Self-healing of Micro-cracks in Engineered Cementitious Composites

Abstract

The performance of an Engineered Cementitious Composite (ECC) to self-heal micro-cracks under a controlled laboratory environment is presented. Ten dog-bone shaped samples were prepared; five of them were preloaded to known strains and then left to heal in water in a temperature-controlled laboratory. Ultrasonic pulse velocity (UPV) measurements were undertaken to monitor the crack-healing process. It was found that all samples exhibited recoveries in UPV and were able to recover to between 96.6% and 98% of their pre-test UPV values over a period of four weeks. An accelerated rate of healing was observed in the initial two-day period immediately following the preloading test.

Full text

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Civil Engineering Dimension, Vol. 17, No. 3, December 2015 (Special Edition), 187-194 CED 2015, 17(3), DOI: 10.9744/CED 17.3.187-194
ISSN 1410-9530 print / ISSN 1979-570X online
Self-healing of Micro-cracks in Engineered Cementitious
Composites
Suryanto, B.1*, Wilson, S.A.1, and McCarter, W.J.1
Abstract: The performance of an Engineered Cementitious Composite (ECC) to self-heal micro-cracks under
a controlled laboratory environment is presented. Ten dog-bone shaped samples were prepared; five of them
were preloaded to known strains and then left to heal in water in a temperature-controlled laboratory.
Ultrasonic pulse velocity (UPV) measurements were undertaken to monitor the crack-healing process. It was
found that all samples exhibited recoveries in UPV and were able to recover to between 96.6% and 98% of
their pre-test UPV values over a period of four weeks. An accelerated rate of healing was observed in the
initial two-day period immediately following the preloading test.
Keywords: Cementitious composite, ECC, self-healing, crack, ultrasonic pulse velocity.
Introduction
There is worldwide concern relating to the ever
increasing cost of repair and maintenance of concrete
structures, primarily as a result of premature dete-
rioration and cracking. In the United Kingdom (UK),
for example, approximately half of the £ 110bn
construction output in 2010 was spent on main-
tenance and refurbishment [1], with a quite a large
portion devoted to concrete structures. More recent-
ly, the Institution of Civil Engineers awarded UK
infrastructure a ‘C’ grade [2] and it was estimated
that £ 383bn in investment is required by 2020 to
upgrade the current infrastructure to an acceptable
standard [3]. The need for large investment is not
exclusive to the UK and is of primary concern in
many developed countries worldwide, including the
US and Japan [4,5]. This issue is partly due to the
brittle nature of concrete and partly caused by poor
construction quality and practice during the
construction boom after the Second World War.
During the service life of these structures, cracking is
therefore inevitable and when this occurs within the
cover-zone of concrete structures, the deterioration
processes are accelerated.
An illustration on how required maintenance over
the lifetime of a reinforced concrete structure im-
pacts on total cost is given in Figure 1. Line 1 repre-
sents the expected performance of a reinforced con-
crete structure built with regular concrete.
A structure is generally designed to deliver perfor-
mance well above the required performance, in order
to make sure that deterioration of the concrete and/
or reinforcing steel will not impair the serviceability
and integrity of the structure (i.e. the performance
level is above the threshold level).
1 School of Energy, Geoscience, Infrastructure and Society; Institute
for Infrastructure and Environment, Heriot-Watt University,
Edinburgh, UNITED KINGDOM.
*Corresponding author’s email: b.suryanto@hw.ac.uk
Figure 1. Schematic of the Potential Cost Benefits of Self-
healing Materials (adapted from [6])
Due to deterioration, however, the structure needs to
be repaired and maintained on a regular basis to
bring the overall performance back to an acceptable
level. The expenditure associated with this main-
tenance will add to the initial construction cost and
therefore increase the total cost. Line 2 shows the
performance of a material which is able to heal
cracks by itself. The lifetime of the structure prior to
any ‘major’ repair is likely to be extended when
compared to a similar structure built with regular
concrete thereby reducing maintenance costs. While
the initial cost of construction using this advanced
material is greater, this would be offset by far lower
required maintenance expenditure and hence such a
material would likely offer economic advantages in
the long term. The enhanced performance of the
material could allow further optimization to be done,
leading to a further cost reduction (Line 3).
Performance
Time
Performance threshold
1
Time
Total cost
Maintenance/repair
1
Inspection
Maintenance/repair
3

