Corrosion Process and Mechanisms of Corrosion-
Induced Cracks in Reinforced Concrete identified
by AE Analysis
M. Ohtsu*, K. Mori† and Y. Kawasaki*
*Graduate School of Science and Technology, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan
†Research & Development Center, Taiheiyo Cement Corp. 2-4-2 Osaku, Sakura 285-8655, Japan
ABSTRACT: Concrete structures could suffer from the corrosion of reinforcing steel bars (rebars)
because of the penetration of chloride ions. For crack detection and damage evaluation in concrete,
acoustic emission (AE) techniques have been extensively applied to concrete and concrete struc-
tures. In the corrosion process of reinforced concrete, it is demonstrated that continuous AE
monitoring is available to identify the onset of corrosion and the nucleation of concrete cracking
because of the expansion of corrosion products. At the latter stage, the expansion of corrosion
products generates corrosion-induced cracks in concrete. The generating mechanisms of these
cracks are studied in accelerated corrosion tests of reinforced concrete beams. Kinematics of
microcracks are identified by SiGMA (Simplified Green’s functions for Moment tensor Analysis)
analysis of AE. It is demonstrated that AE activity at the onset of corrosion and at the nucleation of
corrosion-induced cracks is in remarkable agreement with the phenomenological model of the
corrosion process in steel. Then, mechanisms of corrosion-induced cracks are visually and quanti-
tatively investigated by the SiGMA analysis.
KEY WORDS: acoustic emission, corrosion of rebar, reinforced concrete, SiGMA analysis
Introduction
Corrosion of reinforcing steel bars (rebars) is known
to be one of critical deteriorations in reinforced
concrete structures. When chloride concentration at
the level of rebar in concrete exceeds a range of
values with the probability for onset of corrosion, a
passive layer on the surface of rebar is destroyed and
corrosion is initiated. Then, electrochemical reac-
tion continues with available oxygen and water.
According to a phenomenological model of rein-
forcement corrosion in marine environments [1], a
typical corrosion loss during the corrosion process is
illustrated as shown in Figure 1 [2]. At phase 1, the
onset of corrosion is initiated. As the rate of the
corrosion process is controlled by the rate of trans-
port of oxygen and water from the surface of rebar
and the corrosion products build up on the cor-
roding surface, the flow of oxygen is eventually
inhibited, and thus the rate of the corrosion loss
decreases at phase 2. The corrosion process proceeds
further corrosion loss at phases 3 and 4 because of
anaerobic corrosion. The corrosion penetrates inside
the steel and the growth of corrosion products
occurs. The phenomenological model of steel pre-
sents a two-step process of the onset of corrosion
and the growth of corrosion products.
Acoustic emission (AE) techniques have been
extensively studied in concrete engineering for
approximately five decades [3]. They are applied to
practical applications [4] and are going to be stan-
dardised. This is because the increase in ageing
structures and disastrous damages caused by the
recent earthquakes urgently demand for mainte-
nance and retrofit of reinforced concrete structures
in service. It results in the need for the development
of advanced and effective inspection techniques.
Thus, AE techniques draw a great attention to
diagnostic applications in concrete. Studies on fun-
damentals of AE activity and the effects of mixture
proportion were conducted [5–7]. A frequency
analysis and a source location analysis were also
reported [8–11]. In due course, applications to
reinforced concrete structures were investigated
[12]. These studies have resulted in practical appli-
cations to monitor microcracks in concrete struc-
tures and going to be made practical as diagnostic
applications [13].
� 2010 Blackwell Publishing Ltd j Strain (2011) 47 (Suppl. 2), 179–186 179
doi: 10.1111/j.1475-1305.2010.00754.x
An International Journal for Experimental Mechanics
An application of the moment tensor analysis to
AE waves was first reported on cracking mechanisms
of glass because of the indentation [14], where only
diagonal components of the tensor were assumed.
This was because they dealt with only tensile cracks.
From the definition, however, it is realised that the
presence of all components is not associated with the
type of a crack, but related with the directions of the
coordinate system. Although the crack orientations
are often assumed as parallel to the coordinate sys-
tem, they are generally inclined to the coordinate
system mostly because of the configuration of the
specimen. As a result, the presence of all the com-
ponents is consequent in AE waveform analysis. Both
tensile motion of diagonal components and shear
motion of off-diagonal are definitely present in crack
motions as an AE source. Consequently, general
treatment on the moment tensor components of
diagonal and off-diagonal components has been
developed as SiGMA (Simplified Green’s functions for
Moment tensor Analysis) software [15, 16]. The pro-
cedure is already applied to clarify fracture mecha-
nisms in concrete members [17, 18].
