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Self-Healing Capability of Fibre Reinforced Cementitious Composites

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In order to investigate the self-healing capability of fibre reinforced cementitious composites (FRCC), mechanical properties and surface morphology of crack in FRCC were studied. Three types of FRCC specimens containing (1) polyethylene (PE) fibre, (2) steel cord (SC) fibre, and (3) hybrid fibres composite (both of PE and SC) were prepared. These specimens, in which cracks were introduced by tension test, were retained in water for 28 days. The self-healing capability of the specimens was investigated by means of microscope observation, water permeability test, tension test and backscattered electron image analysis. It was found that many very fine fibres of PE were bridging over the crack and crystallization products became easy to be attached to a large number of PE fibres. As a result, water permeability coef-ficient decreased and tensile strength was improved significantly. Therefore amount of the PE fibre per volume was indicated to have a great influence on self-healing. Furthermore, by means of backscattered electron image analysis, it was also shown that the difference of hydration degree in each FRCC has only little influence on the self-healing capa-bility in case of the employed test series.
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Journal of Advanced Concrete Technology Vol. 7, No. 2, 217-228, J une 2009 / Copyright © 2009 Japan Concrete Institute 217
Scientific paper
Self-Healing Capability of Fibre Reinforced Cementitious Composites
Daisuke Homma1, Hirozo Mihashi2 and Tomoya Nishiwaki3
Received 23 February 2009, acc epted 8 May 2009
Abstract
In order to investigate the self-healing capability of fibre reinforced cementitious composites (FRCC), mechanical
properties and surface morphology of crack in FRCC were studied. Three types of FRCC specimens containing (1)
polyethylene (PE) fibre, (2) steel cord (SC) fibre, and (3) hybrid fibres composite (both of PE and SC) were prepared.
These specimens, in which cracks were introduced by tension test, were retained in water for 28 days. The self-healing
capability of the specimens was investigated by means of microscope observation, water permeability test, tension test
and backscattered electron image analysis. It was found that many very fine fibres of PE were bridging over the crack and
crystallization products became easy to be attached to a large number of PE fibres. As a result, water permeability coef-
ficient decreased and tensile strength was improved significantly. Therefore amount of the PE fibre per volume was
indicated to have a great influence on self-healing. Furthermore, by means of backscattered electron image analysis, it
was also shown that the difference of hydration degree in each FRCC has only little influence on the self-healing capa-
bility in case of the employed test series.
1. Introduction
Cracking is inherent in reinforced concrete (RC) struc-
tures and leads to serious damage in service period.
Cracking is caused by various sources such as drying
shrinkage, external force and freezing and thawing. No
matter what the reason, the crack in RC structures allows
the early access of aggressive agents (like chloride ions
and CO2), and as a result the reinforcement’s corrosion
occurs at an early age. This corrosion brings not only the
decrease of strength, but also the expansion of the rein-
forcement that enlarges the crack width and causes
spalling of the cover concrete. So we have to avoid this
cracking in RC structures as much as possible. One of the
most popular and widely used technique to address this
problem is the use of short fibres. The randomly distrib-
uted fibres bridge over cracks and are able to decrease the
crack width and to block aggressive agent. Research
results and design code indicate that about 0.1 mm or
smaller width crack is relatively safe due to self-healing
autogenously caused by immersing moisture (e.g. Ed-
vardsen 1999). In this self-healing phenomenon, FRCC
that enables to keep the crack width smaller can be
greatly effective.
Since considerable improvement in the post-cracking
behaviour of concrete containing fibres is achieved,
FRCC has been widely used since the middle of 20th
century. Even recently fundamental studies about the
mechanisms of toughening which may contribute to
further development of advanced FRCC were presented
(for example, Fantilli et al. 2007; Dick-Nielsen et al.
2007; Yang et al. 2008). In the meanwhile, some works
of practical application of FRCC for developing high
performance structural members were presented, too.
Some studies on ductile high strength concrete columns
toughened by containing steel fibre were reported
(Sharma et al. 2007; Sugano et al. 2007; Kimura et al.
2007). Ultra ductile fibre reinforced cementitious com-
posites (so called strain hardening cementitious com-
posite: SHCC) have been also developed (Li 2003;
Kanda and Li 2006) and a simplified inverse method for
determining the tensile properties of SHCC that is es-
sential for quality control in practice was proposed
(Qian and Li 2007; Qian and Li 2008).
Self-healing, in another word autogenous healing, of
concrete and reinforced concrete is a phenomenon that
has been often studied by many researchers (for exam-
ple, Ramm and Biscoping 1998; Li et al. 1998; Edvard-
sen 1999; Reinhardt and Jooss 2003; Yang et al. 2005;
Heide and Schlangen 2007; Granger et al. 2007).
Many experimental results and practical experiences
have demonstrated that cracks in concrete have the abil-
ity to heal themselves and water flow through cracks
was reduced with time. The following reasons for the
self-healing have been cited (Ramm and Biscoping
1998): 1) further reaction of the unhydrated cement; 2)
expansion of the concrete in the crack flanks; 3) crystal-
lization of calcium carbonate; 4) closing of the cracks
by solid matter in the water; 5) closing of the cracks by
spalling-off loose concrete particles resulting from the
cracking. Among these reasons, however, it was clari-
fied that crystallization of calcium carbonate within the
crack was the main mechanism for self-healing of ma-
tured concrete (Edvardsen 1999). It was also reported
1Graduate student, Dept. of Architecture and Building
Science, Tohoku University, Japan.
2Professor, Dept. of Architecture and Building Science,
Tohoku University, Japan.