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This paper presents an investigation into the self-
healing performance of an advanced cement-based
material termed an Engineered Cementitious Com-
posite (ECC). An ECC is known for its high tensile
strain capacity, typically well excess 1%, and a
controllable crack width, typically less than 0.1 mm
under service load [7]. It is these unique properties of
ECC that enable micro-cracks in ECC to heal
without human intervention. Studies have shown
that the self-healing performance of ECC is robust
[8–13], with healing being found more effective in
crack widths of less than 50μm. These studies have
also shown that self-healing in ECC offers recovery
of mechanical properties such as strength and
stiffness. The authors have recently developed an
ECC mixture [14,15] and it is the interest of this
paper to assess the self-healing performance of this
mixture under a controlled laboratory environment.
It is well known that small crack width is a key
factor in encouraging self-healing in cement-based
systems [16,17]; therefore, this study will be useful
not only to confirm self-healing capability of the
mixture, but also to confirm whether or not the deve-
loped mixture can exhibit controlled crack width. A
parallel study was undertaken to assess the perfor-
mance of the mixture in the natural environment
and the results are presented in reference [15].
Experimental Program
The experimental program undertaken involved two
series of tests (see Table 1). In the first series, three
samples with dimensions of 50 × 50 × 250 mm (long)
were prepared (Figure 2). These prism samples were
simply cured in water for 28 days to investigate the
influence of cement hydration. In the second series,
ten dog-bone samples with dimensions to JSCE
recommendations [18] (Figure 2) were produced to
allow direct assessment of the self-healing perfor-
mance of ECC in a controlled laboratory environ-
ment. Of these ten samples, samples 1–3 were
deliberately loaded to failure to obtain the tensile
strain capacity; samples 4 and 5 were cured in water
and served as control samples; samples 6 and 7 were
precracked to 30% to the tensile strain capacity; and
samples 8–10 to 60% of this value. In addition, three
50 mm cubes were prepared to assess the compres-
sive strength.
Table 1. Summary of Experimental Programme
Shape
Sample No
Remarks
Prism
P1-P3
Used to monitor the influence
of hydration
Dog-bone
1–3
Tested to failure to obtain
stress-strain curves
4–5
Control samples
6–7
Precracked at 30% tensile
strain capacity
8–10
Precracked at 60% tensile
strain capacity
Figure 2. Dimensions of Dog-bone and Prism Test Samples
Materials
The binder comprised CEM I 52.5N cement to BS
EN197-1:2011 [19] and a fine fly-ash (Superpozz
SV80, ScotAsh). The fly-ash-to-cement (FA/C) ratio
was set constant at 1.8, while the water/binder (w/b)
ratio was 0.28 (see Table 2). A fine silica sand
(RH110, Minerals Marketing) with an average
particle size of 120m was used at a constant sand-
to-cement ratio of 0.6 by mass. Their oxide analyses
are presented in Table 3. A polycarboxylate high-
range water-reducing admixture (Glenium C315,
BASF) was added at a fixed dosage rate of 1% by
mass of cement. Polyvinyl alcohol (PVA) fibers
(RECS15, Kuraray) were used at a fixed dosage of
2% by volume. The PVA fibers had an average
length of 12 m, a diameter of 39m and a tensile
strength of 1600 MPa.
Table 2. Materials Mix Proportions
CEM I
FA
Silica
sand
w/b*
HRWR
PVA&
F28
(kg/m3)
(kg/m3)
(kg/m3)
(kg/m3)
(kg/m3)
(MPa)
454
818
273
0.28
4.54
26
33#
Notes: * binder includes cement and fly-ash.
& RECS15 from Kuraray.
# based on 50 mm cubes.
Sample preparation and curing
The mixing process was performed using a 10-litre
planetary motion mixer. A strict mixing regime was
incorporated to ensure that all ingredients, particu-
larly the PVA fibers, thoroughly distributed through-
hout the ECC mix. This process began with mixing
all dry components excluding the fibers along with
80% of the water at ‘low’ speed. After one minute, the
superplasticizer was introduced and after a further
two minutes, the remaining 20% of the water was
added and the mixer speed was increased to ‘high’.
One and a half minutes later, the PVA fibers were
added and the mixer was again switched to the ‘low’
speed and allowed to mix for one minute. The mixer
speed was increased to ‘high’ again and the compo-
nents were mixed for another four minutes to ensure
that the fibers were thoroughly mixed. The mixer
was finally switched to ‘low’ speed and mixing
continued for 30 seconds.