By applying AE techniques, recently it has been
reported that concrete cracking arising out of rebar
corrosion is effectively detected [19], [20]. AE detec-
tion because of the corrosion of rebar is illustrated in
Figure 2 [21]. Recently, it is demonstrated that high
AE activities are observed twice during the corrosion
process [2]. As seen in Figure 3, a curve of total AE
hits (counts) is in remarkable agreement with the
curve shown in Figure 1. In the case of reinforced
concrete, AE activity at phase 1 reasonably corre-
sponds to the onset of corrosion in reinforcement.
During phases 3 and 4, not only the growth of cor-
rosion products, but also corrosion-induced cracks in
concrete could be generated because of the expan-
sion of corrosion products in reinforced concrete. In
the figure, these periods are compared with the
chloride concentration at rebar, where two threshold
values of 1.2 and 0.3 kg per 1 m3 concrete are de-
noted. The latter is equivalent to the lower-bound
value for nucleation of corrosion in the Japanese
standard [22] and is very low compared with the
threshold values assigned for corrosion initiation in
many reports. But, right after the chloride concen-
tration becomes higher than the lower-bound, 1st
high AE activity is observed which corresponds to
phase 1 and the onset of corrosion. At the stage over
the upper-bound value of chloride concentration,
2nd high AE activity is observed as phases 3 and 4. It
is easily realised that AE hits arising out of concrete
cracking is detected at the stage, because the expan-
sion of corrosion products could occur. By applying
the two-domain boundary-element method (BEM),
extension of the corrosion-induced crack in an arbi-
trary direction was analysed [23]. Here, experiments
were conducted by simulating corrosion-induced
cracks in expansion tests. With respect to the orien-
tations of crack extension, results of the BEM analysis
were compared with those of SiGMA, introducing the
normalised stress intensity factors. It is demonstrated
that extension of the corrosion-induced crack is
governed by the mode-I failure in the meso- and
macroscale.
Based on these findings, here the SiGMA analysis is
applied to an accelerated corrosion test of a rein-
forced concrete beam, and thus kinematics of AE
sources in the corrosion process are clarified and
discussed.
Figure 2: AE generation because of the corrosion of rebar
Figure 3: Total number of AE hits and chloride concentration
per 1 m)3 concrete [4]
Figure 1: Typical corrosion loss for steel in sea water immer-
sion [1]
180 � 2010 Blackwell Publishing Ltd j Strain (2011) 47 (Suppl. 2), 179–186
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Corrosion Process and Mechanisms of Corrosion-Induced Cracks : M. Ohtsu et al.
Experiments
A reinforced concrete specimen tested was of
dimensions 100 mm · 100 mm · 400 mm. One de-
formed steel-bar (rebar) of 13-mm nominal diameter
was embedded with 45-mm cover-thickness. Config-
uration of the specimen is illustrated in Figure 4. In
total, six specimens were prepared and tested. These
were applied to estimate chloride concentration in
concrete and measure half-cell potentials during
accelerated tests. For chloride concentration, core
samples of 30-mm diameter were taken, and sliced
and crashed.
Mixture proportion of concrete was that water:
cement: sand: gravel = 0.55, 1.0, 2.15: 3.58 by
weight. The maximum size of gravel was 20 mm, and
the slump value (6 cm) and air content (5%) were
controlled by using air-entrained admixture. The
compressive strength at 28-day standard curing was
35.4 MPa. Right after the standard curing for 28 days,
all surfaces of the specimen were coated by epoxy,
except the bottom surface.
An accelerated corrosion test conducted is shown
in Figure 5. The specimen was placed on a copper
plate at the bottom of a container filled with 3% NaCl
solution. Between the copper plate and the rebar,
40 mA electric current was constantly charged. The
current density chosen corresponds to a current
density of 245 lA cm)2 charged into rebar, which is
very common for accelerated tests. Previously, the
accelerated corrosion tests of reinforced concrete
were conducted by applying wet–dry cycles and by
applying the electric charge [21]. It is realised that
corrosion on rebar because of the electric charge is
different from that of the wet–dry test. The former
generated corrosion cracking inside rebar, because of
the electric current and heat inside. In contrast, only
the surface of rebar was corroded because of the wet
and dry cycles. Consequently, a fairly low current
density was selected, although the accelerated test
was adopted to encounter two high AE activities
earlier.