E
-mail: mihashi@timos.str.archi.tohoku.ac.jp
3Associate Professor, Faculty of Education, Art and
Science, Yamagata University, Japan.
218 D. Homma, H. Mihashi and T. Nishiwaki / Journal of Advanced Concrete Technology Vol. 7, No. 2, 217-228, 2009
that the growth rate of calcium carbonate within the
crack was dependent on crack width and water pressure,
but that it was independent of concrete composition
(type of cement and aggregate) and type of water (i.e.
hardness of water). On the other hand, Granger et al.
(2007) carried out an experimental program of me-
chanical test on ultra high performance concrete and
concluded that the self-healing of the pre-existing crack
was mainly due to hydration of anhydrous clinker on the
crack surface and that the stiffness of newly formed
crystals is close to that of primary C-S-H. While there
are many studies of self-healing of concrete, there are
quite few of such studies on FRCC. Li et al. (1998) car-
ried out experimental studies on self-healing capability
of SHCC and concluded that self-healing of the induced
micro cracks was observed not only by formation of a
distinct deposit inside the cracks but also by almost
complete recovery of the pre-cracking stiffness. Homma
et al. (2008a, 2008b) reported the summary of a series
of experimental study on the self-healing capability of
FRCC.
In this paper, the experimental study of the FRCC’s
self-healing capability is reported. The capability is
evaluated by water permeability test, microscope meas-
urement, tension test and backscattered electron image.
2. Self-healing
Self-healing is the natural process of crack repair that can
occur in concrete in the presence of moisture. The depo-
sition of calcium carbonate is said to be generated ac-
cording to the following reactions (Edvardsen 1999).
-2
3
-
33222 CO2HHCOHCOHCOOH +++ ++
(1)
8)(pHCaCOCOCa water3
-2
3
2+
+
(2)
8)pH(7.5HCaCOHCOCa water3
-
3
2<<++ ++
(3)
The carbon dioxide CO2 in the air is dissolved in water
and the calcium ion Ca2+ is derived from concrete.
These CO2 and Ca2+ are combined with each other to
produce calcium carbonate crystals. Then the calcium
carbonate crystallization made in this way is attached to
crack surface and fibers. As a result, crack width is re-
duced and after a certain time the crack is repaired.
3. Experimental procedure
In this study, the self-healing capability is evaluated by
the microscope observation, water permeability test,
tension test and backscattered electron image analysis.
Tab l e 1 shows the employed materials and Tab le 2
shows the mix proportions together with the fresh prop-
erty. In this study three types of FRCC were studied: (1)
containing micro polyethylene fibre (φ=12µm,
length=6mm) (FRCC(PE)), (2) containing steel cord
fibre (φ=0.4mm, length=32mm) (FRCC(SC)), and (3)
containing both of PE and SC fibres (i.e. hybrid fibre
composite: HFRCC).
In all series, four specimens of 25×75×75[mm] were
prepared. After standard curing for one week, cracks
were introduced in each specimen by means of a
uni-axial tension test. Figure 1 shows the outline of the
tension test. During this test, each specimen was
stretched to different strain levels in order to have dif-
ferent maximum crack width. In Fig. 2, the relations
between elongation and stress under uni-axial tension are
shown. The nonlinearity is corresponding to the accu-
mulation of multiple cracking. After the tension test, the
Table 1 Employed materials in this study.
Cement High-early-strength
Portland cement ρ = 3.14 g/cc
Aggregate Silica sand (no.7) ρ = 2.61 g/cc
Diameter: 180μm
Mixing
Materials Silica fume ρ = 2.2 g/cc
Diameter: 0.15μm
Viscous
Agent
Water-soluble
cellulose ρ = 1.1 g/cc
Polyethylene (PE) Mentioned below
Fiber Steel cord (SC) Mentioned below
Screw Bar Steel screw bar Nominal diameter:
6mm (M6)
PE fiber (length=6mm) SC fiber (length=32mm)
Types of
Fiber
Length
(mm)
Diameter
(mm)
Modulus
(GPa)
Tensile
strength
(MPa)
Density
(g/cc)
PE fiber 6 0.012 73 2580 0.97
SC fiber 32 0.4 200 2850 7.84
Table 2 Mix proportion of FRCC specimens investigated
in this paper.
Types of
Mix
Water/
Binder
Sand/
Binder
Silica
fume/
Binder
SP/
Binder
PE fiber
(Vol. %)
SC fiber
(Vol. %)
fiber
content
(piece/m3)
FRCC(SC) - - 0.75 187×104
FRCC(PE) 1.5 - 221×108
HFRCC
0.45 0.45 0.15
0.09
0.75 0.75 111×108
Note: Binder = Cement + Silica fume; SP = S uperplasticizer (Polycarboxylat e)
Types of Mix Table flow
(cm×cm) Air content (%)
FRCC(SC) 16.3×16.0 0.90
FRCC(PE) 13.6 ×13.8 5.13
HFRCC 14.7×14.7 3.36
D. Homma, H. Mihashi a nd T. Nishiwaki / Journal of Advanced Concrete Technology Vol. 7, No. 2, 217-228, 2009 219
crack surface was observed by means of a digital mi-
croscope and the crack width was measured as shown in
Tab l e 3 . These values were obtained on the inlet side of
permeability test specimens, while microscopic obser-
vation to measure the thickness of crystallization prod-
ucts was carried out on both sides of each specimen. As
shown in 4.2, the permeability coefficient is a function of
w3. Therefore only one crack of the maximum width on
the inlet side among the multiple cracks is dominant for
the water permeability. After the microscope observation,
the specimens were tested for water permeability. The
schematic description of the water permeability test
Screw bar (M6)
Fixing nut
Fig. 1 Test set-up of uni-axial tension test.