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Table 3. Oxide Analysis and Physical Properties of Fly-ash
and Silica Sand (wt%)
Fly-ash
Silica
sand
Chemical analysis
SiO2
52.7
98.8
Al2O3
26.6
0.21
Fe2O3
5.6
0.09
K2O
–
0.03
CaO
2.4
–
MgO
1.2
–
Na2O equivalent
1.7
–
SO4
0.3
–
Free CaO
0.03
–
Total phosphate
0.5
–
Loss on Ignition (LOI)
<2.0
0.14
Physical properties
Specific gravity
Surface area (m2/kg)
Fineness (% retained on 25 m)
2.20
1300
<25
2.65
–
–
Size distribution (m) and cumulative retained (%)
500
–
0.1
355
–
0.5
250
–
1.5
180
–
6.0
125
–
46.0
90
–
83.0
63
–
96.5
Upon completion of the mixing process, the ECC was
scooped and then poured into ten equally sized
Plexiglas molds (see Figure 3). Extra care was taken
when pouring the mixture to ensure even fiber
distribution, particularly at the center narrower
section of the molds of which the testing would be
focused. Once the samples were cast, they were
covered with polythene sheeting for 24 hours before
being removed from the molds and then placed in a
small curing tank for 13 days. The temperature at
which the curing took place was controlled to 20±1oC.
Figure 3. Plexiglass Mold used to Produce the Dog-bone
Samples
Testing and instrumentation
A 100 kN Instron 4206 machine was used to carry
out the tensile tests. Each sample was gripped at
either end by pneumatic grips and care was taken to
ensure that the sample was aligned so as to mini-
mize the eccentricity of the imposed tensile forces.
To measure the axial deformation at the central
section, two LVDTs were mounted at either side (see
Figure 4(a)) and tensile strains were then deter-
mined from the displacement changes monitored
from these two LVDT readings. Tensile stress was
calculated from the imposed tensile loading divided
by the cross sectional area of the slender section.
Bluehill-2 computer software was used to control the
testing apparatus and to enable continuous moni-
toring of the applied load and displacement. In
addition, a 16-bit data acquisition device was used to
record the output signals from the two LVDTs at a
rate of 1 Hz. Figure 4(b) displays the tensile stress-
strain responses obtained from samples 1–3. The
average tensile strain capacity was found to be 2.7%.
Accordingly, tensile strains of 0.8% and 1.6% were
determined as target initial strain values for samples
preloaded to 30% and 60% of the ultimate tensile
strain, respectively. These samples were then placed
in the same curing tank as before and left to heal.
(a)
(b)
Figure 4. (a) Dog-bone Sample during Uniaxial Tensile
Testing and (b) Tensile Stress-strain Responses of Samples
1–3
0
1
2
3
4
5
0 1 2 3 4
Sample 1
Sample 2
Sample 3
Tensile strain (%)
Tensile stress (MPa)

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190
To monitor the self-healing process in the ECC
samples, ultrasonic pulse velocity (UPV) measure-
ments were undertaken immediately prior to pre-
loading, immediately after preloading, and over four
weeks during the self-healing process. A PUNDIT
apparatus was used, together with two 54 kHz
transducers. This apparatus was calibrated prior to
taking any UPV measurements using a 26.0μs refe-
rence bar. Intimate contact with the specimen
surface was achieved using a viscous gel. Measure-
ments were undertaken by coupling the transducers
with opposite ends of each sample (see Figure 5) and
then by recording the indicated signal transit time.
Data Analysis
The UPV was calculated using Equation 1:
t
L
v
(1)
where v is the UPV (m/s), L is the distance between
the transducers (0.33 m) and t is the measured pulse
transit time in (s). The ratio of the UPV recorded in a
control sample before and after being submerged in
water (Rc) was computed using Equation 2:
c
wc
cv
v
R,

(2)
in which νc,w is the UPV of control samples during
the self-healing test and νc is the UPV of the control
samples before the test. This parameter gives an
indication of continued hydration. A similar relation-
ship (Equation 3) can be used for preloaded samples:
p
wp
pv
v
R,