Acoustic emission measurement was continuously
conducted, by using AE analyser (DiSP, PAC). Six AE
sensors (R15, PAC) of 150 kHz resonance were at-
tached to the surface of one specimen as shown in
Figure 5. The choice of six sensors resulted from the
fact that the region for the analysis is as small as
targeted area was of dimensions 100 mm · 100 mm ·
100 mm. Frequency range of the measurement was
10 kHz–2 MHz, and total amplification was 60 dB
gain. For AE counting, the dead-time was set to 2 ms.
and the threshold level was 40 dB gain. The SiGMA
analysis was applied to the one specimen, because AE
activity during the corrosion process was already
known. To monitor just the activity, other specimens
were monitored by employing one-channel AE sys-
tem.
AE Analysis
Parameter analysis
Acoustic emission activity was analysed by AE hits
and AE event. Here, AE hit is the term to indicate that
a given AE channel has detected and processed one
AE transient signal. Counting methods of AE signals
are ringdown-counting by setting the threshold. By
employing a multichannel system, AE wave can be
detected in the form of hits on one or more channels.
One event is a group of AE hits received from a single
source by two or more channels, of which spatial
coordinates could be located.
Characteristics of AE signals were estimated by
using two indices of RA value and average frequency
[24]. These are defined from such waveform param-
eters as rise time, maximum amplitude, counts and
duration shown in Figure 6. AE sources of active
cracks are classified, based on the relationship be-
tween these indices.
Two indices are defined as follows:
RA ¼ Rise time=Maximum amplitude; (1)
Average frequency ¼ Counts=Duration: (2)
It is reported that the relationship between two
indices is effective to classify cracks into tensile cracks
and shear cracks [24]. Here, when the RA value is
small and the average frequency is high, AE source is
Figure 4: Sketch of reinforced concrete specimen tested
Figure 5: Experimental set-up of accelerated corrosion test
� 2010 Blackwell Publishing Ltd j Strain (2011) 47 (Suppl. 2), 179–186 181
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M. Ohtsu et al. : Corrosion Process and Mechanisms of Corrosion-Induced Cracks
classified as a tensile crack. In the other case, AE
source is referred to as other cracks than a tensile
crack. This criterion is applied to classify AE events
detected in the corrosion process.
To evaluate the size distribution of AE sources, the
amplitude distribution of AE events is applicable. A
relationship between the number of AE events, N,
and the maximum amplitudes, A, is statistically rep-
resented as
Log10N ¼ a� bLog10A; (3)
where a and b are empirical constants. The latter is
called the improved b value (Ib value), which is pro-
posed on the basis of cumulative distribution [25].
For 100 hits, the Ib value is defined, assuming aver-
aged amplitude l and standard deviation r,
Ib ¼ ½log10Nðl� rÞ � log10Nðlþ r�=2r; (4)
where N(l ) r) and N(l + r) represent the number of
hits with the amplitudes higher than l ) r and l + r,
respectively. In the case that the Ib values are large,
small AE events are mostly generated. In contrast, the
case where the Ib values become small implies
nucleation of large AE events.
SiGMA analysis with AIC picker
The SiGMA analysis consists of 3-D (three-dimen-
sional) AE source location procedure and moment
tensor analysis for AE source. Two parameters of the
arrival time (P1) and the amplitude of the first mo-
tion (P2) are read and applied to the analysis. In AE
source location procedure, AE source is located from
the arrival time differences ti between the observation
point xi and xi+1, by solving equations,
Ri � R iþ1 ¼ jxi � x0j � jxiþ1 � x0j ¼ mpti: (5)
Here, vp is the velocity of P wave.
After determining the AE source location, the
amplitudes of the first motion (P2) are substituted
into the following equation.
AðxÞ ¼ CS � Refðt; cÞ
R
� cpcqMpq �DA (6)
Here, A(x) is the amplitude of the first motion and CS
is the calibration coefficient of the sensor sensitivity
and material constants. The reflection coefficient
Ref(t,c) is obtained as t is the direction of sensor
sensitivity. DA is area of crack surface, Mpq is the
moment tensor and c is the direction vector of dis-
tance R from the source to the observation point x. As
the moment tensor Mpq is symmetric and of the sec-
ond rank, the number of independent unknownsMpq
is six. To determine the moment tensor components,
waveforms are to be detected at more than six sen-
sors. The classification of a crack is performed by the
eigen-value analysis of the moment tensor [15].
Eventually, microcracks are visualised by employing
the Light Wave 3D software (New Tek) as shown in
Figure 7. Here, an arrow vector indicates a crack
motion vector, and a circular plate corresponds to a
crack surface, which is perpendicular to a crack nor-
mal vector.
