "C. They are:
- thermal conductivity k = 0.9 W m-' K-' (approximately independent of temperature);
- density p = 2085 kg m-3 (approximately independent of temperature);
-
specific heat c = 787 + 0.897(T-
273) Jkg-' K-', where T is temperature, K.
In the first test, the "furnace temperature" was controlled from nine shielded thermo- couples, as specified in ASTM E119, and the ASTM temperature-time relation was traced. Also recorded were the temperature history of the furnace, as measured by bare thermo- couples specified in I S 0 834, and the temperature history of the specimen at five points (at the centre and the four centres of its quarter sections) at 40 mm from the exposed surface.
In the second test, the "furnace tempera- ture" was controlled from twelve bare thermocouples, as specified in I S 0 834, and
the I S 0 temperature-time relation was traced. Also recorded were the temperature history of the furnace, as measured by shielded thermocouples specified in ASTM E119, and the temperature history of the specimen at five points (at the centre and the four centres of its quarter sections) at 40 mm from the exposed surface.
The following were plotted:
- In Fig. 1 : average temperatures obtained from the ASTM (control) furnace thermo- couples, the I S 0 furnace thermocouples, and the slab thermocouples.
0 3 0 6 0 9 0 1 2 0 1 5 0 TIME, m i n
Fig. 1 . Average temperatures recorded during the ASTM E l 1 9 test.
-In Fig. 2: average temperatures obtained from the ASTM furnace thermocouples, the IS0 (control) furnace thermocouples, and the slab thermocouples.
THEORETICAL DETAILS
Since the "furnace temperatures", mea- sured by either the ASTM or the I S 0 tech- nique, are nominal temperatures, they are not suitable for a quantitative evaluation of exposure severities. In contrast, the specimen temperatures have welldefined meanings and,
2
-
-
"FURNACE TEMPERATURE".-
ASTM THERMOCOUPLES---
"FURNACE TEMPERATURE", I S 0 THERMOCOUPLES-
--
-
STANDARD !SO 834 CURVE-
SLAB TEMPERATURE40 mm BELOW SURFACE
-
I I I I [ I
0 30 6 0 9 0 1 2 0 150 T I M E , m i n
Fig. 2 . Average temperatures recorded during the I S 0
834 test.
having t o do with the heat absorbed by the specimen, can be used for developing informa- tion on the severity (destructive potential) of test fires.
It has been shown (see, e.g., refs.
4
and 5)that the so-called "normalized heat load" is a measure of the maximum level of temperature rise at a critical depth inside the boundaries of an enclosure on fire (irrespective of the pattern of fire exposure), and therefore it is a convenient parameter for comparing the destructive potential of fires (real-world fires br test fires). The normalized heat load is defined as
where H (s1I2K) is the normalized heat load, d k p c (J mP2 sP1l2 K-') is the thermal absorp- tivity (or thermal inertia) of the enclosure boundaries (k is thermal conductivity, p is density, and c is specific heat), q (W mP2) is the heat flux that penetrates the enclosure boundaries, t (s) is time, and T (s) is the dura- tion of fire exposure (more exactly, the dura- tion of heat penetration).
An important characteristic of the normalized heat load is that, for standard test fires (owing to the uniqueness of the fire exposure), the H versus T relation (where T is now the length of the test fire, or, on that account, the fire endurance of the test speci- men) is only slightly dependent on the nature of the specimen. Consequently, the severities of the ASTM E l 1 9 and I S 0 834 fire tests can be directly compared if the H(T) relation for the two tests is known from tests conducted on any specimen (e.g., on the refractory brick specimen used in the present studies).
The problem is now how to determine the H(T) function for the two kinds of tests from the Ta versus T relations shown in Figs. 1 and 2, where Ta (K) is the temperature of the test specimen at some distance a (m) (a =
0.04
m in the present tests) below the fire-exposed surface.
A trial-and-error technique was employed. Starting with an assumed q(t) relation, the T,(t) relation was calculated, using the finite difference method described in the Appendix. After the first trial, the q(t) relation was repeatedly adjusted until satisfactory agree- ment between the calculated and experi- mentally determined T,(t) curve was achieved. The q(t) relation by which agree- ment was achieved was then used in eqn. (1) to calculate the H(T) function.
Figure 3 shows the final forms of the q(t) relations for the ASTM E l 1 9 and I S 0
834
tests, and Fig.
4
the closeness of the agree- ment between the calculated and measured T,(t) curves. Because of the steep rise in the temperature of the furnace gases, not properly recorded by the shielded furnace thermocouples, the test specimen seems t o have absorbed much more heat during the first fifteen minutes in the ASTM test than in the I S 0 test.Figure 5 shows the H(T) relations for the two tests, as calculated with the aid of eqn. (1) and the q(t) relations shown in Fig. 3.
Clearly, the curves indicate that the ASTM test is somewhat more severe than the I S 0 test for the entire 2-h period of testing. The gain in fire endurance by conducting the fire test according t o the IS0 practice can be determined by reading the difference between the two curves along the abscissa axis. As
-
A S T M E l 1 9 T E S T----
I S 0 8 3 4 T E S T0 3 0 60 9 0 1 2 0 1 5 0
T I M E , m i n
Fig. 3 . Rate of heat absorption by the slab.