(3)
where νp,w is the UPV of the preloaded samples
during the self-healing test and νp is the UPV of the
same samples prior to preloading. This parameter
provides an indication of the combined influence of
micro-cracks, continued hydration and crack-heal-
ing. The normalized ratio Rn can then be used to get
an indication of the extent of (Equation 4) crack-
healing:
c
p
nR
R
R
(4)
Figure 5. A Schematic Diagram of the UPV Tests
Results and Discussion
Prism Samples
Figure 6(a) presents the change in UPV with time
over a period of 28 days after casting. It is apparent
that there is a significant increase in UPV over the
initial three days of hydration. At 1-day hydration,
for example, the UPV attains an average value of
2828 m/s with a COV of 0.5% and at 3-days hydra-
tion, this value has been increased to 3342 m/s with
a COV of 0.1%. This rapid increase in UPV reflects
intense microstructural changes as a result of cement
hydration. After this, the hydration process has less
effect on bulk UPV and the UPV appears to plateau.
A slight increase in UPV can still be seen, with
values of 3449 m/s attained at 7 days, 3478 m/s at 14
days and 3497 m/s at 28 days. This slight increase in
UPV can be attributed to pore-structure refinement
resulting from on-going hydration and pozzolanic
activity.
It has been generally established that UPV can be
related to the physical properties of material such as
the dynamic elastic modulus and density. If the ECC
can be regarded as a homogeneous isotropic mate-
rial, the dynamic elastic modulus of the material can
be calculated using Equation 5 [20].
Figure 6. The Influence of Curing Time on (a) Bulk UPV
and (b) Dynamic Modulus of Elasticity of Samples P1-P3
Transmitter
Receiver
ECC sample
2500
2750
3000
3250
3500
3750
0 7 14 21 28
(a)
Time (days)
UPV (m/s)
10
12
14
16
18
20
22
0 7 14 21 28
(b)
Time (days)
Ed (GPa)

Suryanto, B. et al. / Self-healing of Micro-cracks in Engineered Cementitious Composites / CED, Vol. 17, No. 3, December 2015, pp. 187–194
191
  
 
2
1
211 vE
d
dd
d






(5)
where Ed is the dynamic modulus of elasticity,

d is
the dynamic Poisson’s ratio and

is the mass
density. Assuming a constant mass density of 1921
kg/m3, which was measured at 28 days after casting,
and a constant dynamic Poisson’s ratio of 0.23 [16],
the dynamic modulus of elasticity, Ed, can be
computed using Equation 5 and the results are
presented in Figure 6(b). It is apparent that
increasing hydration time has a notable influence on
the dynamic modulus of elasticity. Over the initial
three days of curing, the dynamic elastic modulus
increases rapidly from an average value of 13.3 GPa
at 1-day hydration to 18.5 GPa at 3-days hydration.
This value plateaus at approximately 20 GPa at 10
days.
Dog-bone Samples
The normalized UPV taken from three control
samples, Rc, is presented in Figure 7. As has been
observed in the prism samples, a very slight increas-
ing trend with exposure time can be observed (<1%),
which can be related to continual refinement of pore-
structure. To obtain the extent of initial damage and
self-healing, the UPV values for the preloaded sam-
ples were normalized, in accordance with Equation
3, and the results are presented in Figure 8. In
general terms, the response can be divided into three
distinct regions comprising: (i) a valley region at the
left-hand side of the curve; (ii) a transition region of
rapidly increasing normalized UPV values, resem-
bling the early hydration response; and, (iii) a
plateau on the right-hand side which extends over
the remaining duration of the test.
Figure 7. Normalized UPV Values for Samples 4 and 5
The depth of the valley can be associated with the
extent of damage following a loading event. It is
evident from Figure 8 that the normalized UPV
decreases as the test samples are subjected to a
greater initial strain, resulting in a downward dis-
placement of the valley. As can be seen, the normali-
zed UPV values drop to approximately 86% and 75%
of their normalized pre-test values as the samples
are damaged to 30% and 60% of the strain capacity,
respectively. The larger drop in the normalized UPV
values in samples tested to 60% of the tensile strain
capacity is attributed to the presence of a larger
number of micro-cracks which was also observed
visually.
The rapid increase in UPV over the initial few days
of exposure (see Figure 8), which resembles the early
hydration shown previously (Figure 6(a)), reflects an
intense self-healing activity. This pronounced heal-
ing activity may occur as a result of the hydration of
either unhydrated or partially hydrated cement
particles which were previously encapsulated by
hydration products and thus unreachable by mois-
ture. It is postulated that cracking can create
channels through which moisture can then access
these unhydrated/partially hydrated cement parti-
cles, allowing the hydration process to take place,
thereby allowing these particles to grow and even-
tually seal the micro-cracks. Visual inspection of the
sample surface after 14 days immersion in water
show the presence of white striations (see Figure 9),
indicating that the micro-cracks have been effec-
tively healed. A more detailed analysis of the
morphology and compositions of these healing pro-
ducts is currently being carried out.
Figure 8. Normalized UPV for samples precracked at 30%
and 60% ultimate tensile strain (