Pi
p
ette
Distilled Wate
r
Plate specimen
Plexi
g
las
Screw bar
Rubber tube
Fig. 3 Schematic description of water permeability test.
Table 3 Maximum crack width evaluated by means of
microscope observation in the permeability test
specimens. (unit : mm)
No. of specimens
No. 1 No. 2 No. 3 No.4
FRCC(SC) 0.035 0.076 0.088 0.757
FRCC(PE) 0.019 0.038 0.119 0.368
Typ e
of
mixes HFRCC 0.017 0.081 0.407 0.71
0
0.5
1
1.5
2
2.5
3
3.5
4
00.511.522.53
Stress MP a
Elongationmm
MORTAR1
MORTAR2
MORTAR
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.5 1 1.5 2 2.5 3
Stress MP a
Elongationmm
FRCC(SC)1
FRCC(SC)2
FRCC(SC)3
FRCC(SC)4
FRCC(SC)
0
0.5
1
1.5
2
2.5
3
3.5
4
00.511.522.53
Stress MPa
Elongationmm
FRCC(PE)1
FRCC(PE)2
FRCC(PE)3
FRCC(PE)4
FRCC(PE)
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.5 1 1.5 2 2.5 3
StressMPa
Elongation mm
HFRCC1
HFRCC2
HFRCC3
HFRCC4
HFRCC
Fig. 2 Retation between elongation and stress under
uni-axial tension.
220 D. Homma, H. Mihashi and T. Nishiwaki / Journal of Advanced Concrete Technology Vol. 7, No. 2, 217-228, 2009
(Nishiwaki and Mihashi 2003) is shown in Fig. 3. The
water permeability coefficient was evaluated by the wa-
ter flow speed through the plate specimen. After the
water permeability test, all specimens were kept for 28
days in water tank of 20ºC which is usually used for
standard curing. In order to investigate the effect of the
self-healing of crack, water permeability tests and mi-
croscopic measurement were conducted again at 3, 14
and 28 days. After 28 days, the uni-axial tension test was
performed again, too.
4. Experimental Results and Discussion
4.1 Microscopic observation
The formation of crystallization products (Fig. 4) that
might be the source of self-healing was monitored just
after the tension test and at the end of 3, 14, and 28 days
after water permeability test. Additionally the thickness
of the crystallization products on the crack surface was
also measured by means of a digital microscope. Tab l e 4
shows the typical crack surface photographs in FRCC
(PE), FRCC (SC), and HFRCC at the elapsed time of 0
day (immediately after tension test and before immersion
in water), at 3 days, 14 days and 28 days.
The reason why some values of the maximum crack
width are different from ones shown in Tab l e 3 is that not
only the inlet side for water permeability test but both
sides of each specimen were observed by the digital
microscope for measuring the crack width and the thick-
ness of crystallization products. It can be clearly seen in
Tab l e 4 that the crack surface of all specimens immedi-
ately after tension test is clear and there are no chemical
products on the surface. At 3 days, however, in the crack
surfaces of FRCC(PE) and HFRCC, many crystallization
products were confirmed to be attached and this attach-
ment is done not only to crack surface but also to many
PE fibres. Furthermore in FRCC(PE), even though crack
width reached 100μm, crack was confirmed to be re-
paired by the crystallization products attached to many
fibres bridging the crack. However in the place of little
fibre bridging, especially in almost all FRCC(SC), the
deposition of crystallization products was not seen. Even
if there were fibres bridging the crack, in the case when
crack width was too wide and PE fibres were dropped out,
little attachments of the crystallization products could be
confirmed. After 3 days, in FRCC(PE) and HFRCC, the
thickness of crystallization products attached to the crack
surface and PE fibres increased as the time advanced.
However, in FRCC(SC), the crack surface remained to be
clear. Therefore it can be concluded that amount of the
PE fibres per volume has a great influence on the
self-healing.
After 3 days, it was also observed that steel cords on
the crack surface in FRCC(SC) and HFRCC corroded
and the volume expanded.
Figure 5 shows the time dependence of mean thick-
ness of the crystallization products attached to the crack
surface (summation of products thickness on both sides
of the crack surface). The increasing rate of the mean
thickness during the first 3 days is higher than that after 3
days. This could be because Ca2+ diffusion speed from
the inside of FRCC was reduced as the time advanced
since the formation of the crystallization product layer
may prevent the diffusion (Edvardsen 1999). Order of the
amount of the attached products in each FRCC material
was FRCC(SC) < HFRCC < FRCC (PE). This order is
corresponding to that of the piece number of mixed fibres
per unit volume shown in Ta b l e 2 (i.e. FRCC(SC) <
HFRCC < FRCC (PE)). Therefore it can be concluded
that the piece number of mixed fibre per unit volume has
the dominant influence on the thickness of the crystalli-
zation products attached to the crack surface.
Figure 6 shows the relationship between the maxi-
mum crack width and the mean attachment thickness at
28 days. In each type of mix, totally 8 data of the maxi-
mum crack width measured from both sides of each
specimen are plotted. In this figure, when the slope of
regression line equals to 1, the attachment thickness is
thought to reach the crack width and it can be said that
the crack was repaired. According to the results shown in
this figure, when crack width is less than 100μm, the
Fig. 4 Example of crystallization products.
0
20
40
60
80
100
120
0 5 10 15 20 25 30
Mean thi ckness of cristalli zation
productsμm
Time [days]
FRCC(SC)
FRCC(PE)
HFRCC
Fig. 5 Time dependence of mean thickness of crystalli-
zation products attached to the crack surface.
