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Technical Note (National Research Council of Canada. Division of Building Research), 1967-01-01
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Determining Moisture Content and Calculating its Effect on Fire
Endurance
Harmathy, T. Z.
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478
NOTJE
'fE
C
JHI
N ][ CAlL
PREPARED BY T. Z. Harmathy CHECKED BY G. W. S. APPROVED By N. B. H.
DATE January 1967
PREPAREP FOR Inquiry and record
SUBJECT DETERMINING MOISTURE CONTENT AND CALCULATING ITS
EFFECT ON FIRE ENDURANCE
For some time there has been an effort to set, by the time of a fire test, the moisture content in fire test specimens close to a level corresponding to that in equilibrium with a surrounding atmos-phere of 50 per cent relative humidity (at room temperature). For a great many test specimens, however, it is impossible, at least with the present conditioning techniques, to achieve this moisture condition within a reasonably short time (say in six months) without inhibiting some chemical processes related to the maturing of the specimen, or in other words, without falsifying the characteristics of the material. To eliminate this difficulty, it has been suggested that fire te st standards be modified and allow tests to be carried out on specimens of moisture contents significantly higher than that pertaining to 50 per cent equilibrium relative humidity. The
authorization of such relaxation of the test requirements would not be possible, however, without knowing how the moisture content in excess of the "standard" level affects the result of the fire test. Fortunately there is now enough information to make possible the calculation of the effect of moisture content on the re sult of fire tests. Details of experimental observations and theoretical
considerations concerning this subject have been given elsewhere (1,2). The application to practical problems of the conclusions
derived in these publications will be discussed and illustrated through examples.
J Iij
•
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'(,:i.biffllrd ｾｲＧｩ !r:I"'! It"Ii,-dHillP;
i \ : -:G ...,.1rｾｾ ｲＮｾＩ bｾＮＧ 1 fl.) 1ｾＮＧ ｾfq 1",d "1rioｾt 1i L ;-; ! ,i· i ' ! .. ,.- II I ' ! <.,ｾ1 ; , ! : , . )ｦｾＬ ＺＭｾ >:)'f 'rｾＭＧ f') rt , ; )1rI . ) •J t ;ｾ • \ )... h,; }:'-,
I"
r' t.l "';' \ ;-cｾ ｾh: ' rIi.'If) j q " ) ..ｾ '-)"Ir.-i . i : ) I ' l I;1 ｩﾷｾｾｩＱＬＧｴｾ［ﾷ｜ＬＧ Ｇｾ､［ ｾＮ ｾ'" i ;4.ｾ. . . .r) ; e,dlIi ''',:1, II! ',,(fbi 71"rri ,>I!I ".,
! ' ｦ［ｾＡｾｾＬ［Ａ :",,:';, ,.! ＮＧＩｾＺｴｬＡ
SOME BASIC CONSIDERATIONS
During the past few years it has become generally accepted practice to measure and report at least the maximum value of the equilibrium relative humidity inside fire test specimens prior to fire tests.
The practice of expressing the degree of dampness qf fire
test specimens in terms of equilibrium relative humidity was
adopted on Menzel's suggestion (3). The advantages of this
practice are twofold: the equilibrium relative humidity has an
unambiguous meaning, and it can be measured easily and with relatively high aCC1Jracy.
The shortcomings of this practice, however, soon became
obvious. A priori considerations, as well as theoretical (1) and
experimental work (2), clearly indicated that the effect of moisture on fire endurance i's not related directly to the equilibrium relative humidity, but to the moisture content expressed in per cent by
volume within the apparent boundaries of the solid constituents. As
the relationship between the volumetric moisture content and equilibrium relative humidity, the so-called "sorption relation," depends very markedly on the individual material, and even for a
given material is not unique (due to the hysteresis in sorption), the
author has suggested (4) that, in addition to the equilibrium relative humidity, the absolute moisture content of fire test specimens should also be determined and reported.
Determining the volumetric moisture content of fire test specimens is, unfortunately, not always a routine procedure.
Difficulties may arise for two reasons: (i) in the case of some
materials (generally those formed by hydration) the moisture content
may not be uniquely definable; and (ii) sometimes it is not possible
to obtain samples of the test specimen itself for moisture analysis, therefore the analysis must be based, at least partially, on
measure-ments carried out on representative samples.