tu). The error bars on the
data points represent ±1 standard deviation from the mean
value and where the error bar appears to be missing, the
marker is larger than the error bar
Figure 9. White striations observed on the surface of
sample 10 after being immersed in water for 14 days. The
average distance between the striations (average crack
spacing), s, is ±3 mm.
0.98
0.99
1.00
1.01
1.02
0 7 14 21 28
Exposure time (days)
Rc
0.7
0.8
0.9
1.0
0 7 14 21 28
30%

tu
60%

tu
Exposure time (days)
Rn
1
s

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192
It is also apparent from Figure 8 that the initial self-
healing rate is largely dictated by the extent of initial
damage. In samples damaged to 30% and 60% of the
tensile strain capacity, for example, average reco-
veries of 9.8% and 15.9% can be observed and this
difference can be associated with the difference in
the number of micro-cracks. This can be explained by
the fact that in samples experiencing a greater initial
tensile strain, a larger number of micro-cracks deve-
lop which then provides a larger surface area, in the
form of crack surfaces. As the crack width remains
small, healing products can therefore precipitate
simultaneously on these crack surfaces, thereby lead-
ing to an apparent accelerated healing. This finding
confirms the ability of the developed mixture to
exhibit controlled crack width. Should the micro-
cracks have widened significantly, the rate of healing
would decrease, resulting in a much slower recovery
in UPV [15].
From the results presented in Figure 8, it is also
apparent that the normalized UPV in samples pre-
loaded to 30% tensile strain capacity plateaus after
three days immersion in water whereas in the other
series of samples, this occurs after 10 days. The
normalized UPV attain the values of 98% and 96.6%
of the pre-test values for samples tested to 30% and
60% strain capacity, respectively. Further investiga-
tion is required to confirm the recovery in strength
and stiffness.
Conclusions
The following conclusions can be drawn from the
work presented:
1. The newly developed ECC mixture incorporating
materials locally available in the UK exhibited
the desired tensile strain hardening behavior and
formation of micro-cracks of controlled width
under tensile loading. It has been demonstrated
that the material is proven capable of self-healing
micro-cracks under a controlled laboratory envi-
ronment.
2. From the test investigating the effect of continued
hydration on bulk UPV, it was found that the
PUNDIT apparatus was sensitive enough to
monitor the early hydration of ECC. At 1-day
hydration, the UPV attains an average value of
2828 m/s and further increases to 3342 m/s at 3-
days hydration. The UPV plateaus after appro-
ximately 10 days of curing at a value of
approximately 3500 m/s.
3. It was found that samples damaged to 30% and
60% of the tensile strain capacity were able to
recover to between 96.6% and 98% of their pre-
test UPV values over a period of four weeks.
4. All samples showed significant recovery in the
two-day period after the tensile test, with reco-
very being particularly profound in samples
damaged to a greater extent. The faster recovery
in samples damaged to a greater extent reflects
simultaneous healing of a greater number of
micro-cracks, indicating that the crack widths in
these samples are comparable to those in sam-
ples damaged to a lesser extent. This finding
supports the notion that the developed mixture
exhibits controlled crack width. However, it was
found that the more damaged samples did not
exhibit the same total recovery extent of those
damaged to a lesser extent.
Acknowledgements
The authors wish to acknowledge the support of
Kuraray Japan and Kuraray Europe GmbH for
providing the PVA fibers and BASF UK for donating
the admixture. Financial support from the School of
Energy, Geoscience, Infrastructure and Society,
Heriot-Watt University, is gratefully acknowledged.
Thanks also expressed to Ms. L. L. Teodoro for
assistance in the experimental work as part of the
Science without Borders Programme.
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