D. Homma, H. Mihashi a nd T. Nishiwaki / Journal of Advanced Concrete Technology Vol. 7, No. 2, 217-228, 2009 221
slope was close to 1 and crack was repaired easily in each
series. However, when crack width is wider than 100μm,
the slopes of the regression lines are much lower than 1.
Among these three regression lines shown in Fig. 6(b),
the slope of FRCC(PE) is highest. It confirms that
FRCC(PE) is repaired most easily.
4.2 Water permeability test
The water permeability coefficient was calculated as
follows. Since the water flow in this study is assumed to
be continuous through the laminar, Darcy’s law can be
applied. Darcy’s law states:
l
h
kAQ = (1)
where Q = the flow rate through the specimen (dV/dt); V
= the total volume of water that travels through the
sample; k = the water permeability coefficient and the
parameter under this study, l = the thickness of the
specimen, and A = the surface area of the plate specimen.
Then, because the flow is continuous, the amount of
Table 4 Microscopic observation of crystallization products at a crack surface.
Elapsed time (day)
0* 3 14 28
*
I
m
m
e
d
i
t
e
l
y
f
t
e
r
t
e
n
s
i
o
n
t
e
s
t
b
e
f
o
r
e
i
m
m
e
r
s
i
o
n
i
n
w
t
e
r
.
FRCC (SC)
Steelcord
fiber speci-
men No. 2
(crack width:
1.976mm)
FRCC (PE)
HFRCC
Polyethylene
fiber speci-
men No. 4
(crack width:
0.368mm)
Steel-PE
hybrid fiber
specimen
No. 1 (crack
width:
1.532mm)
Corrosion of steel cord
Formation of healing products
Formation of healing products
Corrosion of steel cord
FRCC(SC)
y = 0.0287x
FRCC(PE)
y = 0.3673x
HFRCC
y = 0.1037x
y = x
0
100
200
300
400
500
600
0 1000 2000 3000 4000
Attachment thic kness [μm]
Crack width [μm]
y = x
0
50
100
150
200
250
300
0 200 400 600 800
Attachment thic kness [μm]
Crack width [μm]
FRCC(SC)
FRCC(PE)
HFRCC
(a) (b)
(a) (b)
Fig. 6 Relationship between the maximum crack width and the mean thickness of crystallization products observed on
each specimens surface .
222 D. Homma, H. Mihashi and T. Nishiwaki / Journal of Advanced Concrete Technology Vol. 7, No. 2, 217-228, 2009
water flowing out of the pipe is given by Equation (2):
= t
h
A
dt
dV
d
d
'
(2)
where dh/dt= the differential of pressure between the
initial pressure head h0 and the remaining pressure head
h1 at the measured time t; A’ = the cross-sectional area of
the pipe.
By combining Equations (1) and (2), Equation (3) is
obtained.
t
hA
A
l
h
k
d
d'
=
(3)
By integrating Equation (3), Equation (4) is obtained.
=
1
0
d'
d
0
h
h
t
h
h
A
A
ltk
(4)
The water permeability coefficient is finally given by
Equation (5).
1
0
ln
'
h
h
At
A
lk =
(5)
By the use of this coefficient of water permeability, the
self-healing capability of each FRCC was evaluated.
Figure 7 shows the relationship between the coeffi-
cient of water permeability and residual elongation just
after the tension test. The coefficient of water perme-
ability tends to increase as residual elongation increases,
but this tendency is quite variable and dependent on the
type of fibre. This could be because the residual elonga-
tion doesn’t indicate the maximum crack width but re-
flects the summation of crack widths and fibre reinforced
cementitious composites make multiple cracking gener-
ated.
In the meanwhile, Fig. 8 shows the relationship be-
tween the coefficient of water permeability and the
maximum crack width just after the tension test without
any self-healing. In addition to four data obtained from
four specimens of each type of mix, some data of
HFRCC obtained in a preliminary test are also plotted. In
the figure, regression curves based on the following
equation are also shown, which was obtained according
to a parallel board theory of fluid dynamics (Tsukamoto
and Woener 1991).
3
12 w
v
gaI
kA
=
(6)
where wcrack width [mm]; aflow rate coefficient
indicating smoothness of crack surface ( 10
<a) to
estimate the influence of the crack geometry ; I : pressure
gradient (h/d) ; h : height of fluid column on the inlet
side ; d : length of crack in flow direction ; v : kinematic
viscosity [m2/s]; g : gravity acceleration. l : length of
crack at a right angle to the flow direction [m].
From Equation (6), the coefficient of water perme-
ability is proportional to cube of crack width: w and the
coefficient a. The result of Fig. 8 is corresponding to this
theory basically. This tendency, however, also has vari-
ability in the small range of the maximum crack width
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
00.511.52
Coeff icient of Water Permeability
m/s
Residual elongation mm
FRCC(SC)
FRCC(PE)
HFRCC
Fig. 7 Relationship between the coefficient of water
permeability and residual elongation.
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
00.20.40.60.8
Coefficient of Water Permeability
m/s
The maximum crack width mm
FRCC(SC)
FRCC(PE)
HFRCC
FRCC(SC) HFRCC FRCC(PE)
Fig. 8 Relationship between the coefficient of water
permeability and the maximum crack width.
y = 0.0059x
-1.209
0
25
50
75
100
125
0 0.00025 0.0005 0.00075 0.001 0.00125
Mean at tachment wid t h [μm]
FRCC(SC)
FRCC(PE)
HFRCC
coefficient of smoothness a
Fig. 9 Relationship between the coefficient of smooth-
ness and mean attachment thickness.