To overcome the first difficulty, the "dry weight" to which the moisture content of building materials is referenced must be
defined unambiguously. It is suggested that the weight of building
materials after being heated in an oven at 221 ± 1°F (105 ± O. SOC)
for a sufficiently long period so that no further weight change can be
detected should be regarded as their dry weight. (This method of
determining the dry weight is not applicable to gypsum products.
,'I:" -r' 1'1 '11.1,1 ", r, . ,0 •• 1 • I .i t (I ｾ II1 '0 I I I ; , II' ., ., , f
"
r' I •. 1(""\" I •• I, "I'
1.1 . , . " ., ' t f ' j • 1 tl· :s " , 'I ;'r 'I t ", ... 1,';..
Ｇｾ ",
",
. i1iｾ fl I . , d; IIJ' I r t ", .i.l·
I . ., I "!. ,1:'I"lt,ll1, IＬｾｲＱＱ hI!r;I;.' r,-'[Jill' P
Ｇ｜ﾷｾＮＧＩｌｦＧ ｻｴｾＱ ｾ｜ＢｪＧｾｉＮＧＺＮｾｲｈＧＧＧＧ r':.J ＡＮＩＢｾＧ ｾｊ ｾｩ ｾ ''1 V Ｍ［Ｎｮｾ ti .) .."!"li,tCiVj ＡｾｾＨＭＢＢＧＱｴｾｾｉｲＺｾＬ［ＺＬＡｴ
"
,n t ｾ Ｎｾ q ｾ .. ,rJ ...) ｾＢｉＢＩ j , r.,"\ ',IｾＮ ",'! ( ;tf",t '3;"; .1 II. P f. ". IIi I,
.'
..
the presence of moisture usually has no significant effect on the
fire test.) Dependent on the size of the specimen the drying
procedure may take from one day to one week.
In all the work done in the DBR laboratory in connection with studying the effect of moisture on fire endurance the moisture content was referenced to dry weight measured in the way just described, therefore some conclusions may not be applicable if·
the moisture content is determined by some different drying method. In selecting this method the DBR laboratory tried to comply with the
most commonly followed practices. In the case of concretes,
which are practically the only.materials sensitive to the drying procedure, drying at 221°F gives approximately the same dry weight as that obtained by the "dry ice method" (5) which is tacitly accepted as the standard drying method in scientific laboratories.
It is important to understand that although a sample oven-dried at 221°F can be regarded as containing no more moisture or as it is often termed "evaporable water, " it may still contain a considerable amount of "non-evaporable water, " i. e., water
attached to the crystalline lattice by various chemical bonds. The
non-evaporable water may also have a marked effect on the fire endurance this effect, however, will not be considered in this Note.
As already mentioned, obtaining samples representative of the test specimen may present another difficulty in determining its
volumetric moisture content. Whether or not this difficulty arises,
depends largely on the type of material used in the construction. Before discussing some methods of overcoming these difficulties, it may be useful to examine briefly how important the "moisture problem" really is in the fire testing of various building materials.
If spalling is not liable to occur, the presence of moisture.
is beneficial from the point of view of fire endurance. As a first
approximation, one may expect about 5 per cent increase in fire
endurance due to every per cent (by volume) of moisture. Since
the reproducibility of fire test results is generally'not much better
than ± 5 per cent (2,6), the effect of moisture on fire endurance
Clannot bl .xillliietiCi to ihOW up ｾｬ .... rly 1n the
ea.11
ofeOfilu"uction.
made from materials which at the time of the fire test do not hold more than 1 per cent (by volume) of moisture.