D U R A T I O N O F F I R E E X P O S U R E , T rnin
Fig. 5. Normalized heat load as function at time of testing.
D 60 9 0 120
T I M E , m i n
Fig. 4 . Average temperature of slab 40 mm below surface.
DURATION OF FIRE EXPOSURE (ACCORDING TO ASTM E1191, T rnin
Fig. 6. Gain in fire resistance time if test is conducted according to I S 0 854 instead of ASTM E119.
Fig. 6 shows, that difference may be as high as 7 min for very short tests. Yet, for tests of realistic duration, 45 min and longer, the gain is about 5 min or less.
CONCLUSION
The severities of the ASTM
El19
and I S 0 834 tests were compared, based on tempera-ture measurements taken during 2-h fire endurance tests, and evaluated using the normalized heat load concept. It was found that of the two tests the IS0 test is slightly less severe, but differences in the fire endurance values yielded by the two tests are not expected t o amount to more than about five minutes.
ACKNOWLEDGEMENT
The authors wish t o thank J. W. MacLaurin for the conduct of the experiment work.
LIST OF SYMBOLS
given 'distance from fire exposed surface
(m)
specific heat (J kg-' K-')
normalized heat load (s1I2K) thermal conductivity (W m-' K-') thermal absorptivity (or thermal inertia) (J mP2 s-'I2
Kpl
1
number of elementary slabs of Ax thickness in the finite difference scheme (-)
heat flux penetrating the test speci- men (W mP2)
time (s)
temperature (K)
time increment (s)
thickness of elementary slab (m)
density (kg m--3)
duration of heat penetration, fire endurance (s)
Subscripts
a at a distance a i initial
n of the nth elementary slab Superscripts
j, j
+
1 at a time t = jAt, t = ( j+
l ) A t , respec- tively ( j = 0, 1, 2,. .
.)REFERENCES
1 M. A . Sultan, T. Z. Harmathy and J . R. Mehaffey, Heat transmission in fire test furnaces, Fire
Mater., 1 0 (1986) 47
-
55.2 T. Z. Harmathy, M. A. Sultan and J . W. Mac- Laurin, Comparison of severity of exposure in ASTM E l 1 9 and I S 0 834 fire resistance tests,
J. T e s t . Eval., 1 5 (1987) 3 7 1
-
375.3 Supporting D a t a for E 1 1 9 , Furnace T h e r m o -
couples, File N o . R R E 5 - 1 0 0 1 , March, 1977,
available from American Society for Testing and Materials, Philadelphia, PA.
4 T. Z. Harmathy, The possibility of characterizing the severity of fires by a single parameter, Fire
Mater., 4 (1980) 7 1 - 76.
5 T. Z. Harmathy and J . R. Mehaffey, The normalized heat load concept and its use, Fire
S a f e t y J . , 1 2 (1987) 75 - 81.
APPENDIX
The purpose of the calculation is to develop, by trial and error, information on the history of heat flux, q (W mP2), at the exposed surface of a slab-like test specimen, which causes the temperature at a depth a (m) (a = 0.04 m in the present study),
T,
(K), t o follow a prescribed (experimentally determined) course.It has been shown [5] that, under circumstances characteristic of standard fire endurance tests, the heat flux absorbed by the specimen does not depend noticeably on the boundary conditions at the unexposed side of the test specimen. Thus a slab-like test specimen can be modeled as a semi- infinite solid or, in numerical studies, as one of substantial thickness. With the problem t o be studied, it is a good approximation t o regard the temperature of the slab at 0.15 m from the heated surface as constant and equal t o the initial temperature.
Figure 7 shows the subdivision of the slab-like specimen into N + 1 elementary slabs (N inside slabs of Ax thickness, and one surface slab of Ax12 thickness). After writing energy balances for the elementary slabs, the following equations are obtained:
for the Nth inside slab (n = N )
I
3 6 ... ..."....""B.' .. ... .. ...Fig. 7 . The finite-difference scheme.
for N - 1 inside slabs ( 1
<
n<
N - 1 ) At [k:,-'
+
k ;p:,c;i, Ax2 2
x
( T i - Ti.')] (3)initial condition
where Ax ( m ) is the width of the (inside) elementary slabs, At (s) is the time increment, T (K) is the temperature and Ti (K) the initial temperature of the elementary slabs, k (W m-I K-' ) is thermal conductivity, p (kg mP3) is density, c (J kg-' K-') is specific heat. The subscript n (or n + 1, etc.) refers t o some characteristic (temperature, thermal conductivity, etc.) of the nth (or (n + l ) t h , etc.) elementary slab ( 0
<
n<
N). The j andj
+
1 superscripts mean at t = jAt and...
(j
+
l ) A t , respectively, where j = 0, 1 , 2, 3, As pointed out in the text, for the refrac- tory brick specimen used in the experiments,k and p are approximately constant (inde- pendent of temperature), so that k: = k and
pi = p for any value of n and j. The specific heat is a function of the temperature, so that
c i should be interpreted as c at T;, etc. Numerical studies indicated that the criterion of stability was satisfied by selecting Ax = 0.005 m and At = 20 s.
T h i s p a p e r