Coefficient of water permeability Coefficient of water permeability
D. Homma, H. Mihashi a nd T. Nishiwaki / Journal of Advanced Concrete Technology Vol. 7, No. 2, 217-228, 2009 223
where crack width is less than 0.1mm and it may mean
that the simple parallel board theory can’t be applied in
the range where elongation is small. This is because fibre
reinforced cementitious composites generically make
multiple crack in the range of small elongation.
Figure 9 shows the relationship between the coeffi-
cient indicating the smoothness of crack surface and
attachment thickness at 28 days. From this figure, it was
confirmed that as the surface crack becomes smoother,
the thickness of the attachment becomes thinner. This
could be because attaching of crystallization products on
crack surface becomes difficult due to no or little PE
fibres remained on the crack surface. Therefore the
smoothness of crack surface is thought to have strong
influence on the self-healing.
Figure 10 shows the result of the water permeability
test of the cracked and uncracked specimens. As can be
seen in this figure, the coefficient of water permeability
for all specimens decreased until 3 days except the un-
cracked specimens. However, after 3 days the decreasing
rate of the coefficient of water permeability slowed down
significantly in all specimens. This corresponds to the
microscopic observation results shown in Fig. 5 in which
the crystallization products attaching speed is faster until
3 days than after 3 days. It can also be noticed that, in the
specimens with a wider crack, the coefficient of water
permeability was almost constant after 3 days. It means
that the crystallization products are the source of the
self-healing mechanism but that the mechanism doesn’t
work well in wider cracks. On the other hand, in case of
specimens with a smaller crack width, the coefficient of
water permeability decreased as time advances even after
3 days and in some cases reached to the same values to
those obtained in uncracked specimens. This clearly
indicates that, in such a small crack width, the crystalli-
zation products have a great effect on self-healing of
crack. Figure 11 shows the influence of time dependence
of water permeability coefficient due to type of FRCC.
The decrease of the water permeability coefficient in
FRCC(PE) and HFRCC is more than that in FRCC(SC).
This can be because the self-healing products could at-
tach to the crack surface more in FRCC(PE) and HFRCC
than in FRCC(SC).
4.3 Raman spectroscopy analysis
In order to examine the chemical composition of the
crystallization products that is the source of self-healing
of crack, FRCC(PE) specimens were analysed by Raman
spectroscopy. Figure 12 shows the Raman spectroscopy
of FRCC (PE) samples. In this figure, upper, middle, and
lower profiles were taken from uncracked area, cracked
area, and pure calcium carbonate crystals, respectively. It
can be seen from this figure that the peaks of cracked
area coincide with those of pure calcium carbonate crys-
tal. On the other hand, in the case of uncracked area, such
a peak couldn’t be observed. Therefore the formation of
crystallization products, that is, the formation of
self-healing products in FRCC was confirmed to be cal-
cium carbonate crystals.
4.4 Uni-axial tension test of self-healed speci-
mens
In order to evaluate the effect of self-healing on the ten-
sile properties, the self-healed FRCC specimens were
tested by uni-axial tension test again. Figure 13 shows
the typical relations between tensile stress and elongation
before and after the self-healing. Figure 14 shows the
schematic description of general tensile stress-elongation
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
0 5 10 15 20 25 30
Coefficient of Water Permeability
m/s
Tim e day
Uncracked
No.1 0.017mm
No.2 0.081mm
No.3 0.407mm
No.4 0.710mm
HFRCC
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
0 5 10 15 20 25 30
Coefficient of Water Permeability
m/s
Tim e day
Uncracked
No.1 0.035mm
No.2 0.076mm
No.3 0.088mm
No.4 0.757mm
FRCC(SC)
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
0 5 10 15 20 25 30
Coeffi cient of Water Permeability
m/s
Ti m e day
Uncra cked
No.1 0.019mm
No.2 0.038mm
No.3 0.119mm
No.4 0.368mm
FRCC(PE)
Fig. 10 Time dependence of water permeability coefficient in each FRCC.
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
0 5 10 15 20 25 30
Coefficient of Water Permeability
m/s
Ti m e day
FRCC(SC)3 w=0.088
FRCC(PE)3 w=0.119
HFRCC2 w=0.081
Fig. 11 Influence of time dependence of water perme-
ability coefficient due to type of FRCC.
Coefficient of water permeability
Coefficient of water permeability
Coefficient of water permeability
Coefficient of water permeability
224 D. Homma, H. Mihashi and T. Nishiwaki / Journal of Advanced Concrete Technology Vol. 7, No. 2, 217-228, 2009
response of FRCC. In case of FRCC(PE), specimens
showed a tendency of quasi-brittle fracture because
the length of the used polyethylene fibres was short.
After the first crack occurred, tensile stress soon de-
creased accompanying the strain softening behaviour.
Multiple cracking was observed in the range of small
strain, though a dominant crack was localized as the
strain increased. In case of HFRCC, the length of steel
cord had the concavity and convexity to create the high
bond strength while the short polyethylene fibres made
the matrix tough enough to prevent the spalling of matrix
when the steel cord was pulled out from the crack surface
(Mihashi and Kohno 2007). As a result, HFRCC showed
a very ductile stress versus elongation relation. Multiple
cracking was observed but finally a dominant crack was
localized as observed in FRCC (PE). FRCC (SC)
specimens which contained only steel cord showed lower
strength and a little less ductility than those of HFRCC.
This is because FRCC (SC) didn’t contain any polyeth-
ylene fibres. In case of FRCC (SC), some specimens
showed multiple cracking in a small strain range but the
others didn’t. While there was a scatter in the cracking
behaviour in the small strain range, a dominant crack was
easily localized in any cases.