I ' ", i:t":I" 'I, iI ;r0; •ii ,) ; . , I • t :ｾ I I : 'fｾ • , t ItfI f , I ｾ If -,tI 'I-!' I
n,
J I} I ,,, ' ' ' fL r ' j } '1" It.' :-.'·rd: n'" I . . . Ip"'," 1 . ' )!' I I : l" i," ,I I :'" I t : ' 1'1 , I r ' l l " ,I, • 1'1 H'· • frII ') \, t. 'i I I· t!" r: t ., " f t't', ;.'f ｾｦｬ ｲＬＺｾｦｦｾＮｲ nIl, ·....' · ' f l ...(J ｾｉｉＨ ) , 1 ' " , ) I • ., • I ｾ I l , I i"l 'J ("\ 'i I I Iｾ , ! It J I I • . ) vll,,," ＺｬｴｬＧＧＧｉｾ , ,;- • .. I 1, I " 'l . \ :1 T'I " -, Ｌｾ rrj(I] , , titi(IftIIf: 'I I t 1 0 1qf"f1ＬｾＺ -, 'J':! 10 i';1 I •rr ,,,
lJ,JI 'r' III I Jt ' 1t t'=' ;f i 10 ll ':; I )( f " . t,. T' ....l • ' t ·commonly used building materials indicated (7) that hydrated portland cement is the only inorganic building material of practical importance which, in equilibrium with a 50 per cent relative humidity environment, can hold significantly more than
1 per cent (by volume) of moisture. It is also the only important
building material, for which the "air dry" condition (which corresponds roughly to the 50 per cent equilibrium relative humidity condition) is not readily attainable without falsifying its
basic properties. Consequently, determining the effect of moisture
on the fire endurance is of practical interest only in the case of constructions containing a significant amount (more than 5 per cent
by volume) of hydrated portland cement. Constructions made with
less than 5 per cent cement paste. e. g., brick walls made with
cement mortar, or floor constructions containing less than
20
percent (by volume) concrete, usually cannot hold more than 1 per cent moisture after conditioning for two or three months, consequently can be subjected to fire tests without paying attention to their actual moisture condition.
The above considerations narrow the range of constructions
for which the effect of moisture on fire endurance definitely ｾ･ｳ･ｲｶ･ｳ
attention to those built predominantly from portland cement products.
Thus the problem of obtaining ｲｾｰｲ･ｳ･ｮｴ｡ｴｩｶ･ samples also narrows
down to fire test specimens employing portland cement products, generally in the form of concrete.
Several year s ago a simple technique was described (4) for obtaining samples of material of the test specimen for moisture .
analysis. This technique has been successfully used in connection
with specimens built from various lightweight concretes.
Unfortu-nately this method could not be applied easily in the case of test specimens built from normal (dense) concrete.
1£ the material of the specimen cannot be readily sampled,
there are four possible methods that the testing laboratory may follow to determine the moisture content of fire test specimens.
1. "Calculation method". - It is possible to obtain a fair estimate of the desorption curve of concrete by calculations described
in detail elsewhere (7). The knowledge of this curve makes it
possible to calculate the moisture content and its distribution
in the fire test specimen, if the values of the equilibrium
relative humidity are known, e.g., by means of the method
I· 0 ! I .
,.,
.' t,r: t • '> "i ".
I I, I I··....
,..
I ,Ii.
.
" 0 . f Jdrt '..
ｾ f I I , :l' .' J • ,cI "''''q I ,,\ " 1" ' I r',
Ｎｾ,
;, #11, I , ",
Ii .; ? 11 , I " I " !.••
-, 0' ; jｾ , t,"\, I, I :ｾ '/ II;V'l I L.'f' .,
II'.F.r:" ·0 ,,
16.ti ", 11 I.,"
h i I.
, IｾI fｾ ｾ.1 . , ' ' . S· t, I '--;I : .ｾ,
\ .:: 6\ ' 0i II ｾＱ : fI t: t·t " 'Oo ·.:1· r; ".,.
, I I )< .'., ,. ,.,I - ,I: III I " ,1,,'\ I ,.---\", I . fl :16 r .•2. "Method of embedded samples". - At the time of placing the
concrete, cylindrical holes of I
t
to 2 in. in diameter, reachingto sufficient depths, are left in the test specimen at a few well
selected locations. Into these holes, after the removal of the
forms, cylindrical sample s of convenient size s, prepared of
the same concrete will be sealed, e. g., with some putty. On
the day of the fire test these cylindrical samples have to be removed and analysed both for equilibrium relative humidity and moisture content in a way described in Ref. (4).
3. "Substitution method". - The testing laboratory may build a "sample specimen" which is large enough to represent
accurately the cross-sectional details of the fire test specimen, but is scaled down in one or two directions so that it can be
oven-dried in a larger laboratory oven. If this sample specimen
has been conditioned together with the fire test specimen, one can expect that the value of the average moisture content
obtained by performing the necessary weight measurements on the specimen will agree reasonably well with what could be obtained if the test specimen could be subjected to the same measurements.