In the conventional SiGMA, determination of the
two parameters of P1 and P2 for the SiGMA analysis
has been carried out one-by-one via a software
package named ‘‘wave-monitor’’. To process many
AE waveforms, easy and quick determination of the
first motion is in great demand. An auto-picker is
developed by combining the auto-regressive model
with AIC (Akaike Information Criterion) method
[26]. Here, a direct AIC method is applied to deter-
mination of the arrival time for the SiGMA analysis
[27]. As the number of amplitudes of a digitised AE
wave is N and values of amplitudes are Xi (i = 1, 2,
..N), AICk at point i = k is represented as
AICk ¼ k � logfvarðX½1;k�Þg þ ðN � kÞ � logfvarðX½k;N�Þg;
(7)
where var(X[1,k]) indicates the variance between X1
and Xk, and var(X[k,N]) is also the variance between
Figure 6: AE waveform parameters
Figure 7: Crack models in SiGMA analysis
182 � 2010 Blackwell Publishing Ltd j Strain (2011) 47 (Suppl. 2), 179–186
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Corrosion Process and Mechanisms of Corrosion-Induced Cracks : M. Ohtsu et al.
Xk and XN. The AIC method defines the onset point
as the global minimum. In the automated detection
of the first motion (auto-picker), this value was
adopted. Then, the parameter P1 of arrival time is
determined by applying an equation,
P1 ¼ Tk MinðAICkÞgf � DT ; (8)
where Tk[Min(AICk)] represents the period when
AICk becomes the minimum value at i = k and DT is
sampling time which is set to 1 ls in our experi-
ments. As for the determination of the parameter
P2, amplitude Xi which satisfies the following
equation is adopted.
P2 ¼ Xi ; when ðXi � Xi�1ÞðXiþ1 � XiÞh0 ; (9)
where small index i represents between k + 1 and N.
Results and Discussion
AE activity
Total number of AE hits and AE events, which were
located reasonably inside the specimen, are shown in
Figure 8. Generating process of AE hits observed is
classified into four stages, referring to the curve of
corrosion loss in Figure 1. After 48 h elapsed, the
increase in AE hits is observed as Stage 1. Following
the decrease in AE hits at Stage 2, high AE activity is
again observed at Stage 3. AE events, which were
simultaneously detected at six channels and suc-
cessfully located, are mostly observed at Stage 4. The
number of these events observed is denoted in the
figure. It is confirmed that the curve of AE activity
(total AE hits) is in remarkable agreement with the
curve in Figure 1. This implies that the stages in
Figure 8 reasonably correspond to the phases in Fig-
ure 1. Accordingly, it leads to the fact that the onset
of corrosion started at Stage 1 after 48 h elapsed, and
the expansion of corrosion products occurred at
Stage 3, continuing at Stage 4. It clearly implies that
AE events observed at Stage 4 result from corrosion-
induced cracking in concrete. It suggests that the
occurrence of corrosion-induced cracking in concrete
is readily detected and located by AE measurement.
Parameter analysis
Variations of the RA values and the averaged fre-
quency are given in Figure 9. Although trends of the
variations are not clear at Stages 1, 2 and 3, the RA
values start to decrease and the averaged frequencies
are increasing from Stage 3. At Stage 4, the RA values
are constantly low and the average frequencies are
high, suggesting the nucleation of tensile cracks.
From 72 to 96 h elapsed at Stage 1, an abrupt de-
crease in the average frequency is observed, corre-
sponding to the period when the 1st increase in AE
hits is observed in Figure 8. This suggests that cracks
other than tensile cracks are nucleated because of the
onset of corrosion in rebar. At Stage 4, as tensile
cracks are actively nucleated, the generation of cor-
rosion-induced cracks is evident.
According to the Ib values in Figure 10, the values
suddenly decrease at around 288 h elapsed, when the
most activity of AE events is observed in Figure 8.
This implies that large-scale tensile cracks are gener-
ated as corrosion-induced cracks. The Ib values be-
come large after the sudden decrease in Figure 10,
while the RA values are consistently low in Figure 9.
It suggests that large-scale tensile cracks are domi-
nantly generated around 288 h elapsed. It has been
reported [2] that at the 1st high AE activity of Stage 1,
AE sources were of small amplitudes and classified as
other-type cracks. At this stage, chloride concentra-
tion at the level of rebar was just higher than the
lower-bound level for the initiation of corrosion. At
Figure 8: AE activity during the test
� 2010 Blackwell Publishing Ltd j Strain (2011) 47 (Suppl. 2), 179–186 183
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M. Ohtsu et al. : Corrosion Process and Mechanisms of Corrosion-Induced Cracks
the 2nd high AE activity
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