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Subcritical crack growth in low-porosity cement systems
Beaudoin, J. J.
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Journal of Materials Science Letters, 6, 2, pp. 197-199, 1987-02-01
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Natlonai Research
Council Canada
Conseil national
de recherches Canada
Institute for
Research in
Construction
lnstitut de
recherche en
construction
Subcritical Crack Growth in
Low-Porosity Cement Systems
by J.J. Beaudoin
Reprinted from
Journal of Materials Science Letters
Vol. 6, 1987, p. 197- 199
(IRC Paper No. 1449)
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ABSTRACT
Mechanisms of s u b c r i t i c a l c r a c k growth i n p o r t l a n d cement p a s t e
a r e d i s c u s s e d and e v i d e n c e i s p r e s e n t e d o f c h e m i c a l
m o d i f i c a t i o n of crack t i p s i n cement p a s t e t e s t e d i n a l c o h o l
media. Environmental e f f e c t s on c r a c k growth are d i f f e r e n t f o r
low-porosity p a s t e s .
L'auteur Ctudie l e s m6canismes i n t e r v e n a n t dans l a c r o i s s a n c e
s u b c r i t i q u e des f i s s u r e s dans l a p a t e d e ciment P o r t l a n d , e t il
p r g s e n t e des donnCes dgmontrant l a m o d i f i c a t i o n chimique d e s
extrSmitSs d e s f i s s u r e s dans l a p a t e de ciment test& e n m i l i e u
alcoolis6.
Les e f f e t s du m i l i e u ambiant s u r l a c r o i s s a n c e d e s
f i s s u r e s s o n t d i f f g r e n t s dans l e c a s des p a t e s 3 f a i b l e
porositC.
J O U R N A L O F MATERIALS SCIENCE LETTERS 6 (1987) 197-199
Subcritical crack growth in low-porosity cement systems
J . J . BEAUDOIN
Institute for Research in Construction, National Research Council, Ottawa, Canada
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Hydrated portland cement or portland cement paste is
a microporous, moisture-sensitive material that forms
the binder in conventional concrete. Numerous studies
have demonstrated the dependence of engineering
properties of concrete on microstructural characteristics including crack formation and growth in cement
paste [I]. The dependence of subcritical crack growth
in paste on humidity, temperature and test media has
been reported at higher waterlcement (w/c) ratio pastes
[2, 31. Evidence from crack growth studies in alcohol
media suggests that stress corrosion processes are
operative [4] at crack tips.
Low-porosity portland c h e n t paste (generally
prepared at waterlcement ratios of 0.25 or less) has
several characteristics that are different from those of
pastes having higher porosity. These include large
quantities of unhydrated cement grains, less Ca(OH),,
lower surface area, and different pore structure. This
letter reports the general effect of such differences
on subcritical crack growth in water, methanol and
decanol.
Log V-K, diagrams for low porosity (w/c = 0.25)
and higher porosity (w/c = 0.35) cement paste, along
with non-porous soda lime glass, are presented in
Fig. 1 (where V is the crack velocity and K, the stress
intensity factor). Apparatus and experimental techniques have been described elsewhere [2]. All paste
samples are dried at 110°C for 3 h and vacuum
saturated in the test fluid for a minimum of 48 h prior
to test. For the low-porosity paste (Fig. la) the curves
for tests in water, decanol and the dry state are close
to each other (0.42 < K, < 0.47MPamL12),crack
growth occurring at the lowest stress values in decanol.
In contrast, in high-porosity paste (Fig. lb) crack
growth occurs at much higher stress levels in decanol.
K, values for subcritical crack growth in the different
media are in the following order: decanol > dry >
methanol > water. The curves (Fig. lc) for soda lime
glass [5] are of the same order with respect to test
media as the curves in Fig. lb. The position of the
curves (K, axis) for methanol (low-porosity paste) and
decanol (high-porosity paste and glass) relative to
those in the dry state appears to be anomalous.
These results may be expained as follows: methanol
interacts with CH (cement chemistry notation is used;
C = CaO; H = H 2 0 ; S = SiO,) and C-S-H [6-81.
In reacting with methanol, the surface area increases
from 13.5 to 60 x lo3m2k g L .Methanol treatment of
synthetically prepared C-S-H can result in reduction
of surface area by a factor of 4 [7]. Methanol interacts
with C-S-H to a greater extent in paste with higher
porosity; for example, the N2 surface area of methanoltreated paste, w/c = 0.25, is similar to the control,
whereas at w/c = 0.50 the surface area is 30% less
than that of the control. Thus, changes in surface area
of the paste are controlled by. those of modified CH
and C-S-H.
It has been suggested that water attacks Si-0-Si
S T R E S S - I N T E N S I T Y F A C T O R . K, (1OMPo rn1I2)
0261-8028/87 $03.00
+ .12 0 1987 Chapman and Hall Ltd.
Figure 1 Log V-K, diagrams for cement
paste (w/c = (a) 0.25, (b) 0.35) and (c)
soda lime glass. Data for glass after
Wiederhorn et al. [S].
197
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Figure 2 Pore size distribution curves for cement paste (w/c = (a)
0.25, (b) 0.50) specimens treated w~thmethanol and decanol.
bonds in silicate structures [5]. The rate of interaction
in paste is controlled primarily by the chemical potential of the reactants (water and C-S-H) and the
permeability of the material.