4. "Combined weighing method". - The testing laboratory may take samples from the concrete used in the test specimen, which are significantly smaller than the one described before, and do not
follow the geometrical details of the test specimen. In this case
the average moisture content of the test specimen can be determined from weight measurements performed on both the
samples and the test specimen.
The first two methods excel in their simplicity. They have
the common disadvantage, however, that the testing laboratory must rely on the assumption that the average moisture content of the entire concrete test specimen can be calculated from data
pertaining to
a
few particular locations.This difficulty, however, is obviously not characteristic of
these methods alone. The testing laboratories have to face the
same problem even when using Menzel's method (3) or the technique
described in Ref. (4). With the "method of embedded samples"
there 18 tlfiothtu" ､ｩﾣｦｩｾｕｬｴｹ thAt tho "mall c::ylindl'ieo,l fif1Ufiplej "dUAlly
contain somewhat less coarse aggregate than the concrete in the test
specimen. This difficulty, however, can be eliminated by considering
",
" '" co ""
"
•: 1-' ,' f " ,'..
,
,"
'" . ,II 'e I'"
" , !' • ",
,•
,
' ',
·
"
t'I. ,.,
...
; • , ; -I " r ' ,, .. I,,
• ; " ',
.;.
• , , I ; r'I ; ,..
',",
,
"
,'0"
r..
',
;. ;"•
""
t" , ",
..
,.
, , 'I '0,
·
,.
, ".-
,'''',"
"
"
,
; ',.'.
•
is always negligible. By determining the density in green condition of both the specimen concrete and sample concrete the volume percentage of portland cement paste in both the specimen and the
saxnples can be calculated [see Eqs. (6) and (7)]. Then the xnoisture
content of the specimen-concrete is obtained as
= en't' -VVi
With the aid of both the "substitution method" and the
"combined weighing method" suitable values of average moisture
contents can be established. Although the first of these two seexns
to be simpler and more convenient, under certain circumstances the latter can be expected to give higher accuracy.
To be able to employ the combined weighing method the testing laboratory must have facilities to weigh the entire fire test
specimen. The weighing may be conveniently done by suspending the
specimen on a load cell. The method of weighing a wall specimen
in the DBR laboratory is shown in Figure 1.
A problexn that xnay arise in connection with the coxnbined weighing xnethod is that the test specixnen is often built in a fraxne or on a sill the weight of which xnay also be subject to changes
induced by variations in the atxnospheric conditions. The testing
laboratories have to study how significant these weight changes xnay
be thr,oughout the year. If it is found that these changes xnay
significantly falsify the results of moisture measurement, vapour barrier coatings may be applied on these frames or sills, or they xnay be replaced by others built from materials of xnore favourable
water sorption properties. (It is often possible to build these
components with protected or unprotected steel.) The Combined Weighing Method
The principal advantage of the combined weighing method is
that the process of drying is deterxnined from measurements .
performed on the (concrete) fire test specimen itself. The principal
difficulty is that the "dry weight" of the test specimen is not
available so that without auxiliary xneasurements, weight changes
QA.fingt b5l ｾｸｰｴｇ IiliHHl in t€lJ'ffii!i @! ｦｦｩｄｩｾｴｕｩＧＴｩ e@fthult. TMi in!Ol'ffiiidOI'\
has to be deterxnined by using a sample of the concrete (conveniently a standard compression test sample) which has been conditioned
7
-its small size, is probably of slightly different aggregate content and will probably dry slightly faster than the test specimen. This problem can be overcome by weighing both the specimen and sample in the "green" condition, immediately after the removal of forms (or molds), and making use of the fact that at this stage the portland cement paste is still completely saturated with water, thus it is in
a
well-defined condition.To be able to calculate the average moisture content of the test specimen, the following weights should be determined.
(a) By suspending the test specimen on a load cell:
W = weight of the green concrete in the test specimen, lb,
g
W = weight of concrete in the test specimen on the day prior to the fire test, lb.