In order to compare the strength recovery due to the
self-healing of each FRCC specimen, the strength re-
covery rate (c) was defined as follows:
100
01
02 ×
=
σσ
σ
σ
c
(6)
where σ0 = the stress at the unloading in the first tension
test, σ1 = the tensile strength in the first tension test, and
σ2 = the tensile strength after the self-healing.
By use of this c, it can be possible to judge the recov-
ering capability of the strength in each FRCC as follows.
500 1000 1500 2000
Raman shift / cm-1
0
10000
20000
30000
40000
50000
Counts
50000
40000
30000
20000
10000
0
[Counts]
[Raman shift/ cm
-1
]
500 200015001000
Sample taken from uncracked
portion a fter self-healing
Sample tak en from cracked
portion a fter self-healing
Pure Calcium
Carbonate sample
CaCO
3
CaCO
3
CaCO
3
CaCO
3
CaCO
3
CaCO
3
CaCO
3
CaCO
3
Fig. 12 Raman spectroscopy of FRCC (PE) specimens.
0
0.5
1
1.5
2
2.5
3
3.5
4
0123456
Tensile stress [MPa]
Elongation [mm]
Before Self-healing
After Self-healing
0
0.5
1
1.5
2
2.5
3
3.5
4
0123456
Tensile stress [MPa]
Elongation [mm ]
Before Self-healing
After Self-healing
0
0.5
1
1.5
2
2.5
3
3.5
4
0123456
Tensile stress [MPa]
Elongation [mm]
Before Self-healing
Af ter Self -healing
(a) FRCC(SC) (b) FRCC(PE) (c) HFRCC
Fig. 13 Comparison of tensile property of specimens before and after 28 days of self-healing.
D. Homma, H. Mihashi a nd T. Nishiwaki / Journal of Advanced Concrete Technology Vol. 7, No. 2, 217-228, 2009 225
If c is over 100, this specimen can be considered to be
recovered perfectly. If c is over 0 and under 100, this one
was recovered a little. If c is equal to 0, this one was not
recovered. If c is under 0, this one was deteriorated.
Figure 15 shows the relationship between the strength
recovery rate c and residual elongation in each FRCC
material. It can be seen in Fig. 15 that after 28 days the
recovery rate of the FRCC (SC) was almost zero or even
minus. This could be because the self-healing products
were not attached to the crack surface and steel cords
were corroded during 28 days.
In the case of FRCC (PE), the recovery rate c is plotted
over 0 and under 100. The tensile strength after
self-healing didn’t reach the first tensile strength, but
could reach the first unloading stress. This could be be-
cause a lot of calcium carbonate crystals attached to
many fibres. This indicated that, if the calcium carbonate
crystals attach to the crack surface, the tensile strength
after self-healing can be recoverable. Moreover it can
also be seen in Fig. 15 that the strength recovery rate
becomes higher as residual elongation is smaller.
In the case of HFRCC, the recovery rate was higher
than 100 when the residual elongation was less than even
2mm. It means that the tensile strength after self-healing
could reach not only the first unloading stress, but also
the first tensile strength. This could be because, as well as
FRCC (PE), a lot of calcium carbonate crystals attached
to crack surface with very fine fibres. Moreover the bond
property of the steel cord damaged by pull out stress
might be recovered by the self-healing products.
Figure 16 shows a comparison of crack surfaces after
the second tension test. By observing the crack surface, it
was confirmed that corrosion of steel cord was observed
not only on the surface of specimens with crack but also
on the surface of inside of the crack in case of FRCC
(SC). In case of HFRCC, however, the corroded steel
fibre was observed only on the surface of the specimen
(i.e. the external surface of the crack) but it was not ob-
served on the internal surface of the crack. This could be
because in HFRCC a lot of calcium carbonate crystals
not only covered the surface of the crack but also at-
tached to the matrix around steel cord fibres. It corre-
sponds to the increase of tensile strength of HFRCC after
the self-healing.
4.5 The measurement of hydration degree
In order to make sure whether this difference of tensile
strength healing is due only to the crystallization of
calcium carbonate or the difference of the hydration
degree of each FRCC has any effects on the difference of
tensile strength healing, the hydration degree of each
FRCC was measured. The measurement of hydration
degree was done by mean of the backscattered electron
image (Igarashi et al. 2004) as shown in Fig. 17. The
backscattered electron image was converted to a binary
image on each divided constituent phase and the volume
fraction of the constituent phase was measured. By use of
this volume fraction, the hydration degree of FRCC was
calculated as follows.
First tension test (FRCC)
Tensile St ress [MPa]
Elongation [mm]
Second tension test (self-healing FRCC)
1
σ
2
σ
0
σ
Residual
Elongation
First tension test
Aft er s elf-healing
Fig. 14 Schematic of the relationship between tensile
stress and tensile elongation of FRCC.
recovery
Perfect
recovery
Deterioration
-50
0
50
100
150
200
250
300
350
00.511.522.5
Recovery rate c[%]
Residual elongation [mm ]
FRCC(SC)
FRCC(PE)
HFRCC
Fig. 15 Relationship between recovery rate and residual
elongation.
Corrosion
FRCC (PE)
FRCC (SC) HFRCC
Corrosion
Fig. 16 Comparison of crack surface after second tension test.
Tensile stress [MPa]
226 D. Homma, H. Mihashi and T. Nishiwaki / Journal of Advanced Concrete Technology Vol. 7, No. 2, 217-228, 2009
0
1
UH
UHi
=
α
(7)
where UHi is area fraction of unhydrated cement particle
at the age of ti; UH0 is the initial area fraction of unhy-
drated cement particles.
In Fig. 18, volume fractions of constituent phases and
the hydration degrees in each FRCC specimen are shown.