In low-porosity pastes there is less CH owing to
lower degree of hydration, less methanol due to lower
pore volume, and a reduced amount of C-S-H. It is
possible, therefore, that the difference in the position
of the log V-K, curves (K,-axis) for methanol and
water media is greater at low porosity because there is
a reduced methanol-CH interaction. This argument
would apply if the product that forms when methanol
interacts with C-S-H were to facilitate crack growth,
e.g. in higher porosity paste where a greater amount of
the complex forms.
In low-porosity paste, however, CH-rich interfaces
are formed at the boundaries of unhydrated cement
particles because of the closeness of the products.
A small amount of reaction product from a CHmethanol interaction deposited at the interface might
actually inhibit crack growth.
Additional factors to consider are the total porosity
of the system and the effect of microstructural changes,
e.g. surface area changes in the cement paste. The total
porosity of methanol-treated paste samples is less for
the low-porosity system and more for the high-porosity
system than that of the control (see pore-size distributions in Fig. 2). How microstructural changes
directly affect crack growth is not known.
Decanol interacts with C-S-H in paste to a much
greater extent than CH [6, 71. Scanning electron
micrographs (Fig. 3) reveal that fracture surfaces of
decanol-treated C-S-H compacts (porosity similar to
paste with w/c = 0.35) and porous glass have much
rougher topography than do the controls. Cracks
propagate through a more tortuous path and higher K
values would be expected. This observation is in
agreement with the higher K, values required for
crack growth in decanol-treated paste and glass (Fig. l b
I
b
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b
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Agure 3 Scannlng electron micrographs: fracture surfaces of (a) C-S-H untreated, (b) C-S-H treated In decanol, (c) porous glass untreated,
(d) porous glass treated In decanol.
1 98
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Figure 4 Pore size distribution of porous glass: (-)
(---) treated in decanol.
untreated and
and c). The extent to which decanol treatment modifies
the microstructure of cement paste is reflected in poresize distributions (Fig. 2). At w/c = 0.25 the change is
large; the population of coarse pores is significantly
increased and that of fine pores decreased. Increase in
the volume concentration of coarse pores shifts the log
V-K, curves to lower values of K, [3]. This may be the
predominant reason for the low values of KI necessary
for crack growth in w/c = 0.25 paste (Fig. la).
At w/c = 0.50 the differences in the pore-size distribution of the decanol-treated sample and the control
are less than those for the w/c = 0.25 sample. The
increase in coarse pores is less and fine pores are not
eliminated. The log V-K, curve for w/c = 0.50 shifts
to higher K, values after decanol treatment. As the
pore-size distribution is less affected at higher w/c
ratios, this shift may be due to other changes. Micro-
structural changes to the paste include, for example,
small changes in surface area at w/c = 0.25 and large
decreases in surface area for w/c = 0.35 and 0.50.
Preparations of C-S-H treated with decanol also
have large decreases in surface area.
A pore-size distribution for decanol-treated porous
glass is given in Fig. 4. The histogram covers a very
narrow pore-size range, 1.50 to 3.80 x 10-3pm.
Major changes occur in the pore radius range, 2.25 to
2.70 x ~ O - ~ p m
decanol
;
treatment more than doubles
the pore volume in the range 2.35 to 2.50 x 10-3pm,
and in the finest pore range, 1.5 to 2.0 x 10-3pm,
decanol treatment also increases pore volume. Chemical modification of the glass itself (i.e. attack of
Si-0-Si bonds at crack tips) has been cited as a
reason why KI values for crack growth are higher in
glass treated with decanol than in dry glass [5]. There
is no direct evidence that pore structure change is
related to this type of chemical modification. No SEM
evidence of chemical modification of crack tip geometry
(e.g. blunting) was obtained for pastes at any w/c ratio
and porous glass. This toughening mechanism, if
operative, is probably not a predominant factor.
It is concluded that chemical modification of crack
tips can occur in cement paste tested in alcohol media
and that media effects on crack growth are different
for low-porosity pastes (w/c = 0.25).
References
1. V . S. R A M A C H A N D R A N , R. F. F E L D M A N and J. J.
B E A U D O I N , "Concrete Science" (Heyden, 1981) p. 398.
2. J . J . B E A U D O I N , Cem. Concr. Res. 15 (1985) 871.
3. Idem, ibid. 15 (1985) 988.
4. Idem, Proceedings International Conference on Fracture
Mechanics of Concrete, Lausanne, October, 1985 (Elsevier).
5.
S.
M . WIEDERHORN,
D. W. F R I E M A N ,
E. R.
F U L L E R J r and C J . SIMONS, J. Muter. Sci. 17 (1982)
3460.
6. J . J B E A U D O I N , Materials and Struclures
submitted.
7. Idem, I1 Cemenlo (1985), submitted.
8. R . L. D A Y , Cem. Conr. Res. 11 (1981) 341.
Received 16 July
and accepted 19 August 1986
(1985),
T h i s paper i s being d i s t r i b u t e d i n r e p r i n t
form by t h e I n s t i t u t e f o r R e s e a r c h i n
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