(b) By conventional weighing methods: WI g
W'
d d g=
weight of green concrete sample, lb,=
weight of sample, after having been kept close to the test specimen during the conditioning period andoven-dried (at 2.2.1 ± 10
F) at about the time of fire test, lb,
= bulk density of green concrete, measJlred during the placement of concrete by using a 1 ft container, lb/ft 3 •
With the aid of this information, the following can be calculated. (The formulae given below are based on obvious principles; the reader can easily prove their correctness.)
Volume of concrete in the test specimen:
v
= W /d g gSpecific gravity of green concrete in the test specimen and in the sampl,e: pi
=
w.
/62.. 4 V' g g (2.) (3) (4)Specific gravity of green portland cement paste: 1
+
w/c
o=
II
p+
wIc
c 0 (5)Volume fraction of cement paste in the concrete of the test specimen and in the sample concrete:
v
=
(p - p >/(p - p )a g a pg
v'
= (p - pi )/(P _ P )a g a pg
Specific gravity of sample in oven-dry condition:
Specific gravity of cement paste in oven-dry condition: P
pd
=
Pa - (pa - pl)/vldSpecific gravity of concrete in the test specimen:
p = vp
+
(I-v) Pd pd a
Oven-dry weight of the concrete of the test specitnen:
( 6)
(7)
(8)
(10)
(11) Volumetric moisture content of the concrete in the fire test specimen at ,the time of fire test:
w-w
. d
c:p = 100Pd W d
It may be added that, in general, the volumetric moisture content can be calculated from the moisture content (by weight) as:
=
apd
(12)
(13)
Obviously it has not been assumed in deriving these formulae, that the sample concrete and the specimen concrete are of the same
ｾ t 't:. .-,J • :,' "0" ," 1 , 0 . ' . . . . .ｾ '6nbｾ ｾ Jt ＧｩＺＭｾ ｾ i'."1 #"• • •/ " • ",jI I , ; ,I ,. "' I t ;/ rdfOrl ; ..., ' , ' 'r/ 1X(It· J.l" 1 : . , I ,I I ｾＮ I
w
£t' \ . • ' V,JIassumed, however, that the hydration of cement paste proceeds in
the sample at the same rate as in the test specimen. As the rate of
hydration reactions depends on the equilibrium relative humidity in the concrete (8), and in turn on the moisture content, one can
count on the approximately equal maturity of the cement paste in
the two concretes only if their moisture contents at the time of the
fire test (as estimated from their percentage decrease in weight during the conditioning period) are not too significantly different.
The calculation procedure may be better understood by studying the ,following sample calculation.
Example I. - For a wall fire test assembly built from dense
concrete, the following weights (of concrete only) have been measured:
W = _6760 lb WI = 29.505 lb g g W = 6666
l"p
W = 27.862 lb d d = 151.73 lb/ft3• gA standard compression test specimen has been used as a sample, for which
A water-cement ratio of
wd
c = 0.572 was measured. It is known,furthermore, that the values
p
=
3. 15 and p=
2ｾ 65c a
are applicable to most commonly met cases. CALCULATIONS V
=
6760/151.73 = 44.55 ft3Pg
= 151. 73/62.4 = 2.4316 pi = 29.505/62.4 x 0.19635 = 2.4082g
from Eq. (2) from Eq. (3)' from Eq. (4) 1+
O. 572=
1/3.15+
0.572=
1.7673 from Eq. (5). "ｾ \: ,J .. c·
r
"5 •\I ＨｲｾＮ '. ｾ . , - ｲｾIt
n, ')
I)ｾ [) of f\i "i :.-; •IｰＢＮｾ -I-: ttl S "
, \,) ｾ Ｈｾ
..
'....,dt '16rf; ｲＩｾＢｉｅＬＧＩｬ｢Ｌ､ Ｘｮｏｉｊｦｾｲ［ＬﾷＮﾷＺ
. ""j"l3 'Hf:! tlO V11rfgiJ'3 '(luI!