The hydration degree of each FRCC shows about 65%
and there was no change in the hydration degree of each
FRCC. Therefore it could be shown that the influence of
the hydration degree difference of each FRCC on the
tensile strength healing was little. It means that the
dominant source of the self-healing is the crystallization
of calcium carbonate.
5. Conclusions
This paper presented some results of an experimental
study on the self-healing capability of the fibre rein-
forced cementitious composites. Based on the experi-
mental study, the following conclusions were obtained:
1) Self-healing of cracks in FRCC materials was con-
firmed.
2) In the case of FRCC containing a lot of very fine
fibres such as polyethylene, the formation of
self-healing products becomes easy to be attached
to the crack surface due to the tight space between
fibres.
3) The self-healing products were confirmed to be
calcium carbonate crystals by means of raman
spectroscopy analysis.
Duotone
Backscattered Electron Image
Pores
Others
Pores, Sand
Others
Others
Unhydreted
cement
Sand
Unhydrated
cement
Pore
Binarized Image
Fig. 17 Backscattered electron image of FRCC(PE).
34.9
12.8
0.0
69.2
65.1
18.0
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 day 7 day
FRCC(SC)
α = 63.2
空隙+硅砂
反応生成物
未水和結合材
34.7
12.0
0.0
70.2
65.3
17.7
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 day 7 day
FRCC(PE)
α = 65.3
空隙+硅砂
反応生成物
未水和結合材
34.7
12.0
0.0
69.9
65.3
18.1
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 day 7 day
HFRCC
α = 65.4
Pores+Sand
Hyd ratio n
products
Unhydrated
cement
Volume f ractio ns o f co nstituent phases
Fig. 18 Volume fractions of constituent phases in each FRCC specimen.
D. Homma, H. Mihashi a nd T. Nishiwaki / Journal of Advanced Concrete Technology Vol. 7, No. 2, 217-228, 2009 227
4) The self-healing by calcium carbonate crystals
leads to reducing water permeability and recovery
of tensile strength.
5) By combining polyethylene fibre and steel cord in a
FRCC (i.e. HFRCC), the tensile strength after the
self-healing was recovered most significantly.
Acknowledgement
The authors would like to express their thanks for the
financial support by Grant-in-Aid for Scientific Re-
search of Japan Society for the Promotion of Science
(Project No. 18206058).
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Microbially induced calcium carbonate precipitation (MICP) technology has attracted widespread research attention owing to its application in crack healing for cement-based materials in an intelligent and environmentally friendly manner. However, the high internal alkalinity, low nutrient content, and dense structure of cement-based materials have restricted its application in self-healing cement-based materials. Various carrier materials have been widely used for the immobilization of microorganisms in recent years. Carrier materials have significantly increased the ability of microorganisms to withstand extreme conditions (high temperature, high alkali, etc.) and have provided new ideas for the compatibility of microorganisms with cement-based materials. In this study, the basic principles of microbial self-healing technology in cement-based materials and microbial immobilization methods and the influencing factors are introduced, followed by a review of the research progress and application effects of different types of carrier materials, such as aggregate, low-alkali cementitious materials, organic materials, and microcapsules. Finally, the current problems and promising development directions of microbial carrier materials are summarized to provide useful references for the future development of microbial carriers and self-healing cement-based materials.
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Crack formation is inevitable on the surface of the cement-based material, considerably leading to shortening service life. A sustainable self-healing technique based on microbial induced calcite precipitate (MICP) is recognized as a promising solution to increase durability and prevent structural failure. This research investigated the potential of bleaching earth immobilized bacteria incorporated normal and glass fiber mortars to enhance autogenous crack-healing ability. The viability of immobilized bacteria in the cement slurry and the influence of self-healing agents on the mechanical strength of mortar were examined first. The crack healing efficiency was evaluated by comparing the recovery of watertightness, the crack width and area healing ratio. Experimental results showed that the permeability coefficient of the specimens with immobilized bacteria (77.4%–93.3%) decreased more than in those with free bacteria (10.1%–12.6%). The maximum crack width healed and fracture area repair ratio were 0.44 mm and 96%, respectively. The crack healing compounds was calcite crystals with dense accumulation according to X-ray diffraction and scanning electron microscopy. This crack healing method from inside to outside has important guiding significance for the sustainable development of cement-based materials.
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The use of nanomaterials in cementitious mixtures has received growing interest in the last few decades owing to their ability to enhance a wide range of material properties, such as an increase in mechanical performance and the improvement in the self-healing capacity. The latter feature is highly important in order to increase a construction’s durability and service life as a cementitious material under tensile stress may exhibit cracking, endangering the durability once substances enter the crack and penetrate into the inner layers. Various mechanisms that contribute to the self-healing ability of cementitious materials are discussed, in addition to the influence of various nano-additions to obtain crack sealing or water tightness and/or to promote healing. These additives are able to contribute to crack healing in different ways. While reactive nano-particles are responsible for the production of materials inside the crack, inert nano-fillers act as nucleation sites for healing products to form. Finally, some future perspectives concerning the use of nanomaterials in self-healing cementitious composites are summarized. The use of nanomaterials is a promising path to obtain cementitious materials with the enhanced capabilities to either heal damage or at least promote a healing mechanism, in combination with other improved material characteristics.