'J t.,
",i
o.t Pflri.J9? ll:,rdw ,""\ ..ｾＩ ｾ t :'1! ; F 'J IＨｾ <7j (f'"tq
r:,
Itiｾ .j•
v=
2.65 - 2.4316 2.65 - 1.7673=
0.2474 from Eq. (6) VI = 2.65 - 2.4082 6 6=
0.2739 2. 5 - 1. 7 73 from Eq. (7) Pd
= 27.862/62.4 x 0.19635=
2.2741 P pd = 2.65 - (2.65 - 2.2741)/0.2739 = 1.2776 P d r=
0.2474 x 1.2776+
(1-0.2474) x 2. ,65 ± 2.3105 W d = 62.4 x 44.5 x 2.3105=
6423lb 6666 - 6423 CD=
100 x 2.3105 6423 = 8. 74%byvol. from Eq. (8) from Eq. '(9) from Eq. (l0) from Eq. (11) from Eq. (12)Some additional calculations indicated that the end re suIt of these calculations depends only slightly on the selection of the value
of Pa • Selecting Pa
=
2.65, which seems to be a good average for avariety of aggregates, will thus probably suffice in most practical
cases. It was also found, however, that the end result depends very
significantly on the accuracy of the load cell measurements. With the
load cell used in the DBR laboratory (20, OOO-lb capacity) an accuracy
of ± 10 lb can be expected.
\
Calculation of the Effect of Moisture
The gain in fire endurance due to moisture (as referred to moistureless, i. e., oven-dry condition) can be determined with the
aid of the nomogram in Figure 2. This same nomogram can also be
used to correct the result of fire endurance tests, when the test was carried out at a non-standard moisture level, or to determine the value of fire endurance for any other moisture level of interest.
When the moisture is supposedly symmetrically distributed with regard to the central plane of the fire test specimen, the
pattern of the actual moisture distribution is immaterial, and should
be interpreted as the average volumetric moisture content of the test
specimen.
JC ":ｾｮＢｦ ＧｉＧｦｾＩＧＩ • • • ,r(rl I. • 1 " t • " II
•
" ' t ! : , -ri·-Ji ' '-t . , ' . ' ; ' f ' , 1i 1 •ｾ ｾ •f •• ,fJI \. . i i ii!""I . ｾ ｾ. . f • : (: .: •ｾII : , ') I ( ,Ii-ｾＬ 1 Ｇｩｩｬｬｾｾ｣ 'l,·;t .to I.J,)fl, i ＺｾＮＬ 'I II) ＧｩｽｾＺﾷ ,.ｾ( f ; r ' • Iｾ• " Ｇｾ Ｚｾ iIJ (; fｾ I! , ,.: :1 i!ＧＮｾＩ ", <j'fｾ [ ｾ Imoisture content, cP , the fire endurance of a test specimen was found to be 1"0.' whato.would be its fire endurance at an average
moisture contentrp S? To calculate 1"
13'
the fire endurance of thespecimen in moistureless (oven-dry) condition, 1"d' has first to be determined.
The calculation procedure is as follows. First calculate
bcp
0.
where b
=
5.5 for dense and gun-applied concretes,8.0 for lightweight concretes, 10.0 for cellular concretes.
Then in the nomogram in Figure 2 draw a line from point A to the
appropriate value of brOa. on the right-hand side scale. Draw another
line, parallel to this, from the point indicating the experimentally
obtained fire endurance value, 1"0.' on the left-hand side scale. A
horizontal line drawn from the point where the latter line intersected the curve of the nomogram, will yield the value of 1"d (the fire
endurance in moistureless condition) on the left-hand side scale. To find the fire endurance of the same test specimen, 1" , at some other moisture content CPS' calculate berS and connect this Svalue
on the right-hand side scale with point A. Draw a line parallel to
this from the point obtained previously on the curve of the nomogram. The intersection of this line with the left-hand side scale will yield 1" S.
If the moisture distribution in the test specimen is markedly
asymmetrical (this is generally due to the geometry of the construction, not to the conditioning procedure), calculate first the "equivalent
symmetrical" moisture content as
(14)
where M, the "moisture moment, " is referred to the exposed side
(where x
=
0) and is defined asJ.
M
=
f
.xcpdx. (15)o
This may also :be written as
n
M Rl l:
i=l
x.co. box.•
\ "
..
V If.,
, IlJI 1,-· " t " I I In .1I ｾ ., ,j , " JI iｾ lq« , I" I Ir.
) I"" Iｾ｜ I • n I , 1r; l}t • d I ,n.;,:,lth'w.'·J '"Ii ",';'!'"f.; i.'. \ t't •no.i :'q'J.0 Poｾｩ i.., ()j bｴｾ , >.;i,i
.' • [ I'; ｾＩ IiJ ) ') lif;If 'tｾＩＨ｝ 'pJ !:)rll ,:i' ;
) J;;-:'T'rInl'3 ,'>'rYi II［ｬｦｉ｛ｊＺＩＡｾ 'J 'TO ,.• 1' : f:) " 1 '
n
i{d J-o911'; '1:tl'lUUｾ G IIn these calculations a continuous air layer ( of any thickness) should be regarded as a I-in. -thick moistureless layer.
The moisture level of particular interest is that pertaining to
50 per cent equilibrium relative humidity. The moisture content of a
reasonably mature portland cement paste in equilibrium with an atmosphere of 50 per cent relative humidity at room temperature is
generally between 10 and 20 per cent by volume. Since, as mentioned
earlier, in a concrete the moisture -holding capacity of the aggregates' is usually very small, one can write that the volumetric moisture content of concretes in "standard" (air-dry) condition is
IOv ｾ cp ｾ 20v. (17)
The wideness of this range is due primarily to hysteresis effects, and only in a lesser degree to the characteristics of the
anhydrous cement used. If during the conditioning period the (ire test
specimen was always subjected to desorption, in other words, was continuously losing weight, the upper half of the above range should be used.
The procedure of "correcting" fire endurance test results for
"standard" moisture level is illustrated by the following two examples.
Example 2. - The wall fire test assembly described in Example
1 yielded Ta. = 3.33 hr fire endurance. What would be its fire endurance
at "standard" moisture level?
The moisture distributioninwall test specimens is always I
reasonably closely symmetrical. For heavy concrete b
=
5.5, thus,bcpa.
=
48.1. With, this and Ta.=
3.33 hr one obtains (see Figure 2) thatthe fire endurance in moistureless condition is Td
=
2.58 hr.For this concrete v = 0.2474 (volume per cent of cement
paste) was previously calculated. Assuming (by means of Eq. (17»
that in the air-dry condition the volumetric moisture content is l5v.
one gets cp
S
Rl 15xO. 2474=
3.71%
by vol. (A closer estimate couldbe obtained by calculating (see Ref. (7» or experimentally determining
the desorption curve of the material.) With this b.:p ,
=
20.4, andagain using Figure 2 one obtains T
S
= 2.90 hr,ｷｨｩｾｨ
is therefore theexpected performance of the wall in the "standard condition."
Example 3. - A steel-jointed floor construction, sketched in
'Figure 3a, yielded To.
=
2.6 hr (thermal) fire endurance. With the ,13
-has be en found in the concrete layer prior to the fire test. What
would be the fire endurance of this construction if tested in
air-dry condition?
The model of the construction, as simplified for these kinds
of calculations, is shown in Figure 3b. In this sketch the thickness
of the air layer has been reduced to 1 in. in accordance with what
was said in connection with Eq. (16). The moisture in the gypsum
plaster has been neglected.
With the notation used in Figure 3b the moisture moment can be calculated as follows:
M
=
Xl CPlt.x
1+
x 2CP2t.x
2+
ｸＳ｣ｰＳｾｸＳ+
x 4ｃｐＴｾｸＴ+
ｸＵｃｐＵｾＵ where Xl=
0.3125 in. CPl=
0t.x
1=
0.625 in. X=
0.8125 in. CP2=
0 ｾＲ=
1.0 in. 2 X=
2.125 in. CP3=
5.80t.x
3=
1.0 in. 3 x=
3.125 in. SP 4=
4.65t.x
4=
1.0 in. 4 x=
4. 125 in. CP5=
3.80t.x
5=
1.0 in. 5 Therefore M = 2.125 x 5.80 x 1.0+
3.125 x 4.65 x 1.0+
4.125 x 3.80 x . 2 .1.0
=
42.531 In.vol
0/0
It has been established by experiments that the moisture content of the concrete used in the construction in air-dry condition
is 3.0 per cent by volume. Thus the moisture moment pertaining to
air -dry conditions is (see Figure 3b):
2 one and the equivalent symmetrical moisture content is
2
2 x 42.531/4.625
=
3.98 % by vol.co
a.
=Since b = 5.5 again,. ber = 21.9. By making use of Figure
a.