Chapter
Cementitious composite materials are the most used material in the realization of transport infrastructures. In order to increase the service life of these structures, it is necessary to design cementitious composite materials with special properties, such as the self-healing capacity, which allows the occurrence of a self-closing phenomenon after cracking. This paper shows the performance recorded for two cementitious composites with self-healing capacity induced by the use of an integral waterproofing admixture by mass crystallization. The experimental results obtained indicate a good self-healing capacity, quantified by a degree of healing of at least 57% after 192 h of conditioning, respectively, total closure of cracks after 336 h of conditioning. This work contributes to the increase of knowledge in the field of cementitious materials with self-healing capacity, indicating a possibility of obtaining this effect through the use of a waterproofing admixture by mass crystallization, simultaneously with the presentation of the possibilities of use of industrial waste such as fly ash and limestone slurry.
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As an emerging advanced construction material, strain hardening cementitious composite (SHCC) has seen increasing field applications in recent years. Reliable data on tensile properties, including tensile strength and tensile strain capac- ity, are needed for structural design and for quality control. However, existing uniaxial tensile tests are relatively com- plicated and sometime difficult to implement, particularly for quality control purpose in the field. A simple inverse method based on beam bending test was presented by the authors (Qian and Li, 2007) for indirect determination of ten- sile strain capacity, aimed at quality control of SHCC in field applications. This paper extends this method to also de- termining the tensile strength based on beam bending test data. This proposed method (UM method) has been validated with uniaxial tensile test results with reasonable agreement. In addition, this proposed method is also compared with the Japan Concrete Institute (JCI) method. Comparable accuracy is found, yet the present method is characterized by much simpler experiment setup requirement and data interpretation procedure. Therefore, it is expected that this proposed method can greatly simplify the quality control of SHCCs both in execution and interpretation phases, contributing to the wider acceptance of this type of new material in field applications.
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The well-known practical phenomenon of autogenous healing in cracks plays a significant role in relation to the functional reliability of structures subjected to water-pressure loads. Due to the autogenous healing, the water flow through the cracks gradually reduces with time, and in extreme cases, the cracks seal completely. In the past, there has been no deliberate technical exploitation of self-healing, because too little is known about the phenomenon itself and about the chemical/physical processes involved. Based on theoretical and experimental research, the effect of crack healing was investigated on a larger scale for the first time. The experimental studies showed the formation of calcite in the crack to be almost the sole cause for the autogenous healing. A comprehensive theoretical discussion of the laws, which govern the calcite nucleation and the subsequent crystal growth of water-bearing cracks in concrete, revealed that the crystal growth responds to two different crystal growth processes that are determined by the changes in the chemical and physical conditions in the crack. Further, the crystal growth rate is dependent on the crack width and water pressure, whereas concrete composition and water hardness have no influence on autogenous healing. On the basis of the experimental studies, an algorithm that can be used to estimate the reduction in water over time as a result of autogenous healing was developed.
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The nonlinear behavior of fractured quasi-brittle materials is conventionally modeled with a fictitious crack model, which relates stresses on the crack surfaces to the corresponding crack widths. Its definition for fiber reinforced concrete is only possible by introducing a cohesive model for the matrix, and by modeling the pullout of randomly oriented fibers. To this aim, a new cohesive interface model, able to predict effectively the pullout response of inclined fiber, is presented in this paper. Based on the nonlinear behavior of steel fibers and cementitious matrixes, the proposed approach also takes into account the bond-slip relationship between the materials. By means of an iterative procedure, numerical results similar,to experimental data can be obtained. In particular, maximum pullout forces at given inclination angles, as well as the complete pullout load vs. displacement diagrams, can be correctly predicted. Moreover, according to test results, the proposed approach shows, from the first pullout stage, the dependence of the response both on crushing of cementitious matrix and on yield strength of steel fibers.
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Cyclic loading tests were carried out on ultra-high strength RC columns under high axial load conditions. Effective concrete strength of 200 MPa with high-strength steel fibers (SF) was used. The investigated parameters were: 1) volumetric ratio of steel fiber, 2) lateral reinforcement ratio, and 3) axial loading type. The main characteristics of the tested columns (maximum strength, deformation capacity and equivalent viscous damping factor) are presented and the influence of various parameters is discussed. The test results revealed the advantage of using SF in terms of strength and damage control. The maximum strength of the tested columns using SF, assessed using a number of different formulas, proved to be on the safe side. On the other hand, in the case of columns in which SF was not used, the estimated maximum strength was found to be larger than the tested value, except when using NZS3101 equation. This difference is probably due to early spalling off of the cover concrete.
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A theoretical study was undertaken to propose the procedure and guidelines for the design of transverse steel confinement for the potential plastic hinge regions of plain (non-fibre) and steel fibre reinforced high strength concrete columns. To achieve this, confinement reinforcement for a few selected square and circular concrete column cross sections were designed for varying concrete compressive strengths (70-100 Mpa) and axial load levels (0.15 - 0.5). Theoretical moment curvature relations were found and curvature ductility factors were computed for all the columns. An attempt has also been made to evaluate the relevant confinement requirements of the Indian Standard Code IS-13920: 1993. The results indicate that the design equations of the Code are inadequate to provide satisfactory ductility for high strength concrete columns especially at high axial load levels. However, it has been shown that the use of steel fibre reinforced concrete results in a significant improvement in the structural response of such column cross sections. A new set of refined equations have been proposed in the present study to design the confining steel of non-fibre and steel fibre reinforced high strength concrete columns, which take into account the effects of high concrete strengths and axial load levels also. The confinement reinforcement design has been made performance based by relating the required quantity of reinforcement with the ductility demand.
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Tests have been carried out on a high-strength concrete establishing permeability and self-healing behaviour of cracked concrete as a function of temperature between 20 and 80 °C and crack width between 0.05 and 0.20 mm. The results show a considerable increase of water transport with temperature. Theoretical prediction on the basis of thermodynamics showed reasonable to good agreement between theory and experiment.