.
obtains 1"d
=
2. 3 hr.M = (2.125
+
3.125+
4.125) x 3.0 x 1.0=
28.125 in. 2 vo1 %and the equivalent symmetrical moisture content in air-dry condition is:
cp
=
2 x 28.125/4.6252=
2.63% by vol.With this bcp = 14.5. Again using Figure 2 one obtains. for the
air -dry
｣ｯｮｾｲｵ｣ｴｩｯｮＬ
'T"=
2. 5 hr.S
NOTATION
I
Symbols denoted by the superscript refer to a sample.
a
=
b=
d=
t=
M=
n = v = V =wjc
=
W = x = t9' =moisture content.
%
by weightempirical value 3
bulk density. lb/ft
over-all thickness of fire test specimen (corrected when continuous air layer is present). in.
moisture moment, in. 2 vol.
0/0
number of strips (see Figure 3b)
volume fraction of portland cement paste volume of concrete, ft 3
water -cement ｲ｡ｴｩｾＬ lb/lb
weight of concrete; without subscript:
weight of concrete on day of fire test, lb. distance from surface exposed to fire, in. width of strip (see Figure 3b). in.
Greek Letters
p = specific gravity
'T"
=
fire endurance, hrcp = moisture content.
%
by volumecp
=
"equivalent symmetrical" moisture content.%
by volumeSubscripts a = c = d
=
g=
i=
pg=
pd=
a.
=
f3 =e
of aggregateof anhydrous portland cement
of dry concrete, in moistureless (oven-:-dry) condition of green concrete
1,2,3, ••.
of green portland cement paste of dry portland cement paste
in some
a.
condition (generally in the condition arisingduring fire test)
" 0' !I. I i i .-... "\I ' •':!1ｾＮ［ '}; .. ,I ; p ,)
..
'I' , J ' I 'I -,' r, lJ'H . '.ｾ .I, ｜ﾷＢｾｮｲｾﾷＮＩﾷﾷｌＩｩＧＡｩ ｾｴﾷｴｦＩｱ ｢ｱＡｪｾＮＮＮＬｩｬﾷｲＡﾷｾﾷｩ ｾ ｾ ｾＮ • {}ｾ • CI .:.: 'ｾ fJj . :.:ｾＧ ｾ(f °t"O!,f: I, \jI,I:IH .ｾｪ ,I' e i .fl,or I} 1'}'3 ＢＧ［Ａｉｊｬｾｾ［ＭＬＬＬＬｪ｜REFERENCES
1. Harmathy, T. Z. Effect of moisture on the fire endurance of
building elements. ASTM Spec. Tech. Publ. No. 385, 1965,
p. 74.
2. Harmathy, T. Z. Fire Technology, Experimental study on
moi sture and fire endurance.
..£,
52 (l966).3. Menzel, C. A. A method for determining the moisture condition
of hardened concrete in terms of relative humidity'.
p.l085-1109, Proc. ASTM,
22.,
1 (1955).4. Harmathy, T. Z. and E. O. Porteous. Sampling method for
measuring the moisture distribution in fire test specimens. Natl. Res. Council, Division of Building Research, Building Research Note 42, July 1963.
5. Copeland, L.E. and J. C. Hayes. Determination of
non-evaporable water in hardened portland-cement paste. ASTM
Bulletin, No. 194, Dec. 1953, p. 70-74.
6. Shorter, G. W. and J. A. C. Blanchard. Some factors influencing
the performance of concrete masonry unit walls in ASTM E119 fire tests (in preparation).
7. Harmathy, T. Z. Moisture sorption of building materials
(paper in preparation).
8. Copeland, L.E.
cement pastes. p. 34-39.
and R. H. Bragg. Self-desiccation in
- - - -- --- - - ' - - - . '
4
1
0
10
20
30
40
50
ｾ
60
....Q
70
80
90
100
--
-
--
---
---
--.---
"
...---
---
"""""'-
---:::::
0"'-
...
A
o'__ＮＮＺＺＺＺＭｾ｟Ｍ｟ＭＭＭＭＭＭＭＭＭＭＭＭＭＭＭＭＭＭ ...- _ ...----.-...
----
--
---...
---ｾ
--
----
--FIGURE 2NOMOGRAM FOR CALCULATION OF THE EFFECT OF MOISTURE
"\
